Lipid Vesicle Fusion on μCP Patterned Self-Assembled Monolayers: Effect of Pattern Geometry on Bilayer Formation A. Toby A. Jenkins,* ,† Richard J. Bushby, ‡ Stephen D. Evans, ‡ Wolfgang Knoll, § Andreas Offenha ¨ usser, ⊥ and Simon D. Ogier ‡,# Department of Chemistry, University of Bath, Bath, BA2 7AY U.K., Centre for Self-Organising Molecular Systems, University of Leeds, Leeds, LS2 9JT, UK, Max-Planck-Institut fu ¨ r Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany, and Forschungszentrum Ju ¨ lich, 52425 Ju ¨ lich, Germany Received October 3, 2001. In Final Form: January 14, 2002 Microcontact printing (μCP) of lipophilic self-assembled monolayers (SAMs) has been used to fabricate micron dimensioned patterned surfaces that can be used as a means of attaching lipid bilayers to solid surfaces. This communication addresses how variation in the patterned SAM geometry affects vesicle adsorption. The substrates consisted of circular hydrophilic regions functionalized with mercaptoethanol surrounded by octadecanethiol (ODT). Three geometries were studied in which the diameter of the hydrophilic portion was varied between 4 and 16 μm and the center-to-center separation was varied between 10 and 40 μm. Thus, it was possible to study lipid adsorption on SAM systems with the same total hydrophilic surface area, but with different sized hydrophilic patches. Impedance spectroscopy showed that for the films of the same total hydrophilic area greater lipid coverage was obtained for SAMs with smaller diameter hydrophilic patches. To understand this observation, tapping mode AFM and surface plasmon microscopy were used to study lipid vesicles adsorption on such surfaces. A new mode of vesicle adsorption on micropatterned hydrophobic/hydrophilic surfaces is presented which shows that the simple picture of a bilayer spanning the hydrophilic patches is overly simplistic and that the real situation is considerably more complex and that the hydrophobic-hydrophilic edge plays an important role. Introduction The plasma membrane exists within cells to control the flow of ions, metabolytes, etc., between the extra- and intracellular regions. This important function is achieved by the incorporation of membrane spanning proteins and peptides. The mode of action of these varies. Many form an ion channel through the membrane on application of an external trigger, such as potential change (voltage gating) or analyte interaction (ligand gating). The ultimate aim of the work presented here is to utilize this effect by creating a biomimetic membrane on a solid surface. Such surfaces would be of interest for two reasons: First, they would form the basis for the next generation of biosensors, utilizing Nature’s own biosensing mechanisms, and second, they would provide ideal systems for cell mem- brane protein structure-function studies. A number of researchers have effectively reviewed this area in recent years, especially Sackmann and Groves. 1,2 The micropatterning of solid substrates with self- assembled monolayers (SAMs) has been shown to be an effective way of producing an array of functionally active bilayer patches on a surface. 3,4 The use of micropatterning provides a high degree of control of the spatial distribution of lipophilic support molecules, in this case octadecanethiol (ODT). The remaining surface is then functionalized with a hydrophilic moiety mercaptoethanol. During vesicle adsorption it is believed that a lipid monolayer is adsorbed at hydrophobic surfaces while a “true” lipid bilayer is adsorbed on the hydrophilic regions. Furthermore, studies have shown that membrane proteins will self-locate into such “bilayer patches”, where the lipids are mobile. 5 Figure 1 presents schematically the view generally presented in the literature for vesicle interactions with nonpatterned hydrophobic (a) and hydrophilic surfaces (b). 6 This paper is concerned with understanding the nature of lipid adsorption at micropatterned SAMs. In particular, we are interested in the hydrophilic regions, where the driving force for bilayer formation vs unruptured vesicle adsorption may be weak. Measurements presented in this paper show that lipid adsorption in the hydrophilic, mercaptoethanol SAM regions is far from simple. To understand this properly requires consideration of the edge effects at the interface between the hydrophobic and hydrophilic regions as well as vesicle interaction with hydrophilic surfaces. 3,4,7 The experimental results presented here suggest that on relatively large hydrophilic patches, some of the adsorbed lipid may not be unrolled but be sitting on the mercapto- ethanol surface as lipid vesicles. The evidence for comes † University of Bath. ‡ University of Leeds. § Max-Planck-Institut fu ¨ r Polymerforschung. ⊥ Forschungszentrum Ju ¨ lich. # Present address: Avecia, PO Box 42, Manchester M8 8ZS U.K. * Corresponding author: e-mail a.t.a.jenkins@bath.ac.uk. (1) Sackmann, E. Science 1996, 271, 43-48. (2) Groves, J. T.; Ulman, N.; Boxer, S. G. Science 1997, 275, 651- 653. (3) Jenkins, A. T. A.; Bushby, R. J.; Boden, N.; Evans, S. D.; Knowles, P. F.; Miles, R. E.; Ogier, S. D. Langmuir 1998, 14, 4675-4678. (4) Jenkins, A. T. A.; Bushby, R. J.; Boden, N.; Evans, S. D.; Knowles, P. F.; Miles R. E.; Ogier, S. D.; Scho ¨nherr, H.; Vancso, G. J. J. Am. Chem. Soc. 1999, 121, 5274. (5) Groves, J. T.; Ulman, N.; Boxer, S. G. Science 1997, 275, 651- 653. Groves, J. T.; Wulfing, C.; Boxer, S. G. Biophys. J. 1996, 71, 2716- 2723. (6) Lingler, S.; Rubinstein, I.; Knoll, W.; Offenha ¨ usser, A. Langmuir 1997, 13, 7085-7091. (7) Ra ¨ dler, J.; Strey, H.; Sackmann, E. Langmuir 1995, 11, 4539- 4538. 3176 Langmuir 2002, 18, 3176-3180 10.1021/la011510p CCC: $22.00 © 2002 American Chemical Society Published on Web 03/09/2002