Chitosan-Based Self-Healable and Adhesive Hydrogels for Flexible Strain Sensor Application

A design strategy to regulate the physical and chemical cross-linking interactions in double network (DN) hydrogels to improve conductivity, mechanical strength, and self-recovery is described in the present study. One of the challenges in generating robust DN hydrogels is the integration of the desirable toughness with multifunctionality such as good self-healing capability, self-adhesion on irregular surfaces, and high strain sensitivity. This work addresses this challenge through a design approach, which leads to the formation of a conductive, self-healing, and self-adhesive DN hydrogel from chitosan, tetraethylene glycol, and poly(acrylic acid). Three types of DN hydrogels (CTA-1, CTA-2, and CTA-3) are synthesized in the present study, which contain a common chemical cross-linker, and the nature of the second cross-linker is varied among the three hydrogels. The obtained DN hydrogels exhibited tunable mechanical properties that could be conveniently modified in a range of tensile stress (180 to 1170 kPa) and strain (870 to 1175%) values. Further, the hydrogels show self-healing properties with self-healing efficiency as high as 90-95% due to the dynamic cross-linking nature of the polymer networks. The hydrogels are self-adhesive on a variety of materials, which include skin, metal, glass, wood, rubber, coin, copper, plastic, aluminum, and polytetrafluoroethylene, even at underwater conditions, which is ascribed to the abundance of hydroxyl functional groups in the monomers. The results in the present study indicate that the hydrogels can be used as soft human motion sensors in real time.