Antimicrobial Nanostructured Coatings: A Gas Phase Deposition and Magnetron Sputtering Perspective

Multi-functional bioactive silver- and copper-doped diamond-like carbon coatings for medical implants

Diamond-like carbon (DLC) coatings doped with bioactive elements of silver (Ag) and copper (Cu) have been receiving increasing attention in the last decade, particularly in the last 5 years, due to their potential to offer a combination of enhanced antimicrobial and mechanical performance. These multi-functional bioactive DLC coatings offer great potential to impart the next generation of load-bearing medical implants with improved wear resistance and strong potency against microbial infections. This review begins with an overview of the status and issues with current total joint implant materials and the state-of-the art in DLC coatings and their application to medical implants. A detailed discussion of recent advances in wear resistant bioactive DLC coatings is then presented with a focus on doping the DLC matrix with controlled quantities of Ag and Cu elements. It is shown that both Ag and Cu doping can impart strong antimicrobial potency against a range of Gram-positive and Gram-negative bacteria, but this is always accompanied so far by a reduction in mechanical performance of the DLC coating matrix. The article concludes with discussion of potential synthesis methods to accurately control bioactive element doping without jeopardising mechanical properties and gives an outlook to the potential long-term impact of developing a superior multifunctional bioactive DLC coating on implant device performance and patient health and wellbeing. STATEMENT OF SIGNIFICANCE: Multi-functional diamond-like carbon (DLC) coatings doped with bioactive elements of silver (Ag) and copper (Cu) offer great potential to impart the next generation of load-bearing medical implants with improved wear resistance and strong potency against microbial infections. This article provides a critical review of the state-of-the-art in Ag and Cu doped DLC coatings, beginning with an overview of the current applications of DLC coatings in implant technology followed by a detailed discussion of Ag/Cu doped DLC coatings with particular focus on the relationship between their mechanical and antimicrobial performance. Finally, it ends with a discussion on the potential long-term impact of developing a truly multifunctional ultra-hard wearing bioactive DLC coating to extend the lifetime of total joint implants.

Exploring multielement nanogranular coatings to forestall implant-related infections

Introduction

As we approach the post-antibiotic era, the development of innovative antimicrobial strategies that carry out their activities through non-specific mechanisms could limit the onset and spread of drug resistance. In this context, the use of nanogranular coatings of multielement nanoparticles (NPs) conjugated to the surface of implantable biomaterials might represent a strategy to reduce the systemic drawbacks by locally confining the NPs effects against either prokaryotic or eukaryotic cells.

Methods

In the present study, two new multielement nanogranular coatings combining Ag and Cu with either Ti or Mg were synthesized by a gas phase physical method and tested against pathogens isolated from periprosthetic joint infections to address their potential antimicrobial value and toxicity in an in vitro experimental setting.

Results

Overall, Staphylococcus aureus, Staphylococcus epidermidis and Escherichia coli displayed a significantly decreased adhesion when cultured on Ti-Ag-Cu and Mg-Ag-Cu coatings compared to uncoated controls, regardless of their antibiotic resistance traits. A dissimilar behavior was observed when Pseudomonas aeruginosa was cultured for 30 and 120 minutes upon the surface of Ti-Ag-Cu and Mg-Ag-Cu-coated discs. Biofilm formation was mainly reduced by the active effect of Mg-Ag-Cu compared to Ti-Ag-Cu and, again, coatings had a milder effect on P. aeruginosa, probably due to its exceptional capability of attachment and matrix production. These data were further confirmed by the evaluation of bacterial colonization on nanoparticle-coated discs through confocal microscopy. Finally, to exclude any cytotoxic effects on eukaryotic cells, the biocompatibility of NPs-coated discs was studied. Results demonstrated a viability of 95.8% and 89.4% of cells cultured in the presence of Ti-Ag-Cu and Mg-Ag-Cu discs, respectively, when compared to negative controls.

Conclusion

In conclusion, the present study demonstrated the promising anti-adhesive features of both Ti-Ag-Cu and Mg-Ag-Cu coatings, as well as their action in hampering the biofilm formation, highlighting the safe use of the tested multi-element families of nanoparticles as new strategies against bacterial attachment to the surface of biomedical implants.

Effect of Bacterial Cellulose Plasma Treatment on the Biological Activity of Ag Nanoparticles Deposited Using Magnetron Deposition

New functional medical materials with antibacterial activity based on biocompatible bacterial cellulose (BC) and Ag nanoparticles (Ag NPs) were obtained. Bacterial cellulose films were prepared by stationary liquid-phase cultivation of the Gluconacetobacter hansenii strain GH-1/2008 in Hestrin-Schramm medium with glucose as a carbon source. To functionalize the surface and immobilize Ag NPs deposited by magnetron sputtering, BC films were treated with low-pressure oxygen-nitrogen plasma. The composition and structure of the nanomaterials were studied using transmission (TEM) and scanning (SEM) electron microscopy and X-ray photoelectron spectroscopy (XPS). Using electron microscopy, it was shown that on the surface of the fibrils that make up the network of bacterial cellulose, Ag particles are stabilized in the form of aggregates 5-35 nm in size. The XPS C 1s spectra show that after the deposition of Ag NPs, the relative intensities of the C-OH and O-C-O bonds are significantly reduced. This may indicate the destruction of BC oxypyran rings and the oxidation of alcohol groups. In the Ag 3d_5/2 spectrum, two states at 368.4 and 369.7 eV with relative intensities of 0.86 and 0.14 are distinguished, which are assigned to Ag⁰ state and Ag acetate, respectively. Nanocomposites based on plasma-treated BC and Ag nanoparticles deposited by magnetron sputtering (BCP-Ag) exhibited antimicrobial activity against Aspergillus niger, S. aureus and Bacillus subtilis.

Enhancing antibacterial property of porous titanium surfaces with silver nanoparticles coatings via electron-beam evaporation

Antibacterial activity is one of the most vital characteristics for Titanium (Ti) dental implants. Coating antibacterial material onto Ti surfaces is an effective approach to enhance their intrinsic antibacterial ability. However, a cost-effective but efficient coating strategy for realizing this objective still remains challenging. In this study, we proposed a novel implant surface modification strategy for coating silver nanoparticles onto the porous Ti surface via a facile electron beam evaporation (EBE) approach. Porous Ti surfaces were firstly prepared by sand-blasting large grit acid-etching (SLA) process. Then, the silver nanoparticles coating thickness on the porous Ti surface was adjusted and optimized by altering the duration of EBE process. Consequently, composite porous Ti surfaces with different silver thicknesses were synthesized. Polished Ti (PT) surface without SLA or EBE process was also prepared as the controlled blank group. The surface characterizations were analyzed by SEM, AFM, and XPS. After that, the antibacterial properties of all groups were tested with bacteria counting method, bacterial viability test, live/dead bacterial staining, and SEM examination. Results show that silver nanoparticles were uniformly distributed on the porous Ti surfaces after the SLA and EBE processes. After being incorporated with silver nanoparticles, the composite surfaces successfully inhibited the growth of Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). The antibacterial ratio (AR) values of SLA-Ag groups increased with the increasing silver thickness and are significantly higher than those of PT and SLA groups. Therefore, by the SLA and EBE processes, the composite porous Ti surfaces modified with silver nanoparticles coatings demonstrate superior antibacterial property compared with pure Ti surfaces, which is highly promising for enhancing the antibacterial functions of dental implants. Graphical abstract.

Mechanical Properties of Nanoporous Metallic Ultrathin Films: A Paradigmatic Case

Nanoporous ultrathin films, constituted by a slab less than 100 nm thick and a certain void volume fraction provided by nanopores, are emerging as a new class of systems with a wide range of possible applications, including electrochemistry, energy storage, gas sensing and supercapacitors. The film porosity and morphology strongly affect nanoporous films mechanical properties, the knowledge of which is fundamental for designing films for specific applications. To unveil the relationships among the morphology, structure and mechanical response, a comprehensive and non-destructive investigation of a model system was sought. In this review, we examined the paradigmatic case of a nanoporous, granular, metallic ultrathin film with comprehensive bottom-up and top-down approaches, both experimentals and theoreticals. The granular film was made of Ag nanoparticles deposited by gas-phase synthesis, thus providing a solvent-free and ultrapure nanoporous system at room temperature. The results, bearing generality beyond the specific model system, are discussed for several applications specific to the morphological and mechanical properties of the investigated films, including bendable electronics, membrane separation and nanofluidic sensing.

The Role of Substrate on Thermal Evolution of Ag/TiO2 Nanogranular Thin Films

In multicomponent thin films, properties and functionalities related to post-deposition annealing treatments, such as thermal stability, optical absorption and surface morphology are typically rationalized, neglecting the role of the substrate. Here, we show the role of the substrate in determining the temperature dependent behaviour of a paradigmatic two-component nanogranular thin film (Ag/TiO2) deposited by gas phase supersonic cluster beam deposition (SCBD) on silica and sapphire. Up to 600 °C, no TiO2 grain growth nor crystallization is observed, likely inhibited by the Zener pinning pressure exerted by the Ag nanoparticles on the TiO2 grain boundaries. Above 600 °C, grain coalescence, formation of However, the two substrates steer the evolution of the film morphology and optical properties in two different directions. anatase and rutile phases and drastic modification of the optical absorption are observed. On silica, Ag is still present as NPs distributed into the TiO2 matrix, while on sapphire, hundreds of nm wide Ag aggregates appear on the film surface. Moreover, the silica-deposited film shows a broad absorption band in the visible range while the sapphire-deposited film becomes almost transparent for wavelengths above 380 nm. We discuss this result in terms of substrate differences in thermal conductivity, thermal expansion coefficient and Ag diffusivity. The study of the substrate role during annealing is possible since SCBD allows the synthesis of the same film independently of the substrate, and suggests new perspectives on the thermodynamics and physical exchanges between thin films and their substrates during heat treatments.

Coating Technologies for Copper Based Antimicrobial Active Surfaces: A Perspective Review

Microbial contamination of medical devices and treatment rooms leads to several detrimental hospital and device-associated infections. Antimicrobial copper coatings are a new approach to control healthcare-associated infections (HAI’s). This review paper focuses on the efficient methods for depositing highly adherent copper-based antimicrobial coatings onto a variety of metal surfaces. Antimicrobial properties of the copper coatings produced by various deposition methods including thermal spray technique, electrodeposition, electroless plating, chemical vapor deposition (CVD), physical vapor deposition (PVD), and sputtering techniques are compared. The coating produced using different processes did not produce similar properties. Also, process parameters often could be varied for any given coating process to impart a change in structure, topography, wettability, hardness, surface roughness, and adhesion strength. In turn, all of them affect antimicrobial activity. Fundamental concepts of the coating process are described in detail by highlighting the influence of process parameters to increase antimicrobial activity. The strategies for developing antimicrobial surfaces could help in understanding the mechanism of killing the microbes.

Sputter-Deposited Ag Nanoparticles on Electrospun PCL Scaffolds: Morphology, Wettability and Antibacterial Activity

Porous scaffolds made of biocompatible and environmental-friendly polymer fibers with diameters in the nano/micro range can find applications in a wide variety of sectors, spanning from the biomedical field to textiles and so on. Their development has received a boost in the last decades thanks to advances in the production methods, such as the electrospinning technique. Conferring antimicrobial properties to these fibrous structures is a primary requirement for many of their applications, but the addition of antimicrobial agents by wet methods can present a series of drawbacks. In this work, strong antibacterial action is successfully provided to electrospun polycaprolactone (PCL) scaffolds by silver (Ag) addition through a simple and flexible way, namely the sputtering deposition of silver onto the PCL fibers. SEM-EDS analyses demonstrate that the polymer fibers get coated by Ag nanoparticles without undergoing any alteration of their morphological integrity upon the deposition process. The influence on wettability is evaluated with polar (water) and non-polar (diiodomethane) liquids, evidencing that this coating method allows preserving the hydrophobic character of the PCL polymer. Excellent antibacterial action (reduction > 99.995% in 4 h) is demonstrated against Escherichia coli. The easy fabrication of these PCL-Ag mats can be applicable to the production of biomedical devices, bioremediation and antifouling systems in filtration, personal protective equipment (PPE), food packaging materials, etc.

Photocatalytic Activity of Cellulose Acetate Nanoceria/Pt Hybrid Mats Driven by Visible Light Irradiation

A photocatalytic system for the degradation of aqueous organic pollutants under visible light irradiation is obtained by an innovative approach based on ceria/platinum (Pt) hybrid nanoclusters on cellulose acetate fibrous membranes. The catalytic materials are fabricated by supersonic beam deposition of Pt nanoclusters directly on the surface of electrospun cellulose acetate fibrous mats, pre-loaded with a cerium salt precursor that is transformed into ceria nanoparticles directly in the solid mats by a simple thermal treatment. The presence of Pt enhances the oxygen vacancies on the surface of the formed ceria nanoparticles and reduces their band gap, resulting in a significant improvement of the photocatalytic performance of the composite mats under visible light irradiation. Upon the appropriate pretreatment and visible light irradiation, we prove that the most efficient mats, with both ceria nanoparticles and Pt nanoclusters, present a degradation efficiency of methylene blue of 70% and a photodegradation rate improved by about five times compared to the ceria loaded samples, without Pt. The present results bring a significant improvement of the photocatalytic performance of polymeric nanocomposite fibrous systems under visible light irradiation, for efficient wastewater treatment applications.

Ag Functionalization of Al-Doped ZnO Nanostructured Coatings on PLA Substrate for Antibacterial Applications

Developing smart, environmentally friendly, and effective antibacterial surfaces is fundamental to contrast the diffusion of human infections and diseases for applications in the biomedical and food packaging sectors. To this purpose, here we combine aluminum-doped zinc oxide (AZO) and Ag to grow nanostructured composite coatings on bioplastic polylactide (PLA) substrates. The AZO layers are grown by RF magnetron sputtering, and then functionalized with Ag in atomic form by RF magnetron sputtering and in form of nanoparticles by supersonic cluster beam deposition. We compare the morphology, wettability, and antimicrobial performance of the nanostructured coatings obtained by the two methods. The different growth modes in the two techniques used for Ag functionalization are found to produce some differences in the surface morphology, which, however, do not induce significant differences in the wettability and antimicrobial response of the coatings. The antibacterial activity is investigated against Escherichia coli and Staphylococcus aureus as representatives of Gram-negative and Gram-positive bacteria, respectively. A preferential antimicrobial action of Ag on the first species and of AZO on the second one is evidenced. Through their combination, we obtain a hybrid composite coating taking advantage of the synergistic dual action of the two materials deposited, with a total bacterial suppression within few minutes for the first species and few hours for the second one, thus representing a valuable solution as a wide-spectrum bactericidal device.

Metallic Antibacterial Surface Treatments of Dental and Orthopedic Materials

The oral cavity harbors complex microbial communities, which leads to biomaterial-associated infections (BAI) during dental and orthopedic treatments. Conventional antibiotic treatments have met great challenges recently due to the increasing emergency of drug-resistant bacteria. To tackle this clinical issue, antibacterial surface treatments, containing surface modification and coatings, of dental and orthopedic materials have become an area of intensive interest now. Among various antibacterial agents used in surface treatments, metallic agents possess unique properties, mainly including broad-spectrum antibacterial properties, low potential to develop bacterial resistance, relative biocompatibility, and chemical stability. Therefore, this review mainly focuses on underlying antibacterial applications and the mechanisms of metallic agents in dentistry and orthopedics. An overview of the present review indicates that much work remains to be done to deepen the understanding of antibacterial mechanisms and potential side-effects of metallic agents.