A Microfabrication-Based Dynamic Array Cytometer
Original entry: Warren Lloyd Ung, APPHY 225, Fall 2009
"A Microfabrication-Based Dynamic Array Cytometer"
Joel Voldman, Martha L. Gray, Mehmet Toner, and Martin A. Schmidt.
Analytical Chemistry (2002).
Soft Matter Keywords
dielectrophoresis, dielectrophoretic trapping, single-cell assay
Voldman et al used microfabrication methods to construct an array of traps, which can be used to independently examine single cells in a programmable and controlled manner (see Figure 1). These traps use dielectrophoresis to confine cells within a defined area of a microfluidic flow device. Each trap consists of four metal posts, which extend into the flow channel. When coupled with an optical system, that probes the responses of cells in real time, this device provides a suitable basis for studying single cells. Cells can be sorted depending on whether they exhibit a desired fluorescent response; the chip additionally provides a dynamic capability to capture or discard cells based on these fluorescent indicators.
Fluorescence markers are powerful tools for biological studies. Markers can be designed to pinpoint many aspects of cellular behaviour. Although flow cytometry methods allow cells to be sorted quickly based on fluorescence, this device offers the additional ability of holding a particular cell in place, and examining its dynamic behaviour over time. This may reveal a response, which is not observable during the short time scales required by typical methods of flow cytometry.
As mentioned previously, the cells are held in place by dielectrophoresis. The dielectrophoretic traps create a non-linear, time-varying electric field pattern. Cells within this field are polarized, and move towards the field minimum, because they are less polarizable than the ionic medium, in which they are suspended. The parameters of the traps have been finely designed, modeled, and tuned to hold single cells stationary against the flow of fluid through the device. A trapezoidal trap geometry was chosen to facilitate the loading of cells into each trap, while retaining the traps' confining strength. Each trap within the device can be switched on and off independent of each of the other traps. This allows multiple single cells to be captured and observed simultaneously. If a trap contains multiple cells, extraneous cells can be ejected from the trap by modulating the flow rate. This allows the array to maintain single cell resolution. This device also allows cells to be released at the appropriate time, whether this cell is to be discarded, or captured for further study. By holding the cells in place, different types of fluorescence assays can be performed on the same set of cells, simply by flowing the appropriate fluorescent labels and/or reagents through the microfluidic flow channel.
Soft Matter Discussion
In order to trap cells by dielectrophoresis, it is necessary to select a frequency at which the applied electric fields do not induce a voltage across the cell membrane. The cell membrane has the same structure as a vesicle, which is essentially a double layer of surfactants. Electric fields, which induce such voltages, may break holes in the membrane of the cell, compromising its integrity. Since this particularly device uses negative dielectrophoresis, it is also necessary to select a frequency at which the cell is less polarizable than the medium which surrounds it. Fortunately, operating the trap at high frequencies in the megahertz range fulfills both these requirements.