Effects of Silicon Compounds on Biomineralization, Osteogenesis, and Hard Tissue Formation

Abstract

Bioinspired stem cell-based hard tissue engineering includes numerous aspects: The synthesis and fabrication of appropriate scaffold materials, their analytical characterization, and guided osteogenesis using the sustained release of osteoinducing and/or osteoconducting drugs for mesenchymal stem cell differentiation, growth, and proliferation. Here, the effect of silicon- and silicate-containing materials on osteogenesis at the molecular level has been a particular focus within the last decade. This review summarizes recently published scientific results, including material developments and analysis, with a special focus on silicon hybrid bone composites. First, the sources, bioavailability, and functions of silicon on various tissues are discussed. The second focus is on the effects of calcium-silicate biomineralization and corresponding analytical methods in investigating osteogenesis and bone formation. Finally, recent developments in the manufacturing of Si-containing scaffolds are discussed, including in vitro and in vivo studies, as well as recently filed patents that focus on the influence of silicon on hard tissue formation.

Keywords: alveolar bone; biomineralization; osteoblast; osteogenesis; scaffold; silicates; silicon; stem cells; tissue engineering.

Figure 1

Setup of wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS).

Figure 2

SAXS setup: Transmission (T)-SAXS and gracing incidence (GI)-SAXS measurement setup.

Figure 3

Characterization of materials. (a) XRD analysis showing that both Si-free and Si-doped HA groups exhibited the main diffraction peaks of HA in comparison to the HA standard card (JCPDS-PDF 09-0432). (b) FTIR analysis indicating that each group of template-induced materials displayed the typical phosphate group and main functional groups of amides I and II due to the presence of organic protein templates. (c) Thermogravimetric (TGA) analysis confirming that the template-induced materials were organic–inorganic composites consisting of 20%–30% organics and 70%–80% inorganics. (d) Si-molybdenum blue spectrophotometry confirming that the actual value of silicon content was quite close to the theory value. (e,f) X-ray photoelectron spectroscopy (XPS) analysis showing the typical peaks of Ca and P in a Si–O group and the typical peaks of Si in Si-doped groups. The mole ratio of n(Ca)/n(P) or n(Ca)/[n(P) + n(Si)] of the sample was close to the theoretical value of HA (n(Ca)/n(P) = 1.67). Reprinted from [140], with permission from Springer Nature, 2017.

Figure 4

Five percent calcium silicate (CS)/β-TCP scaffolds stimulated human bone marrow stromal cell (hBMSC) osteogenesis and human umbilical cord vein endothelial cell (HUVEC) angiogenesis in vitro. (A) Alkaline phosphatase (ALP) staining in hBMSCs cultured for 7 and 14 days with β-TCP scaffolds or 5% CS/β-TCP scaffolds in transwell inserts. (B) 5% CS/β-TCP scaffolds stimulated ALP activity at 7 days and 14 days in comparison to pure β-TCP. (C) Schematic representation of transwell experiments. Scaffolds in transwell inserts (upper chamber), cells in the lower chamber. (D) Tube formation by HUVECs, as observed by light microscopy (top) and calcein acetoxymethyl (AM) staining (bottom), after 4 h on Matrigel with β-TCP or 5% CS/β-TCP scaffolds in transwell inserts; (E) 5% CS/β-TCP scaffolds enhanced tube formation in comparison to β-TCP scaffolds. * p < 0.05; ** p < 0.01. Scale bar represents 10 μm. Reprinted from [161], published under a Creative Commons Attribution 4.0 International License, 2017.