Inspired by the excellent wear resistance and lubrication of articular joints, a novel bionic interfacial system was proposed by combining thixotropic hydrogel with surface porous Ultrahigh Molecular Weight Polyethylene (UHMWPE). Thixotropic hydrogel, synthesised by gelatin, alginate sodium, tannic acid and weak crosslinking by Ca2+ (Gel-TA-Alg@Ca2+), was used as a lubricant due to its shear-thinning when loaded, then the recovery viscosity to be benefitted for reserving in surface pores on UHMWPE when unloaded. Surface porous UHMWPE was fabricated by using hydroxyapatite particles as porogen to control its porosity, pore size, surface roughness and surface energy (PE-HA). Gel-TA-Alg@Ca2+ significantly reduced average coefficients of friction and wear factors compared to those under normal saline and calf serum solution lubricating after reciprocating tribological testing. Notably, Gel-TA-Alg@Ca2+ still maintained thixotropy and was stored in surface pores of UHMWPE even after tribological testing for 7200 min. Thus, durable lubrication could be realised due to the synergistic effect of surface porous structure and thixotropy. Stribeck curves showed the characterisations of mixed, elastohydrodynamic and hydrodynamic, but without boundary lubrications for PE-30HA under three lubricants. The present results might provide the potential application to construct the durable lubrication bionic articular joint interfacial system for artificial joints.

Osteochondral defects include the damage of cartilage and subchondral bone, which are still clinical challenges. The general replacements are difficult to simultaneously repair cartilage and subchondral bone due to their various requirements. Moreover, appropriate printable bioactive materials were needed for 3D bioprinting personalized scaffolds for osteochondral repairing. Herein, the novel hydrogel was developed by hybridizing the alginate sodium (SA) and gellan gum (GG) with the inorganic thixotropic magnesium phosphate-based gel (TMP-BG) in the pre-crosslinking of Mg2+ to enhance osteochondral repairing. SA-GG/TMP-BG hybrid hydrogels possessed controllable rheological, injectable, mechanical properties and porosities by tuning their ratio. The shear-thinning of SA-GG/TMP-BG was responsible for its excellent injectability. SA-GG/TMP-BG hybrid hydrogels displayed good cell compatibility, on which MG-63 and BMSCs cells attached and spread well with the high proliferation and up-regulated osteogenic genes. In addition, the inorganic TMP-BG gel hybridized with SA-GG hydrogel released Mg2+ was conducive to recruiting BMSCs and promoting the osteogenic and chondrogenic differentiation of BMSCs. Histological results confirmed that SA-GG/TMP6040 significantly promoted the osteogenesis of subchondral bone and then further facilitated the cartilage repairing after being implanted in osteochondral defects of rabbits for 6 and 12 weeks. Our finding revealed that the inorganic TMP-BG endowed the excellent osteogenic activity of the hybrid hydrogels, which played a key role in successful osteochondral repairing. The newly SA-GG/TMP-BG hybrid hydrogels appeared to be promising materials for osteochondral repairing and the further 3D bioprinting.

3D Bioprinting is expected to become a strong tool for regenerative medicine, but satisfactory bioinks for the printing of constructs containing living cells are lacking due to the rigorous requirement of high printability and biocompatibility, which are often contradictory. Here, we have reported the development of a novel hybrid bioink by combining rigid gellan gum (GG), flexible sodium alginate (SA), and a bioactive substance thixotropic magnesium phosphate-based gel (TMP-BG). The ratio of these components was first optimized to obtain satisfactory gelating, mechanical, rheological, and printing properties. The formulated hybrid GG-SA/TMP-BG bioink had a good printability due to the shear-thinning and its multiple cross-linking by Mg2+ and Ca2+. The tunable mechanical performance of the hybrid bioink could simulate various extracellular matrices of the different tissues and support integrity of 3D printing constructs. Moreover, the hybrid bioink induced apatite deposition during immersion in simulated body fluids, and also promoted cell proliferation in vitro. MG-63 osteosarcoma cells were dispersed in the bioink and printed into 3D constructs. The cells exhibited good cell survival due to the shear-thinning property of the bioink and the ion concentration used for cross-linking. The proliferation rate of the cells also significantly exceeded those in non-printed samples. Confocal microscopy revealed a homogeneous distribution of cells in the printed constructs, and survival for more than 7 d. In vivo animal experiments showed that the hybrid bioink without cells could induce osteochondral repair. Therefore, this hybrid bioink has good printability, biocompatibility, mechanical support, and bioactivity, which is expected to have promising applications in 3D bioprinting.

Although the toughening of Calcium phosphate (CaP) scaffold by the addition of fiber has been well recognized, integrated mechanical, structural and functional considerations have been neglected in the design and fabrication of CaP scaffold implant. The emerging 3D printing provides a promising technique to construct CaP scaffold with precise size and elaborate microstructure. However, the most challenge is to extrude smoothly the CaP paste containing fibers for frequently-used extrusion-based 3D printing. In this study, frozen section and chemical dispersant (Pluronic F127, F127) were employed jointly to prepare non-aggregated polylactic-co-glycolic acid (PLGA) fibers. The injectability of CaP pastes with well dispersed PLGA fibers was more than 90% when the content of PLGA fibers was no more than 3 wt%. Meanwhile rheological property of CaP pastes with well dispersed fibers showed shear thinning, which were both beneficial to extrude CaP paste with well dispersed fibers for 3D printing. Moreover, these CaP scaffolds showed ductile fracture behavior due to the pullout and bridging effect of PLGA fibers. The cell proliferation and alkaline phosphatase (ALP) activity indicated that 3D printed CaP scaffold containing PLGA fibers possesses excellent biocompatibility and facilitate osteogenic differentiation ability. Thus, it was feasible to print CaP pastes with well dispersed PLGA fibers to construct toughening CaP scaffolds with the higher shape fidelity and complex structures, which had significant clinical potentials in osteoanagenesis due to their higher toughness and excellent biocompatibility.