Highly Accurate Wearable Temperature Sensors for Skin Temperature Monitoring

Body temperature is a primary health marker of metabolic health, circadian rhythm, physiological activities, infection, and disease. Accurate and real-time monitoring of skin temperature changes offers a new way for disease diagnosis, infection monitoring, fitness tracking, and athlete performance. Wearable temperature sensors for healthcare and fitness monitoring are required to meet the high resolution (±0.1-0.2 °C) of medical-grade thermometers. However, achieving this goal has proven challenging because wearable sensors have been found to respond strongly to mechanical deformation, primarily associated with skin-stretching and touch pressure. This thesis presents the progresses that I have made towards high-accuracy wearable temperature sensors by minimizing cross-interferences of wearable temperature sensors. To achieve this aim, two complementary strategies have been developed: (1) increasing the temperature sensitivity and (2) reducing the deformation-induced cross-interferences. These strategies have been applied to two different types of sensors: resistive and capacitive. In the first strategy, several approaches have been used to enhance the sensors’ sensitivity depending on their sensing mechanisms, as briefly summarized below • New methods for controlling the density and pattern of microcracks have been developed to improve the temperature sensitivity of a resistive sensor, which consists of a Poly(3,4-ethylenedioxythiophene) Polystyrene Sulfonate (PEDOT:PSS) sensing layer deposited on a thin Poly(dimethylsiloxane) (PDMS) substrate. The temperature sensitivity is found to be strongly influenced by the crack morphology in the PEDOT:PSS layer, which can be controlled by three fabrication parameters: pre-stretching strain, substrate roughness, and acid treatment time. The maximum temperature sensitivity achieved is 0.042 °C-1 when crack length is 185.2 μm and crack density is 22.84 mm-1. This is a 42-times improvement as compared to the reference sensor without microcracks. • New methods have been developed to improve the sensitivity of capacitive temperature sensors by rational design of dielectric separates. Since the sensitivity of capacitive temperature sensors depends on the coefficient of thermal expansion and the temperature coefficient of permittivity, it is found that by selecting the low-melting-temperature (65°C) thermoplastic polyurethane (TPU) with significant thermo-responsive dielectric properties around skin temperature (30 – 45 °C) as the dielectric layer, the temperature sensitivity can achieve 0.007 °C-1. Replacing the TPU dielectric layer with Polyvinyl Alcohol (PVA)-based organogel, the temperature sensitivity can be further improved to 0.093 °C-1. In the second strategy, the following new approaches to suppress strain and pressure interferences have been investigated. • Patterning the electrodes of capacitive sensors into kirigami cut to suppress the cross-interference of stretching. The electrodes of capacitive sensors consist of silver nanowires (AgNWs). The kirigami pattern is demonstrated to effectively reduce strain interference. Computational modeling of kirigami geometry’s effects on strain and temperature sensitivities of capacitive sensors reveals that tailoring kirigami design can reduce the strain cross-sensitivity by a factor of 3125, which enables the capacitive temperature sensors to achieve the resolution of 0.14 °C. • A pressure-insensitive supercapacitive sensor has been develop by synthesising PVA organogels consisting of Na+ and Cl- ions to be used as redox-active separator. The resulting temperature sensors show very low sensitivity to surface pressure. The results reveal that the pressure interference is reduced to 0.068% (with the external pressure of 10 kPa) and the wearable temperature sensor can achieve an accuracy of 0.2 °C, matching typical medical grade thermometers. In summary, this thesis presents several major advances in creating high-accuracy wearable temperature sensors that can match the detection limits of of medical-grade thermometers. The high-precision wearable sensors have been demonstrated to offer reliable monitoring of daily skin temperature rhythm when subjected to stretch exerted by body movements and the pressure from medical compression garment. The results show that sensors can perform skin temperature monitoring with a minor error of 0.1°C compared to infrared devices. The new insights and understandings of the strategies to suppress cross-sensitivities of wearable temperature sensors to in-plane stretch and out-of-plane pressure will generate cross-cutting benefits to the design of wearable health sensors