Omnidirectionally Stretchable Electronics Substrate Film Having Negative Poisson’s Ratio Using Auxetic-Structured Glass-Fabric Reinforcement

Nowadays, as the application of electronic devices is broadening into portable, deformable, and wearable forms, non-rigid material-based electronics with free form factors are required. To realize the wearable and stretchable devices, stretchable substrate is an essential component. Elastomers are widely used as the substrate of stretchable electronics due to their high stretchability and resilience. However, the elastomeric substrates suffer from severe distortion during stretching because of their high Poisson’s ratio (PR), which is a demerit for its usage such as a display substrate. In addition, there has been limitation to wearable devices because of the different behavior with human skin that expands along multiple direction with 1-degree of freedom bending of joint. To overcome these, meta-structures having negative PR (NPR), namely the auxetics, has been propsed. Though, general auxetic materials exhibit NPR characteristic by transversal expansion of empty spaces upon elongation, which hinders its usage as a substrate because of the structural discontinuity and a limited practical area.<br/>Here, we propose an omnidirectionally stretchable substrate film without any discontinuity and empty spaces on the surface using an auxetic-structured selective reinforcement with glass-fabric into elastomeric matrix. The film is composed of two domains: i) auxetic-structured glass-fabric reinforced domain that induces NPR characteristic and ii) pristine elastomer domain that induces resilience and surface continuity. To achieve an ideal NPR characteristic (i.e. theoretical maximum value of unfilled auxetic structure) while maintaining a continuous surface, the two domains should have a substantially different elastic modulus. To investigate the effect of modulus difference between two domains and geometrical parameters of auxetic structure such as thickness, shape, and dimension, an experimental study is performed using digital image correlation (DIC) analysis. By modifying the mechanical properties of component materials and auxetic structure, a controllable PR between -1 to 0.5 is achieved. The optimized glass-fabric reinforced substrate exhibits NPR characteristics with maximum NPR value of -0.95, stretchability of 20% with 10,000 cycles of repetition. The proposed substrate also exhibits high thermal stability (&gt;350°C), dimensional stability (&lt;50 ppm/°C), and low surface roughness (R_a &lt; 100nm), showing its capability to high temperature and solution processes. We demonstrate the omnidirectionally stretchable substrate as a wearable biosensor platform such as ECG sensor, providing stable conformal contact to human joint. Having NPR, the substrate expands omnidirectionally under uniaxial extension like a human skin. We also fabricate a distortion-free stretchable light-emitting devices using the omnidirectionally stretchable glass-fabric reinforced substrate.