Plasma surface modification of degradable magnesium alloy and non degradable stainless steel

: enhanced corrosion resistance, antimicrobial properties, cytocompatibility and covalent immobilization of protein molecules

Abstract

Biomaterials are being widely investigated which are divided into degradable and non-degradable materials and which further classified into various types such as metals, metal alloys, and polymers. The surface properties such as corrosion resistance, biocompatibility, antimicrobial activity, protein binding capability etc of currently used biomedical materials are not perfect. Surface modification is the viable approach to solve these problems by modifying the surface properties of the current biomaterials. Plasma treatments used in this thesis are plasma ion implantation, cathodic arc deposition and chemical vapor deposition (CVD) which is explained detail in chapter 1. In plasma ion implantation process energetic ions are bombarded into the surface layer of the substrate. The energetic ions used in this process provides the feasibility of introducing a broad range of atomic species independent of thermodynamic factors which leads to feasibly obtain impurity concentrations and distributions of particular interest. In cathodic arc deposition, a well adherent metal layers can be achieved by this process due to the high ion content of the depositing materials The process generates a flux of metal ions that can be accelerated resulting in implantation into the surface of the substrate materials. In CVD process a non-volatile compound is deposited on the substrate because of the chemical reactions between chemicals in the gas phase and the substrate surface. Two kinds of biomedical materials are used in this thesis. One is Mg alloys which possess good mechanical properties and degrades in physiological environment which makes them attractive biodegradable biomaterials. However, their degradation rate is rapid in physiological environment which limits its wider application. In chapter 2, we investigate the corrosion behavior of WE43 and ZK60 Mg alloys in Ringer's solution and simulated body fluids (SBF) up to 96 h. Ca10(PO4)6(OH)2, Ca3(PO4)2·3H2O, and Mg(OH)2 are predominant corrosion products formed on both Mg alloys in SBF whereas crystalline aragonite form of CaCO3 and Mg(OH)2 dominate in Ringer's solution. After 96 h immersion in both the solutions, WE43 offers better corrosion resistance than ZK60 because of the alloying element Y. In chapter 3, we investigate the corrosion resistance of WE43 Mg alloy after Si plasma ion implantation. Barrier layers containing SiO2 are produced after Si implantation which enhanced the corrosion resistance of WE43 in SBF. In chapter 4, we investigate the corrosion behavior of WE43 Mg alloy in SBF and cell culture medium after Zr & N plasma ion implantation. After Zr & N implantation on WE43, enhanced corrosion resistance is observed in both SBF and cell culture medium where the enhancement is more pronounced in cell culture medium. Calcium phosphate is formed on the corrosion product. In chapter 5, we investigate the corrosion resistance of ZK60 Mg alloy after Zr & O plasma ion implantation. After Zr & O implantation barrier layers containing ZrO2 are produced which improve the corrosion resistance of ZK60 in SBF. After 30 h immersion, protective corrosion products comprised of Mg(OH)2, Ca10(PO4)6(OH)2, Ca3(PO4)2·3H2O, and zirconium phosphate are formed. In chapter 6, we investigate the properties and performance of Mg–Ca and Mg–Sr alloys after Zr & O plasma ion implantation in terms of the chemical composition, corrosion resistance, cytocompatibility, and antimicrobial properties. ZrO2 containing layers are formed on the surface of Mg–Ca and Mg–Sr alloys after Zr & O implantation which enhanced the corrosion resistance, cytocompatibility, and antimicrobial properties. In chapter 7, we investigate the corrosion resistance of Mg–Nd–Zn–Zr alloy after C2H2 plasma immersion ion implantation (PIII). Barrier layers containing graphite are produced after C2H2 implantation which enhanced the corrosion resistances of Mg–Nd–Zn–Zr alloy in NaCl. Another kind of biomedical biomaterial used in this thesis is stainless steel which is used in implants such as stents because of their desirable mechanical properties. However implant materials subject to adverse responses such as inflammation, clotting, activation of cellular immune response, excessive fibrosis, or rejection because of the weak interaction with the protein molecules. A surface which strongly binds proteins without denature its function is necessary. Carbon plasma polymers can solve this problem. However adhesion between carbon-based plasma polymers and the metal is a major problem whereas a high degree of adhesion is desirable for biomedical implants such as cardiovascular stents. In chapter 8, we demonstrated that the deposition of a Ti & C composite graded film by cathodic arc deposition combined with plasma immersion ion implantation, and the subsequent deposition of an amorphous carbon film by PACVD. A graded Ti & C film without carbon top coat bonded organic molecules. The molecules retained their functionality after washing. The graded coating was shown to have good adhesion even when subjected to mechanical stress.

Date of Award	15 Jul 2014
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Paul Kim Ho CHU (Supervisor)