Objectives: Bioactive glasses (BAG) are used in clinics as granules or rigid implants with different shapes. Polymer-BAG composites could exhibit improved handling characteristics, and could overcome the problem of brittleness associated with bioceramic materials. The purpose of this study was to compare differentiation of bone marrow stromal cells on poly(epsilon-caprolactone/D,L-lactide) / BAG composites in different stages of calcium phosphate (CaP) deposition. Materials and methods: CaP layer was formed on composite discs by immersing them in simulated body fluid (SBF) in a shaking water bath at 37°C for 5 (Comp5) and 14 (Comp14) days. Osteogenic differentiation on untreated composites (Comp0), neat co-polymer (Pol0) discs and standard tissue culture polystyrene (TCPS) wells was also analyzed. Results: In the beginning of cultures the fastest expansion of cells was observed on TCPS. No differences between the other substrate types were evident. TCPS also had high initial alkaline phosphatase (ALP) activity, together with Pol0 substrates, whereas Comp14 showed delayed ALP activity. Depletion of calcium from culture medium was first observed with SBF-treated composites, followed by untreated ones. However, the untreated composites showed great variation between individual specimens. The plain polymers Pol0 and TCPS started to mineralize later than the composites. There was large variation in bone sialoprotein (BSP) and osteocalcin (OC) gene expression levels, with no clear differences between the materials. However, there was strong positive correlation between the expression of these two osteogenic markers within each substrate type. Conclusions: Inclusion of BAG to the co-polymer had only minor influence in the early cultures of bone marrow stromal cells. However, the mineralization of cultures was much faster with composites than with plain polymers. The SBF-treatment further expedited the onset of mineralization. The fastest and most reliable osteogenic differentiation was observed on the composites immersed in SBF for 5 days.

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