addae-mensah_loc_c002349c

Actuation of elastomeric microvalves in point-of-care settings using handheld,
battery-powered instrumentation†
Kweku A. Addae-Mensah, Yuk Kee Cheung, Veronika Fekete, Matthew S. Rendely and Samuel K. Sia*
Received 3rd February 2010, Accepted 16th March 2010
First published as an Advance Article on the web 12th April 2010
DOI: 10.1039/c002349c
Although advanced fluid handling using elastomeric valves is useful for a variety of lab-on-a-chip
procedures, their operation has traditionally relied on external laboratory infrastructure (such as gas
tanks, computers, and ground electricity). This dependence has held back the use of elastomeric
microvalves for point-of-care settings. Here, we demonstrate that microfabricated microvalves, via
liquid-filled control channels, can be actuated using only a handheld instrument powered by a 9 V
battery. This setup can achieve on–off fluid control with fast response times, coordinated switching of
multiple valves, and operation of a biological assay. In the future, this technique may enable the widely
used elastomeric microvalves (made by multilayer soft lithography) to be increasingly adopted for
portable sensors and lab-on-a-chip systems.
Introduction
In recent years, the use of multilayer soft lithography for making
microfabricated pneumatic valves has provided a reliable and
versatile platform for advanced fluid handling in lab-on-a-chip
(LOC) devices, offering advantages of rapid prototyping and
biocompatibility compared to silicon-based MEMS. Such
pneumatic valves have been used for a wide range of research
applications taking place in centralized facilities.1–4
A drawback in the most widely adopted implementation of
pneumatic valves is that they require laboratory infrastructure
such as gas tank, computers, and ground electricity. Hence,
advanced fluid handling with pneumatic valves is difficult to
accomplish for point-of-care (POC) systems in health diagnos-
tics, epidemiology, autonomous sensors, and homeland security.
This limitation applies especially to resource-limited settings
such as developing countries, where even reliable external power
is lacking. For example, while a variety of valves (such as screw-
based valves,5
stimuli-responsive hydrogels,6,7
passive valves,8
and burst valves9
) are useful for different applications performed
in a research lab, they typically do not have the combination of
automated operation, fast response times, resistance to leakage,
independence of external infrastructure, and ability to be used
multiple times for an ideal POC system.
Previous studies on implementing valves for remote settings
include a system used on the planet Mars,10,11
although fabrica-
tion of the chip was relatively complex. In another approach
pioneered by Takayama’s group, moving pins from a Braille
display12,13
driven by a computer (running a text editor or Braille
screen reading software) have been used to successfully demon-
strate hydraulic-principle based two-layer valve control.14
However, the requirement for the computer may introduce
limitations for use at the POC.
Here, we develop a handheld instrument to operate
membrane-based microvalves, based on the hydraulic principle
for actuation.14
We sought to replace the requirement for
external instrumentation and a computer with a portable device
that can maintain high control over valve operation.
Materials and methods
Microfluidic chamber and control layer fabrication
The microfluidic flow and control layers were fabricated using
standard methods as described by Unger et al.,15
with a push-up
configuration where the control layer is located below the fluid
flow layer.
The control layer was filled with water, which served as the
hydraulic fluid. Before filling the control layer, we sonicated
the water for 15 minutes to eliminate micro-bubbles. With the
control channels being closed at one end, they were filled by
placing the chip under vacuum for 20 minutes, while water
droplets covered the control layer inlets. Once the control
channels were completely filled, we sealed their inlets with a thin
PDMS membrane as follows. First, we coated a cured PDMS
membrane (about 3 mm2
in area) with PDMS prepolymer. We
then placed the membrane on the water droplet covering the
control channel inlet, and gently pressed the membrane down
until it covered the inlet and was firmly in place, while ensuring
that we did not trap any air bubbles between the membrane and
the control channel inlet. To cure the sealing membrane, we
placed the completed device in a humid chamber (Petri dish
containing water-soaked Kim wipes) for 48 hours at room
temperature. We did not detect any loss of water from the control
channels for up to 7 days when the device was kept in the sealed
Petri dish. We also repeated the above experiment using ionic
Department of Biomedical Engineering, Columbia University, 351
Engineering Terrace, 1210 Amsterdam Ave, New York, NY, 10027,
USA. E-mail: ss2735@columbia.edu
† Electronic supplementary information (ESI) available: Fig. S1, Circuit
diagram of the microcontroller; Fig. S2, Circuit diagram for measuring
the response times of microvalves; Fig. S3, Circuit diagram for
measuring the power drawn by solenoids; Fig. S4, Response time of
valves using ionic liquid inside control channels; Movie S1, A movie of
colored dye moving through two parallel channels in a microfluidic
chip with the valves actuated in an arbitrary sequence described in
Fig 3. See DOI: 10.1039/c002349c
1618 | Lab Chip, 2010, 10, 1618–1622 This journal is ª The Royal Society of Chemistry 2010
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liquid (1-butyl-3-methylimidazolium tetrafluoroborate, Acros
Organics) instead of water as the fluid in the control channels.
Design and construction of solenoid-containing portable actuation
unit
Solenoids (F110C-6V GeePLus, Inc) with cylindrical plungers of
diameter 1.2 mm were used to drive the hydraulics. They can be
operated between 6.5 and 9 volts, and are rated for force between
0.07 and 1.2 N and stroke lengths from 0.25 to 1.25 mm. They
were modified by inserting springs (rates between 0.014 and
0.17 N mmÀ1
) between the plunger and the solenoid body
(Fig. 1B) to provide a return mechanism for the solenoid
plungers. These modified solenoids were attached to a movable
plate which could be moved freely in the vertical direction such
that the plungers aligned with the positions of the access holes
punched for the control layer (Fig. 1). To ensure alignment of the
inlets in the control layer with the positions of the solenoids, we
imported the design drawings used for making the movable plate
into the designs used for making the fabrication masks for the
control layer.
Design of microcontroller-based electronic circuit
The solenoids were controlled by an Atmel Mega32 micro-
controller along with peripheral circuitry (Fig. S1†). The actua-
tion sequence of the solenoids and valves could be
pre-programmed into the microcontroller, which contained 32
programmable input/output lines. The operating voltage for the
microcontroller is 4.5 to 5.5 V. The microcontroller-based circuit
also controls a small micropump (Hargraves Advanced Fluidic
Solutions E219-12) with operating voltage of 3–6 V. A liquid trap
was also built to interface the micropump to the microfluidic
network to prevent liquid from destroying the pump. The trap
consisted of a 500 mL well with two outlets one connected to the
micropump and the other connected to the outlet of the micro-
fluidic network. The speed/strength of the micropump could be
adjusted by setting a value in the microcontroller which deter-
mined the final voltage applied to the pump using a pulse width
modulation (PWM) algorithm. The speed was adjusted to
account for the latency introduced by the liquid trap. The overall
footprint of the device was 13 Â 7.6 Â 5.8 cm, and could be
operated by a 9 V dry-cell battery.
Operation of sample biological assay: detection of horseradish
peroxidase (HRP)
To demonstrate the fluid handling capability of the micro-
controller based electronic control system, we ran an assay to
detect horseradish peroxidase (as a mock sample) using Amplex
Red. The Amplex Red reagent was prepared in 1Â reaction
buffer to a final concentration of 100 mM, and mixed with 2 mM
H2O2. We drew 200 mL of the resulting solution into 5 cm long
Tygon tubing (0.05000
Â 0.09000
OD), and connected it to the
input port of the microfluidic network. Similarly, we connected
tubings containing 100 mL of sodium phosphate buffer
(pH z 7.4) and horseradish peroxidase in 1Â reaction buffer to
two other inlet ports. We connected a small battery-powered
micropump to the outlet and used to draw the fluids through the
microfluidic network. After mixing, the valves were closed to
confine the mixed reagents within the detection zones. We
measured fluorescence by analyzing mean intensities of light
emanating from three rectangular regions within the detection
zone. We subtracted the background from the signal region to
obtain corrected values of fluorescence.
Results and discussion
We constructed a handheld instrument for actuating elastomeric
microvalves as follows. Four linear pull-type solenoids for
controlling the microvalve system were attached to a plate that
could move vertically (in order to accommodate a microfluidic
chip) relative to a fixed base plate (Fig. 1A). The positions of the
Fig. 1 Schematic diagram, picture and mode of operation of the actu-
ation system. (A) Simplified schematic and picture of instrument.
A simplified schematic diagram of the instrument showing the main
components. A cross-sectional view of a single solenoid showing a typical
arrangement of the coil assembly, and plunger is also shown. The picture
shows the fully constructed instrument and a close up of the interface
between the solenoids and the microfluidic chip. The whole unit is
designed to be battery operated with a microcontroller and peripheral
electronic components assembled on a custom made printed circuit
board. (B) Schematic showing how the hydraulic valves operate.
The solenoids are modified by inserting solenoid return springs between
the solenoid body and the plunger.
This journal is ª The Royal Society of Chemistry 2010 Lab Chip, 2010, 10, 1618–1622 | 1619
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solenoids were designed such that they aligned with positions of
inlets into the control layer of the microfluidic chip (Fig. 1A).
The positions of the inlets were spaced apart (at least 12 mm) to
accommodate interfacing with multiple solenoids. The micro-
fluidic channels in the control layer were filled with water.
Each solenoid was a linear pull-type solenoid, consisting of
a plunger (movable iron component) and a coil assembly
(Fig. 1A). Initial contact of the solenoid plungers with PDMS
membranes ($250 mm thick) covering the inlets to the control
channels resulted in the downward deflection of the membranes,
thereby transmitting force through the fluid-filled control chan-
nels to the thin membrane ($30 mm) between the control and the
fluid layer. As a result, the microvalves in the fluid layer closed.
When a voltage was applied to the coil assembly, a magnetic
attractive force was produced between the plunger and the coil,
pulling the plunger into the metal housing; in turn, the micro-
valves in the fluid layer opened (Fig. 1B).
In these commercially available solenoids, the plunger stayed
in the metal housing even after switching off the voltage. We
inserted a return spring between the plunger and the metal
housing (Fig. 1B), such that the plunger moved out of the
housing (closing the microvalves) immediately after switching off
the voltage. This design resulted in two useful features: (i) the
microvalves would be closed in the absence of voltage supplied to
solenoids, and (ii) the plunger was primed for movement upon
re-application of the voltage.
We first characterized the ability of the solenoids to actuate the
valves to high precision. In order to monitor the response times
of the valves, we used a MATLAB script to apply voltage to the
solenoids (Fig. S2 and S3† show electronic circuit and connec-
tions) and a data acquisition and control card that control image
Fig. 2 Demonstration of operation of the valve. (A) Images of the valve
in the closed and open state. Colored dye was flowed through the micro-
fluidic channel using a Kent Scientific pump operating at 1 mL minÀ1
and
the valve was opened and closed at various times. The images represent
bright field images of the valve closed at 90 seconds and opened at
5 seconds. The rectangular area represents the region of interest that is
processed for generation of mean intensity values used for the plot in (B).
(B) Valve opening and closingwith time. A graphshowing the opening and
closing of the valve recorded over 100 s (16 cycles). (C) Time response of
the valve. Time response of valves 100 mm Â 100 mm Â 12 mm and $8.6 mm
long filled with water which is used as the hydraulic fluid. The average time
for the valve to close for devices filled with water was 53.4 ms.
Fig. 3 Parallel operation of multiple valve locations. (A) Bright field
images of multiple valve locations. Bright field images taken with
a QImaging Retiga 2000R 16bit monochrome camera showing multiple
valve locations. The images represent snapshots of the fluid and control
channels at 19 and 56 seconds with the valves designated as grey, dashed
black, and black. (B) A plot showing valve opening and closing with time.
We pumped colored dye through two parallel microfluidic channels and
closed and opened the valves in the following arbitrary sequence: open
grey (t ¼ 3 s), open dashed black (t ¼ 4 s), open black (t ¼ 5 s), hold all
three valves open (t ¼ 5 to 9 s), close black (t ¼ 9 s), close dashed black
(t ¼ 10 s), close grey (t ¼11 s), hold all three valves closed (t ¼ 11 to 13 s),
and repeat the cycle. We used a Kent Scientific syringe pump operated as
a vacuum pump with a 30 mL BD syringe. The fluid flow rate was
approximately 4 mL min-1.
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acquisition on the camera. Two bright field images (Fig. 2A)
show colored dye in the fluid channel, with a valve in closed and
open states. As expected, we saw fluid flow in the open state, and
saw no flow in the closed state, indicating that the microvalves
could be controlled by movement of the plungers. We estimated
the degree of ‘‘valve opening’’ by expressing the mean pixel
intensity value of the dye region as a percentage between the
maximum (255 for 8-bit grayscale values) and minimum (0) pixel
intensities. The valve opened and closed consistently over at least
16 cycles (Fig. 2B).
The response time of microvalves is an important factor in
determining its use. For instance, hydrogel-based microvalves
that respond in seconds to minutes are well-suited for drug
delivery and other applications where slow delivery and control
of fluid flow are required. By contrast, faster response times
(on the order of seconds or less) are needed for analytical systems
where sorting, splitting and merging of fluid are prevalent. We
measured the response time of the valve by measuring the
difference in time for when the control signals were sent to the
valve and when a change in the valve opening state was observed
(see ESI†). For water-filled control channels, we observed
a response time of 53 Æ 5 ms (Fig. 2C). (Since a previous study
also reported the use of ionic liquids in the control layer,14
which
can potentially be kept longer at room temperature without
evaporation through the PDMS, we also measured the response
time of our microvalves with control channels filled with ionic
liquid. The response time was 128 Æ 14 ms, which is also fast.
In this study, we used water for the rest of the experiments.)
Next, we tested the ability to coordinate the operation of
multiple microvalves via solenoid-driven control. We tested the
ability of three solenoids to control six microvalves (each sole-
noid controlled two valves). As in the previous experiment, we
used a data acquisition module in order to accurately monitor
the response times of the valves. We flowed color dye through
two parallel microchannels in the fluid layer; three microvalves
regulated flow at different sections of a single microchannel
(Fig. 3A). With the three valves designated as grey, dashed black,
and black (Fig. 3A), we sought to operate the valves in an
arbitrary sequence (Fig. 3). The microvalves operated as
designed (see Fig. 3A and Movie S1, ESI†). We also measured
the degree of opening and closing of the three valves, and
observed a high degree of control over operation (Fig. 3B).
Fig. 4 Detection of horseradish peroxidase. (A) Design of the microfluidic network. Outline of microfluidic network showing the fluid and control
layers, inlets and outlets and an image of the microfluidic chip filled with the fluid and control channels filled with food coloring making them easier to
visualize. The regions within the detection zones that are imaged and analyzed have been shown. We programmed the MCU to turn on the solenoids in
the following sequence: turn on the micropump (t ¼ 0 s), open valves 1, 2, 3 and 4 (t ¼ 5 s) and keeping them open (t ¼ 5 to 25 s) to allow mixing of the
fluorescent dye with a negative control (i.e. sodium phosphate buffer), and close valves 1, 2, 3, and 4 (t ¼ 25 s) to confine the mixed solution to the
detection zone. The MCU repeated this operating sequence with valves 5, 6, 7 and 8 to mix the fluorescent dye with the sample (horseradish peroxidase),
and confine the mixed sample to the detection zone. Finally the MCU would turn off the micropump one second later, and the valves would be closed off
during a 30 minute incubation period. (B) Epifluorescence images. Images of the detection zone showing epifluorescence produced from mixing 100 mM
Amplex Red reagent, 2 mM H2O2 and 20 mU of HRP in 50 mM sodium phosphate buffer pH 7.4 and from mixing 100 mM Amplex Red reagent, 2 mM
hydrogen peroxide and 50 mM sodium phosphate buffer pH 7.4 which is used as a negative control. The mixtures were incubated for 30 minutes confined
within the respective detection zones. The rectangular regions of interest analyzed for mean intensity values for the plot in (C) are also shown. (C) Plot of
relative intensities. A plot of the relative intensity (l ¼ 585 nm) of the fluorescence produced by the reaction of Amplex red, hydrogen peroxide and HRP
and for a no-HRP (sodium phosphate buffer) control reaction. Background fluorescence, determined for the no-HRP control region, was subtracted
from the HRP values.
Table 1 The solenoids used to drive the hydraulics are rated to operate from 6 V to 12 V with a power rating of 2.3 W below a 50% duty cycle (maximum
on time of 100 seconds at 8.5 V). From calculations based on data sheets provided by manufacturer, the current drawn by each solenoid is about 250 mA.
For a typical assay, we assume that the valves are open for 10% of the time for an assay that runs for 20 minutes
Component Voltage/V Current/mA Quantity Power/W Avg % on time per run Usage/W Usage/mA h
Microcontroller 5.0 12 1 0.06 100 0.06 4.0
Solenoid 9.0 233a
4 8.40 10 0.83 31.1
4N35 Optoisolator 1.2 10 4 0.05 10 0.01 0.3
TIP 31 Switching Transistor 9.0 — 4 — 10 — —
2N3902 Switching Transistor 5.0 — 4 — 10 — —
Totals 255 8.51 0.90 35.4
a
Measured value. All other current values are estimates from manufacturer’s data sheets.
This journal is ª The Royal Society of Chemistry 2010 Lab Chip, 2010, 10, 1618–1622 | 1621
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Finally, we sought to use the solenoid-control system to conduct
a biological assay. In the previous experiments, we had used a data
acquisition module in order to measure the response times. Here,
we substituted the data acquisition and control module card (and
the accompanying computer needed for running the MATLAB
scripts) with a microcontroller unit (MCU) pre-programmed with
a solenoid-actuation sequence. We translated the schematic
diagram of the electronic circuit (Fig. S1†) into a printed circuit
board layout (6.5 Â 6.5 cm), in order to minimize the footprint of
thedevice (Fig.1A).Inthismanner,wecouldoperatethesolenoids
without computer control, and since the solenoids and MCU
could be powered by a 9 V battery, without ground electricity.
In addition, a small micropump (also controlled by the micro-
controller and also powered by the 9 V battery), rather than an
external syringe pump, was used to pump the fluids through the
microfluidic chip (Fig. 1A).
We tested the use of this compact setup to operate the valves in
the microfluidic chip. We programmed the MCU to turn on the
solenoids in a predetermined sequence (Fig. 4, legend). We first
tested the sequence of valve operation by using water and colored
dyes as substitutes for the actual reagents, and observed the
expected fluid movement. For the biological assay, we used
buffer as negative control and horseradish peroxidase as sample,
and measured the fluorescence intensity in the two detection
regions. The assay produced a strong fluorescent signal for the
case with enzyme and a weak background signal for the case of
the negative control (Fig. 4B), with significant differences in the
fluorescence intensities (Fig. 4C), as expected.
Since the instrument is designed to operate without ground
electricity, we sought to minimize power consumption of the
electrical and mechanical components. We believe that an
important feature of this design is that the solenoids can close the
microvalves without a continuous supply of power, since many
biological assays feature incubation times where valves would
need to stay closed to confine samples or reagents. Moreover, we
estimated how many assays can be run using a single 9 V battery,
by calculatingthe power requirements of allthe components in our
device. We determined the current drawn by a single solenoid
(using a circuit described in Fig. S3†) for 33 consecutive on–off
cycles to be an average of 233 mA per cycle. Using estimates of
power consumption from the manufacturers’ data sheets for other
components, our estimate of energy consumption for a 20 minute
biological assay is 35 mA hour (Table 1). Assuming 80% of
capacity of a 9 V dry-cell battery rated for $625 mA hour, our
handheld instrument for controlling the pneumatic microvalves
can operate for $36 assays on a single battery. Previous valve
technologies10–14
that require a computer will consume at least
about 11 W h for a 30 minute assay, considering only the power
consumed by the computer such as a laptop. This power require-
ment is substantially higher than our current design (0.45 W h).
Over a long time period, water-filled control channels in
PDMS may pose problems due to evaporation of water through
the PDMS. Future designs using PDMS could make use of ionic
liquids rather than water in the control channels,14
or other
combinations of fluids and elastomeric materials where fluid
adsorption to the material is low.16
In conclusion, we developed a setup that retained the advan-
tages of PDMS-based pneumatic microvalves, but enable their use
for POC diagnostics, including in resource-limited settings.17,18
Acknowledgements
We acknowledge an NIH STTR grant (5R41NR010753) and the
Wallace H. Coulter Foundation for financial support. We thank
Professor Bruce Land (Cornell University) whose layout of the
printed circuit board we adapted for our design. We also thank
Keith Yeager for help with machining and construction of the
instrument.
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