Lithography-Free Fabrication of Periodic Metal Network for Ultra-Sensitive Transparent and Unidirectional Strain Sensor in Health-Monitoring Applications

Tools with the ability of continuous health monitoring have received immense interest in biomedical engineering and the internet of things (IoT). Ultrasensitive wearable strain sensors are such tools having the potential to bring cost-effectiveness in the therapeutic field. The present work describes the development of a transparent strain sensor comprising parallel Au microwires semi-embedded in polydimethylsiloxane (PDMS). A lithography-free technique is employed for the cost-effective fabrication, where a parallel-crack network, formed by the gravity-flow of a crackle precursor on an inclined silicon substrate, serves as a stencil for the parallel micro-wire formations. The ultimate device is realized by transferring the wires into PDMS substrate. The sensor with ~85% transmittance shows a resistance change of ~370% for an applied strain of 2%. The mechanism relies on the formation of nano-break junctions in the micro-wires due to applied strain leading to discontinuation in the electron percolation. The unidirectional sensor having such ultra-high sensitivity is exploited for the detection of resonance and dissonance oscillations of a coupled cantilever. Besides, the sensor is utilized for speech recognition, gesture detection and radial augmentation index calculation from the blood-pulse pattern, supporting its feasibility as a potential tool in robotics and biomedical applications.