Page 294 - Optofluidics Fundamentals, Devices, and Applications

268 Cha pte r Ele v e n

Since OFM images are naturally digitalized, we can perform
large volume and automatic quantitative information extraction by
computer assisted postprocessing. We developed a MATLAB pro-
gram to trace the contour of the C. elegans and determine the area
and length of the C. elegans in batches (Fig. 11-2d). From those two
quantities, we then computed an effective width for each nematode
by dividing the area by the length. In Fig. 11-5d and 11-5e, the col-
umns represent the mean length and width of the three C. elegans
strains; the hatched areas correspond to the confidence intervals of
our mean length and width estimates. The standard deviations
(error bars) of the measurement indicate the variation between indi-
viduals within the strain.

11-3-2 Cell Imaging
The imaging of cells with the OFM method requires a different flow
mechanism. This is because a pressure-based microfluidic flow has a
parabolic velocity profile (Poiseuille flow) that arises from the non-
slip boundary condition on the channel side-walls. Objects flowing
under such condition tend to rotate and tumble due to the torque
they receive from the nonuniform fluid push. While C. elegans simply
do not have the space to rotate in the microfluidic channel, ellipsoi-
dal/spherical cells do not have such constraints.
Fortunately, we found that the use of dc (direct current) electroki-
netics provides a simple and direct way to control the motion of bio-
logical cells in the on-chip OFM system as to suppress rotation and to
allow a constant translational motion in the microfluidic channel.
This method is simple to implement—apply an electric field along
the channel by introducing a potential difference between the two
ends of the microfluidic channel. We typically apply approximately
25-V difference along a 3-mm-long channel. By varying the potential
difference, we can easily alter the speed of the objects.
There are three mechanisms involved. First, the electric field
causes the translation of the electric double layer at the channel walls
(electrosmosis). This in turn drags the entire fluid column uniformly
through the channel. Second, a cell would typically carry a net elec-
tric charge and the interaction of this charge with the electric field
will likewise actuate the cells (electrophoresis). Third, the electric
field will induce a dipole moment on a cell. Alternately, the heteroge-
neous distribution of electric charge on a cell can also create a natural
dipole. The interaction of the dipole with the electric field will cause
the cell to orientate itself in the channel to minimize the associated
electric potential energy (electro-orientation).
Using this method, we were able to control cell motions well
and achieve good-quality OFM imaging of cells. Figure 11-6
shows comparison images acquired by the OFM and a conven-
tional microscope.