Analytical Chemistry Chemical Cytometry Quantitates Superoxide

Figure 2. Influence of a surfactant (Tween 20) on microfluidic
transport. The distance indicates that of the meniscus of the liquid
column from the reservoir for the H2O2 solution in a straight flow
channel of 500 µm × 75 µm in cross-section. Concentration of the
surfactant: [, 0wt%;×, 0.005 wt %; 2, 0.01 wt %; b, 0.05 wt %; 9,
0.1 wt %.
channels. In addition, the movement occasionally became irregular,
possibly due to the morphological or chemical nonuniformity
of the channel. In the worst case, the column stopped
midway in its journey and did not reach the lower compartment
of the micropump. This problem was solved by adding a surfactant
to the solution, which facilitated smooth movement. Figure 2
shows the dependence of the movement of the solution on the
concentration of the surfactant (Tween 20) that was added. The
influence of the surfactant was dramatic, and the solution’s
movement became smoother and faster with increasing concentration
of surfactant. At concentrations higher than 0.01 wt %, the
flow velocity almost leveled off. In the following experiments, the
concentration was therefore fixed at 0.01 wt %.
In biochemical analyses in microsystems, the length of time
required for a reaction is often on the order of seconds or minutes.
In view of this, an additional requirement in such cases is the
presence of structures that can slow the flow velocity of solutions.
For this reason, we then examined how the velocity of the column
changed with changes in the width of the flow channel. In Figure
3A, flow channels 2-4 are straight and have widths of 250 µm,
500 µm, and 1.0 mm, respectively. With a change made only in
the width, a marked difference in flow velocity was observed that
demonstrated accelerated movement of liquid plugs in wider flow
channels. The presence of compartments positioned along the flow
channel exerts an additional similar influence. 34,35 Therefore,
rectangular compartments with dimensions of 1.5 mm × 880 µm
and 6.0 mm × 3.5 mm were attached to the 250 µm wide and 1.0
mm wide flow channels (flow channels 1 and 5, respectively). A
portion of the solution, however, also penetrated into the extending
controlling flow channel while the solution filled each compartment.
As a result, the movement of the column was not
significantly different from the case in which there were no
compartments. This result indicated that branched compartments
are not effective for this purpose.
We then tried a sequential arrangement. Figure 3B shows 500
µm wide flow channels. Flow channel 1 is straight, and flow
channels 2 and 3 have compartments of different sizes (2.0 mm ×
2.0 mm and 3.5 mm × 3.5 mm, respectively). By locating the exit
at an appropriate position in the compartment, the transport in
the flow channel was resumed after the compartment was filled
completely, and the effect of the structures was more significant
than that in Figure 3A. There was a tendency for bubbles to
remain in the corners of the square compartments. Although these
bubbles were small and had no adverse effect on the transport of
solutions, circular or elliptic compartments might be better, both
to avoid this potential problem and to realize a more accurate
adjustment of timing.
The movement of the column could also be delayed by the
introduction of constrictions. The width of the flow channels in
Figure 3C is 500 µm. For flow channels 2 and 3, the constrictions
were positioned near the inlet and had widths of 300 and 200 µm,
respectively. The constrictions also had an effect, and the movement
of the column was slowed with narrower constrictions.
Although we used only simple delaying structures because of the
limited space, microfluidic transport can be delayed further
through the use of a more complicated network of flow channels. 22
The movement of a solution in a flow channel with compartments
and/or constrictions can be understood using numerical
Figure 3. Movement of solutions in the flow channels with various delaying structures. (A) Effect of changing the width of the flow channel and
attaching rectangular compartments on the sides. (B) Effect of adding rectangular compartments in series. (C) Effect of creating constrictions.
The images were taken 1, 3, and 6 s (from left to right) after the introduction of the solution from the left. Scale bars correspond to 2 mm.
Analytical Chemistry, Vol. 82, No. 16, August 15, 2010
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