ICMCTF2011 Session D2: Coatings for Biomedical Implants

In an articulating joint simulator, we analyzed the wear behavior of DLC coated CoCrMo implant pairs up to 80 million loading cycles in simulated body fluid. The results indicate that low wear and lifetime performance may be obtained in-vivo, whenever the interfaces and interlayers between the coating and the implant are stable under biological conditions. Especially the only a few nm thick carbidic reaction layer between the DLC and the CoCrMo implant has a crucial influence on long term adhesion. We will show an XPS analysis of the stoichiometry of these reactively formed interfaces. The slow advancement of cracks in this carbidic interface layer was controlled by the laws of stress corrosion cracking, in highly stressed coatings when exposed to a corrosive liquid such as body fluid. We will also present analysis of in-vivo failed DLC coated implants and show that the primary cause of failure was mainly a slow corrosive attack of the interlayers by the body fluid.

In recent years there has been a great deal of research in many laboratories worldwide on the properties of carbon layers which have shown their potential usefulness in medicine. Various medical implants are covered by carbon coating which creates a barrier between the implant and the tissue environment. Studied of diamondlike (DLC) films have proved that such films show high biocompatibility. The biocompatibility of DLC is due to their chemical stability, biostability, and appropriate mechanical and adhesive characteristics of the implant/layer system.

However DLC are not specially applicable in the medical implants, with blood contact. DLC are generally thought to be chemically inert but their biological activity has also been reported.

So, better results has been reported with nanocrystalline diamond (NCD) coating. In this paper the results of experimental studies of nanocrystalline diamond coatings obtained by a method of RF plasma CVD are presented. Nanocrystalline diamond coatings have a thickness of about micrometer and are composed of diamond. They are created from crystallites of the order of few nanometer in size, so they are referred to as nanocrystalline diamond.

They were reported to decrease the level of induced hemolysis and to inhibit lipid peroxidation in blood plasma. NCD Nanocrystalline Diamond Coating (NCD) was found to have positive effects on cells and tissues in the human organism with little adverse reactions. Antiinflammatory action of NCD has been reported.

At present examinations are assigned to acquiring new properties of NCD through their chemical modification which is creating covalent bond with organic moieties. Comparison of different functional groups obtained nanodiamonds depending on their origin causes their different behaviour in biological environment. Therefore examining functional groups of carbon powders coming from technological various processes, also will be recommended of the ones commercially available, next modifying through Fenton treatment, showing changes of functional groups after the reaction and of changes created after the application in the living cell.

The chemistry of the surface and near surface region is well known to control the biocompatibility of materials intended for in-vitro use. The distribution of the active substance within polymer film coatings of drug eluting stents (DES) has been the focus of considerable research as the controlled release of the drug over extended periods is of great importance in the functioning of the device. This work presents x-ray photoelectron spectroscopy (XPS) concentration depth profiles using a cluster ion source of flat film models of DES devices containing a drug and poly(l-lactic acid) (PLA) spun cast onto cleaned silicon wafers. Samples were initially characterised using spectroscopic ellipsometry and determined to be ~100nm thick. XPS sputter profiling using a coronene (C₂₄H₁₂⁺) primary ion source has provided quantitative elemental distribution as a function of depth through the polymer film whist retaining the chemical information. Results indicate that there is a depletion of codeine from the surface with segregation to the bulk of the sample rather than a uniform distribution of the bulk drug loading for these model systems. Parameters known to affect cluster ion depth profiles of DES model systems, including sample temperature and drug loading within the polymer matrix have been studied.

Atomic Layer Deposition (ALD) is a promising technique for the production of biologically safe, wear resistant and corrosion protective coatings for dental and orthopedic applications. In this work, we investigate the impact of coating thickness and surface preparation on the hardness (H), elastic modulus (E), wear resistance, and delamination of ALD Al₂O₃ films.

Al₂O₃ was deposited via ALD at 300°C using Al(CH₃)₃ and H₂O. 200 nm, 600 nm, and 1000 nm thick Al₂O₃ films were coated on polished 305 stainless steel (SS) substrates. Prior to deposition, SS substrates were cleaned using one of three methods: i) sonication in acetone, isopropyl alcohol and deionized water (AID), ii) AID followed by argon plasma treatment, iii) AID followed by oxygen plasma treatment. Nanowear, nanoscratch, and nanoindentation testing were performed using a Hysitron UBI-1 nanomechanical test system. A Berkovich diamond tip was used for nanoindentation testing to calculate the average H and E. A conical diamond tip was used to perform scratch testing in order to determine the delamination force and coefficient of friction values. The same conical diamond tip was also used for wear testing.

Nanoscratch testing and nanowear testing indicates excellent adhesion –Al₂O₃ films do not delaminate even when penetration depth is beyond the film thickness. Nanoindentation results indicate that the H and E of coated substrates are higher than that of the uncoated substrates. In addition, H and E exhibit a trend of modulus and hardness changing from bulk Al₂O₃ values to bulk stainless steel values with increasing penetration depth. Nanowear testing demonstrates that ALD Al₂O₃ films offer effective protection of the SS substrate. Although no distinguishable difference in the wear resistance was observed due to plasma treatment, films of increasing thickness exhibit greater resistance to wear due to reduced coefficient of friction.

In conclusion, we find that ALD exhibits great promise for protective coating of dental and orthopedic devices.

Nowadays, medical implants are widely used to repair or replace damaged organs or parts of the human body, improving people’s quality of life or even preserving life. In order to develop new medical implants and to improve the existing ones, it is necessary to develop novel biomaterials. One strategy to develop new biomaterials is to design the bulk material to fulfill the required properties for the implant’s performance and then tailor its surface properties towards biocompatibility by coating it with an adequate thin film.

Whenever a material comes in contact with blood or physiological fluids, as it is upon implantation, a cascade of biological events is triggered; the right development of this cascade of events determines the material’s biocompatibility. One of the first biological events to occur is the adsorption of a layer of proteins onto the material’s surface. This protein layer mediates the subsequent biological events such as cell adhesion. Cells adhere preferentially to specific proteins in specific conformations; thus, the composition and the conformation of the protein layer is a key factor dictating whether the material is biocompatible or not. Due to the importance of protein adsorption to define the biocompatibility of a material, it is essential to develop a fundamental understanding of protein-surfaces interactions in order to design novel and successful biocompatible coatings. Transition metal oxide thin films can be considered as promising biocompatible coatings, based on the successful biocompatibility displayed by TiO₂ and the good mechanical, corrosion properties and bioinert response displayed by transition metal oxides.

To evaluate sputtered deposited TiO_x, TaO_x, NbO_x and ZrO_x thin films as biocompatible coatings, the in-situ adsorption of albumin, fibrinogen and collagen onto these films was studied by ellipsometry. Protein adsorption onto solid surfaces is affected by the physicochemical properties of the surface; therefore the chemical composition (X-ray photoelectron spectroscopy), wettability and surface energy (contact angle) and roughness and thickness (profilometry) of the thin films were characterized in order to identify any possible relation between these properties and the protein adsorption process. The results showed that for each protein, there is different adsorption kinetics depending on the material, clearly observed by the changes in the ellipsometer spectra. Moreover, the surface mass density of the protein layer, calculated from the fitting of the spectroscopic ellipsometer spectra, was observed to increase with both the average roughness (Rq) of the surface and the polar component of the surface energy.