Osteogenic differentiation of hBMSCs on porous photo-crosslinked poly(trimethylene carbonate) and nano-hydroxyapatite composites

Large bone defects are challenging to repair and novel implantable materials are needed to aid in their reconstruction. Research in the past years has proven the beneficial effect of porosity in an implant on osteogenesis in vivo. Building on this research we report here on porous composites based on photo-crosslinked poly(trimethylene carbonate) and nano-hydroxyapatite. These composites were prepared by a temperature induced phase separation of poly(trimethylene carbonate) macromers from solution in ethylene carbonate. By controlling the ethylene carbonate content in viscous dispersions of nano-hydroxyapatite in poly(trimethylene carbonate) macromer solutions, composites with 40 wt% nano-hydroxyapatite and 27 to 71% porosity were prepared. The surface structure of these porous composites was affected by their porosity and their topography became dominated by deep micro-pore channels with the majority of pore widths below 20 µm and rougher surfaces on the nano-scale. The stiffness and toughness of the composites decreased with increasing porosity from 67 to 3.5 MPa and 263 to 2.2 N/mm2, respectively. In cell culture experiments, human bone marrow mesenchymal stem cells proliferated well on the composites irrespective of their porosity. Furthermore, differentiation of the cells was demonstrated by determination of ALP activity and calcium production. The extent of differentiation was affected by the porosity of the films, offering a reduced mechanical incentive for osteogenic differentiation at higher porosities with topographies likely offering a reduced possibility for cells to aggregate and to elongate into morphologies favourable for osteogenic differentiation. This ultimately resulted in a 3-fold reduction of calcium production of the differentiated cells on composites with 71% porosity compared to those on composites with 27% porosity.