Surface modification of stainless steel by grafting of poly(ethylene glycol) for reduction in protein adsorption
Introduction
Over the last two decades, research in materials science and engineering has led to the development of numerous metals and alloys for biomedical applications. Among them, stainless steel has been extensively used as orthopedic implants owing to its corrosion resistance and superior mechanical properties [1]. However, the reaction of human body to the implanted foreign material is usually very complex. Once the implanted metal or alloy is brought into contact with blood, protein adsorption, platelet adhesion and complement activation are provoked by the human defence system. The adsorption of proteins is the initial step in a series of events leading to thrombosis and failure of the biomaterials [2], [3], [4], [5].
Surface modification is a method applicable to implant technology because it can enhance the biocompatibility of a material surface through proper molecular design, while keeping the bulk properties intact [6], [7], [8]. A common approach to metal surface modification is coating of materials bearing biological properties via chemical or physical adsorption, chemical vapor deposition (CVD), physical vapor deposition (PVD) and sputter coating [9], [10], [11]. Another well-known method for creating protein-resistant surfaces is through the immobilization of water-soluble polymers onto the substrate surfaces [12]. The water-soluble polymers include the neutral hydrophilic polymers, such as poly(acrylamide), poly(N,N-dimethylacrylamide), poly(vinyl alcohol), ethylene–vinyl alcohol copolymer, poly(hydroxyethyl methacrylate), poly(ethyene oxide) (PEO), and poly(ethylene glycol) (PEG). PEG is the low molecular weight equivalent (MW<10,000) of PEO [13]. Among them, PEO is wide recognized as a biomaterial due to its non-interacting nature with proteins and cells. Thus, the preparation of PEO-bearing surfaces is currently one of the most promising approaches to the generation of biocompatible surfaces [14].
Physical adsorption of the homopolymer of PEO directly onto a substrate is the simplest, though not the most reliable, way to achieve a PEO-modified surface [15], [16], [17]. Many proteins and cells in the biological fluid can readily displace the adsorbed PEO from the surface. Greater adhesion strength for physical adsorption can be obtained by using PEO-containing amphiphilic block copolymers. Covalent bonding of PEO to substrates is an effective way to prepare the permanent, PEO-modified surface. However, direct coupling method requires functional groups or coupling agent on the substrate surface with which derivatized PEO can react. The interfacial coupling reaction has limited the applicability of the technique to certain substrate materials. Furthermore, the reaction procedures are usually very complex and time-consuming. An alternative method for obtaining PEG-modified surface is through graft-copolymerization with PEG-containing macromonomer. Methoxy poly(ethylene glycol) monomethacrylate (PEGMA) and related macromonomers are commonly used to synthesize this type of PEG-modified surface. This method is more economical than the coupling reaction. It can also generate stable bonds between the polymer and the substrate [18], [19], [20].
In spite of the wide spread use of the grafting and graft polymerization technique on polymer surfaces [13], [21], [22], [23], graft polymerization of PEG-containing and other functional monomers on metals and alloys has yet to be examined in detail. Since protein adsorption at the physiological fluid/stainless-steel interface will lead to unfavorable foreign-body response or even serious clinical problem, it seems justified to explore alternative and novel ways to create the protein-repellent and blood-compatible surface for stainless steel. The purpose of the present work is to further modify the (3-mercaptopropyl)trimethoxysilane-treated stainless-steel surface by Ar plasma pretreatment and then UV-induced graft polymerization with the PEGMA macromonomer.
Section snippets
Materials
AISI type 304 stainless-steel foils of 0.05 mm in thickness were purchased from Goodfellow Ltd. of Cambridge, UK. Prior to silane deposition, the foils were cleaned ultrasonically in a distilled water bath at room temperature for 5 min, and then dipped into acetone for 5 min. After that, they were chemically treated in a sulfochromic bath for 10 min, rinsed thoroughly with distilled water, and blown dry with purified nitrogen. The silane coupling agent (SCA), (3-mercaptopropyl)trimethoxysilane of
Results and discussion
The processes of silanization, argon plasma pretreatment and the graft polymerization of PEGMA onto the silianized and plasma-pretreated stainless-steel surface are shown schematically in Fig. 1. Thus, the stainless-steel coupon is first treated in SCA solution to allow the formation of the SCA overlayer on its surface. The silanized-stainless-steel (SCA-SS) surface is pretreated with argon plasma for a brief period of time, followed by exposure to the atmosphere to effect the formation of
Conclusion
Surface modification of the stainless-steel coupons was achieved via three steps: (1) silanization of the alloy surface by a silane coupling agent (SCA), such as (3-mercaptopropyl)trimethoxysilane, (the SCA-SS surface); (2) argon plasma pretreatment of the SCA-SS surface, followed by air exposure; (3) UV-induced graft polymerization of PEGMA on the plasma-activated SCA-SS surface (the PEGMA-g-SCA-SS surface). A brief plasma treatment of 5 s was found to be optimum in initiating the UV-graft
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