Titanium alloys are widely usedin biomedical application. This is the most commonly used material for surgical tools and biological devices. However, some of the important features limiting the applications of this material in cardiovascular system, such as relatively low wearresistance and high thrombogenicity.The effective method to overcome the above mentioned limitation therefore concerns a surface modification, which would improve biological functionwith a hard coating.
Among the various coating materials, Ti-Si-N coating shows excellent mechanical properties, corrosion resistance and superior physiochemical properties. In this study, superhard ternary Ti-Si-N coatings are deposited on titanium alloy (Ti-6Al-4V) substrates by novel arc-enhanced magnetron sputtering. This thesis focuses on microstructure, tribologicalperformance, corrosion resistance, cytocompatibility and hemocompatibility of Ti-Si-N coatings. The main results of the thesisare described in the following.
The Ti–Si–N coatings are prepared by AEMS and the effects of Si contents on the surface characterization, microstructure, hemocompatibility of the coatings have been investigated. The coatingshowed poly-orientation from (1 1 1), (2 0 0) and (2 2 0) planesand was found to be nc-TiN/a-Si3N4 structure.Calculated from AFM images,the surface roughness of the Ti–Si–N coatingsdecreases with varying Si content. This is possibly due to the small TiN grain size. The hardness values of Ti–Si–N coatings were observed to increase up to 41 GPa with 10 at.% Si content but decreases afterwards with an increase in Si content. The improvement in the hardness of the coating is due to ion bombardment effect and absence of dislocations. With Si contents increasing beyond 10 at.%, hardness of the coatings decreases becausedislocations can move easily.
Hemocompatibility of the coatings was evaluated by protein adsorption, hemolysis testand platelet adhesioninvestigation.The Ti-Si-N coatings with 10 at.% Siadsorbed more human plasma protein than that of other titanium alloys.With an increasing of Si content, adsorption level on the coating will slightly decrease. This may be due to the Si-N bonds has the smaller surface energy.The hemolysis ratio of the titanium alloys were also improved greatly through the AEMS treatment. The Ti-Si-N coatings have a relatively smooth surface which can reduce the damage of blood cells.The results of the platelet adhesion tests had shown that the effect of varying Si content revealed an intimate relationship between the component of the Ti-Si-N coatings and its hemocompatibility. The titanium alloys were rougher than the Ti-Si-N coatings with higher Si contents and these extra surface structures may trap platelets.
The endothelialisation of Ti-Si-N coatings with different Si contents has been investigated to assess the influence of Si content on the cell biocompatibility. Titanium alloys are associated with the risk ofthromboemboli formation.We propose to solve this problem by lining the titanium alloys blood-contacting surfaceswith endothelialisation of the Ti-Si-N coatings. A CCK-8 assay was performed to study the promotion of endothelial cell growth, which was also complemented by fluorescent staining. Endothelial cells showedbetter proliferation and anti-platelet adhesion ability compared to titanium alloy control. Ti-Si-N coating with 10 at.% Si exhibited excellent endothelialization. Therefore endothelialisation of the Ti-Si-N coatings may be used to ameliorate thrombus formation.
The microstructure and tribological properties of Ti-Si-N coatings were determined. The friction coefficient of Ti-Si-N coatingsis smaller in human serum than in ambient air and decreases gradually with Si contents. Protein gel electrophoresis shows that the smaller friction coefficient is due to adsorbed proteins from the human serum under sliding conditions. The cytocompatibility of the coatings is assessed in vitro by arelative nitrite assay. The Ti-Si-N coatings have a positive effect on nitric oxide synthesis on the endothelial cells. The cell morphology and its spreading on the coatings are examined by fluorescence staining. The Ti-Si-N coating with 12 at% Si exhibits the best effects in promoting actin cytoskeleton formation and cell spreading compared to coatings with other Si contents and titanium alloys.
Various negative bias voltageswere used to prepared Ti-Si-N coatings by AEMS and then the coatings properties and endothelial cytocompatibility were studied. Ti-Si-N coatings showed crystalline TiN and amorphous Si3N4 phases which confered a better protection against the corrosion effects compared with that of the titanium alloy. Reduced platelet activation was observed. The coatings exhibited prolonged clotting time and the lowest platelet activation at a negative bias voltage of -100 V. These results indicats thatthe Ti-Si-N coating deposited at -100 V exhibits desirable cytocompatibility and hemocompatibility which are beneficial for surface modificationof cardiovascular implants.