Stick-On Large-Strain Sensors for Soft Robots

Soft robots require sensors that are soft, stretchable, and conformable to preserve their adaptivity and safety. Here the successful application of hydrogels as large-strain sensors for elastomeric structures such as soft robots is reported. Following a simple surface preparation step based on silane chemistry, prefabricated sensors are strongly bonded to elastomers via a "stick-on" procedure. This method separates the construction of the soft robot's structure and sensors, expanding the potential design space for soft robots that require integrated sensing. This work demonstrates that the adhesion strength exceeds that of the hydrogel itself and then characterizes the sensor via quasistatic, fatigue, and dynamic response tests. The sensor exhibits exceptional electrical and mechanical properties: it can sense strains exceeding 400% without damage, maintain stable performance after 1500 loading cycles, and has a working bandwidth of at least 10 Hz, which is sufficient for rapidly actuated soft robots. The hydrogel-based large-strain sensor into a soft pneumatic actuator is further integrated and the sensor effectively measures the actuator's configuration while allowing it to freely deform is demonstrated. This work provides "stick-on" large-strain sensors for soft robots and will enable novel functionality for wearable robots, potentially serving as a "sensing skin" through stimuli-responsive hydrogels.