Hybrid Response Pressure Sensor (HRPS) for Wearable and Robotic-Finger-Based Pulse Wave Sensing

Soft pressure sensors with high sensitivity over a wide pressure range are required for various applications such as e-tattoos for mechanophysiology measurements and e-skins for human-mimetic robotics. In the last decade, most research aiming at increasing the sensitivity of capacitive pressure sensors focused on developing dielectric materials with added air gaps and/or higher dielectric constants. After extensive research, sensitivity has been significantly improved at low pressure range, e.g. 1 kPa, but still decays drastically as the pressure increases. To overcome this bottleneck, we engineered a hybrid response pressure sensor (HRPS) by laminating an electrically conductive porous nanocomposite (PNC) with an ultrathin (500-nm-thick) insulating layer. The PNC was fabricated by molding the mixture of functionalized carbon nanotubes (f-CNT) and Ecoflex out of a nickel (Ni) foam. The PNC was 600-µm thick and 86% porous with open cells and tubular ligaments. With enough f-CNT doping (e.g. 0.25 wt%), the PNC became electrically conductive with hybrid piezoresistive and piezocapacitive characteristics but the whole device was still capacitive due to the ultrathin insulating layer. This HRPS was measured to have a sensitivity of 3.13 /kPa within 0-1 kPa, 1.65 /kPa within 1-5 kPa, 1.16 /kPa within 5-10 kPa, 0.68 /kPa within 10-30 kPa, and 0.43 /kPa within 30-50 kPa of pressure ranges, representing up to 423% improvement over existing capacitive pressure sensors. The mechanism of HRPS was fully understood through a simplified circuit model, which has been validated by experimental measurements and could be used to determine the optimal f-CNT doping concentration. We have successfully applied HRPS as an e-tattoo on human skin to noninvasively measure pulse waves over the radial artery, the common carotid artery, and the temporal artery. Even under large preloads such as a VR (virtual reality) headset, the temporal arterial pulse wave was still successfully measured by HRPS.[1] With improvement in the fabrication process, HRPS could be made stretchable, which we call SHRPS (stretchable hybrid response pressure sensor). We found that given the enormous pressure sensitivity, the capacitance change of SHRPS due to out-of-plane pressure is significantly larger than that due to in-plane stretch. Therefore, SHRPS is insensitive to stretch and has been applied as an intrinsically stretchable pressure sensor with 4 x 4 electrodes on an inflatable robotic finger. When the robotic finger is inflated, it is stiff and curved, which can be applied to successfully palpate human radial pulses, even with wrist movement. When the robotic finger is deflated, it is soft and flat, which is ideal for grabbing both stiff and soft brittle materials without causing any damage.[2] The inflatable robotic finger has indicated the potential of applying SHRPS as an e-skin on other highly deformable soft robots.<br/>References<br/>[1] Ha, K.-H., Zhang, W., Jang, H., Kang, S., Wang, L., Tan, P., Hwang, H., and Lu, N.: ‘Highly sensitive capacitive pressure sensors over wide pressure range enabled by the hybrid responses of a highly porous nanocomposite’, Adv Mater, 2021, pp. 2103320<br/>[2] Ha, K.-H., Huh, H., Li, Z., Kim, S., Kang, S., Wang, Z., Scalco de Vasconcelos, L., and Lu, N.: ‘E-Skin based on Stretchable Hybrid Response Pressure Sensors (SHRPS)’, in preparation, 2022