Hybrid pesponse pressure sensors as an electronic skin for bio-sensing and soft robotics

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Soft pressure sensors are critical components of e-skins, which are playing an increasingly significant role in two burgeoning fields: soft robotics and bio-electronics. Capacitive pressure sensors are popular given their mechanical flexibility, high sensitivity, and signal stability. After two decades of rapid development, e-skins based on soft capacitive pressure sensors are able to achieve human-skin-like softness and sensitivity. However, there remain two major roadblocks in the way of the practical application of soft capacitive pressure sensors: 1) the decay of sensitivity with increased pressure and 2) the coupled response between in-plane stretch and out-of-plane pressure. This dissertation aims to tackle the two critical challenges. To increase the sensitivity of a capacitive pressure sensor, past research has mostly focused on developing dielectric layers with surface/porous structures or higher dielectric constants. However, such strategies have only been effective in improving sensitivities at low pressure ranges (e.g. up to 3 kPa). To overcome this well-known obstacle, I devised a flexible hybrid response pressure sensor (HRPS) composed of an electrically conductive porous nanocomposite (PNC) laminated with an ultrathin dielectric layer. Using a nickel foam template, the PNC was fabricated with carbon nanotubes (CNT) doped Ecoflex to be 86% porous and electrically conductive. The PNC exhibits hybrid piezoresistive and piezocapacitive responses, resulting in significantly enhanced sensitivities (i.e., more than 400%) over wide pressure ranges, from 3.13 kPa⁻¹ within 0-1 kPa to 0.43 kPa⁻¹ within 30-50 kPa. The effect of the hybrid responses was differentiated from the effect of porosity or high dielectric constants by comparing the HRPS with its purely piezocapacitive counterparts. Fundamental understanding of the HRPS and the prediction of optimal CNT doping were achieved through simplified analytical models. Second, coupled pressure and stretching responses disable capacitive sensors to differentiate out-of-plane and in-plane inputs. Due to this critical limitation, intrinsically stretchable and accurate capacitive pressure sensors have not been successfully demonstrated, while other types of soft sensors have already achieved stretchability beyond flexibility. To discriminate the compression and stretching responses, I proposed a stretchable hybrid response pressure sensor (SHRPS). SHRPS is a stretchable version of HRPS, whose sensitive pressure response trivializes the stretching response and enables the SHRPS to accurately read applied pressure even under stretching. Analytical models for conventional capacitive pressure sensors, HRPS and SHRPS have been developed and provide fundamental electromechanical understanding for the difference among the three. As a demonstration, a 3 x 3 SHPRS array has been glued to an inflatable manipulator which is capable of diverse tasks such as pulse palpation and object grabbing given the tunable inflation. A perspective for the future directions of HRPS is provided at the end of the dissertation.


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