DLC coatings: Effects of physical and chemical properties on biological response
Introduction
Atherosclerosis remains the foremost cause of mortality in developed countries [1]. One-third of patients with coronary atherosclerosis are treated by coronary angioplasty and stenting. However, the chance of restenosis is as high as 25% after 3–6 months of the procedure. This is because some biomaterial surfaces stimulate activation of the coagulation cascade, scar tissue overgrowth due to excessive wound healing response, and the release of metal ions which are potentially toxic for cells [2]. The most biocompatible blood-interfacing material is pyrolitic carbon; however, carbon is too brittle to be reliably used in stenting applications [3]. Therefore, intense research is directed towards modifying the surfaces of the currently used metallic stents to achieve better outcomes [4]. Diamond-like carbon (DLC) films have been used to coat the surfaces of blood-interfacing prostheses of biocompatible metals such as titanium and nitinol [5]. DLC is a metastable form of amorphous carbon containing both sp2 (graphite like) bonds and sp3 (diamond-like) bonds. Both hydrogenated (a-C:H) and unhydrogenated tetrahedrally coordinated amorphous carbons (ta-C) are referred to as DLC.
Previous investigations of DLC as a biomaterial have focused mainly on the biological performance of DLC coatings, without detailed understanding of the composition and structural effects of the coating for specific biomedical applications [4]. It has been shown that DLC has better biocompatibility and wear resistance than stainless steel [6], titanium and Ti-alloys [7], [8], cobalt chrome alloys, and alumina ceramics. It has also been shown that DLC, with appropriate structural and surface properties, is superior in terms of haemocompatability and anti-thrombogenicity to pyrolytic carbon [9], stainless steel and polyurethane [10]. However, the limited understanding to-date of the role of deposition technique, coating composition, and coating surface properties on the biological performance of the coatings has been a barrier to their optimization for a given biomedical application.
More than ten DLC deposition techniques have been reported, and each technique, depending on coating conditions, produces different levels of sp3 fraction, hydrogen content and surface properties [11]. Amongst all known deposition techniques, DC plasma-assisted chemical vapor deposition (DC-PACVD) is the most amenable to industrial scale deposition of hydrogenated amorphous carbon coatings (a-C:H) [12]. Filtered arc deposition (FAD) is known to produce higher sp3 fractions, and hydrogen-free DLC coatings [12]. It has been shown that the sp3 fraction and the hydrogen content are the determining factors in controlling the mechanical functionality of DLC. It is known that high sp3 content can lead to coating delamination [12], and for this reason, coatings in this study were produced with moderate sp3 content. The current study was aimed at investigating the role of deposition technique and physical and compositional properties of DLC-coated silicon wafers on the physical properties of the coating, and on two biological response parameters: macrophage attachment and viability; protein adsorption.
Section snippets
Coating preparation
DC-PACVD was used to prepare low sp3 fraction hydrogenated amorphous carbon coatings. Methane (CH4) and acetylene (C2H2) were used to deposit DLC films (denoted as a-C:H(CH4) and a-C:H(C2H2), respectively, in the text) in order to vary the hydrogen content and sp3 fraction under similar processing conditions. Higher sp3 fraction unhydrogenated amorphous carbon coatings (denoted as ta-C in the text) were deposited by the FAD method. Semiconductor grade Si (1 0 0) wafer disks of 15 mm diameter were
Coating structure and surface composition
High-resolution TEM observation showed that while the hydrogenated coatings were completely amorphous as shown in Fig. 1, Fig. 2, the unhydrogenated amorphous, ta-C coating contain nanoscale crystalline regions of graphite (Fig. 3) as confirmed by electron diffraction analysis (Fig. 3C). These graphite inclusions were mainly caused by incomplete filtering during the coating deposition process.
The EELS spectra of all the coatings are shown in Fig. 4, revealing that both types of a-C:Hs have a
Discussion
Macrophages are known to produce a number of inflammatory mediators, which have an effect on the surrounding tissue [16]. They play a vital role in the development of atherosclerosis as well as in-stent restenosis. A minor population of macrophages can proliferate in the atherosclerotic lesions themselves, particularly in the early stage. Activated macrophages can accumulate cholesterol esters in the cytoplasm, which leads to foam cell formation in lesion development [17]. In the current study,
Conclusions
Minimal macrophage attachment, and maximal albumin:fibrinogen adsorption ratio, are important goals in optimizing blood-interfacing implants. DLC coatings deposited by PACVD and FAD, were analysed with respect to surface energy (sessile drop), surface composition (XPS), hydrogen content (ERDA), sp3 content (EELS), surface roughness (AFM), albumin:fibrinogen adsorption ratio, and macrophage viability and attachment. Our results showed a significant enhancement in macrophage viability and
Acknowledgements
The authors would like to acknowledge the Australia National Health, Medical Research Council and the Australian Research Council and AINSE postgraduate award for funding this research.
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