Published on Web 06/21/2003
A Nanofluidic Switching Device Roger Karlsson,# Anders Karlsson,# and Owe Orwar*,§ Department of Physical Chemistry and Microtechnology Center, Chalmers UniVersity of Technology, SE-412 96 Go¨teborg, Sweden, and Department of Chemistry, Go¨teborg UniVersity, SE-412 96 Go¨teborg, Sweden Received February 26, 2003; E-mail:
[email protected] Controlled aqueous flow in channels with characteristic length scales in the low nanometer range, ∼10-100 nm (nanofluidics), has a potential to provide new powerful tools that can give unique insight into and understanding of how chemical reactions occur and how fluids behave in confined geometries.1,2 Appropriately designed nanofluidic systems could serve as important platforms for studies of single-molecule dynamics,3 enzyme-catalyzed reactions,4 single-file diffusion,5 and single-molecule sequencing and synthesis. Furthermore, such systems can provide an understanding of materials transport and reactions in biological systems that occur on these length scales and, as of today, are poorly understood.6,7 We have constructed networks in lipid bilayer membranes consisting of surface-immobilized containers (5-10 µm radius) conjugated with suspended nanotubes (∼100 nm in diameter and 20-30 µm in length). A system of spherical containers connected by thin tubes is energetically favored due to minimization of the elastic energy in the lipid bilayer membrane, where in-plane tension or stretching competes with bending of the membrane. The networks have controlled geometry and topology,8-10 and fluid flow can be controlled by controlling the surface free energy, e.g., the membrane tension (σ) of the device material itself.11 The membrane tension can, for example, be locally increased by using carbon microfibers working as tweezers, controlled by micromanipulators.8 Here, we present a nanofluidic switching function based on creating a membrane tension gradient using a two-point perturbation technique, which allows directed transport between any two or several containers in a network. This capability is essential for controlled routing and switching of fluid directions in networks with more than two containers and will thus open up possibilities for large-scale integration in nanofluidic systems. If surface tension is increased at a single container, in a system of n containers we would have one drain (at the point of increased tension) and n - 1 sources for the lipid membrane in a symmetrical case. For example, if surface tension is increased, +σ, in vesicle no. 4 in Figure 1 a, membrane and fluid will symmetrically be moved from vesicles 1-3. To be able to route material with directionality in such multicontainer networks, we developed a two-point perturbation technique. With this method, the membrane tension of one surface-adhered vesicle was decreased by adding excess membrane material through insertion of membrane from a nanotube-connected donor vesicle12 (denoted by δ in Figure 1a and d), simultaneously as the membrane tension in another vesicle was increased by mechanical excitation using a carbon microfiber as tweezers. We thereby created a difference in tension between two containers that was much larger than for all the other connected containers (Figure 1b). For the addition of membrane material, we § #
Chalmers University of Technology. Go¨teborg University.
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J. AM. CHEM. SOC. 2003, 125, 8442-8443
used 5-µm-diameter donor vesicles, δ, which were allowed to merge (by size expansion or micromanipulator-controlled transport of δ) with the surface-adhered vesicles having a typical size of 15 µm in diameter. This will lead to an 8% surplus membrane area of the resulting product vesicle. The excess membrane drops the membrane tension below the limit where membrane bending dominates (
