Highly flexible and stretchable silk composite films with manipulated hierarchical hydrogen bonds for on-skin electronics.

As the rapid evolution of modern electronics, wearable electronics that can non-invasively capture signal have attracted increasing attention in healthcare system. The ideal wearable electronics for healthcare should to be on-skin electronics with mechanical compliance and biocompatibility. Inspired by its natural biocompatibility, silk is elected to integrate with wearable electronics. However, its inherent stiffness and brittleness restrict on-skin applications. In this work, we present flexible silk composite films (SCFs) with lower binding energy through disrupting the ordered hydrogen (H) bonds in silk for on-skin electronics. Waterborne polyurethane and bis(2-hydroxyethyl) disulfide are incorporated to manipulate the H bonds which modulates the hierarchical structural transitions of silk, thus softening the SCF. The resulting SCF demonstrates markedly improved stretchability (~97.30%) and reduced elastic modulus (~37.85 MPa), representing a 1796.69% increase and a 98.15% reduction, respectively, compared to pure silk films. Benefiting from these improved mechanical properties, the SCFs are employed as electrodes for capturing electromyogram (EMG) and electrocardiogram (ECG) signals. Furthermore, a pressure sensor is constructed by integrating the SCF electrodes with a micro-structured SCF layer, which exhibits a wide sensing range (0-200.8 kPa) with an impressive sensitivity of 0.493 kPa-1 in the range of 0-12 kPa and a rapid response time of 120 ms. Therefore, the sensor enables reliable monitoring of various physiological signals, including pulse, swallowing, respiration, and writing of English letters. This study provides a new avenue for developing flexible silk-based films and expands their application in advanced wearable bioelectronics.