Paul V. Jones, Sarah J. R. Staton, Noah G. Weiss, and Mark A. Hayes
Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287
Today we are enjoying a renaissance of the rather unique and complicated phenomenon of dielectrophoresis (DEP). In a large part, this is driven by the limited number of effective tools available for manipulating small particles (in the range of 10 microns—10 nanometers) for analytical or preparative uses. Various forms of DEP, combined with electrokinetics and flow fields, provide distinct capabilities to selectively move, stop, or capture particles. New strategies using DEP and microfluidics are rapidly being applied to environmental analysis and key biological and biomedical applications.
Dielectrophoresis (and microfluidics) presents certain unique advantages over alternative techniques for particle manipulation . Working on a microfluidic scale offers lowered costs, rapid response times, reduced reagent consumption, smaller sample sizes, micro/nano-scale phenomena, and device portability. Dielectrophoretic forces arise when dipoles and multi-poles are exposed to spatially non-uniform electric fields. The force exerted depends on complex variables such as particle size, structure, and dielectric properties. These properties can vary dramatically, even between similarly sized biological targets. By exploiting these dielectrophoretic disparities, seemingly similar particles can be differentiated based on subtle distinctions.
Insulator-based DEP (iDEP) has fueled the renaissance and has recently emerged as an important technique for manipulating micro- to nano-scale particles . Various implementations of iDEP have been demonstrated using features such as glass beads, insulating hurdles, and curved channels [3-5]. Other approaches use arrayed features like uniformly sized insulating posts within a straight microchannel . In each of these cases, insulating features induce non-uniformities in the electric field to create the dielectrophoretic forces. The subtle and particular properties of a particle define how it interacts with an electric field gradient; these are captured with a characteristic dielectrophoretic mobility (µDEP). Most applications of iDEP result in bifurcation of a sample population, where particles above a certain cutoff value of µDEP are trapped, while all others pass the insulating features unhindered. Alternatively, applications utilizing streaming DEP divert particles down different outlet channels based on their characteristic mobility .
In comparison, insulator-gradient dielectrophoresis (g-iDEP) utilizes a series of varied insulating features along a microchannel wall. In 2007 Pysher and Hayes introduced a globally tapered sawtooth design with aligned, opposing teeth creating a series of successively narrower gaps, or gates through which fluid and particles must pass . The slight difference in geometry at each set of teeth causes particles to experience increasing local DEP forces as they travel down the channel. A given particle driven down-channel by electrokinetic forces will encounter multiple, distinct DEP traps of increasing strength, until it reaches a trap strong enough to block its progressive translational motion. In this way, multiple classes of particles may be captured and spatially resolved within the confines of a single continuous channel and within a single experimental run.
Initial work with the tapered sawtooth channel demonstrated separation and capture of viable and non-viable Bacillus subtilis cells within a single channel. Cells were stained using a two-color fluorescence assay and electrokinetically injected into the iGDEP channel. Live cells were captured first, in weaker DEP traps, while dead cells were captured downstream in stronger DEP traps. These findings were reproduced using two other species of bacteria: Escherichia coli and Staphylococcus epidermidis .
Research performed by Staton et al. further characterized differential particle capture within a tapered sawtooth channel by investigating the behavior of fluorescent polystyrene spheres of varying diameter (20, 200, and 1000 nm) . Results from these studies proved consistent with mathematical models formulated using similar channel geometry. They established that specific particulates could be isolated within distinct portions of the device, while simultaneously concentrating particle species by factors of 103 to 106 above background.
The most recent work utilizing DC g-iDEP extended the approach to complex, heterogeneous samples of body fluids . Human blood cells were successfully captured and concentrated from small-volume samples of diluted whole blood. The channel design enabled reproducible capture of blood cells at specific locales within the channel, while plasma and debris flowed continuously towards the outlet. The technique was shown to operate with isotonic buffers, allowing capture of intact cells, and at relatively low voltage ranges of 200–700 V.
Comparison of theoretical constructs to real-world data is only beginning. Current assessments in the field of DEP are unable to assign a theoretical basis for subtle particle properties and discussions of real data are generally limited to an analysis of positive DEP (directed toward greater field strength) versus negative DEP (toward lesser field strength). Improving theoretical foundations and gaining valid physical descriptions of dielectrophoretic behavior based on first principles will enable mathematical fine-tuning of g-iDEP devices for specific, multiplexed particle separations . Such efforts are currently underway, utilizing a converging microfluidic channel to quantitatively determine particle µDEP . Dielectrophoretic and electrokinetic mobilities of polystyrene particles and human erythrocytes have been determined simultaneously using unique streak-based image processing to generate the velocity profile of particles which then allows measurement of µDEP. In the future, this work will aid in the creation of a new class of devices, capable of clinically significant separation of multiple bioanalytes from complex samples.