Effects of DBD plasma operating parameters on the polymer surface modification Chaozong Liu a , Naiyi Cui b , Norman M.D. Brown b , Brian J. Meenan a, * a Northern Ireland Bioengineering Centre, University of Ulster, Shore Road, Newtownabbey, Co. Antrim BT37 0QB, Northern Ireland, UK b Surface Science Laboratory, University of Ulster, Coleraine BT52 1SA, UK Received 7 July 2003; accepted in revised from 22 January 2004 Available online 15 April 2004 Abstract This paper, based on an orthogonal experimental design and analysis method, reports the effects of a dielectric barrier discharge (DBD) plasma surface treatment on polytetrafluoroethylene (PTFE), polyimide (PI) and poly (lactic acid) (PLA) films in terms of changes in surface wettability and surface chemistry. The purpose was to study the influence of the main operating parameters, i.e. plasma power, treatment period duration (treatment cycles) and electrode gap on the resultant surface properties. Statistical analysis was carried out to develop an equation which expresses surface properties (water contact angle and oxygen enrichment, as observed by XPS analysis) in terms of these operational parameters. It was observed that the plasma parameters have a selective effect on the changes observed for the polymers processed. In particular, plasma processing time (treatment cycles), plays an important role in the treatment of PTFE and PI in this study, whereas the size of the electrode gap plays the dominant role in the treatment of PLA. Fast surface activation can be achieved in all cases after only a few seconds of treatment duration. The wettability improvement observed in all cases was attributed to changes in both surface chemistry and surface micro-structure. D 2004 Elsevier B.V. All rights reserved. Keywords: Dielectric barrier discharge plasma processing; Polymer surface modification; Water contact angle; Oxygen enrichment; XPS 1. Introduction The interaction between the biological environment and biomaterials (and thereby medical devices) takes place mainly at the materials surface. Moreover, the biological response from living tissues to such biomaterials depends on their intrinsic or directed surface properties, e.g. chemical composition, cleanliness, texture, surface energy, corrosion/ erosion resistance, etc. and the attendant influence of these properties on biological species. The surface attributes of medical implants can be designed purposely to enhance their functionality and biocompatiblity through various surface modification methods, such as grafting of functional groups, chemical treatment, ion irradiation, blending, thin film deposition and plasma treatment [1–3]. Plasma mod- ification techniques have advantages compared to other methods, especially their ability to uniformly modify the surface without affecting the bulk properties. This approach has proved to be a very promising method for achieving polymer surface treatments without changing bulk proper- ties. As a result, plasma treatment of bio-polymers (poly- mers used in biomaterial applications) is gaining popularity in biomedical engineering. Applications include cleaning/ sterilization, coating or deposition and induced grafting of functional groups by plasma polymerization. For example, Liu et al. [4] have reported that the surface properties of polytetrafluoroethylene (PTFE), including chemistry and microstructure, can be modified by different types of plas- ma. Specifically, hydrophobic PTFE surfaces can be con- verted to a more hydrophilic condition [5–9]. Moreover, the radicals that are introduced by plasma treatment can be utilized subsequently for post-plasma reactions, such as grafting processes [10] and, if required, for covalent protein immobilization [11]. Coen et al. have modified several polymers, including polypropylene (PP), polymethylmetha- 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.01.024 * Corresponding author. Tel.: +44-28-9036-8928; fax: +44-28-9036- 6863. E-mail addresses: bj.meenan@ulster.ac.uk (B.J. Meenan), c.liu@ulster.ac.uk (C. Liu). www.elsevier.com/locate/surfcoat Surface & Coatings Technology 185 (2004) 311–320