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B2.05 - Scalable, Transparent, Flexible, Thin-Film Pressure Sensors Utilizing Ultrasoft Elastomers 
December 1, 2014   11:15am - 11:30am

Thin-film pressure sensors based on elastomers have required either foams or microstructuring. Solid films of elastomer have not been used due to low sensitivities mostly due to relatively high moduli on the order of megapascals. However, solid elastomeric films are more easily incorporated into electronics, are more scalable and can be transparent. Here, we demonstrate elastomers with Young’s moduli on the order of 1 kPa while maintaining elastic recovery. Typically, materials with kPa moduli are viscoelastic. Within a pressure sensor, a viscoelastic material will have a substantial hysteresis and will recover from pressure slowly and incompletely. Off-stoichiometry thiol-click chemistry with thiol-functionalized polydimethylsilane (PDMS) and vinyl-terminated PDMS oligomers is used to form elastomers with low crosslink densities and long, flexible side chains. These side chains plasticize the elastomer, leading to a much smaller Young’s modulus and helping to maintain the elastic behavior. These elastomers are transparent, ultrasoft, and have little to no hysteresis in compression cycles, indicating mostly elastic behavior.

Pressure sensors using these elastomers are made by depositing electrodes onto films with thicknesses below 100 nanometers. The thin films are made by diluting the PDMS oligomers into a good solvent and rod coating the solutions onto glass. The resulting pressure sensors act as variable resistors. With the application of pressure and voltage, the film is compressed and current flows between the electrodes. For 2 mm x 2 mm pressure sensing pixels and an applied voltage of 1V, the current increases from a picoamp baseline to 10’s of picoamps to nanoamps depending on the electrode design and the amount of applied pressure from 1 g to 1 kg of force. This range of sensitivity matches that of finger pressure, which makes these sensors suitable for pressure-sensitive touchscreens. These pressure sensors are incorporated into a matrix-addressable array by using a transistor array with the variable resistors in series with the drain and the drain current is measured. When using transparent electrodes, these arrays are transparent, low-power, highly sensitive and have high 2-dimensional spatial resolution. This allows these pressure sensor arrays to be used in many applications including pressure-sensitive, multi-touch touchscreens.

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