Functional properties improvement of Ag-ZnO thin films using Inconel 600 interlayer produced by electron beam evaporation technique
Introduction
Microorganisms, especially the pathogenic bacteria and fungi are a major threat for the human health since they are ubiquitary and can cause infectious diseases. Microorganisms commonly attach to the surface of indwelling medical devices forming a biofilm that is, by definition, an accumulation of microorganisms and of their extracellular products forming a structured community highly resistant to antimicrobial treatment and tenaciously bond to the surface [1,2]. Healthcare-associated infections (HCAIs) are infections acquired through contact with any aspect of health care and may involve a wide variety of resistant or emergent microorganisms [3]. Surgical site infections (SSIs) are the result of pathogenic microorganisms' proliferation at a surgical incision site and can be associated with the medical devices use [4]. SSIs represent about one-fifth of HCAIs and at least 5% of patients undergoing open surgery develop a superficial or deep SSI [3]. Some of the most common pathogenic bacteria and fungi that account for HCAIs and SSIs, respectively, include Staphylococcus species (S. aureus, S. epidermidis), Enteroccocus species (E. faecalis, E. faecium), Escherichia coli (E. coli), Pseudomonas aeruginosa (P. aeruginosa), Klebsiella pneumoniae (K. pneumoniae), Acinetobacter baumannii (A. baumannii), Enterobacter cloacae (E. cloacae) and Candida albicans (C. albicans) [5]. The development of multidrug resistance in the pathogenic microorganisms and bacterial biofilms represent major concerns for clinical and public health units due to their negative impact on the quality of life and increased healthcare costs. The regular medical devices that have a direct contact with the human body must be an efficient barrier against pathogens, too. Therefore, there is a real need to develop efficient antimicrobial and antibiofilm agents and coatings with proper strategies to prevent, control, reduce and eradicate microbial infections [6,7].
At present, the most investigated methods for biocidal functionalization of different products are the surface modifications and improvement with inorganic metal nanoparticles (NPs), metal oxide NPs or their compounds as antimicrobial agents and coatings. It is known that inorganic antimicrobial materials show superior durability and less toxicity than organic ones [8]. Among inorganic NPs can be mentioned silver (Ag) NPs and zinc oxide (ZnO) NPs that have very attractive chemical, optical and photocatalytic properties and highly effective antimicrobial activity. Moreover, Ag-ZnO NPs exhibit better antimicrobial activity than Ag NPs and ZnO NPs when these NPs are used separately due to the synergistic effect of their own antimicrobial activity [9,10].
Bare ZnO and Ag-ZnO thin films can be grown on different types of metal, glass or plastic substrates with simple or complex geometrical shapes by physical and chemical vapor deposition techniques. Among surface engineering techniques, electron beam (e-beam) evaporation can be an efficient technique for obtaining uniform, homogeneous and stable thin films and coatings on large area substrates with a very high yield of the utilization of the evaporation materials at controlled deposition rates ranging 1 nm/min to several μm/min and relatively low temperature of substrates [11]. However the properties of the starting deposition materials and substrate along with the process parameters and annealing treatment can lead to different results among producers of Ag-ZnO based coatings. The challenge in nanostructured composite coatings for medical applications consists in obtaining stable and robust coatings with antimicrobial and antibiofilm activity.
This study opens an original insight on the development of e-beam evaporated and annealed Ag-ZnO and IN600 nanostructured stable films deposited on the surface of austenitic 316 L stainless steel (SS) substrate. The surface morphology and elemental composition along with some functional properties of the samples were investigated by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX) spectroscopy, Atomic Force Microscopy (AFM), X-ray photonelectron spectroscopy (XPS), Ultraviolet-visible (UV–Vis) spectroscopy, Raman spectroscopy, microscratch and electrochemical tests. The antimicrobial activity tests were assessed against pathogenic microorganisms using both planktonic and adherent bacterial and fungal cells. The breakthrough of our works consists in using of a stable 150 nm thick IN600 interlayer and reporting its effects on the properties enhancement of 500 nm thick Ag-ZnO films. The obtained results endorse the using of Ag-ZnO/IN600 films with superior functional properties in medical applications as antimicrobial and robust coatings on SS surface-contacting medical devices with limited exposure (contact duration up to 24 h) for reducing SSIs and improving human health.
Section snippets
Materials
Thermal evaporation disk compacts with 8 mm in diameter and 4 mm in height were prepared by powder metallurgy techniques from carboxymethyl cellulose capped Ag-ZnO composite powders containing up to 0.6 weight (wt.) % Ag NPs that were chemically synthesized by us and described elsewhere [10]. The disk compacts were used for the deposition of Ag-ZnO films from Mo e-gun evaporation crucibles. Commercial pure Inconel 600 wire (Umicore) with 1 mm in diameter was used for the deposition of IN600
XRD analysis
Fig. 1 depicts the XRD patterns of the annealed Ag-ZnO and Ag-ZnO/IN600 films.
The XRD analysis revealed that all Ag-ZnO films are of polycrystalline nature having sharp diffraction peaks corresponding to ZnO zincite with a hexagonal wurtzite crystal structure with space group P63mc (186) and lattice parameters of a = 3.2498 Å and c = 5.2066 Å according to the International Centre for Diffraction Data (ICDD) PDF card No. 00–036-1451. For all the peaks, Ag-ZnO/IN600/SS sample revealed higher
Conclusions
The development and investigation of engineered surfaces with antimicrobial and antibiofilm activity and robust coatings are of increasing significance. In our study, e-beam evaporated and annealed 500 nm thick Ag-ZnO films on mirror surface finished 316 L SS substrate and on 150 nm thick IN600 interlayer were achieved as stable films with improved surface properties and without substrate distortion. XRD, AFM and SEM analysis revealed nanocrystalline grains with maximum size of 40 nm for Ag-ZnO
Acknowledgements
This work was supported by the Ministry of Research and Innovation (MCI) and Executive Unit for Financing Higher Education, Research, Development and Innovation (UEFISCDI), Romania, and co-supported by SC MGM STAR CONSTRUCT SRL, Romania [grant no. 215/2014].
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