Fast <i>in vivo</i> fixation of double network hydrogel to bone by monetite surface hybridization

Abstract

Double network (DN) hydrogels, possessing biocompatibility, low sliding friction, high strength and toughness, are promising as artificial cartilages for next-generation joint disease treatment. For such application, a fast and robust fixation of DN hydrogel to bone tissue in vivo is indispensable. However, bonding the DN hydrogel that contains ∼90 wt % of water to bone is a grand challenge since glues do not work on hydrated surfaces. Recently, we reported that a DN hydrogel of its subsurface hybridized with low crystalline hydroxyapatite (HAp) can achieve robust fixation to bone after 4 weeks implantation in rabbit knees, owing to the HAp-induced osteogenesis penetration into the hydrogel matrix. For clinical application, achieving a quick fixation at the early stage of implantation remains as a next subject. In this study, instead of HAp, we hybridized calcium monohydrogen phosphate (monetite), which is a HAp precursor calcium phosphate salt, in the subsurface of the DN hydrogel and we observed an increase in the pushout resistance of the DN hydrogel to bone after 1 week implantation, prior to the HAp-induced osteogenesis penetration. In physiological environment, the monetite hybridized in the subsurface of the DN gel spontaneously dissolved to calcium and phosphate ions and then re-crystallized to more stable HAp. We consider that the HAp formed in the boundary between the gel and the bone forms physical interlocking that significantly enhances the frictional resistance against the pushout force. The fast temporally pre-fixation to the bone by monetite surface hybridization makes one step closer to the clinical application of the DN gels as artificial cartilages.