Online proceedings - EDA Publishing Association

11-13
May 2011, Aix-en-Provence, France
A Microfluidic Chip with Single-particle-based Arrays
Using Electroosmotic Flow
Chun-Ping Jen and Ju-Hsiu Hsiao
Department of Mechanical Engineering,
National Chung Cheng University,
Chia Yi, Taiwan, R.O.C.
Abstract- Microfabrication technologies achieving precise
manipulation of biological cells provide the potential for
individual characterization, detection and assay to cells at the
single-cell level. The main purpose of the present study was to
develop a microfluidic chip with microwells for
single-particle-based positioning by using electroosmotic flow.
Therefore, the process could not only be reliable, but also simple
without a syringe pump. A biocompatible material of
polydimethylsiloxane (PDMS) was adopted as a structure in the
microfluidic chip for single-particle-based array. The sample of 6
μL with latex particles (17 μm in diameter) was suspended in the
sucrose medium with a concentration of 10 6 particles/mL and
dropped into the microchannel for micropatterning. The DC
(direct current) voltages for electroosmotic flow were set as 10, 15
and 20 volts, respectively. The velocity of electroosmotic flow
increased with the applied voltages. 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. Furthermore, there was only one single particle
within the individual microwell in most of occupied microwells
with 20 μm in diameter, which was much higher than that for the
30-μm-diameter microwells. Micropatterned latex particles in
microwells were successfully achieved in this preliminary study.
The microfluidic chips with microwells with different diameters
were fabricated herein, which was suitable for measurements at a
single-cell level.
Keywords: microarray, single-particle, electroosmotic flow.
I. INTRODUCTION
Microfabrication technologies achieving precise
manipulation of biological cells or microparticles provide the
potential for individual characterization, detection and assay
to cells at the single-cell level. It also has stimulated research
to understand the fundamental cell biology and
pharmaceutical analysis by exposure of cells to drugs and
environmental perturbations [1]. Numerous methods, such
as microcontact printing, microfluidic patterning and
photolithography have been employed to create
micropatterned surfaces containing adhesive and
non-adhesive regions for cells, as reviewed previously [2-5].
There approaches are limited to adherent cells and additional
surface chemistry procedures are often required. Alternative
methods including dielectrophoresis [6], optical tweezers [7]
and selective dewetting [8] are adopted for trapping single
cells and do not require that the cells are adherent. However,
these methods are not suitable for high-throughput
applications. The approach that cells are confined inside
microwells passively becomes attractive because of its
simplicity and easy-handling [9]. However, the injection by
a syringe pump is still required for introducing the particles
into the closed microchannel in this approach. Otherwise, a
suspension of cells is pipetted onto the surface of the chip
with opened microwells immersed in the medium in a culture
dish [10], which required manually handling and was not
reliable. The main purpose of the present study is to develop
a microfluidic chip with microwells for single-particle-based
positioning by using electroosmotic flow. Therefore, the
process could not only be reliable, but also simple without a
syringe pump.
II. EXPERIMENTAL SECTION
A biocompatible material of PDMS was adopted as a
structure in the microfluidic chip for single-cell-based arrays,
as illustrated in Fig. 1. The main channel formed on the top
PDMS was 15 mm wide, 160 μm in height and 26 mm long.
The main channel is divided into four microchannels with
800 μm wide and 10 mm long at the center region. Each
microchannel contains six 10×10 microwells with 20 μm or
30 μm in diameter and 20 μm in depth on the substrate. The
mold masters were fabricated by spinning SU-8 (SU-8 50,
MicroChem Corp., Newton, MA, USA) on the silicon wafer
to define the microwells and microchannel, respectively.
The mold master of microfluidic channels (around 160 μm in
height) were fabricated by spinning SU-8 at 500 rpm for 20
seconds and then at 800 rpm for 35 seconds on the silicon
wafer. The resist was soft baked on a hotplate at 65 °C for 10
minutes and then at 95 °C for 30 minutes. The resist was
then allowed to cool to room temperature. The SU-8 was
exposed to ultraviolet (UV) radiation at a dose of 200 mJ/cm 2 .
The post-exposure baking was done at 65 °C for 3 minutes
and 95 °C for 10 minutes. The exposed samples were
developed with the SU-8 developer for 5 minutes. Moreover,
the mold master of microwells (around 20 μm in height)
were fabricated by spinning SU-8 at 500 rpm for 20 seconds
and then at a higher spin speed of 4500 rpm for 35 seconds on
the silicon wafer. The resist was developed with the SU-8
developer for about 2 minutes after baked and exposed to UV
radiation under the same conditions mentioned above. The
PDMS prepolymer mixture (Sylgard-184 Silicone Elastomer
Kit, Dow Corning, Midland, MI, USA) was poured and
cured on the mold masters to replicate the patterned
structures. After peering off the PDMS replica with the
microchannel, the inlet and outlet ports were made by a
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