Fabrication of polymeric films and coatings on nickle-titanium shape memory alloy and their biomedical applications

Abstract

Biomedical shape memory nickel-titanium (NiTi) alloys are widely used in biomedical devices such as intravascular stents, orthodontic components, and orthopedic implants due to the unique shape memory effect, superelasticity, and good biocompatibility. However, several post-implantation problems should be considered and solved. Corrosion of NiTi alloys in long-term clinical use can be hazardous because of release of toxic Ni and ensuing inflammatory reactions, and serious implant-related bacterial infection induces possible post-surgical complications. Moreover, the osteoconductivity or hemocompatibility of NiTi alloys need to be improved for their specific use in implants or stents. Deposition of polymeric thin films is a versatile modification technique to overcome the aforementioned problems. Homogeneous coatings with good surface coverage offer excellent corrosion resistance. With the proper design and fabrication, functional polymeric coatings can alter the surface wettability and properties of the NiTi alloys, thus changing the behavior of osteoblast cells, bacteria, and platelets. In the studies presented in this thesis, different functional polymeric films or coatings are deposited on NiTi alloys by plasma polymerization and electrochemical deposition technique to selectively achieve better corrosion resistance, antibacterial properties, osteoconductivity, and hemocompatibility. The thesis consists of four parts as described in the following. Firstly, polymeric thin films are fabricated on NiTi alloys using plasma polymerization with allylamine (AAm) and acrylacid (AAc) precursors. The chemical composition and structure of the films confirm successful introduction of bioactive carboxyl and amino functional groups (-COOH and -NH2) onto the surface. The corrosion behavior of the coated NiTi alloys in simulated body fluids is studied by polarization test and electrochemical impedance spectroscopy (EIS) and the results disclose improved corrosion resistance. The cell adhesion and proliferation tests reveal enhanced cytocompatibility in vitro. Secondly, hydrophobic polymeric coatings are deposited by plasma polymerization to improve the corrosion resistance and hemocompatibility of biomedical NiTi alloys. This process takes place at a low temperature in air in the presence of a fluorine-containing precursor using an atmospheric-pressure plasma jet. The composition and chemical states of the polymeric coatings are characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The corrosion behavior of the coated and bare NiTi samples is assessed and compared by means of polarization tests and electrochemical impedance spectroscopy (EIS) in physiological solutions including simulated body fluids (SBF) and Dulbecco's Modification of Eagle's Medium (DMEM). The corrosion resistance of the coated NiTi alloys is evidently improved. Protein adsorption and platelet adhesion tests reveal that the adsorption ratio of albumin to fibrinogen is increased and the number of adherent platelets on the coatings is greatly reduced. The plasma polymerized coatings give rise to better hemocompatibility in vitro and is promising as a protective and hemocompatible coating on cardiovascular implants. Thirdly, chitosan coatings are deposited on the NiTi alloy with electrochemical deposition method. The chemical structure of chitosan coating is confirmed with XPS. The corrosion resistance of NiTi alloy in SBF solution is improved with the chitosan coating. Moreover, the adsorbed albumin to fibrinogen ratio is increased and number of the adherent platelets is greatly reduced with the chitosan coating. The chitosan coating with this convenient and simple method renders NiTi alloy enhanced hemocompatibility and potential applications in cardiovascular stent technology. Last but not least, electrochemical deposition technique is also utilized to deposit chitosan/Ag complex coatings on NiTi alloys, with the purpose to enhance the antibacterial characteristics. The composition andproperties of the coatings are determined systematically. The thickness of the chitosan/Ag complex coating is around 7.5 μm and it adheres well to the NiTi substrate. The chemical state of silver in the chitosan/Ag complex coating is found to be different from that in the deposited silver without chitosan by X-ray photoelectron spectroscopy (XPS) and leaching tests. Furthermore, the antibacterial properties of the chitosan/Ag complex coatings are assessed and there are significant effects against the model bacteria Escherichia coli (E.coli). The results suggest that the complex antibacterial coatings are promising in orthopedics, dentistry, and other biomedical applications.

Date of Award	3 Oct 2014
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Paul Kim Ho CHU (Supervisor)