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J. Micromech. Microeng. 10 (2000) 415–420. Printed in the UK
PII: S0960-1317(00)11716-4
A pneumatically-actuated three-way
microvalve fabricated with
polydimethylsiloxane using the
membrane transfer technique
Kazuo Hosokawa† and Ryutaro Maeda
Surface and Interface Technology Division, Mechanical Engineering Laboratory,
AIST/MITI, 1-2 Namiki, Tsukuba, Ibaraki, 305-8564 Japan
E-mail: [email protected]
Received 8 February 2000, in final form 14 April 2000
Abstract. In this paper, a three-way microvalve system composed of three independent
one-way valve units is presented. Each valve unit has a membrane, which is actuated by
external negative air pressure. Intervals between the valve units are smaller than 780 µm,
which opens up the possibility of realizing a high-density microvalve array. The small
intervals were realized by providing the system with a layer of microchannels to conduct the
air pressure to the valve units. In spite of the extra layer of microchannels, the device has
been fabricated through a simple process by adopting polydimethylsiloxane (PDMS) as the
material for the microchannel chips as well as the membrane. In particular, a newly
developed technique for wafer level transfer of a PDMS membrane has been proven to be
effective. Flow characteristics of the microvalve system for water are presented. The
microvalve works in an on–off manner with hysteresis. No leakage has been observed in the
closed state. In the open state, measured flow resistances (pressure drops) are within the
range of 1.65–2.29 kPa (µl min−1 )−1 , and consistent with an electric circuit model.
1. Introduction
Recently, microfluidic devices such as microvalves have
been attracting more and more attention, especially for
(bio)chemical applications including micro total analysis
systems (µTAS) [1, 2]. For example, miniaturization of
a flow injection analysis (FIA) system requires a set of
microvalves to control a pulsed sample flow [3, 4]. In a
typical design, a microvalve has a microfabricated channel
interrupted by a valve seat. The fluidic route is opened/closed
at the valve seat with a membrane, which is actuated via
various methods [5]. A key issue in the valve design is
material for the membrane. A large deflection comparable
to the channel height—several tens of micrometers in many
cases—is required for switching the liquid, which is essential
in biochemical applications. It has recently been recognized
that silicone elastomer is one of the most suitable materials
for the microvalve membrane because of its low Young’s
modulus and excellent sealing property [6–10].
On the other hand, a disadvantage of the silicone
elastomer membrane is the limited variety of actuation
methods.
Only two methods—pneumatic [6–9] and
thermopneumatic [10]—have been reported so far. These
methods are problematic for fabricating a high-density array
† Author to whom correspondence should be addressed.
0960-1317/00/030415+06$30.00
© 2000 IOP Publishing Ltd
of microvalves. In the case of thermopneumatic actuation,
insulation of heat between neighboring valves is difficult
in a high-density array system. In the case of pneumatic
actuation, microfabricated pneumatic channels are required
to conduct the air pressure to the membranes. This means
an extra layer of microchannels besides those for the fluid
to be switched. In general, a complicated fabrication
process is required for such a device containing multilayered
microchannels.
Pneumatically driven three-way [7] and four-way
[8] microvalves have already been reported. They are
composed of three and four independent one-way valve units,
respectively. However, they are not equipped with pneumatic
microchannels. Hence, the intervals between the valve
units seem to be limited by the sizes of non-microfabricated
pneumatic connectors. The intervals in these systems are
larger than 2.5 mm.
This paper describes a three-way microvalve system
composed of three one-way valve units. Each valve unit has
a silicone elastomer membrane, which is actuated by external negative air pressure. Intervals between the valve units
are smaller than 780 µm. These small intervals have been
achieved by providing the system with pneumatic microchannels. To simplify the fabrication process for this relatively
complicated device containing multilayered microchannels,
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K Hosokawa and R Maeda
(A)
(B)
(C)
Figure 1. (A) Schematic drawing of the design of the three-way microvalve system made with three one-way valve units. The fluidic chip
has three fluidic ports (In1 and 2 and Out), and the pneumatic chip has three control ports (C1–3). The main working region A is detailed in
(B), where the dashed lines indicate microchannels between the pneumatic chip and the membrane. Sectional views (section B–B ) of a
one-way valve unit are shown in (C).
we adopted polydimethylsiloxane (PDMS)—a kind of silicone elastomer—as material for the device.
Recently, the replica molding technique of PDMS has
been widely used to simplify the fabrication process of
microfluidic devices [9, 11–19]. Commonly used procedures
are: (1) a PDMS chip with grooves on its surface is molded
against a microfabricated negative master, and (2) the grooves
are sealed with a flat substrate to make microchannels. The
simple fabrication process is enabled by two features of
PDMS. One is the submicrometer replica fidelity [11], which
is sufficient for most applications. The other is the selfadhesion property. To seal the grooves, a PDMS surface
can be reversibly bonded to various surfaces—glass, silicon,
PDMS itself, and so on—without the need for an elaborate
bonding technique. If necessary, they can also be bonded
irreversibly by treating the surfaces with an oxygen plasma
before they are brought into contact [13, 14].
The device described here is composed of two PDMS
microchannel chips and a PDMS membrane. They have
been assembled using both the bonding techniques, reversible
and irreversible. The latter has been utilized to transfer the
PDMS membrane to one of the PDMS chips from a silicon
wafer, on which the membrane was originally spin-coated.
Besides the simple fabrication process, the use of PDMS has
additional advantages, as follows. This material is tough,
inexpensive, transparent (suitable for optical detection),
and biocompatible (even implantable into human bodies).
However, it is swelled by many organic solvents and hence
unsuited for use with such solvents.
The device consists of two PDMS chips—referred to as
‘fluidic chip’ and ‘pneumatic chip’—and a PDMS membrane
sandwiched between the two chips. Although the device has
no specific flow direction, three access ports in the fluidic chip
are referred to as ‘inlet ports’ (In1 and 2) and ‘outlet port’
(Out) for convenience. On the other hand, the pneumatic
chip has three control ports (C1–3). On the fluidic and
the pneumatic chips, there are microfabricated grooves with
depths of 25 µm and 70 µm, respectively. The grooves are
sealed with both sides of the membrane to make two layers
of microchannels. The fluidic chip is reversibly bonded to
the membrane, whereas the pneumatic chip is irreversibly
bonded to it.
The functional microstructures are magnified in
figure 1(B). Three 100 µm wide fluidic channels from the
inlet and the outlet ports meet at the center of the chip. All
the channels have one-way valve units, which are equivalent
in essence. Sectional views of a valve unit are illustrated in
figure 1(C). A 50 µm gap interrupting the fluidic channel
works as a valve seat. The valve unit is normally closed
by sealing the valve seat with the membrane. External
negative air pressure, supplied via a control port and a 200 µm
wide pneumatic channel, deflects an active area (450 µm
by 200 µm) of the membrane to open the valve unit. For
example, the fluidic route from In1 to Out opens by applying
negative air pressure to C1 and C3 at the same time. The
dead volume of each valve unit is smaller than the cavity on
the pneumatic chip: 450 µm × 200 µm × 70 µm = 6.3 nl.
2.2. Fabrication
2. Design and fabrication
2.1. Design
Figure 1 shows the design of the three-way microvalve
system made with three independent one-way valve units.
416
The device fabrication process is outlined in figure 2. First,
the pneumatic chip was fabricated by a molding technique
as follows. To obtain a negative pattern for the pneumatic
channels with a height of 70 µm, an ultrathick photoresist
(SU-8; MicroChem, USA) was spin-coated on a silicon wafer
A pneumatically-actuated PDMS three-way microvalve
A
Closed
0 kPa
0 kPa
Closed
Closed
0 kPa
B
Open
-60 kPa
-60 kPa
Open
Closed
Figure 2. Schematic diagram of the fabrication process of the
three-way microvalve.
0 kPa
and processed according to the manufacturer’s instructions
(figure 2(A)). After development, the wafer was baked at
150 ◦ C for 5 min in an oven to reinforce the adhesion, and
then it was gradually cooled to room temperature over 1–2 h.
For mold release, the wafer was coated with a fluorocarbon
layer polymerized by a CHF3 plasma in a reactive ion etching
(RIE) machine (System VII SLR 730/740; Plasma-Therm,
USA) for 2 min under the following conditions: CHF3 gas
flow of 50 sccm, pressure of 160 mTorr, and electric power of
200 W. A prepolymer solution of PDMS (Sylgard 184; Dow
Corning, USA, base:curing agent = 10:1) was poured onto
the wafer with a frame for holding the solution (figure 2(B)). It
was cured in the oven at 65 ◦ C for 1 h, followed by the second
cure at 100 ◦ C for 1 h. The cured PDMS chip was peeled from
the wafer, and three access holes of 1.5 mm diameter were
punched in the chip using a metal pipe (figure 2(C)).
Second, the PDMS membrane was formed on another
silicon wafer and transferred onto the pneumatic chip using
a new technique, as follows. In advance, the silicon wafer
was coated with a CHF3 -plasma-polymerized fluorocarbon
layer using the process described above. The prepolymer
solution of PDMS was spin-coated on the wafer at 3000
rpm for 30 s and cured in the oven at 100 ◦ C for 1 h. As
a result, a 25 µm thick PDMS membrane was obtained.
For the irreversible bonding of the pneumatic chip and the
membrane, both surfaces were treated with an oxygen plasma
in the RIE machine for 1 min under the following conditions:
oxygen gas flow of 100 sccm, pressure of 300 mTorr, and
electric power of 200 W. Immediately after removal from the
plasma chamber, the two surfaces were brought into contact,
and baked in the oven at 100 ◦ C for 2 h (figure 2(D)). Since
the two parts were irreversibly bonded, they could be peeled
off together from the silicon wafer without crumpling the
membrane (figure 2(E)).
Finally, all the parts were assembled. The fluidic chip
was fabricated in the same way as the pneumatic chip,
C
Open
-60 kPa
-60 kPa
Open
Open
-60 kPa
500 µm
Figure 3. Optical micrographs of the three-way microvalve.
(A) All the fluidic routes are closed. (B) The fluidic route from In1
to Out is opened by applying a negative pressure of −60 kPa to the
control ports C1 and C3. (C) All the fluidic routes are opened by
applying a negative pressure of −60 kPa to all the control ports.
except that the thickness of the SU-8 photoresist was 25 µm.
The fluidic chip was reversibly bonded to the composite
of the pneumatic chip and the membrane only by bringing
the two surfaces into contact. Since PDMS is transparent,
alignment is fairly easy using a video microscope (VH-6300;
KEYENCE, Japan) and a homemade tool based on an x–y–z
stage. Six glass pipes were inserted into the access holes,
and glued with PDMS (figure 2(F)). Figure 3 shows optical
micrographs of the fabricated microvalve system in different
states.
3. Flow characteristics
3.1. Experimental set-up and methods
Flow characteristics of the three-way microvalve system for
water were evaluated using the set-up depicted in figure 4.
417
K Hosokawa and R Maeda
Figure 4. Experimental set-up for evaluation of flow
characteristics of the three-way microvalve for water: (1) vacuum
pump, (2 and 3) vacuum regulators, (4–7) manual three-way
valves, (8) trap for water, (9) the device, (10 and 11) silicone tubes
filled with pure water.
Negative pressure was used not only for controlling the
valve membranes but also for pumping the water, because
the reversible bonding between the fluidic chip and the
membrane is not perfect against positive internal pressure
higher than 10 kPa. The negative pressure was supplied by
a vacuum pump (DA-5D; ULVAC Sinku Kiko, Japan), and
regulated for water pumping and valve control independently
using two vacuum regulators (VR200-G; Koganei, Japan).
Four manual three-way valves were used for switching the
pressures in the ports from vacuum to atmospheric pressure
and vice versa. Pressure in each port is denoted by pport name .
The water pumped from the outlet port was trapped in a
bottle to prevent the regulator from sucking in the water.
The inlet ports were connected to 1 m long, 1 mm internal
diameter silicone tubes filled with pure water. The ends of
the tubes were exposed to the atmosphere. Neglecting the
height difference of several centimeters between the device
and the tubes lying on a table, we regard the pressures pIn1
and pIn2 as atmospheric. We measured the moving velocities
of the water menisci in the tubes to calculate the volumetric
flow rates denoted by q1 and q2 for In1 and In2, respectively.
Figure 5. Flow characteristics of the three-way microvalve for
water. (A) Opening/closing behavior of the fluidic route In1–Out.
The other route, In2–Out, was kept closed. (B) Flow rate against
pumping pressure for the route In1–Out in the open state. The
other route, In2–Out, was kept closed. (C) Flow rates against
pumping pressure for both of the fluidic routes in the open states.
3.2. Results and discussion
In figure 5(A), the flow rate q1 is plotted against the control
pressures pC1 and pC3 , which were varied together from
0 to −70 kPa. The pressures pC2 and pOut were kept
constant as shown in the insert. This curve illustrates the
opening/closing behavior of the fluidic route In1–Out when
the other route In2–Out is closed. The curve has almost no
linear range in practice. In other words, the set of the two
valve units, controlled by C1 and C3, works as an on–off
valve. Since all three valve units are virtually equivalent,
other combinations (C2–C3 and C1–C2) should have on–off
characteristics similar to figure 5(A). The hysteresis between
the opening and the closing pressures is considered to be
caused by sticking between the membrane and the valve seats.
No leakage could be detected in the closed state.
Although the current device does not stand positive
internal pressure higher than 10 kPa, it is worthwhile to
estimate the behavior of the valve operated with a high
418
positive pressure. As shown in figure 5(A), the required
control pressure to open the valve is −30 to −40 kPa,
which means a 10–20 kPa pressure difference across the
membrane, because pOut was kept at −20 kPa. Therefore,
in the case of pIn1 > 20 kPa and pOut = 0, the valve would
become a ‘normally-open’ valve which could be closed by
a positive control pressure. Another conceivable operation
mode is the ‘passive mode’ with variable inlet pressure and
constant control pressure. When the inlet pressure is lower
than a threshold value (10–20 kPa higher than the control
pressure), the valve would be closed. By increasing the
inlet pressure, the valve would be abruptly opened at the
threshold value. To render the microvalve resistant to a high
positive pressure, all the parts should be irreversibly bonded
except for the active areas of the membrane. Such selective
irreversible bonding would be possible by using a protective
layer, such as polyimide or Parylene, patterned onto the active
areas.
A pneumatically-actuated PDMS three-way microvalve
4. Conclusions
Figure 6. (A) Electric circuit model of the three-way microvalve
system. (B) Designed lengths of the fluidic channels.
In figure 5(B), the flow rate q1 is plotted against the
pumping pressure pOut , which was varied from 0 to −30 kPa.
The control pressures were kept constant as pC1 = pC3 =
−60 kPa and pC2 = 0 kPa, which rendered the fluidic
route In1–Out open and the route In2–Out closed. A
photograph of this state is presented in figure 3(B). As shown
in figure 5(B), the flow rate is proportional to the pumping
pressure. Therefore, the flow resistance (pressure drop) can
be defined as the factor of proportionality. By fitting the
curve in figure 5(B), the flow resistance is calculated as
RA = 1.65 kPa (µl min−1 )−1 .
Figure 5(C) shows the results of an experiment similar
to the above. By applying −60 kPa pressure to all the
control ports, all the fluidic routes were kept open, as shown
in figure 3(C). As expected, the two flow rates q1 and q2
in figure 5(C) are mutually balanced and proportional to
the pumping pressure. By fitting the two curves, the flow
resistances are calculated as RB = 2.24 kPa (µl min−1 )−1
and RC = 2.29 kPa (µl min−1 )−1 for the fluidic routes
In1–Out and In2–Out, respectively.
The above flow resistances are consistent with the
electric circuit model shown in figure 6(A), where the symbol
R denotes the flow resistance of each channel. Neglecting
the small difference between RB and RC calculated above, we
assume that the system is symmetric with respect to the two
inlet ports. In this case, the two resistance values should be
represented by the mean value RB = (RB +RC )/2 = 2.27 kPa
(µl min−1 )−1 . Considering the combined resistances, the
experimental results are expressed as:
R0 + R1 = RA
R0 + (R1 /2) = RB /2.
Rearranging the above equations, we obtain:
R0 = RB − RA = 0.62 kPa (µl min −1 )−1
R1 = 2RA − RB = 1.03 kPa (µl min −1 )−1 .
The difference between R0 and R1 probably originates from
the difference in the channel length illustrated in figure 6(B),
because, in general, the flow resistance of a microchannel
with a uniform cross section is proportional to its length. This
relationship is formulated as Hele–Shaw flow or Poiseuille
flow in the low Reynolds number regime [20]. As for
the above experiments, the ratio of the flow resistances,
R0 /R1 = 0.60, roughly agrees with the ratio of the channel
lengths, L0 /L1 = 0.66. This agreement suggests that the
flow resistances in the open states are dominated by the
straight regions of the microchannels, not by the valve units.
A pneumatically-actuated three-way microvalve system
composed of three one-way valve units was constructed.
Intervals between the valve units are smaller than 780 µm,
which opens up the possibility of realizing a high-density
microvalve array. The device containing multilayered
microchannels was fabricated through a relatively simple
process, which was enabled by the use of PDMS. In
particular, a new technique for wafer level transfer of a PDMS
membrane has been proven to be effective. Since PDMS can
be irreversibly bonded to various materials including glass
and silicon [13], this technique is considered to be widely
applicable to multilayer microchannel systems.
There are at least three issues for future study. First,
resistance to positive internal pressure should be improved,
as discussed in section 3.2. Second, the robustness for
practical applications, such as lifetime and particle tolerance,
should be tested. Finally, the geometrical design should
be optimized. The miniaturization is currently limited
by alignment inaccuracy—tens of micrometers—in the
assembly process. To reduce it to several micrometers, a
more sophisticated technique using a mask aligner is under
development. As for the thickness of the membrane, it
has been reported that a 2 µm thick PDMS membrane can
be spin-coated and peeled off [21]. Therefore, from the
fabrication point of view, we consider that the microvalve
system can be further miniaturized by a factor of ten.
However, from the application point of view, there are some
trade-off relationships. For example, miniaturization would
cause an increase in the flow resistance, and an extremely
thin membrane would be problematic for robustness. These
relationships should be quantitatively characterized to enable
optimized valve design according to its application.
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