Alireza Salmanzaheh1,2 and Rafael V. Davalos1,2,*
1 Bioelectromechanical Systems Laboratory, School of Biomedical Engineering and Sciences, Virginia Tech — Wake Forest University, Blacksburg, VA, 24061
2 Engineering Science and Mechanics Department, Virginia Tech, Blacksburg, VA, 24061.
*E-mail: email@example.com; Tel: 1-540-231-1979
Particle manipulation in microfluidic devices, including isolation, enrichment, and mixing, has an important role in biological and chemical applications. However, there are still wide areas of opportunity for existing approaches; current approaches fall short to fulfill the needs for making rapid medical and scientific advances. For instance, there are numerous situations in which it is highly desirable to separate very similar yet distinct cells. Applications include separating stem cells from adipose tissue, removing circulating tumor cells or bacteria from blood, and isolating cancer cells based on stage for individualized medicine.
Dielectrophoresis (DEP), the electrokinetic motion of a particle due to its polarization in the presence of a non-uniform electric field, is one of the successful particle manipulation techniques that can be used to differentiate between cells based on their intrinsic properties. The dielectrophoretic force is a function of cell volume and polarization, the conductivity and permittivity of the surrounding media, and the frequency and spatial gradients of the magnitude of the generated electric field.1 Traditionally, DEP is applied with the use of electrode arrays patterned on the surface of a silicon wafer slide under a microfluidic channel through which the sample fluid passes. The electrode array produces the nonuniform electric field needed to generate the dielectrophoretic force on the cell. Although DEP has been a very successful technique2, it has some difficulties such as bubble formation (due to electrolysis), electrode fouling3 and delamination, and expensive fabrication4. Modified DEP techniques, such as insulator-based dielectrophoresis (iDEP) have been developed to overcome drawbacks of conventional DEP techniques. In iDEP, insulating obstacles are straddled between two electrodes to create a non-uniform electric field5. Although electrode fouling and delamination are eliminated, sending an electric current across the sample fluid in iDEP still results in electrolysis and may induce large temperature increases for applications requiring highly conductive biological samples, which is damaging to cells and alters the DEP response.
Contactless dielectrophoresis (cDEP) is a recently developed technique, which capitalizes on the sensitivity of dielectrophoresis while eliminating many of its challenges6. In cDEP, an electric field is generated in the sample channel using electrodes inserted into two other side microchannels (filled with a conductive solution) that are separated from the sample channel by thin insulating barriers (Figure 1). These insulating barriers exhibit a capacitive behavior, and therefore an electric field can be produced in the sample channel by applying an AC field across the side microchannels. The geometry of the insulating membranes and insulating structures within the sample channel, e.g., an array of insulating posts, can be customized to enhance electric field gradients. The side channels and the fluidic channels are fabricated on the same layer, and there is no need for electrode patterning; thus, fabrication of the cDEP microdevices, which are disposable, has a relatively low cost and is readily amenable to mass fabrication techniques such as hot embossing and injection molding.
While cDEP will have wide-ranging applications as we further develop this technology, we have focused so far on cell isolation and mixing enhancement. The absence of a contact between the electrodes and the sample inside the fluidic channels avoids any contaminating effects the electrodes may have on the sample, making cDEP an ideal “isolate-and-culture” platform for investigating the biological processes of a target cell type. Moreover, in contrast to techniques such as flow cytometry, chemically functionalized pillar-based microchips, or magnetic bead cell separation, antibodies are not required for the cDEP cell identification technique, which eliminates extensive sample preparation. The ability of cDEP to isolate a heterogeneous mixture of live and dead THP-1 human leukemia monocytes,7 to isolate cancer cells from erythrocytes8, and to separate breast cancer cells based on their metastatic stage9 has been demonstrated recently by our group.
We have also developed an active micromixer based on cDEP10. Rapid mixing is necessary in lab-on-a-chip and micro total analysis systems devices for chemical and biological processes. Due to the small length scale of the Reynolds number, the ratio between inertia and viscous forces is also small making mixing dominated by molecular diffusion, and therefore rapid, efficient mixing is difficult to achieve. To address this need, we invented cDEP-based micromixers, in which pressure-driven flow of deionized water containing beads was mixed in various chamber geometries by imposing a dielectrophoretic force on the beads. Several system designs with rectangular and circular mixing chambers were fabricated in PDMS. When the time scales of the bulk fluid motion and the dielectrophoretic motion were commensurate, rapid mixing was observed (Figure 2). This approach shows potential for mixing low diffusivity biological samples, which is a very challenging problem in laminar flows at small scales.
|Figure 1. (a) 30 seconds after applying the electric field (100 Vrms and 152 kHz). The live (green) cells were trapped due to positive DEP force, but the dead (red) cells pass by the trapping area, (b) releasing the trapped live cells by turning off the power supply (the field superimposed reveals how cells trap along gradient).|
|Figure 2. cDEP micromixer showing stretching and folding of interface of two flows due to DEP force exerted on 0.5 μm beads (300 Vrms and 600 kHz).|