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Vascular endothelial growth factor (VEGF) plays an essential role in angiogenesis and vascular homeostasis. Endothelial progenitor cells (EPCs) are primitive bone marrow cells participating in neovascularization and revascularization processes, which also promote bone regeneration. Synthetic hydroxyapatite (HA) has been widely used in bone repair and implant coatings. In HA-based materials, small levels of ionic substitution by silicon (Si) have significant effects on osteoclastic and osteoblastic responses. Moreover, nanocrystalline hydroxyapatites (nano-HA) display enhanced bioreactivity and beneficial effects in bone formation. In this work, the angiogenic potential of VEGF-121 adsorbed on crystalline and nanocrystalline HAs with different Si proportion is evaluated with endothelial-like cells derived from EPCs cultured on nano-HA, nano-SiHA0.25, nano-SiHA0.4, HA, SiHA0.25 and SiHA0.4 disks. The Si amount incorporated for x ¼ 0.25 is enough to yield changes in the textural parameters and surface charge without decomposing the HA phase. Si substitution for x ¼ 0.4 does not result in pure Si-substituted apatites. Si probably remains at the grain boundaries as amorphous silica in nano-SiHA0.4 and SiHA0.4 is decomposed in a-TCP and HA after 1150 �C treatment. Immobilized VEGF on nano-HA, nano-SiHA0.25, nano-SiHA0.4, HA, SiHA0.25 and SiHA0.4 maintains its function exerting a local regulation of the cell response. The crystallite size and topography of nanocrystalline HAs could produce insufficient and weak contacts with endothelial-like cells triggering anoikis. Concerning Si proportion, the best results are obtained with SiHA0.25/VEGF and nano- SiHA0.25/VEGF disks. All these results suggest the potential utility of SiHA0.25/VEGF and nano-SiHA0.25/VEGF for bone repair and tissue engineering by promoting angiogenesis.

In the present study, the effects of nanocrystalline hydroxyapatite (nano-HA) and nanocrystalline Si-substituted hydroxyapatite (nano-SiHA) on osteoclast differentiation and resorptive activity have been evaluated in vitro using osteoclast-like cells. The action of these materials on proinflammatory and reparative macrophage populations was also studied. Nano-SiHA disks delayed the osteoclast differentiation and decreased the resorptive activity of these cells on their surface, as compared to nano-HA samples, without affecting cell viability. Powdered nano-SiHA also induced an increase of the reparative macrophage population. These results along with the beneficial effects on osteoblasts previously observed with powdered nano-SiHA suggest the potential of this biomaterial for modulating the fundamental processes of bone formation and turnover, preventing bone resorption and enhancing bone formation at implantation sites in treatment of osteoporotic bone and in bone repair and regeneration.