The role of silicon on the bioactivity of Skelite(TM) bioceramic: A material and biological characterization of silicon alpha-tricalcium phosphate based ceramics

Skelite(TM) bioceramics are novel synthetic skeletal replacement materials that participate in the full remodeling process of bone. Skelite contains a high fraction of Silicon Stabilized alpha-Tricalcium Phosphate (Si-TCP), a novel phase to which the unique bioactive properties of Skelite have been attributed. The role of Si in the development of the microporous, interconnected microstructure and mixed phase composition of Skelite was investigated using crystallization kinetics and defect characterization studies. The kinetics of the phase transformation to Si-TCP were studied using rapid thermal processing of thin films on quartz substrates. The results, interpreted using a novel Avrami model, show that Si acts as a nucleation agent for Si-TCP, and also that Si pins the microstructure of the films at higher concentrations. Characterization of defects induced by Si substitution into the phases of Skelite material utilized electron spin resonance (ESR) and thermoluminescence (TL) techniques. These results identify two unique paramagnetic defect centers associated with Si substitution in the hydroxyapatite lattice. Quantification of the relative level of these centers supports a novel chemical model that describes the development of the mixed phase system of Skelite as a function of silica addition.; The significance of the Si-TCP phase, sample morphology, and surface chemistry on the activity of osteoclast and osteoblast cells was investigated using cell culture and protein functionalized atomic force microscopy techniques. The biological characterization identifies three interaction mechanisms between Skelite and the biological system. Skelite releases a soluble molecular complex containing Si to the extracellular media, which has a significant bioactive effect on osteoclast and osteoblast growth and activity. Using protein functionalized atomic force microscopy the surface chemistry and reactivity of samples is shown to influence osteopontin affinity for Skelite surfaces with associated effects on osteoclast growth on the samples. Ultimately, it is the bulk acid solubility of the material that influences the efficiency of the osteoclastic resorption process.; The unique feature of Si-TCP is proposed to lie in its rapid reactivity in electrolytic media, resulting in a rapid conversion to hydroxyapatite or possibly a biologically equivalent hydroxyapatite in physiological media, with a corresponding release of bioactive aqueous Si to the extracellular environment.