Flex-to-Stretch Hybrid Electronics—Bonding-Free Robust Interface for Wearable Wireless Physiological Monitoring

Hybrid electronics require a robust mechanical interface between externally fabricated stretchable sensors and flexible printed circuit boards (FPCBs) to obtain stable electrophysiological information. The most advanced technique for the integration of FPCBs with stretchable sensors is anisotropic conductive film bonding. Fabricating high-performance sensors requires microfabrication techniques, such as photolithography and etching, which are cumbersome and expensive. Therefore, a sensor fabrication process that supports FPCB manufacturing with lower complexity and cost is required. Herein, we propose a bonding-free approach for fabricating FPCBs and stretchable sensors on a single substrate. This approach utilizes in- and out-of-plane mechanical gradients to obtain a robust and durable mechanical interface for a smooth transition of the internal mechanical stress compliance, as confirmed by experiments and simulations. The gradient mesh patterns, without ACF bonding, can withstand tensile strains of over 30% before experiencing electrical breakdown. Additionally, Kirigami-inspired mesh patterns can extend stretchability by over 100%. The electrical performance of temperature sensors (linear response to temperature changes) and electrocardiography (ECG) sensors (clear visibility of PQRST peaks) remains stable under various physical activities. User-accessible, facile laser ablation and cutting techniques compatible with the FPCB manufacturing process were employed to fabricate stretchable sensors. This approach enables the development of FPCB-compatible on-skin stretchable sensors with robust mechanical properties.