Two-Dimensional Nanocellulose-Enhanced High-Strength, Self-Adhesive, and Strain-Sensitive Poly(acrylic acid) Hydrogels Fabricated by a Radical-Induced Strategy for a Skin Sensor

Hydrogels have attracted extensive attention in recent years, while it remains a great challenge to integrate excellent mechanical properties, self-adhesion, and strain sensitivity into a single hydrogel, especially in the wearable sensor field. In this work, a two-dimensional (2D) cellulose nanosheet (CNS)-enhanced flexible multibond cross-linked poly(acrylic acid) (PAA) hydrogel was fabricated through a facile radical-induced grafting method gelation using Fenton reagents as an initiator. The introduction of 2D CNSs, which originated from the ball-milling exfoliation of cotton cellulose, enormously improved the mechanical properties of the PAA hydrogel because of its effective dispersion of the applied external force along the polymer chains. At a premium Fe3+/acrylic acid ratio of 2.5 parts per thousand and a cellulose content of 14% (Cel(14)/PAA-Fe-2.50(3+)), benefiting from the abundant multibonds including various covalent bonds and coordinate bonds, the mechanical and self-adhesive properties of the hydrogel were superior to those of other hydrogels. The hydrogel exhibited excellent self-adhesive ability (elongation at break = 1800%) and a durability of 500 cycles under 20, 100, and 400% strains. The existence of Fe3+ from the Fenton reagent endowed the hydrogel with excellent conductivity, evidenced by the fact that the strain sensor based on Cel(n)/PAA-Fe-m(3+) hydrogels demonstrated stable conductivity and strain sensitivity (the gauge factor = 7.6 at 600-1100% strains), which could monitor both large human motions and subtle motions such as finger knuckle and knee joint motion and wrist pulse beating. These findings indicated that Cel(n)/PAA-Fe-m(3+) hydrogels laid the foundation of developing ultrasensitive and highly stretchable hydrogels holding potential applications in wearable strain sensors, electronic skin, and human machine interfaces.