Electrohydrodynamic size stratification and flow separation of giant vesicles
Original Entry by Michelle Borkin, AP225 Fall 2009
S. Lecuyer, W. D. Ristenpart, O. Vincent, and H. A. Stone, Appl. Phys. Lett., 92, 104105, 2008
An electrohydrodynamic (EHD) method for separating small from giant unilamellar vesicles (GUVs) is presented in this paper. GUVs are fragile and common suspension separation techniques (e.g. centrifugation) can damage them so are ineffective. Thus having an effective way to separate them is desirable. GUVs are of particular interest due to their ability to model biophysical systems since GUVs have similar sizes and structures (e.g. lipid bilayers, membranes) as living cells. There is also interest in using GUVs for new technology including nanoreactors and designable drug carriers. In summary, the process for separating the vesicles involves applying an oscillatory electric field which generates an EHD flow around each vesicle close to an electrode. The result is that the smaller vesicles are pulled underneath the larger ones thus lifting the larger ones off the electrode and shielding them. A brief spike in the electric field is applied to keep the smaller vesicles on the bottom while a flow is applied to push the larger vesicles into a separate container. The result is the removal of >90% of the small vesicles from the GUVs.
The unilamellar vesicles studied in this paper are self-assembled phospholipid vesicles in which spherical molecular bilayers separate a specific internal volume from the external environment. The giant unilamellar vesicles (GUVs) that the researchers are attempting to isolate are on the order of tens of micrometers in diameter. When an external electric field is applied, a dipole field is induced around each vesicle and this field distorts the charge polarization layer near the electrode thus giving rise to an electrohydrodynamic (EHD) flow. A schematic of this experimental set-up and a sketch of the EHD streamlines can be seen in Figure 1.
When a field of 1V or greater is applied with a frequency in the range of 10-100 Hz, the following phenomena were observed: similarly sized vesicles formed planar clusters, and small vesicles (<10 micrometers) near larger vesicles (>20 micrometers) would either “orbit” the larger vesicle or more commonly “lift” the larger vesicle as they went underneath. For a schematic of these behaviors, see Figure 2 (b-c). After the lifting occurred (~10 minutes of an applied field applied), a dc field (1 V) was applied for ~10 seconds in order to make the smaller vesicles adhere to the electrode (the larger ones don't stick since they are shielded by the smaller vesicles below them). A flow was then applied to push the larger vesicles into a separate collection chamber resulting in >90% of the smaller vesicles being separated from the larger ones, as shown in Figure 2 (d). The effectiveness of this separation technique can also be seen in Figure 3 which shows the distribution of vesicle sizes before and after the EHD process was applied to a sample.
This paper also includes excellent videos displaying the observed vesicle behavior as part of its supplemental material. They are as follows:
aggregation.avi: Example of vesicles aggregating when the electric field is applied (i.e. Figure 2 (b)).
transition.mpg: Example of a transition from the small vesicles "orbiting" to "lifting".
reversibility.avi: Demonstration of the frequency dependence (frequency is increased in one step from 30 Hz to 500 Hz) and reversibility of the aggregation process.
flow.avi: Demonstration of the removal of larger vesicles after a spike in the field has been applied (i.e. Figure 2 (d)).