FABRICATION OF POLYMERIC FILMS AND COATINGS ON NICKEL-TITANIUM SHAPE MEMORY ALLOY AND THEIR BIOMEDICAL APPLICATIONS

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1 FABRICATION OF POLYMERIC FILMS AND COATINGS ON NICKEL-TITANIUM SHAPE MEMORY ALLOY AND THEIR BIOMEDICAL APPLICATIONS LI PENGHUI DOCTOR OF PHILOSOPHY CITY UNIVERSITY OF HONG KONG AUGUST 2014

2 CITY UNIVERSITY OF HONG KONG Fabrication of Polymeric Films and Coatings on Nickel-Titanium Shape Memory Alloy and Their Biomedical Applications Submitted to Department of Physics and Materials Science in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy by Li Penghui August 2014

3 i 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.

4 ii 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

5 iii 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 and properties 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.

6 iv coli). The results suggest that the complex antibacterial coatings are promising in orthopedics, dentistry, and other biomedical applications.

7 vii TABLE OF CONTENTS ABSTRACT... i ACKNOWLEDGEMENTS... v TABLE OF CONTENTS... vii LIST OF FIGURES... x LIST OF TABLES... xv LITS OF SYMBOLS AND ABBREVIATIONS... xvi Chapter 1 Introduction Nickel-titanium shape memory alloy Shape memory effect and superelasticity of NiTi SMA Biocompatibility of NiTi SMA Medical applications of NiTi SMA Deposition techniques for polymeric thin films and coatings Plasma polymerization Electrophoretic deposition Spin coating and dip coating Biomedical applications of polymeric thin films and coatings Coatings for orthopaedic implants Polymer coated stents Antimicrobial coatings Motivation and objectives References Chapter 2 Corrosion inhibition and biocompatibility of plasma-polymerized films on biomedical shape memory NiTi alloy Introduction Experimental details Sample preparation Surface characterization Corrosion resistance test... 38

8 viii Cell proliferation and morphology Results and dicussion Surface morphology Surface chemical composition Corrosion resistance Surface wettability Cell adhesion and proliferation Summary References Chapter 3 Enhanced corrosion resistance and hemocompatibility of biomedical NiTi alloy by atmospheric-pressure plasma polymerized fluorine-rich coating Introduction Experimental details Sample preparation Surface characterization Corrosion resistance test Surface energy measurement Protein adsorption and platelet adhesion tests Results and discussion Chemical structure Corrosion resistance Contact angle and surface energy Protein adsorption Platelet adhesion Summary References Chapter 4 Electrochemical deposition and properties of chitosan coatings on biomedical NiTi alloy Introduction Experimental details... 96

9 ix Sample preparation Surface characterization Corrosion resistance test Protein adsorption and platelet adhesion tests Results and discussion Surface chemistry Corrosion resistance Protein adsorption and platelet adhesion Summary References Chapter 5 Electrochemically deposited chitosan/ag complex coatings on biomedical NiTi alloy for antibacterial application Introduction Experimental details Sample preparation Characterizations Antibacterial property tests Results and discussion Chemical structure Surface morphology Antibacterial property Summary References Chapter 6 Conclusions and future work Conclusions Future work Appendix: List of publications Journal Papers Conference Presentations

Materials Science and Engineering C

Materials Science and Engineering C 28 (2008) 1430 1434 Contents lists available at ScienceDirect Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec Surface structure and