Artificial bone substitutes include metals, polymers and ceramics that are selected for clinics by their ability to support bone cell growth and osteogenic differentiation of progenitor cells, i.e. they display osteoconductive properties. Yet, one-phase materials are not able to fulfil the requirements for the ideal bone substitute and very few biomaterials are osteoinductive, i.e. they induce osteogenesis on their own. We have tested the hypothesis the incorporation of gel-derived bioactive glasses (SBG) into polymer (i.e. poly(L-lactide-co-glycolide; PLGA) or titanium dioxide (TiO₂) will modify the nanostructure of resulting composite materials, improve their mechanical properties, surface activity (i.e. bioactivity) and result in composite osteoinductivity in human mesenchymal stem cell (hMSC) cultures. Composites made of either TiO₂ or PLGA were combined with either A2 or S2 SBG of respective CaO/SiO₂ ratios: 54/40mol% (A2) or 16/80mol% (S2). PLGA was combined with 21%vol of A2 or S2 S-BG, whereas TiO₂ with 25, 50 or 75 wt% of A2 or S2 SBG. PLGA-SBG composites were next fabricated into 3D scaffolds, whereas TiO₂-SBG composites into cell growth surfaces. Human MSC were harvested from bone marrow of adult patients, seeded onto scaffolds and surfaces and stimulated in culture with either dexamethasone (Dex) or recombinant human BMP-2 up to 20 days. Bone-like hydroxyapatite structures formed mainly inside the PLGA-SBG scaffolds. Human MSC cultures on bioactive PLGA-A2 scaffolds increased ALP activity, osteopontin and BMP-2 mRNAs without Dex or BMP-2 treatments. When hMSC on PLGA-A2 scaffolds were treated with BMP-2, cells elevated ALP activity, RANK-L, osteoprotegrin and osteocalcin mRNAs compared to plain PLGA.
In addition, BMP-2 treated hMSC cultured on PLGA-SBG scaffolds elevated several osteogenic genes compared to Dex-treated hMSCs. Either Dex or BMP-treated hMSC cultured on PLGA-SBG scaffolds elevated collagen type I and matrix mineralization compared to plain PLGA. TiO₂-SBG surfaces developed hydroxyapatite-like forms mostly on the surface of composites containing 50 or 75wt% of either SBG. Higher content of A2 or S2 SBG in TiO₂ decreased the viability of hMSC and thiswas observed regardless of the time and culture conditions. Incorporation of 25wt% S2 SBG increased almost twice ALP activity in Dex-treated hMSC compared to Dex-treated cells on plain TiO₂.
In contrast, increased A2 SBG contents in the TiO₂ composites significantly enhanced both matrix mineralization and cell-mediated collagen production. Our data indicate the human MSC osteogenic response can be enhanced by bioactive composites made of either PLGA or TiO₂ enriched with gel-derived bioactive glasses A2 or S2. We believe the osteoinductive properties of the reported PLGA- or TiO₂-based SBG composite scaffolds in human MSC cultures may prove useful in several bone tissue engineering strategies and therapies where cell and/or growth factor delivery are not sufficient to induce proper bone formation.

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