**3.1. Fundamentals of DEP**

Dielectrophoretic force, *FDEP*, for a stationary alternative current (AC) field is given by

$$F\_{\rm DEP} = 2\pi a^3 \varepsilon\_n \operatorname{Re} [f\_{\rm Col}] \nabla \left| E \right|^2 \tag{1}$$

where *E* is the electric field, *εm* is the permittivity of the suspending medium, *a* is the particle radius, *f CM* is the Clausius-Mossotti factor which describes relationship between dielectric constants of two different media, and the Re[*f CM*] is the real part of the factor [7].

For a spherical particle, *f CM* is governed by

$$f\_{\rm CM} = \frac{\varepsilon\_p^\* - \varepsilon\_m^\*}{\varepsilon\_p^\* + 2\varepsilon\_m^\*} \tag{2}$$

where *ε\** is the complex permittivity, which is determined by

$$
\varepsilon\_{p,m}^{\*} = \varepsilon\_{0}\varepsilon\_{p,m} - j\frac{\sigma\_{p,m}}{\omega\nu} \tag{3}
$$

translational movement in direction of electrode array. The electrorotation employs quadrupole electrodes with 90° phase-shifted electric signal. The electrodes generate a rotating electric field

As bioparticles are typically multilayered with the presence of multilayer membranes, Clausius-Mossotti factor calculation needs to consider total permittivity of the bioparticle

)]/ [(

is the permittivity that includes the outermost layer of the bioparticle, while *εpn-1* is per-

 is the second outermost layer (e.g., membrane) radius. The denotation *n =* 0*,* 1*,* 2*,* 3*…* is the corresponding layer calculation number that starts from the innermost layer (*n* = 0). The total per-

Transition between the pDEP and nDEP responses of bioparticle happens across the point when the polarization of the particle and the suspending medium are equal, which occurs at a par-

> *xo* <sup>=</sup> <sup>√</sup> \_\_ 2 *σ* \_\_\_\_\_\_*<sup>m</sup>* 2*a Cm*

where *σm* is the conductivity of the surrounding medium, *a* is the particle radius, and *Cm* is the

Manipulation of model organism has been demonstrated by Chen et al. [18], who perform detection and trapping of *Shewanella oneidensis*, a model organism for electrochemical activity bacteria, using DEP microfluidic chip, which equipped with real-time imaging for trapping process observation. The trapping is enhanced by providing hole arrays on top of the electrode, in which the hole size-bacteria count relation is also studied. Sang et al. [19] demonstrate blood manipulation using DEP. A portable microsystem for separation of living/dead RBCs by DEP which integrated with subsequent evaluation using surface stress sensor for living/dead RBC detection has been developed. This multifunctional portable microsystem is of potential application into diagnostic of hemolytic anemia disease, i.e., the situation where RBCs are damaged and removed from the bloodstream before their normal lifespan is over. Chiu et al. [20] use optically induced DEP (ODEP) to enhance conventional microfluidic isolation and purification of CTCs from whole blood sample. In this system, light image is projected on photoconductive materials which coated on indium tin oxide (ITO) glass substrate to generate dielectrophoretic force field. Hollow circular images are used for targeted CTC

*a* \_\_\_*<sup>n</sup> an*−1) 3 − ( *εpn*−1 <sup>−</sup> *<sup>ε</sup><sup>p</sup>* \_\_\_\_\_\_\_*<sup>n</sup> εpn*−1 + 2 *εpn*

Biological Particle Control and Separation using Active Forces in Microfluidic Environments

*CM*] < 0) for

87

*CM*] [16].

)] (5)

*xo* is governed by

(6)

, for a spherical multilayered

http://dx.doi.org/10.5772/intechopen.75714

is the outermost layer (e.g., cell wall) radius, and

*xo*. For a spherical structure, *f*

, in which *n* is the final layer number [16].

when excited with this multiphase AC field. Electrorotation is achieved by nDEP (Re[*f*

bioparticle levitation during the rotational motion which is determined by Im[*f*

comprising of the permittivity of all layers. Total permittivity, *ε<sup>p</sup>*

*a* \_\_\_*<sup>n</sup> an*−1) 3 + 2 ( *εpn*−1 <sup>−</sup> *<sup>ε</sup><sup>p</sup>* \_\_\_\_\_\_\_*<sup>n</sup> εpn*−1 + 2 *εpn*

, is the final layer calculated permittivity, *εpn*

particle is given by

where *εpn*

mittivity, *ε<sup>p</sup>*

*an−*<sup>1</sup>

*<sup>ε</sup><sup>p</sup>* <sup>=</sup> *<sup>ε</sup>pn*[(

mittivity that excludes the outermost layer. *an*

ticular frequency known as crossover frequency, *f*

capacitance of the bioparticle plasma membrane [17].

**3.2. Dielectrophoretic manipulation of bioparticles**

*f*

**Figure 1.** (a) Dielectrophoresis. At a certain frequency, particle A (purple) experiences higher polarizability than the suspending medium (*f CMA* > 0), attracting it to the region with higher electric field gradient, while at the same time particle B (yellow) experiences less polarizability than the suspending medium (*f CMB* < 0), pushing it away to the region with lower electric field gradient (FpDEP, positive dielectrophoretic force; FnDEP, negative dielectrophoretic force) (Reprinted with permission from Md Ali et al. [13]. Copyright 2016 Royal Society of Chemistry). (b) Manipulation of bacteria by dielectrophoresis. Blood specimen mixed with a permeabilizing agent sample is injected into the microfluidic device for desalination process by membraneless dialysis before subsequent enrichment of target bacteria, *E. coli*, at dielectrophoretic electrodes (Adapted with permission from D'Amico et al. [14]. Copyright 2017 Royal Society of Chemistry).

where *ε*<sup>0</sup> is the vacuum permittivity (8.854 × 10−12 F/m, *j* is the unit imaginary number, i.e., √(−1), *σ* is the conductivity, and *ω* is the angular frequency of electric signal supplied. The subscripts "*p*" and "*m*" stand for the particle and the medium, respectively.

The dielectrophoretic force direction is determined by the sign of the *f CM*, as presented in **Figure 1a**. For *f CM* with positive sign (*f CM* > 0), the force pulls particles to the region with high electric field gradients, and the situation is termed as positive DEP (pDEP). While for *f CM* with negative sign (*f CM* < 0), the force pushes particles away from those regions, and this is termed as negative DEP (nDEP). Hence, the dielectrophoretic force is proportional to particle volume and is highly dependent on the electrical properties of the particle, the medium, and the frequency of the AC field [15].

Moreover, for an AC field with spatial variation, the dielectrophoretic force is given by

$$F\_{\rm DEP} = 2\pi\varepsilon\_{\rm m} \operatorname{Re} \left[ f\_{\rm CM} \right] a^3 \nabla \left| E \right|^2 + 2\pi\varepsilon\_{\rm m} \operatorname{Im} \left[ f\_{\rm CM} \right] a^3 \times \left( \left| E\_j \right|^2 \nabla \Phi\_j \right) \tag{4}$$

where Re[*f CM*] and Im[*f CM*] are the Clausius-Mossotti real part and imaginary part, respectively; *ϕ* is the AC field phase, and subscript "*i"* represents each element of the field as well as the phase [15]. AC field with spatial variation is being employed in (1) traveling wave DEP and (2) electrorotation.

In general, the traveling wave DEP is created by application of 90° phase-shifted electric signal, i.e., 0, 90, 180, and 270°, on an array of planar parallel electrodes, causing generation of a traveling wave of electrostatic potential which can vertically suspend a lossy dielectric sphere while at the same time propels it along the array. The Re[*f CM*] determines whether the bioparticles are levitated (nDEP) or attracted to the electrodes (pDEP), while Im[*f CM*] determines the translational movement in direction of electrode array. The electrorotation employs quadrupole electrodes with 90° phase-shifted electric signal. The electrodes generate a rotating electric field when excited with this multiphase AC field. Electrorotation is achieved by nDEP (Re[*f CM*] < 0) for bioparticle levitation during the rotational motion which is determined by Im[*f CM*] [16].

As bioparticles are typically multilayered with the presence of multilayer membranes, Clausius-Mossotti factor calculation needs to consider total permittivity of the bioparticle comprising of the permittivity of all layers. Total permittivity, *ε<sup>p</sup>* , for a spherical multilayered particle is given by

$$\varepsilon\_p = \varepsilon\_{p\_\cdot} \left[ \left( \frac{a\_n}{a\_{n+1}} \right)^3 + 2 \left( \frac{\varepsilon\_{p\_{r\cdot i}} - \varepsilon\_{p\_\cdot}}{\varepsilon\_{p\_{r\cdot i}} + 2 \, \varepsilon\_{p\_\cdot}} \right) \right] \Bigg/ \left[ \left( \frac{a\_n}{a\_{n+1}} \right)^3 - \left( \frac{\varepsilon\_{p\_{r\cdot i}} - \varepsilon\_{p\_\cdot}}{\varepsilon\_{p\_{r\cdot i}} + 2 \, \varepsilon\_{p\_\cdot}} \right) \right] \tag{5}$$

where *εpn* is the permittivity that includes the outermost layer of the bioparticle, while *εpn-1* is permittivity that excludes the outermost layer. *an* is the outermost layer (e.g., cell wall) radius, and *an−*<sup>1</sup> is the second outermost layer (e.g., membrane) radius. The denotation *n =* 0*,* 1*,* 2*,* 3*…* is the corresponding layer calculation number that starts from the innermost layer (*n* = 0). The total permittivity, *ε<sup>p</sup>* , is the final layer calculated permittivity, *εpn* , in which *n* is the final layer number [16].

Transition between the pDEP and nDEP responses of bioparticle happens across the point when the polarization of the particle and the suspending medium are equal, which occurs at a particular frequency known as crossover frequency, *f xo*. For a spherical structure, *f xo* is governed by

$$f\_{\nu\nu} = \frac{\sqrt{2}\,\sigma\_n}{2\pi a \,\mathbb{C}\_n} \tag{6}$$

where *σm* is the conductivity of the surrounding medium, *a* is the particle radius, and *Cm* is the capacitance of the bioparticle plasma membrane [17].

#### **3.2. Dielectrophoretic manipulation of bioparticles**

where *ε*<sup>0</sup>

Chemistry).

**Figure 1a**. For *f*

suspending medium (*f*

86 Microfluidics and Nanofluidics

negative sign (*f*

where Re[*f*

and (2) electrorotation.

quency of the AC field [15].

*FDEP* = 2*πε<sup>m</sup>* Re[*f*

*CM*] and Im[*f*

is the vacuum permittivity (8.854 × 10−12 F/m, *j* is the unit imaginary number, i.e.,

*CMA* > 0), attracting it to the region with higher electric field gradient, while at the same time

*CM* < 0), the force pushes particles away from those regions, and this is termed

*CM* > 0), the force pulls particles to the region with high

*CM*] *a* <sup>3</sup> × (|*Ei*

*CM*] are the Clausius-Mossotti real part and imaginary part, respec-


*CM*] determines whether the biopar-

*CM*, as presented in

*CMB* < 0), pushing it away to the

) (4)

*CM*] determines the

*CM* with

√(−1), *σ* is the conductivity, and *ω* is the angular frequency of electric signal supplied. The

**Figure 1.** (a) Dielectrophoresis. At a certain frequency, particle A (purple) experiences higher polarizability than the

region with lower electric field gradient (FpDEP, positive dielectrophoretic force; FnDEP, negative dielectrophoretic force) (Reprinted with permission from Md Ali et al. [13]. Copyright 2016 Royal Society of Chemistry). (b) Manipulation of bacteria by dielectrophoresis. Blood specimen mixed with a permeabilizing agent sample is injected into the microfluidic device for desalination process by membraneless dialysis before subsequent enrichment of target bacteria, *E. coli*, at dielectrophoretic electrodes (Adapted with permission from D'Amico et al. [14]. Copyright 2017 Royal Society of

as negative DEP (nDEP). Hence, the dielectrophoretic force is proportional to particle volume and is highly dependent on the electrical properties of the particle, the medium, and the fre-

electric field gradients, and the situation is termed as positive DEP (pDEP). While for *f*

Moreover, for an AC field with spatial variation, the dielectrophoretic force is given by

2

tively; *ϕ* is the AC field phase, and subscript "*i"* represents each element of the field as well as the phase [15]. AC field with spatial variation is being employed in (1) traveling wave DEP

In general, the traveling wave DEP is created by application of 90° phase-shifted electric signal, i.e., 0, 90, 180, and 270°, on an array of planar parallel electrodes, causing generation of a traveling wave of electrostatic potential which can vertically suspend a lossy dielectric sphere

+ 2*πε<sup>m</sup>* Im[*f*

*CM*] *a* <sup>3</sup> ∇|*E*|

ticles are levitated (nDEP) or attracted to the electrodes (pDEP), while Im[*f*

subscripts "*p*" and "*m*" stand for the particle and the medium, respectively. The dielectrophoretic force direction is determined by the sign of the *f*

particle B (yellow) experiences less polarizability than the suspending medium (*f*

*CM* with positive sign (*f*

while at the same time propels it along the array. The Re[*f*

Manipulation of model organism has been demonstrated by Chen et al. [18], who perform detection and trapping of *Shewanella oneidensis*, a model organism for electrochemical activity bacteria, using DEP microfluidic chip, which equipped with real-time imaging for trapping process observation. The trapping is enhanced by providing hole arrays on top of the electrode, in which the hole size-bacteria count relation is also studied. Sang et al. [19] demonstrate blood manipulation using DEP. A portable microsystem for separation of living/dead RBCs by DEP which integrated with subsequent evaluation using surface stress sensor for living/dead RBC detection has been developed. This multifunctional portable microsystem is of potential application into diagnostic of hemolytic anemia disease, i.e., the situation where RBCs are damaged and removed from the bloodstream before their normal lifespan is over. Chiu et al. [20] use optically induced DEP (ODEP) to enhance conventional microfluidic isolation and purification of CTCs from whole blood sample. In this system, light image is projected on photoconductive materials which coated on indium tin oxide (ITO) glass substrate to generate dielectrophoretic force field. Hollow circular images are used for targeted CTC trapping, while long rectangular light bar is used to attract other untargeted cells. Adams et al. [21] perform sorting of neural stem and progenitor cells using microfluidic DEP device. The sorting is based on the cell membrane capacitance variation, which specifically determines the future forming cells, either neuron or astrocyte. Bacteria manipulation using DEP microfluidic device is shown by D'Amico et al. [14], in which they isolate and enrich *Escherichia coli* (*E. coli*) which spiked into whole blood sample. The device integrates membraneless dialysis process and dielectrophoretic trapping, presented in **Figure 1b**. Ding et al. [22] demonstrate the capture and enrich of *Sindbis* virus in a gradient insulator-based DEP microfluidic device, i.e., electric signal source is applied across both microchannel ends while insulating polydimethylsiloxane (PDMS) used to distort electric field, hence forming a nonuniform electric field. By tuning the voltage of applied signal, they can capture or release the virus from the saw-shaped electrode tip. Nucleic acid manipulation has been conducted by Jones et al. [23], in which they perform size-based sorting of a wide range of nucleic acid analytes, i.e., 1.0, 10.2, 19.5, and 48.5 kbp double-stranded DNA (dsDNA) analytes, including both plasmid and genomic DNA in a continuous flow microfluidic platform, by using an insulator-based DEP to tune the deflection of the nucleic acids. Viefhues et al. [24] perform dielectrophoretic mobility shift assay of DNA complexes as well as pure DNA in a nanofluidic DEP system to demonstrate new technique in detection of different DNA variants, including protein-DNA complexes. Manipulation of proteins has been achieved by Mohamad et al. [25]. They use DEP to capture and characterize the electrical properties of colloidal protein molecule, i.e., bovine serum albumin (BSA), based on the molecule dispersion impedance exhibited at particular frequencies, which are influenced by the electrical double layer surrounding the molecule. Liao et al. [26] developed a DEP nanofluidic device to perform selective pre-concentration of functional proteins within bio-fluid medium, which in general is a high ionic strength medium. The device is advantageous in the enhancement of DEP trapping forces against electrothermal flow which is challenging in nanoscale device design.
