ICMCTF2009 Session TS3-2: Bioactive Coatings and Surface Biofunctionalization

There has been much advancement in the development of ceramics for biomedical use, expanding the use of high strength materials (e.g. alumina and zirconia) in varying prosthetic and reconstructive applications. In dentistry, high strength ceramics have become a popular alternative to traditional porcelains, due to their superior fracture resistance and long-term viability. A major clinical problem with the use of indirectly placed alumina and zirconia restorations is the difficulty in achieving adequate bonding with the underlying substrate (tooth structure, implant abutment). Traditional adhesive bonding techniques used with silica-based ceramics like porcelain do not work effectively with these higher strength materials. Initial investigations focused on functionalizing alumina and zirconia surfaces with an ultra-thin Si-based layer (1-25 nm thick) allowing for silane treatments to tether hydro-carbons. These hydro-carbon groups are a common reactant with acrylic poly mers, allowing for strong covalent bonding between prosthesis and the underlying structure. As a proof of concept, several planar test structures, consisting of dental alumina and zirconia, were prepared, treated, and analyzed, before and after Si-O modification, using x-ray photoelectron spectroscopy (XPS). XPS studies indicated that the pretreatment process deposited less than five monolayers of Si_xO_y on the surfaces. A commercially available silane was then applied to the treated surfaces. High resolution core scans of the C(1s) and Si(2p) supports successful chemical bonding of the organo-silane to the Si-O treated surfaces. Mechanical property analysis, consisting of micro-tensile testing of zirconia bonded to a dental composite, revealed that pretreatment of bonding surfaces increased the failure strength over traditional bonding techniques.

Research was supported by NIH/NIDCR DE013511-09 and an internal research grant provided by RTI International.

Alpha alumina thin films are excellent candidates for implantation due to the material’s resistance to corrosion, wear, and protein adsorption. Applying such films to a suitable bulk material can be problematic however, as current methods reach high temperatures unsuitable for commonly used alloys or involve undesirable template materials such as chrome.

Thin films of reactively sputtered aluminum oxide were deposited at 480°C on Ti-6Al-4V ELI substrates with no template layer. Process parameters (including magnetron power, substrate biasing, process pressure, oxygen partial pressure, film thickness, and substrate orientation) were varied to investigate the influence of deposition conditions on film quality, crystallography, and mechanical and corrosion behaviors. Film phase has been determined by XRD. Corrosion behavior was assessed using electrochemical impedance experiments. Tribological testing of film hardness and wear resistance supports the XRD resul ts. A detailed discussion of the effect of varying deposition parameters on the properties and performance of the coatings will be presented as well as a comparison to previous literature results.

Superhydrophobic surfaces can be utilized to prevent cells proliferation that causes most of failures in biomedical devices. We have shown novel method to fabricate superhydrophobic surfaces based on 3-D hierarchical wrinkle structures combined with the replica molding to make the micro-scale pillar array on PDMS as the soft base material, covered by nanoscale wrinkle patterns formed with hydrophobic diamond-like carbon (DLC) using RF-CVD to make nanometer scale roughness. Nanoscale wrinkle patterns were controlled by varying the thickness of film, causing different pattern width.

Superhydrophobicity of the surfaces can be achieved when we have 3-D hierarchical structures on pillars with spacing ratio of 1 to 4. It was found that theoretical wetting analysis was well matched the experimental results that the wetting states are Cassie and Cassie for microscale roughness and nanoscale roughness, respectively (C^m-Cⁿ) at spacing ratio of 1 to 4, while transformed into Wenzel and Wenzel (W^m-Wⁿ) at spacing ratio higher than 5.

The superhydrophobic surface with hierarchical wrinkled patterns was applied for the cell template with the Calf Pulmonary Artery Endothelial (CPAE) cells. It was observed that the CPAE cells were very difficult to adhere to the 3-D wrinkled surface and the rejection of filopodia extension was observed on the protrusion of nanoscale wrinkles. Further more, as the spacing between the pillars close to 1, the filopodia spreading was highly hindered by the small focal adhesion point of the cells on hierarchical structures since the filopodia spread from top to top of the pillars. This limited focal point adhesion of the cells to superhydrophobic surfaces would prevent the growth and proliferation.

Parylene (or poly-para-xylene) coating known to be a transparent, uniform and effective barrier. There are however some drawbacks, including expensive starting material, high thermal energy consumption for monomer generation, high vacuum requirement and low growth rate. In this study, low-cost para-xylene was used as the starting material to form plasma-polymerized films. A pulsed-dc plasma power supply was used and the deposits were examined their microstructure, mechanical properties and fibroblast cytotoxicity.

Experimental results reveal that the coatings present an amorphous structure with their deposition rate widely ranging from 50 to 480 nm/h, depending on the pulse frequency (ω_p) of the input power and argon flow rate (f_Ar, para-xylene monomer carrier gas). At high f_Ar and low ω_p, the plasma-polymerized para-xylene (PPX) films exhibit alkane and alkene features, indicating a more organic character. In contrast, the films obtained at low f_Ar and high ω_p present inorganic features. For its mechanical properties, a pencil hardness of 7H-8H of the coated specimen is higher than that of the conventional parylene coating, and the film adhesion graded at 4B determined using the cross-cut test is similar to that of the conventional parylene coating. The water contact angle of PPX films was measured and ranged from 60° to 85°, which is more hydrophilic than the conventional parylene coating. Cell cultures on PPX deposited specimens present a higher cell count than the parylene deposited film. The PPX films deposited at high ω_p and low f_Ar exhibit a lower water contact angle (hydrophilic), which accounts for its better cell compatibility. These quantitative indications imply that the plasma-polymerized para-xylene is a possible alternative to the conventional parylene coating for biomedical device surface modification.

The β-titanium alloys have recently become important in bone implant applications. In this study, micro-arc oxidation (MAO) was used to modify the surface of β-titanium alloy (Ti-13Cr-3Al-1Fe) using NaH2PO4 solution as an electrolyte. The microstructure of the oxidized layer as a function of the applied voltage and treatment time were investigated. The cellular adhesion, proliferation and alkaline phosphatase (ALP) activity of MC3T3-E1 pre-osteoblast were measured to reveal the cell compatibility of the treated specimens.

The experimental results show that micro-structure of the oxidized layer can be greatly affected by the discharge voltage and treatment time during MAO treatment as α-titanium metal and α+β titanium are. The oxidized layer can gradually increase its thickness, pore size and surface roughness as a function of the discharge voltage and treatment time, which at the same time increases the rutile phase constituent. The osteoblast cell compatibility presented by the treated specimens is increased but less sensitive to the process parameters, even though the associated difference in morphology and crystalline form are found to be tremendous.