Designed Biosilica and Collagen-Coated Carrier to Deliver Growth Factor for Bone Regeneration

Since the silica surface is generally composed of hydroxyl groups and ether bonds, it is considered to have a negatively charged surface that readily adsorbs oppositely charged molecules. The heparin-binding domain (HBD), mainly composed of positively charged residues, is present in many growth factors (GFs). It plays an essential role in the binding with the anionic sulfate of heparin/heparan sulfate. Based on this, GFs containing HBD may have an affinity for the anionic silica surface. We hypothesized that the silica coating would increase the association for the growth factors. To identify this, hydroxyapatite, currently used as a bone graft material and BMP2 carrier, was coated with bio-silica on the surface using a silica-forming peptide. By atomic force microscopy, we measured the binding force between GF, BMP2, and the silica surface, demonstrating that the biosilica surface showed an 8-fold higher binding strength than the HA alone. While the loading efficiency was increased due to the strong binding force, there was a problem in that the release rate was low. For effective delivery of GF by increasing diffusion rate, collagen was introduced. HA was coated with collagen and biosilica layer by layer according to collagen concentration and the number of coating layers. In the final biosilica-coated surface, the diffusion rate of two successive coatings of collagen and biosilica was higher than the one-time coating. Meanwhile, triple layers lowered the loading efficiency (LE%) and diffusion rate than the former coating. Double successive coatings of collagen and biosilica with 0.01% collagen displayed the highest LE% and diffusion rate. Hence, this formulation was selected as an optimal carrier for GFs. The carrier above demonstrated increases in cell proliferation, alkaline phosphatase activity, osteogenic gene expression, and calcium mineral precipitation in the osteogenesis of MC3T3 E1 cells compared to those of HA and one-time biosilica-coated HA. Therefore, the biosilica/collagen-coated HA can be an effective carrier for GF-employed therapy in bone regeneration.