Stéphane Turgeon

A study of atmospheric pressure plasma discharges for surface functionalization of PTFE used in biomedical applications

Plasma polymer surface modification is widely used in the biomedical field to tailor the surface properties of materials to improve their biocompatibility. Most of these treatments are performed using low pressure plasma systems but recently, filamentary dielectric barrier discharge (FDBD) and atmospheric pressure glow discharge (APGD) have appeared as interesting alternatives. The aim of this paper is to evaluate the potential of surface modifications realized with FDBD and APGD in different atmospheres (N2+ H2 and N2+ NH3 mixtures) on poly(tetrafluoroethylene) to determine the relative influence of both the discharge regime and the gas nature on the surface transformations. From XPS analysis, it is shown that the discharge regime can have a significant effect on the surface transformation; FDBDs operating in H2/N2 lead to a high concentration of amino-groups with high specificity but also important damaging on the surface. Glow discharges in both H2/N2 and NH3/N2 lead to lower concentrations of amino-groups with lower specificity but lower surface damaging. Therefore, this simple surface treatment seems to be an effective, low cost method for the production of uniform surface modification with amino-groups that can subsequently be used to graft various chemical functionalities used for biomaterial compatibility.

Characterization of Multilayer Anti-Fog Coatings

Fog formation on transparent substrates constitutes a major challenge in several optical applications requiring excellent light transmission characteristics. Anti-fog coatings are hydrophilic, enabling water to spread uniformly on the surface rather than form dispersed droplets. Despite the development of several anti-fog coating strategies, the long-term stability, adherence to the underlying substrate, and resistance to cleaning procedures are not yet optimal. We report on a polymer-based anti-fog coating covalently grafted onto glass surfaces by means of a multistep process. Glass substrates were first activated by plasma functionalization to provide amino groups on the surface, resulting in the subsequent covalent bonding of the polymeric layers. The anti-fog coating was then created by the successive spin coating of (poly(ethylene-maleic anhydride) (PEMA) and poly(vinyl alcohol) (PVA) layers. PEMA acted as an interface by covalently reacting with both the glass surface amino functionalities and the PVA hydroxyl groups, while PVA added the necessary surface hydrophilicity to provide anti-fog properties. Each step of the procedure was monitored by XPS, which confirmed the successful grafting of the coating. Coating thickness was evaluated by profilometry, nanoindentation, and UV visible light transmission. The hydrophilic nature of the anti-fog coating was assessed by water contact angle (CA), and its anti-fog efficiency was determined visually and tested quantitatively for the first time using an ASTM standard protocol. Results show that the PEMA/PVA coating not only delayed the initial period required for fog formation but also decreased the rate of light transmission decay. Finally, following a 24 hour immersion in water, these PEMA/PVA coatings remained stable and preserved their anti-fog properties.

Effects of chemical composition and the addition of H2 in a N2 atmospheric pressure dielectric barrier discharge on polymer surface functionalization.

We examined the effect of hydrogen content in various polymers in a N2/H2 discharge for surface amine functionalization. Three polymers (polyethylene (PE), polyvinylidene fluoride (PVDF), and poly(tetrafluoroethylene) (PTFE)) containing various amounts of hydrogen and fluorine were treated with an atmospheric pressure dielectric barrier discharge (DBD). While surface modification was observed on the PE and the PVDF in a pure N2 discharge, adding H2 in a N2 discharge was necessary to observe the fluorine etching on the surface of the PVDF and PTFE polymers. The presence of a slight amount of hydrogen in the gas mixture was also a prerequisite to the formation of amino groups on the surface of all three polymers (max NH2/C approximately 5%). Aging revealed that the modified polymer surfaces treated in a N2-H2 discharge were less prone to hydrophobic recovery than were surfaces treated in pure N2, due primarily to the presence of a higher density of polar groups on the surfaces. We demonstrated that H atoms in the discharge are necessary for the surface amine functionalization of polymers in a N2 atmospheric pressure DBD, regardless of polymer chemical composition. It is therefore possible to control the plasma functionalization process and to optimize the concentration and specificity of NH2 grafted onto polymer surfaces by varying the H2 concentration in a N2 atmospheric pressure DBD.

Study of the adhesion of thin plasma fluorocarbon coatings resisting plastic deformation for stent applications

Metallic intravascular stents are medical devices (316L stainless steel) used to support the narrowed lumen of atherosclerotic stenosed arteries. Despite the success of bare metal stents, restenosis remains the main complication after 3–6 months of implantation. To reduce the restenosis rate of bare metal stents, stent coating is an interesting alternative. Firstly, it allows the modification of the surface properties, which is in contact with the biological environment. Secondly, the coating could eventually act as a carrier for drug immobilization and release. Moreover, the in vivo stent implantation requires in situ stent expansion. This mandatory step generates local plastic deformation of up to 25% and may cause coating failures such as cracking and delamination. Fluorocarbon films were selected in this study as a potential stent coating, mainly due to their chemical inertness, high hydrophobicity, protein retention capabilities and thromboresistance properties. The aim of this study was to investigate the adhesion properties of fluorocarbon films of three different thicknesses deposited by plasma polymerization in C2F6/H2 on 316L stainless steel substrates. A previously developed small punch test was used to deform the coated samples. According to atomic force microscopy, field emission scanning electron microscopy and x-ray photoelectron spectroscopy characterizations, among the coatings with different thicknesses studied, only those with a thickness of 36 nm exhibited the required cohesion and interfacial adhesion to resist the stent expansion without cracking or delaminating. Otherwise, cracks were detected in the coatings having thicknesses equal or superior to 100 nm, indicating a lack of cohesion.

X-ray photoelectron emission microscopy and time-of-flight secondary ion mass spectrometry analysis of ultrathin fluoropolymer coatings for stent applications.

Fluoropolymer plasma coatings have been investigated for application as stent coatings due to their chemical stability, conformability, and hydrophobic properties. The challenge resides in the capacity for these coatings to remain adherent, stable, and cohesive after the in vivo stent expansion, which can generate local plastic deformation of up to 25%. Plasma-coated samples have been prepared by a multistep process on 316L stainless steel substrates, and some coated samples were plastically deformed to mimic a stent expansion. Analyses were then performed by X-ray photoelectron spectroscopy (XPS), X-ray photoelectron emission microscopy (X-PEEM), and time-of-flight secondary ion mass spectrometry (TOF-SIMS) to determine the chemical and physical effects of such a deformation on both the coating and the interfacial region. While XPS analyses always showed a continuous coating with no significant effect of the deformation, TOF-SIMS and near-edge X-ray absorption fine structure (derived from X-PEEM) data indicated the presence of a certain density of porosity and pinholes in all coatings as well as sparse fissures and molecular fragmentation in the deformed ones. The smallness of the area fraction affected by the defects and the subtlety of the chemical changes could only be evidenced through the higher chemical sensitivity of these latter techniques.

Influence of cross-rolling on the micro-texture and biodegradation of pure iron as biodegradable material for medical implants.

Iron-based biodegradable metals have been shown to present high potential in cardiac, vascular, orthopaedic and dental in adults, as well as paediatric, applications. These require suitable mechanical properties, adequate biocompatibility while guaranteeing a low toxicity of degradation products. For example, in cardiac applications, stents need to be made by homogeneous and isotropic materials in order to prevent sudden failures which would impair the deployment site. Besides, the presence of precipitates and pores, chemical inhomogeneity or other anisotropic microstructural defects may trigger stress concentration phenomena responsible for the early collapse of the device. Metal manufacturing processes play a fundamental role towards the final microstructure and mechanical properties of the materials. The present work assesses the effect of mode of rolling on the micro-texture evolution, mechanical properties and biodegradation behaviour of polycrystalline pure iron. Results indicated that cross-rolled samples recrystallized with lower rates than the straight-rolled ones due to a reduction in dislocation density content and an increase in intensity of {100} crystallographic plane which stores less energy of deformation responsible for primary recrystallization. The degradation resulted to be more uniform for cross-rolled samples, while the corrosion rates of cross-rolled and straight-rolled samples did not show relevant differences in simulated body solution. Finally, this work shows that an adequate compromise between biodegradation rate, strength and ductility could be achieved by modulating the deformation mode during cold rolling.

Effect of grain sizes on mechanical properties and biodegradation behavior of pure iron for cardiovascular stent application

ABSTRACT Pure iron has been demonstrated as a potential candidate for biodegradable metal stents due to its appropriate biocompatibility, suitable mechanical properties and uniform biodegradation behavior. The competing parameters that control the safety and the performance of BMS include proper strength-ductility combination, biocompatibility along with matching rate of corrosion with healing rate of arteries. Being a micrometre-scale biomedical device, the mentioned variables have been found to be governed by the average grain size of the bulk material. Thermo-mechanical processing techniques of the cold rolling and annealing were used to grain-refine the pure iron. Pure Fe samples were unidirectionally cold rolled and then isochronally annealed at different temperatures with the intention of inducing different ranges of grain size. The effect of thermo-mechanical treatment on mechanical properties and corrosion rates of the samples were investigated, correspondingly. Mechanical properties of pure Fe samples improved significantly with decrease in grain size while the corrosion rate decreased marginally with decrease in the average grain sizes. These findings could lead to the optimization of the properties to attain an adequate biodegradation-strength-ductility balance.

On the long term antibacterial features of silver-doped diamondlike carbon coatings deposited via a hybrid plasma process

Environmental surfaces are increasingly recognized as important sources of transmission of hospital-acquired infections. The use of antibacterial surface coatings may constitute an effective solution to reduce the spread of contamination in healthcare settings, provided that they exhibit sufficient stability and a long-term antibacterial effect. In this study, silver-incorporated diamondlike carbon films (Ag-DLC) were prepared in a continuous, single-step plasma process using a hybrid, inductively coupled plasma reactor combined with a very-low-frequency sputtering setup. The average Ag concentration in the films, ranging from 0 to 2.4 at. %, was controlled by varying the sputtering bias on the silver target. The authors found that the activity of Escherichia coli was reduced by 2.5 orders of magnitude, compared with the control surface, after a 4-h contact with a 2.4 at. % Ag-DLC coating. The coatings displayed slow release kinetics, with a total silver ion release in the sub-ppb range after 4 h in solution, as measured by graphite furnace-atomic absorption spectroscopy. This was confirmed by Kirby-Bauer diffusion tests, which showed limited diffusion of biocidal silver with a localized antibacterial effect. As a slow and continuous release is mandatory to ensure a lasting antibacterial effect, the newly developed Ag-DLC coatings appears as promising materials for environmental hospital surfaces.

Influence of the 316 L stainless steel interface on the stability and barrier properties of plasma fluorocarbon films.

Coatings are known to be one of the more suited strategies to tailor the interface between medical devices and the surrounding cells and tissues once implanted. The development of coatings and the optimization of their adhesion and stability are of major importance. In this work, the influence of plasma etching of the substrate on a plasma fluorocarbon ultrathin coating has been investigated with the aim of improving the stability and the corrosion properties of coated medical devices. The 316 L stainless steel interface was subjected to two different etching sequences prior to the plasma deposition. These plasma etchings, with H(2) and C(2)F(6) as gas precursors, modified the chemical composition and the thickness of the oxide layer and influenced the subsequent polymerization. The coating properties were evaluated using flat substrates submitted to deformation, aging into aqueous medium and corrosion tests. X-ray photoelectron spectroscopy (XPS), time of flight-secondary ion mass spectrometry (ToF-SIMS), ellipsometry, and atomic force microscopy (AFM) were performed to determine the effects of the deformation and the aging on the chemistry and morphology of the coated samples. Analyses showed that plasma etchings were essential to promote reproducible polymerization and film growth. However, the oxide layer thinning due to the etching lowered the corrosion resistance of the substrate and affected the stability of the interface. Still, the deformed samples did not exhibited adhesion and cohesion failure before and after the aging.

Toward High-Performance Coatings for Biomedical Devices: Study on Plasma-Deposited Fluorocarbon Films and Ageing in PBS

High performance coatings tailored to medical devices represent a recognised approach to modulate surface properties. Plasma-deposited fluorocarbon films have been proposed as a potential stent coating. Previous studies have shown promising adhesion properties: the 35 nm-thick film sustained plastic deformation up to 25% such as induced during the clinical implantation. In this study, the compositional and morphological changes of plasma-deposited fluorocarbon films were examined during ageing in a pseudo-physiological medium, a phosphate buffer solution (PBS), by angle-resolved XPS, FT-IR data and AFM images. The evolution of the ageing process is discussed: defluorination and crosslinking yielded an oxidized protective top layer onto the films, which showed further degradation.