Design and fabrication of highly flexible tactile sensors based on green hydrogel

The current technological development requires new ways of interfacing between humans and machines. Soft tissues typical of human body actually need to be linked to the rigid electronics and mechanical components of machineries thanks to devices which reproduce the features of both realms. Drawing inspiration from the human skin, tactile or smart sensors are devices able to sense external stimuli, such as deformations, temperature, moisture and light. These devices find applications in a wide range of fields like world of wearables, soft robotics, prosthetic and e-skin. In particular, the reproduction of haptic sensation is a challenge which still yielded no applicable results, despite its considerable importance in biomedicine. Herein, a sandwich-like tactile sensor composed of a cellulose based hydrogel encapsulated by a flexible substrate, is presented. A methacrylated carboxymethylcellulose (M-CMC) hydrogel endowed of ionic conductibility by sodium chloride (NaCl) was used as sensitive part. Both the sensible layer, electrodes and shells were investigated to find the best setup. Finally, sensors with different M-CMC concentrations were subjected to stretching and bending stimuli and their electrical properties variations were measured through an LCR meter. Devices shows a very high sensitivity to external deformation and an extreme stretchability up to 600\%. After the selection of the best configuration, sensors were used to monitor different human body signals. The results demonstrated that the performances of these sensors could be a promising starting point for the development of transparent, low-cost, eco-friendly and easy manufacturable devices.