ICMCTF2010 Session TS3-1: Bioactive Coatings and Surface Biofunctionalization

Implants and tissue substitutes are made from (bio)materials that have one common property, i.e. biocompatibility. A promising application of nanotechnology is the development of better functioning biomaterials, as e.g. applied in craniofacial surgery for the replacement of lost teeth, bone and temperomandibular joints. The currently available biomaterials as used for the manufacturing of implants and tissue substitutes do not fulfill completely to its intended function. This is due to an un-natural response between the biomaterial and the surrounding tissue cells.

A recent approach to the design of next-generation tissue regeneration supporting biomaterials is focusing on the more dimensionally intricate characteristics of surfaces, i.e. structure at the nanometer scale. The underlying hypothesis is that nanometer structure matches with the natural extracellular matrix resulting in an improved interaction of the tissue-forming cells compared with conventional biomaterials. Recent developments in the field of nanotechnology offer powerful tools to modify the surface of biomaterials by introducing artificial topography and specific surface chemistry on the material. It is well-known that both topography and surface chemical composition affect the reactions of the biological environment to the device.

The current lecture will deliver the relevant knowledge of the biomaterial surface parameters that control the biological response and can be used for implant surface functionalization.

Nanocomposite coatings including CrN/Ag, ZrN/Ag, TiN/Ag and CrN/Cu w ith varying silver or copper contents were produced by co-deposition in a dual pulsed magnetron sputtering system. The composition and structure of the coatings were characterised using energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM) and X-ray diffraction (XRD), and the physical and tribological properties were assessed by means of nanoindentation, scratch adhesion testing and thrust washer wear testing. Although increasing silver content provided a reduction in the coefficient of friction, this was accompanied by reductions in hardness and wear resistance. Zones of inhibition were used to determine the extent of silver ion release from the coating surfaces, and a NBT (nitro-blue tetrazolium) redox dye was used to determine the anti - microbial effectiveness of the coatings following incubation. The microorganisms tested were Pseudomonas aeruginosa, Escherichia coli (E. coli) and Staphylococcus aureus. For the NBT assays, significant reductions in the number of viable cells were observed with increasing Ag or Cu content, compared to the ‘pure’ nitride surfaces. Whilst no zones of inhibition were observed for S. aureus, on any of the surfaces, the diameter of the ‘kill’ zones generally increased with increasing silver content for the other microorganisms.

A contributing factor to orthopaedic implant and dental prosthesis failure is inadequate bonding between the device and underlying structure. Resin cements and adhesives have long been developed to increase bond strength through specific chemical bonding. More recently, there has been a significant push to use high strength ceramics (i.e. zirconia and alumina), but because of the non-reactive nature of such materials, a long-term, reliable chemical bond is difficult to achieve. An earlier study presented a promising surface-modification technology, whereby a silica-like thin film (<10nm) can be deposited on the surface thus enabling covalent bonding with conventional cements and silanation techniques [1]. A point of concern for such a thin coating is optimizing the coverage and correlating it with surface roughness. It was thus hypothesized that a surface with near 100 percent coverage would be optimal for bonding improvement. This study investigated the effect of silica coating thickness and post deposition treatments on the bonding structure and coverage of zirconia ceramics. X-ray photoelectron spectroscopy (XPS) was used to evaluate the chemical binding of the surface coating and atomic force microscopy (AFM) was used to analysis surface roughness. A novel surface modeling algorithm was also used to determine the percent surface coverage of the functional molecules in the context of a rough, non-planar surface. Initial data indicate that the chemical structure of the Si_xO_y surface modifications is thickness dependant and stochiometry may be modified via the aforementioned post-treatment. These data help to explain the earlier reported response to film thickness on zirconia-to-composite adhesion and enable the development of a more robust thickness and roughness independent pretreatment.

1. J.R. Piascik, E.J. Swift, J.Y. Thompson, S. Grego, BR Stoner, “Surface modification for enhanced silanation of zirconia ceramics”, Dent. Mat. 25 (2009) 1116-21.

TiO₂ thin films have been deposited at different O₂ / (Ar + O₂) gas ratios (0.2, 0.3, 0.5 and 0.6) by R.F reactive magnetron sputtering at a constant power of 200 watt. The formation of TiO₂ was confirmed by X-Ray Photoelectron Spectroscopy (XPS). The oxygen percentage in the films was found to increase with increase in oxygen partial pressure during deposition. Band gap of the films were calculated from the UV-Visible transmittance spectra. Increase in oxygen content in the films showed substantial increase in optical band gap from 2.2 eV to 3.6 eV. Optical contact angle and surface free energy of the films was found to vary with oxygen partial pressure. The films turned hydrophobic to hydrophilic (from 100.8⁰ to 48⁰) due to decrease in oxygen partial pressure during deposition. The surface free energy increased from 22.15 mN/metre to 56.36 mN/metre with the decreasing O₂ partial pressure. The bactericidal efficiency of the deposited films was investigated using Escherichia coli cells under 1hr UV irradiation. The growth of E coli cells was estimated through the Optical Density measurement by UV-Visible absorbance spectra. The qualitative analysis of the bactericidal efficiency of the deposited films after UV irradiation was observed through SEM. A correlation between the surface free energy, optical band gap and bactericidal efficiency of the TiO₂ films at different oxygen partial pressure have been studied.