Coatings for Implants
The implantation of biomaterials into the human body allows it to re-establish biological and mechanical functions and therefore to increase the quality of life. Biomaterials are synthetic materials intended to function appropriately in contact with a living tissue and body fluids. Metals, ceramics, polymers and composites are used as biomaterials. These materials alone cannot satisfy all the requirements for bio-implant applications such as superior mechanical properties, wear resistance, corrosion resistance from the body plasma and, biocompatibility etc. Hence enormous research work is being carried out on biocompatible coatings with superior mechanical, tribological and biocompatible properties towards developing bioceramic and biomimetic coating on both metallic and non-metallic substrates. A variety of medical devices, such as coronary stents, intraocular lenses, heart valves, hip and knee joints, bio-chips, dental implants and pacemakers are implanted in the human body. The materials of the implants are exposed to the interaction with the body cells and fluids and to potential corrosive activity from the materials of the body. The body fluid contains 1% sodium chloride and constitutes a corrosive environment for the implants. Joint implants are exposed to sliding wear. The interactions of the implants with the body cells, the products of the corrosion, and of the wear debris can have adverse effects on the body and on the implants. These effects can include cellular damage, infections, blood coagulation (potentially leading to thrombosis) and failure of the implants. In order to accomplish their function the implants should not cause infections, prevent uncontrolled cell growth, maintain their integrity inside the body, and avoid formation of debris. In certain cases it is useful to have the implants interact in a controllable way with the biological environment, e.g. to promote growth of bone cells on implants. The biological behaviour of an implant is strongly influenced by the chemical situation present at the implant surface. Therefore, bio reactions can be tuned by tuning the surface chemistry of an implant, especially the elemental composition, to create a specific surface yielding a defined biological response. Metallic implants can release metal ions and wear debris into the surrounding tissue and these can lead to osteolysis (bone resorption, loss) and loosening and failure of the implant. Hip and knee implants are exposed to sliding movements, which can cause wear of the surfaces in contact. Ti and its alloys are among the most biocompatible metals but their wear resistance is relatively low. The debris particles generated by the wear can cause inflammations of the tissue and can lead to osteolysis around the implant. Coating the implants with protective films, which can reduce corrosion and wear, may prevent or alleviate the problems described above and extend the lifetime of implants to the benefit of the patients. Hence a coating to be used for implants should have the following properties
- Excellent Mechanical Properties
- Low Coefficient of Friction
- Low Wear
- Low Surface Roughness
- Low Internal Stress (Compressive Stress)
- Good Adhesion towards both Metallic and Non-metallic Surfaces
- Hemocompatibility and
- Longer Life Time
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Extensive research work has been carried out in the field of biocompatible coatings based on metal coatings, diamond-like carbon coatings (DLC) and heteroxyapatite (HA) coatings. During the past two decades, DLC films have attracted an overwhelming interest from both industry and the research community. These films offer a wide range of exceptional physical, mechanical, biomedical and tribological properties that make them scientifically very fascinating and commercially essential for numerous industrial applications. DLC films are primarily made of carbon atoms that are extracted or derived from carbon-containing sources, such as solid carbon targets and liquid and gaseous forms of hydrocarbons and fullerenes. Depending on the type of carbon source being used during the film deposition, the type of bonds (i.e. sp1, sp2, sp3) that hold carbon atoms together in DLC may vary a great deal and can affect their mechanical, electrical, optical and tribological properties. The major disadvantage of the deposition of super hard diamond-like carbon films and therefore of the realization of technical applications is often a relatively low adhesion of DLC-films on metallic and ceramic substrates caused by very high internal compressive stress of these coatings. In the case of ion-beam-assisted deposition characteristic properties of the coatings like hardness, elastic modulus and residual stress are strongly correlated with the amount of sp3 bonding in carbon films. This problem has now been overcome by ensuring that there are no stress concentrations near the coating/substrate interface. A frequently used method deals with a design of a tailored interface in the form of a chemically gradient interlayer or a multilayer coating between the substrate material and the carbon film. On a multilayer interface with an optimized layer sequence of Ti, TiN, TiCN, TiC, DLC to reduce residual stress and increase the critical load of failure in the scratch-test of the DLC films. An interesting method to decrease the internal stress in the DLC coating is to introduce metal nanoparticles in between the DLC matrix.
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In an earlier work (M. Sridharan et al., Surface and Coating Technology, 202 (2007) 920), g-alumina nanoparticles have been embedded in the amorphous alumina matrix yielded increased hardness and decreased internal stress in the alumina films. The films were deposited by ionized magnetron sputtering (inductively coupled plasma magnetron sputtering). The magnetron was powered with a pulsed DC power supply, and the substrate was negatively biased with a pulsed DC voltage supply. An RF powered coil was located above the substrate to vary the ion flux hitting the substrate.
As measured with transmission electron microscopy and X-ray diffraction, the films were amorphous in the temperature range 200 to 600 °C, when only a low flux of ions hit the film during growth. At 200 °C, with increasing ion bombardment, 100 nm large clusters of small crystalline grains of g-alumina with sizes of the order of 5 nm were embedded in the amorphous phase. Increasing the flux of ions and/or the temperature further enabled us to make films consisting only of crystalline g-alumina. A systematic study was carried out of the dependence of the nanostructure on a) the RF power of the coil creating the ions bombarding the growing film, on b) the bias voltage, and on c) the substrate temperature. The hardness values of the films were measured and correlated to the film nanostructure. The hardness increased with the crystalline-volume fraction. The hardness values of the amorphous alumina films were 7-9 GPa and for the completely crystalline gamma alumna films the hardness value was 22 GPa. When the substrate temperature was increased further resulted in reduction of hardness value caused by the larger grain sizes.
Work to be carried out:
DLC nanocomposite coatings with superior mechanical, tribological and biocompatible properties will be synthesised towards meeting the requirements for implants (heart valves and hip and knee joints). The coatings will be deposited by magnetron sputtering (inductively coupled plasma magnetron sputtering) onto steel and silicon substrates. The metal targets (for example Cr and Ti) will be sputtered (sputtering gas: argon; reactive gas: methane or acetylene) to get metal nanoparticle embedded DLC coatings (nc-DLC). The deposition parameters, such as, gas flow rate, partial pressure, substrate bias, substrate temperature and power applied to the cathode will be systematically varied and the nanoparticle concentration, grain size and grain distribution will be varied to obtain nc-DLC coatings with superior mechanical, tribological and biocompatible properties. Also coatings / multilayers based on Ti and Cr will be deposited onto steel and silicon substrates by magnetron sputtering aiming towards the applications for implants. Also the hydroxyapatite coatings will be synthesised by magnetron sputtering, which is very much used in orthopaedic and dental implants. These nc-DLC coatings, Ti and Cr based coatings and the hydroxyapatite coatings with superior properties will be used for heart valves, dental implants and other protective coatings to be used for the implants.