Bioactive and biodegradable silica biomaterial for bone regeneration

Biosilica, a biocompatible, natural inorganic polymer that is formed by an enzymatic, silicatein-mediated reaction in siliceous sponges to build up their inorganic skeleton, has been shown to be morphogenetically active and to induce mineralization of human osteoblast-like cells (SaOS-2) in vitro. In the present study, we prepared beads (microspheres) by encapsulation of beta-tricalciumphosphate [beta-TCP], either alone (control) or supplemented with silica or silicatein, into the biodegradable copolymer poly(D,L-lactide-co-glycolide) [PLGA]. Under the conditions used, approximate to 5% beta-TCP, approximate to 9% silica, and 0.32 mu g/mg of silicatein were entrapped into the PLGA microspheres (diameter approximate to 800 mu m). Determination of the biocompatibility of the beta-TCP microspheres, supplemented with silica or silicatein, revealed no toxicity in the MTT based cell viability assay using SaOS-2 cells. The adherence of SaOS-2 cells to the surface of silica-containing microspheres was higher than for microspheres, containing only beta-TCP. In addition, the silica-containing beta-TCP microspheres and even more pronounced, a 1:1 mixture of microspheres containing beta-TCP and silica, and beta-TCP and silicatein, were found to strongly enhance the mineral deposition by SaOS-2 cells. Using these microspheres, first animal experiments with silica/biosilica were performed in female, adult New Zealand White rabbits to study the effect of the inorganic polymer on bone regeneration in vivo. The microspheres were implanted into 5 mm thick holes, drilled into the femur of the animals, applying a bilateral comparison study design (3 test groups with 4-8 animals each). The control implant on one of the two hind legs contained microspheres with only beta-TCP, while the test implant on the corresponding leg consisted either of microspheres containing beta-TCP and silica, or a 1:1 mixture of microspheres, supplemented with beta-TCP and silica, and beta-TCP and silicatein. The results revealed that tissue/bone sections of silica containing implants and implants, composed of a 1:1 mixture of silica-containing microspheres and silicatein-containing microspheres, show an enhanced regeneration of bone tissue around the microspheres, compared to the control implants containing only beta-TCP. The formation of new bone induced by the microspheres is also evident from measurements of the stiffness/reduced Young's modulus of the regenerated bone tissue. The reduced Young's modulus of the regenerating bone tissue around the implants was markedly higher for the silica-containing microspheres (1.1 MPa), and even more for the 1:1 mixture of the silica- and silicatein-containing microspheres (1.4 MPa), compared to the beta-TCP microsphere controls (0.4 MPa). We propose that based on their morphogenetic activity on bone-forming cells in vitro and the results of the animal experiments presented here, silica/biosilica-based scaffolds are promising materials for bone repair/regeneration. (C) 2014 Elsevier Inc. All rights reserved.