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.