A facile method to prepare transparent and stretchable epidermal thin film heaters

A stretchable and semi-transparent epidermal thin film heater (TFH) has been fabricated by using silver nanowire (Ag NW) networks as a Joule heat generator and polydimethylsiloxane (PDMS) as a stretchable and transparent matrix. The device is a conductive layer of interconnected Ag NWs sandwiched with two layers of PDMS. The output power and transparency of the device were rationally tuned by adjusting the Ag NW amount. Our results show that the device transparency at 600 nm decreases from 70% to 46% when we tuned the sheet resistance of Ag NW layer from 30.0 Omega/square to 5.4 Omega/square. The TFH shows a favorable stretchability, and it can almost completely recover when we applied a strain up to 200% or after a fatigue test of 1000 cycles, 50% in strain. The ultimate temperature in air reached up to 81 degrees C when we applied an electric field of 1.0 V/cm. Its epidermal heating performance was simulated by warming a piece of pork, and it can make the subcutaneous temperature increase 12 degrees C. A model to evaluate the heating performance of TFHs with thermal capacity per-unit area, device resistance and energy conversion efficiency was also proposed, which will fatally determine their heating rate and ultimate temperature.