Enhancing Comfort and Functionality in Stretchable Thermotherapy Thin Film Heaters through Breathable Design

Abstract

Stretchable thin film heaters (TFHs) are essential for localized thermotherapy, conforming to the skin and joints. However, conventional TFHs made from nonbreathable elastomers often cause discomfort and increase infection risks. We propose a sample TFH design that is both breathable and stretchable. By using nickel (Ni) foam as a sacrificial template, we deposit Ag nanowires (Ag NWs) to form the heating element, which is then encapsulated in polydimethylsiloxane (PDMS). Crucially, the PDMS coats only the inner surfaces of the micropipes, leaving the interstitial spaces unfilled, creating a breathable 3D conductive network. This contrasts with traditional TFHs that are typically nonbreathable and limited in flexibility, often leading to heat accumulation and discomfort. Our TFH maintained consistent performance over 1000 cycles of bending, stretching, and water immersion. Even with up to 25% stretching, resistance changes remained under 13%. Breathability tests revealed a 5:1 ratio in deionized water permeability between an uncovered bottle and one covered with our film with a permeation rate of 7 mg/cm²·h. Also, the TFH effectively reached 67 °C within 1 min under a 3.5 V bias. Unlike existing methods that neglect breathability or require complex fabrication, our strategy offers a simple yet robust solution to the limitations of conventional TFHs, combining both breathability and stretchability.