Porous Polydimethylsiloxane-Silver Nanowire Devices for Wearable Pressure Sensors

Abstract

We demonstrate a simple, nonlithographic method for fabricating piezoresistive pressure sensors with a broad range of working pressures and low detection limit. Our wearable pressure sensor is fabricated by using two metallized, porous polymer layers which undergo a change in resistance as a function of pressure. This sensor has a sandwich structure composed of top and bottom sheets of porous polydimethylsiloxane (PDMS) fabricated by using a simple templating method. The inner face of each of these layers is coated with a layer of conductive silver nanowires (AgNWs). The AgNW layers are initially in light contact, and the device undergoes a change in resistance when pressure is applied perpendicular to the plane of the sheets. In these devices, the size of the pores is an important determinant of the device sensitivity and range. Powder templates of KCl, NaCl, and sugar are used to create different sized pores (60-90, 200-275, and 400-550 μm, respectively) in the elastomeric PDMS layers, which in turn affects the mechanical deformability of the devices. The sensors fabricated with KCl porous layers (which had the smallest pores) demonstrate the best performance, with a sensitivity of 14.1 kPa-1 (up to 3.5 kPa), 4.8 kPa-1 (up to 10 kPa), and 1.84 kPa-1 (up to 40 kPa) and good stability over 1000 loading and unloading cycles. Pressure sensors are also fabricated by decreasing the ratio of PDMS base to curing agent from the standard 10:1 formulation, which increases the stiffness of the viscoelastic PDMS layer and thus shortens the response time. The stiffest formulation (5:1 (base:curing agent)) sample gives the shortest response time of ∼47 ms. Wearable electronic applications of these devices, including pulse measurement, facial movements, and sound tracking, are demonstrated in this work.