| Due to their strong corrosion resistance,mechanical properties and good biocompatibility,titanium and its alloys have been widely used in biomedical field,in particular in cardiovascular stents,heart valves and grafts and other blood-contacting devices.Nanostructured titanium materials have potential advantages in promoting cell growth and osseointegration,among which TiO2 nanotube arrays(TNTs)have attracted a lot of attention because of their high specific surface area,controllable tube diameter and morphology.When devices are implanted in the human body,the first event occurs is adsorption of proteins in the blood to the surface of the material,accompanied by competitive adsorption,desorption,and rearrangement.These behaviors of the proteins further determine the adhesion behavior and activation state of platelets,thereby affecting the blood compatibility of the material surface.At present,a large body of literature exists on the blood compatibility of TNTs,but the majority of which focuses on the effects of the chemical properties,topological structure,crystal form and wettability of the material,and some findings are controversial or even inconsistent.This thesis focuses on the influence of the intermediate protein layer instead,and explores the relationship between the protein adsorption and the platelet adhesion on TNTs surfaces from molecular perspective,the factors investigated including the adsorption kinetics,adsorption amount and protein conformation.The work includes the following three aspects:1.Protein adsorption on TNTs surface and its effects on platelet adhesionProtein adsorption plays an important role in the interactions between implants and the bio-environment and is directly related to the hemocompatibility of implanted devices.In this chapter,we study the adsorption behavior of two plasma proteins,bovine serum albumin(BSA)and fibrinogen(Fib),on the surface of TNTs,as well as its effects on subsequent platelet adhesion and blood compatibility.We first investigated the adsorption kinetic of BSA and Fib on flat titanium substrate,and found that both proteins adsorbed to the surface in mixed orientation of side-on and end-on,following the Langmuir isotherm model.At the same time,we studied the influence of crystal form on the protein adsorption,and found that changing the amorphous to mixed crystal form upon annealing had little effect.On TNTs,proteins existed as aggregates,and there were interactions between adsorbed proteins,the adsorption following the Freundlich model.The amount of adsorption increased with the tube diameter.Next,conformation of the proteins adsorbed on the substrates was explored,and significant difference was observed between that on the TNTs and that on the flat titanium,and the ratio of ?-sheet to ?-turn was found to depend on the tube diameter.Then,platelet adhesion was studied,which revealed that pre-adsorbed BSA can further reduce the adhesion and activation of platelets on the TNTs,whereas the Fib effectively reduced the number of platelets adhered but promoted the platelets activation.2.Covalent grafting of protein on TNTs surface and its effects on blood compatibilityAmino or carboxyl groups were introduced to TNTs surface by silane coupling agents,and then BSA and Fib were fixed to the two types of surface.By regulating the conformation of the two proteins,the relationship between their secondary structure and the adhesion behavior of platelets was further explored.We found that the ?-sheet/?-turn ratio of the BSA grafted to the TNTs surface was smaller than that of the physically adsorbed one,hence the blood compatibility of the former surface was lower than the latter one.The BSA covalently bonded to the aminated surface tended to expose its platelet receptors,and the platelets attached were more activated.On the other hand,the Fib grafted to the surface of TNTs with outer diameter of 100 nm exhibited a smaller?-sheet/?-turn ratio than that adsorbed,but the ratio for the former was slightly higher than that for the latter for TNTs of 50 nm size;however,the loss of ?-helix in the secondary structure of Fib induced exposure of its binding sites for platelet,resulting in significant reduction in the blood compatibility when the TNTs surface was modified by the Fib grafting.The results show that covalent bonding leads to changes in protein conformation,promoting the inert BSA to expose its platelet receptors and resulting in exposure of more platelet binding sites for the Fib.Out finding indicates that reduction of protein adsorption on the surface and preservation of protein conformation upon its adsorption is an important means to improve blood compatibility.3.Hemocompatibility of polyzwitterion-modified TNTsChemical modification not only can change the chemical properties of a surface,but also is the main method to fabricate protein-resistant surfaces.In this chapter,two types of zwitterionic polymers,poly(sulfobetaine methacrylate)(pSBMA)and poly(carboxybetaine methacrylate)(pCBMA),were grafted onto the TNTs with different diameters using ARGET-ATRP(activators regenerated by electron transfer atom transfer radical polymerization)method to construct surfaces that can resist protein adsorption and maintain the natural conformation of the adsorbed ones.The results show that both polyzwitterions can effectively reduce adsorption of BSA and Fib on the TNTs surface,showing excellent protein resistance.FT-IRRAS analysis revealed that on the polymer-modified TNTs the ratio of ?-sheet to ?-turn in the BSA adsorbed was equivalent to that on the unmodified surface and higher than that on the BSA-grafted surface,whereas the conformation of the adsorbed Fib basically remained unchanged.The polyzwitterion-modified TNTs were found to be non-hemolysis and significantly reduce platelet adhesion and activation,showing excellent blood compatibility. |
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