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The common theme of our research is investigating dynamics at interfaces. We are interested phenomena at both biological and physical interfaces. Often our work focuses on dynamics occuring at interfaces between phases of matter like lipid membrane phases in the cell and the solid,liquid, and vapor interface that controls the contact lines of sessile droplets when they are perturbed by vibration. Within these themes, we aim to uncover fundamental information about processes that occur at these interfaces. Details on specific projects are provided below.
1) Organization, dynamics, and function of the cell membrane and its constituents
The cell membrane is composed of myriad proteins and biomolecules existing in a patchwork lipid matrix of different phases. It is hypothesised that the purpose of this heterogeneity is to regulate if and when certain species can interact, by either sequestering or excluding species from regions of specific membrane composition until some stimuli changes partitioning preference. Partitioning preference is governed by hydrophobic, electrostatic, and steric forces and mediated by the properties of the lipids composing the phase. These lipid regions are called microdomains or lipid rafts. Current biochemical methods are inadequate for identifying residents of these regions or monitoring their changes between them. By combining microfluidic patterning and supported lipid bilayers, our group invented an approach that allows us to control the spatial and temporal location of different lipid phases within a convenient platform that allows us to determine when species interact with each other. Using this technology we study:
- the kinetics of phase changes & partitioning... Read a current paper by our group here
- stimuli that change partitioning of species between phases
- how changes in protein-lipid interactions impact protein structure, activity level, and biological function
(Above, Left) Angela Hsia and Prof. Daniel set up a microfluidic assay to study membrane microdomains. (Right) An image of a two-phase membrane inside a microfluidic device. Red denotes a cholesterol-rich phase that exhibits more ordering in the membrane and green denotes a phospholipid-rich phase that is less ordered and a more fluid membrane environment.
2) Kinetics of virus binding and membrane fusion
We study viral-host membrane binding and fusion using individual virion imaging (IVI) and statistical analysis techniques. We employ a supported lipid bilayer as a host membrane mimic and total internal reflection fluorescence microscopy to image individual influenza viruses binding and fusing to it. We integrate a uniform and rapid fusion trigger (pH drop) into our assay to coordinate the initiation of many individual fusion events, revealing high resolution kinetic information for influenza virus. This approach distinguishes several key steps within the fusion pathway, namely hemifusion (stalk formation) from pore opening and expulsion of viral contents. These distinctions are essential details than cannot be readily provided by other methods. Using this assay, our current research focus is studying:
- the binding and fusion kinetics of viruses... Read a recent paper by our group here
- the influence of virus strain and membrane properties on fusion... Read our recent story in the Olin Hall News here
- mechanisms of fusion inhibition by antibodies and anti-viral compounds
(Above, Left) Donald Lee and Prof. Daniel set up a microfluidic assay to study virus infection. (Right) Time lapse images of virus fusion triggered by proton uncaging to rapidly drop the local pH. Each fluorescence "burst" marks an individual fusing virus.
3) Droplet dynamics and interfacial wetting behavior
An alternative to conventional continuous flow microfluidics is digital, or droplet-based fluidics. Miniaturizing batch processes into droplets has several advantages for applications ranging from protein folding, crytallization, and manufacture of scarce, expensive products. However, moving sessile droplets on surfaces, especially aqueous solutions, is challenging because of significant contact angle hysteresis. In our approach, we use surface patterning techniques, vibration, and imaging tools to study the shape deformations that occur in the drops and can be used to overcome hysteresis forces leading to controllable drop motion on surfaces. Currently we are:
- developing imaging approaches for determining resonance modes from drop footprints
- determining the influence of drop volume, surface contact angle, and wetting on drop resonance modes
- Studying the droplet surface instabilities created by vibration, contact angle hystersis, and contact line mobility
(Above, Left) Deirdre Costello and Prof. Daniel examine droplets on a silicon wafer treated with a gradient chemical coating. (Right) A droplet mechanically oscillating at its natural resonance frequency as seen from the top view.