2.1 Dielectrophoretic movement
Pohl and co-workers detailly explained the theory of the
dielectrophoresis41-43. DEP force (\(F_{\text{DEP}}\))
can be defined according to the difference between the dielectric
properties of the particle and its suspension
medium44-46.
\(F_{\text{DEP}}=2\pi r^{3}\varepsilon_{m}Re(K\left(\omega\right))\nabla E^{2}\)(1)
The \(F_{\text{DEP}}\) is expressed in Eq (1) . Here, ris the radius of the cell (particle), \(\varepsilon_{m}\) is the
permittivity of the surrounding medium, (\(Re(K\left(\omega\right))\)is the real part of the Clausius-Mossotti factor\(K\left(\omega\right)\), and \(E\) is the applied electric field. The
Clausius-Mossotti factor is defined as given by in Eq (2) . The
subscripts “m ” and “c ” indicate suspending medium and
cells, respectively.
\(K\left(\omega\right)\ =\ \frac{(\varepsilon_{c}^{*}\ -\ \varepsilon_{m}^{*}\ )\ }{(\varepsilon_{c}^{*}+\ 2\varepsilon_{m}^{*}\ )}\)(2)
Here, \(\varepsilon_{c}^{*}\) is known as the complex permittivity of a
cell and \(\varepsilon_{m}^{*}\) is the complex permittivity of the
surrounding medium. The complex permittivity can be expressed as shownEq (3) . Here, \(\varepsilon\) is the permittivity, \(\sigma\)is the conductivity and \(\omega\) (\(\omega=2\pi f\)) includes the
electric field frequency (f).
\(\varepsilon^{*}\ =\varepsilon+\frac{\text{jσ}}{\omega}\) (3)
Reaction of the cells to dielectrophoretic forces can be classified
according to sign of the \(Re(K\left(\omega\right))\). When strong
electric field regions attract cells, the value of the\(Re(K\left(\omega\right)\) is positive, it is referred as positive
DEP (pDEP). Contrary, when strong electric field regions repel cells,
the value of the \(Re(K\left(\omega\right)\) is negative, and negative
DEP (nDEP) occurs. The crossover frequency can be described as the
cessation of the particle motion, which is specific for the particles.
Mostly, cells experience very weak DEP forces which does not generate a
clear movement.