Robust Full‐Surface Bonding of Substrate and Electrode for Ultra‐Flexible Sensor Integration

Abstract The integration of multiple flexible electronics is crucial for the development of ultra‐flexible wearable and implantable devices. To fabricate an integrated system, robust and flexible bonding throughout the connection area, irrespective of the electrode or substrate, is needed. Conventional methods for flexible direct bonding have primarily been confined to metal electrodes or substrate‐only bonding due to varying material properties. Consequently, the mechanical and electrical properties of the connections deteriorate based on their shape and size. This study introduces a bonding technique for wearable electronics, achieving strong, flexible connections between materials like gold and parylene at a low temperature (85 °C). This hybrid direct bonding method ensures strong bonding across both the Au electrode and parylene substrate within electronic interconnections. Additionally, a 3D‐stacked flexible structure that maintains robustness and high flexibility without an adhesive layer is successfully developed. An ultrathin photoplethysmography sensor developed by stacking an ultrathin organic photodetector atop an organic light‐emitting diode is demonstrated. Unlike traditional methods requiring adhesives or high pressure, this approach maintains flexibility essential for deformation, withstanding bending at a radius of 0.5 mm. The technique's robustness suggests promising applications in durable, ultra‐flexible electronics integration.