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

Abstract 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 100 mol% by substituting the CaCl 2 -stock solution (0.1 M) with SrCl 2 (0.1 M) 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, Sr 6 H 3 (PO 4 ) 5  ∗ 2 H 2 O, CS) and highly crystalline strontium hydroxyapatite (Sr 5 (PO 4 ) 3 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 2 + ) were released in a sustained manner from the modified scaffolds, with a clear correlation between the released Sr 2 + concentration and the degree of Sr-substitution. The accumulated specific Sr 2 + release over the course of 28 days reached 141.2 μg (~ 27 μg mg − 1 ) from Sr50 and 266.1 μg (~ 35 μg mg − 1 ) from Sr100, respectively. Under cell culture conditions this led to maximum Sr 2 + concentrations of 0.41 mM (Sr50) and 0.73 mM (Sr100) measured on day 1, which declined to 0.08 mM and 0.16 mM, respectively, at day 28. Since Sr 2 + 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.