1. Introduction

Electrokinetics (EK), as the name implies, is a technique of using electric field to cause motion. The motion can be that of a liquid or colloidal particle (microscopic solid particles suspended in a fluid). This concept of electrokinetics has four main features: Electro-osmosis (EO), electrophoresis (EP), streaming potential (StP), and sedimentation potential (SeP). In the disease diagnostics arena—an integral part of the application of microfluidics in healthcare, electro-osmosis and electrophoresis are the main electrokinetic forces that particles experience. Electro-osmosis involves the bulk motion of a liquid through a solid surface under the influence of electric field. On the other hand, electrophoresis is the motion of a solid material through a liquid under an electric field effect. Both electrophoresis and electroosmosis require that the surface of the solid be charged. As shown in Figure 1, when the surface of the particle, placed within a uniformly distributed electric field, is positively charged (A), the particle (suspended in a characterized liquid) moves to the left but to the right (against the field direction) when the charge on the surface

In dielectrophoresis (DEP), the applied electric field must be non-uniform. The movement of the particles in non-uniform electric field does not depend primarily on the particle charge but on the ability of the particle to become polarized relative

Generation of non-uniform electric fields for dielectrophoretic applications; (A) simple unequal spatial electrode-pair arrangement connected to an AC source with the lower electrode acting as the high-field region while the upper electrode on the low-field region. The field low-high regions remain the same even when the electrode terminals are interchanged owing the frequency component of the AC source (B) insulating constriction (or obstacle) changed the uniform field between the electrodes to non-uniform field at its location.

Applications of Electrokinetics and Dielectrophoresis on Designing Chip-Based Disease…

Therefore, DEP force is always in effect when the particle is charged or not. This means, dielectrophoresis, therefore, safely be defined as a technique of using nonuniform electric field to induce the motion of a charged or an uncharged particle. Electric field can be rendered non-uniform in different ways. Figure 2 shows some examples of how to generate non-uniform electric fields using AC or DC source. For the AC field, a simple unequal spatial electrode-pair arrangement (Figure 2A) would generate non-uniform field (AC DEP or classical DEP). A second method is to place an insulating constriction within a simple straight channel operating under DC condition that would make the field non-uniform (DC-iDEP or iDEP). There is never any hard and fast rule regarding how the electrodes should be arranged or the constrictions be distributed but simulation could assist in formulation of the device architecture. Every researcher has a purpose in mind and that purpose drives the architecture of the channel or device without denying the underlining physics.

Characterization of biological entities like cells involves the utilization of various

methods including but not limited to electrical, magnetic, acoustic, and optical characterization to explore cell properties. In this section, electrical method will be discussed (with focus on dielectrophoresis) since dielectrophoretic force is related, in part, to the electrical properties of the biological cell. In the utilization of electrical method for bioparticle characterization, it is not uncommon to use impedance cytometry, dielectrophoresis, and electrorotation. Impedance cytometry works on the principle that when a particle suspended in a conductive fluid passes through a small orifice (comparable to the size of the particle) created by two electrodes, the passage of the particle through the (usually AC) electric field between the electrodes results in the generation of electric signal, which can be processed to provide

valuable information about the electrical properties of the particle. In

electrorotation, four electrodes are each charged with AC voltage of different phases to generate a rotating electric field, thus setting up an electrical torque. When a (spherical) particle is placed within this rotating field, it becomes polarized inducing a dipole. The dipole moment induced within this particle rotates with the electric field at certain velocity. However, the multiphase nature of the four electrodes causes the particle to lag behind the field by a factor that depends on the

to that of the suspending medium [2].

DOI: http://dx.doi.org/10.5772/intechopen.82637

Figure 2.

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2. Characterization of biological cells

### Figure 1.

The representation of electro-osmosis and electrophoresis as the two main electrokinetic phenomena in microfluidic channels: (A) positively charged particle under a uniform field region effect will move toward the negative electrode according to the fundamental attraction-repulsion laws. (B) Negatively charged particle placed under the same electric field region will migrate toward the positive electrode. (C) A no-charge (neutral) particle placed in the same field region will experience no electrophoretic motion because electrophoresis only applies to charged particles. (D) Representation of the deprotonation of the terminal (SiOH)/surface chemical group of the channel wall (treated poly(dimethyl siloxane) (PDMS) or uncoated glass), which births the electro-osmotic pumping within the channel.

is reversed (B). If the particle is a neutral body (C), no electrophoretic effect will be seen. Figure 1D is a case where liquid is flowing through a charged solid surface. The solid surface (usually glass or surface-treated polymer) become deprotonated when in contact with the liquid such that counter (positive) ions from the liquid goes into the channel surface to firmly replace the detached positive ions forming what is referred to as the Stern layer. Columbic force of attraction causes the Stern layer to, in turn, attract negative ions from the liquid forming the diffuse layer (together with some unattached positive charges) beyond which the region of electro-neutrality is initiated. A plane, called the slipping plane, formed around the loosely-bound diffuse layer is very crucial to how the bulk of the liquid would move when the electric field if applied across the static upstream and downstream flow region. The potential of this slipping plane is called the Zeta potential, ζ [1]. The same concept of Zeta potential applies to a charged solid moving through a liquid. For a charged solid moving through a liquid under an electric field effect, the rate of motion of the solid, rep, depends on the Zeta potential at its slipping plane, ζp, viscosity of the liquid in which it is moving, η, dielectric constant of the liquid, εm, as well as the strength of the applied electric field, E. Mathematically, this has been represented as

$$r\_{ep} = f\left(\zeta\_p, \varepsilon\_m, \eta, E\right) = \left(\zeta\_p \varepsilon\_m E\right) / \eta \tag{1}$$

On the other hand, when a liquid is flowing through a charged wall of Zeta potential, ζw, the positional rate of flow of the liquid, reo, is given by

$$r\_{\text{ev}} = f(\zeta\_w, \varepsilon\_m, \eta, E) = -\zeta\_w \varepsilon\_m E \big/ \eta \tag{2}$$

Applications of Electrokinetics and Dielectrophoresis on Designing Chip-Based Disease… DOI: http://dx.doi.org/10.5772/intechopen.82637

### Figure 2.

is reversed (B). If the particle is a neutral body (C), no electrophoretic effect will be seen. Figure 1D is a case where liquid is flowing through a charged solid surface. The solid surface (usually glass or surface-treated polymer) become deprotonated when in contact with the liquid such that counter (positive) ions from the liquid goes into the channel surface to firmly replace the detached positive ions forming what is referred to as the Stern layer. Columbic force of attraction causes the Stern layer to, in turn, attract negative ions from the liquid forming the diffuse layer (together with some unattached positive charges) beyond which the region of electro-neutrality is initiated. A plane, called the slipping plane, formed around the loosely-bound diffuse layer is very crucial to how the bulk of the liquid would move when the electric field if applied across the static upstream and downstream flow region. The potential of this slipping plane is called the Zeta potential, ζ [1]. The same concept of Zeta potential applies to a charged solid moving through a liquid. For a charged solid moving through a liquid under an electric field effect, the rate of

The representation of electro-osmosis and electrophoresis as the two main electrokinetic phenomena in microfluidic channels: (A) positively charged particle under a uniform field region effect will move toward the negative electrode according to the fundamental attraction-repulsion laws. (B) Negatively charged particle placed under the same electric field region will migrate toward the positive electrode. (C) A no-charge (neutral) particle placed in the same field region will experience no electrophoretic motion because electrophoresis only applies to charged particles. (D) Representation of the deprotonation of the terminal (SiOH)/surface chemical group of the channel wall (treated poly(dimethyl siloxane) (PDMS) or uncoated glass), which births the

motion of the solid, rep, depends on the Zeta potential at its slipping plane, ζp, viscosity of the liquid in which it is moving, η, dielectric constant of the liquid, εm, as well as the strength of the applied electric field, E. Mathematically, this has

On the other hand, when a liquid is flowing through a charged wall of Zeta

¼ ζpεmE 

reo ¼ fð Þ¼� ζw; εm; η; E ζwεmEÞ=η (2)

=η (1)

rep ¼ f ζp; εm; η; E 

potential, ζw, the positional rate of flow of the liquid, reo, is given by

been represented as

56

Figure 1.

Bio-Inspired Technology

electro-osmotic pumping within the channel.

Generation of non-uniform electric fields for dielectrophoretic applications; (A) simple unequal spatial electrode-pair arrangement connected to an AC source with the lower electrode acting as the high-field region while the upper electrode on the low-field region. The field low-high regions remain the same even when the electrode terminals are interchanged owing the frequency component of the AC source (B) insulating constriction (or obstacle) changed the uniform field between the electrodes to non-uniform field at its location.

In dielectrophoresis (DEP), the applied electric field must be non-uniform. The movement of the particles in non-uniform electric field does not depend primarily on the particle charge but on the ability of the particle to become polarized relative to that of the suspending medium [2].

Therefore, DEP force is always in effect when the particle is charged or not. This means, dielectrophoresis, therefore, safely be defined as a technique of using nonuniform electric field to induce the motion of a charged or an uncharged particle. Electric field can be rendered non-uniform in different ways. Figure 2 shows some examples of how to generate non-uniform electric fields using AC or DC source. For the AC field, a simple unequal spatial electrode-pair arrangement (Figure 2A) would generate non-uniform field (AC DEP or classical DEP). A second method is to place an insulating constriction within a simple straight channel operating under DC condition that would make the field non-uniform (DC-iDEP or iDEP). There is never any hard and fast rule regarding how the electrodes should be arranged or the constrictions be distributed but simulation could assist in formulation of the device architecture. Every researcher has a purpose in mind and that purpose drives the architecture of the channel or device without denying the underlining physics.
