Biomedical Engineering Reference
In-Depth Information
which is simply the Helmholtz-Smoluchowski equation from the electroosmosis
phenomenon. One interesting thing to note about Eqs. [6] and [7] is that, even
though they are developed for opposite limiting cases, they differ only by the
constant factor 2/3. When the Debye length is neither large nor small relative to
the particle radius, the dynamics of the particle motion are significantly more
difficult to calculate. However, even in these cases Eq. [7] is still a reasonable
estimate of particle velocity.
As with the case of electroosmosis, the effectiveness of electrophoresis is
quantified using an
electrophoretic mobility
parameter defined as
u
E
ep
N
=
,
[8]
ep
el
where N
ep
can be thought of as motion produced per unit field.
2.3. Applications
Because of the net negative charge associated with DNA molecules, elec-
trophoresis can be used to manipulate DNA molecules. A classic example is
capillary gel electrophoresis, where an electrical field is used to pull tagged
DNA molecules through a gel matrix. The gel effectively filters the DNA mole-
cules according to size, since the shorter DNA segments can travel through the
gel much quicker than the longer segments.
Nanogen's biochip (8) is an example of using electrophoresis-enhanced hy-
bridization of DNA in a microfluidic chip (see Figure 3). The electrodes have a
positive potential, thereby inducing the DNA molecules toward specific hybridi-
zation sites. The microfluidic chip shown in Figure 3 (8), contains 100 microlo-
cation test sites, which are approximately 80
m in size.
3.
AC ELECTROKINETICS
AC electrokinetics has received limited attention in the microfluidics
community compared to its DC counterpart. AC electrokinetics refers to induced
particle and/or fluid motion resulting from externally applied AC electric fields.
A primary advantage of AC electrokinetics is that the alternating fields sign-
ificantly reduce electrolysis at the electrodes. In addition, the characteristic
voltages are typically on the order of tens of volts, which are typically much
smaller than those used in DC electrokinetics. AC electrokinetics can be
classified into three broad areas: dielectrophoresis (DEP), electrothermal forces,
and AC electroosmosis (17).
