LipoGels: Robust Self-Lubricating Physically Cross-Linked Alginate Hydrogels Embedded with Liposomes

Abstract

The exceptional lubricity in living systems has stimulated wide scientific interest in the design of biomimetic lubricants. Here, a sodium alginate hydrogel physically cross-linked with divalent cations (Ca2+), incorporating ≈100 nm 1,2-dipalmitoyl-sn-glycero-3-phosphocholine liposomes in trace amounts (≈0.004–0.08 wt% lipids), which are termed “LipoGels”, is optimized to overcome gelation shrinkage to generate a homogeneous and smooth surface. Cryogenic scanning electron microscopy reveals the compact porous gel structure with embedded liposomes, and confocal fluorescent microscopy reveals a uniform distribution of liposomes in the gel. The LipoGels demonstrate robust mechanical strength, with an elastic modulus Gʹ > 200 kPa and a Young's modulus E ≈1 MPa from rheology and microindentation measurements, respectively. Small-angle neutron scattering deduces a classic de Gennes’ mesh size of ξ ≈3.8 nm for the polymer network, consistent with that estimated from the rheology results. The underwater friction coefficient of liposome–alginate hybrid LipoGels, evaluated in the sphere-on-flat geometry with a tribometer, is as low as μ ≈0.02 at a maximum contact pressure of 0.45 MPa. These results offer nanostructural insights into the hybrid liposome–alginate hydrogels, prepared by facile physical cross-linking, demonstrating their potential as biomimetic lubricants, also with encapsulation capacities for functional additives.