1-s2.0-S0167931708005388-main

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 a r t i c l e i n f o Article history: Received 1 October 2008 Rec eiv ed in rev ise d form12 Nov emb er 200 8 Accepted 14 November 2008 Available online 3 December 2008 Keywords: PDMS Nanostructures Plasma Hydrophobic recovery Protein adsorption a b s t r a c t Poly(dimethylsiloxane) (PDMS) is an inherently hydrophobic material widely used in microfluidic and lab- on-a -chip devices. In this work , plas ma- induced surfa ce nano struc turin g of PDMS in SF 6 was expl ored 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 treatment of the surfaces. Immobilization of biotinylated-BSA (b-BSA) on treated surfaces was also investigate d. The results indicate that inte nse surface topogra phy leads to signi ficant dela y in hydro phob ic recovery of the surface and an enhanced capacity for protein adsorption. 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 iner tnes s and bioc ompati bility [1,2]. PDMS is inherently hydrophobic exhibiting a water contact angle of 110, however, plasma treatments can modify surface wettability, as a result of both chemical and morphological modification of the surface. Hydrophilization of PDMS surface is of great interest, as for examp le, hydrop hilic microchannel s exhibit increased cell adhesion or reduced air bubble trapping during filling with aque- ous solutions. Ageing of hydrophilized PDMS has also been widely reported afte r vari ous pla sma trea tme nts [3–5], whi le ther mal treatment [6] and surface nanoroughness [7] have also been reported 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 immobili zation on such surfaces is investigated. 2. Experimental A commer cial PDMS mat eria l (Syl gard 184, supp lied by Dow Cor ning ) was used. Thin films of PDMS wer e prepared by spin - co at ing a 10:1 mi xt ur e of ba se and its curi ng ag ent on Si subs tr at es, to form PDMS films of 10 lm thickness after thermal cross-linking of the material (baked at 150 C for 15 min ). The prepar ed PDMS samples were treated in SF 6 plasmas generated in an inductively coup led plasma (ICP) reacto r with a heli con source of 13.56 MHz (Mic rom achi ning Etch ing Too l, MET , from Alcatel ). The etchi ng pla sma was produced at cond itio ns (190 0 W, 10 0 V, 10 mT or r SF 6 ) ensuring anisotropic etching of the exposed PDMS surface. A reactive ion etch er (RIE) fr om Nex tral (NE 330) was used to furthe r hydrophilize the nanostructured PDMS surfaces in O 2 plasmas under mild conditio ns (100 mTor r, 100 W). Topog raphical chara cter- ization of the surfaces was performed on a Nanoscope III Digital Instruments Atomic Force Microscope, used in the tapping mode, for surfa ces expo sed to the plasm as for shor t 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 surf aces was probed by stati c and dynamic water cont act angl e measurements, performed with a GBX Digidrop System. In ord er to eval uate adsorp tion cap acity of PDMS surf aces , bo vin e ser um alb umin (BS A, Cohn fracti on V, RIA grade) wa s 0167-9317/$ - see front matter 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: [email protected] (A. Tserepi). Microelectronic Engineering 86 (2009) 1321–1324 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee

1-s2.0-S0167931708005388-main

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