Abstract
Introduction
Silicon nitride (Si3N4) is a ceramic material presently implanted during spine surgery. It has a fortunate combination of material properties such as high strength and fracture toughness, inherent phase stability, scratch resistance, low wear, biocompatibility, hydrophilic behavior, easier radiographic imaging and resistance to bacterial biofilm formation, all of which make it an attractive choice for orthopaedic applications beyond spine surgery. Unlike oxide ceramics, (e.g., alumina or Al2O3) the surface chemistry and topography of Si3N4 can be precisely engineered to address in vivo demands. Si3N4 can be manufactured to have an ultra-smooth, or highly fibrous, or porous morphology. Its chemistry can be varied from that of a silica-like surface composed of silanol moieties to one which is predominately comprised of silicon-amine functional groups.
Methods
In the present study, a Si3N4 bioceramic formulation was exposed to thermal, chemical, and mechanical treatments in order to induce changes in surface composition and features. The treatments included grinding and polishing, etching in hydrofluoric acid solution, and heating in nitrogen or air. Resulting surfaces were characterized using a variety of microscopy techniques to assess morphology. Surface chemical and phase composition were determined using x-ray photoelectron and Raman spectroscopy, respectively. Streaming potential measurements evaluated surface charging, and sessile water drop techniques assessed wetting behavior.
Results
Induced changes to surface morphology and wetting behavior are shown in Figure 1. A wide variety of wetting behavior was observed, ranging from moderate hydrophilicity (θ ∼60°) in the case of untreated surfaces to extreme hydrophilicity (θ <10°) in the case of surfaces subjected to heat treatments in different atmospheres. Figure 2 shows the zeta potential as a function of solution pH for the surfaces shown in Figure 1. All samples exhibit strong negative surface charging at homeostatic pH (−40 mV or more), and the oxidized sample exhibits extremely strong charging (−75 mV).
Conclusions
Our data show that Si3N4 is a facile biomaterial whose material properties can be engineered and optimized for specific applications. This work provides a basis for future in vitro and in vivo studies which will examine the effects of these treatments on important orthopaedic properties such as friction, wear, protein adsorption, bacteriostasis and osseointegration.
