Biomedical Engineering Reference
In-Depth Information
9.5.5
Fluidic Transportation
There are two basic paradigms in microfluidics: one is continuous flow in mi-
crochannels and the other is droplet microfluidics without channel, the so-called
digital microfluidics. In either paradigm, superhydrophobic coatings have been
widely applied for fluidic transportation. Washizu [
198
] have reported that water
droplets can be transported in a controlled manner on a superhydrophobic surface
with electrostatic methods. The surface is constructed by arrays of microelectrodes
and actuation was achieved by sequentially applying voltages to the electrodes. The
author also demonstrated that deflection of a droplet in either of the bifurcating
paths, and the mixing of two droplets serving as droplet reactor. The driving
force in this system was proposed as to field lines connecting the energized
electrode resulting in Maxwell stress by induced charge. This prototype device
may find various applications in microfluidic transportation, chemical analyses,
and lab-on-chip devices. Alternatively, rapid manipulation of discrete microdroplets
was demonstrated on a microactuator composed by two sets of opposing planar
electrodes. Pollack et al. [
199
] demonstrated droplet transportation over 1,000's
cycles at switching rates up to 20 Hz. The average droplet velocity is 3.0 cm/s,
which is nearly 100 times faster than a previously report for electrical manipulation
of droplets. Furthermore, Takeda et al. [
200
] have investigated the effect of vertical
electric field on the movement of water droplets on a superhydrophobic surface.
It was found that a water droplet can jump up from the superhydrophobic surface
without splitting itself. However, to obtain the jump of a 5 mg water droplet without
splitting, precise control of Coulomb force and high voltage of 9.0 KV DC were
required. Therefore, further investigation and new design of electrode are desired
for the practical application of fluidic transportation.
Since high voltage and electric heating may limit the applications in bioactive
component detection, researchers have found that magnetic fields can be an
alternative candidate for activation source to control the fluidic transportation.
For instance, Garcia et al. [
201
] have demonstrated magnetic fields controlled
movement of aqueous drops on non-patterned, silicon nanowire superhydrophobic
surfaces by introducing magnetizable carbonyl iron microparticles into the liquid.
Standard key operations of movement, coalescence, and splitting of biological fluid
drops were presented. Such droplet-based digital magnetofluidics system further
demonstrated the ability to performance reliable electrochemical measurements
[
202
]. The advantage of this method is that droplets are moved in air without
needing for a water-immiscible carrier fluid. Meanwhile, the lack of oil facilitates
electrochemical measurement by minimizing electrode fouling and viscous drag.
Another way of water droplets transportation is conducted from one sur-
face to another according to the different wetting performance of water droplets
on special surfaces under varied wetting conditions. Jin et al. [
94
]havere-
ported a special superhydrophobic-like surface composed of greater than 6 million
aligned polystyrene (PS) nanotubes per square millimeter. Contrary to normal
superhydrophobic surfaces with large contact angle and small adhesion, this
