ICMCTF2011 Session D3: Coatings for Mitigating Bio-Corrosion, Tribo-Corrosion and Bio-Fouling

Tuesday, May 3, 2011 1:30 PM in Room Royal Palm 4-6

Tuesday Afternoon

Start	Invited?	Item
1:30 PM	Invited	D3-1 Metal Oxide Coatings for Dental Implants: What is Important? Perla Rojas, Giovanni Ramírez, Argelia Almaguer (Universidad Nacional Autonoma de Mexico); Rene Olivares-Navarrete (Georgia Institute of Technology); Phaedra Silva-Bermudez, Stephen Muhl, Sandra Rodil (Universidad Nacional Autonoma de Mexico) Metal alloys are widely used in biomedical devices and components, especially as hard tissue replacements as well as in cardiac and cardiovascular applications, because of their desirable properties, such as relatively low modulus, good fatigue strength, toughness, formability and machinability and bioinertia. However, they are still far from the ideal biomaterial. Some of the issues that need to be improved are corrosion resistance in body fluids, improving the cell-surface interactions in order to promote a specific cell response (bioactivity instead of bioinertia), such as, bone-growth or antibacterial properties. Similarly, for some applications the tribological response needs to be improved, reducing friction coefficients and increasing the wear resistance. Remarkably, all these requirements are associated to surface-related-properties instead of bulk properties. Therefore a recent proposal has been the development of different surface treatments for the currently used metallic alloys. One popular method of surface modification involves the deposition of coatings or thin films, which allows the selective modification of the surface-mechanical and biological properties of conventional materials retaining their bulk properties. Obviously, the purpose of the coating material is to improve all the fails of the metallic implants, otherwise the costs will not be attractive. It means that the coatings have to be smartly engineered to fulfill the different requirements. The actual development of multilayered or nanocomposite coatings is an attractive method to obtain surfaces with multifunctional properties. In this review, we present our research work on sputtering deposition of metal-transition oxide films on stainless steel substrates as candidates for dental implants. Titanium, zirconium, tantalum and niobium oxides were produced and their corrosion resistance has been evaluated using potentyodynamic polarization and electrochemical impedance spectroscopy using physiological solutions. Similarly, the oral bacterial attachment and biofilm formation on the different surfaces was studied, as well as the effect of the bacterial attachment on the electrochemical response. The biocompatibility of the coatings have also been evaluated directly in-vitro using osteoblast-like cells or indirectly by studying the protein adsorption on the surfaces, which dictates the later cell-surface interactions. Moreover, the effect of the film thickness, structure, surface energy and topography on the functional response has also been analyzed in order to determine what is important for the future development of biomedical coatings.
2:10 PM		D3-3 Synthesis, Characterization and Performance of Silver Nanoparticle Coatings on Bioimplants Christos Takoudis (University of Illinois at Chicago) In 2003, 220,000 Total Hip Replacements (THR) were performed and this figure is expected to rise. Of the surgeries performed, 8% of the THR’s resulted in infections and required revisions. Biofilms are matrices of bacteria enveloped in proteins and polysaccharides which evade the immune system. The goal of this work is to inhibit bacterial colonization with a silver nanoparticle infused linseed oil coating on Ti90Al6V4 alloy which could prevent biofilm formation and avert infection. Silver nanoparticles with diameters less than 30 nm are believed to be non-toxic to living tissues. In this project, silver nanoparticles are formed by means of the autoxidation process that occurs during the drying of the linseed oil rich in polyunsaturated fatty acids. The abstraction of hydrogen from the fatty acid chains creates free radicals that react with silver benzoate to release silver nanoparticles. The confirmation of nanoparticle formation and range of diameters are determined with transmission electron microscopy. Three groups consisting of coated Ti90Al6V4 pins, linseed oil-coated Ti90Al6V4 pins and a control group consisting of uncoated Ti90Al6V4 pins are exposed to Staphylococcus aureus and Escherichia coli, respectively. The bacterial growth in these three groups is quantified at 8, 24 and 48 hours by phase contrast microscopy. Furthermore, coated pins are stored and continually tested for long term stability during three weeks of experimentation. Our results seem to support the infection-free properties of the silver nanoparticle coating. Over the course of 48 hours exposure to bacterial growth, the silver nanoparticle coated pins are found to contain the least amount of bacteria out of the three groups proving the bactericidal effects of the silver nanoparticle coating.
2:30 PM		D3-5 Tribo-Corrosion Mechanisms of Ti Based PVD Coatings on Y-TZP Dental Implants Margaret Stack (University of Strathclyde, UK); Wang-Long Li (National Cheng Kung University, Taiwan) In studies of tribo-corrosion of dental replacement materials, there have been few evaluations of the performance of coatings. This is despite the fact that coatings may provide a significant opportunity to reduce wear in such environments. In addition, there have been few studies of the characterization of mechanisms of tribo-corrosion in such conditions for coated systems. Recent work on Y-TZP materials (a dental replacement material) has shown that the effect of pH and exposure time may change the erosion-corrosion mechanism on the surface. Wear and phase transformation maps have been generated for the material. The results have identified that at low pH values, and longer exposure times, the wear rate significantly decreases in such conditions. In this work, a range of Ti based PVD coatings on Y-TZP were evaluated under combined erosion-corrosion conditions. Wear maps were produced using the results. The maps indicate the mechanisms of wear and the conditions under which the coatings may modify the wear rate in such environments.
2:50 PM	Invited	D3-7 Biotribolayer Formation in Metal-on-Metal Hip Prostheses- a Beneficial Coating Process? Markus Wimmer, Mathew Mathew, M.P. Laurent (Rush University Medical Center); Alfons Fischer (University of Duisburg-Essen); J. Jacobs (Rush University Medical Center) One of the bearing combinations for total hip replacement is a self-mating metal-on-metal (MoM) articulation based on cobalt-chromium-molybdenum alloy. Wear of these implants is of concern, because they are intended to stay a life-time in the patient’s body Wear particles can cause detrimental biological effects if released in too high quantities into the tissues. Studies have suggested that among the major wear mechanisms of MoM joints tribochemical reactions (TCR) are predominant. These TCRs result in the reaction of the metal surface with the interfacial medium (i.e., synovial fluid), and form a tribomaterial at the surface. The tribomaterial consists of a mixture of nanocrystalline ceramic and metallic constituents together with organic matter and forms a patchy coating on the surface. While this mechanism has shown advantageous effects on MoM wear, its corrosive (and tribocorrosive) behavior is unknown. In one of the studies, three different surface conditions were electrochemically investigated in a protein containing solution mimicking synovial fluid: (i) new, out of box, (ii) retrieved MoM head with tribomaterial, (iii) retrieved MoM head after enzymatic digestion of the tribomaterial. Because the number of retrievals was limited, we substantiated the study using samples with artificially prepared tribomaterial. Its nanocrystalline structure was verified using TEM before the experimental investigation. Results depicted a shift in the corrosion potential to higher anodic potential of the MoM samples with tribomaterial, suggesting a more noble behavior with better corrosion resistance than samples without tribomaterial coverage. However, the capacitive behavior of the tribomaterial under impedance measurements was inferior to samples without coverage. It was concluded that, in addition to being protective, the tribomaterial delays the corrosion kinetics and protective oxide films are generated slower than on samples without tribomaterial. This is an important finding because the tribomaterial is at risk of being abraded during articulation. Therefore, the talk will highlight the need of further investigations to better understand the formation and retention of this natural, dynamic coating layer. Further, we will look into necessary studies to better understand the role of the tribolayer on corrosive and tribocorrosive challenges in vivo.
3:30 PM		D3-10 Fabrication of Superhydrophobic Surfaces on Stainless Steel Substrates for Potential Biomedical Applications Samuel Beckford, Min Zou (University of Arkansas) The benefits of superhydrophobic surfaces are commonly seen in the biomedical field. For example, bacterial adhesion to biomedical implant surfaces can be greatly reduced if the surface is superhydrophobic. Since the superhydrophobicity of water repellant leaves in nature is often attributed to their microstructures combined with the presence of water repellant waxy nanocrystals on these microstructures, mimicking nature to create micro-/nano-scale topography and chemistry on material surfaces to render superhydrophobic surfaces has attracted much attention. This poster reports the fabrication of micro and nano-textured surfaces on stainless steel surfaces to produce superhydrophobic surfaces for possible use in biomedical applications. Various fabrication techniques, including sandblasting, thermal evaporation, aluminum induced crystallization (AIC) of amorphous silicon (a-Si), and deep reactive ion etching C₄F₈ passivation, were investigated to change the topography as well as chemistry of the stainless steel surfaces. The topographies resulting from these surface modifications were analyzed through scanning electron microscopy and surface profilometry. The wetting properties of these surfaces were characterized by water contact angle measurement. This study determined that the most effective process for creating superhydrophobic surfaces is to perform AIC of a-Si on plain stainless steel, leading to a water contact angle of more than 160°. The results of this study also showed that sharp nano textures fabricated by AIC of a-Si produced higher water contact angles than smooth nano textures produced by thermal evaporation.
3:50 PM		D3-11 What is the Role of Lipopolysaccharide (LPS) on the Tribocorrosive Behavior of Titanium in Dentistry? Valentim Barao (Univ Estadual Paulista, Brazil); Mathew Mathew (Rush University Medical Center); Judy Yuan (University of Illinois at Chicago); Wirley Assuncao (Univ Estadual Paulista, Brazil); Markus Wimmer (Rush University Medical Center); Cortino Sukotjo (University of Illinois at Chicago) Titanium has been widely used to fabricate dental implants due to its resistance to corrosion and biocompatibility. However, during mastication, implants are exposed to mechanical, chemical and microbiological actions, which can be considered a continuous complex degradation process. Studies have looked at chemical corrosion of titanium; however, very few have reported on the effect of combined chemical, mechanical and microbiological actions, which resemble the oral environment. A new multi-disciplinary research area, tribocorrosion, can address such issues. This study aimed to investigate the tribocorrosive nature of titanium in artificial saliva (pH 6.5) with the presence of Escherichia coli lipopolysaccharide (LPS). A total of 24 titanium discs, 12-mm diameter and 7-mm thickness, were used. Samples were divided into 8 groups (n=3) as a function of different titanium type (commercially-pure titanium and Ti-6Al-4V alloy) and LPS concentration (0, 0.15, 15 and 150µg/ml).The sliding duration (2000 cycles), frequency (1.2Hz) and load (20N) parameters were selected to simulate the oral environment and mastication process. Electrochemical impedance spectroscopy was conducted before and after tribocorrosion to comprehend the changes in the corrosion kinetics. Worn surfaces were examined using white-light-interferometry microscopy and scanning electron microscopy. Total weight loss (K_wc) and roughness values were calculated. Data were analyzed using one-way ANOVA, and Tukey’s HSD and paired-T tests were used where appropriate (α=.05). LPS influences the tribocorrosive behavior of both titanium types. LPS statistically accelerated the ions exchange between titanium and saliva, and reduced the resistance of titanium surface against corrosion (p<.05). Sliding tends to decrease the protectiveness of titanium surface against corrosion. In general, Ti-6Al-4V alloy exhibited better corrosion behavior. Both titanium types showed similar K_wc values (p>.05). LPS increased the weight loss of commercially-pure titanium (p=.041), and the roughness values (p<0.001). The study clearly indicates that LPS negatively affect the corrosion/wear behavior of titanium, which might increase the failure rate of dental implants.
4:10 PM		D3-9 Bio-Tribological Properties of UHMWPE Against Surface Modified Titanium Alloy Dangsheng Xiong, Lei Xiong (Nanjing University of Science and Technology, China) Wear is primarily cause of failure of artificial joints. In this study, two–step plasma immersion ion implantation technique, thermal oxidation (TO) treatment, laser surface texturing (LST) treatment and TO+LST treatment were developed and applied for modification of Ti6Al4V alloy. The surface layer structures and the wettability of the modified alloys were investigated. A pin-on-disc tribometer has been used to evaluate the tribological response of UHMWPE to untreated, two–step ion implanted, TO treated, LST treated and TO+LST treated Ti₆Al₄V counterfaces under distilled water and serum lubricated. The result showed that the after the ion implanted or TO-Treated, the TiO,Ti₂O film was formed in the subsurface of Ti₆Al₄V alloys. The wettability of the modified alloy surfaces were increased significantly. The friction coefficients and the wear rates of UHMWPE against the modified Ti₆Al₄V alloy decreased significantly.