“Autonomous alignment and healing in multilayer soft electronics using immiscible dynamic polymers”

Speaker: Sam Root, Postdoctoral Scholar, Chemical Engineering, Stanford University

Abstract: Self-healing soft electronic and robotic devices, like human skin, can recover autonomously from some forms of damage. Existing multi-layer devices generally employ a single type of dynamic polymer embedded with different functional nano–micro materials in each layer, to provide a cohesive interface between layers. In such devices, successful healing from damage requires precise manual alignment and re-contacting of the fractured interfaces, limiting functional recovery from diverse forms of damage, such as processes leading to imperfect registry of device layers. These limitations are especially prevalent for devices containing thin layers (< 100 µm). To overcome these limitations, we have designed a pair of dynamic polymers, which have immiscible polymer backbones (poly(dimethylsiloxane) and poly(propylene glycol)), but identical dynamic bonding units (a mixture of strong and weak bisurea-based hydrogen bonding interactions). This strategy allows for the maintenance of interlayer adhesion, while providing selectively self-healing layers with similar viscoelastic behavior (and thus self-healing dynamics) over a convenient range of temperatures (25–100°C). Upon lamination, these dynamic polymers exhibit a weakly interpenetrating and adhesive interface, whose width and toughness are reversibly tunable with processing temperature. When multilayered polymer films are misaligned after damage, these structures autonomously realign during the healing process. We propose a mechanism in which the polymer chains diffuse along gradients of chemical potential to minimize overall interfacial free energy of the layered system. Our experimental observations are captured by both coarse-grained molecular dynamics simulations and continuum phase field simulations, providing evidence of the proposed mechanism, and suggesting generality of the observed phenomenon to other pairs of polymer backbones or reversible bonding chemistries. As a demonstration of the potential utility of our approach, we fabricated several demonstrative devices with conductive, dielectric, and magnetic particles that functionally heal after damage, enabling thin film pressure sensors, magnetically assembled soft robots, and underwater circuit assembly. Moving forward, we expect that the improved capacity for functional recovery of such multilayered devices will open the door for entirely new applications such as damage-sensing electronic skins.

“What makes wearable devices and flexible electronics safe and comfortable”

Speaker: Bryan Cho, M.D., Ph.D., CEO & Founder, BOHLD Consulting LLC; Board-certified Dermatologist

Abstract: Wearable devices can incorporate cutting-edge technology and be designed brilliantly but if they cannot be worn safely and comfortably for their intended purpose they are unlikely to be widely embraced. This talk will describe what it means to use biocompatible materials and why this can erroneously make one assume the device can be worn on the skin safely. Allergens that have been identified with wearable devices will be reviewed to help identify potential future roadblocks. Even when all biocompatibility criteria are met, other human factors and material properties can impact device tolerability. Parameters to optimize skin tolerance and comfort will be presented.