Homogeneous hydroxyapatite/alginate composite hydrogel promotes calcified cartilage matrix deposition with potential for three-dimensional bioprinting

Calcified cartilage regeneration plays an important role in successful osteochondral repair, since it provides a biological and mechanical transition from the unmineralized cartilage at the articulating surface to the underlying mineralized bone. To biomimic native calcified cartilage in engineered constructs, here we test the hypothesis that hydroxyapatite (HAP) stimulates chondrocytes to secrete the characteristic matrix of calcified cartilage. Sodium citrate (SC) was added as a dispersant of HAP within alginate (ALG), and homogeneous dispersal of HAP within ALG hydrogel was confirmed using sedimentation tests, electron microscopy, and energy dispersive spectroscopy. To examine the biological performance of ALG/HAP composites, chondrocyte survival and proliferation, extracellular matrix production, and mineralization potential were evaluated in the presence or absence of the HAP phase. Chondrocytes in ALG/HAP constructs survived well and proliferated, but also expressed higher levels of calcified cartilage markers compared to controls, including Collagen type X secretion, alkaline phosphatase (ALP) activity, and mineral deposition. Compared to controls, ALG/HAP constructs also showed an elevated level of mineralized matrix in vivo when implanted subcutaneously in mice. The printability of ALG/HAP composite hydrogel precursors was verified by 3D printing of ALG/HAP hydrogel scaffolds with a porous structure. In summary, these results confirm the hypothesis that HAP in ALG hydrogel stimulates chondrocytes to secrete calcified matrix in vitro and in vivo and reveal that ALG/HAP composites have the potential for 3D bioprinting and osteochondral regeneration.