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The Use of DBD Plasma Treatment and Polymerization for the Enhancement of Biomedical UHMWPE

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The Use of DBD Plasma Treatment and Polymerization for the Enhancement of Biomedical UHMWPE View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Ghent University Academic Bibliography The use of DBD plasma treatment and polymerization for the enhancement of biomedical UHMWPE By Pieter Cools, Stijn Van Vrekhem, Nathalie De Geyter and Rino Morent P. Cools, Dr. N. De Geyter, Prof. Dr. R. Morent Research Unit Plasma Technology (RUPT), Department of Applied Physics, Faculty of Engineering, Ghent University, St-Pietersnieuwstraat 41 B4, Ghent, 9000, Belgium E-mail: [email protected] Tel: +32 9 264 38 38 1 The use of DBD plasma treatment and polymerization for the enhancement of biomedical UHMWPE Pieter Cools*, Stijn Van Vrekhem, Nathalie De Geyter, Rino Morent P. Cools, S. Van Vrekhem, Dr. N. De Geyter, Prof. Dr. R. Morent Research Unit Plasma Technology (RUPT), Department of Applied Physics, Faculty of Engineering, Ghent University, St-Pietersnieuwstraat 41 B4, Ghent, 9000, Belgium E-mail: [email protected] Abstract Surface modification of polymers for biomedical applications is a thoroughly studied area. The goal of this paper is to show the use of atmospheric pressure plasma technology for the treatment of polyethylene shoulder implants. Atmospheric pressure plasma polymerization of methyl methacrylate will be performed on PE samples to increase the adhesion between the polymer and a PMMA bone cement. For the plasma polymerization, a dielectric barrier discharge is used, operating in a helium atmosphere at ambient pressure. Parameters such as treatment time, monomer gas flow and discharge power are varied one at a time. Chemical and physical changes at the sample surface are studied making use of X-ray photoelectron spectroscopy and atomic force microscopy measurements. Coating thicknesses are determined by making use of optical reflectance spectroscopy. After characterization, the coated samples are incubated into a phosphate buffered saline solution for a minimum of one week at 37°C, testing the coating stability when exposed to implant conditions. The results show that PMMA coatings can be deposited with a high degree of control in terms of chemical composition and layer thickness Keywords: Dielectric barrier discharge, XPS, plasma activation, plasma polymerization, PMMA 2 1. Introduction Total shoulder arthroplasty (TSA) is the third most common arthroplasty in the world.[1] It is a commonly used treatment for diseases of the glenohumeral joint, such as advanced osteoarthritis, rheumatoid arthritis or avascular necrosis.[2] An anatomical TSA mimics the anatomy of the shoulder joint and therefore consists of a humeral and glenoid component. The humeral component contains a stem and a rounded head or ball, usually made out of titanium or cobalt-chromium. The glenoid component is a concave socket made of ultra-high molecular weight polyethylene (UHMWPE). The major problem concerning total shoulder replacement today is the loosening of these components.[3] Literature states that loosening of the glenoid component is occurring more frequently than loosening of the humeral component.[1, 3] The fixation of both components has initially been performed by using a bone cement, usually a polymethyl methacrylate (PMMA) resin. Nowadays, next to bone cement fixation, also metal-backed glenoid components are used. These components are based on bone ingrowth fixation and do not need bone cement.[4] However, in spite of these metal-backed components and the fact that the glenoid component can come in different geometrical designs, e.g. pegged or keeled, the incidence of glenoid component loosening is still too high. This is mostly due to the nature of the material that is used. Although UHMWPE has adequate mechanical properties, it is an inert and non-polar material. Therefore, it is difficult to chemically bind UHMWPE in order to get an adhesive fixation.[5] Improving the adhesive properties of medical grade UHMWPE by means of surface modification could consequently lead to an improved performance of a shoulder prosthesis. Literature shows that research is going into ameliorating the adhesive properties of UHMWPE by using, amongst others, radiation methods, gas-phase based treatments or simple mechanical abrasion. Radiation methods include ion-, gamma- or electron irradiation. Gas- 3 phase based treatments include UV/ozone treatment, corona treatment or glow discharge treatment. [5, 6] In the past, extensive research, using plasma activation, has been successfully done on the improvement of the wettability of polymer surfaces.[7-14] Functional groups, such as hydroxyls and alkanoates, are introduced onto the surface, making it more polar and less inert. The introduction of these functional groups occurs through a two-step process. Starting with the formation of free radicals on the polymer surface during the plasma treatment, it is followed by the reaction of these free radicals with the medium that is present. [15-19] Past research has shown that the increase in wettability can lead to an increase in cell adhesion and proliferation. [12, 20, 21] Next to plasma activation, also plasma polymerization is an interesting technique to change the properties of a material’s surface. Plasma polymerization is an easy method to produce polymer nano-films on a substrate. These plasma polymerized films are pinhole-free, completely amorphous, insolvable in most solvents and highly cross-linked, which results in a good thermal, chemical and mechanical stability. Plasma polymers also have a strong adherence on numerous substrates.[17-19, 22] The aim of this study is to use both plasma treatment and plasma polymerization of methyl methacrylate (MMA) on an UHMWPE surface to improve its bioactivity and adhesive properties. As mentioned before, plasma treatment will turn an inert UHMWPE surface into an activated surface. Plasma polymerization of MMA will deposit a plasma-PMMA layer on the pre-activated UHMWPE surface. Since this intermediate layer then consists of the same material as the bone cement that is used in TSAs, the adhesion between the implant material and the bone cement should be improved. Both plasma activation and plasma polymerization are performed by using a dielectric barrier discharge (DBD) at medium pressure and 4 atmospheric pressure respectively. DBDs are known for their easy formation of a stable discharge and their scalability. The efficiency of medium pressure DBDs has already been demonstrated [9-12, 14, 23-25]. The fact that the polymerization process can be performed at atmospheric conditions highlights its simplicity.[26, 27] In order to get an insight in the modifications made to the surface, a varied set of surface analysis techniques are used. Hence, for both plasma methods, static water contact angle goniometry (WCA), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and optical reflectance spectroscopy (OPS) are performed. 2. Materials and methods 2.1.Chemicals and films MMA (99%, 30 ppm MEHQ as inhibitor) was purchased from Sigma-Aldrich and used as such. Helium, dry air, argon and nitrogen (Alphagaz 1) were purchased from Air Liquide. The salts used to make the PBS solution(sodium chloride (99.8%), potassium chloride (99%), sodium hydrogenphosphate (98%), potassium dihydrogenphosphate (98%) and sodium azide (99%)) were purchased from Carl Roth and mixed to a pH of 7.4. A roll of UHMWPE film with a thickness of 0.5 mm was ordered from Goodfellow Cambridge UK. Samples of 1 cm² were cut out for treatment. 2.2.Plasma methods All plasma treatments presented in this paper are performed with a dielectric barrier discharge (DBD) reactor, which is schematically presented in Fig. 1. The discharge is generated between two circular copper electrodes (Ø: 65 mm), which are both covered with a glass plate. The gas gap between these glass plates is 7 mm. The upper electrode is connected to a 50 kHz AC high voltage source, while the lower electrode is connected to earth through a resistor R (50 Ω) or a capacitor C (10 nF). After fixing the UHMWPE sample on the lower 5 glass plate using double sided tape, the plasma reactor is pumped to 0.1 kPa using a rotary vane pump and subsequently filled with the gas of choice at 3 slm (standard litre per minute). After reaching sub-atmospheric pressure (90 kPa), the plasma reactor is flushed at 3 slm with the working gas for 3 min to obtain a controllable gas composition. After this purging step, the pressure in the plasma reactor is lowered to 5.0 kPa. At this medium pressure, plasma activation is performed with a gas flow of 1 slm and for various treatment times ranging from 0 s to 1 min, as shown in Table I. Plasma polymerization is done on the pre-activated UHMWPE substrates in a second DBD reactor, as monomer contamination during the activation step has to be avoided and is schematically presented in Fig. 2. The discharge is generated between two circular copper electrodes (Ø: 55 mm), which are both covered with a glass plate. The gas gap between these glass plates is 3.5 mm. The upper electrode is connected to a 50 kHz AC high voltage source, while the lower electrode is connected to earth through a resistor R (50 Ω) or a capacitor C (10 nF). After fixing the UHMWPE sample on the lower glass plate using double sided tape, the plasma reactor is pumped to 0.1 kPa using a rotary vane pump and subsequently filled with He at 3 slm. After reaching atmospheric pressure (100 kPa) the reactor is flushed at 3 slm with He for 3 min to obtain a controllable gas composition. After the purging step, a mixture of He and monomer gas is allowed to flow between the two electrodes. The main He flow is controlled using a mass flow controller (MKS Instruments, the Netherlands) and is adjustable in the range between 0-10 slm. The monomer is introduced in the gas chamber using a glass bubbler containing the MMA monomer, carried by a secondary He flow.
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