Material Matters - Sigma
Volume 11, Number 2
Three-dimensional Science
Printing the Future in
Multiple Dimensions
3D PRINTING GRAPHENE INK:
CREATING ELECTRONIC AND BIOMEDICAL STRUCTURES
AND DEVICES
BIOPRINTING
FOR TISSUE ENGINEERING AND REGENERATIVE MEDICINE
3D AND 4D PRINTING TECHNOLOGIES:
AN OVERVIEW
3D PRINTABLE CONDUCTIVE
NANOCOMPOSITES
OF PLA AND MULTI-WALLED CARBON NANOTUBES
NANOPARTICLE-BASED ZINC OXIDE
ELECTRON TRANSPORT LAYERS
FOR PRINTED ORGANIC PHOTODETECTORS
Introduction
Welcome to the second issue of Material Matters™ for 2016, focusing on
multi-dimensional printing technologies and printing materials.
A number of dramatic technological innovations have added a great
deal of character and dimension to the rapidly developing story of threedimensional (3D) printing technologies. Seemingly all at once, new printing
technologies have the potential to change everything from daily life to
the global economy. Various two-dimensional (2D) printing techniques
like inkjet and screen printing are already being used for the fabrication of
Jia Choi, Ph.D.
flexible electronic devices. 3D printing is rapidly emerging to attract interest
Aldrich Materials Science
from both the academic community and the business world. Numerous
studies have been performed to improve the methods and instrumentation for 3D printing using
a wide range of new and existing materials, including plastic, metal, ceramic, wood, nanomaterials
(like graphene), and even biomaterials. In this issue of Material Matters, we concentrate on recent
advances in multi-dimensional printing technologies, from 2D to 4D, and the promising applications
employing these printing techniques in multiple disciplines.
In our first article, Prof. Ramille N. Shah et al. (Northwestern University, USA) highlight novel graphene
inks for 2D and 3D printing. Developments in 2D and 3D-printable graphene-based materials began
with the development of ready-to-use graphene materials for device research and engineering. The
authors demonstrate that their 3D graphene ink can be used to print large, robust 3D structures
containing 60–70% graphene and exhibit unique mechanical and biological properties.
Prof. Peter Yang et al. (Stanford University, USA), in the second article, review the use of bioprinting
for tissue engineering and regenerative medicine. Bioprinting is a new biofabrication technology
used to create cellular constructs through the printing of polymer, ceramic, or other scaffolds, or
even through the printing of the cells themselves. An increasing demand for new disease models,
more predictive toxicity screening methods, and the emerging potential of tissue and organ printing
is stimulating the development of bioprinting. The article introduces bioprinting approaches based
on materials and discusses the current challenges, potential solutions, and bioprinting trends.
In the third article, Dr. Wonjin Jo et al. (Korea Institute of Science and Technology, South Korea)
provide a brief overview of multi-dimensional printing technologies. The authors highlight recent
advances in printing processes and printing materials development for 3D printing. They also
introduce the concept of “4D printing” in which the form or function of a printed structure changes
with time in response to stimuli such as temperature, light, or pressure.
Prof. Daniel Therriault et al. (École Polytechnique Montréal, Canada) discuss the promises offered
by nanomaterial-based nanocomposites in the fourth article. The authors specifically focus on
conductive carbon nanotubes and polymer nanocomposites for 3D printing. This research shows
the strong potential for 3D printing as a novel method for manufacturing nanocomposites with
promising applications such as reinforced structural parts, flexible electronics, electromagnetic
shielding grids, and liquid sensors.
The final article by Dr. Gerardo Hernandez-Sosa et al. (Karlsruher Institut für Technologie, Germany)
describes printed organic photodiodes which utilize ZnO nanoparticle-based electron transfer layers.
The organic photodiodes featured in the article were fabricated by inkjet or aerosol printing of active
organic materials and ZnO nanoparticle inks. The authors point out that using nanoparticles instead
of sol-gel precursor-based layer depositions offers significant processability advantages for the
manufacture of ideal flexible printed electronics.
Each article in this publication is accompanied by a list of relevant materials available from Aldrich®
Materials Science. For additional product information, visit us at aldrich.com/matsci. As always,
please bother us with your new product suggestions as well as thoughts and comments for Material
Matters™ at [email protected]
About Our Cover
Three-dimensional (3D) printing is the process of creating a 3D object from a digital file. There
is currently intense innovation and energy focused on 3D printing, making it an area worthy of
attention and investment for many years to come. New approaches are continually being introduced
that facilitate faster printing with higher resolution and using a wider array of materials, allowing
advancement in many areas of research. The cover art for this issue expresses 3D printing, providing
new design freedom to shape the future.
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Vol. 11, No. 2
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Table of Contents
Your Materials Matter
Articles
3D Printing Graphene Ink: Creating Electronic and Biomedical
Structures and Devices
41
Bioprinting for Tissue Engineering and Regenerative Medicine
49
3D and 4D Printing Technologies: An Overview
56
3D Printable Conductive Nanocompositesof PLA and Multi-walled
Carbon Nanotubes
61
Nanoparticle-based Zinc Oxide Electron Transport Layers for Printed
Organic Photodetectors
67
Featured Products
Bryce P. Nelson, Ph.D.
Materials Science Initiative Lead
We welcome fresh product ideas. Do you have a material or compound you
wish to see featured in our Materials Science line? If it is needed to accelerate
your research, it matters. Send your suggestion to [email protected] for
consideration.
Prof. Lei Zhai of the University of Central Florida (USA) recommended
the addition of edge-oxidized graphene oxide (EOGO, Aldrich Prod.
No. 794341) to our catalog for use as a two-dimensional filler to improve
the mechanical, thermal, and electrical properties of thermoplastics.
3D Printable Graphene Ink
A list of Graphene inks for 3D printing
46
Nanocarbon Inks for Printing
A list of graphene and CNT inks
46
Graphene
A list of graphene, graphene nanoplatelets and graphene nanoribbons
47
Reduced Graphene Oxide
A list of RGO materials
47
Graphene Oxide
A list of GO materials
47
Graphene Nanocomposites
A selection of graphene and reduced graphene oxide based nanocomposites
48
Biodegradable Polymers
A selection of PLA, PCL, and poly(lactides) for printing
52
Poly(ethylene glycol)s (PEG)
A list of PEGs for 3D printing
55
3D Printing Filaments
An assortment of 3D printing filaments
59
3D Printing UV Curable Resins
A selection of UV curable resins for 3D printing
60
Carbon Nanotubes
A selection of single, double, multi-walled, and functionalized CNTs
64
Nanoparticle Inks for Printing
A selection of ZnO, Al-doped ZnO, TiO2, and Ag nanoparticle inks for printing
69
Organic Conductive Inks
A list of organic conductive inks
71
Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)
A list of PEDOT:PSS conductive materials
71
Polythiophene (PT)
A list of PT materials
71
Fullerenes
A selection of fullerenes
72
•• Form: Brown/Black suspension
•• Bulk Density: ~1.8 g/cm3
•• Number of layers: 15–20
Product of Garmor Inc.
Indium Tin Oxide (ITO) Coated Substrates
A selection of ITO on glass and PET substrates
72
794341-50ML
50 mL
794341-200ML
200 mL
This few-layer graphene oxide has a high aspect ratio (1–5 nm thick
and 400 nm in diameter) and high electrical conductivity, making it an
effective and conductive filler. As a result, a conductive EOGO network
can be created in a polymer matrix with low percolation thresholds and,
therefore, with low filler loading. This is an important feature for functional
composites with high electrical conductivity because composites typically
become embrittled at a high filler content.1,2 A variety of methods
including melt blending, compression/grind blending, solvent blending,
and in situ polymerization have been used to produce EOGO/polymer
composites with greatly enhanced electrical conductivity—up to 1012
times of that of the host polymer. The enhanced performance is due
to the unique EOGO structure that consists of hydroxyl and carboxyl
groups on the perimeter and a non-oxidized, graphitic basal plane. This
ambipolarity allows for facile dispersion of EOGO into a wide range
of polymeric hosts and solvents, while preserving the useful electrical
properties of few-layer graphene.
References
(1) Du, Jinhong; Cheng, Hui-Ming. Macromol. Chem. Phys. 2012, 213, 1060–1077
(2) Singh, Virendra; Joung, Daeha; Zhai, Lei; Das, Soumen; Khondaker, Saiful I.; Seal, Sudipta. Prog.
Mater. Sci. 2011, 56(8), 1178–1271.
Gra­phene oxide
CxOyHz
15–20 sheets, 4–10% edge-oxidized, 1 mg/mL, dispersion in H2O
41
TRANSPARENT
CONDUCTIVE CNT INKS
Printable • Environmentally Stable • Stretchable
Formulated using patented CoMoCAT™ CNTs, aqueous and
solvent-based (V2V™) conductive inks are setting a new standard
for transparent conductor performance in applications where
durability and environmental stability are paramount.
A)
B)
C)
D)
V2V™: Print. Dry. Done.
Conductive CNT Ink Systems are optimized for screen printing:
yy
Dries quickly, evenly at low temperature
yy
Contains no surfactants or electrically active
dispersants, binders1
yy
Adheres strongly to common screen printing substrates
A) Thermoformed CNT touch sensor prototype, B) Capacitive CNT touch screen array,
C) TEM scan of rod-coated CNT network (~10 mg/mm2), D) CNT TCF at 95% VLT.
SWeNT® Conductive Inks
Sheet Resistance2 (Ω/sq)
AC100
Description
Purpose
SWCNT in aqueous surfactant solution
Spray Coating
85% VLT
90% VLT
92% VLT
137
237
330
791490
Prod. No.
AC200
SWCNT in aqueous surfactant solution
Meyer-Rod/Slot-Die Coating
166
251
317
791504
VC101
SWCNT in proprietary solvent system (V2V)
Screen Printing
783
1,466
2,206
792462
1. V2V inks (e.g., VC101) contain electrically inert sulfonated tetrafluoroethylene (Nafion).
2. SR measurements for AC100, 200 taken with top coat.
For detailed product information on SWeNT® Conductive Inks, visit
aldrich.com/swnt
3D Printing Graphene Ink: Creating Electronic and Biomedical Structures and Devices
3D PRINTING GRAPHENE INK:
CREATING ELECTRONIC AND
BIOMEDICAL STRUCTURES AND DEVICES
2D Ink
• Low viscosity
• Moderate drying rates
• Extended shelf-life
Deposition
Characteristics
INKJET
Discrete
Droplets
The growing interest in graphene has led to commercial efforts to
produce graphene and its derivatives at scale. As a result, graphene is
now available in a variety of forms, including unmodified and modified
powders, films, liquid suspensions, and more. More recently, the
development and availability of new, easy-to-utilize graphene-based 2D
and three-dimensional (3D) printing inks8–10 (Prod. Nos. 798983, 793663,
796115, and 808156) provide researchers with the necessary tools to
develop and engineer graphene-based devices and applications in many
areas such as flexible electronics and sensors,9,10 bioelectronics, and nerve,
muscle, and bone tissue engineering constructs and devices.8
2D vs. 3D Graphene Printing Inks
It is important to distinguish between 2D inks intended for the fabrication
of planar devices and 3D inks intended for the fabrication of volumetric
constructs and devices (Figure 1).8,11,12 The rapidly emerging worldwide
interest in 3D printing from consumers, researchers, and those interested
in the industrial production of end-use parts has generally outpaced
an adequate understanding of the underlying technologies, their uses,
restrictions, and requirements, particularly 3D printing materials. This has
frequently resulted in unintentional but false equivocation of established
2D printing technologies with newer 3D printing technologies and
associated materials and their uses.
3D Ink
Thermo or piezo-electric driven
Since its discovery little more than a decade ago,1 the two-dimensional
(2D) allotrope of carbon—graphene—has been the subject of intense
multidisciplinary research efforts. These efforts have not only revealed the
exceptional electrical,2 mechanical,3 thermal,4 and biological5,6 properties
of graphene, but have also lead to the discovery of an entire class of 2D
materials with unique and potentially highly advantageous properties.7
As the knowledge and understanding of graphene and its properties has
grown, so too has the interest in elevating this material from a scientific
curiosity to a material that can be widely and readily applied to a broad
range of applications and devices.
• Moderate viscosity
• Extremely rapid
drying rates
• Self-supporting upon
deposition
• Extended shelf-life
Deposited Material
Cross Sections
SYRINGE
EXTRUSION
Plastic or metal
nozzle
Room temperature
Continuous
fibers
Pneumatically or
mechanically driven
Extrusion Pressures = 5–800 kPa
Substantially wets substrate
(Difficult to remove from
substrate)
50–200 µm
Introduction
Graphene Ink
Characteristics
<10 µm
Adam E. Jakus, Ph.D.1,3*, Ramille N. Shah, Ph.D.1,2,3
1
Materials Science and Engineering, McCormick School of Engineering,
Northwestern University, Evanston, IL 60208 USA
2
Comprehensive Transplant Center and Department of Surgery, Feinberg School of Medicine,
Northwestern University, Chicago, IL 60611 USA
3
Simpson-Querrey Institute for BioNanotechnology in Medicine,
Northwestern University, Chicago, IL 60611 USA
*Email: [email protected]
2D graphene inks generally make use of well-established inkjet10 and
gravure-related9 printing hardware and processes. The 2D processes
utilize 2D inks with similar requirements as those used for inkjet printing,
including low viscosity (Figure 1). The need for low viscosity limits the
maximum graphene content within the ink, but enables very rapid,
highly precise deposition of material onto flat substrates. The resulting
small or large area 2D graphene structures retain mechanical flexibility,
high electrical conductivity, and thermal and chemical stability after
deposition,9,10 making these materials exceptionally useful for a variety of
current and future electronic and energy applications.
Partially wets substrate prior
to rapid drying
(Easy to remove from substrate)
Printed Material
Characteristics
• Extremely high
electrical conductivity
• Mechanically
flexible (if on flexible
substrate)
Subsequent material
can span gaps
• High electrical
conductivity
• Mechanically
flexible
• Highly bioactive
Figure 1. Characteristics comparison of 2D and 3D graphene inks, their deposition processes, and
printed material characteristics.
Graphene has also demonstrated substantial promise for 3D materials
fabrication.8 For example, an extensive variety of graphene-containing
composite foams,13 hydrogels,14 and thermoplastics15 are already in use.
3D printing graphene inks are a new class of graphene materials that can
be utilized to rapidly create user-defined 3D graphene structures.8 Unlike
2D graphene inks, which do not need to be mechanically self-supporting
upon deposition onto a substrate, 3D inks must satisfy a much broader
set of requirements,11 such as retaining significant material functionality
as well as having the ability to be printable into volumetric structures
comprised of one to many thousands of layers. We have developed 3D
printing inks that are printed via room-temperature extrusion from a
mechanically or pneumatically driven syringe (Figure 1) in much the
same way an individual uses a standard syringe to extrude material from a
needle or nozzle by hand. The 3D inks and associated room-temperature
extrusion 3D printing process are distinct from the widely used fuseddeposition modeling (FDM) 3D printing approaches, which utilize
thermoplastic filaments extruded at elevated temperatures.
3D printing graphene inks8 and related materials employ an evaporation
driven solidification mechanism, whereby an ink is formulated by
dissolving a polymer in a fast-evaporating solvent such as chloroform
or dichloromethane. The ink is extruded at ambient or near ambient
temperatures and rapidly solidifies as the solvent evaporates and the
polymer comes out of solution. These 3D inks are user friendly, easily
printable, print very rapidly, and exhibit highly advantageous functional
materials properties.
For questions, product data, or new product suggestions, contact us at [email protected]
43
aldrich.com/matsci
3D Printing Graphene Ink Characteristics
Versatility and Handling
3D printing graphene ink8 (Prod. No. 808156) is a moderate viscosity
(25–35 Pa.s) graphene suspension comprised of graphene, dissolved
elastomeric polymer binder, and a mixture of solvents that can be 3D
printed (or used with any standard syringe) from a nozzle (50–2,000 µm
in diameter; Figure 2A) under ambient conditions to rapidly create
3D graphene-based constructs that can be handled immediately (no
drying time required; Figure 2B). The resulting 3D printed material is
comprised of 60 vol.% graphene and 40 vol.% elastomeric polymer. Due
to the rapid solidification of the 3D printing graphene ink, it must not
be exposed to an open environment for an extended period of time
when not in use. It may be possible to recover dried inks by adding a
small volume of dichloromethane (Prod. Nos. 270563, 676853, 320269,
and more) followed by mechanical stirring or shaking to homogenize.
However, inks that are reconstituted in this way tend to suffer from
more nozzle clogging events than the unreconstituted native inks. The
solvent contents of the 3D inks also require that they not be exposed
to materials that are soluble in dichloromethane, including polystyrene
and low density polyethylene, prior to washing (more on washing in the
following sections).
A 3D-printed graphene object can be physically handled immediately
after production and is surprisingly robust despite the high graphene
content. Residual solvents can be removed by washing objects in 70%
ethanol.8 Few-layer objects, such as sheets, can be rolled, folded, and cut
in a similar manner to standard paper (Figure 3A). Even thick objects
can be cut or “punched” to yield many samples of defined sizes from a
single 3D-printed object (Figure 3B). Finally, the nature of the 3D printing
graphene inks enables independently 3D-printed graphene objects to
be fused together to create larger and/or much more complex objects
than could be 3D-printed directly (Figure 3C). This is achieved through
conservative application of small volumes of graphene inks at contact
surfaces of one or both of the parts to be joined. The freshly deposited
ink locally dissolves the polymer matrix in the 3D-printed graphene and
rapidly evaporates, bringing the just-dissolved polymer out of solution
and creating a mechanically and electrically seamless interface between
the joined objects.8
A)
B)
2
3D-printed Graphene Objects
Fold
100 µm
200 µm
410 µm
610 µm
1,200 µm
B)
150-Layer
Triple Helix
t
hee
er S
-Lay
C)
1-Layer
“Graphene”
100 µm
200 µm
Multi-Layer
Cylinders
D)
3
48-Layer
Octopus
1 cm
25-Layer
Square
5 µm
5 µm
Figure 2. A) Photograph of graphene fibers extruded from nozzles with indicated diameters.
Typically, the rapid drying of the 3D printing graphene ink upon deposition results in a 10%
diameter reduction (i.e., Ink extruded from 100 µm tip results in ~90 µm fiber). B) Photograph of a
variety of few- and many-layer graphene architectures 3D-printed from the 3D printing graphene
ink using a range of nozzle diameters. C) SEM of 0–90° alternating and 0–120–240° alternating
graphene structures produced from 3D printing graphene inks and extruded from 100 and
150 µm nozzles, respectively. D) Scanning electron micrograph (SEM) of surface and interior
3D-printed graphene fiber of 100 µm diameter. Modified from Reference 8.
44
3
Cut/Punch
A syringe-based 3D printer is not required to use the 3D printing
graphene ink, although it is desirable for precision X, Y, Z spatially
controlled deposition of the material. In place of a 3D printing platform,
a standard hand or mechanically driven syringe may be used to extrude
the 3D printing graphene inks and produce solid structures. Due to
the elongated morphology of the comprising graphene flakes and
shear forces from nozzle extrusion, the 3D printed graphene adopts
a micro-texture defined by the lengths of the individual graphene
particles aligned with, and stacked perpendicular to the fiber direction
(Figure 2D).8 Fabrication of multi-layer graphene objects is done through
deposition of sequential materials in a pre-defined pattern and predefined object geometry (Figure 2C). The software associated with
defining internal patterns and overall object geometry varies greatly and
is typically specific to individual 3D printing platforms.
A)
1
C)
5
4
Roll
Fuse
5 mm
1 cm
Figure 3. A) Graphene structures from 3D-printed graphene inks are flexible, and in the case of
sheets (8 cm diameter), can be easily rolled, folded, cut, etc. B) Smaller graphene objects can also
be directly cut or punched from larger 3D-printed graphene objects. This photo illustrates circular
graphene samples punched from the 25-layer graphene square shown in Figure 2. C) Photograph
of anatomically correct, scaled human skull, mandible, and upper spine produced by fusing five
independently 3D-printed graphene parts together by lightly applying 3D printing graphene inks
at points of contact.
Mechanical, Thermal, and Electrical Properties
3D-printed graphene objects derived from 3D printing graphene inks are
mechanically plastic in nature (Figure 4A,4B),8 can undergo substantial
strain (>80%) prior to failure, and exhibit yield and ultimate tensile
strengths of less than 1 MPa. Thus, 3D-printed graphene objects are
relatively soft in nature and can be shaped and modified after 3D printing
to suit individual requirements. Although graphene itself is compatible
with high temperatures,4 the elastomeric matrix (responsible for the highly
versatile mechanical properties), comprising 40% of the solids volume of
the material, is not. The polymer will decompose at temperatures at or
above 150 °C, which causes the material to become mechanically brittle
(Figure 4B),8 even though the 3D-printed architecture is maintained.
Due to the high graphene content, objects created from 3D printing
graphene inks are electrically conductive, exhibiting as-printed
conductivities in excess of 650 S/m, which can be improved to >870 S/m
if the material is thermally annealed in air at 50 °C for approximately
30 minutes (Figure 4C).8 This is the highest recorded conductivity for a
3D-printed material that is not a metal or alloy. The nature of the ink and
3D printing process also ensures that printed layer boundaries do not act
as electrical defects, which would otherwise inhibit conductivity across
small and large objects.
TO ORDER: Contact your local Sigma-Aldrich office or visit aldrich.com/matsci.
3D Printing Graphene Ink: Creating Electronic and Biomedical Structures and Devices
Tension
Tensile Stress (kPa)
400
B)
300
E = 3.0 ± 0.4 MPa
σYield = 231 ± 18 kPa
σUTS = 373 ± 36 kPa
StrainFailure = 81.3 ± 9.7%
200
100
0
0
0.2
0.4
0.6
0.8
1
Biological Properties
Compression
700
Compressive Stress (kPa)
A)
<150 °C
600
500
>150 °C
400
300
200
100
0
Tensile Strain (-)
0
0.1 0.2 0.3 0.4 0.5 0.6
Compressive Strain (-)
C)
1 cm
Figure 4. A) Despite the exceptionally high graphene content, 3D-printed graphene objects
are relatively soft and can withstand upwards of 80% tensile strain. B) Under compression,
3D-printed graphene objects plastically deform if not previously heated to temperatures ≥150 °C.
C) Photograph illustrating electrical conductivity of a 3D-printed graphene object (triple helix
shown in Figure 2). Adapted from Reference 8.
N=4
Fold Increase over Day 0 hMSCs
B)
In vitro (hMSCs)
A)
50
40
Bioactivity and the potential of biocompatibility are the most exceptional
aspects of 3D-printed graphene from 3D printing graphene inks.8 Once
washed to remove residual solvents, 3D-printed graphene contains
only graphene flakes, and a biocompatible elastomeric polymer. In
vitro studies using bone marrow derived adult human mesenchymal
stem cells (hMSCs) and cultured in standard DMEM (Dulbecco’s
Modified Eagle’s medium) growth medium with fetal bovine serum (no
biochemical, mechanical, or electrical differentiation cues) illustrate that
3D-printed graphene not only supports stem cell viability (Figure 5A)
and proliferation over the course of at least weeks, but that the stem
cells begin differentiating into glial and neuron-like cells, as indicated
by both gene expression and cell morphology (Figure 5B).8 This is
remarkable, and the first time a material alone (without additional
biological factors) has induced such strong neurogenic behavior in adult
human stem cells. Preliminary in vivo experiments using a BULBc mouse
subcutaneous model reveal that over the course of 7 and 30 days, native
tissues rapidly integrate with and vascularize the implanted 3D-printed
graphene constructs (Figure 5C–F) with no significant immune response.8
Combined with its ability to be 3D-printed into nearly any form, its
ability to be mechanically manipulated, its electrical conductivity, and
its bioactivity, 3D-printed graphene is an excellent addition to the 3D
printing biomaterials palette,11 with many fundamental and translational
applications on the horizon.
Upregulation of
glial and neuronspecific
genes
= 20 vol.%
3D Printing
Graphene
30
20
10
0
GFAP
Tuj1
Nes
Day 7
MAP2
GFAP
Tuj1
Nes
Day 14
50 µm
MAP2
100 µm
10 µm
In vivo (Mouse SubQ)
C)
E)
F)
D)
100 µm
10 µm
Figure 5. A) Top: Confocal microscopy reconstruction, top-down view of live (green) and dead (red) human mesenchymal stem cells on 3D-printed graphene 21 days after initial cell seeding;
Bottom: confocal microscopy reconstruction showing cytoskeletal extensions (red) and cell nuclei (blue). B) Glial and neurogenic-relevant gene expression of hMSCs on 3D-printed graphene and lower
content graphene material, 7 and 14 days after initial cell seeding and cultured in simple DMEM + FBS medium. Corresponding images are live/dead confocal reconstructions of hMSC derived neuronlike cells on 3D-printed graphene 14 days after initial seeding. Modified from Reference 8. C) H&E histological micrograph of 3D-printed graphene scaffold explanted 7, and D) 30 days after subcutaneous
implantation into the backs of female BULBc mice. Black is the cross-section of individual 3D-printed graphene struts comprising the scaffold, pink is new cellular and extracellular tissue, and purple/
blue are cell nuclei. E) SEM of 3D-printed graphene scaffold and integrated tissue 7 days after subcutaneous implantation into mice. Cross-section of comprising graphene structures, outlined by yellow
dotted lines. F) SEM micrograph of day 30 explanted in vivo 3D-printed graphene sample showing tight interface between 3D-printed graphene material and new, integrated tissue.
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Conclusions and Future Prospects
Recent developments in 2D and 3D printable graphene-based materials
are beginning to bring the story of graphene full circle; from the discovery
of graphene little more than decade ago, to the extensive fundamental
research into its properties and their underlying mechanisms, to
large-scale synthesis, to the development of ready-to-use graphene
materials for device research and engineering. Based on these rapid
developments and continued interest, it is safe to say that graphene
materials are successfully making the transition from scientific curiosity
to an indispensable tool, forming the foundation for the development
of a broad array of new, advanced electronic, bioelectronics, and
biomedical technologies.
Acknowledgments
The authors acknowledge the support and the use of the following
facilities: Northwestern University Cell Imaging Facility supported by NCI
CCSG P30 CA060553 awarded to the Robert H. Lurie Comprehensive
Cancer Center; EPIC facility (NUANCE Center_Northwestern University)
supported by NSF DMR-1121262 and EEC-0118025|003; Northwestern
University Mouse Histology and Phenotyping Laboratory and Cancer
Center supported by NCI CA060553; and the Equipment Core Facilities at
the Simpson Querrey Institute for BioNanotechnology at Northwestern
University developed by support from The U.S. Army Research Office, the
U.S. Army Medical Research and Material Command, and Northwestern
University. This research was also supported by Northwestern
University’s International Institute for Nanotechnology (NU# SP0030341),
Northwestern University’s McCormick Research Catalyst Award and the
Office of Naval Research MURI Program (N00014-11-1-0690).
References
(1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.;
Firsov, A. A. Science. 2004, 306, 666.
(2) Castro Neto, A. H.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K. Rev. Mod. Phys. 2009,
81, 109.
(3) Lee, C.; Wei, X. D.; Kysar, J. W.; Hone, J. Science. 2008, 321, 385.
(4) Balandin, A. A.; Ghosh, S.; Bao, W. Z.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N. Nano Lett.
2008, 8, 902.
(5) Shen, H.; Zhang, L. M.; Liu, M.; Zhang, Z. J. Theranostics. 2012, 2, 283.
(6) Zhang, Y.; Nayak, T. R.; Hong, H.; Cai, W. B. Nanoscale. 2012, 4, 3833.
(7) Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L.-J.; Loh, K. P.; Zhang, H. Nat. Chem. 2013, 5, 263.
(8) Jakus, A. E.; Secor, E. B.; Rutz, A. L.; Jordan, S. W.; Hersam, M. C.; Shah, R. N. ACS Nano. 2015, 9,
4636.
(9) Secor, E. B.; Lim, S.; Zhang, H.; Frisbie, C. D.; Francis, L. F.; Hersam, M. C. Adv. Mater. 2014, 26,
4533.
(10) Secor, E. B.; Prabhumirashi, P. L.; Puntambekar, K.; Geier, M. L.; Hersam, M. C. J. Phys. Chem. Lett.
2013, 4, 1347.
(11) Jakus, A. E.; Rutz, A. L.; Shah, R. N. Biomed. Mater. 2016, 11, 014102.
(12) Jakus, A. E.; Taylor, S. L.; Geisendorfer, N. R.; Dunand, D. C.; Shah, R. N. Adv. Funct. Mater. 2015,
25, 6985.
(13) Zhao, Y.; Liu, J.; Hu, Y.; Cheng, H.; Hu, C.; Jiang, C.; Jiang, L.; Cao, A.; Qu, L. Adv. Mater. 2013, 25,
591.
(14) Chen, J.; Sheng, K.; Luo, P.; Li, C.; Shi, G. Adv. Mater. 2012, 24, 4569.
(15) Leigh, S. J.; Bradley, R. J.; Purssell, C. P.; Billson, D. R.; Hutchins, D. A. PLOS ONE 2012, 7.
3D Printable Graphene Ink
For a complete list of available materials, visit aldrich.com/3dp.
Particle Size (μm)
Viscosity (Pa.s)
Resistivity (Ω/cm)
Prod. No.
1 - 20 (length and width)
1 - 15 (thick)
25-45 (At low shear stresses. Shear thins to ~10-15 Pa.s at
Shear Stress = 100 Pa)
0.12-0.15 (as 3D-printed fibers, not ink, 200-400 μm
diameter)
808156-5ML
Nanocarbon Inks for Printing
For a complete list of available materials, visit aldrich.com/inks.
Graphene Inks
Name
Particle Size
Viscosity
Resistivity
Prod. No.
Graphene dispersion, with ethyl cellulose
in cyclohexanone and terpineol, inkjet
printable
≤3 μm
8-15 mPa.s at 30 °C
resistivity 0.003-0.008 Ω/cm (thermally annealed 250 °C
for 30 minutes, film thickness >100 nm)
793663-5ML
Graphene dispersion, with ethyl cellulose
in terpineol, gravure printable
≤3 μm
0.75-3 Pa.s at 25 °C
resistivity 0.003-0.008 Ω/cm (thermally annealed 250 °C
for 30 minutes, film thickness >100 nm)
796115-10ML
Graphene dispersion, with ethyl cellulose
in terpineol, screen printable
≤3 μm
5-50 Pa.s at 25 °C
resistivity 0.003-0.008 Ω/cm (thermally annealed 300 °C
for 30 minutes, film thickness >100 nm, 25 °C)
798983-10ML
Graphene ink in water, Inkjet printable
80-500 nm (exfoliated graphene flakes)
1 cP (100 s-1)
sheet resistance 4k Ω/sq (80 nm thickness)
808288-5ML
Graphene ink in water, flexo/gravure/
screen printable
500-1,500 nm (exfoliated graphene flakes)
570 cP (100 s-1)
140 cP (1,000 s-1)
sheet resistance 10 Ω/sq (25 μm thickness)
805556-10ML
Graphene ink in water, screen printable
500-1,500 nm (exfoliated graphene flakes)
350 cP (100 s-1)
1,800 cP (1,000 s-1)
sheet resistance 10 Ω/sq (25 μm thickness)
808261-10ML
Single-walled Carbon Nanotube Inks
SWCNT Concentration
Viscosity
Sheet Resistance
Prod. No.
1.00 ± 0.05 g/L (by Absorbance at 854 nm) dispersion in H2O (black liquid)
Form
3.0 mPa.s
sheet resistance <600 Ω/sq (at 85% VLT (ohm/sq), by
4-point probe on prepared film by spray)
791504-25ML
791504-100ML
1 mg/mL
17.7 Pa.s at 25 °C
sheet resistance <1,000 Ω/sq (by 4-point probe on
prepared, at 87.5% VLT (ohm/sq))
792462-25ML
792462-100ML
~1.0 mPa.s resistance <400 Ω/sq (by 4-point probe on prepared film 791490-25ML
791490-100ML
by spray)
viscous liquid (black)
0.20 ± 0.01 g/L (by Absorbance at 854 nm) dispersion in H2O (black liquid)
46
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3D Printing Graphene Ink: Creating Electronic and Biomedical Structures and Devices
Graphene
For a complete list of available materials, visit aldrich.com/graphene.
Graphene and Graphene Nanoplatelets
Name
Form Composition
Description
Prod. No.
Graphene nanoplatelets
powder
Graphene nanoplates as produced
Surfactant type: Anionic Surfactant
799084-500MG
powder
Carbon >95 wt. %
Oxygen <2 wt. %
hydrophobic
806668-25G
powder
Carbon >85 wt. %
Oxygen >3 wt. %
Dispersibility: dichloromethane, N-methyl-2pyrrolidone, and other non-polar solvents
hydrocarbon functionalized, hydrophobic
806633-25G
powder
Carbon >95 wt. %
Oxygen >1 wt. %
Dispersibility: Water, THF, DMF
oxidized
806641-25G
powder
Carbon >70 wt. %
Oxygen >10 wt. %
Dispersibility: water (high stability in aqueous
medium)
polycarboxylate functionalized, hydrophilic
806625-25G
dispersion in H2O, 1 mg/mL
Graphene 0.1 wt. %
Water 99.9 wt. %
Surfactant type: Anionic Surfactant
799092-50ML
dispersion (in NMP), 10 mg/mL
Graphene 1 wt. %
NMP 99 wt. %
dispersion in NMP
803839-5ML
Graphene dispersion
Graphene Nanoribbons
Name Purity
Dimension (L × W)
Surface Area (BET m2/g)
Prod. No.
Graphene nanoribbons, alkyl functionalized
≥85% carbon basis, TGA
2-15 μm × 40-250 nm
38
797766-500MG
Graphene nanoribbons
≥90.0% carbon basis, TGA
2-15 μm × 40-250 nm
48-58
797774-500MG
Reduced Graphene Oxide
For a complete list of available materials, visit aldrich.com/graphene.
Description
Composition
Conductivity
Prod. No.
chemically reduced
Carbon >95 wt. %
Nitrogen >5 wt. %
>600 S/m
777684-250MG
777684-500MG
chemically reduced by hydrizine
Carbon >75%
Nitrogen <5%
7,111 S/m (pressed pallet)
805424-1G
amine functionalized
Carbon >65 wt. %
Nitrogen >5 wt. %
-
805432-500MG
octadecylamine functionalized
Carbon >78 wt. %
Nitrogen >3 wt. %
6.36 S/m (pressed pellets)
805084-500MG
tetraethylene pentamine functionalized
Carbon >65 wt. %
Nitrogen >8 wt. %
-
806579-500MG
piperazine functionalized
Carbon >65 wt. %
Nitrogen >5 wt. %
70.75 S/m (pressed pellets)
805440-500MG
Graphene Oxide
For a complete list of available materials, visit aldrich.com/graphene.
Name
Form
Description
Prod. No.
Graphene oxide
film
4 cm (diameter) × 12-15mm (thickness), non-conductive
798991-1EA
powder
15-20 sheets
4-10% edge-oxidized
796034-1G
powder or flakes
sheets
763713-250MG
763713-1G
dispersion in H2O
1 mg/mL, 15-20 sheets
4-10% edge-oxidized
794341-50ML
794341-200ML
dispersion in H2O
2 mg/mL
763705-25ML
763705-100ML
dispersion in H2O
4 mg/mL, dispersibility: Polar solvents
monolayer content (measured in 0.5 mg/mL): >95%
777676-50ML
777676-200ML
Graphene oxide nanocolloids
dispersion in H2O
2 mg/mL
795534-50ML
795534-200ML
Graphene oxide, alkylamine functionalized
dispersion in toluene
2.0 mg/mL
<0.5 % (w/w)
809055-50ML
Graphene oxide, ammonia functionalized
dispersion in H2O
1 mg/mL
791520-25ML
791520-100ML
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Graphene Nanocomposites
For a complete list of available materials, visit aldrich.com/graphene.
Reduced Graphene Oxide-based Nanocomposites
Name
Particle Size (nm)
Composition
Prod. No.
Fe3O4/reduced graphene oxide nanocomposite
5-25 (Fe3O4 nanocrystal)
reduced graphene oxide 10-17%
Fe3O4 nanocrystal 3-8%
acetone ~80%
803804-5ML
Mn3O4/reduced graphene oxide nanocomposite
5-25 (Mn3O4 nanocrystal)
reduced graphene oxide 10-17%
Mn3O4 nanocrystal 3-8%
acetone ~80%
803812-5ML
Pd/reduced graphene oxide nanocomposite
5-50 (Pd nanocrystal)
reduced graphene oxide 5-20%
Pd nanocrystal <5%
acetone ~80%
803790-5ML
Pt/reduced graphene oxide nanocomposite
2-5 (Pt nanocrystal)
reduced graphene oxide 5-20%
Pt nanocrystal <5%
acetone ~80 wt. %
803782-5ML
PtCo/reduced graphene oxide nanocomposite
2-5 (PtCo nanocrystal)
reduced graphene oxide 10-18%
PtCo nanocrystal 2-10%
acetone ~80%
803901-5ML
PtPd/reduced graphene oxide nanocomposite
5-50 (PtPd nanocrystal)
reduced graphene oxide 10-18%
PtPd nanocrystal 2-10%
acetone ~80%
803820-5ML
Graphene-based Nanocomposites
Name
Particle Size (nm)
Composition
Prod. No.
Fe3O4/graphene nanocomposite
5-25 (Fe3O4 nanocrystal)
graphene 3-8%
Fe3O4 nanoparticle 4-9%
acetone ~80 wt. %
803715-5ML
Mn3O4/graphene nanocomposite
5-25 (Mn3O4 nanocrystal)
graphene 3-8%
Mn3O4 nanoparticle 4-9%
acetone ~80 wt. %
803723-5ML
Pd/graphene nanocomposite
5-50 (Pd nanocrystal)
graphene 6-10%
Pd nanoparticle 2-6%
acetone ~80 wt. %
803707-5ML
Pt/graphene nanocomposite
2-5 (Pt nanocrystal)
graphene 6-10%
Pt nanoparticle 1-4%
acetone ~80 wt. %
803693-5ML
PtCo/graphene nanocomposite
2-5 (PtCo nanocrystal)
graphene 10-15%
PtCo nanoparticle 5-10%
acetone ~80 wt. %
803766-5ML
PtPd/graphene nanocomposite
5-50 (PtPd nanocrystal)
graphene 10-15%
PtPd nanoparticle 5-10%
acetone ~80 wt. %
803758-5ML
48
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Bioprinting for Tissue Engineering and Regenerative Medicine
BIOPRINTING
FOR TISSUE ENGINEERING AND REGENERATIVE MEDICINE
Chi-Chun Pan,1,2 Arnaud Bruyas,1 Yunzhi Peter Yang1,3,4
Departments of 1Orthopedic Surgery, 2Mechanical Engineering,
3
Materials Science and Engineering, and 4Bioengineering
Stanford University, 300 Pasteur Drive, Stanford, CA 94305
Email: [email protected]
Introduction
In the past two decades, tissue engineering and regenerative medicine
have become important interdisciplinary fields that span biology,
chemistry, engineering, and medicine.1,2 These new fields promote the
healing and restoration of lost function in damaged or diseased tissues
and organs by combining scaffolds, cells, and biological signaling
molecules to recreate functional biological substitutes and mimic
native tissues and functions.3 One objective of tissue engineering and
regenerative medicine is the fabrication of viable tissues and organs
for transplantation, but with the exceptions of thin skin and avascular
cartilage,4 limited success in human patients has been achieved due
to the complexity of tissue biology. The traditional tissue engineering
approach includes loading cells onto a solid porous biomaterial, called
a scaffold, in the presence or absence of growth factors that encourage
cells to form the desired tissues with biomimetic complexity.5 However,
the desired result is rarely achieved because the three component
mixture does not adequately promote formation of a well-defined spatial
distribution of cells, growth factors, and biomaterials at the microscale
level that is characteristic of a tissue-like structure. Three-dimensional (3D)
printing, also known as additive manufacturing (AM), holds great promise
to overcome this limitation in tissue engineering. Because it is a layer-bylayer process, 3D printing enables the formation of complex geometries
using multiple materials (Figure 1). 3D printing for tissue engineering
has evolved into a new technology, called bioprinting, defined as
“the use of material transfer processes for patterning and assembling
biologically relevant materials, molecules, cells, tissues, and biodegradable
biomaterials with a prescribed organization to accomplish one or more
biological functions.”6 In particular, bioprinting enables personalizable
and precision medicine by engineering anatomically shaped implants
with tissue-like complexity using a patient’s own cells. Currently, 3D
bioprinting technologies can be classified into two categories: acellular
and cellular constructs.7 Acellular bioprinting is used to manufacture the
scaffold and biomaterial itself in the absence of cells during the printing
process. Acellular bioprinting offers higher accuracy and greater shape
complexity than cellular constructs because the fabrication conditions
are less restrictive than methods that require maintenance of cell viability.
For cellular bioprinting, cells and other biological agents are integrated
into the material during manufacturing in order to fabricate living tissue
constructs. It is clear that the printing parameters, biomaterials, and
properties of the 3D-printed constructs are, therefore, different in each
category because of the presence or absence of cells and biological
substances. Here we briefly introduce and discuss these two approaches
based on the suitable materials for these constructs and the fabrication
processes used to manufacture them. We also discuss current limitations,
potential solutions, and future directions in bioprinting.
Manufacturing of Acellular Scaffolds
An acellular scaffold consists of a porous structure that mimics the
mechanical and biochemical properties of the extracellular matrix
(ECM) and provides mechanical integrity as well as a template for cell
attachment in order to stimulate tissue formation.8 Acellular scaffolds
must present biocompatible and bioresorbable properties as well as
biochemical, biophysical, biomechanical, bioelectrical, and biomagnetic
signals.9 Since pores provide room for cell migration and tissue ingrowth,
facilitate vasculature formation, and improve cell viability,10 porosity and
porous structures are other key features for the scaffold. Thus, the use of
AM is highly beneficial, allowing very accurate and repeatable control of
the scaffold geometry (and, thus, porosity) while allowing for the potential
assembly of tissue-like spatial complexity. A wide range of applications
of bioprinted acellular scaffolds have been reported, such as muscular
tissues, liver tissues, cartilage, bone, skin, etc.2 The specific material that
composes the scaffold and any potential biological agents must be
selected to recreate the nature of the engineered tissue. In this section,
we focus on constructs with high mechanical strength, typically for
engineered bone. The materials and AM processes for acellular scaffolds
based on soft engineered tissues (e.g., skin, liver) are similar to the cellladen ones and are, therefore, described in the section “Manufacturing
Soft Materials for Cell Encapsulation” later in this article.
…
G1 X1 Y1 E0.01
G1 X1 Y1.2 E0.01
G1 X1.2 Y1.2 E0.02
G11 X1.2 Y1.4 E0.01
G1 X1 Y1.4 E0.01
3D CAD
Model
Pre-processing
Manufacturing
3D-printed
Constructs
Figure 1. Overview of the 3D printing process.
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49
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Materials
Four categories of materials are highlighted based on their chemical
nature. The first category is polymers,11 such as collagen (Prod.
Nos. C5483, C7624, H4417, etc.), fibrin (Prod. No. F5386), alginate,
chitosan (Prod. Nos. 448869, 448877, 419419, etc.), poly(lactic acid) (PLA)
(Prod. Nos. 764590, 765112, 764698, etc.), poly(glycolic acid) (PGA) (Prod.
Nos. 457620 and 46746), polycaprolactone (PCL), and poly(propylene
fumarate) (PPF). They can be highly bioresorbable and highly flexible in
terms of chemical composition and processability. However, polymerbased scaffolds show a rapid decrease in stiffness over time once
implanted. Calcium phosphate (CaPs) (Prod. No. 21218) based ceramic
scaffolds, such as hydroxyapatite (HA) (Prod. Nos. 289396, 677418,
693863, etc.) and β-tricalcium phosphate (β-TCP) (Prod. Nos. 13204,
21218, and 49963)12 have been extensively studied and used in clinical
applications.13 CaPs scaffolds, being a major constituent of bones, exhibit
high osteoconductivity. They also present high compressive strength,
which can even be improved using dopant additives such as SiO2
(Prod. Nos. 805890, 806587, 806765, etc.) or ZnO (Prod. Nos. 14439,
96479, etc.). But their processability is reduced and, therefore, possible
geometries are limited. Metals are also used, usually titanium or stainless
steel to ensure biocompatibility. They present a high mechanical strength
but are non-biodegradable.13 Finally, composite materials have been
developed by mixing two or more materials with the goal of combining
the advantage of each individual material into one. One example is
a polymer/ceramic composite, such as PCL/TCP or PCL/HA, in which
the ceramic is integrated into the polymer to improve the mechanical
integrity and bioactivity of the polymer.14 Composites show promising
results for acellular scaffolds, with many potential combinations remaining
to be explored.
Manufacturing Process
Numerous 3D printing and AM processes have been developed and
commercialized since the 1980s,15,16 such as Stereolithography (SLA),
Selective Laser Sintering (SLS), and Fused-deposition Modeling (FDM).
These printing techniques can be used for the bioprinting of acellular
scaffolds because they require less restrictive precautions like speed,
temperature, toxicity and pressure for printing.
SLA consists of deflecting a laser beam in a horizontal plane to cure a
photosensitive material in order to form a fixed layer.16 This layer is then
moved along the vertical axis to allow the next adjoining layer to be
created, as shown in Figure 2. This technology permits high resolution
printing, with a layer thickness as small as 20 µm. In the horizontal plane,
the resolution is defined by the diameter of the laser (around 250 µm);
the use of Digital Light Projection (DLP) in place of a laser can improve the
resolution to 70 µm. However, SLA limits the biochemical composition of
the constructs to a single material, which must also be photosensitive.
SLS, in contrast, uses a high powered laser (Figure 2) to heat and fuse a
powder-based material. By rastering the laser over the powder bed, the
successive layers are fabricated. Once each layer is complete, another
layer of powder is added to the top of the previous one, to be sintered
Pre-polymer
Solution
Build Stage
Laser Source
by the laser to form the next layer. This is repeated until the entire part
is produced.16 Scaffolds manufactured using SLS show high mechanical
strength and shape complexity, since sintering provides better bonds
between each layer and the presence of unsintered powder gives support
for each successive layer. The resolution and the surface finish can vary
depending on the powder.
3D printing using FDM consists of the positioning of an extruding
nozzle in order to deposit strands of material in 3D space. The extrusion
material is thermally melted inside the nozzle, solidifying after cooling
upon deposition to create a layer (Figure 2). Materials used for FDM
must exhibit a molten phase, making certain polymers and composites
well-suited for this process. Since the process is strand-based, it is highly
suitable for porous structures. However, complex geometries, such as
overhanging layers, are difficult to manufacture.
Manufacturing Soft Materials for
Cell Encapsulation
Although acellular scaffolds can provide mechanical support and
structural guidance for the growth of cells, post-processing cell seeding
and/or biomolecule loading are required if cells and/or biomolecules
need to be attached to the scaffold. This is a delicate task and does not
allow for controlled attachment and spatial distribution of cells and
biomolecules within the scaffold. However, loading is easier to achieve
by encapsulating the cells and/or biomolecules directly in the printed
material. By combining different cell types and growth factors according
to designed biomimetic patterns, highly complex tissue constructs can
be achieved.2 Such tissue constructs have many applications, allowing
significant progress toward 3D miniature tissue models for drug delivery
tests. To meet the needs of these demanding applications, however,
sterile conditions, non-toxic materials, mild fabrication processes, and
relatively short processing time windows are required, impacting both the
choice of the material and the printing process.
Materials
Cell encapsulation requires the printing material have high water content
and sufficient porosity to enable cells to receive nutrients and oxygen
from the environment as well as remove waste in order to stay alive. The
material should be soft and biodegradable to allow the cells to spread,
migrate, proliferate, and interact with each other.17 The most commonly
used materials for cell encapsulation are hydrogels, which can be either
natural or synthetic. Natural hydrogels, such as gelatin and collagen, are
extracted from animal or human tissues, presenting intrinsic molecular
interactions with cells. Synthetic hydrogels, such as poly(ethylene glycol)
or PEG, are widely used in bioprinting because of the flexibility of their
physical properties. Depending on the gelation principle, hydrogels can
be divided into two categories: physical and chemical hydrogels.15 A
hydrogel is formed “physically” by changing the temperature, pH value,
or other physical properties, while a “chemical” hydrogel is produced by
Roller
Scanner
Molten
Material
Heater
Build Vat
Powder
Light
Light Source
Photocrosslinking
SLA
Stage
SLS
Figure 2. 3D printing processes for the manufacturing of acellular scaffolds.
50
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FDM
Bioprinting for Tissue Engineering and Regenerative Medicine
Current Challenges and Perspectives
crosslinking through covalent bonds. In the first case, the hydrogel can
reverse back to its liquid state if the physical property is changed back to
its initial state; whereas in the second case, the gelation is irreversible due
to the water-insoluble network created by the covalent bonds formed.
Chemical crosslinking is achieved by mixing two mutually reactive
chemicals, while photocrosslinking is performed by exposing a solution
composed of a photosensitive polymer and a photoinitiator to visible or
UV light.16
We have provided a brief overview of the current state of the art in
bioprinting based on the two predominant approaches. In the first
approach, acellular scaffolds are used to provide high resolution and
highly reproducible implantable templates to promote cell function
and tissue regeneration. In the second approach, cells are encapsulated
directly in the material for integration inside a construct during printing.
Materials used for this method contain a high ratio of water and are,
thus, considered to be soft materials in terms of stiffness. While the first
approach has been deliberately dedicated to stiffer materials in this
section, it should be noted that acellular scaffolds based on soft materials
can also be achieved using processes described in the earlier section
“Manufacturing Soft Materials for Cell Encapsulation.”
Manufacturing Process
Inkjet-based bioprinting has been widely used to form 3D cell-laden
constructs by continuously ejecting cell-laden droplets onto a destination
stage using a thermal or an acoustic actuator. The nature of the print head
designates the 3D construct be built dot by dot for each layer. Inkjetbased bioprinters are common for bioprinting applications because they
have a fast printing speed, are compatibile with biological components,
and are low cost. The viscosity of the printing material should be
considered when choosing this printing method in order to reduce
clogging the print head.
While already showing very promising results, bioprinting is still in its
early stage, and several challenges remain to be addressed to move the
field forward. For example, as previously discussed, current 3D bioprinting
approaches have limited capability for integrating the soft and rigid
multifunctional components required for tissues and organs because
of their inherent heterogeneity in mechanical, physical, chemical, and
biological properties and functions. As a step toward this goal, our lab
has developed a 3D hybrid bioprinter (Hybprinter) that can continuously
and rapidly integrate cell-laden soft materials and rigid frame materials
using FDM, SLA, and extrusion-based techniques in a controllable and
automated manner under a single platform.18 By taking advantage of
each process, we are able to manufacture constructs with both acellular
scaffolds and cell-laden hydrogels, a step toward highly complex multimaterial constructs.
SLA can also be used to manufacture cell-laden constructs by adding cells
to an uncrosslinked pre-polymer material. Visible light is the preferred
light source for crosslinking when using DLP due to cell sensitivity to UV
exposure and changes in temperature. While SLA offers high resolution,
the tradeoff between printing quality and total processing time should
be considered to yield optimal conditions for cell viability. SLA requires
a larger amount of material than other bioprinting methods due to the
requirement of filling the vat (Figure 3) with the printing material, which
presents a major drawback if using expensive materials.
Another way to print hydrogel constructs is extrusion. To do this, a
chamber is filled with cell-laden biomaterial, then using either pneumatic
or piston-driven extrusion, the material is propelled through the print
head. In order to create cell-laden constructs layer by layer, the print head
robotically follows the desired path. For a physically formed hydrogel, the
struts are extruded and gelled on stage upon change in pH, temperature,
or other physical condition. Photocrosslinkable materials can also be
used for this process. Once a layer of pre-polymer solution is extruded, it
is crosslinked by exposure to light. Although the printing speed and the
amount of extrusion can be precisely controlled, significant shear stress
on the material can impact cell viability and should be carefully avoided.
While bioprinting shows promise in tissue engineering, improvements
in the printing processes are required. Increased speed should be
considered in order to ease scale-up for acellular scaffolds and improve
cell viability for cell-laden constructs. Moreover, higher resolution
is required, in particular for the manufacturing of heterogeneous
composite tissues and vascularized tissues. Vascularization is one of
the key components and arguably the greatest challenge of tissue
engineering.19 Such tissues consist of a highly complex vascular network,
from millimeter-sized vessels to micrometer-scale capillaries. Reproducing
such a network is a huge challenge, and up to now it has mainly been
addressed by providing sufficient space in the porous scaffold for
vascular tissues to spontaneously develop. A 3D printing process such
as two-photon polymerization20 allows the manufacturing of parts with
a micron-to-millimeter range of size and is, therefore, considered as a
promising bioprinting process for the manufacturing of vascularized
tissue constructs.
Laser-based bioprinting is a commonly used technique in which a laser is
used to transfer cell-laden materials from a source plate to the deposition
stage. To achieve this transfer, the source plate is coated in a double layer
with a laser-absorbing layer and a donor layer of biomaterial (Figure 3).
When the laser pulse focuses on the laser-absorbing layer, the heated
region generates a bubble to propel and deposit a droplet of biomaterial
onto the destination stage. Use of a laser instead of a nozzle allows for the
deposition of highly viscous materials with high accuracy. However, the
heat generated by the laser hinders cell viability, and this process is the
most limited in terms of vertical constructs.
Vapor
Bubble
Pre-polymer
Solution
Build Stage
Heater
Light
Light
Source
SLA
Laser Source
Strut
Laserabsorbing Layer
Glass
Build Vat
Photocrosslinking
Inkjet Based
Research should also focus on the development of new materials that
have improved biological properties and are suitable for bioprinting.
Additionally, research should focus on using the current or future
technologies to improve assembly of existing biomaterials to better
mimic the complexity of the ECM, or the combination of both. Available
materials for cell-laden tissue constructs dedicated to bioprinting are
Biomaterial
Stage
Extrusion Based
Laser Based
Figure 3. 3D printing process for the manufacturing of soft materials for cell encapsulation.
For questions, product data, or new product suggestions, contact us at [email protected]
51
aldrich.com/matsci
currently limited, but improved imaging capability and a fundamental
understanding of the complexity of tissues and developmental biology
will contribute to the development of new materials and bioprinting
technologies. It remains to be seen to what extent biomimetic complexity
of the bioprinted constructs either in chemistry or in physical structure is
necessary to achieve better healing and restoration of lost functions.
(6) Mironov, V.; Reis, N.; Derby, B. Tissue Engineering 2006, 12(4), 631–4. doi: 10.1089/
ten.2006.12.631. PubMed PMID: WOS:000237494400001.
(7) Sears, N. A.; Seshadri, D. R.; Dhavalikar, P. S.; Cosgriff-Hernandez, E. Tissue Eng. Part B, Rev. 2016.
Epub 2016/02/10. doi: 10.1089/ten.TEB.2015.0464. PubMed PMID: 26857350.
(8) Bose, S.; Vahabzadeh, S.; Bandyopadhyay, A. Mater. Today 2013, 16(12), 496–504. doi:
10.1016/j.mattod.2013.11.017. PubMed PMID: WOS:000328640100018.
(9) Hutmacher, D. W. Biomaterials 2000, 21(24), 2529–43. doi: 10.1016/s0142-9612(00)00121-6.
PubMed PMID: WOS:000089861700006.
(10) Bose, S.; Roy, M.; Bandyopadhyay, A. Trends Biotechnol. 2012, 30(10), 546–54. doi: 10.1016/j.
tibtech.2012.07.005. PubMed PMID: WOS:000309946600007.
(11) Liu, X. H.; Ma, P. X. Ann.Biomed. Eng. 2004, 32(3), 477–86. doi: 10.1023/B:ABME.0000017544.360
01.8e. PubMed PMID: WOS:000222465100019.
(12) Sweet, L.; Kang, Y; Czisch, C.; Witek, L.; Shi, Y.; Smay, J.; et al. Plos One. 2015;10(10). doi: 10.1371/
journal.pone.0139820. PubMed PMID: WOS:000362510600085.
(13) Lichte, P.; Pape, H. C.; Pufe, T.; Kobbe, P.; Fischer, H. Injury 2011, 42(6), 569–73. Epub
2011/04/15. doi: 10.1016/j.injury.2011.03.033. PubMed PMID: 21489531.
(14) Lu, L.; Zhang, Q.; Wootton, D.; Chiou, R.; Li, D.; Lu, B.; Lelkes, P.; Zhou, J., Biocompatibility
and biodegradation studies of PCL/beta-TCP bone tissue scaffold fabricated by structural
porogen method. Journal of Materials Science-Materials in Medicine 2012, 23 (9), 2217-2226.
(15)Hoffman, A. S. Adv. Drug Deliv. Rev. 2002, 54(1), 3–12. doi: http://dx.doi.org/10.1016/S0169409X(01)00239-3.
(16) Elomaa, L.; Pan, C. C.; Shanjani, Y.; Malkovskiy, A.; Seppala, J. V.; Yang, Y. J. Mater. Chem. B. 2015,
3(42):8348–58. doi: 10.1039/c5tb01468a.
(17) Elomaa, L.; Kang, Y.; Seppälä, J. V.; Yang, Y. J. Polym. Sci. A Polym. Chem. 2014, 52(23), 3307–15.
doi: 10.1002/pola.27400.
(18) Shanjani, Y.; Pan, C. C.; Elomaa, L.; Yang, Y. Biofabrication 2015, 7(4), 045008. Epub 2015/12/20.
doi: 10.1088/1758-5090/7/4/045008. PubMed PMID: 26685102.
(19) Mercado-Pagan, A. E.; Stahl, A. M.; Shanjani, Y.; Yang, Y. Ann. Biomed. Eng. 2015, 43(3), 718–29.
Epub 2015/01/27. doi: 10.1007/s10439-015-1253-3. PubMed PMID: 25616591.
(20) Weiß, T.; Berg, A.; Fiedler, S.; Hildebrand, G.; Schade, R.; Schnabelrauch, M.; et al. Two-Photon
Polymerization for Microfabrication of Three-Dimensional Scaffolds for Tissue Engineering
Application. In: Dössel O, Schlegel WC, editors. World Congress on Medical Physics and
Biomedical Engineering, September 7–12, 2009, Munich, Germany: Vol 25/10 Biomaterials,
Cellular and Tussue Engineering, Artificial Organs. Berlin, Heidelberg: Springer Berlin
Heidelberg; 2010. p. 140–2.
A growing number of AM applications are emerging, with bioprinting
emerging as one of the most promising and challenging manufacturing
processes due to the potential impact on global healthcare concerns
like aging, organ transplantation, cancer therapy, and personalized
and precision medicine. In the future, bioprinting has the potential to
become both a source of miniature disease and toxicology models for
the pharmaceutical industry and as a source of life-sized tissue/organ
replacements for clinical treatments.
Acknowledgments
We would like to acknowledge the financial support of the following
agencies: NIH R01AR057837 (NIAMS), NIH R01DE021468 (NIDCR), DOD
W911NF-14-1-0545 (DURIP), DOD W81XWH-10-1-0966 (PRORP), and
Stanford Coulter Translational Seed Grant.
References
(1) Stock, U. A.; Vacanti, J. P. Annu. Rev. Med. 2001, 52, 443–51. doi: 10.1146/annurev.med.52.1.443.
PubMed PMID: WOS:000167302900025.
(2) Langer. R.; Vacanti, J. P. Science. 1993, 260(5110), 920–6. Epub 1993/05/14. PubMed PMID:
8493529.
(3) Griffith, L. G.; Naughton, G. Science. 2002, 295(5557), 1009+. doi: 10.1126/science.1069210.
PubMed PMID: WOS:000173793000043.
(4) Dababneh, A. B.; Ozbolat, I.T. J. Manuf. Sci. E-T ASME. 2014, 136(6), 11. doi: 10.1115/1.4028512.
PubMed PMID: WOS:000344393000018.
(5) Smith, I. O.; Liu, X. H.; Smith, L. A.; Ma, P. X. WIREs Nanomed. Nanobiotechnol. 2009, 1(2),
226–36. doi: 10.1002/wnan.026. PubMed PMID: WOS:000276839400008.
Biodegradable Polymers
For more information on these products, visit aldrich.com/biopoly.
Polycaprolactones
Name
Structure
Polycaprolactone
O
Molecular Weight
Transition Temperature
Prod. No.
average Mn ~10,000
Tg −60 °C
440752-5G
440752-250G
440752-500G
average Mn 45,000
-
704105-100G
704105-500G
average Mn 80,000
-
440744-5G
440744-250G
440744-500G
average Mn 550
-
802115-2G
average Mn 2,250
-
802158-2G
average Mn ~530
softening point 35 °C
189405-250G
189405-500G
average Mn ~2,000
softening point 50 °C
189421-250G
189421-500G
average Mn 950
-
799556-2G
average Mn ~300
softening point 10 °C
200387-250G
200387-500G
average Mn ~900
softening point 30 °C
200409-250G
200409-500G
O
n
Polycaprolactone dimethacrylate
O
O
O
O
O
n
O
O
n
O
O
Polycaprolactone diol
O
O
O
H
Polycaprolactone trimethacrylate
O
O
O
n
RO
O
n
O
H3C
OR
O
R=
n
O
OR
Polycaprolactone triol
RO
H3C
O
OR
OR
52
H
O
H
R=*
n
TO ORDER: Contact your local Sigma-Aldrich office or visit aldrich.com/matsci.
Bioprinting for Tissue Engineering and Regenerative Medicine
Block PCL Polymers
Name
Structure
Poly(dl-lactide-co-caprolactone)
O
O
O
CH3
Poly(l-lactide-co-caprolactoneco-glycolide)
O
x
O
CH3
Transition Temperature
Prod. No.
0.7-0.9 dL/g in chloroform
Tg 16 °C
457647-5G
dl-lactide
40 mol %
0.7-0.9 dL/g in chloroform
Tm 31 °C, DSC, onset
457639-5G
-
-
568562-1G
568562-5G
glycolide 10%
l-lactide 70%
caprolactone 20%
O
O
Inherent Viscosity
86 mol %
y
O
O
Composition
dl-lactide
O
y
x
z
Poly(lactide-co-glycolide) Copolymers
Name
Feed Ratio
End Group
Molecular Weight
Degradation Time (months)
Prod. No.
Resomer® RG 502 H, Poly(d,l-lactide-co-glycolide)
lactide:glycolide 50:50
acid terminated
Mw 7,000‑17,000
<3
719897-1G
719897-5G
Resomer® RG 503 H, Poly(d,l-lactide-co-glycolide)
acid terminated
Mw 24,000‑38,000
<3
719870-1G
719870-5G
Resomer® RG 504 H, Poly(d,l-lactide-co-gylcolide)
acid terminated
Mw 38,000‑54,000
<3
719900-1G
719900-5G
Resomer® RG 502, Poly(d,l-Lactide-co-Glycolide)
ester terminated
Mw 7,000‑17,000
<3
719889-1G
719889-5G
Resomer® RG 503, Poly(d,l-lactide-co-glycolide)
ester terminated
Mw 24,000‑38,000
<3
739952-1G
739952-5G
Resomer® RG 504, Poly(d,l-lactide-co-glycolide)
ester terminated
Mw 38,000‑54,000
<3
739944-1G
739944-5G
Resomer® RG 505, Poly(d,l-lactide-co-glycolide)
ester terminated
Mw 54,000‑69,000
<3
739960-1G
739960-5G
Resomer® RG 653 H, Poly(d,l-lactide-co-glycolide)
lactide:glycolide 65:35
acid terminated
Mw 24,000‑38,000
<5
719862-1G
719862-5G
Resomer® RG 752 H, Poly(d,l-lactide-co-glycolide)
lactide:glycolide 75:25
acid terminated
Mw 4,000‑15,000
<6
719919-1G
719919-5G
ester terminated
Mw 76,000‑115,000
<6
719927-1G
719927-5G
ester terminated
Mw 50,000‑75,000
<6
430471-1G
430471-5G
ester terminated
Mw 190,000‑240,000
<9
739979-1G
739979-5G
Resomer® RG 756 S, Poly(d,l-lactide-co-glycolide)
Poly(d,l-lactide-co-glycolide)
lactide:glycolide 85:15
Resomer® RG 858 S, Poly(d,l-lactide-co-glycolide)
Well-defined Poly(l-lactide)s
Name
Molecular Weight (Mn)
PDI
Degradation Time (years)
Prod. No.
Poly(l-lactide)
5,000
≤1.2
>3
764590-5G
10,000
≤1.1
>3
765112-5G
20,000
≤1.1
>3
764698-5G
Well-defined Poly(d,l-lactide)s
Name
Molecular Weight (Mn)
PDI
Degradation Time (months)
Prod. No.
Poly(d,l-lactide)
5,000
≤1.1
<6
764612-5G
10,000
≤1.1
<6
764620-5G
20,000
≤1.3
<6
767344-5G
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53
aldrich.com/matsci
Poly(l-lactide)s
O
O
CH3
n
Name
End Group
Molecular Weight
Degradation Time (years)
Prod. No.
Resomer® L 206 S, Poly(l-lactide), ester terminated
ester terminated
-
>3
719854-5G
719854-25G
Poly(l-lactide)
ester terminated
average Mn 50,000
>3
94829-1G-F
94829-5G-F
ester terminated
Mn 59,000
Mw 101,000
>3
93578-5G-F
ester terminated
Mw ~260,000
>3
81273-10G
ester terminated
Mn 103,000
Mw 259,000
>3
95468-1G-F
95468-5G-F
Name
End Group
Molecular Weight
Degradation Time (months)
Prod. No.
Resomer® R 202 H, Poly(d,l-lactide)
acid terminated
Mw 10,000‑18,000
<6
719978-1G
719978-5G
Resomer® R 203 H, Poly(d,l-lactide)
acid terminated
Mw 18,000‑24,000
<6
719943-1G
719943-5G
Resomer® R 202 S, Poly(d,l-lactide)
ester terminated
Mw 10,000‑18,000
<6
719951-1G
719951-5G
Resomer® R 203 S, Poly(d,l-lactide)
ester terminated
Mw 18,000‑28,000
<6
719935-1G
719935-5G
Molecular Weight
PDI
Prod. No.
average Mn 2,500
≤1.2
775991-1G
average Mn 5,500
≤1.2
775983-1G
average Mn 2,500
≤1.3
776378-1G
776378-5G
average Mn 4,000
≤1.2
776386-1G
776386-5G
average Mn 5,000
<1.2
774146-1G
average Mn 2,000
≤1.2
746797-1G
746797-5G
average Mn 5,000
<1.2
746517-1G
746517-5G
average Mn 2,000
≤1.1
771473-1G
771473-5G
average Mn 5,500
≤1.2
766577-1G
766577-5G
average Mn 2,000
≤1.1
774162-1G
average Mn 5,000
≤1.1
774154-1G
average Mn 2,500
≤ 1.2
747386-1G
747386-5G
average Mn 5,000
≤ 1.2
747394-1G
747394-5G
Poly(d,l-lactide)s
O
O
CH3
n
End-functionalized Low PDI Poly(l-lactide)
Name
Structure
Poly(l-lactide), acrylate terminated
O
H
O
O
O
CH3
n
Poly(l-lactide), amine terminated
O
O
H
O
CH3
Poly(l-lactide), azide terminated
n
O
O
CH3
Poly(l-lactide), N-2-hydroxyethylmaleimide
terminated
N3
n
O
O
N
O
O
O
CH3
CH3
H
O
Poly(l-lactide), propargyl terminated
H
n
O
O
O
CH2
O
CH3
n
O
H
O
O
CH3
Poly(l-lactide), thiol terminated
CH
n
CH3
H
O
O
O
54
NH2
O
H
Poly(l-lactide), 2-hydroxyethyl, methacrylate
terminated
CH2
O
n
SH
TO ORDER: Contact your local Sigma-Aldrich office or visit aldrich.com/matsci.
Bioprinting for Tissue Engineering and Regenerative Medicine
Poly(ethylene glycol)s (PEG)
For a complete list of available materials, visit aldrich.com/peg.
Structure
O
O
H2C
CH2
O
n
O
O
CH3
H2C
O
O
n
CH3
O
H2C
n
O
O
O
CH2
O
O
O
O
H2C
CH2
O
n
Molecular Weight
Prod. No.
average Mn 2,000
701971-1G
Acrylate-PEG3500-Acrylate
average Mn 3,500
JKA4048-1G
Poly(ethylene glycol) diacrylate
average Mn 6,000
701963-1G
Poly(ethylene glycol) diacrylate
average Mn 10,000
729094-1G
Poly(ethylene glycol) diacrylate
PEG average Mn20,000 (n~450)
average Mn 20,000
767549-1G
Poly(ethylene glycol) dimethacrylate
average Mn 2000
687529-1G
Poly(ethylene glycol) dimethacrylate
average Mn 6,000
687537-1G
Poly(ethylene glycol) dimethacrylate
average Mn 10,000
725684-1G
4arm-PEG10K-Acrylate
average Mn 10,000
JKA7068-1G
4arm-PEG20K-Acrylate
average Mn 20,000
JKA7034-1G
8arm-PEG20K-Acrylate, hexaglycerol core
average Mn 20,000
JKA8005-1G
8arm-PEG10K-Acrylate, tripentaerythritol core
average Mn 10,000
JKA10021-1G
O
O
O
O
O
O
O
Name
Poly(ethylene glycol) diacrylate
CH2
O
n
n
O
R=
R
R
O
O
R
O
n
CH2
O
R
O
O
R
O
R
O
R
R
O
R
O
O
n
CH2
O
8
R = tripentearythritol core structure
For questions, product data, or new product suggestions, contact us at [email protected]
55
aldrich.com/matsci
3D AND 4D PRINTING TECHNOLOGIES:
AN OVERVIEW
Wonjin Jo,* Kyung Sung Chu, Heon Ju Lee, Myoung-Woon Moon
3D Printing Group, Computational Science Research Center
Korea Institute of Science and Technology, 02792, Seoul, Republic of Korea
*Email: [email protected]
Introduction
Three-dimensional (3D) printing technology, also called additive
manufacturing (AM), has recently come into the spotlight because
of its potential high-impact implementation in applications ranging
from personal tools to aerospace equipment. Even though 3D printing
technology has only recently emerged as a hot topic, its history can be
traced back to 1983 when the first 3D printer was created by Charles
W. Hull, co-founder of 3D Systems. Since then, new and wide-ranging
applications and markets for 3D printers have appeared rapidly, especially
with the expiration of a number of core 3D printing patents owned
by Stratasys Inc. and 3D Systems Inc. Users can easily build or modify
3D printers by themselves or take advantage of the rapidly growing
availability of inexpensive 3D printers. The recent availability of highly
capable 3D design software and 3D design websites (e.g., Shapeway and
Thingiverse) allows the sharing of user-created free 3D digital design
files or models, leading to more access to 3D printers and additional
proliferation of 3D printing technology. When compared to traditional
manufacturing technologies such as casting, machining, and drilling,
3D printing is considered an efficient technology in the areas of energy
and materials, utilizing up to 90% of materials and providing up to
50% energy savings.1
As 3D printing becomes more than just a simple production process, it
has come to support a convergence of technologies and applications
such as sports equipment, food packaging, and jewelry, as well as
products in the high tech fields of aerospace, medicine, architecture,
education,2,3 automotive industry, military support, and others. At the 2016
New York Fashion Week, two unique 3D printed dresses were unveiled.
These masterpieces were produced through a collaboration between
fashion designers and the 3D printing company, Stratasys.4 The complex
designs (e.g., mixing a variety of interlocking weaves, biomimicking
natural animal textures) and cutting-edge material (e.g., nano-enhanced
elastomeric 3D printing material) gave the dresses durability and
flexibility. The area of regenerative medicine has also achieved impressive
applications within the 3D printing field. Dr. Anthony Atala’s team from
the Wake Forest Institute for Regenerative Medicine has successfully used
3D printing technology to fabricate living organs and tissue (including
muscle structures, and bone and ear tissue).5,6 These bioprinted body
parts are capable of generating functional replacement tissue.7 NASA
56
has also been implementing 3D printing techniques and 3D printers
to develop materials that allow astronauts to repair or replace essential
parts and build structures in space. NASA recently collaborated with
researchers at Washington State University to fabricate a replica of a moon
rock using raw lunar regolith simulant and 3D laser printing technology.8,9
The assembly of modular construction materials using giant 3D printers
for use in the housing industry has gained significant interest, especially
for poorer countries, during natural disasters, or sudden emergencies.
Some 3D companies have succeeded in building houses or bridges with
cement, sand, or concrete materials.10–12
The rapidly decreasing cost, improved software design, and increasing
range of printable materials have helped to bring about a new
technology called four-dimensional (4D) printing. 4D printing provides
printed objects with the ability to change form or function with time
according to various stimuli such as heat, water, current, or light
(Figure 1A).13 The essential difference between 4D printing and 3D
printing is the addition of smart design, or responsive materials that cause
time-dependent deformations of objects. This review covers both 3D
and 4D printing processes and shows the materials related to different
printing types.
A)
1D
2D
3D
2
3
4D
B)
1
4
5
Figure 1. A) Schematic of 1-, 2-, 3-, and 4D concepts. B) The process of 3D and 4D printing
technology involves three general stages: (1–2) modeling; (3–4) printing; and (5) finishing.
The Process of 3D and 4D Printing
Technology
3D printing is the process of fabricating objects by building up materials
layer by layer. Figure 1B shows the 3D printing process from modeling
to final printing. Based on the use of computer-aided design (CAD)
describing the geometry and the size of the objects to be printed, a
complicated 3D model is created in a printable standard tessellation
language (STL) file format (Figure 1B1,1B2). Then, it is sliced into a
series of digital cross-sectional layers in accordance with the layer
TO ORDER: Contact your local Sigma-Aldrich office or visit aldrich.com/matsci.
3D and 4D Printing Technologies: An Overview
thickness setting (Figure 1B3). Upon completion of the model, the
object is fabricated by a 3D printer through the layer-by-layer fabrication
process based on a series of 2D layers to create a static 3D object
(Figure 1B4,1B5). 3D printing can involve different types of materials such
as thermoplastic polymer, powder, metal, UV curable resin, etc.
Four-dimensional printing incorporates a time component to the 3D
printed objects, making the design process more important. 4D-printed
structures must be preprogrammed in detail based on the transforming
mechanism of controllable smart materials that incorporate timedependent material deformations.13 Figure 2A–C show 3D structures that
self-fold based on the thermal activation of spatially variable patterns
printed with a variety of shape memory polymers. Each polymer has a
different thermal-dependent behavior that can make the box self-fold in
a time-sequential manner based on smart design and thermomechanical
mechanisms.14 The choice of materials for 4D printing is significant,
however, because most 3D printing materials are designed only to
produce rigid, static objects. Recently, some smart shape alloy/polymer
memory materials have been developed to utilize their self-assembled
behaviors driven by heat, UV, or water absorption-driven as shown
in Figure 2D–F.13,15 For example, the temperature-responsive artificial
hand shown in Figure 2F was printed with a temperature-responsive
TPU (thermal polyurethane) filament. It has the ability to contract or
expand in response to specific temperatures. In addition, multi-materials
having different environmental behaviors are also useful in 4D printing.
A research group at the Massachusetts Institute of Technology used
two different materials with different porosities and water-absorption
abilities to print transformable structures.16,17 It was composed of a porous
water-absorbing material on one side and a rigid waterproof material
on the opposite side. When exposed to water, the water-absorbing side
increased in volume while the other side remained unchanged, resulting
in shape deformation.
A)
C)
B)
D)
Classification of 3D and 4D Printing
Technologies
The 3D and 4D printing technologies are classified into different printing
processes, defined mainly by the types of materials used. The selection of
materials has a direct influence on the mechanical or thermal properties,
as well as the transformation stimuli of the finished objects. This section
describes the three most common types of 3D and 4D printing and
reviews the most frequently used materials for these processes.
Fused-deposition Modeling (FDM)
The FDM method operates by extruding thermoplastic materials and
placing the semi-molten materials onto a stage to fabricate a 3D structure
layer by layer.18 More specifically, the thermoplastic filament is first led
to an extruder which feeds and retracts the filament in precise amounts.
The filament is melted by a heater block set to the melting temperature
and moved through the extrusion nozzle tip by two rollers. The extruded
filament is deposited as the print head traces the design of each defined
cross-sectional layer of the desired structure by a digitally positioned
mechanism. Then, the stage moves to the Z position in accordance with
the setting value of layer thickness. These steps are repeated to complete
fabrication of the 3D structure.
One advantage of FDM is the availability of a variety of filament materials
as shown in Figure 3. A wide selection of FDM filaments are commercially
available with different strength and temperature properties, such as ABS
(acrylonitrile butadiene styrene, Prod. Nos. 3DXABS001–3DXABS0016),
nylon (Prod. Nos. 3DXION001–3DXION004), PET (polyethylene
terephthalate, Prod. Nos. 900095 and 900125), TPU (thermal
polyurethane, Prod. Nos. 900126 and 900128), POM (polyoxymethylene),
PC (polycarbonate), HIPS (high impact polystyrene), and PVA (polyvinyl
alcohol), among others. In addition, some materials can be used as a raw
material for mixing with other functional materials to improve specific
functions. Among them, the PLA (polylactic acid) filament is a popular
choice due to the many available properties as shown in Figure 3. Due to
the thermoplastic behavior, many FDM filaments can also be used as 4D
materials under applied heat change.
E)
ABS
Nylon
PET
TPU
PLA
• Biocompatible (Derived
from corn starch)
• Biodegradable
• Hard or soft/flexible
variants
• Various colors including
translucent color
• A variety of finishing
• Unnecessity of heating
bed
F)
POM
PC
HIPS
PVA
Functionalized Filament
4D Printing Element
Conductive Filament
Color-changed Filament
• Carbon black
• Graphene
Natural Filament
• By UV exposure
• By temperature
No Sunshine
Sunshine
• Wood
• Coconut
• Bamboo
Metal Alloy Filament
Figure 2. A–B) The design of the folding box with different materials assigned at different
hinges. C) Upon heating, the programmed 3D printed sheet folds into a box with a self-locking
mechanism. Copyright 2015, rights managed by Nature Publishing Group. D–E) The resulting
swollen flower structures were generated by biomimetic 4D printing with composite hydrogel
and cellulose fibrils. Copyright 2016, rights managed by Nature Publishing Group. F) The
temperature-responsive artificial hand was made with temperature-responsive TPU filament.
•
•
•
•
Bronze
Stainless steel
Iron
Copper
Mechanical Strength
Improved Filament
Water-reactive Filament
Layfomm:
• Made from a rubber-elastomeric
polymer and a PVA-component
• Is a foamy and gel-like material
• Carbon fiber
Figure 3. Thermoplastic filaments for Fused-deposition Modeling (FDM). The FDM-printed flower
was made with a color changed filament under UV exposure.
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57
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Powder Bed and Inkjet Head 3D Printing (PBP)
The PBP process is an adaptation of inkjet printing. In this process, a layer
of powder is first deposited and rolled to ensure uniform thickness, then
the inkjet print head drops binder in a specified pattern as it moves, to
form a single layer of a printed object across the bed of powder. The
next powder layer is distributed over the deposited liquid binder, and
this process is repeated, with each layer adhering to the last. Support
structures are not required in PBP due to the ease of removing unbound
powder using an air gun, after solidification of the finished object. The use
of multiple print heads with colored binder, allows printing in full color.
Among the variety of powders available, calcium sulfate (CaSO4, Prod.
Nos. 255696 and 237132) is one of the most widely-used because of its
ability to react with water-based binders. It can react with water-based
solutions rapidly and change into gypsum (CaSO4 ∙ 2H2O) in a solid state.19
In this method, the binding strength is the key factor in determining the
physical and chemical properties of the printed device. Therefore, the
proper combination of powder and binder should be carefully considered.
Very recently, Voxeljet developed the world’s largest industrial PBP
system (VX4000) for sand molds. The largest cohesive build space is
4,000 × 2,000 × 1,000 mm (L × W × H) with 300 µm of a layer applied in
one cycle.20
Stereolithography (SLA)
SLA combines ultraviolet (UV) or visible laser light with curable liquid
photopolymer resins. To create each layer, a laser beam illuminates a 2D
cross-section of the object in a vat of resin, allowing the resin to solidify.
Next, the object is raised by an equal distance of layer thickness to fill
resin under and maintain contact with the bottom of the object. This
process is repeated until the entire model is completed, at which point
the platform is raised out of the vat and the excess resin is drained. Finally,
the SLA object is finished by washing and curing under UV light. SLA
produces a smoother surface on the final product compared to other 3D
printing methods, as a result of using liquid photopolymers. Although SLA
can produce a wide variety of shapes, its drawbacks include a significant
amount of resin waste and the need for extensive cleaning after
fabrication. Furthermore, resins used in the process are limited to either
epoxy or acrylic bases, most of which can shrink upon polymerization.
A recent advance in SLA significantly decreases printing time. Carbon
3D Inc. announced a new continuous liquid interface production (CLIP)
method that can print an object 100 times faster than existing methods
by creating an oxygen depletion zone (dead zone) in liquid resins as
shown in Figure 4.21 The introduction of a unique oxygen-permeable
window in the resin reservoir creates a thin liquid interface of uncured
resin between the window and printing part. This oxygen-depleted dead
zone allows for continuous translation and curing of the resin above the
dead zone to form a consistent solid part.
A)
Build
Support
Plate
Continuous Elevation
Future Prospects
Three-dimensional printing technology is highly versatile and efficient
with respect to design, fabrication, and applications. 4D printing may
be of great importance in the future due to its potential to redefine
manufacturing-related industries. However, the technology must
be further refined before it can replace conventional manufacturing
methods. Therefore, future research and investment in 3D and 4D printing
technologies are imperative to bring about improvements in essential
areas including materials, printer systems, and product markets.
References
(1) Pangaea Blog, posted by Matthew Cohen on May 2013. http://www.pangaeaventures.com/
blog/additive-manufacturing-printing-a-better-and-cleaner-world
(2) Science News section of ScienceDaily, posted on July 2014. http://www.sciencedaily.com/
releases/2014/07/140702102601.htm.
(3) Jo, W.; I, J. H.; Harianto, R. A.; So, J. H.; Lee, H.; Lee, H. J.; Moon, M.-W. J. Vis. Impair. Blind. 2016,
110, in press.
(4) 3Ders.org, posted by Tess on Feb 2016. http://www.3ders.org/articles/20160216-threeasfourunveils-two-spectacular-3d-printed-dresses-at-new-york-fashion-week.html
(5) Kang, H. W.; Lee, S. J.; Ko, I. K.; Kengla, C.; Yoo, J. J., Atala, A. Nat. Biotechnol. 2016, 34(3),
312–319.
(6) Shafiee, A.; Atala, A. Trends Mol. Med. 2016, 22(3), 254-265.
(7) The Gaurdian, posted by Tim Radford on Feb 2016. https://www.theguardian.com/
science/2016/feb/15/bioprinter-creates-bespoke-lab-grown-body-parts-for-transplant
(8) SPACE.com, posted by Megan Gannon on Jan 2012. http://www.space.com/18694-moondirt-3d-printing-lunar-base.html
(9) Balla, V. K.; Roberson, L. B.; O’connor, G. W.; Trigwell, S.; Bose, S.; Bandyopadhyay, A. Rapid
Prototype J. 2012, 18, 451–257.
(10) WASP, posted on Mar 2015. http://www.wasproject.it/w/en/new-extruder-wasp-can-trulyrealize-house-dream/
(11) 3DPrint.com, posted by Eddie Krassenstein on Aug 2014. http://3dprint.com/12933/3dprinted-castle-complete/
(12) 3Ders.org, posted by Kira on Jan 2015. http://www.3ders.org/articles/20150118-winsunbuilds-world-first-3d-printed-villa-and-tallest-3d-printed-building-in-china.html
(13) Choi, J.; Kwon, O. C.; Jo, W.; Lee, H. J.; Moon, M.-W. 3D Printing and Additive Manufacturing
2015, 2, 159–167.
(14) Mao, Y.; Yu, K.; Isakov, M. S.; Wu, J.; Dunn, M. L.; Jerry Qi, H. Sci. Rep. 2015, 5, 13616.
(15) Sydney Gladman, A.; Matsumoto, E. A.; Nuzzo, R. G.; Mahadevan, L.; Lewis, J. A. Nat. Mater.
2016, advance online publication.
(16) Tibbits, S. Archit. Des. 2014, 84, 116–121.
(17) Raviv, D.; Zhao, W.; McKnelly, C.; Papadopoulou, A.; Kadambi, A.; Shi, B.; Hirsch, S.; Dikovsky, D.;
Zyracki, M.; Olguin, C.; Raskar, R.; Tibbits, S. Sci. Rep. 2014, 4, 7422.
(18) Jo, W.; Kim, D. H.; Lee, J. S.; Lee, H. J.; Moon, M.-W. RSC Adv. 2014, 4, 31764–31770
(19) Liu, W.; Wu, C.; Liu, W.; Zhai, W.; Chang, J. J. Biomed. Mater. Res. B Appl. Biomater. 2013, 101B,
279–286.
(20)http://www.voxeljet.de/en/systems/vx4000/
(21) Tumbleston, J. R.; Shirvanyants, D.; Ermoshkin, N.; Janusziewicz, R.; Johnson, A. R.; Kelly, D. et al.
Science 2015, 347, 1349–1352.
B)
Part
Dead Zone
Liquid Resin
O2
Permeable
Window
Mirror
Imaging Unit
Figure 4. A) Schematic of a CLIP printer. B) The resulting parts via CLIP at print speeds of
500 mm/hour. Copyright 2015, The American Association for the Advancement of Science.
58
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3D and 4D Printing Technologies: An Overview
3D Printing Filaments
For a complete list of available materials, visit aldrich.com/3dp.
ABS
Nylon/ABS Alloy
Weight = 2.2 lb spool
Weight = 1.1 lb spool
Color
Diameter (mm)
Prod. No.
Color
Diameter (mm)
Prod. No.
black
1.75 3DXABS001-1EA
black
1.75 3DXION001-1EA
2.85 3DXABS002-1EA
blue
1.75 3DXION002-1EA
1.75 3DXABS003-1EA
red
1.75 3DXION003-1EA
2.85 3DXABS004-1EA
natural
1.75 3DXION004-1EA
1.75 3DXABS005-1EA
2.85 3DXABS006-1EA
1.75 3DXABS007-1EA
2.85 3DXABS008-1EA
1.75 3DXABS009-1EA
Color
Diameter (mm)
Prod. No.
2.85 3DXABS010-1EA
forest green
1.75 900114-1EA
1.75 3DXABS011-1EA
3 900115-1EA
2.85 3DXABS012-1EA
1.75 900116-1EA
1.75 3DXABS013-1EA
3 900117-1EA
2.85 3DXABS014-1EA
1.75 900118-1EA
1.75 3DXABS015-1EA
3 900119-1EA
2.85 3DXABS016-1EA
1.75 900120-1EA
3 900121-1EA
1.75 900122-1EA
3 900123-1EA
1.75 900124-1EA
3 900125-1EA
blue
green
natural
orange
red
white
yellow
PLA
PET+®
Weight = 1 lb spool
gold (goldenrod)
orange (tangerine)
light brown (almond)
ruby
Weight = 2.2 lb spool
blue (sapphire)
Color
Diameter (mm)
Prod. No.
black
1.75 3DXPLA001-1EA
2.85 3DXPLA002-1EA
1.75 3DXPLA003-1EA
2.85 3DXPLA004-1EA
1.75 3DXPLA005-1EA
Color
Diameter (mm)
Prod. No.
2.85 3DXPLA006-1EA
clear
1.75 900126-1EA
1.75 3DXPLA007-1EA
2.85 900127-1EA
2.85 3DXPLA008-1EA
3 900128-1EA
1.75 3DXPLA009-1EA
2.85 3DXPLA010-1EA
1.75 3DXPLA011-1EA
2.85 3DXPLA012-1EA
1.75 3DXPLA013-1EA
2.85 3DXPLA014-1EA
1.75 3DXPLA015-1EA
2.85 3DXPLA016-1EA
blue
green
natural
orange
red
white
yellow
FlexSolid
Weight = 1 lb spool
Carbon Fiber Reinforced Filaments
Weight = 1.65 lb spool
Description
Diameter (mm)
Prod. No.
3DXMAX™ CFR-PLA carbon fiber reinforced PLA 3D printing filament
1.75 3DXCFR002-1EA
3DXMAX™ CFR-PLA carbon fiber reinforced PLA 3D printing filament
2.85 3DXCFR004-1EA
3DXMAX™ CFR-ABS carbon fiber reinforced ABS 3D printing filament
1.75 3DXCFR003-1EA
3DXMAX™ CFR-ABS carbon fiber reinforced ABS 3D printing filament
2.85 3DXCFR001-1EA
Description
Diameter (mm)
Prod. No.
3DXNANO™ ESD CNT-ABS carbon nanotube reinforced ABS 3D printing filament
1.75 3DXCNT001-1EA
3DXNANO™ ESD CNT-ABS carbon nanotube reinforced ABS 3D printing filament
2.85 3DXCNT002-1EA
3DXNANO™ ESD CNT-PETG carbon nanotube reinforced polyethylene terephthalate glycol copolymer 3D printing filament
1.75 3DXCNT003-1EA
3DXNANO™ ESD CNT-PETG carbon nanotube reinforced polyethylene terephthalate glycol copolymer 3D printing filament
2.85 3DXCNT004-1EA
Carbon Nanotube Reinforced Filaments
Weight = 1.65 lb spool
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59
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3D Printing UV Curable Resins
For a complete list of available materials, visit aldrich.com/3dp.
Name
Color
Size
Prod. No.
MS Resin®
blue
1 L
900129-1EA
black
1 L
900130-1EA
white
1 L
900132-1EA
red
1 L
900133-1EA
red (LittleRP red)
1 L
900134-1EA
black
500 mL
900136-1EA
clear
500 mL
900138-1EA
gray (grey)
500 mL
900139-1EA
orange
500 mL
900135-1EA
white
500 mL
900137-1EA
yellow (translucent)
500 mL
900140-1EA
Vorex®
CastSolid® resin
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3D Printable Conductive Nanocomposites of PLA and Multi‑walled Carbon Nanotubes
3D PRINTABLE CONDUCTIVE
NANOCOMPOSITES
OF PLA AND MULTI‑WALLED CARBON NANOTUBES
Vincent Hughes, Ilyass Tabiai, Kambiz Chizari, Daniel Therriault*
Laboratory for Multiscale Mechanics (LM2)
École Polytechnique Montréal, QC H3T1J4, Canada
*Email: [email protected]
Introduction
A nanocomposite is typically defined as a mixture between a host
material (e.g., polymer matrix) and nanofillers with at least one dimension
of less than 100 nm. The addition of nanoparticles to a host material
very often leads to significant improvements in material properties (e.g.,
mechanical, electrical, thermal) at relatively small loadings. The utilization
of nanocomposites is being increasingly reported in a wide variety of
fields including electronics,1,2 sporting goods,3 and aerospace.4,5 Some of
the most commonly studied nanomaterials include carbon nanotubes,
nanowires, buckyballs, graphene, metal particles, and quantum dots.
Nanofillers can be used to transform an insulating material such as a
polymer into a highly conductive material through the use of percolation
pathways generated by the nanoparticles. Percolation pathways are
routes in which a current can pass through the material. The size of the
percolation pathway is often dependent upon nanoparticle characteristics
such as aspect ratio and their alignment within the host matrix. As a
host material, polymer matrices are widely used due to their ease of
processability, low cost, and light weight. The addition of nanofillers into
a polymer matrix comes with various challenges and difficulties including
processing problems, dispersion and alignment complications, longer
lead times, and higher overall cost.
The fabrication of nanocomposite structures through additive
manufacturing is extremely promising for a myriad of applications
such as tissue engineering scaffolds6,7 and liquid8 or strain sensors.9
Additive manufacturing (also referred to as three-dimensional (3D)
printing) consists of joining materials to make complicated objects from
a 3D computer-aided-design (CAD) model in a layer-by-layer fashion.
Extrusion-based 3D printing methods create 3D structures by extruding
material from a small diameter nozzle and depositing it onto a printing
platform. This technique extrudes the material in a low viscosity state and
solidifies post-extrusion. Two extrusion-based methods appropriate to
the manufacture of nanocomposite thermoplastics are fused-deposition
modeling (FDM)10 and solvent-cast printing.11
FDM uses heat to melt the fabrication material and is a widely
used method for the 3D printing of thermoplastics. The addition of
nanoparticles to thermoplastic resin is known to increase the material
viscosity to the point where clogging can occur more frequently within
the nozzle even at high temperatures. Instead of using heat to soften the
thermoplastic material, solvent-cast printing uses a solvent to dissolve
the material and produce a printable ink solution at room temperature.
This ink solution is extruded through a micro nozzle under constant
applied pressure; the solution rapidly transitions from liquid-like to a
solid-like structure due to fast evaporation of the solvent in air. This steep
rigidity gradient makes conventional layer-by-layer printing possible but
also provides the additional ability to fabricate freeform self-supporting
curved shapes, such as reported by Guo, et al.12 (Figure 1), which is not
possible when printing via FDM. This, along with no material degradation
due to heat, makes the solvent-cast process very encouraging as a 3D
printing method.
A)
B)
C)
D)
Figure 1. Solvent-cast 3D printing of freeform structures. A) Schematic representation of the
process.12 The solution is extruded from the syringe barrel through the nozzle. B) Close-up view
of rapid solvent evaporation.12 C) Example of freeform structure fabricated from this process.12
D) LED bulb lit up by using 3D-printed freeform spirals made out of PLA and 5 wt% loading of
MWCNT using the solvent-cast process.13
Here we present the solvent-cast printing of thin nanocomposite
fibers made of thermoplastic poly(lactide) acid (PLA) reinforced with
multi-walled carbon nanotubes (MWCNTs). PLA is suitable for this
method because it is a widely available, low-cost thermoplastic that is
biocompatible and biodegradable with good processability. MWCNTs
are nanoparticles with large aspect ratios that exhibit good mechanical,14
electrical,15 and thermal16 properties. The solvent used to create a dilute
ink solution of PLA/MWCNT is dichloromethane (DCM), chosen because
of its ability to dissolve PLA and its low boiling point (39.6 °C). This article
explores the methodology used for 3D printing nanocomposite fibers
of PLA/MWCNT using the solvent-cast method. The steps include (1)
the nanocomposite ink fabrication, (2) the solvent-cast printing process,
and (3) the characterization of the printed fibers. This research shows
the strong potential of 3D printing as a novel method for manufacturing
nanocomposites with promising applications as reinforced structural
parts, flexible electronics, electromagnetic shielding grids, and
liquid sensors.
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61
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Experimental Method
Results and Discussion
Nanocomposite Fabrication
Conductivity Results
The MWCNTs (Nanocyl NC7000) are dispersed within PLA (PLA 4032D,
Natureworks LLC) with a ball mill mixer method (SPEX Sample Prep 8000M
Mixer/Mill).17 First, PLA is mixed with the solvent DCM to create a 10 wt%
mixture and allowed to mix for 24 h for full dissolution. The PLA/DCM
mixture is poured into a ball mill container with the desired amount of
MWCNTs. We investigated the loading of MWCNTs of 0, 5, 10, and 20 wt%.
The container is placed into the ball mill machine for 15 min to allow for
proper dispersion of the nanofillers within the polymer matrix. Afterward,
the mixture is left to dry for 24 h. After the solvent fully evaporates, the
dry ink mixture is removed from the container and weighed. DCM is
added at an amount based on the desired wt% of MWCNTs to this dry
nanocomposite to yield a PLA/DCM concentration in the range of 25 to
30%. The mixture is left for 24 h to achieve full dissolution.
Figure 2 shows the electrical conductivity of the 3D-printed
nanocomposite fibers with respect to the MWCNT loadings. Pure PLA
experiences no conductivity as expected. Increasing the MWCNT wt%
from 5 to 20 causes the conductivity to go from 39.7 to 1,206 S/m,
corresponding to a ~3,000% increase. This important conductivity is
mainly attributed to the numerous particle-to-particle interactions, which
increases the number of percolation pathways within the material. The
conductivity of our material at 20 wt% performs extremely well against
other polymer nanocomposites (Figure 2B) with the closest being
400 S/m for 3D printable nanocomposite of rubber/silver (Ag).18 Increasing
the MWCNT wt% past 20% increases the conductivity; however, the
increase in viscosity makes the printing process significantly harder.
Shrinkage Ratio Measurements
0
5
10
15
MWCNT Loading (wt%)
B)
20
1200
1000
800
600
400
200
0
%)
wt
)
e
en
ph
Gra
k
A+
lac
PL
(68
nB
Ag
rbo
Ca
er+
bb
)
t%
t%
%)
0w
T(2
CN
A+
PL
Ru
A+
PL
wt
0w
T(1
62
0
T(5
Each fiber was mounted in the Insight® MTS electromechanical testing
machine equipped with a 1 kN load cell and tested according to the
ASTM D3822 standard test method for tensile properties of single textile
fibers. Each combination of MWCNT percentage and nozzle diameter was
tested at least three times.
200
CN
Tensile Mechanical Measurements
400
CN
The conductivity measurements of the printed fibers were performed
using a two-point probe method. The applied electrical current varied
from 1 to 5 mA using a Kiethley 6221 current source. The volume
conductivity was calculated from the resistance values resulting from the
two-point probe measurements and by knowing the length and diameter
of each fiber. Measurements were done on each combination of MWCNT
(0, 5, 10, and 20 wt%) and nozzle diameter (150, 200, 330 µm) at least
six times.
600
A+
Conductivity Measurements
800
A+
The density of the fibers was measured using a helium pycnometer
(Micromeritics, AccuPyc II 1340). The mass of the fibers was measured first
and input into the pycnometer, which then measured the density ten
times. An average was calculated. The measurement was performed for
the fibers at 0, 10, and 20 wt% PLA/MWCNTs concentrations.
1000
PL
Density Measurements
1200
PL
We defined the shrinkage ratio as the actual diameter of the fibers over
the printing nozzle inner diameter. The fibers printed were cut into six
60-mm long sections for each MWCNT wt% and nozzle tip diameter. The
actual diameter of the fibers was measured through digital image analysis
taken with an optical microscope (Olympus, BX61). The diameter of each
60 mm fiber is measured at twelve different positions spaced by about
5 mm. An average diameter value for each fiber is calculated.
Conductivity (S/m)
The nanocomposite ink is poured into a syringe barrel connected to a
micronozzle mounted onto the head of a computer-controlled dispensing
robot (I&J2200-4, I&J Fisnar). The extrusion pressure was controlled by a
pressure regulator (HP-7X, EFD). The pressure ranged from 200–500 kPa
depending on the desired linear flow rate of the ink coming out of the
micronozzle, the nozzle size, and the nanocomposite ink concentration
(i.e., viscosity of the ink). Continuous cylindrical fibers were extruded at 0
wt% CNT with a 150 µm inner diameter micronozzle, 5 wt% and 10 wt%
with 200 µm and 20 wt% with 330 µm tip. The fibers were extruded onto
a platform and left to dry for 10 min, then stored in airtight containers
before performing subsequent characterization measurements.
A)
Conductivity (S/m)
Solvent-cast 3D Printing
Printable Conductive
Polymer Nanocomposites
Figure 2. A) Conductivity of PLA/MWCNT fibers (wt%/nozzle diameter (µm) = 0/150; 5/200;
10/200; 20/330). B) Comparison of the conductivity of the 5, 10, and 20 wt% MWCNT (shaded bar
charts) compared to other conductive polymer composites.18,19,20
Tensile Mechanical Measurements
Figure 3 shows the mechanical properties of the nanocomposite fibers
along with the shrinking and density measurements. Figure 3A shows
the stress-strain curves of the pure PLA and the three MWCNT loadings.
The measurements show a much lower strain-at-break when adding
MWCNTs. The values change from 0.239 at pure PLA to 0.058 at 5 wt%,
0.046 at 10 wt%, and 0.009 at 20 wt% MWCNT, the largest drop being at
20 wt% with a 96% decrease in the strain-at-break from pure PLA. The
tensile strength at pure PLA is 44 MPa; 5 and 10 wt% increases to 58
MPa and 56 MPa (31% and 27% improvement), respectively. The tensile
strength at 20 wt% decreases from pure PLA by 28% to 32 MPa. Pure PLA
has a ductile behavior with a large elongation at failure. Therefore, it is
expected that the addition of MWCNTs would reduce the strain-at-break
and make the material more brittle. The increase in tensile strength is also
anticipated due to the covalent sp2 bonds formed between the individual
carbon atoms, which make carbon nanotubes one of the strongest and
stiffest materials ever discovered.21 The decrease in tensile strength for the
20 wt% could be due to the weak van der Waals bonding of PLA-CNT. By
increasing the loading of MWCNTs to 20 wt%, the stronger crosslinked
bonds within the PLA are replaced by weaker bonds. PLA-CNT bonds
are not as entangled, so the whole structure falls apart more easily at
lower stress.
TO ORDER: Contact your local Sigma-Aldrich office or visit aldrich.com/matsci.
3D Printable Conductive Nanocomposites of PLA and Multi‑walled Carbon Nanotubes
A)
40
30
Pure PLA
5 wt% MWCNT
10 wt% MWCNT
20 wt% MWCNT
20
10
0
0
2
4
6
8
Strain (%)
600
Young Modulus (MPa)
Stress (MPa)
50
500
400
300
200
100
0
10
C)
0
5
10
15
MWCNT Loading (wt%)
20
D)
1.2
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4
1.5
Density (g/cm3)
Shrinkage Ratio
Conclusion
B)
1.45
1.4
1.35
0
5
10
15
MWCNT Loading (wt%)
20
0
5
10
15
MWCNT Loading (wt%)
20
Figure 3. Mechanical results of the printed nanocomposite fibers (wt%/nozzle diameter (µm) =
0/150; 5/200; 10/200; 20/330). A) Stress-at-Strain curve of printed fibers. B) Plot of Young’s Modulus
against MWCNT loading. C) The calculated shrinkage ratios of each printed fiber. D) Helium
pycnometer density measurements for pure PLA and reinforced MWCNT nanocomposite fibers.
Figure 3B presents the Young’s Modulus for the different printed fibers.
The modulus increases from 351 MPa for the pure PLA to 546 MPa for
the 20 wt% fiber, corresponding to a 55% increase. It is possible that the
extrusion of the ink inside the small nozzle helps the alignment of the
nanotubes in the fibers, which would affect the resulting properties of
the material.22
Figure 3C displays the shrinkage ratio of PLA/MWCNTs for each MWCNT
wt%. The plot clearly shows increasing shrinkage with higher loadings of
MWCNT, whereas pure PLA actually increased in size by 6%. Maximum
shrinkage occurred at 20 wt% and fiber diameter shrank 52%. The
increased shrinkage is most likely related to the amount of DCM in the
mixture. More solvent is needed at higher nanofiller concentrations
to achieve suitable viscosity for printing. It is necessary to know this
shrinkage so that the geometrical changes can be anticipated and
correctly planned for the 3D printing of microstructures. Figure 3D shows
the density with increasing MWCNT wt%. The density was measured at 0,
10, and 20 wt% of MWCNT and found to be 1.33, 1.452, and 1.48 g/cm3,
respectively. Linear interpolation was used to find 5 wt% MWCNT of
1.391 g/cm3. The increasing density is directly related to the higher
loadings of MWCNTs. This data will be useful for mechanical modeling
purposes of larger structures made out of these 3D printed fibers.
In this work, the solvent-cast 3D printing technique is used to fabricate
nanocomposite materials made of PLA and 5, 10, and 20 wt% MWCNTs
concentrations. The addition of the nanotubes significantly affects the
properties of the host material while preserving a very good solventcast printability. The best improvement was observed for the electrical
conductivity where a maximum of 1,206 S/m at 20 wt% MWCNT was
measured. To the best of our knowledge, this conductivity is superior to all
the current printable commercial polymer-based composite materials. In
addition to outstanding electrical properties, the nanocomposite material
exhibits better stiffness than the pure PLA. However, the more electrically
conductive fibers show a significant decrease of the strain-at-break
making the fibers slightly more fragile. The tensile strength increased to
58 MPa for the 5 wt% fiber while it decreased for the 20 wt% MWCNT.
The decrease may be attributed to the poor van der Walls bonding
between PLA-CNT.
This study shows that it is possible to 3D print highly conductive polymer
nanocomposites. While more work is needed to print more complex
3D structures, these materials have the potential to produce finished
products in many niche applications. The high conductivity achieved
opens the door for on-the-fly 3D-printed electrical components.
3D-printable conductive inks can be further enhanced when combined
with other materials for integrated functionality (embedded sensors, EMI
shielding, tactile surfaces). This research provides only a small insight into
the possibilities enabled by 3D printing. The endless opportunities will
only be realized with further work and future collaboration among other
fields of science and technology.
References
Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J.; Zheng, Y. Nature Nanotech. 2010, 5(8), 574-578.
Jiang, G. Appl. Mech. Mater. 2014, 484-485, 114-117.
Nogi, M.; Yano, H. Adv. Mater. 2008, 20(10), 1849-1852.
Wu, N. J. Aeronaut. Aerospace Eng. 2012, 01(04).
Njuguna, J.; Pielichowski, K. Adv. Eng. Mater. 2003, 5(11), 769-778.
Farahani, R. D.; Dubé, M.; Therriault, D. Adv. Mater. 2016, doi:10.1002/adma.201506215
Karageorgiou, V. D. Biomaterials 2005, 26(27), 5474-5491.
Guo, S.; Yang, X.; Heuzey, M.; Therriault D. Nanoscale 2015, 7(15), 6451-6456.
Farahani, R.; Dalir, H.; Le, Borgne V.; Gautier, L.; El Khakani, M.; Lévesque, M. et al.
Nanotechnology 2012, 23(8), 085502.
(10) Leigh, S.; Bradley, R.; Purssell, C.; Billson, D.; Hutchins, D. PLoS ONE 2012, 7(11), e49365.
(11) Farahani, R.; Chizari, K.; Therriault, D. Nanoscale 2014, 6(18), 10470.
(12) Guo, S.; Heuzey, M.; Therriault, D. Langmuir. 2014, 30(4), 1142-1150.
(13) Guo, S.; Yang, X.; Heuzey, M.; Therriault, D. Nanoscale 2015, 7(15), 6451-6456.
(14) Treacy, M. M. J.; Ebbesen, T. W.; Gibson, J. M. Nature 1996, 381, 678–680.
(15) Ebbesen, T. W.; Lezec, H. J.; Hiura, H.; Bennett, J. W.; Ghaemi, H. F.; Thio, T. Nature 1996, 382,
54–56.
(16) Berber, S.; Kwon, Y. K.; Tománek, D. Phys. Rev. Lett. 2000, 84, 4613–4616.
(17) Chizari, K.; Therriault, D. In Fabrication of Conductive Microfilaments and Liquid Sensor from
CNTs/PLA Nanocomposites, Design, Manufacturing and Applications of Composites Tenth
Workshop 2014: Proceedings of the Tenth Joint Canada-Japan Workshop on Composites,
August 2014, Vancouver, Canada, DEStech Publications, Inc: 2015; p 214.
(18) Blackmagic3D, Conductive Graphene 3D Printing PLA Filament. URL http://www.
blackmagic3d.com//ProductDetails.asp?ProductCode=GRPHN%2D175
(19) Strümpler, R., Glatz-Reichenbach, J. J. Electroceram. 1999, 3, 329–346.
doi:10.1023/A:1009909812823
(20) ProtoPlant, Composite PLA - Electrically Conductive PLA. URL https://www.proto-pasta.com/
products/conductive-pla
(21) Salvetat, J.; Bonard, J.; Thomson, N.; Kulik, A.; Forró, L.; Benoit, W.; et al. App. Phys. A-Mater. 1999,
69(3), 255-260.
(22) Thostenson, E.; Chou, T. J. Phys. D: Appl. Phys. 2002, 35(16), L77-L80.
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
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63
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Carbon Nanotubes
For a complete list of available materials, visit aldrich.com/cnt.
Single-walled Carbon Nanotubes
Production Method Dimensions Purity
Prod. No.
CoMoCAT® Catalytic Chemical Vapor Deposition (CVD) Method
(6,5) chirality
diameter 0.7-0.9 nm (by fluorescence)
≥93% (carbon as SWNT)
773735-250MG
773735-1G
diameter 0.7-0.9 nm (by fluorescence)
L ≥700 nm
≥77% (carbon as SWNT)
704148-250MG
704148-1G
CoMoCAT® Catalytic Chemical Vapor Deposition (CVD) Method
(7,6) chirality
diameter 0.7-1.1 nm
L 300-2,300 nm (mode: 800 nm; AFM)
≥77% (carbon as SWNT)
704121-250MG
704121-1G
CoMoCAT® Catalytic Chemical Vapor Deposition (CVD) Method
diameter 0.6-1.1 nm
>95% (carbon as SWCNT)
775533-250MG
775533-1G
diameter 0.7-1.4 nm
≥80.0% (carbon as SWNT)
724777-250MG
724777-1G
diameter 0.7-1.3 nm
L 450-2,300 nm (mode: 800 nm; AFM)
≥70% (carbon as SWNT)
704113-250MG
704113-1G
Catalytic Carbon Vapor Deposition (CCVD) Method
average diameter 2 nm
L 3 μm
>70%
755710-250MG
755710-1G
Electric Arc Discharge Method
diameter 1.2-1.7 nm
L 0.3-5 μm
30% (Metallic)
70% (Semiconducting)
750492-100MG
diameter 1.2-1.7 nm
L 0.3-5 μm
30% (Metallic)
70% (Semiconducting)
750514-25MG
diameter 1.2-1.7 nm
L 0.3-5 μm
2% (Metallic)
98% (Semiconducting)
750522-1MG
diameter 1.2-1.7 nm
L 0.3-5 μm
98% (Metallic)
2% (Semiconducting)
750530-1MG
D × L 2-10 nm × 1-5 μm
(bundle dimensions)
1.3-1.5 nm (individual SWNT diameter)
40‑60 wt. %
698695-1G
698695-5G
Single-walled Carbon Nanotube Inks
Form
SWCNT Concentration
Viscosity
Sheet Resistance
Prod. No.
dispersion in H2O (black liquid)
0.20 ± 0.01 g/L (by Absorbance at 854 nm)
~1.0 mPa.s <400 Ω/sq (by 4-point probe on prepared film
by spray)
791490-25ML
791490-100ML
1.00 ± 0.05 g/L (by Absorbance at 854 nm)
3.0 mPa.s (at 10 s-1 shear rate)
<600 Ω/sq (at 85% VLT (ohm/sq), by 4-point probe
on prepared film by spray)
791504-25ML
791504-100ML
1 mg/mL
17.7 Pa.s at 25 °C (at 10 s-1 shear rate)
<1,000 Ω/sq (by 4-point probe on prepared, at
87.5% VLT (ohm/sq))
792462-25ML
792462-100ML
viscous liquid (black)
Double-walled Carbon Nanotubes
Production Method
Dimensions
Purity Prod. No.
Catalytic Carbon Vapor Deposition (CCVD) Method
avg. diam. × L 3.5 nm × >3 μm
(TEM)
Metal Oxide ≤10% TGA
755141-1G
avg. diam. × L 3.5 nm × 1-10 μm
(TEM)
Metal Oxide <10% TGA
755168-1G
O.D. × I.D. × L 5 nm × 1.3-2.0 nm × 50 μm
50‑80%
637351-250MG
637351-1G
Chemical Vapor Deposition (CVD) Method
Multi-walled Carbon Nanotubes
Production Method
Description Purity Prod. No.
CoMoCAT® Catalytic Chemical Vapor Deposition (CVD) Method
O.D. × I.D. × L 10 nm ±1 nm × 4.5 nm ±0.5 nm × 3-~6 μm
(TEM)
≥98% carbon basis
773840-25G
773840-100G
O.D. × L 6-9 nm × 5 μm
diam. 6.6 nm (median)
diam. 5.5 nm (mode)
>95% (carbon)
724769-25G
724769-100G
O.D. × I.D. × L 10 nm × 4.5 nm × 4 μm
Aspect ratio (L/D) 350-550
Tubes typically have 6-8 tube walls.
70‑80% (carbon)
791431-25G
791431-100G
avg. diam. × L 9.5 nm × <1 μm
(TEM)
thin and short
Metal Oxide <5% TGA
755117-1G
avg. diam. × L 9.5 nm × 1.5 μm
(TEM)
thin
Metal Oxide <5% TGA
755133-5G
Catalytic Carbon Vapor Deposition (CCVD) Method
64
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3D Printable Conductive Nanocomposites of PLA and Multi‑walled Carbon Nanotubes
Production Method
Description Purity Prod. No.
Chemical Vapor Deposition (CVD) Method
O.D. × L 6-13 nm × 2.5-20 μm
12 nm (average diameter, HRTEM)
10 μm (average length, TEM)
>98% carbon basis
698849-1G
D × L 110-170 nm × 5-9 μm
>90% carbon basis
659258-2G
659258-10G
O.D. × L 7-12 nm × 0.5-10 μm
powdered cylinder cores
20‑30% MWCNT basis
406074-500MG
406074-1G
406074-5G
O.D. × L 7-15 nm × 0.5-10 μm
as-produced cathode deposit
>7.5% MWCNT basis
412988-100MG
412988-2G
412988-10G
diam. × L 100-150 nm × 30 μm
(SEM)
vertically aligned on silicon wafer substrate
>95 atom % carbon basis (x-ray)
687804-1EA
Electric Arc Discharge Method
Plasma-Enhanced Chemical Vapor Deposition (PECVD) Method
Functionalized Nanotubes
Name
Purity (%)
Dimensions (D × L)
Production Method
Prod. No.
O
OH
Structure
Carbon nanotube, single-walled,
carboxylic acid functionalized
>90
4-5 nm × 0.5-1.5 μm
(bundle dimensions)
Electric Arc Discharge Method
652490-250MG
652490-1G
O
OH
Carbon nanotube, multi-walled,
carboxylic acid functionalized
>80
9.5 nm × 1.5 μm
Catalytic Carbon Vapor
Deposition (CCVD) Method
755125-1G
O
O
Carbon nanotube, single-walled,
poly(ethylene glycol) functionalized
>80
4-5 nm × 0.5-0.6 μm
(bundle dimensions)
Electric Arc Discharge Method
652474-100MG
O
NH 2
Carbon nanotube, single-walled,
amide functionalized
>90
4-6 nm × 0.7-1.0 μm
(bundle dimensions)
Electric Arc Discharge Method
685380-100MG
O
H
N
Carbon nanotube, single-walled,
octadecylamine functionalized
80‑90
2-10 nm × 0.5-2 μm
(bundle dimensions)
Electric Arc Discharge Method
652482-100MG
1.1 nm × 0.5-1.0 μm
(bundle dimensions)
Electric Arc Discharge Method
639230-100MG
H
N
Carbon nanotube, single-walled,
polyamino­benzene sulfonic acid
functionalized
75‑85
O
H
O n
CH 2 (CH 2 ) 16 CH 3
SO3H
NH
n
For questions, product data, or new product suggestions, contact us at [email protected]
65
CARBON NANOTUBE
3D PRINTING FILAMENTS
3DXNANO™ ESD Carbon Nanotube (CNT) Printing Filaments
are used in critical applications that require electrostatic
discharge (ESD) protection and a high level of cleanliness.
The compounded filaments are formulated with premium
Acrylonitrile Butadiene Styrene (ABS) or Polyethylene
Terephthalate Glycol (PETG) resin and multi-walled CNTs in
the presence of dispersion modifiers, resulting in a filament
with excellent printing characteristics and consistent
ESD properties. Properties of 3DXNANO™ ESD 3D Printing Filaments
yy
Consistent surface resistivity (target 1.0 × 107–109 Ω)
yy
Amorphous polymers – low and near isotropic shrinkage
yy
ABS suitable for water-based conformal coatings
yy
Low moisture absorption of PETG
yy
PETG preferred for solvent-based conformal coatings
yy
Excellent retention of base-resin ductility due to low loading
rate of CNTs needed to achieve ESD
yy
Ultra-low particulate contamination vs. carbon black
compounds
Name
3DXNANO™ ESD CNT-ABS 3D printing filament
Diameter (mm)
1.75
3DXCNT001
Prod. No.
2.85
3DXCNT002
3DXNANO™ ESD CNT-PETG 3D printing filament
1.75
3DXCNT003
2.85
3DXCNT004
For the complete nanomaterials offering, visit
aldrich.com/3dp
Nanoparticle-based Zinc Oxide Electron Transport Layers for Printed Organic Photodetectors
NANOPARTICLE-BASED ZINC OXIDE
ELECTRON TRANSPORT LAYERS
FOR PRINTED ORGANIC PHOTODETECTORS
Unlike solar cells, photodiodes are generally operated in reverse bias in
order to achieve a larger collection of photo-excited carriers. The On–Off
ratio of photodiodes is defined as the ratio between the measured current
under illumination and dark conditions at a certain bias voltage. The dark
current density (Jdark) can be used to estimate the electrical noise (Snoise),
which is dominated by the shot noise of the OPDs, as seen in Equation 2.
A lower Snoise compared to the obtained SR will result in a higher D* as
evident from Equation 3. D* is a figure of merit of photodetectors which
characterizes the smallest detectable signal of the device.
Gerardo Hernandez-Sosa,1,2* Ralph Eckstein,1,2 Tobias Rödlmeier,1,2 Uli Lemmer1,2,3
1
Lichttechnisches Institut, Karlsruher Institut für Technologie,
Engesserstrasse 13, 76131 Karlsruhe, Germany
2
InnovationLab, Speyerer Str. 4, Heidelberg, Germany
3
Institut für Mikrostrukturtechnik, Karlsruher Institut für Technologie,
Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
*Email: [email protected]
D* =
Introduction
Recent progress in the area of solution-processed functional materials
has led to the development of a variety of thin-film optoelectronic
devices with significant promise in the industrial and consumer
electronics fields.1,2 These devices are becoming core technologies for
the development of novel applications in sensing,3 energy harvesting,4
and energy conversion, exhibiting a unique combination of mechanical
flexibility and light weight.5 Optical sensor research is growing in the
area of organic photodiodes, with a special focus on the development of
high-performance multilayer device architectures printed on a variety of
substrates.6,7 We present the fabrication of printed organic photodiodes
(OPDs) based on bulk-heterojunction (BHJ) active materials utilizing zinc
oxide (ZnO) hole blocking layers deposited from nanoparticle (NP)-based
ink. In contrast to sol gel or precursor-based layer deposition, the use of
NPs offers great benefits with regard to processability and, in particular,
the ability to precisely adjust the electronic and optical properties
of the NPs without influence from the required conditions of thin
film deposition.
Figures of Merit of Photodetectors
Organic BHJ photodetectors and solar cells generally have the same
device architecture and material set. However, they differ in application
focus which requires the optimization of different figures of merit. While
solar cells are generally characterized by the short circuit current density
(JSC), open circuit voltage (VOC), fill factor (FF), and device efficiency (η),
photodiodes are characterized by the current On–Off ratio in reverse bias,
specific detectivity (D*), bandwidth (BW), and spectral responsivity (SR).
SR is given in amperes per watt (A/W) and is analogous to the external
quantum efficiency (EQE) in solar cells, as seen in Equation 1:
SR =
λ·q
Iph
= EQE ·
h·c
Popt(λ)
Snoise = √ 2 · q · Jdark · ∆f · A
(2)
SR · √ ∆f · A
Snoise
(3)
=
SR
√ 2 · q · Jdark
The dynamic response of the photodiode is characterized by its BW,
which can be obtained by a transient photocurrent measurement or by
varying the frequency of the excitation source. BW is usually defined by
the cut-off frequency at –3 dB, which corresponds to a power drop of
~50%.
Results of Fully Printed Photodetectors
In practice, the main leverage for increasing the On–Off ratio and,
consequently, D* of a photodetector is the reduction of the dark
current. This can be achieved by introducing interlayers that selectively
block electrons in the device architecture in order to avoid charge
recombination at the active layer/electrode interface and simultaneously
adjusting the energy levels of the electrode to the active material.
ZnO NPs have shown exceptional electron transport properties within
optoelectronic devices.8,9 The photodiodes presented in this work were
fabricated in an inverted device architecture onto indium-doped tin
oxide (ITO)-covered glass or PET substrates. The electron transport layer
(ETL) using ZnO NP dispersions was fabricated by spin casting, inkjet
printing, and aerosol printing. A thermal treatment at 120 °C for 5 minutes
was applied to fully dry the ETL. The active material comprising P3HT
(poly(3-hexylthiophene-2,5-diyl)) (Prod. Nos. 698997 and 698989) and
PCBM ([6,6]-phenyl C61 butyric acid methyl ester) (Prod. No. 684457)
diluted in dichlorobenzene (40 g/L) was blended in a ratio of 1:0.9 and
was spin cast, resulting in a ~200 nm thick layer. A 20 nm thick poly(3,4ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) layer was spin
cast and used as the hole transport layer (HTL). Afterward, the samples
were transferred to a nitrogen-filled glovebox and thermally treated at
145 °C for 15 minutes on a hotplate in order to remove humidity from the
PEDOT:PSS layer and improve BHJ morphology.
(1)
where Iph is the generated photocurrent per wavelength λ, q is the
electron charge, c the speed of light, and h the Planck constant.
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67
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The 100 nm thick silver top electrode was thermally evaporated through
a shadow mask in a vacuum system with a base pressure of 10–7 mbar. For
the aerosol-printed OPDs, an active layer of poly({4,8-bis[(2-ethylhexyl)oxy]
benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]
thieno[3,4-b]thiophenediyl}) (PTB7, Prod. No. 772410) was blended with
[6,6]-Phenyl-C71-butyric acid methyl ester (PC70BM, Prod. No. 684465)
using a ratio of 1:1.5 (10 g L–1 in 1,2-dichlorobenzene) with 3% volume
of diiodooctane printed on top of a PEDOT:PSS/AZO cathode. The film
was then dried in a vacuum (15 mbar) for 15 minutes. The conductive
transparent top anode was aerosol-printed using a diluted PEDOT:PSS
dispersion and finally dried in vacuum (15 mbar) in an antechamber,
then transferred into a glovebox. All devices were encapsulated with an
adhesive barrier foil to avoid the penetration of oxygen and moisture into
the device.
A)
100
Current Density (mA/cm2)
B)
45.4 mn
20.0
15.0
10.0
l Je
oso
Aer
–4.2
E)
–6.7
–4.3
–6.1
5.0
20.0
10.0
0.0
The J-V characteristic of devices with spin cast (SC), inkjet (IJ), and aerosol
(AJ) printed A) aluminium-doped ZnO (AZO) and B) ZnO nanoparticlebased layers are presented in Figure 2. The device characteristics can be
found in Table 1. A dark current density on the order of 10–4 mA/cm2 at
–1 V was observed for the spin-cast AZO and ZnO layers, demonstrating
the suitability of these ETLs for photodiode application. IJ-printed
AZO layers show results as good as the spin-cast AZO layers; however,
Jdark for AJ-printed ZnO, and IJ- or AJ-printed AZO-comprising devices
is increased.
–1
0
1
–1
0
1
Voltage (V)
1000
N11
1
SC
AJ
IJ
0.1
0.01
0.001
1E-4
1E-5
1E-7
–3
52.7 nm
Figure 1. A) Inverted organic photodiode stack. B) Corresponding band diagram. AFM
image (10 × 10 µm2) of C) inkjet-printed N11 Jet and D) aerosol-jet printed N11 Slot on ITOcovered glass.
68
–2
1E-6
0.0
30.0
–5.2 –5.1
1E-5
–2
40.0
–3.7
ITO
–4.8
1E-4
t
–3.3
0.001
10
25.0
D)
0.1
0.01
100
30.0
B)
1
1E-7
–3
35.0
Inkjet
SC
AJ
IJ
1E-6
Current Density (mA/cm2)
C)
N21x
10
The OPD architecture and the relative energy level arrangement of
the materials is shown in Figure 1A. Atomic force microscope (AFM)
measurements were conducted on inkjet and aerosol-jet printed ZnO
nanoparticle layers on ITO as shown in Figure 1B. Inkjet-printed ZnO using
N11 Jet (Prod. No. 808202) formulation and aerosol-jet printed ZnO using
N11 Slot (Prod. No. 808199) produced the best results. Both techniques
delivered a very homogeneous ZnO dense layer with a Root Mean Square
(RMS) roughness on the order of 2.7 to 3 nm.
A)
1000
Voltage (V)
Figure 2. J-V curves of inverted stack comprising A) spin cast (SC) AZO (N21x), inkjet (IJ) (N21x Jet),
and aerosol jet (AJ) (N21x slot) layers and B) for ZnO (N11, N11 Jet, N11 Slot), respectively.
This suggests that some low resistant pathways are formed during drying
of the printed films. In both digital printing technologies, a sequential
printing path is used. A local inhomogeneity of the surface energy on the
ITO substrate can lead to incomplete wetting and even to pin holes in
the ZnO layer. The layer thickness of the spin-cast ZnO and AZO layers is
~30 nm, while the necessary thickness of the IJ and AJP ZnO/AJP layers
is below 50 nm. In the case of inkjet printing, the layer thickness was
adjusted by the drop spacing or the NP concentration in the ink. Film
thicknesses of AJ-printed ZnO layers are adjusted through a variety of
parameters including the mist density, printing velocity, atomizer, and
sheath gas flows. Further information about the aerosol working principle
can be found elsewhere.10 All devices showed solar cell efficiencies >3%,
with fill factors exceeding 53% as shown in Table 1.
Table 1. Inverted solar cell parameters of devices comprising SC, IJ, and AJ-printed ZnO
and AZO layers.
Device
Jsc (mA/cm²)
Jdark @–1 V (µA/cm²)
Efficiency (%)
Fill factor (%)
N21x SC
–10.3
–1,94
3.27
54.94
N21x IJ (Jet)
–10.8
–3.02
3.53
54.63
N21x AJ (slot)
–10.5
–41.00
3.23
53.79
N11 Sc
–10.0
–9.20
3.31
57.42
N11 IJ (Jet)
–10.7
–11.13
3.46
53.90
N11 AJ (slot)
–10.8
–64.80
3.56
57.53
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Nanoparticle-based Zinc Oxide Electron Transport Layers for Printed Organic Photodetectors
Fully AJ-printed semi-transparent photodiodes are presented in Figure 3.
The inverted device stack, shown in Figure 3A, comprises a transparent
conductive PEDOT:PSS/AZO bottom electrode printed on PET foil.
PTB7:PC70BM was used as the active material. Conductive PEDOT:PSS
was AJ-printed in a similar manner as the top electrode. Figure 3B shows
a device that achieved a 2 mm bending radius; smaller bending radii
led to irreversible deformation of the device due to delamination of the
barrier foils. The EQE and SR of the device under bending conditions and
in normal state are shown in Figure 3C. In stress-free conditions the fully
printed OPD showed an average SR of ~0.25 A/W from 400 to 700 nm at
–3 V reverse bias. The bended device showed only a marginal reduction of
<5%. The D* for both cases was found to be ~1011 Jones, with BW on the
order of 200 kHz.7
C)
–3V bias
0.2
B)
0.1
initial
bended (2 mm radius)
20
400
D)
Detectivity (Jones)
Acknowledgment
References
40
0
We have presented the properties of printed organic photodiodes
utilizing ZnO nanoparticle-based ETLs. The ZnO nanoparticles allow low
dark currents and specific detectivities comparable to spin cast devices,
regardless of whether they were produced by AJ or IJ printing. We also
showed that the devices can be processed on flexible substrates and
can be operated under mechanical stress. These results highlight the
applicability of nanoparticle-based inks to flexible printed thin-film
electronics and the potential use of organic photodiodes as optical
sensors manufactured by digital printing technologies.
The authors acknowledge financial support of the German Federal
Ministry of Education and Research via project FKZ: 13N13691.
0.0
EQE (%)
PEDOT:PSS
PTB7:PC70BM
AZO NP
PEDOT:PSS
PET
0.3
SR (AW–1)
A)
Conclusion
500
600
700
Wavelength (nm)
800
1E11
1E10
1E9
initial
bended (~2 mm radius)
400
500
600
700
Wavelength (nm)
(1) Punke, M.; Valouch, S.; Kettlitz, S. W.; Gerken, M.; Lemmer, U. J. Lightwave Technol. 2008, 26,
816–823.
(2) Ho, C. K.; Robinson, A.; Miller, D. R.; Davis, M. J. Sensors. 2005, 5, 4–37.
(3) Sprules, S. D.; Hart, J. P.; Pittson, R.; Wring, S. A. Electroanal. 1996, 8, 539–543.
(4) Wöhrle, D.; Meissner, D. Adv. Mater. 1991, 3, 129–138.
(5) Kaltenbrunner, M.; White, M. S.; Głowacki, E. D.; Sekitani, T.; Someya, T.; Sariciftci, N. S.; Bauer, S.
Nat Commun. 2012, 3, 770.
(6) Baeg, K. J.; Binda, M.; Natali, D.; Caironi, M.; Noh, Y. Y. Adv. Mater. 2013, 25, 4267–4295.
(7) Eckstein, R.; Rödlmeier, T.; Glaser, T.; Valouch, S.; Mauer, R.; Lemmer, U.; Hernandez-Sosa, G. Adv.
Electron. Mater. 2015, 1, 1500101.
(8) Yang, T.; Cai, W.; Qin, D.; Wang, E.; Lan, L.; Gong, X.; Peng, J.; Cao, Y. J. Phys. Chem. C 2010, 114
(14), 6849–6853.
(9) Letellier, P.; Mayaffre, A.; Turmine, M. J. Colloid Interface Sci. 2007, 314 , 604–614.
(10) Eckstein, R.; Hernandez-Sosa, G.; Lemmer, U.; Mechau, N. Org. Electron. 2014, 15, 2135–2140.
800
Figure 3. Fully AJ-printed organic photodiodes. A) Scheme of inverted stack, comprising a
PEDOT:PSS/AZO anode and a PEDOT:PSS cathode. B) Flexible OPD on PET substrate at bending
radius of 2 mm. C) Spectral response (SR) and EQE before and during bending at –3 V reverse bias.
D) Specific detectivity (D*) before and during bending at 2 mm radius. Adapted and reproduced
with permission from Reference 7, ©2015 John Wiley & Sons Inc.
Nanoparticle Inks for Printing
For a complete list of available materials, visit aldrich.com/inks.
Zinc Oxide
Particle Size (nm)
Concentration (wt. %)
Viscosity (cP)
Work Function (eV)
Prod. No.
10-15
2.7 (crystalline ZnO in 2-propanol)
-
-
793361-5ML
793361-25ML
8-16
2.5 (crystalline ZnO in isopropanol)
1.6-2.6
-3.7 to -4.1
808253-10ML
2.5 (crystalline ZnO in isopropanol and propylene glycol)
2.4-3.8
-4.1 to -4.5
807648-10ML
2.5 (crystalline ZnO in isopropanol and propylene glycol)
2.4-3.7
-3.7 to -4.1
808199-10ML
2.5 (crystalline ZnO in isopropanol and propylene glycol)
8-14
-4.1 to -4.5
807613-5ML
2.5 (crystalline ZnO in isopropanol and propylene glycol)
8-14
-3.7 to -4.1
808202-5ML
2.5 (crystalline ZnO in α-terpineol)
26-36
-4.1 to -4.5
807621-10ML
2.5 (crystalline ZnO in α-Terpineol)
32-48
-3.7 to -4.1
808075-10ML
7-17
For questions, product data, or new product suggestions, contact us at [email protected]
69
aldrich.com/matsci
Aluminum-doped Zinc Oxide
Particle Size (nm)
Concentration (wt. %)
Viscosity (cP)
Work Function (eV)
Prod. No.
<50 (BET)
2.5 (crystalline Al-doped ZnO (98 wt% ZnO; 2 wt% Al2O3) in 2-propanol)
-
-
793388-5ML
793388-25ML
8-16
2.5 (crystalline Al doped ZnO (3.15 mol% Al) in mixture of alcohols)
1.7-2.7
-3.7 to -4.1
808237-10ML
2.5 (crystalline Al doped ZnO (3.15 mol% Al) in 2-propanol)
1.9-3.1
-4.1 to -4.5
807729-10ML
2.5 (crystalline Al doped ZnO (3.15 mol% Al) in 2-propanol and propylene glycol)
2.4-4.0
-4.1 to -4.5
807656-10ML
2.5 (crystalline Al doped ZnO (3.15 mol% Al) in 2-propanol and propylene glycol)
2.5-3.7
-3.7 to -4.1
808164-10ML
2.5 (crystalline Al doped ZnO (3.15 mol% Al) in 2-propanol and propylene glycol)
8-14
-3.7 to -4.1
808180-5ML
2.5 (crystalline Al doped ZnO (3.15 mol% Al) in 2-propanol and propylene glycol)
8-14
-4.1 to -4.5
808172-5ML
2.5 (crystalline Al doped ZnO (3.15 mol% Al) in α-terpineol)
25-37
-4.1 to -4.5
808210-10ML
2.5 (crystalline Al doped ZnO (3.15 mol% Al) in mixture of alcohols)
32-48
-3.7 to -4.1
808229-10ML
Titanium Dioxide
Name
Particle Size (nm)
Concentration (wt. %)
Description
Prod. No.
Titania paste, active opaque
20 (active)
≤450 (scatter)
27.0
>99% anatase
791555-5G
791555-20G
Titania paste, reflector
150-250 (scatter)
20.0
>99% anatase
791539-5G
791539-20G
Titania paste, transparent
20 (active)
19.0
>99% anatase
791547-10G
791547-20G
Titanium dioxide
22-25 (BET)
16
nanocrystalline colloidal paste for transparent film
798495-25G
18-20 (BET)
16
>95% anatase (XRD)
nanocrystalline colloidal paste for transparent film
798509-25G
22 and >150 (BET)
16
>95% anatase (XRD)
colloidal paste for opaque film
798517-25G
18-20 (BET)
16
nanocrystalline colloidal paste for opaque film
798525-25G
Silver
Name
Particle Size (nm)
Concentration
Viscosity at 25 °C
Prod. No.
Silver, dispersion
≤10
50-60 wt. % in tetradecane
8-14 cP 736511-25G
736511-100G
≤50
30-35 wt. % in triethylene glycol
monomethyl ether
10-18 cP 736473-25G
736473-100G
≤10
50-60 wt. % in tetradecane
7-14 cP 736503-25G
736503-100G
≤50
30-35 wt. % in triethylene glycol
monomethyl ether
10-18 cP 736465-25G
736465-100G
≤50
30-35 wt. % in triethylene glycol
monoethyl ether
10-18 cP 736481-25G
736481-100G
115 (d90 (by Brookhaven))
70 (d50 (by Brookhaven))
30 wt. % dispersion in ethylene glycol
28 cP
798738-10G
798738-25G
70 (d50)
115 (d90)
50 wt. %
24 cP 796042-5G
796042-20G
Silver
<5 (20%)
200 (80%)
≥75%
100,000-300,000 cP 735825-25G
Conductive silver printing ink, resistivity 30-35 μΩ/cm
-
-
6,000-9,000 mPa.s
791903-10G
791903-20G
Conductive silver printing ink, resistivity 5-6 μΩ/cm
-
-
13,000-17,000 mPa.s
791873-10G
791873-20G
Conductive silver printing ink, resistivity 9-10 μΩ/cm
-
-
9,000-12,000 mPa.s
791881-10G
791881-20G
Reactive silver ink
-
-
10-12 cP 745707-25ML
Silver nanoparticle ink
Other Nanoparticles
Name
Particle Size (nm)
Concentration
Viscosity
Prod. No.
Molybdenum oxide nanoparticle ink
8-16
-
1-3 cP
900151-10ML
Platinum paste, screen printable
-
-
2,500-4,500 mPa.s
791512-20G
Tungsten oxide nanoparticle ink
11-21
2.5 wt. %
8 cP
807753-5ML
Tungsten oxide (WO3-x) nanoparticle ink
<50 (BET)
2.5 wt. % in 2-propanol
-
793353-5ML
793353-25ML
70
TO ORDER: Contact your local Sigma-Aldrich office or visit aldrich.com/matsci.
Nanoparticle-based Zinc Oxide Electron Transport Layers for Printed Organic Photodetectors
Organic Conductive Inks
For a complete list of available materials, visit aldrich.com/conductiveink.
Name
Viscosity at 25 °C (cP)
Resistivity (Ω/cm)
Work Function (eV)
Prod. No.
Plexcore® OC AQ-1100 Organic Conductive Ink
4-7
500-5,000 (Film)
-5.6 ± 0.1 eV (Film)
805068-20ML
Plexcore® OC AQ-1200 Organic Conductive Ink
3.3-5.3
1,000-5,000 (Film)
-5.3 ± 0.1 eV (AC2, Film)
805785-20ML
Plexcore® OC AQ-1250 Organic Conductive Ink
3.3-5.5
400-5,000 (Film)
-5.3 ± 0.1 eV (AC2, Film)
805793-20ML
Poly(thiophene-3-[2-(2-methoxyethoxy)ethoxy]-2,5-diyl), sulfonated
4.0-10.0 (Brookfield)
500-3,000
-5.1 to -5.2
699780-25ML
Poly(thiophene-3-[2-(2-methoxyethoxy)ethoxy]-2,5-diyl), sulfonated
7-13 (Brookfield)
25-250
-5.1 to -5.2
699799-25ML
Ink Kits
For a complete list of available materials, visit aldrich.com/conductiveink.
Name
Application
Prod. No.
Organic photovoltaic ink system
Ready-to-use organic ink system for bulk heterojunction solar cells and spin coating.
711349-1KT
Organic photovoltaic ink system, PV 2000 kit
Ready-to-use organic ink system for bulk heterojunction solar cells and spin coating.
772364-1KT
Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)
For a complete list of available materials, visit aldrich.com/pedot.
Description
Sheet Resistance (Ω/sq)
Viscosity
pH
0.8% in H2O, conductive inkjet ink
-
7-12 cP at 22 °C
1.5-2.5
Prod. No.
739316-25G
1.1% in H2O, neutral pH, high-conductivity grade
resistance <100 (>70% visible light transmission,
40 μm wet)
<100 cP at 22 °C
5-7
739324-100G
1.1% in H2O, surfactant-free, high-conductivity grade
resistance <100 (<80% visible light transmission,
40 μm wet)
<100 cP at 22 °C
<2.5
739332-100G
1.0 wt. % in H2O, high-conductivity grade
resistance 50-120
7-12 mPa.s at 22 °C (typical)
1.8-2.2
768642-25G
5.0 wt. %, conductive screen printable ink
resistance 50-150
30,000-90,000 mPa.s at 22 °C
1.5-2.0
768650-25G
dry re-dispersible pellets, high conductivity
sheet resistance <200 (by addition of 5% diethylene
glycol)
-
-
900208-1G
high-conductivity grade
sheet resistance <200 (coating: 40 μm wet,
drying: 6 min 130 °C)
<30 mPa.s at 20 °C
-
900181-100G
3.0-4.0% in H2O, high-conductivity grade
resistance 1,500 (4 point probe measurement of dried
coating based on initial 6 μm wet thickness.)
10-30 cP at 20 °C
1.5-2.5 at 25 °C
(dried coatings)
655201-5G
655201-25G
resistance 500 (4 point probe measurement of dried
coating based on initial 18 μm wet thickness.)
2.8 wt % dispersion in H2O, low-conductivity grade
-
<20 cP at 20 °C
1.2-1.8
560596-25G
560596-100G
dry re-dispersible pellets, dry re-dispersible pellets
resistance 200-450
-
-
768618-1G
768618-5G
Polythiophene (PT)
For a complete list of available materials, visit aldrich.com/polythio.
Name
Structure
Poly(3-butylthiophene-2,5-diyl), P3BT
CH3
S
Poly(3-hexylthiophene-2,5-diyl), P3HT
Regioregularity
Molecular Weight
Prod. No.
regioregular
Mw 54,000 (typical)
495336-1G
regiorandom
-
511420-1G
regioregular
average Mn 54,000‑75,000
698997-250MG
698997-1G
698997-5G
regioregular
average Mn 15,000‑45,000
698989-250MG
698989-1G
698989-5G
regioregular
-
445703-1G
regiorandom
-
510823-1G
n
CH 2 (CH 2 ) 4 CH 3
S
n
For questions, product data, or new product suggestions, contact us at [email protected]
71
aldrich.com/matsci
Fullerenes
For a complete list of available materials, visit aldrich.com/fullerene.
Structure
O
60
OCH3
Name
Purity (%)
Prod. No.
[6,6]-Phenyl C61 butyric acid methyl ester
>99.9
684457-100MG
[6,6]-Phenyl C61 butyric acid methyl ester
>99.5
684449-100MG
684449-500MG
[6,6]-Phenyl C61 butyric acid methyl ester
>99
684430-1G
[6.6] Diphenyl C62 bis(butyric acid methyl ester) (mixture of isomers)
99.5
704326-100MG
[6,6]-Phenyl C71 butyric acid methyl ester, mixture of isomers
99
684465-100MG
684465-500MG
ICMA
97
753947-250MG
ICBA
99
753955-250MG
O
60
OCH3
O
H3CO
70
O
OCH3
Indium Tin Oxide (ITO) Coated Substrates
For a complete list of available materials, visit aldrich.com/ito.
Description
Dimension (L × W × thickness)
Surface Resistivity (Ω/sq)
Prod. No.
Indium tin oxide coated PET
1 ft × 1 ft × 5 mil
60
639303-1EA
639303-5EA
1 ft × 1 ft × 5 mil
100
639281-1EA
639281-5EA
1 ft × 1 ft × 5 mil
200
749745-1EA
749745-5EA
1 ft × 1 ft × 5 mil
250
749761-1EA
749761-5EA
1 ft × 1 ft × 5 mil
300
749796-1EA
749796-5EA
1 ft × 1 ft × 7 mil
60
749729-1EA
749729-5EA
1 ft × 1 ft × 7 mil
100
749737-1EA
749737-5EA
1 ft × 1 ft × 7 mil
200
749753-1EA
749753-5EA
1 ft × 1 ft × 7 mil
250
749788-1EA
749788-5EA
1 ft × 1 ft × 7 mil
300
749818-1EA
749818-5EA
25 × 25 × 1.1 mm
8-12
703192-10PAK
25 × 25 × 1.1 mm
30-60
703184-10PAK
25 × 25 × 1.1 mm
70-100
703176-10PAK
Indium tin oxide coated boro-aluminosilicate glass slide
75 × 25 × 1.1 mm
5-15
576360-10PAK
576360-25PAK
Indium tin oxide coated glass slide, rectangular
75 × 25 × 1.1 mm
8-12
578274-10PAK
578274-25PAK
75 × 25 × 1.1 mm
15-25
636916-10PAK
636916-25PAK
75 × 25 × 1.1 mm
30-60
636908-10PAK
636908-25PAK
75 × 25 × 1.1 mm
70-100
576352-10PAK
576352-25PAK
Indium tin oxide coated glass slide, square
72
TO ORDER: Contact your local Sigma-Aldrich office or visit aldrich.com/matsci.
MATERIALS
TO DRIVE INNOVATION
Across a Variety of Applications
Energy
Electrode and electrolyte materials for batteries, fuel cells; hydrogen storage
materials including MOFs; phosphors; thermoelectrics; nanomaterials; precursors
for nanomaterials and nanocomposites
Biomedical
Materials for drug delivery, bone and tissue engineering; PEGs, biodegradable and
natural polymers; functionalized nanoparticles; block copolymers and dendrimers;
and nanoclays
Electronics
Nanowires; printed electronics inks and pastes; materials for OPV, OFET, OLED;
nanodispersions; CNTs and graphene; precursors for PVD, CVD, and sputtering
Find more information on our capabilities at
aldrich.com/matsci
Inkjet printed silver image provided by Xerox®. We are the proud
distributor of Xerox silver nanoparticle ink.
MATERIALS RESEARCH SOCIETY
MID-CAREER
RESEARCHER AWARD
Dr. Hongjie Dai Receives the 2016 Materials Research Society Mid‑Career Researcher Award
The Materials Research Society (MRS) awarded Hongjie Dai, professor of chemistry at Stanford University, the MidCareer Researcher Award “for seminal contributions to carbon-based nanoscience and applications in nanoelectronics,
renewable energy, and biological systems.” The MRS Mid-Career Researcher Award, endowed by Aldrich® Materials
Science, recognizes exceptional achievements in materials research made by mid‑career professionals.
About Dr. Dai
Dr. Dai pioneered the controlled growth of carbon nanotubes using metal-catalyzed chemical vapor deposition,
showing for the first time that high-quality single-walled nanotubes could be synthesized using a method that
enables control over the growth process. He used his knowledge of nanotube growth to demonstrate hierarchical
organization over multiple length scales. Dai also exploited this unique control over nanotube growth to uncover
basic electronic properties of metallic and semiconducting nanotubes.
Dr. Hongjie Dai
Professor Dai and his group have defined the fundamental limits of nanotube transistors. Pioneering the use of
nanotubes as intracellular molecular transporters for biological molecules and cancer drugs, demonstrating that key spectroscopic properties unique to
nanotubes and other carbon nanostructures make them ideal for biological detection, fluorescence imaging in the second near-infrared window, drug
delivery, and cancer therapy via in vivo photothermal tumor destruction.
Hongjie Dai is the J. G. Jackson and C. J. Wood Professor of Chemistry at Stanford University. He earned his Ph.D. in applied physics/physical chemistry
from Harvard University. He is the Honorary Chair Professor of the National Taiwan University of Science and Technology, a Fellow of the American
Association for the Advancement of Sciences and the American Academy of Arts and Sciences, and serves on the editorial boards of eight publications.
Dr. Dai has written more than 250 papers and is ranked as one of the most cited chemists (in materials chemistry) by Thomson Reuters.
For more information, visit
aldrich.com/mrsaward
SAL
84314-517860
07-2016
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