(306g) Insulator-Based Dielectrophoresis with Direct Current Electric Fields

Dielectrophoresis (DEP) is an efficient electrokinetic technique employed in microdevices for the concentration and manipulation of particles by means of polarization effect under non uniform electric fields, and it can occur in AC or DC electric fields.^1-3 DEP has been successfully implemented for the manipulation and concentration of a wide array of particles, ranging from macromolecules to parasites.^{1, 4, 5} Traditionally, dielectrophoresis has been used employing arrays of microelectrodes and AC electric fields. Electrode-based DEP allows obtaining high electric field gradients employing low applied voltages.^{4, 6, 7} However, significant electrolysis can be obtained, and massive parallel systems aimed to produce high throughput are prohibited due to high cost of electrode fabrication.⁸

Cummings and Singh proposed a different approach for carrying dielectrophoresis, by employing an array of cylindrical posts embedded in a mcirochannel.⁹ In insulator-based DEP (iDEP) the voltage is applied across an array of insulating structures, employing only two electrodes that straddle the array of insulating structures. When an electric field is applied, the electric field lines are bent between the insulating structures, creating the nonuniform electric field necessary for DEP to occur.⁹  When iDEP is applied employing DC electric fields, other electrokinetic transport mechanisms are also present: electrophoresis and electroosmotic flow. In order to concentrate and immobilize particles, iDEP has to overcome electrokinetics (electrophoresis and electroosmosis).⁹

In this study, the performance of an iDEP microdevice was modeled implementing FEA through the computational software COMSOL Multiphysics®. The microdevice modeled consisted of a microchannel that contained an array of cylindrical insulating structures, where DC electric fields were applied across the array. Inert 1-mm-diameter polystyrene microspheres were selected as probe particles. The main purpose of the mathematical simulations was the prediction of the locations and magnitude of the zones, within the insulating structures array, where dielectrophoretic trapping of the microparticles occurs as a function of the operating conditions. Furthermore, simulation results were confirmed by carrying out iDEP experiments employing the same microchannel and microparticles.

Trapping of particles occurs when the DEP contribution to the total flow of particles overcomes the electrokinetic contribution. So, DEP trapping when the relation

is satisfied along the region where the electric field E  is applied. Here, m_dep and m_ek are the dielectrophoretic and electrokinetic mobilities respectively. These parameters characterize the particle flow along the microchannel.

The microchannel used in this work was 10.12-mm-long, 1-mm-wide, 10-mm deep, and had an array of 8 columns x 4 rows of cylindrical insulating posts 470-μm in diameter and arranged 510-μm center-to-center (Figure 1).

Figure 1. Geometry of the microchannel employed for dsimulation an experimentation

Figure 2. Predicted trapping zones with simulation and experimental observations, with and applied electric potential of 750 V and under different suspending mediums. (a) and (b)  pH=6 and sm=50 mS/cm. (c) and (d) pH=9 and sm=100 mS/cm.

Result from simulation and from experiment are presented in Figure 2. It has been reported how pH and conductivity of the suspending medium have an effect on the development of trapping regions along an array of insulating posts. Figure 2a presents the trapping regions obtained in the simulations for a suspending medium with pH 6 and conductivity of 50 mS/cmwhen an electric potential of 750 V is applied. Figure 2b presents the experimental result under the same conditions. Figure 2c displays the trapping regions along the microchannel when a suspending medium with pH 9 and conductivity of 100 mS/cm is used and a potential of 750 V is applied. Figure 2d shows the regions obtained in the experiment under the same conditions.  As it can be observed, the model is able to predict with excelellen accurary how the dielectrophoretic behavior of particles is affected by the suspending medium properties.

It is expected that these findings will be employed as a tool for the design and selection of operating conditions of dielectrophoretic microdevices for bioparticles concentration and manipulation.

References

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