Electrical and Mechanical Properties of Intrinsically Flexible and Stretchable PEDOT Polymers for Thermotherapy

For wearable applications such as electronic skin and biosensors, stretchable conductors are required (similar to 30% strain to follow the skin extension). Owing to its high conductivity, good flexibility, low cost, and ease of processing, poly(3,4-ethyl-enedioxythiophene) (PEDOT) appears as a promising candidate. However, destructive cracks come out above 10% strain in the case of PEDOT:PSS, the most common form of PEDOT. Different strategies have already been investigated to solve this problem, including the design of specific structures or the addition of plasticizers. This article presents a different approach to obtain highly conductive and stretchable PEDOT materials based on doping with small counteranions. We indeed demonstrate the intrinsic stretchability (up to 30% strain) of thin films (35 nm) of PEDOT-based materials with small counterions. Both thin-PEDOT:OTf (triflate counter-ion) and thin-PEDOT:Sulf (sulfate counter-ion) films remain structurally resilient up to 25-30% strain, and their electrical conductivity remains remarkably stable over more than 100 cycles. Under limited strain (<30%), polarized UV-vis-NIR measurements (parallel and perpendicular to the stretching direction) show that the conductivity of the material is improved by chain alignment in the stretching direction. As a proof of concept, a thermotherapy patch is presented. It shows a fine temperature control (stability around 40 degrees C at 9 V bias) and a uniform heating across the surface.