Surfactant-induced lysis of lipid-modified microgels.

Lipid layers supported on hydrophilic polymer cushions have attracted considerable attention, not only because they provide a cell-like environment for transmembrane protein incorporation, but also because the lipid membrane creates a thin barrier that is able to maintain a chemical gradient.1 These properties have inspired the preparation of novel types of hybrids that combine lipids and hydrogels.2 For sensory applications, a simple method for the integration of these objects within a microfluidic device and a means to chemically trigger ion barrier disintegration would be highly advantageous. Previously, we described the in situ creation of a hydrogel object within a microchannel that was covalently modified with a thin ion-impermeable fatty acid layer.3 Herein, we report that the barrier permeability can be chemically induced with a surfactant solution, triggering the complete expansion of the hydrogel by an unusual process. pH-sensitive hydrogel cylinders (μgels), 180 μm tall by 400 μm in diameter, were photopolymerized within microchannels following our reported method.4 They were constrained by the glass channel at their top and bottom interfaces. The μgel copolymers consisted of 2-hydroxyethyl methacrylate, acrylic acid (4:1 vol ratio), and ethyleneglycol dimethacrylate (1 vol %) photoinitiated with 2,2-dimethoxy-2-phenylacetophenone (3 wt %). To monitor the pH inside of the μgel, a pH-sensitive indicator (phenolphthalein or fluorescein) was entrapped within the hydrogel matrix during polymerization. The dried μgel was bathed in benzene and esterified with palmitoyl chloride, resulting in the covalent attachment of a fatty acid layer to the μgel surface.5 This modification procedure creates an ion barrier that enables a pH-sensitive μgel to remain contracted while bathed in a pH solution that otherwise expands an unmodified μgel. By analogy to the lysing behavior of surfactants on cells and vesicles, we investigated the ability of the ionic surfactant sodium dodecyl sulfate (SDS) and the nonionic surfactant Triton X-100 (TX-100) to disrupt the lipophilic ion barrier. Prior to the addition of detergent, each modified μgel was exposed to an elevated buffer (pH 12) for a few hours to demonstrate that the fatty acid layer was impermeable to ions, as evidenced by the lack of μgel swelling. A solution of the surfactant dissolved in a pH 12 buffer was then flowed into the channel, and the diameter of the μgel was measured as a function of time. At surfactant concentrations above the critical micelle concentration (cmc) (1 mM for SDS,6 0.24 mM for TX-1007), localized regions of expanded hydrogel were visible within minutes at the surface of the object. These areas grew larger until the entire μgel expanded and the phenolphthalein indicator changed from colorless to pink (Figure 1). At lower surfactant concentrations, the behavior was similar but surface disruptions appeared more slowly. The pH change within the μgel was imaged with confocal microscopy to determine if surface disruptions initially formed at the μgel-glass interface or in the μgel interior. This was accomplished by monitoring the increase in the emission intensity of the pH-sensitive fluorescein dye entrapped in the μgel. For an unmodified μgel, the emission increase, and therefore μgel expansion caused by the inward diffusion of buffer, began uniformly on the surface and symmetrically progressed to a point at its center (Figure 2a-d). In contrast, surfactant-induced expansion of the modified μgel proceeded unsymmetrically. Prior to addition of surfactant to the buffer solution, the fluorescein emission intensity was minimal and the μgel was contracted since the high-pH solution could not penetrate the μgel exterior. After * To whom correspondence should be addressed. (1) Sackmann, E. Science 1996, 271, 43-48. (2) (a) Jin, T.; Pennefather, P.; Lee, P. I. FEBS Lett. 1996, 397, 70-74. (b) Ng, C. C.; Cheng, Y.; Pennefather, P. S. Macromolecules 2001, 34, 57595765. (c) Kiser, P. F.; Wilson, G.; Needham, D. Nature 1998, 394, 459-462. (3) Beebe, D. J.; Moore, J. S.; Yu, Q.; Lui, R. H.; Kraft, M. L.; Jo, B.; Devadoss, C. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 13488-13493. (4) Beebe, D. J.; Moore, J. S.; Bauer, J. M.; Yu, Q.; Lui, R. H.; Devadoss, C.; Jo, B. Nature 2000, 404, 588-590. (5) See Supporting Information for experimental details. (6) Wanless, E. J.; Ducker, W. A. J. Phys. Chem. 1996, 100, 3207-3214. (7) Lichtenberg, D.; Robson, R. J.; Dennis, E. A. Biochim. Biophys. Acta 1983, 737, 285-304. Figure 1. A pH sensitive hydrogel was modified by covalently linking palmitoyl chloride to the surface. When the μgel was bathed in a pH 12 buffer solution, it remained stable for hours (b) while an unmodified μgel rapidly expanded in the same solution ([). The addition of a 0.1 M solution of SDS in a pH 12 buffer to the modified μgel at the indicated time (v) triggered localized areas of expansion on the exterior of the μgel (a), which propagated to adjacent regions (b) until the entire surface of the μgel expanded (c). Full μgel expansion was complete when the pH indicator phenolphthalein changed from colorless to pink at the interior of the μgel. fD is the fractional change in diameter, ∆d/d0, where d0 is the total diameter change for the fully expanded gel. The scale bar is 250 μm.