3.3. ICEK microvalves
Microvalves are vital elements of complex microfluidic devices to control fluid flow along with a network of microchannels. Various types of microvalves have been developed for basic fluid flow mechanistic studies (Shoji & Kawai, 2011), in vitro diagnostics (Tikka, Faulkner, & Al-Sarawi, 2011), and biological sample delivery (Samiei, Tabrizian, & Hoorfar, 2016). Rapid response and ability to be integrated into microfluidic systems are the essential criteria for selecting appropriate microvalves. The potential of ICEK phenomenon for controlling the direction of fluid flow through microchannels was first demonstrated by Sugioka (Sugioka, 2010). A rotating elliptical metal cylinder placed within a normally open microchannel was actuated using MVs around it under both DC and AC electric fields. This actuation induced the closure of the liquid flow in microchannels (Fig. 4A ). In another design, Daghighi and Li (Daghighi & Li, 2011) utilized a Janus particle within a microchamber with three side channels, where the direction of fluid flow inside the channels was controlled under the motion of the Janus particle in the microchamber as a result of external electric field actuation (Fig. 4B ). Zhang et al. (Capretto, Cheng, Hill, & Zhang, 2011) employed electrically a series of conducting surfaces embedded at the corners of a microchannel network to use ICEK actuation for controlling the flow direction toward desired outlets (Fig. 4C ). A similar technique was used by Wang et al. (C. Wang et al., 2016) to control fluid motion in a more complex fluidic network. Applying the electric field to the flow using conducting surfaces placed on a Y-junction wall generated MVs to control the flow direction toward the outlets (Fig. 4D ). Li et al. (Li & Li, 2018) employed an electrically induced Janus droplet to produce ICEK microvalves functioning under DC electric field and to control fluid flow direction within a microchamber with several outlets (Fig. 4E ). The Janus droplet was made of an oil droplet coated with aluminum oxide nanoparticles. Among different ICEK microvalves, this new system was demonstrated to be highly controllable and easy to manipulate for adjusting the fluid flow in microfluidics.
Unlike many other On/Off or disposable valves used in microfluidic systems, most ICEK microvalves act like a control valve to control the flow direction in microchannels. Among different electrode configurations used for ICEK microvalves, the polarizable plates patterned on the microchannel walls are shown to be a simple and effective ICEK method for controlling the fluid flow direction in microchannels (Figs. 4C and 4D ). The response time for switching the flow direction reaches down to one second with the capability of controlling the flow direction in desired branches (Capretto et al., 2011; Daghighi & Li, 2011; Li & Li, 2018). All ICEK microvalves so far functioned based on the ICEO mechanism. There have not been any ACEO-based microvalves yet proposed.