Soft, stretchable thermal protective substrates for wearable electronics

Wearable electronics hold immense promise for medical applications, including healthcare monitoring, wound management, and drug delivery. However, the heat generated by functional components, such as LEDs, can cause skin discomfort or damage due to the poor thermal insulation of commonly used substrates like PDMS and Ecoflex. While strategies such as using insulation fillers (e.g., hollow microspheres, aerogel) or phase change materials (PCMs) have been explored, issues like rigidity and liquid leakage remain. Existing orthotropic and functional soft composite designs also compromise flexibility and stretchability. This research addresses this challenge by designing a soft, stretchable thermal protective substrate (TPS) that effectively insulates while maintaining intimate skin contact. The TPS incorporates heat-absorbing microspheres containing n-eicosane encapsulated in a melamine-formaldehyde resin shell. This encapsulation prevents PCM leakage and inhibits PDMS cross-linking, resulting in a soft, stretchable, and adhesive substrate. The study aims to demonstrate the effectiveness of this design through experimental and numerical analyses, culminating in in vivo testing on mouse skin.

The introduction extensively reviews existing literature on wearable electronics and thermal management strategies. It highlights the limitations of current materials and designs, emphasizing the need for a solution that balances thermal insulation, flexibility, and stretchability. The review covers various approaches, including the use of insulation fillers and phase change materials, and notes the drawbacks of each, paving the way for the proposed innovative solution.

The researchers fabricated the TPS by mixing heat-absorbing microspheres (phase change temperature ~37°C) with liquid PDMS (10:1 base-to-curing agent ratio) at varying mass ratios (0-80%). The mixture was molded, degassed, and cured in a multi-stage oven process. The mechanical properties (elastic modulus, maximum tensile strain) were characterized at various temperatures. Thermal properties (thermal conductivity, specific heat capacity, latent heat) were measured using a high-temperature synchronous thermal analyzer. The thermal insulating performance was evaluated using a wearable heater device on the TPS. Temperature distributions were measured experimentally using an infrared thermal imager and modeled using finite element analysis (FEA) in COMSOL. In vivo testing involved a wearable heater applied to depilated mouse skin, comparing the TPS with a conventional PDMS substrate. Skin temperature, damage area, and histological analysis (H&E staining) were used to assess the thermal protection efficacy.

The TPS exhibited remarkable properties. Its elastic modulus decreased by 98% (from 1.56 to 0.023 MPa) with an 80% mass ratio of heat-absorbing microspheres, demonstrating significant softness. While temperature influenced maximum tensile strain, the TPS remained highly stretchable (over 100% even at 100°C). The density remained nearly constant at 970 g cm⁻³. The thermal conductivity was lower after phase change, and the specific heat capacity showed a clear endothermic peak at the phase transition temperature (~37°C), indicating effective heat absorption. The latent heat increased linearly with the mass ratio of microspheres. In thermal insulation tests, the TPS with an 80% mass ratio reduced the peak skin temperature increase by 82% compared to PDMS. FEA results closely matched experimental measurements. Varying substrate thickness and heating power impacted peak temperatures, while external stretch had negligible effects. In vivo studies confirmed the TPS's superior thermal protection, significantly reducing skin damage compared to PDMS.

The results demonstrate the efficacy of the proposed TPS in providing effective thermal protection for wearable electronics. The combination of phase change material and the cross-linking inhibition technique successfully addresses the limitations of previous approaches. The significant reduction in peak skin temperature, confirmed by both in vitro and in vivo experiments, highlights the potential of this technology to improve the safety and comfort of wearable devices. The close agreement between experimental and FEA results validates the methodology and provides a framework for further optimization. The influence of material properties and geometric parameters, such as substrate thickness and heating power, provides valuable design guidelines for future applications.

This study successfully developed a soft, stretchable, and highly thermally insulative substrate for wearable electronics. The TPS, incorporating heat-absorbing microspheres with phase change materials, significantly reduces peak skin temperature increases and minimizes skin damage. Future work could focus on improving the substrate's aesthetics (e.g., addressing its milky white appearance) and exploring its compatibility with various wearable device designs and applications.

The milky white appearance of the current TPS due to the heat-absorbing microspheres may limit its applications where transparency is required. The study focused on a specific type of phase change material and heating element; further investigation into other PCMs and heating sources may be beneficial. The in vivo study was conducted on depilated mice; further research is needed to confirm the findings on human skin.