Supported Membranes

Supported membranes are versatile biomimetic interfaces. We study the structure and dynamics of supported membranes on various solid supports in collaboration with the group of Bert Nickel. The control of fluidity, spacing and lateral organisation is a current research challenge. In the past we have studied the speading of lipid membranes on solid surfaces and vertical spacing in lipid-polymer-solid interfaces.

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Dynamic patterns in a supported lipid bilayer driven by standing surface acoustic waves

M. Hennig, J. Neumann, A. Wixforth, J. O. Rädler & M. F. Schneider
Lab on a Chip (2009)

In the past decades supported lipid bilayers (SLBs) have been an important tool in order to study the physical properties of biological membranes and cells. So far, controlled manipulation of SLBs is very limited. Here we present a new technology to create lateral patterns in lipid membranes controllable in both space and time. Surface acoustic waves (SAWs) are used to generate lateral standing waves on a piezoelectric substrate which create local ‘‘traps’’ in the lipid bilayer and lead to a lateral modulation in lipid concentration and membrane density. We demonstrate that pattern formation is reversible and does not affect the integrity of the lipid bilayer as shown by extracting the diffusion constant of fluid membranes. The described method could possibly be used to design switchable interfaces for the lateral transport and organization of membrane bound macromolecules to create dynamic bioarrays and control biofilm formation.

Transport, Separation, and Accumulation of Proteins on Supported Lipid Bilayers

Jürgen Neumann, Martin Hennig, Achim Wixforth, Stefan Manus, Joachim Rädler, Matthias Schneider
Nano Letters (2010)

Transport, separation, and accumulation of proteins in their natural environment are central goals in protein biotechnology. Miniaturized assays of supported lipid bilayers (SLBs) have been proposed as promising candidates to realize such technology on a chip, but a modular system for the controlled transport of membrane proteins does not exist. We demonstrate that standing surface acoustic waves drive the in-plane redistribution of proteins on planar SLBs over macroscopic distances (3.5 mm). Accumulation of proteins in periodic patterns of about 10-fold protein concentration difference is accomplished and shown to relax into the homogeneous state by diffusion. Different proteins separate in individual fractions from a homogeneous distribution and are transported and accumulated into clusters using beats. The modular planar setup has the potential of integrating other lab-on-a-chip tools, for monitoring the membrane-protein integrity or adding microfluidic features for blood screening or DNA analysis.

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Alignment and Deformation of Lipid Bilayer Domains in Vesicles Adhering to Microstructured Substrates

T. Stögbauer, M. Hennig, J. O. Rädler
Biophysical Reviews and Letters (2010)

In heterogeneous lipid membranes, the lateral organization is coupled to local curvature as the membrane bending energies depend on composition. We investigate phase separated vesicles of ternary lipid composition in contact with structured surfaces, where the distinct elastic properties of the Lo (liquid ordered) and Ld (liquid disordered) phases come into play. We show that fused silica substrates with microstructured grooves induce sorting, alignment and deformation of Lo domains in adhering vesicles. The same phenomenon is observed on flat, chemically modified substrates with alternating stripes of rough and smooth regions. In both cases it is the Lo phase which accumulates over the smooth substrate and membrane spanned groove regions respectively. Deformation of Lo domains occurs when domain diameters grow beyond the width of the microstructured stripes. Domain alignment was also observed in binary membranes featuring gel and fluid phase coexistence showing the generic character of domain sorting on microstructured surfaces.

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DNA-stretching

In the case of substrates with etched channels, we observe DNA stretching at the groove edges: The concave edges form attractive line potentials that bind the diffusing molecules after some time, thus polymer confinement to 1D.
The reason for the DNA stretching results from the geometry: Finally, molecules in a concave edge have closer contact with the positive charges within the membrane and are therefore bound tighter. Convex edges would have a contrary effect. The effect is strong enough to stretch DNA molecules to 95% of their contour length which could make applications like gene mapping possible.

Conformational dynamics of DNA electrophoresis on cationic membranes

V. Kahl, M. Hennig, B. Maier, J. O. Rädler
Electrophoresis (2009)

The conformational dynamics of DNA molecules undergoing electrophoresis on a fluid substrate-supported cationic lipid bilayer is investigated using fluorescence microscopy. At low electrophoretic velocities, drift of 2-D random coils is observed. In contrast, at velocities larger than 0.3 mm/s, the DNA molecules stretch out and assume branched configurations. The cross-over scenario is explained by the observation that cationic lipids segregate underneath the adsorbed DNA and confine the DNA to its counter charge imprint on time scales shorter than the relaxation time of the imprint. The concept of a tube-like confinement of the DNA is corroborated by the observed 1/N size dependence of the electrophoretic mobility in analogy to the biased reptation model in gels. The role of membrane defects and possible applications of membrane-based electrophoresis in microfluidic devices are discussed.