Flexible and Conformable Touchpads and Keyboards Based on Piezoelectric Polymers
Sprache des Vortragstitels:
2009 MRS Spring Meeting, San Francisco, USA
Sprache des Tagungstitel:
We present flexible and conformable position sensitive touchpads and keyboards based on piezoelectric polymers. As we use large area electrodes combined with electronics at the edges, complex sensor matrices are avoided. Cellular polypropylene ferroelectrets are soft, lightweight and electroactive transducer materials which can be used in a wide range of sensors and actuators. They can be obtained from polymer packaging foams by internally charging their voids. As they show a high longitudinal piezoelectric effect and small transverse piezoelectricity, they only react on touch and not on bending. Therefore they are especially advantageous for flexible and conformable transient pressure sensing applications. We fabricated and characterized a large-area ferroelectret touchpad that is position sensitive and allows for a customizable layout. It consists of a ferroelectret film that is placed between two large area electrodes. One of them is highly conductive and grounded. The other one is highly resistive. Electrical ports are fixed to the corners of the device. If a force is applied, electrical charges which are generated by the piezoelectric effect diffuse to the electrical ports. Therefore transient voltage signals can be measured, whose amplitudes and phases depend on the touch position. We fabricated a touchpad of 10x10 cm2 and tested it applying forces between 1 N and 2 N on an area with a diameter of 5 mm. These parameters are typical of touch events in practice. For the flexible keyboards we used a binary or balanced ternary coding technique profiting from the polarization of the ferroelectret. Functional layers with a polarization pattern are covered by large area electrodes and then stacked. The keys are formed by the sequence of the three possible polarization states plus, minus and none. If a force is applied to a key, the corresponding transient voltage signals are measured at the edges of the device for every layer.