Two US chemists have combined electrophoresis and dielectrophoresis to create a microfluidic device that can trap polymer particles and bacterial cells with a high degree of control and specificity.

Both electrophoresis and dielectrophoresis are regularly used to transport particles around microfluidic devices, mainly for separation and analytical purposes, but they have rarely before been combined on the same device. This is because the factors that determine the strength of the electrophoretic and dielectrophoretic forces are very different.

Electrophoresis involves the transport of charged particles in an electric field, with the particles naturally migrating towards the opposite charge. Dielectrophoresis involves the transport of neutral particles in an electric field with a high potential gradient (a large change in electric potential over a short distance). This high field gradient causes the positive and negative charges in a neutral particle to separate, producing a dipole moment. If this dipole moment is greater in the particle than in the surrounding medium then the particle will migrate towards strong electric fields, but if the surrounding medium has a greater dipole moment then the particle will migrate away from strong electric fields.

The strength of the electrophoretic force on a particle depends primarily on the strength of the applied electric field: the stronger the electric field, the greater the electrophoretic force. The strength of the dielectrophoretic force, on the other hand, depends on a complex interaction between a number of different factors. These include the ratio between the dipole moments of the particle and the surrounding medium, and the frequency and gradient of the applied electric field.

All of this explains why researchers have found it quite challenging to combine electrophoresis and dielectrophoresis on the same device, because they each require electric fields with very different properties. But now Michelle Kovarik and Stephen Jacobson at Indiana University, Bloomington, have found a way to trap particles using both electrophoresis and dielectrophoresis.

Traditionally, the high field gradients required for dielectrophoresis have been generated by arrays of metal electrodes, but more recently researchers have discovered that these gradients can be generated by placing tiny structures in a standard electric field. Kovarik and Jacobson wondered whether placing a polymer membrane with nanoscale pores in an electric field would also do the trick.

So they etched tiny conical pores into a poly(ethylene terephthalate) (PET) membrane and placed it at the intersection of two microscale channels (75µm wide and 13mm long). These channels were placed at right angles with one on top of the other, forming a cross shape. The conical pores had a diameter of around 1µm at their base, reducing to 130nm at their tip, and were 10µm deep. Around 60 of these pores were spread across the polymer membrane at the intersection between the two microscale channels.

When the chemists applied a fairly strong electric potential across the membrane, they found it generated high field gradients at the pores. This meant that dielectrophoretic forces dominated, with Kovarik and Jacobson able to trap neutral bacterial cells at the pores. However, when the chemists applied weaker electric potentials across the membrane, they found that the electric fields, although still reasonably strong, had lower potential gradients. This meant that electrophoretic forces began to take over, allowing them to trap negatively-charged polymer microspheres at the pores.

Kovarik and Jacobson conclude that combining electrophoresis and dielectrophoresis on the same device in this way delivers great control over both charged and neutral particles, allowing them to be trapped and separated according to their size, charge and dipole moment. Reducing the size of the membrane pores should allow this control to be extended to single biomolecules, they predict.

Related links:

• Analytical Chemistry, 2008, 80, 657 - 664: "Integrated nanopore/microchannel devices for ac electrokinetic trapping of particles"  

Article by Jon Evans

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.