ICMCTF2011 Session D1-1: Bioactive and Biocompatible Coatings and Surface Functionalization of Biomaterials

High-density, high-purity ceramic materials (i.e. alumina and zirconia) have been investigated for load bearing prostheses and dental restorations because they exhibit excellent corrosion resistance, good biocompatibility, high wear resistance, and high strength. However, one inherent issue with using these high-strength oxides is their inert or non-reactive surface properties. Earlier research presented data of a promising surface pretreatment, whereby yttria-stabilized zirconia (YSZ) surfaces are converted to a more reactive oxyfluoride using gas-phase fluorination. It was shown that employing conventional silanation techniques displayed higher bond strengths when adhered to resin-based composites. The present investigation focuses on the surface modification, via the gas-phase fluorination process, of yttria-stabilized zirconia (YSZ) to increase its wettability and reactivity with arcylate-based resin cements. YSZ plates and cylinders, as-received and roughened, were pretreated in a fluorine-containing plasma and bonded with a commercially available resin cement for simple shear bond adhesion testing. Shear bond tests revealed that bond strength increased with treatment time. Moreover, the pretreated as-received specimen group displayed relatively high bond strengths suggesting surface reactivity and direct chemical bonding with the resin cement. Simple contact angle measurements revealed that a 2 minute exposure reduces the contact angle from ~56° to 6° suggesting that the surface energy has been altered creating a highly hydrophilic surface. X-ray photoelectron spectroscopy (XPS) analysis revealed the surface conversion layer to be a mixture of phases; zirconium oxyfluoride, zirconium fluoride, and yttrium fluoride. It is hypothesized that these new phases have the potential to increase hydroxylation at the surface, creating a more reactive surface, thus allowing for covalent bonding between surface and resin cement. It is believed that this surface treatment has broad reaching impact when using high strength ceramics in a multitude of bio-applications. This research was supported through RTI International research and development fund.

Embryonic stem cells enable pluripotency to replicate indefinitely, making themselves the most multifunctional stem cell than the other stem cells. However, it remains controversial due to ethic problem. Alternatively, human Adipose-derived stem cells (hADSCs) capable of differentiating into various lineages of osteoblast phenotype and adipocyte. It is therefore appealed a widespread attention in tissue engineering and life sciences.Recent researches indicated that the differentiation pathway of hADSCs can be determined by the stiffness (elasticity) of the cultured matrix. Therefore, in this study we evaluate the growth behavior and interaction of cultured hADSCs with various lengths of SiNWs chips fabricated by Electroless Metal Deposition (EMD) process.

Our experimental results showed that the length of SiNWs chips by EMD method are easily controlled due to its linear etch rate of 1.06 μm/min. After culturing ADSCs with various length SiNWs chips, the hADSC surface protrusion would be more active on short SiNWs (10 min treated); meanwhile, gene expression of ADSC shows positive results on short SiNWs, either. Therefore, it indicates that ADSCs prefer growing on short SiNWs chips which are strongly influenced by SiNWs stiffness (elasticity).

Synthesis of vertically aligned carbon nanotube arrays (VANTAs) using chemical vapor deposition (CVD) method has been studied using various different gas precursors such as methane, ethylene, ethane, etc. There are few studies where alcohol precursors are used for their synthesis, as well. In this study, we have synthesized VANTAs through alcohol chemical vapor deposition (ACVD) method using methanol, ethanol, isopropanol and acetone as carbon sources. During the synthesis a range of growth temperatures were used between 625 – 750^°C . The catalyst layers necessary for the nucleation of CNTs were prepared following the sandwich method through e-beam and thermal deposition of Al / Fe or Co / Al bilayers on a pre-oxidized Si (100) wafer. The change in the carbon source types and the growth temperatures resulted in a significant change in the length of CNTs. The highest array length (~200µm) was been achieved using acetone as the carbon source. TEM images of arrays indicated that arrays were consists of single, double or multi-walled CNTs with a diameter range of 5 to 7 nm. Besides the change of length with growth parameters, we have also observed a switch of contact angle of water measured on these arrays from super-hydrophobic to hydrophobic by synthesis temperature. At the high growth temperatures (> 750^°C ), the contact angle was almost 180^° and even after 20 minutes the angle was kept at 160^°. However, the behavior switched to a hydrophobic one (angle ~ 140^°) for CNT arrays grown at lower temperature (< 625^°C ). These results indicated that hydrophobicity can be modified by optimizing the growth parameters. We have also patterned and functionalized CNT arrays using water soluble conjugate polymers with positively and negatively charged end groups. We used the 2-D patterns formed on the CNT arrays after wetting by water as an extracellular support matrix for cell attachment. Finally, we seeded stem cells and cancer cells on the CNT arrays functionalized by various end groups to investigate the possibility of using CNT arrays as scaffold materials for biomaterials applications. The results from these experiments indicated that cells were well-adhered to these surfaces and no-adverse effects of CNTs were observed on the cells. All these results showed that VANTAs would be used as extracellular matrix for tissue engineering in future.

A variety of products promising antibacterial effects are commercially available nowadays. Silver (Ag) containing textiles, household equipment or wound healing products are especially widely present. Next to the question whether all the proposed applications are meaningful, it is important to ask if the antibacterial effectiveness of the products is always correctly adapted to their intended application. Indeed, the antibacterial properties have to be adapted to the usage conditions (environment) and duration, an aspect which often appears to be overlooked.

The combination of radiofrequency plasma-enhanced chemical vapor deposition (PECVD) and a physical vapor deposition processes (PVD) under low pressure conditions enables the production of Ag containing plasma polymer coatings. These Ag nanocomposites consist of an oxygen functional hydrocarbon matrix with embedded Ag nanoparticles. The polymer matrix is deposited by using an carbon dioxide (CO₂)/ ethylene (C₂H₄) mixture and the Ag nanoparticles are produced by the simultaneous sputtering of a Ag cathode in a one-step process.

Different degrees of the polymer matrix functionality were obtained by varying the CO₂/C₂H₄ ratio from 2:1 up to 6:1, the higher ratio leading to a higher content of oxygen functional groups in the matrix and a better wettability of the surface. The Ag content as well as the Ag particle size was adjusted by means of the power input and the gas ratio. Increasing both the power input and CO₂/C₂H₄ ratio results in higher Ag content. The size of the particles behaves differently. A higher CO₂/C₂H₄ ratio leads to smaller and more homogenously distributed nanoparticles whereas an increasing power input favours the formation of agglomerates, as observed by TEM analysis. A strong dependence of the Ag⁺ ion release on the incorporated Ag was measured in bi-destilled water with ICP-OES. These Ag nanocomposite thin films can be used for short term applications where an initial Ag release boost is required e.g. to avoid bacterial inflammation.

Controlling the Ag⁺ ion release over longer periods of time could be done by covering the Ag nanocomposite thin films with an additional Ag-free layer or with a layer containing a lower Ag amount. This approach enables the build-up of Ag reservoirs in the deeper lying coating regions, which are covered by cytocompatible polymer coatings that still allow Ag⁺ ion release. These gradients in the coatings can be produced by changing the gas mixture without interruption of the process. The coatings can be adjusted to be antibacterial over a broad range without losing their cytocompatible properties.