Soft Chemiresistive Sensing Shields Soft Robotic Actuators from Mechanical Degradation due to Critical Solvent Exposure

Abstract

Thermoplastic elastomers (TPEs) are popular for fabricating soft actuators thanks to their compatibility with thermoplastic processing methods, like material extrusion additive manufacturing. However, these TPEs are susceptible to nonpolar solvents, and upon exposure, the mechanical properties can diminish significantly. Herein, soft chemiresistive sensing elements based on thermoplastic elastomers and carbon black (CB) are developed for detecting the presence of nonpolar solvents. A low CB concentration (20% w/w) leads to a faster response for detecting the presence of nonpolar solvents. A threshold value of the electrical current is defined by tensile testing, based on exposure time that does not diminish the mechanical properties. Exposure to solvent vapor above 150 s decreases the elongation at fracture significantly (450%). To achieve nociceptive behavior for a sensorized soft actuator and shield the actuator from structural damage, this threshold value is used for programming a closed-loop control motion system. When the threshold value is reached, the actuator withdraws from the solvent surface autonomously. However, this threshold decreases with parameters like the solvent temperature and sensor distance from the solvent surface. Overall, adding soft chemiresistive elements into elastomer-based soft robots extends their lifetime, by preventing damage in harsh chemical atmospheres.