4. Conclusion
This paper reviews the breakthrough in microfluidic-based applications
of induced-charge electrokinetic (ICEK) phenomenon for developing
various lab-on-chip (LOC) components, including micromixers, micropumps,
and microvalves as well as applications in induced charge
electrophoresis (ICEP) and particle/cell manipulation. The ICEK
micropumps provided ultrafast pumping capability in a wide range of flow
rates for LOC applications. The most efficient AC electroosmotic (ACEO)
micropumps operate based on either biased AC/DC signals or
three-dimensional stepped electrode arrays. The most efficient ICEO
micropumps utilized a circular conducting cylinder for pumping purposes.
The ACEO and ICEO micromixers made based on ICEK have shown above 90%
mixing efficiency. The ACEO micromixers used mostly floating electrodes
on the microchannel walls. Among various ICEK-based mixers, employing a
conducting circular cylinder or a floating conducting particle inside a
microchamber has been widely studied. However, further experimental
characterization is needed to optimize its performance. Besides, ICEK
microvalves can operate as both On/Off and control valves, which are
highly desirable for microfluidic applications. Among the ICEK-based
microvalves developed, the ones that employed conducting plates on the
microchannel walls showed the fastest switching response. There is still
no ACEO microvalve developed operating based on the ICEK.
Despite the broad scope of the applications for ICEK microfluidics,
there are still challenges in technology scaling up. The primary
challenge is the costly fabrication of 2D and 3D electrodes within
microchannels. To address this challenge, new inexpensive, scalable, and
easy to fabricate methods are to be developed to assemble electrodes
with the microchannels. Extensive studies are still needed to enhance
the efficiency of this method and reduce the power required to implement
the system in a feasible point-of-care setting. For example, it would be
worthy to characterize the effect of charged particles (conducting or
Janus) on the ICEO flow and the generated MVs; the effect of conducting
surface properties such as hydrophobic and hydrophilic surfaces on the
slip velocity around the object; the role of ion concentrations in the
electrolyte solution on ICEO flow; and the influence of polarizability
of the induced charges on the object and the flow around it. Most
studied have employed spherical particles in the ICEK microfluidic
devices and the effect of particle shape has not been yet considered for
investigating the ICEP systems. Also, further studies are needed to
characterize the effect of different particle configurations on the
performance of ICEP systems. Moreover, research works have to delve into
the potential of ICEK for particle/cell manipulation. The results showed
that the induced MVs in ACEO and ICEO flows could be utilized for
focusing, trapping, and sorting of particles as small as 50 nm in size.
However, the challenge of separating and sorting of defined particle
size from a wide variety of particulates in complex biofluids needs
further investigation.