A Self-Lubricating Gradient Hydrogel Coating with Integrated Load-Bearing and Anti-Debonding Capabilities

Abstract

Hydrogel coatings, featuring high water content and excellent biocompatibility, represent a highly promising strategy for the surface functionalization of biomedical implants. However, developing a hydrogel coating that simultaneously integrates high mechanical strength, superior lubrication performance, and robust interfacial anchoring on implant substrates remains a long-standing critical challenge. Inspired by the hierarchical anisotropic structure of natural articular cartilage, we report a gradient double-network (GDN) hydrogel coating based on a polydopamine–gelatin–carboxymethyl cellulose (CMC)–tannic acid (TA) system. The fully physically cross-linked GDN hydrogel coating was fabricated via a facile three-step strategy: (i) polydopamine (PDA) pretreatment of the implant substrates; (ii) formation of the primary preorganized physical network through low-temperature refrigeration, where abundant phenolic hydroxyl groups of TA formed multiple hydrogen bonds and hydrophobic interactions; (iii) establishment of the secondary physical cross-linked network through directional diffusion of aluminum ions (Al3+), where Al3+ formed ionic coordination complexes with the −COO– groups on CMC chains via in situ ion exchange. This fabrication process generates a gradient in cross-linking density through the thickness across the coating: the dense bottom layer endows the coating with exceptional load-bearing capacity (a compressive modulus of 455 kPa), while the loosely cross-linked, highly hydrated top layer ensures efficient lubrication. Furthermore, the incorporation of the nonionic surfactant Tween 80 enables the coating to achieve an ultralow coefficient of friction (CoF = 1.9 × 10–3) without additional lubricants. Furthermore, the coating exhibits robust interfacial bonding through multiple interfacial interactions, delivering a high adhesion energy of 285 ± 11 J m–2. This biomimetic gradient structure synergistically breaks the inherent trade-off among load-bearing capacity, lubrication performance, and interfacial adhesion, providing a universal and facile strategy for the surface functionalization of articular cartilage implant materials.