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Abstract

With emerging need for orthopaedic implants and prolonged exposure of organism to implants also problems connected to elution of metal ions that in some cases cause adverse reactions in the human body, emerge. The main aim of our work was therefore to investigate the potential of silicon carbide (SiC) to be used instead of currently employed metallic implants.
Our work consisted of ceramics processing to produce green parts of high density, densification of produced parts and analyses of properties, regarding mechanical and magnetic properties as well as bioactivity, dissolution of ions and response of cells.
Throughout the whole work electrophoretically deposited samples were compared to dry pressed samples. To produce a bulk part with high green density, electrophoretic deposition was proven as more successful technique, for this reason suspension had to be optimized. We aimed at high zeta potential and moderate conductivity and samples of SiC without sintering additives with more than 62 % of theoretical density were prepared. Tetramethyl ammonium hydroxide was chosen as an efficient additive for anodic deposition while cetyltrimethyl ammonium bromide did not result in deposits with sufficient green density.
When sintering additives (i.e. carbon black and boron carbide or magnesium oxide) were added, suspensions had to be further optimized due to dissimilar colloidal behaviour of additives. In the case of boron carbide and carbon black, surface of the latter had to be made hydrophilic but conductivity of resulting suspension with additives was too high for a successful EPD. Therefore this set of samples was only dry pressed.
In the case of MgO ethanol had to be used for suspension preparation due to solubility of MgO in water. Cathodic deposition with addition of polyethyleneimine was employed and deposits with high green density were produced.
Samples were sintered in vacuum at temperatures above 2000 °C or in open air at 1350 °C and their properties were measured. Mechanical properties for samples with addition of boron carbide and carbon sintered in vacuum at 2200 °C exhibited flexural strength 220 MPa, elastic modulus 250 GPa and fracture toughness 3.9 MPa·m1/2 while these properties for air-sintered sample containing MgO were 180 MPa, 80 GPa and 2.9 MPa·m1/2 for flexural strength, elastic modulus and fracture toughness, respectively. It is important to note that these specimens have different porosity levels. Since trabecular bone is a composite material it will always have superior properties to bulk ceramics, namely tensile strength of trabecular bone is 90-140 MPa, elastic modulus 10-40 GPa and fracture toughness between 1 and 15 MPa·m1/2. The properties of our material are still not ideal, especially in terms of fracture toughness, but these properties can be further tailored and improved.
None of the SiC samples showed any notable response to the magnetic field which makes it an appropriate implant material regarding use of magnetic resonance imaging. Samples were immersed in physiological solution to measure the eluted elements. After 7 days silicon, magnesium and vanadium were detected.
Cell tests that were performed showed the most beneficial response towards samples containing MgO; cells attached on the surface and after first day 50 % of cells were
attached compared to the control (glass plate coated with polylysine). Overall, SiC can be considered a potential, though not ideal, material for orthopaedic implants.