Electromechanical and Thermal Characterization of Printed Liquid Metal Ink on Stretchable Substrate for Soft Robotics Multi-Sensing Applications

Electronic skins (e-skins) are widely used in wearables, robotics, and medical field. In this paper we introduce a fabrication and characterization of a bio-inspired robot electronic skin, with multiparameter sensing capabilities that include detection of changes in temperature, and locomotion of a sensi-worm robot. Soft robotics has gained much attention in recent years due to its stretchability and conformability in reaching confined areas and uneven surfaces. Soft robots enabled by advances in flexible and hybrid electronics (FHE) offer the potential to navigate through unusual environments with robust, compliant structures that integrate sensors and actuators suitable for industrial inspection, maintenance, and repair requirements. Liquid metal inks have demonstrated immense promise in highly stretchable electronics, offering good conductivity and stable resistance over large strains. However, these inks present adhesion and wetting challenges during printing. Herein, we developed an e-skin fabrication process for multiparameter sensing using polymerized liquid metal ink printed on styrene-ethylene-butadiene-styrene (SEBS) and Thermoplastic Polyurethane (TPU) substrates, using direct write printing method, followed by an electromechanical evaluation against mechanical and thermal stresses. The thermomechanical behavior of the printed traces was investigated with a series of uniaxial stretching and cycling, thermal shock, and thermal stability. The change in electrical resistance with respect to strain amplitude up to 50% during uniaxial stretching was monitored. The skin strain sensors have gauge factor $\sim \mathbf{1.3}$ for TPU and 1.8 for SEBS. On the other hand, the printed thermistor design showed stable performance over thermal cycling with a thermal coefficient of resistance (TCR) 5.7e-4 ppm/C which is smaller to other regular metal-based inks.