Bio-functionalization of micro-arc oxidized magnesium alloys via thiol-ene photochemistry

Highlights

• Biofunctional multilayer is successfully fabricated on Mg alloy surface.

• The corrosion resistance of modified Mg alloy is significantly improved.

• The modified Mg alloy can prevent platelets adhering to its surface.

• The employed strategy for surface modification is simple to control and inexpensive.

Abstract

To enhance corrosion resistance and hemocompatibility of magnesium alloy (Mg alloy), poly(ethylene glycol) methacrylate (PEGMA) was covalently bonded to micro-arc oxidation treated Mg alloy substrate via thiol-ene photochemistry using self-assembled (3-mercaptopropyl)trimethoxysilane film as spacer. Chemical composition of the modified surfaces was verified by attenuated total reflection Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy analysis and X-ray diffraction. Scanning electronic microscopy was used to show morphologies of the samples at different stages. Results of water contact angle measurement showed that the hydrophilicity of the surface has improved significantly after surface modification. Potentiodynamic polarization, electrochemical impedance spectroscopy and hydrogen evolution test demonstrated that corrosion resistance increased significantly for modified Mg alloy versus unmodified surfaces. In the meantime, the surface bonding of PEGMA imparted anti-platelet adhesion compared to the Mg alloy surface, displaying excellent hemocompatibility. This composite coating technique gives promise in acutely reducing both the corrosion and platelet adhesion, making it attractive for application to blood contacting material, such as vascular stents, made from degradable Mg alloys.

Introduction

Mg alloys were introduced as implant materials for orthopedic and trauma surgery during the 1930s following their excellent biocompatibility [1]. In recent years, magnesium and its alloys are being reconsidered for use as biodegradable implant materials for bone implants as well as for cardiovascular stents [2], [3], [4], [5]. However, Mg alloys are corrosion susceptible in a biological environment [6]. Besides, the high corrosion rate and the low bioactivity of magnesium implants are the challenging problems, which need to be resolved before employing them in clinical applications. As we known, surface modification is one of the major techniques to improve the hemocompatibility and bioactivity of implants in this field. Additionally, suitable coatings can improve bioactivity in vitro and protect the implant from fast corrosion and degradation in vivo [7], [8]. Herein, there is an urgent need that appropriate surface modification is applied to enhance its biocompatibility and anti-corrosive property for further broad application in the future.

Poly(ethylene glycol) (PEG) are known for their biocompatibility and the reduction of unspecific protein adsorption and cell adhesion [9], [10], [11]. Furthermore, they are widely used for imparting hydrophilicity to a substrate [12]. Therefore, many studies have been carried on regarding membrane surface modifications to reduce biofouling by introducing hydrophilic moieties such as PEG on the surfaces. Not only do PEG brushes provide a nonadhesive property, but also the terminal hydroxyl groups on their side chains can be used for the immobilization of biomolecules [13]. Antoine Venault et al. reported a set of new functionalized polyethyleneimine (PEI) polymers, including a neutral PEGylated polymer PEI-g-PEGMA, a negatively charged polymer PEI-g-SA, and a zwitterionic polymer PEI-g-SBMA, and their use as antibiofouling coating agent for human teeth protection [14]. Kurkuri et al. prepared multifunctional coatings based on copolymers made from poly(ethylene glycol) methyl ether methacrylate (PEGMA), immobilization of a selection of proteins and lectins [15]. Donahoe et al. exploited separate, sequential click reactions for the cross-linking of PEG as well as its attachment to glass surfaces to form thin hydrogel coatings, showing that higher cross-linking densities provide modest improvements in resistance to fibrinogen adsorption, but greatly negatively impacted resistance to cell adhesion [16]. Weihua Guo et al. selected the iron-based AGET ATRP strategy to conduct the SI-ATRP of polyethylene glycol methacrylate (PEGMA) on the 316LSS surface, indicating enhanced anticoagulative to acid-citrate-dextrose (ACD) blood [17]. However, even though the antifouling and nonadhesion properties of the PEG-based coatings have been well-documented, these coatings have also been reported not to completely prevent bacterial adhesion [18], [19], [20]. Accordingly, grafting PEG-based material to the surface of Mg alloy is an ideal choice, after which it is prospected to possess excellent antifouling properties for resisting protein adsorption and platelet adhesion.

There recently have been tremendous efforts dedicated to modify Mg and its alloys for biodegradable application. Surface modification technology is widely employed to improve the corrosion resistance of Mg alloy. Without changing the bulk properties of substrates, this technique has a great advantage to other ways [21]. Surface modification usually encompasses two subtypes, i.e., chemical conversion and deposition [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33] and the methods include plasma electrolytic oxidation [23], [24], phosphate treatment [25], alkaline heat treatment [26], micro-arc oxidation coating [27], anodization treatment [28], chemical conversion coating [29], organic coating [30], [31], to name just a few. Among them, micro-arc oxidation (MAO) is a promising surface treatment method, which can remarkably enhance corrosion resistance of magnesium and its alloys [34], [35]. In addition to all of the techniques mentioned above, thiol-ene photochemical reaction has generated considerable interest in recent years, which proceed under benign reaction conditions with good efficiency [36], [37], [38], [39]. It's a pity that this kind of reaction is scarcely applied for metal surfaces, especially for Mg alloys. Integration of micro-arc oxidized and thiol-ene photochemistry, as a versatile way for Mg alloys bio-functionalization is difficult but fascinating task. Herein, we utilize thiol-ene photochemical reaction to graft PEGMA to Mg alloy surface, using a MAO coating as an inter layer and thiol-terminated self-assembled film of (3-mercaptopropyl) trimethoxysilane (MPTS) as initiators. A schematic illustration of the preparation process of anticorrosive and biocomposite coating on Mg alloy surface is shown in Fig. 1.