Cellulose nanocrystals-reinforced mechanically robust and anti-swelling anisotropic composite hydrogels for multifunctional underwater sensing.

Hydrogels, as skin-mimicking soft materials, have shown promising applications in wearable flexible electronics. However, their intrinsic swelling behavior leads to mechanical degradation and unstable sensing performance in aqueous environments, severely limiting their applicability in underwater sensing. Herein, we develop a cellulose-reinforced anisotropic composite hydrogel with excellent mechanical properties and anti-swelling properties through a strategy combining mechanical pre-stretching and Al3+ coordination crosslinking. Specifically, the carboxylated cellulose nanocrystals/poly(acrylic acid) isotropic precursor hydrogel is mechanically pre-stretched to induce the oriented arrangement of polymer chains and cellulose nanocrystals along the stretching direction, and then the oriented structure is fixed through Al3+ coordination bonding, ultimately yielding a cellulose-reinforced anisotropic composite hydrogel with dense coordination bonds and multiple hydrogen-bond networks. This anisotropic and dense network structure design endows the composite hydrogel with significantly improved tensile strength of 0.72 MPa and toughness of 2.87 MJ·m-3, realizing a 14.12-fold and 21.92-fold increase compared to the isotropic precursor hydrogel, respectively. Meanwhile, the composite hydrogel exhibits excellent conductivity of 0.64 S·m-1 and superior anti-swelling performance with a low swelling ratio of 13.02% after immersion in seawater for 6 days. Given these features, the composite hydrogels can function as high-sensitivity (GF = 1.27) and rapid response (120 ms) wearable strain sensors for real-time motion monitoring and Morse-code communication in underwater environments. This study provides a practical strategy for the design of high-performance bio-based hydrogels for underwater sensing electronics.