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puncher. The two PDMS replicas were bonded after
treatment of the oxygen plasma in the O 2 plasma cleaner
(model PDC-32G, Harrick Plasma Corp. Ithaca, NY, USA).
The electric field generating the electroosmotic flow was
applied by inserting electrodes close to the ports
aforementioned and the distance between these two
electrodes was about 16 mm. The sample of 6 μL with latex
particles (17 μm in diameter) was suspended in the sucrose
medium with 8.62 wt% and 10 -4 M of KCl (σ=6.5×10 -3 S/m)
and dropped into the microchannel for micropatterning. The
concentration of particles was 10 6 particles/mL. The DC
(direct current) voltages for electroosmotic flow were set as
10, 15 and 20 volts, respectively. The detailed procedures
were illustrated in Fig. 2.
III. RESULTS AND DISCUSSION
The images of micropatterned latex particles in 30 and
20-μm microwells under different applied voltages were
revealed in Fig. 3. The velocities of electroosmotic flow for
10, 15 and 20 volts were measured as 3.0, 4.5 and 5.9 μm/s,
respectively. The results of micropatterned in Fig. 3
qualitatively showed that the occupancy of particles
decreased with the voltage applied due to the increase of
velocity. The statistical data for particle occupancy in the 20
μm- and 30-μm-diameter microwells for different applied
voltages were plotted in Fig. 4. The experimental data were
based on manual counts of particles in three arrays of 10×10
microwells by an inverted microscope. Each experimental data
point represents the average value, and the error bar depicts
the standard error from the mean. The occupancy of particles
decreased with voltages applied for both the microfluidic
chips containing 20 or 30-μm microwells, which implied that
the higher velocity of electroosmotic flow caused lower
particulate occupancy. For the case of applying 10 volts, the
occupancy of particles on the microchip with 30 μm
microwells was up to 93.67 %, which was higher than that
obtained on the chip with 20 μm microwells (approximately
85.16 %). The data for the particulate occupancy in the
11-13
May 2011, Aix-en-Provence, France
individual microwells were revealed in Fig. 5 to investigate
the performance of the single-particle level. The results
indicated apparently that there was only one single particle
within the individual microwell in approximate 97 % of
occupied 20-μm microwells, which was much higher than
that for the 30-μm-diameter microwells (62.48 %).
IV. CONCLUSIONS
The method of micropatterning latex particles in
microwells at single-particle level was successfully achieved
in this preliminary study. The microfluidic chips with
microwells were fabricated herein, which was suitable for
measurements at a single-cell level. However, the
experimental demonstration of micropatterning biological
cells is required in future work. Microchips with microwells
proposed herein could be used for cellular micropatterning.
The technique has the potential to realize single cell analysis
and to acquire a population of data based on high-throughput
and parallel processing.
ACKNOWLEDGMENT
The authors would like to thank the National Science
Council of the Republic of China for its financial support
under contract No. NSC-99-2923-E-194-001-MY3.
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