Ultrastretchable Helical Conductive Fibers Using Percolated Ag Nanoparticle Networks Encapsulated by Elastic Polymers with High Durability in Omnidirectional Deformations for Wearable Electronics

Stretchable interconnects with invariable conductivity and complete elasticity, which return to their original shape without morphological hysteresis, are attractive for the development of stretchable electronics. In this study, a polydimethylsiloxane-coated multifilament polyurethane-based helical conductive fiber is developed. The stretchable helical fibers exhibit remarkable electrical performance under stretching, negligible electrical and mechanical hysteresis, and high electrical reliability under repetitive deformation (10 000 cycles of stretching with 100% strain). The resistance of the helical fibers barely increases until the applied strain reaches the critical strain, which is based on the helical diameter of each fiber. According to finite element analysis, uniform stress distribution is maintained in the helical fibers even under full stretching, owing to the fibers' true helix structure. In addition, the stretchable helical fibers have the ability to completely return to their original shapes even after being fully compressed in the vertical direction. Cylinder-shaped connecting pieces made using 3D printing are designed for stable connection between the helical fibers and commercial components. A deformable light-emitting diode (LED) array and biaxially stretchable LED display are fabricated using helical fibers. A skin-mountable band-type oximeter with helical fiber-based electrodes is also fabricated and used to demonstrate real-time detection of cardiac activities and analysis of brain activities.