Full length articleNitrogen enriched C:H:N:O thin films for improved antibiotics doping
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.
Section snippets
Samples preparation
C:H:N:O thin films were prepared by RF magnetron sputtering (13.56 MHz) using a 3-inch planar magnetron equipped with a 3 mm thick nylon 6,6 target (Fig. 1a). Base pressure in the deposition chamber was 5 × 10−4 Pa and was acquired by a diffusion pump. The vacuum chamber was pumped for at least 24 h before each experiment to minimise the residual atmosphere inside the chamber. Following chamber evacuation, working gases were introduced to the chamber with a total flow of 20 sccm. Three
Effect of nitrogen on C:H:N:O properties
The main motivation of this work is to study the effect of the chemical structure of produced C:H:N:O thin films on the immobilization of antibiotics, namely bacteriostatic ampicillin and bactericidal ciprofloxacin. One can expect enhanced antibacterial properties of films which can deliver larger amounts of antibiotics in the solution, and this motivated us to determine the minimum inhibitory concentrations of antibiotics against bacteria. Antibiotic immobilization, however, can be affected by
Conclusion
We have demonstrated the positive effect of reactive N2 admixture to the deposition process of C:H:N:O plasma polymers on antibiotics storage/release from the produced coatings. By changing the chemical structure of C:H:N:O coatings, it is possible to increase the amount of immobilized antibiotics by a factor of 5. This effect is attributed to the different swelling mechanism of thin films produced in different working gas environments. The enhanced interaction of natural polar groups of
Acknowledgment
This research was supported by the Czech Science Foundation – grants GACR 19-20168S and GACR 17-10813S. Jiří Kratochvíl acknowledges the support of GAUK 1394217.
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