Strontium-modification of porous scaffolds from mineralized collagen for potential use in bone defect therapy.

The present study describes the development and characterization of strontium(II)-modified biomimetic scaffolds based on mineralized collagen type I as potential biomaterial for the local treatment of defects in systemically impaired (e.g. osteoporotic) bone. In contrast to already described collagen/hydroxyapatite nanocomposites calcium was substituted with strontium to the extent of 25, 50, 75 and 100mol% by substituting the CaCl-stock solution (0.1M) with SrCl (0.1M) during the scaffold synthesis. Simultaneous fibrillation and mineralization of collagen led to the formation of collagen-mineral nanocomposites with mineral phases shifting from nanocrystalline hydroxyapatite (Sr0) over poorly crystalline Sr-rich phases towards a mixed mineral phase (Sr100), consisting of an amorphous strontium phosphate (identified as Collin's salt, SrH(PO)∗2 HO, CS) and highly crystalline strontium hydroxyapatite (Sr(PO)OH, SrHA). The formed mineral phases were characterized by transmission electron microscopy (TEM) and RAMAN spectroscopy. All collagen/mineral nanocomposites with graded strontium content were processed to scaffolds exhibiting an interconnected porosity suitable for homogenous cell seeding in vitro. Strontium ions (Sr) were released in a sustained manner from the modified scaffolds, with a clear correlation between the released Sr concentration and the degree of Sr-substitution. The accumulated specific Sr release over the course of 28days reached 141.2μg (~27μgmg) from Sr50 and 266.1μg (~35μgmg) from Sr100, respectively. Under cell culture conditions this led to maximum Sr concentrations of 0.41mM (Sr50) and 0.73mM (Sr100) measured on day 1, which declined to 0.08mM and 0.16mM, respectively, at day 28. Since Sr concentrations in this range are known to have an osteo-anabolic effect, these scaffolds are promising biomaterials for the clinical treatment of defects in systemically impaired bone.