Improved sensitive conductive sponge sensors with tunnel-crack broadening for pressure, humidity and temperature sensing applications

Nanocrack-based flexible sensors can be fabricated on various stretchable substrates for different applications, such as strain, humidity and temperature sensing. As the initial width of the nanocrack has a great influence on the sensitivity of the flexible nanocrack based sensor, there still are great demands to further improve the sensitivity by broadening the width of the nanocrack. Therefore, we propose a simple method of expanding the width of nanocracks on three-dimensional (3D) conductive sponge sensor by acid etching, which can be used to enhance the sensitivity of pressure, humidity and temperature sensing. The acid-etched tunnel-cracked nickel networks on polyurethane sponge (ATNNPS) samples were prepared by ion sputtering nickel conductive networks on the polyurethane sponge, followed by mechanical compression and acid etching. ATNNPS served as a sensitive platform for pressure, humidity, and temperature sensing, and offered good mechanical elasticity and high compressibility. ATNNPS exhibited a piezoresistive sensitivity of 257.5 kPa-1, which was over 3000 times higher than that of an unetched sample. Similarly, acid etching increased the humidity and temperature sensitivities by over 46,000 times (69.96 kΩ/1%RH) and 41 times (TCR=1.770 °C-1), respectively. This enhanced sensitivity was attributed to efficient adjustment of the initial width of the nanocracks in the conductive network, as tunnel resistance has an exponential relationship with nanocrack width. ATNNPS-based sensors offer appropriate sensing properties, low cost, easy fabrication, and the application potential is shown by using it in a proof-of-principle setup for speech recognition, pulse wave monitoring and automatic switch.