The versatility of silicone polymers, their physical and chemical attributes enable them to be used in a wide band of applications as e.g.: an interconnection between two silicon wafers, a spring material in accelerometers or as the ion selective membranes for ISFETs. Polysiloxanes can also be used in mechanical and chemical decoupling of biomedical micro devices from their environment. Since silicone precursors are viscous it is possible to form substrate or coating films by spin coating method. Polysiloxanes are not photo-definable and cannot be photolithographically treated. Among the possible ways to pattern silicon elastomers is plasma processing. Silicones can be effectively etched and/or permanently modified using fluorine-containing plasma. This treatment, however, may also cause unwanted modification in morphology and surface contamination with fluorine compounds such as C_xF_y or with a mask material e.g. aluminum. Uncontrolled surface changes may turn an initially non-cytotoxic material into a cytotoxic one. The chemical composition, roughness and wettability are believed to directly influence the cell viability and adhesion. That is why the surface purity and proper morphology after processing are key factors for biomedical applications. In this work we studied the fluorine-based plasma treatments of a highly popular silicone elastomer based on poly(methylhydrogensiloxane-co-dimethylsiloxane) and their potential effects on BioMEMS devices. Plasma experiments were performed in both RIE and ICP reactors using CF₄, SF₆ and O₂ gases. The surface morphology and wettability were investigated by means of SEM and contact angle measurements, respectively. The chemical composition of post-etch remnants was analysed by XPS. In order to estimate the cytocompatibility of plasma treated elastomer the standard MTT and CV tests were used. The surface morphology was found to be very dependent on the discharge condition (RIE/ICP) and temperature treatment. The polysiloxane surface appeared to preserve or even improve its hydrophobic properties after fluorine containing plasma exposure. The contact angle values varied between 110° and 140°, depending on the treatment temperature and plasma chemistry. The most hydrophobic surface consisted of tiny, irregular pillars covered with conformal fluorocarbon film. The XPS analysis showed that the polysiloxane surface enriched with fluorine. The amount of fluorine found on the surface was three times higher after the treatment in CF₄ plasma than in SF₆ one. Despite some contamination and morphology changes the results of the preliminary cytotoxicity study appeared to be very promising: cell viability on a raw and all plasma treated polysiloxane was very high and comparable to control. Such a result is important if the plasma treated polysiloxanes are considered for use in BioMEMS application.
This work has been partly supported by the European Commission - the Healthy Aims Project IST-2002-1-001837.

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