Biomedical Engineering Reference
In-Depth Information
A positive DEP force propels particles to the ield maxima; a negative DEP force propels particles
to the ield minima. he ields used can be either AC or DC, but in general, AC ields are used because
insulated electrodes can be used, allowing them to be used indeinitely in the aqueous medium. At
high frequencies on the >1 MHz range, electrolysis-generated H
+
or OH
-
ions recombine before dif-
fusing away, which prevents cell-adverse changes in the local pH. Also, AC ields induce reduced
transmembrane voltages compared with DC ields: at 20 MHz, a 3 V AC ield will only induce a 12
mV transmembrane voltage on HL-60 cells, approximately 20 times smaller than it would at DC.
his force can be used to diferentially levitate particles including cells (see Section 5.3.4 on
dielectrophoretic traps
), and to do so in such a way that particles are sorted in time as they
progress down a channel—those that loat higher than others move at a higher velocity down the
channel by residing in a faster-moving luid stream. It can also be used to concentrate speciic
particles at speciic points on surfaces, and to divert particular particles from one low stream to
another, to cause dynamic concentration.
WHEN SMALLER IS (MUCH) BETTER
Much of the initial microfluidics research impulse
was about making
things smaller for its own sake, or “just” to save reagents, but overall it was thought that
smaller devices would operate in the same way as their macro-counterparts. As it turns
out, microfabricating analytical systems is not simply about following a Moore's type law
down to a smaller and denser “luidic microprocessor” to save costs and improve per-
formance. he physics of luid behavior makes interesting transitions as things reach
the microscale, particularly with respect to the aforementioned Reynolds number, and
the importance of difusion for mass transport. Not all things get worse when things get
smaller, as a consequence; there are entirely new and extremely useful devices possible at
the microscale that have no counterpart at the macroscale. Discovering these new operat-
ing principles has been one of the most exciting features of research in this area.
3.3.4 Electrowetting
Electrowetting
is the phenomenon whereby an electric ield inluences the wettability of an
electrolyte droplet on a surface (which is typically micropatterned with electrodes). he irst
observations were made with mercury (a metal luid) in the 19th century but it has found most
applications in the manipulation of small water droplets in the last 25 years. he electrowetting
efect can be deined as
the change in solid electrolyte contact angle due to an applied potential
diference between the solid and the electrolyte
. he fringing ield at the corners of the electrolyte
droplet tends to pull the droplet down onto the electrode, lowering the macroscopic contact angle
and increasing the droplet contact area. Looking at electrowetting from a thermodynamics per-
spective, the surface tension of an interface (between the liquid electrolyte and the solid conduc-
tor) γ
SL
is equal to the Gibbs free energy required to create a certain area of the droplet's surface,
which contains a chemical and an electrical (charge) term. he electrical component of the Gibbs
free energy is the energy stored in the capacitor formed between the conductor and the electrolyte
and can thus be modulated by changing the voltage of the electrodes. he chemical component is
simply the surface tension of the solid-electrolyte interface in the absence of an electric ield (γ
S
0
).
Deining
C
, as the capacitance of the interface; ε
r
ε
0
/
t
, for a uniform dielectric of thickness
t
and
permittivity ε
r
;
V
, the efective applied voltage, integral of the electric ield from the electrolyte to
the conductor, then we have the total surface tension between the conductor and the electrolyte as:
CV
2
γ
=
γ
0
−
(
3.36
)
L
SL
2
