New Generation of Digital Microfluidic Devices
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Kedzierski, J., S. Berry, and B. Abedian. “New Generation of
Digital Microfluidic Devices.” Microelectromechanical Systems,
Journal of 18.4 (2009): 845-851. ©2009 Institute of Electrical and
Electronics Engineers.
As Published
http://dx.doi.org/10.1109/jmems.2009.2023845
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Institute of Electrical and Electronics Engineers
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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 18, NO. 4, AUGUST 2009
845
New Generation of Digital Microfluidic Devices
Jakub Kedzierski, Shaun Berry, and Behrouz Abedian
Abstract—This paper reports on the design, fabrication, and
performance of micro-sized fluidic devices that use electrowetting
to control and transport liquids. Using standard microfabrication
techniques, new pumping systems are developed with significantly
more capability than open digital microfluidic systems that are
often associated with electrowetting. This paper demonstrates
that, by integrating closed microchannels with different channel
heights and using electrowetting actuation, liquid interfaces can
be controlled, and pressure work can be done, resulting in fluid
pumping. The operation of two different on-chip pumps and devices that can form water drops is described. In addition, a theory
is presented to explain the details of single-electrode actuation in a
closed channel.
[2008-0224]
Index Terms—CYTOP, digital microfluidics, electrocapillary,
electrowetting, microchannels, microfluidics, micropumps, surface tension.
I. I NTRODUCTION
V
ARIOUS techniques have been reported to generate pressure and control fluids in microfluidic systems [1]. In
some systems, pressure has to be supplied from an external
macroscopic pressure source [2]. Systems that generate their
own pressure “on chip” have been based on a number of effects,
including electroosmosis [3], [4], electrophoresis [5], electromagnetism [6], acoustics [7], and thermocapillary effects [8],
[9]. In addition, self-contained microelectromechanical system
micropumps, which consist of moving solid boundaries, have
been developed [10].
One important (and arguably the most versatile) technique
that can be used to manipulate fluids on a microscale is electrowetting [11]–[15]. In electrowetting, the surface energy between a fluid and a dielectric-coated electrode can be controlled
with an applied electric potential. Electrowetting is unique,
because it is a direct way of controlling the surface tension of
a fluid; thus, it is particularly useful for microfluidics where
surface-tension effects are dominant. It is also unique in its
simplicity of fabrication. In the most basic electrowetting system, only a single-electrode level needs to be patterned. There
are numerous microfluidic applications for electrowetting, including variable focus lenses [16]–[19], electronic displays
Manuscript received August 28, 2008; revised March 11, 2009. First published July 7, 2009; current version published July 31, 2009. This work was
supported in part by the U.S. Air Force under Contract FA8721-05-C002.
Opinions, interpretations, conclusions, and recommendations are those of the
authors and not necessarily endorsed by the United States Government. Subject
Editor C.-J. Kim.
J. Kedzierski and S. Berry are with the Lincoln Laboratory, Massachusetts
Institute of Technology, Lexington, MA 02420 USA (e-mail: jakub@ll.mit.edu;
sberry@ll.mit.edu).
B. Abedian is with the Department of Mechanical Engineering, Tufts University, Medford, MA 02155 USA (e-mail: behrouz.abedian@tufts.edu).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JMEMS.2009.2023845
[20]–[22], and biotechnology systems [23], [24]. Electrowetting lends itself to manipulating discrete volumes of fluids; thus,
microfluidics based on electrowetting is often referred to as
digital microfluidics [24]–[27]. The majority of biotechnology
applications that use digital microfluidics for the liquid transport mechanism do so by controlling discrete volumes, i.e.,
droplets [25]. Fluidic functions such as transporting, merging,
splitting, and mixing of drops have been demonstrated [26].
Research in digital microfluidic-based biological applications
includes polymerase for chain reaction for deoxyribonucleic
acid analysis [27], assays [28], and protein analysis [29].
Digital microfluidics has typically been implemented with an
open fluidic design, where water droplets are vertically confined
by two parallel plates but are free to move in both horizontal
directions. Drops are transported by the proper sequencing of a
voltage potential on an array of patterned electrodes [26], [30].
This design is easy to fabricate but severely limits the types
of functions that can be implemented. Closed-channel digital
microfluidic structures, which permit active pumping, have only
been explored by a few research groups [31], [32].
In this paper, we present a new type of electrowetting-based
digital microfluidic device that uses closed microchannels to
control and direct pressure. This new design allows for the
implementation of basic fluidic components such as pistons and
valves, which can further be integrated into a number of pump
designs, droplet generators, and microfluidic transport systems.
Starting with a theory of a single electrode that actuates in a
microchannel, we present pumps for aqueous and nonaqueous
fluids, as well as a digital microfluidic circuit that can generate/destroy droplets, and move them in an arbitrarily long
channel using only a few electrodes.
II. D ESIGN AND FABRICATION
A. Design
The microfluidic structure consisted of two wafers that were
bonded together face to face, with the fluidic channels in
between. The top wafer was 4 Pyrex and was allowed for
optical observation of the fluids in the channels. The bottom
wafer was 6 silicon, with fluidic ports drilled through the
wafer to allow the introduction of fluids from the bottom. The
electrode contacts were fabricated on the silicon wafer outside
the 4 region where the top Pyrex wafer was bonded. The
overall structure is shown in Fig. 1.
B. Fabrication
1) Bottom-Wafer Process: The fabrication process for the
6 silicon bottom wafer is shown in Fig. 2(a). First, a
2-μm-thick layer of SiO2 is deposited by plasma-enhanced
1057-7157/$26.00 © 2009 IEEE
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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 18, NO. 4, AUGUST 2009
Fig. 1. Schematic of the completed bonded wafer pair with attached nanoports
and tubes. Electrode potentials are controlled through a series of pads at the
edge of the silicon wafer.
chemical vapor deposition (PECVD). Next, a 300-nm Al layer
is deposited and patterned to form the electrodes. A 2.7-μmthick layer of SiO2 is then deposited and polished by
chemical–mechanical polishing (CMP) to 2 μm. After CMP, the
contact pad openings are patterned and wet etched in HF. The
microchannel layer is formed from an 11-μm-thick patterned
SU-8 structural resist. A 10-s oxygen plasma is used to treat
the SU-8 surface before the application of the amorphous fluoropolymer (aFP). The aFP film, i.e., 3% CYTOP in CYTOPSolv, is then spun on the wafer that covers the top of the SU-8,
as well as the sidewalls and bottom of the channels. To prevent
fluoropolymer streaking, the spin time is kept very short, i.e.,
only 2 s at 1000 r/min. After spinning, the wafer is baked at
90 ◦ C for 30 s, which evaporates the remaining solvent, resulting in a 200-nm fluoropolymer film. The CYTOP aFP layer is
further hardened by vacuum baking at 150◦ C for 1 min. Finally,
the fluidic ports are drilled through the wafer with a Gatan
601 ultrasonic drill. A thick resist layer is used to protect the
CYTOP layer from damage and is removed in acetone after the
drilling process is complete.
2) Top-Wafer Process: The fabrication process for the
4 Pyrex top wafer is shown in Fig. 2(b). The microchannel
layer is formed from an 11-μm-thick patterned SU-8 structural
resist. A transparent 100-nm-thick indium tin oxide (ITO) is
then sputtered onto the wafer. Finally, the aFP, i.e., 2% CYTOP,
is spun on the wafer and baked. The process is similar to the
one used for the bottom wafer and produces an 80-nm-thick
fluoropolymer film.
3) Bonding: Next, the microfluidic structure is formed by
thermally bonding the CYTOP surfaces of the two wafers.
Wafer-to-wafer alignment prior to bonding is achieved with
a contact lithography tool. The bond is formed with pressure
applied to the top Pyrex wafer at 150 ◦ C. Fig. 2(c) shows
the cross section of the bonded wafer. Finally, Nanoports
(Upchurch Scientific) are epoxied to the bottom of the silicon
wafer and threaded with Teflon tubing.
III. O PERATION OF A S INGLE E LECTRODE
A. Fluidic Fill
Initially, the entire microfluidic structure is filled with the
dodecane oil. This instance spontaneously happens as soon
as the oil is applied to one of the ports, because the oilfluoropolymer surface energy is much smaller than the airfluoropolymer surface energy. Once the structure is filled with
oil, water is applied to the water port with a pressure Pb . Pb
is defined as the difference between the water pressure at the
water port and the oil pressure at the oil port. In our experiment,
Pb was controlled using a manometer with a pressure range
of 0 to 15 kPa. The surfaces of all the fluidic channels are
coated with an aFP, i.e., CYTOP; thus, the microchannels
are hydrophobic, and a certain minimum pressure is required
to fill them with water. The microchannels are much wider
(W ≥ 100 μm) than they are tall (height = 11 μm or 22 μm);
thus, it is the height that determines the dominant radius of
curvature of the oil–water interface in the channel. The minimum pressure that was required to displace oil with water in a
microchannel of height h can be calculated using Young’s and
Laplace equations [33] as
ΔP =
∗
2γw
.
h
(1)
Strictly defined, ΔP is the difference between the water
pressure and oil pressure at the oil–water interface. In addition,
∗
γw
= γwe − γoe , i.e., the difference between the water–aFP
and oil–aFP surface energies, approximately 50 mJ/m2 . There
are two types of channels in our microfluidic structure:
1) oil channels, with a height of ho = 11 μm, and 2) water
channels, with a height of hw = 22 μm. The minimum water
pressure that was required to displace oil with water into the
water channels is Pmin , which is equal to about 4.5 kPa for a
water–dodecane system. The oil channels are half the height of
the water channels; thus, the water pressure that was required
to displace oil in them is twice as high Pmax , i.e., about 9 kPa.
Because of the capillary pressure difference between the two
channel heights, as long as Pb is kept between Pmin and Pmax ,
water will displace oil from the water channels but not the oil
channels. Pb of 5.5 kPa worked well to give a slow controllable
water fill. At this pressure, the water slowly displaces dodecane
from the water channels but stops as soon as it reaches an oil
channel (see Fig. 3).
B. Single-Electrode Actuation
The cross section of a single-electrode device after fluidic fill
and prior to actuation is shown in Fig. 3. The pressure across
the water–oil interface without electrowetting is ΔP , as given
by (1). ΔP depends on the microchannel height h and thus
is dependent on the interface position. We have the following
three cases.
Case 1: Fig. 4(a): When the interface is in the water channel,
∗
/hw = Pmin . This condition typically
ΔP = 2γw
only occurs during fluidic fill. If Pb > Pmin , water
will displace oil from the water channel.
Case 2: Fig. 4(b): When the interface is in the oil chan∗
/ho = Pmax . This condition occurs
nel, ΔP = 2γw
when Pb > Pmax , which is typically avoided. It
will also occur when water is driven into the oil
channel through electrowetting actuation, and then,
the voltage is removed.
Case 3: Figs. 3 and 4(c): This case occurs when the interface is between the oil and the water channels
and, typically, once the structure is filled. For this
condition, the interface will adjust its curvature so
that ΔP = Pb , as long as Pmin < Pb < Pmax and
there is no voltage applied to the electrode.
KEDZIERSKI et al.: NEW GENERATION OF DIGITAL MICROFLUIDIC DEVICES
847
Fig. 2. Cross-sectional schematics for the fabrication process of (a) the bottom wafer, (b) the top wafer, and (c) the final bonded pair. (a) A 6 silicon wafer is
used for the bottom wafer. Fabrication proceeds as follows. A 2-μm oxide is deposited by PECVD, and 300-nm aluminum electrodes are deposited and patterned.
Another oxide is deposited over the electrodes and CMPed to 2 μm over the electrodes (contact holes in the oxide are opened over the contact pads, which are
external to the 4 center diameter, with a patterned HF etch). An 11-μm SU-8 layer is spun on and patterned, a 200-nm CYTOP aFP is spun on, and ports are
drilled with a ultrasonic drill. (b) A 4 Pyrex wafer is used for the top wafer. Fabrication proceeds as follows. An 11-μm SU-8 layer is spun on and patterned, a
100-nm ITO layer is deposited by PECVD, and an 80-nm aFP layer is spun on and baked. (c) Wafers are aligned and bonded at 150 ◦ C.
Fig. 3. Cross-sectional schematic of the filled single-electrode structure. The
water-filled channels have a height of hw , whereas the oil-filled channels have
a height of ho . When the water–oil interface is at the boundary of the two
heights, as shown here and in Fig. 4(c), the effective capillary height h can
change between ho and hw as the water–oil interface curvature changes.
All these cases still ignore electrowetting; thus, they are
only valid when the electrode is at 0 V or when it is not
present. In electrowetting, the voltage that was applied to the
bottom electrode V and its capacitances are important. The
capacitances per area from the top ITO ground to the bottom
electrode are C W in a water-filled channel and C O in the oil
channel (see Fig. 3). We have
−1
bottom
ttop
tOxide
W
aFP + taFP
+
(2)
C =
εr_ox ε0
εr_aFP ε0
−1
bottom
ttop
tOxide
ho
O
aFP + taFP
C =
+
+
. (3)
εr_ox ε0
εr_aFP ε0
εr_oil ε0
It is also convenient to define the difference between these
two capacitances as C ∗ = C W − C O . Note that C W does not
depend on the channel height, because the water phase is treated
as a conductor. When the voltage is switched on, electrowetting
must be included into ΔP through Lippman’s equation [33] in
Case 2, where it is relevant for normal actuation. We have
ΔP =
Fig. 4. Cross-sectional schematics of the water–oil interface at three different
positions. ΔP is the pressure difference across the interface. Note that, if
ΔP = Pb , which is, in general, the case in (a) and (b), the interface moves.
The pressure that drives the movement is Ps = Pb − ΔP . In (c), ΔP = Pb ,
as long as Pmin < Pb < Pmax . No voltage is applied to the electrodes in these
cases.
∗
C ∗V 2
2γw
−
.
ho
2ho
(4)
Here, C ∗ is used as the effective capacitance, because both
the free energy associated with the oil capacitor and water
capacitor must be considered. If ΔP = Pb , there is no interface
or liquid motion, because the applied pressure is balanced by
the pressure drop across the oil–water interface. If ΔP = Pb ,
then it is useful to define a working pressure Ps = Pb − ΔP .
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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 18, NO. 4, AUGUST 2009
Fig. 6. Electrode actuation data for various DC actuation voltages and back
pressures Pb . The channel dimensions are W = 150 μm, ho = 11 μm, and
L = 300 μm. The actuation in Fig. 5 corresponds to the 65-V point at 8 kPa.
Fig. 5. Top–down photographs of a single-electrode (a) actuating and
(b) relaxing. The actuation voltage is 65 V DC, and the back pressure Pb is
8.0 kPa.
Ps is the pressure that causes the interface and liquids in the
system to move. A voltage that is sufficiently large to generate
a positive Ps will result in the movement of water into the oil
channels. This actuation is shown in Fig. 5(a). Upon actuation,
if the voltage is removed, ΔP increases, Ps changes sign, and
now, a negative working pressure forces the interface back to
its original position at the boundary between the water and
the oil channels. This relaxation is shown in Fig. 5(b). The
working pressure during actuation is, in general, not of the same
magnitude as the working pressure during relaxation; thus, the
two processes will generally occur at different speeds.
To calculate a theoretical flow rate and, therefore, the speed
with which actuation occurs, the fluidic resistance from the oil
port to the water port must be considered. Fluidic resistance
can be defined as R = Ps /Q, where Q is the flow rate during
actuation (in liters per second). Using elementary hydrodynamics, each rectangular section of channel with length L, height h,
width W , and viscosity μ can be considered as contributing a
flow resistance of
2
RN =
12μL(h + W )
.
(hW )3
(5)
Fig. 7. Comparison of measured and theoretical ΔP values for different single-electrode actuation voltages. Theoretical ΔP values are obtained
from (4), whereas measured values were obtained from Q = 0 intercepts in
Fig. 6.
To theoretically estimate the flow rate, first, R is calculated
as the sum of all RN along the fluid path, and then the flow rate
can be obtained as Q = Ps /R. For the electrode in Figs. 5–7,
the calculated R is 1.3 kPa · s/nL.
An actual flow rate can directly be obtained by measuring
the actuation time with a CCD camera and considering the microchannel dimensions. A plot of flow rates that was measured
this way for different actuation voltages and Pb values is shown
in Fig. 6. Each voltage has a minimum Pb at which actuation
occurs, where Q ∼ 0 L/s. Q increases with higher Pb and
voltage, except for the relaxation return flow, which decreases
with higher Pb and is independent of the voltage that was
applied during actuation. The point where Q = 0 L/s occurs
when Ps = 0 Pa and Pb = ΔP . Thus, it is possible to obtain a
measurement of ΔP by looking at the intercept of the lines in
Fig. 6 with the axis of zero flow. Fig. 7 shows the comparison
of measured and theoretical [see (4)] values for ΔP .
IV. M ICROFLUIDIC D EVICES
A. Droplet Generator
Multiple electrodes can be integrated into more complex
microfluidic devices. For example, two electrodes can be used
KEDZIERSKI et al.: NEW GENERATION OF DIGITAL MICROFLUIDIC DEVICES
849
Fig. 8. Top–down photographs of a droplet generator that creates a water
droplet (1–4). A reverse process can be used to destroy the droplet (4–1). The
oil phase consists of dodecane, the actuation voltage is 80 V DC, and the back
pressure Pb is 7.5 kPa. Electrode–electrode spacing is 2 μm.
Fig. 10. Top–down photographs of a three-electrode water pump pumping
(1–4). The design consists of two water valves and a water piston. The top
drawing shows top–down schematics and two cross sections along the dotted
lines of the water valves and piston in different positions.
Fig. 9. Top–down photographs of a three-electrode oil pump pumping (1–4).
The design consists of two oil valves and an oil piston. The top drawing shows
top–down schematics and two cross sections along the dotted lines of the oil
valves and piston in different positions.
to form a droplet generator. A droplet generator can be used to
create or destroy water droplets in an oil channel, as shown in
Fig. 8. The spacing between the two electrodes, i.e., 2 μm, is
important in this application; in general, it should be smaller
than or equal to the dielectric thickness between the fluid and
the electrode.
B. Oil and Water Pumps
Perhaps, the most important microfluidic device is a pump.
Figs. 9 and 10 show a three electrode implementation of an
oil pump and a water pump, respectively. The two pumps are
designed in a similar way, with two valve electrodes and one
piston electrode. Considering the oil pump first, the oil valves
on either side of the central oil piston electrode function by
blocking the oil channel with water when a potential is applied
to them. Thus, the oil valves are open when their electrode
potential is off and are closed when it is on. The oil piston
works by using a single-electrode actuation to displace the oil
with water. The oil piston is extended when its electrode is on
and is retracted when it is off. The water pump components
function in a somewhat complementary manner, although there
are important differences in the engineering of the heights of the
appropriate channel regions. When the water valve electrode is
switched on, water fills the valve area, and the water valve is
open. When the electrode is switched off, water retracts from
the valve area blocking water flow in the channel with oil and
thus closing the valve. The water piston is also implemented
with a single electrode, but unlike the oil piston, it is extended
when the potential is switched off and is retracted when it is
switched on. Theoretically, the pumps can generate a pressure
that is somewhat smaller than (Pmax − Pmin ), and when tested
at typical operating conditions, they generated a pressure of
2.0–2.5 kPa.
C. Pressure Regulator
In all the aforementioned devices, the water phase was pressurized at a positive Pb value through an external manometer.
Clearly, it is advantageous to generate the required water pressure on chip and eliminate the need for an external pressure
source during operation. This approach can be done with the
use of a pressure regulator device, as shown in Fig. 11.
The pressure regulator must be primed with water from the
water port at Pb > Pmin . However, once primed, the regulated
water reservoir in the bottom of Fig. 11 is self contained, and
its pressure Pr is independent of Pb . Pr can electrically be
regulated between Pmin , with Vr at 87 V, and Pmax , with Vr at
0 V. The relation for Pr is similar to the one used for ΔP , i.e.,
Pr =
∗
2γw
C ∗ Vr2
−
.
ho
2ho
(6)
Note that in theory, Pr can be reduced to below Pmin with a
sufficiently high voltage. However, in reality, the electrowetting
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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 18, NO. 4, AUGUST 2009
Fig. 13. Top–down photograph of the vibration of the oil–water interface
during AC actuation with impurities in the oil phase. Actuation potential is
a square wave of −85 V to 85 V at 1 kHz. The Pb is 6.5 kPa.
Fig. 11. Top–down photographs of the priming of a water pressure regulator
(1–4). The Pb water terminal is attached to the water port, but after priming,
the Pr water terminal is an independent self-contained water reservoir. The Pr
pressure can electrically be controlled [see (6)] between Pmin and Pmax with
the regulating electrode even after Pb is removed and the water port is sealed.
One important consideration when designing an electrowetting system is the conductivity of the fluids. The water phase
should be conducting, whereas the oil phase should be insulating. The limited conductivity of DI water is sufficient for the
water phase. However, there is a stringent requirement on the
resistance of the oil phase. Impurities that reduce the resistivity
of the oil phase can short circuit the electrowetting effect,
particularly in DC. For example, improper baking of SU-8 can
leave sufficient solvent to make the oil very conductive. Oil
conductivity can induce an interesting effect in AC actuation,
where the oil–water interface will oscillate at the waveform
frequency much like a vibrating string (see Fig 13).
VI. C ONCLUSION
Fig. 12. Top–down photographs of a microfluidic system with a droplet
generator and a four-electrode push–pull oil pump. The system can be used
to generate and pump water droplets in a closed channel.
effect saturates [15] at approximately 90 V when γwe of the
bottom electrode fluoropolymer is reduced to 0 mJ/m2 . Trying
to operate above the saturation voltage will lead to permanent
damage of the fluoropolymer.
Using the pressure regulator, any system that requires a
pressurized water source can be attached to a regulated reservoir. After fluidic fill and regulator priming, the external water
manometer can be removed and the ports can be closed.
The key contribution of this paper has been the integration of
microchannels of various heights in an electrowetting system.
The integration of microchannels allows for the selective filling
of some regions with water and the implementation of critical
microfluidic components such as valves and pistons. These
basic components were further integrated into fluidic devices
such as pumps that semicontinuously pump oil or water, droplet
generators, and pressure regulators. Using basic components,
complex systems can be assembled to perform a variety of functions such as transporting, forming, shaping, and combining
droplets, regulating, and monitoring pressure.
ACKNOWLEDGMENT
D. Integrated Systems
Fig. 12 shows an integrated fluidic system with a droplet
generator and two pumps at the ends of a long microchannel.
With the proper timing sequence water droplets can be created
and then moved in the microchannel. Fig. 12 shows 9 droplets
created and then moved to the right. In general a wide variety of
digital microfluidic components can be integrated into a system
to control fluid flow.
V. T ECHNOLOGY C ONSIDERATIONS
The speed of actuation is dependent on the fluidic resistance R. A typical actuation time of 300 ms can be reduced
by decreasing the fluidic resistance. In this experiment, one
500-μm-long electrode that was designed to reduce R actuated
in less than 50 ms, and further gains are possible with scaling.
The relatively high actuation voltage, which is up to 90 V, is
common for electrowetting systems. This voltage can be reduced by making the dielectric over the electrode thinner [14].
The authors would like to thank the Staff of the Microelectronics Laboratory for their help with fabrication and the
Lincoln Laboratory Advanced Concepts Committee for their
support.
R EFERENCES
[1] G. Whitesides, “The origins and future of microfluidics,” Nature, vol. 442,
no. 7101, pp. 368–373, Jul. 2006.
[2] Z. Yu, Y. Lee, M. Wong, and Y. Zohar, “Fluid flows in microchannels
with cavities,” J. Microelectromech. Syst., vol. 14, no. 6, pp. 1386–1398,
Dec. 2005.
[3] C. Buie, D. Kim, S. Litster, and J. Santiago, “An electro-osmotic fuel
pump for direct methanol fuel cells,” Electrochem. Solid-State Lett.,
vol. 10, no. 11, pp. B196–B200, 2007.
[4] J. Wu, “AC electro-osmotic micropump by asymmetric electrode polarization,” J. Appl. Phys., vol. 103, no. 2, p. 024 907, Jan. 2008.
[5] W. H. Huang, F. Ali, Z. L. Wang, and J. K. Cheng, “Recent advances
in single-cell analysis using capillary electrophoresis and microfluidic
devices,” J. Chromatogr. B, Anal. Technol. Biomed. Life Sci., vol. 866,
no. 1/2, pp. 104–122, Apr. 2008.
[6] N. Pamme, “Magnetism and microfluidics,” Lab Chip, vol. 6, no. 1,
pp. 24–38, 2006.
KEDZIERSKI et al.: NEW GENERATION OF DIGITAL MICROFLUIDIC DEVICES
[7] M. Schneider, Z. Guttenberg, S. Schneider, K. Sritharan, V. Myles,
U. Pamukci, and A. Wixforth, “An acoustically driven microliter flow
chamber on a chip (μFCC) for cell–cell and cell–surface interaction
studies,” ChemPhysChem, vol. 9, no. 4, pp. 641–645, Mar. 2008.
[8] Z. Jiao, N.-T. Nguyen, and X. Huang, “Thermocapillary actuation of
liquid plugs using a heater array,” Sens. Actuators A, Phys., vol. 140, no. 2,
pp. 145–155, Nov. 2007.
[9] A. Darhuber, J. Valentino, S. Troian, and S. Wagner, “Thermocapillary
actuation of droplets on chemically patterned surfaces by programmable
microheater arrays,” J. Microelectromech. Syst., vol. 12, no. 6, pp. 873–
879, Dec. 2003.
[10] H. van Lintel, F. van de Pol, and S. Bouwstra, “A piezoelectric micropump
based on micromachining of silicon,” Sens. Actuators, vol. 15, no. 2,
pp. 153–167, Oct. 1988.
[11] F. Mugele and J.-C. Baret, “Electrowetting from basics to applications,”
J. Phys. Condens. Matter, vol. 17, no. 28, pp. R705–R774, Jul. 2005.
[12] A. Quinn, R. Sedev, and J. Ralston, “Influence of the electrical double
layer in electrowetting,” J. Phys. Chem. B, vol. 107, no. 5, pp. 1163–1169,
Feb. 2003.
[13] T. Jones, “An electromechanical interpretation of electrowetting,”
J. Micromech. Microeng., vol. 15, no. 6, pp. 1184–1187, Jun. 2005.
[14] J. Kedzierski and S. Berry, “Engineering the electrocapillary behavior
of electrolyte droplets on thin fluoropolymer films,” Langmuir, vol. 22,
no. 13, pp. 5690–5696, Jun. 2006.
[15] S. Berry, J. Kedzierski, and B. Abedian, “Low-voltage electrowetting
using thin fluoropolymer films,” J. Colloid Interface Sci., vol. 303, no. 2,
pp. 517–524, Nov. 2006.
[16] S. Kuiper and B. H. W. Hendriks, “Variable-focus liquid lens for miniature
cameras,” Appl. Phys. Lett., vol. 85, no. 7, pp. 1128–1130, Aug. 2004.
[17] B. Berge and J. Peseux, “Variable focal lens controlled by an external
voltage: An application of electrowetting,” Eur. Phys. J. E, Soft Matter,
vol. 3, no. 2, pp. 159–163, Oct. 2000.
[18] Y. J. Chang, K. Mohseni, and V. Bright, “Fabrication of tapered SU-8
structure and effect of sidewall angle for variable focus microlens using EWOD,” Sens. Actuators A, Phys., vol. 136, no. 2, pp. 546–553,
May 2007.
[19] R. Shami, D. Andelman, B. Berge, and R. Hayes, “Water, electricity,
and between: On electrowetting and its applications,” Soft Matter, vol. 4,
no. 1, pp. 38–45, 2008.
[20] R. van Dijk, B. Feenstra, R. Hayes, I. Camps, R. Boom, M. Wagemans,
A. Giraldo, B. Heijden, R. Los, and H. Feil, “Gray scales for video
applications on electrowetting displays,” in SID Symp. Dig. Tech. Papers,
2006, vol. 37, no. 1, pp. 1926–1929.
[21] B. Sun, K. Zhou, Y. Lao, J. Heikenfeld, and W. Cheng, “Scalable fabrication of electrowetting displays with self-assembled oil dosing,” Appl.
Phys. Lett., vol. 91, no. 1, p. 011 106, Jul. 2007.
[22] K. Blankenbach, A. Schmoll, A. Bitman, F. Bartels, and D. Jerosch,
“Novel electrowetting displays,” in SID Symp. Dig. Tech. Papers, 2007,
vol. 38, no. 1, pp. 618–621.
[23] H. Hosono, W. Satoh, J. Fukuda, and H. Suzuki, “On-chip handling of
solutions and electrochemiluminescense detection of amino acids,” Sens.
Actuators B, Chem., vol. 122, no. 2, pp. 542–548, Mar. 2007.
[24] V. Srinivasan, V. Pamula, M. Pollack, and R. Fair, “A digital microfluidic
biosensor for multianalyte detection,” in Proc. IEEE 16th Annu. Int. Conf.
Micro Electro Mech. Syst., 2003, pp. 327–330.
[25] R. Fair, “Digital microfluidics: Is a true lab-on-a-chip possible?”
Microfluid. Nanofluid., vol. 3, no. 3, pp. 245–281, Jun. 2007.
[26] S. K. Cho, H. Moon, and C. J. Kim, “Creating, transporting, cutting, and merging liquid droplets by electrowetting-based actuation for
digital microfluidic circuits,” J. Microelectromech. Syst., vol. 12, no. 1,
pp. 70–80, Feb. 2003.
[27] Y.-H. Chang, G.-B. Lee, F.-C. Huang, Y.-Y. Chen, and J.-L. Lin, “Integrated polymerase chain reaction chips utilizing digital microfluidics,”
Biomed. Microdevices, vol. 8, no. 3, pp. 215–225, Sep. 2006.
[28] D. Jary, A. Chollat-Namy, Y. Fouillet, J. Boutet, C. Chabrol, G. Castellan,
D. Gasparutto, and C. Pepponet, “DNA repair enzyme analysis on EWOD
fluidic microprocessor,” in Proc. NSTI Nanotech Tech., 2006, vol. 2,
pp. 554–557.
851
[29] U.-C. Yi and C.-J. Kim, “Soft printing of droplets premetered by electrowetting,” Sens. Actuators A, Phys., vol. 114, no. 2/3, pp. 347–354,
Sep. 2004.
[30] M. Pollack, A. Shenderov, and R. Fair, “Electrowetting-based actuation of
droplets for integrated microfluidics,” Lab Chip, vol. 2, no. 2, pp. 96–101,
May 2002.
[31] K.-S. Yun, I.-J. Cho, J.-U. Bu, C.-J. Kim, and E. Yoon, “A surfacetension-driven micropump for low-voltage and low-power operations,”
J. Microelectromech. Syst., vol. 11, no. 5, pp. 454–461, Oct. 2002.
[32] F. Malloggi, A. Vanapalli, H. Gu, D. van den Ende, and F. Mugele,
“Electrowetting-controlled droplet generation in a microfluidic flowfocusing device,” J. Phys. Condens. Matter, vol. 19, no. 46, p. 462 101,
Nov. 2007.
[33] A. W. Adamson and A. P. Gast, Physical Chemistry of Surfaces.
New York: Wiley, 1997, ch. II and V.
Jakub Kedzierski received the Ph.D. degree in electrical engineering from the University of California,
Berkeley, in 2001.
After his graduation, he was with the IBM T. J.
Watson Research Center, working on advanced
silicon devices. In 2005, he joined the Lincoln
Laboratory, Massachusetts Institute of Technology,
Lexington, where he is currently the Assistant Group
Leader of the Advanced Silicon Technology Group.
His research interests include FinFETs, silicon
nanowires, fully silicided gates, metal source/drains,
and fully depleted silicon-on-insulator technology. His current research efforts
are focused on graphene transistors, low-power electronics, and electrocapillary
microfluidics.
Shaun Berry received the Ph.D. degree in mechanical engineering, studying microfluidics and the
electrowetting phenomenon, from Tufts University,
Medford, MA, in 2008.
Since 1998, he has been a Staff Member in
the Lincoln Laboratory, Massachusetts Institute of
Technology, Lexington, working on various electrooptical systems. His research interests include fluid
and structural numerical simulations, microfluidics,
and the development of chemical/biological sensors.
Behrouz Abedian received the Ph.D. degree in mechanical engineering from Massachusetts Institute of
Technology, Lexington, in 1980. His Ph.D. research
was focused on electric charging in pipe flows.
Since 1981, he has been a Faculty Member in the
Department of Mechanical Engineering, Tufts University, Medford, MA, where he was the Department
Chair from 1988 to 1993. His research interests include flow-induced dielectric charging phenomenon
and the fundamental and applied aspects of fluid
mechanics, in particular, electrochemical transport,
megnetohydrodynamics, adhesion, piezoelectric effects, and properties of dental polymers. He is the holder of four patents on fluidic devices.