Osmotic Pressure Gradient-Induced In-Plane Dual Orientation: A Versatile Strategy for Ultrastrong, Anti-Swelling Gradient Hydrogels.

Emerging ultrastrong hydrogels hold promise for various applications but often suffer from swelling-induced fragility in wet environments, limiting practical utility. Herein, we report an osmotic pressure gradient-mediated in-plane self-orientation (OPG-ISO) strategy to fabricate ultrastrong, anti-swelling dual self-oriented gradient (DSOG) hydrogels. This approach relies on interfacial diffusion-complexation between concentrated low-molecular-weight (LMW) polycations and a mixture of cellulose nanofibers (CNFs) and polyanions. The concentrated LMW polycations complex with the polyanions to form a gradient matrix while generating an osmotic pressure gradient that induces directional deswelling and in-plane self-orientation of both the polymer networks and CNFs. Concurrently, these LMW polycations also serve as physical cross-linkers to stabilize the aligned microstructure. Benefiting from the dual self-oriented architecture, dense physical cross-linking, and hydrogen bonding, the resulting DSOG composite hydrogels exhibit exceptional mechanical properties (strength: 6.33 MPa, elastic modulus: 112.3 MPa, and toughness: 7.14 MJ/m3) and outstanding swelling resistance, with negligible changes in volume (∼3.14%) and tensile strength (∼6.16%) after equilibration in water. Moreover, the charge gradient imparts a pressure-sensitive self-polarized potential, enabling a highly sensitive self-powered ionic skin capable of detecting subtle vibrations. The OPG-ISO strategy also allows customizable three-dimensional (3D) configurations, thereby offering a versatile platform for constructing ultrastrong, anti-swelling, and multifunctional hydrogels for engineering and biomedical applications.