Embedding ionic hydrogel in 3D printed human-centric devices for mechanical sensing

Flexible sensor applications have increasingly focused on ionically conductive hydrogels due to their notable deformability and easily tunable properties compared to rigid materials. These hydrogels possess electrical properties, thanks to their high water content and porous structure that facilitate effective ion transfer. Despite their attractive features, hydrogels have limitations in terms of water retention and shape fidelity, and they are more typically inspected as two dimensional films and patches. In this paper, 3D printed thermoplastic polyurethane (TPU) elastomer frames with various geometries were injected with ionic conductive polyacrylamide (PAAm) based hydrogels to create durable, robust soft mechanical sensors for detecting strain, pressure, and bending through changes in their electrical resistance. After the effectiveness of the TPU encasement in maintaining the hydrogel water content was demonstrated, hydrogel embedded frames with varying geometries were designed. Their response to mechanical loading was investigated in relation to their dimensions and geometric shape. Finally, glove-shaped frames were fabricated to fit human fingers and injected with ionic hydrogel for sensing abilities. The wearable sensors accommodated free movement of the fingers in multiple directions and were able to detect simultaneous and independent bending and stretching of the fingers. Through comprehensive observation of the electrical behavior of all soft ionic sensors in response to different kinds of mechanical stimuli, it was concluded that the resistance change following mechanical loading was dependent on the specific geometric features of each individual hybrid sensor. Thus, ionic hydrogel-embedded TPU encasement could be designed with targeted geometry to dictate the type and direction of mechanical sensing with regard to its application. This work presents a facile approach to fabricating multi-component soft geometric sensors with potential to be used for wearable electronics and human–machine interactions.