Transparent and conformal 'piezoionic' touch sensor

A polyurethane hydrogel based touch sensor with high transparency and conformability is demonstrated. Polyurethane hydrogels swollen with various electrolytes were compressed at a pressure of 30 kPa, simulating a fingertap on a conventional touch screen device. Unlike ionic polymer metal composite and conducting polymer trilayer sensors, where electrodes render the sensors opaque and relatively rigid, the electrodes used in this work are metal wires or strips, separated from each other by regions of transparent film, enabling transparency and compliance. The voltages and currents observed when the perturbation is above one electrode are on the order of 10^-2 V and 10^-7 A, relative to a second electrode that is approximately 1 cm away. The sign of voltage and current signals detected from perturbations made between electrodes is determined by relative proximity to each electrode, and the magnitude appears to decrease with increasing distance from the electrodes. These observations suggest that it may be possible to discriminate the location of touch based on signals transmitted to the edges of an ionically conductive film. A model to describe the inhomogeneous ionic distribution and predict the resultant voltage and current is presented to qualitatively explain the sensing, based on the Donnan potential.