A Skin-Like Hydrogel for Distributed Force Sensing Using an Electrical Impedance Tomography-Based Pseudo-Array Method

Hydrogels are compliant biomaterials that can be integrated with robotic systems to act like human skin for sensing and perception during interactions with their environments. The conventional method for fabricating skin-like hydrogel sensors is based on an array-type design which contains numerous discrete sensitive elements to obtain touch locations and force information. Array-based sensors are complex, with tiny communication units that make manufacturing complicated and expensive. Electrical impedance tomography (EIT) is a noninvasive imaging technique that can be easily implemented to create large-area tactile sensors into a "one-piece" structure without any internal wires. However, EIT-based tactile sensors suffer from low spatial resolution and sensitivity in areas far from the electrodes. This paper introduces a pseudo-array method to remedy the effect of location-dependent sensitivity on the spatial sensing of EIT-based hydrogel sensors and improve their performance for practical applications in detecting distributed contact forces without any arrays or internal wires. As a preliminary study, a skin-like hydrogel-based tactile sensor with an area of 400 cm2 was fabricated using a simple manufacturing process. The entire piece of the tactile sensor is then divided into a 5 x 5 array, which is referred to as a pseudo-array for simulation and experimental calibration. Each "pseudo-array" unit was calibrated to obtain the mapping relationship between the force and the reconstructed conductivity. Subsequently, a quantitative relationship between the touch force and the EIT measurement for the hydrogel-based tactile sensor for continuous sensing was achieved. Finally, the real-time performance of the EIT-based hydrogel sensor demonstrates that the proposed pseudo-array method can realize more accurate force detection with an error of 1.62 N (10.15% of the maximum force) for sensing distributed force over a large area of 400 cm2 with only 16 boundary electrodes.