Low Frequency Alternating Current Insulator-based Dielectrophoresis (AC-iDEP)

Aytug Gencoglu, Microscale Bioseparations Laboratory, Chemical and Biomedical Engineering Department, Rochester Institute of Technology, Rochester, NY 14623
E-mail: axgbme@rit.edu, Phone: (906)370-9836

Insulator-based dielectrophoresis (iDEP) employs DC electric fields which are made nonuniform by channel cross-section changes caused by insulating posts, constrictions or other means. Its main advantage over electrode-based DEP (eDEP) is that electrodes in iDEP are away from the regions where analytes are subjected to DEP forces. This greatly reduces problems related to water electrolysis and electrode fouling [1]. iDEP is also more suitable for flow-through operation, allowing faster analysis. However, typical voltages are hundreds or thousands of Volts, compared to <10 V for AC-DEP. This causes problems such as Joule heating, cell electroporation and pH changes in the microfluidic channels. For this reason, it is desirable to reduce the applied voltages required in iDEP applications. AC-iDEP can achieve this by reducing or eliminating electroosmotic flow (EOF), which the DEP force needs to overcome [2,3].

Insulating particles experience negative DEP (nDEP) in DC or low frequency AC fields. Therefore, an insulating particle approaching a constriction in a microfluidic channel via EOF will be repelled from the constriction. The particle in this case will be subjected to opposing forces of EOF and DEP, as shown in Figure 1. EOF rate changes linearly with the electric field E⃗, while DEP force changes with the magnitude of ∇(E⃗2). Increasing the applied voltage increases both EOF and DEP forces, but eventually the DEP force dominates, and particles approaching a constriction are trapped as they reach a zone where the net force becomes zero [4]. This has a concentrating effect, where particles are transported by EOF to the high electric field gradient zones, where they become trapped as the DEP force becomes significant and form concentrated bands.

Figure 1: EOF, electrophoresis (EP) and DEP rate (Black) and DEP force (Blue) in an AC-iDEP system over 1 period of AC signal.

Figure 2: Voltage (red), EOF rate (Black) and DEP force (Blue) in an AC-iDEP system over 1 period of AC signal.

With an AC signal, EOF direction changes with that of the electric field. However, since ∇(E⃗ 2) is always positive, DEP force direction does not change, and always repels insulating particles away from the constrictions (Figure 1). Changes in EOF and DEP over the duration of 1 period of an AC signal are shown in Figure 2. The time averaged EOF rate is zero, while the time averaged DEP force is greater than zero. Therefore, when a low frequency AC signal is applied instead of a DC signal, minimum voltage where DEP dominates over EOF is significantly reduced.

In a 1 Hz AC field, particles oscillate between two points. When DEP force is not strong enough to trap particles, they may move back and forth across a constriction with EOF. If the DEP force is strong enough, particles can concentrate in DEP trapping zones during part of the AC waveform, and move as a concentrated sample with EOF in the rest of the waveform [2]. In 10 Hz AC fields, particles that are away from high electric field gradient regions appear as if they are “vibrating in place.” No net particle movement is observed, and samples do not concentrate or become trapped unless the AC signal is made asymmetrical. However, particles in high electric field regions move away from these regions, resulting in depletion zones [2]. Figure 3 shows the behavior of 1 μm diameter polystyrene particles in an array of circular insulating posts were subjected to 1 Hz and 10 Hz AC fields. In the 1 Hz case (Fig. 3a-b), particles were not concentrated, but they moved between two constrictions. Figure 3d (10 Hz) shows the formation of a depletion zone in the constriction, where the DEP force is significant. Electric field gradient is negligible in the region between constrictions, so the particle distribution in this region does not change over time [2].

Figure 3: Movement of 1 μm diameter polystyrene particles near circular insulating post structures. (a-b) ω=1 Hz. Particles were concentrated into “packages” and moved back and forth between constrictions. Individual packages were labeled with letters A-C for comparison between images. Field of view was not moved. (c-d) ω=10 Hz. Near voltage peaks, particles were depleted in high electric field gradient regions. The depletion region became smaller when the voltage was lower (c) as the DEP force magnitude decreased.

Frequency also affects net EOF rate when a DC offset is added to the applied AC signal. The EOF rate becomes nonzero with a DC offset, but the net rate is frequency dependent [3]. Figure 4 shows total displacement of polystyrene particles over 30 s in a straight channel with no insulating structures while a signal sequence was applied where a DC offset was added only to the negative half of an AC signal, gradually increasing the net EOF rate. Total displacement of particles with a 1 Hz signal sequence was more than 3 times greater than the total displacement with a 20 Hz signal sequence.

Figure 4: EOF in a constant cross-section channel under 1 Hz and 20 Hz electric fields. EOF was determined with particle image velocimetry using 1 μm diameter polystyrene particles. (a-b) Displacement of particles relative to t=0 s. (c-d) 1 Hz and 20 Hz AC signals applied.

Low frequency AC fields can be used for iDEP separations at voltages lower than those required for DC-iDEP. In DC-iDEP, a mixture of particles can be separated by first applying a high voltage to trap and concentrate all particles, then lower the voltage in steps so that only one kind of particle is released at a time. In AC-iDEP, the particles are initially trapped with a high amplitude AC signal with a small DC offset, so that all particles are concentrated in the high electric field gradient regions. Particles can then be released by reducing the AC amplitude in steps (Reducing the DEP force, while keeping net EOF constant) or by increasing the DC offset in steps (Increasing net EOF) [3]. Figure 5 shows an example where both effects were combined. A DC offset was applied only to the negative half of a 20 Hz AC signal, and offset value was increased in steps. With each step, time averaged EOF rate increased, while time averaged DEP force decreased. 0.5 μ, 1 μ and 2 μ diameter polystyrene particles were separated in an array of diamond shaped insulating posts with this signal [3].

Figure 5: Separation of 0.5 μm, 1 μm and 2 μm polystyrene beads with an EOF gradient using a 20 Hz AC signal. (a) 1 μm and 2 μm beads trapped, 0.5 μm beads showing both trapping and streaming behavior. Trapped particles of different sizes were in separate bands. (b-d) Negative part of the AC signal was reduced. Particles were released sequentially from smallest to largest. (c) 0.5 μm beads released. (d) 0.5 and 1.0 μm beads released. (e) 2.0 μm beads showing trapping-streaming behavior.

Low frequency (1-20 Hz) AC-iDEP can reduce EOF and achieve separations with lower voltages than DC-iDEP. Frequency, signal shape, offset (applied to whole signal or one half-cycle) and amplitude can all be exploited, providing more ways to manipulate particles. Low frequency AC-iDEP also differs from DC-iDEP because it is more suitable for releasing concentrated “packets” of particles with regular intervals (i.e. 1 packet/s with a 1 Hz signal).

References
  1. Srivastava, S., Gencoglu, A., Minerick, A., Anal. Bioanal. Chem. 2010, 399, 301-321.
  2. Gencoglu, A., Olney, D., LaLonde, A., Koppula, K. S., Lapizco-Encinas, B. H., J. Nanotech. Eng. Med. 2013, 4, 021002.021001-021007.
  3. Gencoglu, A., Olney, D., LaLonde, A., Koppula, K. S., Lapizco-Encinas, B. H., Electrophoresis 2014, 35, 363-373.
  4. Cummings, E. B., IEEE Eng. Med. Biol. Mag. 2003, 22, 75-84.