Dielectrophoretic trapping makes use of dielectrophoresis (DEP) to hold objects in place. The effects of DEP depend highly on the difference between the polarizability of the object of interest and that of the medium in which it is situated. Objects which are more polarizable than the medium around them, will move towards the electric field maxima; this is termed positive dielectrophoresis (pDEP). On the other hand, objects which are less polarizable than the surrounding medium, will move towards the minimum of the electric field. This is called negative dielectrophoresis (nDEP). In the latter case, the polarizable medium is preferentially drawn towards the field maximum and displaces the object of interest (see the attached figure).
DEP only occurs when the applied fields are non-uniform, otherwise there will be no net force exerted on an object's induced dipole. DEP can also be applied over a wide range of frequencies. Since an object's dielectric constant is a function of frequency, this means that there is often an optimal frequency at which the difference in polarizability between the object and the medium is at a maximum.
In the context of trapping, the distinction between pDEP and nDEP is also important. Generally, pDEP can only take place when an object moves towards the maximum electric field, this means that in pDEP, the object is always trapped near the positive electrode or pulled against a surface. By contrast, in nDEP, stable minima can be produced at any point by carefully choosing the field geometry of the trap. For instance, in the figure shown on the right, the minimum of the electric field is in the centre of the four posts, and the object, in this case a cell, will migrate towards the minimum of electric field in the centre. Other factors come into play in determining the utility of each method for specific applications. For instance, nDEP normally takes place in a highly ionic medium. Under the influence of electric fields, such an ionic medium may undergo significant Joule heating, which may be undesirable.
DEP trapping is currently a commonly used method for trapping and isolating single cells. Electric fields at high frequencies are a convenient, non-contact method for handling cells, since they are thought to have little affect on the viability of cells.
- Voldman, J., Gray, M.L., Toner, M., and Schmidt, M.A. "A Microfabrication-Based Dynamic Array Cytometer" Anal. Chem. (2002), 74(16), 3984-3990.