Elsevier

Applied Surface Science

Volume 441, 31 May 2018, Pages 871-883
Applied Surface Science

Full Length Article
New bio-active, antimicrobial and adherent coatings of nanostructured carbon double-reinforced with silver and silicon by Matrix-Assisted Pulsed Laser Evaporation for medical applications

https://doi.org/10.1016/j.apsusc.2018.02.047Get rights and content

Highlights

  • C:Ag:Si films synthesized by Matrix-Assisted Pulsed Laser Evaporation.

  • Post-deposition thermal treatment in vacuum increases sp2/sp3 C species ratio.

  • Adherence values considerably improved after thermal treatment.

  • Enhanced antimicrobial activity, whilst cytotoxic action at acceptable levels.

  • Prevention of infections associated with biofilms developed on medical equipments.

Abstract

We report on Matrix-Assisted Pulsed Laser Evaporation (MAPLE) deposition of Carbon thin films, simple or reinforced with intended concentrations of Ag and Si. A KrF (λ = 248 nm, τFWHM ≤ 25 ns, ν = 10 Hz) excimer laser was used for irradiation. The effect of a post-deposition thermal treatment in vacuum was studied. Besides detailed morphological, compositional, structural and pull-out adherence characterizations, the potential of the carbonaceous films for medical applications was investigated in vitro by anti-biofilm and cytocompatibility assays. The microscopic images evidenced no delaminations. Micro-Raman spectroscopy revealed a graphitization tendency depending on preparation conditions, thermal treatment and reinforcing agents’ presence. Adherence values improved considerably after thermal treatment. In vitro biological evaluation showed that the films containing ∼1.85 at.% Ag were non-cytotoxic for MG63 cells, while eliciting a limited antimicrobial activity. The increase of Ag content to 3.6 at.% results in a significant enhancement of antimicrobial activity, whilst maintaining the cytotoxic action and adherence characteristics at acceptable levels.

We propose a new class of metamaterials based on C reinforced with Ag and Si obtained by MAPLE for medical applications, i.e. the prevention and treatment of various infections associated with biofilms developed on implants and other medical equipments.

Introduction

Amorphous Carbon (C) consists of a disordered network of carbon atoms with a mixture of sp3 (diamond-like), sp2 (graphite-like) and sp1 (polymer-like)-coordinated sites, usually in the presence of hydrogen in different concentrations.

C-based layers are used nowadays in a wide range of applications as protective anti-wear coatings due to their attractive properties, such as: low friction coefficient, chemical inertness, tunable electrical resistivity and optical transparency, ductility, high mechanical hardness, resistance to corrosion and excellent tribological behavior [1], [2], [3], [4], [5], [6], [7], [8], [9]. These properties can be tailored to accommodate various well-defined functionalities, including high biocompatibility and haemocompatibility (anti-thrombogenicity) [10], [11], [12].

Nowadays, the choice of reinforcement agent plays a key-role in designing the coating features. In this respect, the effect of incorporation of various metal and non-metal atoms (e.g., N [13], [14], F [13], [15], [16], Ti [17], [18], Mo [19], Cr [20], [21], Fe [14], W [22], Si [16], [23] or Ag [24]) was systematically evaluated. They proved beneficial to reduce the residual stress in the coating, without compromising the wear and corrosion performances [25].

Metallic species can be incorporated into C-based coatings to form nanocomposites with improved functionality. When used as protective layers, Ag incorporated C films (C:Ag) can support structures with reduced surface free energy and residual internal stress, low coefficient of friction, high wear resistance and hydrophobic behavior [5], [26], [27]. For the biomedical field, C:Ag structures demonstrated attractive haemocompatibility and anti-bacterial properties [27]. It was also shown that incorporation of 5.5 at.% Ag can boost the osseointegration properties [28].

Si incorporation into C-based coatings (C:Si) has been shown to overcome some limitations of simple (undoped) C films, in what concerns the chemical, biological and especially mechanical properties. In this respect, C:Si structures are characterized by low intrinsic compressive stress, good adhesion and mechanical resistance, improved chemical stability and wear resistance, and/or decreased inflammatory reactions [29], [30], [31]. These properties are highly beneficial in view of new, challenging biomedical applications. We note that Si doping of diamond like carbon (DLC) increases the surface roughness and diminishes the ID/IG ratio (which is associated with the stabilization of the C-sp3 phase) [32]. Also, the adsorbed amount of proteins increases with the Si content in DLC [32].

The C-coatings double-reinforced with Si and Ag can prove efficient against nosocomial infections, which can appear in any medical facilities like hospitals, nursing home, rehabilitation service, outpatient clinic or other clinical settings. The infections are spread onto susceptible patients via contaminated equipment, personal uniforms and air particles, among others. The studied coatings can be used to inhibit or completely annihilate these contaminations as an effect of microbes’ apoptosis immediately after contact [33], [34].

Simple and reinforced C-based coatings can be synthesized by different methods, such as: Direct Current and Radio Frequency Magnetron Sputtering [1], [5], [35], Plasma Assisted Chemical Vapor Deposition derived methods [9], [23], [30], [31], [36], [37], [38], Ion Beam Deposition [39], [40], Pulsed Laser Deposition (PLD) [41], [42], [43], Combinatorial PLD [44], [45] or Cathodic Arc Evaporation [46].

In the present study, the influence of different concentrations of Ag and Si used as reinforcement agents on the morphological, structural, bonding strength and biological characteristics of the obtained Matrix-Assisted Pulsed Laser Evaporation (MAPLE) carbonaceous coatings were investigated. MAPLE was chosen because of proved milder deposition conditions, which can push for the growth of better crystallized films and synthesis of complex composite structures [47], [48]. On the other hand, it was demonstrated that MAPLE films accurately reproduce in composition, structure and morphology the raw material, also preserving their functionality and biological behavior [47], [48]. One therefore expects that the combination of the C, Ag, and Si elements in a complex MAPLE process results in the production of a metamaterial [49] with innovative properties for biomedical applications. To the best of our knowledge, this is the first report in dedicated literature on MAPLE C thin films reinforcement with Ag and Si for obtaining biocompatible surfaces resistant to microbial colonization and biofilm development for top medical applications.

Section snippets

MAPLE experiment

MAPLE experiments have been performed inside a stainless steel reaction chamber using a KrF excimer laser source (λ = 248 nm, τFWHM ≤ 25 ns, CompexPRO 205), operated with a frequency repetition rate of 10 Hz. The laser beam delivering an energy of 120 mJ/pulse was directed under an angle of 45° onto the target surface. To assure a top-hat laser spot and a repeatability of the samples, a dedicated beam homogenizer (single axis type) was used in all experiments.

Simple C powders (Sigma–Aldrich,

Results and discussion

The thickness of the synthesized films, inferred from profilometry measurements (data not shown here), was of ∼100 nm.

Conclusions

Continuous and uniform thin Carbon (C) layers with various degrees of Ag and Si reinforcement were synthesized by Matrix-Assisted Pulsed Laser Evaporation (MAPLE). Synthesized surfaces contain closely-packed spheroidal particulates.

Micro-Raman and XPS investigations demonstrated that the post-deposition thermal treatment promotes a higher amorphization and an increase of the sp2/sp3 C species ratio. In our opinion, this evolution can be for at least partly accounted by the metamaterial nature

Acknowledgements

CR, NM, MB and INM acknowledge the support of this work by NATO under the contract SPS G4890. The authors are grateful for support of this work by UEFISCDI under the Contract No. 7-083/2014. LD, CR and INM acknowledge with thanks the partial support by the Romanian National Authority for Scientific Research and Innovation, CNCSUEFISCDI, under the project number PN-II-RU-TE-2014-4-1570 (TE 108/2015) and Core Programme - Contract 4 N/2016. GES, MAH, and CB acknowledge with thanks the support of

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