Nitrogen enriched C:H:N:O thin films for improved antibiotics doping

Highlights

• Ability of magnetron sputtered nylon films to store/release antibiotics was studied.

• The highest storage capability was found for films sputtered in N₂-rich atmosphere.

• MIC of released ampicillin and ciprofloxacin against model bacteria was reached.

• C:H:N:O films impregnated with antibiotics may be used as antibacterial coatings.

Abstract

In this study we investigate the ability of RF magnetron sputtered nylon 6,6 films (C:H:N:O) to store and release two model antibiotics, ampicillin and ciprofloxacin, that differ significantly in their chemical structure. We demonstrate that the addition of nitrogen to the deposition process leads to a substantial enhancement in the abundance of nitrogen-containing functional groups in the resulting coatings as well as to the significant changes in their swelling and dissolution. These variations were found to result in different impregnation and release capabilities of the produced coatings: an almost 5 times higher amount of released antibiotics was observed for the samples prepared in nitrogen-rich atmosphere when compared to films deposited in pure argon. It was also found that the amount of released antibiotics strongly depends on the natural character of the antibiotics used. However, it is demonstrated that minimum inhibitory concentrations against S. epidermidis and E. coli can be exceeded for both antibiotics.

Introduction

The massive usage of antibiotics in the last decades has led to unwanted effects such as the development of resistant bacteria strains [1,2] and observable serious side effects caused by antibiotics' toxicity to the human body. Hence, methods which enable a reduction in the dosage of antibiotics are of high interest in the scientific and broader communities. A typical example where antibiotics are systemically used is that of implant surgery. New strategies have been aimed at the effective utilization of implant surfaces by the introduction of antibacterial coatings [[3], [4], [5], [6]]. Such antibacterial surfaces can serve either against biofilm formation or planktonic bacteria growth. It has already been demonstrated that biofilm formation can be prevented via proper chemical composition or through the topography of the uppermost surface [[7], [8], [9]]. Planktonic bacteria can be destroyed via the release of antibacterial agents such as metal ions (e.g. [[10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]] or see our recent review [23]) or antibiotics [24]. There are three different ways to achieve antibiotic immobilization: (i) bonding of antibiotics onto the functionalized surface [25], (ii) encapsulation of antibiotics [[26], [27], [28]] and (iii) utilization of natural porosity or soaking properties of materials including polymers [29,30], calcium triphosphate ceramics [31], nano-porous aluminium [32] or polymer particles [33]. Asadinezhad et al. demonstrated, for example, the successful activation of PVC surface through the process of coplanar plasma treatment, followed by graft copolymerization of acrylic acid, and finally biocide igrasan immobilization [34]. Camporeale et al. immobilized dispersin B into epoxy-rich thin films in order to disrupt biofilm formation [35]. Another approach utilized plasma assisted spraying polymerization with proper liquid bio-precursor delivered directly to the plasma stream [36,37]. Braceras et al. covered substrates with silylciprofloxacin by means of a spin coating method followed by low-pressure plasma polymerization [38]. The same group successfully grafted azidovancomycin to the plasma polymerized acrylic acid [39]. Another highly appreciated method is the deposition of an antibacterial substance in form of a biodegradable thin film [40].

Yet another strategy is based on the capability of certain plasma polymer films to be impregnated with antibiotics. This strategy was tested by Garcia-Fernandez et al. [41] for oxygen-rich plasma polymers, and more recently by our group using RF magnetron sputtered nylon 6,6 (C:H:N:O) [24], showing that antibiotics can be stored in the volume of the thin film and are subsequently gradually released when the doped coatings are brought into the contact with aqueous media.

Regardless of the method used for the synthesis of antibacterial coatings, several requirements must be fulfilled: the coating itself has to be biocompatible, the amount of stored/released bactericidal agents should be controllable in a way to ensure the desired bactericidal effect, the production of the coatings should be simple, cost-effective, applicable to a wide range of substrate material and, preferably, be environmentally benign. In addition, and with respect to the recent demand for the development of advanced multi-release, multi-action or “smart” antibacterial thin films, e.g. multi-component hybrid organic/inorganic coatings [4,23], the desired fabrication procedure should be compatible with other deposition/modification techniques in order to allow for the production of such materials. Taking into account the aforementioned requirements, the strategy based on the impregnation of magnetron sputtered C:H:N:O plasma polymer films appears to be highly attractive as it offers several principal advantages. The first relates to the fact that the deposition procedure is a purely physical one. Benefits of such a process include the fact that it allows for the coating of virtually any kind of substrate material without need of any liquid or gaseous precursors or solvents, and that it is compatible with other vacuum-based deposition techniques. The latter opens the possibility for the fabrication of multi-functional coatings with improved antibacterial properties. The second key feature is the excellent biocompatibility of magnetron sputter deposited C:H:N:O films [42]. Finally, this strategy offers also the possibility for the precise control of stored/released antibiotics; in the case of C:H:N:O films, the release was found linearly proportional to film thickness [24].

The main aim of this study is to show that the amount of stored/released antibiotics may be tailored not only by the thickness of C:H:N:O coatings but also by their own chemical structure. This should pave the way for the optimization of the loading capacity of produced materials and in combination with bio-fouling properties of nitrogen-rich coatings, their biocompatibility. In order to prove this phenomenon, the loading/release properties of C:H:N:O films deposited by RF magnetron sputtering of nylon 6,6 in different working gas mixtures were studied for two kinds of antibiotics - ampicillin and ciprofloxacin. While ampicillin, a simple penicillin-like drug with negative and positive charged functional groups, was selected in order to allow for the direct comparison with our previous data, ciprofloxacin was used as a model for drugs without the positively charged amino group.