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.