Texas Instruments | Design Challenges of Wearable Temperature Sensing | Application notes | Texas Instruments Design Challenges of Wearable Temperature Sensing Application notes

Texas Instruments Design Challenges of Wearable Temperature Sensing Application notes
Design Challenges of Wireless Patient Temperature
Monitors
Introduction
Monitoring patient vitals in a clinical environment has
traditionally been a job for expensive and heavily
calibrated systems requiring patients to be tethered to
bedside monitors. Wireless patient monitoring systems
provide for both patient comfort and clinical
convenience, provided they can still be made to
operate within strict medical standards.
In the case of wearable temperature monitors, there
are many design tradeoffs that must be considered for
power consumption, size, system performance (in
terms of both RF and accuracy), and patient comfort.
Thinner flexible batteries provide greater comfort but
may require more careful power management.
Smaller, lower cost, designs suffer in terms of thermal
isolation and RF performance. Optimal solutions for
long term monitoring must make good use of board
area to improve accuracy and signal integrity, while
trying to keep current consumption as low as possible.
System designers will have to balance these
requirements alongside the comfort and experience of
the patient.
Standard Compliance for Thermometers
The governing standards for intermittent electrical
patient thermometers are given in ASTM E1112 and
ISO-80601-2-56. For standard compliant clinical
temperature measurement applications under ASTM
E1112, human body temperature monitors must
produce readings within ± 0.1 °C accuracy, and must
read and display temperature from a minimum of
35.8°C to 41.0°C. At a bare minimum, any temperature
monitoring design should include a sensing element
able to meet these requirements after calibration.
TI recommends using the TMP117 ultra-high-accuracy
digital temperature sensor for this purpose. The device
itself has better than 0.1°C accuracy from 25 to 50 °C
and requires no calibration to exceed the requirements
of both ASTM E1112, and ISO-80601. Additionally, the
TMP117’s low overall current draw and one-shot mode
are ideal for battery operated applications. The digital
I2C output of this device also greatly simplifies system
design when compared with RTD or thermistor based
solutions.
Layout Considerations
Even with an appropriate sensing element, ensuring
total system accuracy will still require care in terms of
layout. For monitoring skin temperature an ideal layout
will do all of the following:
1. Maximize thermal isolation between the sensing
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element and the other devices.
2. Minimize thermal mass surrounding the
temperature sensing element for faster response.
3. Provide good thermal contact between the patient
and the sensing element to minimize the
temperature gradient between the sensor and the
target.
Optimizing Thermal Isolation & Mass
For optimal thermal mass and isolation, the
recommendations in Layout Considerations for
Wearable Temperature Sensing should be followed.
Figure 1 shows an example of this for a skin
temperature monitoring system. The TMP117
measuring temperature is extended from the rest of
the PCB using a narrow arm to minimize thermal
conductance from the rest of the board. Figure 2
shows the stack up for the same 2-layer flex PCB.
Using a flex-board also helps to reduce total thermal
mass, which improves thermal response time of the
patient monitor. Copper fills between the top and
bottom side of the board should be ommitted to avoid
drawing heat away from the TMP117, and increasing
the thermal mass.
Figure 1. TMP117 (U1) on Flex PCB, using an
extended arm isolates the IC from being influenced
by heat from other devices.
Thermal Contact
Reliable measurement of the patient's skin
temperature requires good thermal contact between
the patient to be monitored and the sensing device.
This works in conjunction with the thermal isolation
from the rest of the board to ensure that the
temperature being reported is as close as possible to
the actual skin temperature of the patient. With the
TMP117, a solid copper pour and contact vias can be
used to provide a thermal path from the underside of
the board as shown in Figure 3. The pad contacts the
wearer's skin directly, and ensures the primary heat
source for the device is from the person to be
monitored.
Design Challenges of Wireless Patient Temperature Monitors
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1
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System Power
Power requirements will vary based on overall system
design, but most wireless patient monitors will need to
have enough energy storage for several years of shelf
life, and at least 48-72 hours of active life. Coin-cell
batteries can easily exceed these requirements for
energy capacity, but they are entirely rigid and may be
uncomfortable to device wearers. In the case of
patches which are not intended to be reused, a coincell based solution can also be extremely wasteful.
Figure 2. Example flex layer Stack, thickness
should be minimized to reduce thermal mass.
An alternative option for energy storage is to use thinfilm, flexible batteries. Due to small storage capacities,
using these batteries will require that total system
power consumption be minimal. For only intermittent
temperature monitoring, systems powered with flexible
batteries can exceed the requirements for multiple
years of shelf-life and 48-72 hours active time. The
design trade off between current consumption and
additional features must be made by the system
designer.
Making System Tradeoffs
If the system is required to meet the requirements of
ASTM E1112 and ISO ISO-80601-2-56 following the
recommendations on layout is essential, but there are
other system design considerations to be made. For
patient comfort, non-temperature-sensing devices and
the RF region should be kept in as small an area as
possible. Keeping the populated region of the board
compact will reduce the portion of the monitor which
feels rigid to the wearer.
Figure 3. Copper pour underneath TMP117 (Left),
topside layout for TMP117 (U1, Right). The vias
underneath the TMP117 and the copper pour
provide a thermal path between the patient's skin
and the device.
Self-Heating
Regardless of the choice of sensing element and
proper layout, the stringent accuracy requirements for
medical thermometers will require that device selfheating be taken into consideration. Some self-heating
will always be present from the resistive losses of the
chosen sensing element. The TMP117, may be
configured for one-shot mode conversion and be kept
in shutdown mode between successive reads, to
minimize self-heating. Individual temperature readings
(using a configurable number of averaged readings)
can be triggered using the one-shot feature of the
device. Human body temperature will not
conventionally exhibit change on the order of seconds,
so taking these readings at 10 to 60 second intervals
is sufficient to monitor patient temperature over long
periods. This method has the added benefit of
extending the active-battery life of the system.
2
For RF communication, any wireless protocol that can
be made to work on a flex PCB is acceptable. Since
most wearable patient monitors will want to keep
power consumption low, a BLE wireless
communication link is recommended. If the information
being transmitted from the monitor is only temperature,
the monitor can be configured to broadcast the
temperature reading alongside it's pairing ID. Sending
the information in this manner removes the
requirement for an actual connection to be made and
maintained, and will reduce system power
consumption even further.
To get more information on these topics, or for general
tips when measuring temperature, please see the
additional resources linked to in Table 1.
Table 1. Related Materials
Document Type
Description
Application Report
Wearable Temp-Sensing Layout
Consideration Optimized for Thermal
Response
Tech Note
Layout Considerations for Wearable
Temperature Sensing
Tech Note
Precise Temperature Measurements with
TMP116
Design Challenges of Wireless Patient Temperature Monitors
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