Effect of surface nanostructuring of PDMS on wetting properties, hydrophobic recovery and protein adsorption M.-E. Vlachopoulou a,c , P.S. Petrou b , S.E. Kakabakos b , A. Tserepi a, * , K. Beltsios c , E. Gogolides a a Institute of Microelectronics, NCSR Demokritos, 15310 Aghia Paraskevi, Greece b Institute of Radioisotopes and Radiodiagnostic Products, NCSR Demokritos, 15310 Aghia Paraskevi, Greece c Materials Science and Engineering Department, University of Ioannina, 45110 Ioannina, Greece article info Article history: Received 1 October 2008 Received in revised form 12 November 2008 Accepted 14 November 2008 Available online 3 December 2008 Keywords: PDMS Nanostructures Plasma Hydrophobic recovery Protein adsorption abstract Poly(dimethylsiloxane) (PDMS) is an inherently hydrophobic material widely used in microfluidic and lab-on-a-chip devices. In this work, plasma-induced surface nanostructuring of PDMS in SF 6 was explored as a means to control surface wettability modified in SF 6 and O 2 plasmas. In addition, rapid hydrophobic recovery was achieved with a process consisting of washing with de-ionized water and thermal treat- ment of the surfaces. Immobilization of biotinylated-BSA (b-BSA) on treated surfaces was also investi- gated. The results indicate that intense surface topography leads to significant delay in hydrophobic recovery of the surface and an enhanced capacity for protein adsorption. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction PDMS is widely used as a structural material for microfluidic systems and biochips for applications in life sciences, because of its ease in fabrication, its inertness and biocompatibility [1,2]. PDMS is inherently hydrophobic exhibiting a water contact angle of 110°, however, plasma treatments can modify surface wettabil- ity, as a result of both chemical and morphological modification of the surface. Hydrophilization of PDMS surface is of great interest, as for example, hydrophilic microchannels exhibit increased cell adhesion or reduced air bubble trapping during filling with aque- ous solutions. Ageing of hydrophilized PDMS has also been widely reported after various plasma treatments [3–5], while thermal treatment [6] and surface nanoroughness [7] have also been re- ported to affect the rate of hydrophobic recovery. SF 6 plasma treatment of PDMS in a high density plasma reactor under conditions of anisotropic etching results in the formation of columnar-like nanostructures on the surface, as we have reported in previous work [8,9]. We have also reported modification of the surface wettability and ageing after SF 6 plasmas and its effect on protein adsorption [10]. In this work, we investigate the effect of SF 6 plasma-induced nanoroughness on hydrophilization of PDMS surfaces after subsequent treatment with O 2 plasmas and on the following hydrophobic recovery. A process for rapid hydrophobic recovery is also proposed. Protein immobilization on such surfaces is investigated. 2. Experimental A commercial PDMS material (Sylgard 184, supplied by Dow Corning) was used. Thin films of PDMS were prepared by spin- coating a 10:1 mixture of base and its curing agent on Si substrates, to form PDMS films of 10 lm thickness after thermal cross-linking of the material (baked at 150 °C for 15 min). The prepared PDMS samples were treated in SF 6 plasmas generated in an inductively coupled plasma (ICP) reactor with a helicon source of 13.56 MHz (Micromachining Etching Tool, MET, from Alcatel). The etching plasma was produced at conditions (1900 W, À100 V, 10 mTorr SF 6 ) ensuring anisotropic etching of the exposed PDMS surface. A reactive ion etcher (RIE) from Nextral (NE 330) was used to further hydrophilize the nanostructured PDMS surfaces in O 2 plasmas un- der mild conditions (100 mTorr, 100 W). Topographical character- ization of the surfaces was performed on a Nanoscope III Digital Instruments Atomic Force Microscope, used in the tapping mode, for surfaces exposed to the plasmas for short time (up to 2 min). A Leo 440 SEM was used for observation of PDMS surfaces treated in the plasma for longer times. Wettability of the plasma treated surfaces was probed by static and dynamic water contact angle measurements, performed with a GBX Digidrop System. In order to evaluate adsorption capacity of PDMS surfaces, bovine serum albumin (BSA, Cohn fraction V, RIA grade) was 0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.11.050 * Corresponding author. Tel.: +30 210 650 3264; fax: +30 210 651 1723. E-mail address: atserepi@imel.demokritos.gr (A. Tserepi). Microelectronic Engineering 86 (2009) 1321–1324 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee