Laser direct writing of waterproof sensors inside flexible substrates for wearable electronics

The preparation of stable, reliable and multifunctional sensors requires constant research. This study reports a novel approach to fabricate waterproof sensors inside flexible substrate by laser direct writing. Polydimethylsiloxane (PDMS) films were prepared as flexible substrates. A silver ink composed of silver nitrate, sodium citrate, and polyvinyl pyrrolidone (PVP) were used in this work. The silver ink was first coated on the surface of PDMS film. Then a 405 nm laser was used to reduce silver nanoparticles for conductive circuits on PDMS film. After cleaning the unprocessed silver ink another layer of PDMS film was covered on the silver circuits to form a waterproof sensor. The sensor can be used for detection of bending and elongation of fingers. The response time is 0.3 s, and the recovery time is 0.4 s. The sensor can respond well with high frequency and low frequency bending. It can keep its functions normally after five hundred times of bending. Moreover, the sensor can be operated normally underwater with good waterproof performance and stability. It can be safely used to monitor movement in wet conditions, rainy weather and underwater environments. Based on these experimental results, this facile process is promising for preparation of waterproof wearable sensors.