Temperature Problems Solved for You (VI) Design Challenges of Wearable Temperature Sensing

In previous articles, we have covered the basics of temperature sensors. Knowing a patient’s temperature is a critical first step in any clinical diagnosis and is an important concern for athletes. In addition to the need for ultra-high accuracy, the industry is moving toward compact wearable devices to provide continuous temperature monitoring. Temperature sensors with up to 0.1°C accuracy not only meet the American Society for Testing and Materials (ASTM) E1112 requirements for medical thermometers, but are also optimized to keep battery-powered wearable devices compact and comfortable.

Introduction

Monitoring patient vital signs in a clinical setting is typically a task performed by expensive systems that require rigorous calibration and require the patient to be tethered to a clinical monitor. Wireless patient monitoring systems can provide patient comfort and clinical convenience while still meeting strict medical standards.

When designing a wearable temperature monitor, there are many trade-offs to be made between power consumption, size, system performance (both in terms of radio frequency [RF] and accuracy), and patient comfort. For example, thinner, more flexible batteries offer greater comfort but may require more careful power management.

Smaller, lower-cost designs require trade-offs in thermal isolation and RF performance. Solutions for long-term monitoring must make the best use of board area to improve accuracy and signal integrity while minimizing current consumption. System designers must balance these requirements with patient comfort and experience.

Thermometer Compliance

The American Society for Testing and Materials (ASTM) E1112 and the International Organization for Standardization (ISO) 80601-2-56:2017 are the regulatory standards for intermittent electronic patient thermometers. For clinical temperature measurement applications to comply with ASTM E1112, a temperature monitor must be able to produce readings with an accuracy of ±0.1°C and must also read and display temperatures as low as 35.8°C to 41.0°C. At a minimum, any temperature monitoring design should include a sensing element that is able to meet these requirements after calibration.

TI recommends using the TMP117 ultra-high-precision digital temperature sensor for wearable temperature monitors. The device itself has an accuracy of better than 0.1°C over the 25°C to 50°C temperature range and meets the requirements of ASTM E1112 and ISO 80601-2-56:2017 without calibration. In addition, the low total current consumption and single-shot mode of the TMP117 are ideal for battery-powered applications. The digital I2C output of the TMP117 also greatly simplifies system design compared to solutions based on resistance temperature detectors (RTDs) or thermistors.

Layout Considerations

Even with the right sensing element, ensuring overall system accuracy still requires careful board layout. To monitor skin temperature, the optimal layout should meet the following conditions:

•Maximize thermal isolation between the sensing element and other devices.

•Minimize the thermal mass around the temperature sensing element to speed response.

• Provide good thermal contact between the patient and the sensing element, minimizing temperature gradients between the sensor and the target.

Optimizing insulation and thermal mass

The following figure shows an example of a skin temperature monitoring system. The TMP117 digital temperature sensor extends from the rest of the printed circuit board (PCB) using a narrow arm, minimizing heat conduction to the rest of the board.

The TMP117 (U1) is located on the flexible PCB. The extension arm is used to isolate the integrated circuit from the heat generated by other devices.

The following figure shows the stackup of the same two-layer flex PCB. Using a flex board helps reduce the overall thermal mass, thereby improving the thermal response time of the patient monitor. Omitting the copper fill between the top and bottom of the board prevents heat from being drawn away from the TMP117 and increases the thermal mass.

Example of a flexible stackup, minimizing thickness to reduce thermal mass.

Thermal Contact

Reliable measurement of patient skin temperature requires good thermal contact between the monitored patient and the sensor device. This thermal contact, combined with thermal isolation from the rest of the board, ensures that the reported temperature is as close to the patient's actual skin temperature as possible. With the TMP117, a solid copper pour and contact vias provide a thermally conductive path underneath the board, as shown in the following figure. The pads are in direct contact with the wearer's skin and ensure that the primary heat source for the device comes from the person being monitored.

Copper pour under the TMP117 (left); layout on top of the TMP117 (U1, right). The vias and copper pour under the TMP117 provide a thermal path between the patient's skin and the device.

Self-heating

Regardless of the sensing element and layout chosen, the stringent accuracy requirements of medical thermometers require an understanding of the effects of device self-heating. Due to resistive losses in the chosen sensing element, some degree of self-heating will always occur. The TMP117 can be configured for a one-shot mode transition and remain in shutdown mode between consecutive readings, minimizing self-heating. The one-shot feature of the TMP117 can trigger individual temperature readings using a configurable number of averaged readings. Human body temperature typically does not change for more than a few seconds, so taking these readings at intervals of 10 to 60 seconds is sufficient to monitor patient temperature for extended periods of time. This approach also has the added benefit of extending the system's effective battery life.

System power

Power requirements will vary depending on the overall system design, but most wireless patient monitors need to have enough energy storage to ensure a shelf life of several years and a minimum of 48 to 72 hours of active life. Coin cell batteries easily meet these energy requirements, but they are completely rigid and can be uncomfortable for patients. In a disposable patch, a coin cell-based solution can be very wasteful.

Another energy storage option is to use thin-film flexible batteries. Since these batteries have very small storage capacity, the total system power consumption must be minimal if they are to be used. If only intermittent temperature monitoring is required, a system powered by flexible batteries can easily meet the battery requirements of the specific application, ensuring a shelf life of several years and an active use time of 48 to 72 hours.

Making system trade-offs

While following layout recommendations is critical to meeting ASTM E1112 and ISO 80601-2-56 standards, there are other system design considerations. For patient comfort, it is best to keep non-temperature monitoring devices and RF areas as small as possible. Keeping the board’s fillet tight helps reduce areas on the monitor that can cause stiffness to the patient.

For RF communications, any wireless protocol that works on a flexible PCB is viable. Since most wearable patient monitors want to keep power consumption low, TI recommends using a low-power Bluetooth® wireless communication link. If the only information being sent from the monitor is the temperature, the monitor can be configured to broadcast the temperature reading along with its pairing ID. Sending information in this manner eliminates the need to establish and maintain an actual connection, reducing system power consumption even further.

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