Surface modification of ultra-high molecular weight polyethylene (UHMWPE) by argon plasma
Introduction
The use of artificial implants for replacement of the hip, knee, and other articular joints has become very common in light of improving life quality and increasing life expectancies [1]. The ultra-high molecular weight polyethylene (UHMWPE) has the characteristics of notably high abrasion resistance, high impact strength, low friction, excellent toughness, ease of fabrication, biocompatibility and bio-stability [2], therefore, UHMWPE has been used as a bearing material in hip and knee total joint replacement. However, this soft material has a high materials loss in relative motions between the friction partners. The biological reaction to these debris particles is called osteolysis, resulting in a gradual loss of the bone tissue surrounding the stem of the implant and thus the loosening of the prosthesis [3], [4]. Plasma treatment is suggested here as an efficient and environmentally friendly process which has become a frequently used method to modify polymer surfaces without changing the bulk composition.
Argon (Ar) plasma has been often used for crosslinking of molecules on a polymer surface [5], [6]. The structural changes of polymer surface caused by plasma ions could induce improvements in hardness, wettability, surface chemical activity, crosslinking and other properties [3]. In our current investigation, microwave electron cyclotron resonance (MWECR) plasma source has been used to modify the surface of UHMWPE samples. Compared with radio frequency glow discharge (RFGD) plasma frequently used in polymer surface modification, the MWECR plasma has significant advantages, such as high ion density, low substrate temperature, and independent control of ion energy during the treatment. These ensure it a better surface modification of the polymer compared with the conventional plasma surface modification technique [7]. In this paper, the effect of argon plasma treatment on the surface properties of UHMWPE is investigated.
Section snippets
Materials
The medical grade UHMWPE in this study was made by German Ploy Hi Solidur Deutschland GmbH MediTECH Division Company, the detail properties are shown in Table 1. The UHMWPE was cut into 12 mm diameter discs with 3 mm thickness and the disc samples were ground and polished to reduce the surface roughness.
Experimental procedure
Microwave electron cyclotron resonance (MWECR) plasma modification technique was used to modify the polymer surfaces. The schematic diagram of the MWECR source is shown in Fig. 1, which has been
Changes in chemical structure
Chemical structures of UHMWPE samples were characterised by FTIR spectroscopy. Fig. 2 shows the FTIR spectra of the native and argon plasma treated UHMWPE. The absorption peaks of 2917, 2848, 1467, and 722 cm−1 are attributed to methylene non-symmetry stretch vibration, methylene symmetry stretch vibration, methylene non-symmetry changing angle vibration, and methylene swing in plane vibration, respectively. The areas of these absorption peaks, which are corresponding to the vibration strength
Conclusions
In this paper, the feasibility and validity of using MWECR Ar plasma treatment to modify the surface of UHMWPE are discussed. Ar plasma treatment greatly improves surface wettability due to the formation of polar groups (mainly carbonyl, carboxyl and hydroxyl groups) as demonstrated by FTIR spectroscopy. The active electrons and ions in the plasma bombard the surface, break C–H and C–C bondings and produce free radicals. Additional crosslinking among radicals occurs between the molecular chains
Acknowledgements
The authors would like to thank Prof. Linmao Qian, Prof. Jin Wang and Prof. Yong Wang for technical assistance and their help in discussing this paper. This work is supported by the National High Technology Research and Development Program of China (863 Program #2006AA02A135), the Program for New Century Excellent Talents in University (NCET-06-0800) and National Natural Science Foundation of China (#50802077).
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