Wearable Temperature Sensors with Enhanced Sensitivity by Engineering Microcrack Morphology in PEDOT:PSS–PDMS Sensors

Wearable temperature sensors with high sensitivity, linearity, and flexibility are required to meet the increasing demands for unobtrusive monitoring of temperature changes indicative of the onset of infections and diseases. Herein, we present a new method for engineering highly sensitive and flexible temperature sensors made by sandwiching a poly(3,4-ethylenedioxythiophene):polystyrene (PEDOT:PSS) sensing film between two poly(dimethylsiloxane) (PDMS) substrates. Pre-stretching the sensor to a certain strain can create stable microcracks in the sensing layer that bestow high senstivity and linearity. The average length and density of the microcracks, which dictate the sensor's temperature sensitivity, can be engineered by controlling three key processing parameters, incuding (a) pre-stretching strain, (b) sulfuric acid treatment time, and (c) surface roughness of the substrate. For a given acid treatment time and surface roughness condition, the density and average length of the microcracks increase pre-stretching strain. A smooth PDMS substrate tends to yield long and straight cracks in the PEDOT:PSS film, compared to shorter microcracks with higher density on rough surfaces. Crack density can be further increased via sulfuric acid treatment with an optimum duration of approximately 3 h. Prolonged treatment would result in weak adhesion between the PEDOT:PSS film and the PDMS substrate, which in turn reduces the microcrack density but increases the crack length. By optimizing the three design parameters we have designed a high performance PEDOT:PSS–PDMS sensor that provides a combined high temperature sensitivity of 0.042 °C–1 with an excellent linearity of 0.998 (from 30 to 55 °C), better than the highest temperature sensitivity of PEDOT:PSS based sensors reported in the literature. With a good optical transparency, high temperature sensitivity, excellent linearity, and high flexibility, this microcrack-based sensor is a very promising wearable temperature-sensing solution.