by Roberto C. Gallo-Villanueva and Blanca H. Lapizco-Encinas, Tecnologico de Monterrey, Mexico
Miniaturization has significantly driven the advancement of separation technology since it offers important advantages: lower cost, reduced sample and reagent consumption, short response time, greater sensitivity and portability. There is a growing interest on the development of separation techniques that can be applied on microdevices . Dielectrophoresis (DEP) is an efficient electrokinetic technique with great potential for miniaturization. DEP is produced by polarization effects when particles are exposed to nonuniform electric fields; the applications of DEP range from biomolecules to parasites [2, 3]. Most research on DEP has been performed employing arrays of microelectrodes and AC electric fields. Electrode-based DEP allows obtaining high electric field gradients employing low applied voltages. However, there are some drawbacks: high cost of electrode construction, complex fabrication processes and decrease of functionality due to fouling effects, which is a common effect when handling biological samples.
There is an alternative manner of carrying out DEP; insulator-based DEP (iDEP) is a technique where the voltage is applied using only two electrodes that straddle an insulating structures array. When an electric field is applied across such an array, the presence of the structures creates regions of higher and lower field strength, i.e., dielectrophoretic traps . Insulator-based DEP systems do not lose their functionality despite fouling effects, which makes them more suitable for biological applications; iDEP systems can be fabricated from a wide variety of materials, including plastics, leading to inexpensive systems, increasing their potential for high throughput applications. Despite the novelty of iDEP, there have been outstanding designs of microdevices successfully employed for bioparticle manipulation. Below is included a brief description of strategically selected research studies that depict some of the foremost examples of successful applications of iDEP. Figure 1 shows a schematic representation of the microsystems employed in these studies.
|Figure 1. Schematic representation of some microdevices employed in iDEP. (a) Microchannel used by Cummings and Singh , (b) Dielectrophoretic filter employed by Suehiro, et al. , (c) Oil droplet channel developed by Barbulovic-Nad, et al. , (d) Microdevice used by Kang, et al. , (e) Circular channel proposed by Zhang, et al. , (f) Device with oil menisci insulating posts developed by Thwar, et al. , (g) Saw-tooth microchannel employed by Pysher and Hayes , and (h) Microchannel with porous membrane developed by Kovarik and Jacobson .|
In 2003 Cummings and Singh  reported the application of an array of insulating posts inside a microchannel for microparticle manipulation (Fig. 1a). This same configuration was employed later for bacterial and protein manipulation [5, 6]. In that same year Suehiro et al.  developed a dielectrophoretic filter by employing spherical glass beads as insulators between two electrodes for the removal of yeast cells suspended in water using AC fields (Fig. 1b). In 2006, Barbulovic-Nad et al.  produced a nonuniform field by intercalating an oil droplet inside a microchannel (Fig. 1c), where the size of the oil droplet was manipulated to achieve a dynamic iDEP system that was tested by employing polystyrene particles of different sizes and applying DC fields. In 2006, Kang et al.  demonstrated dielectrophoretic separation of particles utilizing an insulating block inside a microchannel, where the particles experienced negative DEP in the corners of the block deflecting from its electrokinetic path according to their size (Fig. 1d). In 2006, Zhang et al.  reported experimental and simulation research on the development of a circular channel with electrodes on its extremes to continuously separate particles according to size (Fig. 1e). A dynamic system where an oil menisci was used to form dynamic dielectrophoretic traps and immobilize particles inside a channel (Fig. 1f) was proposed by Thwar et al. in 2007 , where the immobilization of polystyrene particles was achieved under DC current. In the same year, Pysher and Hayes  separated live and dead bacteria by using a gradient of dielectrophoretic traps in a microchannel with a saw-tooth geometry, where the width of the channel decreased longitudinally to the flow, thus increasing the compression of the electrical field and dielectrophoretic force (Fig. 1g). In 2008, Kovarik and Jacobson  used a membrane with trapezoidal pores (Fig. 1h) to achieve multiple dielectrophoretic traps inside a microchannel and collect polystyrene particles and bacteria.
In conclusion, the development of iDEP has advanced significantly in the last few years. The studies reviewed here demonstrate that this powerful and dynamic technique has an enormous potential for bioparticle manipulation. Nevertheless, much work is needed to achieve a complete iDEP microdevice able to handle complex biological samples. It is expected that the development of iDEP will continue to grow considerably in years to come.