Controlled Formation and Mixing of Two-Dimensional Fluids Ilja Czolkos, ² Yavuz Erkan, ² Paul Dommersnes, ‡ Aldo Jesorka, ² and Owe Orwar* ,² Department of Chemical and Biological Engineering, Chalmers UniVersity of Technology, 412 96 Go ¨teborg, Sweden, and MSC, UniVersite ´ Paris Diderot, 10, rue Alice Domon et Le ´ onie Duquet, F-75205 Paris, France Received March 28, 2007; Revised Manuscript Received May 8, 2007 ABSTRACT We introduce a novel technique for the controlled spreading and mixing of lipid monolayers from multilamellar precursors on surfaces covered by the hydrophobic epoxy resin SU-8. The lipid spreads as a monolayer as a result of the high surface tension between SU-8 and the aqueous environment. A micropatterned device with SU-8 lanes, injection pads, and mixing regions, surrounded by hydrophilic Au, was constructed to allow handling of lipid films and to achieve their mixing at controlled stoichiometry. Our findings offer a new approach to dynamic surface functionalization and decoration as well as surface-based catalysis and self-assembly. In the past few years, the successful formation of two- dimensional lipid interfaces has received numerous contribu- tions because of their potentially useful application in, for example, biotechnology and surface science. It has been shown that different types of planar lipid membranes, such as supported lipid bilayers on hydrophilic surfaces, 1-4 polymer-cushioned lipid bilayers, 5,6 and tethered lipid bi- layers, 5 could be established for biotechnological or biosen- sorical exploitation. Furthermore, lipid monolayers could be formed by the traditional Langmiur-Blodget deposition technique 7 or by lipid vesicle adsorption. 8,9 Self-assembled monolayers (SAMs) represent another class of molecularly thin films that, in contrast to lipid or surfactant systems, do not possess properties of a fluid. SAMs, which typically are composed of alkanthiolates or other alkyl compounds on Au, Ag, and Cu 10 are easily formed 11 and have been shown to be very useful for surface modification and functionalization on the micrometer scale. 12-15 We have developed a concept for controlled formation of liquid films on microfabricated hydrophobic substrates that we call dynamic liquid film formation (DLFF). In contrast to previous methods of fabrication, this method allows for stoichiometric control of the different components included in the film. As hydrophobic substrate, we used SU-8, which is a negative tone photoresist that can vary in hydrophobicity depending on the fabrication procedure. 16 We spin-coated glass coverslips with SU-8, which is ab initio hydrophobic and thereby permits lipid monolayer adsorption. The contact angle of water on SU-8 was determined to be 91.4° ( 1.5°. When multilamellar lipid vesicles suspended in a buffer droplet are placed on the SU-8 substrate, the contained lipid rapidly spreads as a monolayer on the surface. The formed lipid patches are perfectly circular, as shown in Figure 1a. The multilamellar vesicles are eventually entirely depleted and transformed into a lipid monolayer. The tension induced by SU-8 is sufficient to disrupt the structure of the multi- lamellar vesicle. Therefore, the surface adhesion energy of lipids on SU-8, Σ, is larger than the lysis tension of bilayer membranes σ L ≈ 2-9 mN/m. The adsorbed lipid basically screens the hydrophobic surface energy between SU-8 and water, and the gain in surface energy associated with lipid adsorption, Σ, is expected to be roughly equal to the surface tension between SU-8 and water. SU-8 is an epoxy, and it is therefore reasonable to assume that the surface tension SU-8/water could be as high as σ epoxy ≈ 47 mN/m. 17 We quantified the dynamics of the lipid spreading process and found that the wetted area A over time is approximately linear at the beginning of the spreading process (see Figure 1a). In refs 18 and 19, the dynamics of spreading was modeled by balancing the lipid film Marangoni stress ∇σ with the sliding friction force between lipid film and surface (per unit area): ∇σ - V) 0. For lipid film spreading on a lane of SU-8, the spreading velocity is V)  /t where  ) S/2 is the spreading coefficient and the spreading power S is the difference in free energy between lipids on the surface and lipids in the reservoir (per unit area). 19 The lipid film velocity on a lane is uniform over the film, 18,19 whereas for circular spreading, there is a gradient in velocity. For circularly spreading monolayers, we find that the radius of the spreading film is given by R log(R/R 0 )dR/dt ) 2. Taking * Corresponding Author: E-mail: orwar@chalmers.se. Telephone: +46- 31-772-3060. Fax: +46-31-772-6120. ² Department of Chemical and Biological Engineering, Chalmers Uni- versity of Technology. ‡ MSC, Universite ´ Paris Diderot. NANO LETTERS 2007 Vol. 7, No. 7 1980-1984 10.1021/nl070726u CCC: $37.00 © 2007 American Chemical Society Published on Web 06/06/2007