Wearable sensing systems have important applications in areas ranging from medical devices to fitness tracking, offering real-time monitoring of vital signs and physical activities, enhancing healthcare diagnostics, and promoting proactive wellness management. One of the most fundamental metrics captured by these sensors is temperature. Body heat variations play a crucial role in detecting fever, monitoring inflammatory responses, and contributing to overall health assessments.
Despite the importance of this metric, capturing it with wearable devices is not entirely easy. When it comes to flexible temperature sensing materials, most options exhibit a low level of sensitivity, which means that complex sampling equipment is needed to distinguish between the minute changes in resistance that represent temperature. Positive Temperature Coefficient (PTC) thermistors, on the other hand, are very highly sensitive, but unfortunately they are only capable of detecting a very narrow range of temperatures.
Clearly, neither of these solutions is optimal for a wearable device where size and unobtrusiveness are of paramount importance. Either the sensor will need to be supplemented with bulky and expensive supporting components, or many PTC thermistors will need to be included in the design to cover a wide range of temperatures. Further complicating the matter, these sensors also typically require an analog to digital converter, a wireless transceiver, and more as a part of the readout circuit.
Without a doubt, a simpler temperature sensor could benefit many use cases. Recent findings revealed by a team led by researchers at the University of Glasgow could be the breakthrough that the field has long been awaiting. They have developed a simple, flexible temperature sensor that is sensitive over a wide range of temperatures and that can be read out with minimal hardware. And as an added bonus, the sensor does not require any battery power to capture a measurement.
The sensor is composed of polydimethylsiloxane (PDMS), a soft and flexible polymer matrix composite material. PDMS grows in size under the influence of high temperatures. This in turn increases the separation between the electrically conductive carbon microfibers that are embedded within it in a repeatable manner. These differences alter the material’s ability to absorb and reflect radio-frequency (RF) signals. Accordingly, if the surface of the material is exposed to a source of RF radiation it will be reflected back, but that reflected signal will also be modulated in a way that reveals the temperature of the sensor. This signal can be directly read without an analog to digital converter or other preprocessing components at the sensor.
The sensors are inexpensive to produce and can be manufactured via a simple process that utilizes a set of 3D-printed molds. But this simplicity does not mean that the sensor is not a capable piece of equipment — it has been shown that it can accurately distinguish between temperatures ranging from about 86 to 572 degrees Fahrenheit. Nor does it mean that it will not hold up to real-world use. Experiments revealed that the material can withstand many thousands of cycles of stretching and bending without degrading its sensing accuracy.
Looking ahead, the researchers hope that future work will help them to find suitable commercial applications for their technology.