Physically Cross-Linked Gradient Double Network Hydrogel with Self-Lubrication and Load-Bearing Capacity

Hydrogels are promising biomaterials due to their high hydration and biocompatibility. Nevertheless, simultaneously integrating excellent load-bearing capacity and self-lubricating interface into a single hydrogel remains a significant challenging. To address this, we developed a physically cross-linked gradient double-network (GDN) hydrogel based on poly(vinyl alcohol) (PVA) and sodium carboxymethyl cellulose (CMCNa) for biomedical implant applications. The PVA/CMC GDN hydrogel was prepared using a two-step physical cross-linking strategy. First, PVA chains in the mixture formed crystalline domains acting as cross-linking junctions via hydrogen bonding during freezing-thawing cycles, establishing the primary physical network. Subsequently, Al³⁺ ions diffused unidirectionally from an Al₂(SO₄) solution into the hydrogel, complexing with CMCNa chains to create a second physical network. The resulting Al³⁺ concentration gradient generated a spatially varying cross-linking density, with the bottom region developing a dense double-network structure that endowed the GDN hydrogel with superior load-bearing capacity (macroscopic compressive modulus: 401 kPa). In contrast, the top region retained a single-network structure, ensuring the GDN hydrogel's excellent self-lubrication. By incorporating the nonionic surfactant Tween 80, the hydrogel achieved an ultralow boundary coefficient of friction reached (9.8 × 10^-3) even in the absence of external lubricant. This work not only demonstrates the significant potential of PVA/CMC GDN hydrogels as biomedical implants but also presents a versatile strategy for designing biomimetic architectures with spatially tunable properties.