Tough, adhesive, self-healing, fully physical crosslinked κ-CG-K+/pHEAA double-network ionic conductive hydrogels for wearable sensors

The development of multiple-functional conductive hydrogels with combined merits of high mechanical strength, fast self-healing, strong surface adhesion, and strain sensibility has proved to be a highly demanded, but extremely challenging task for different applications. Herein, we designed, synthesized, and characterized fully physically crosslinked double-network (DN) hydrogels of κ-carrageenan/poly(N-hydroxyethyl acrylamide) (κ-CG-K+/pHEAA DN gels) using a facile heating-cooling-photopolymerization process. The resultant κ-CG-K+/pHEAA gels exhibited highly mechanical (tensile strength of 2.02 MPa, tensile strain of 1550%, and elastic modulus of 0.91 MPa) and self-healing properties owing to the interpenetrating DN structures and reversible hybrid ionic-hydrogen bond cross-linking networks. In parallel to high mechanical properties in bulk, κ-CG-K+/pHEAA gels also demonstrated their high surface adhesion of ∼773 J/m2 on different untreated hard substrates (i.e., titanium, aluminum, ceramic, and glass), which stemmed from the rich-functional groups (hydroxyl and amide) in pHEAA polymer chains. Moreover, the DN gels also presented conductivity, strain sensitivity with gauge factor (GF) = 2.48, and sensing stability due to the presence of K+ and stretching-dependent resistance variations. Such a unique combination of high mechanical, adhesive, and conductive properties in κ-CG-K+/pHEAA gels allows us to further fabricate them into gel-based strain sensors with high and rapid sensitivity for detecting subtle strain-induced human motions. The design concept and hydrogel system from this work provide a new angle for broad human-machine interface applications.