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Electrostatic actuator with liquid metal–elastomer compliant electrodes used for on-chip
microvalving
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2012 J. Micromech. Microeng. 22 097001
(http://iopscience.iop.org/0960-1317/22/9/097001)
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IOP PUBLISHING
JOURNAL OF MICROMECHANICS AND MICROENGINEERING
doi:10.1088/0960-1317/22/9/097001
J. Micromech. Microeng. 22 (2012) 097001 (6pp)
NOTE
Electrostatic actuator with liquid
metal–elastomer compliant electrodes
used for on-chip microvalving
Nikola Pekas 1,4 , Qing Zhang 1,5 and David Juncker 1,2,3
1
Department of Biomedical Engineering, McGill University, 3775 Rue University, Montréal,
QC H3A 2B4, Canada
2
Génome Québec Innovation Centre, 740 Dr Penfield Avenue, Montréal, QC H3A 1A4, Canada
3
Department of Neurology and Neurosurgery, McGill University, Montréal, QC H3A 1A4, Canada
E-mail: [email protected] and [email protected]
Received 15 March 2012, in final form 18 May 2012
Published 26 July 2012
Online at stacks.iop.org/JMM/22/097001
Abstract
We describe a new class of electrostatic actuators with a compliant electrode made of liquid
metal alloy contained by a thin elastomeric membrane. We illustrate the use of such actuators
as on-chip microvalves for gas flow control. The microvalve comprises of one fixed electrode
spanning the floor and sidewalls of the trapezoidal gas channel and one corresponding flexible
electrode suspended across the channel. Details of fabrication and preliminary characterization
of on/off and proportional valving are presented.
S Online supplementary data available from stacks.iop.org/JMM/22/097001/mmedia
(Some figures may appear in colour only in the online journal)
1. Introduction
less power to operate. Relatively high voltages required to
achieve electrostatic actuation can be readily provided by highvoltage integrated circuitry [3].
Electrostatically actuated microvalves generally consist
of a pair of electrodes, one fixed and one flexible that is
moved by an electrostatic force to open or close the fluid path.
Compliant electrodes in electrostatic microvalves described
to date are realized either by depositing and patterning thin
metal films onto flexible membranes [4–6], or by doping
the bulk elastomeric material with conductive nanoparticles
[7, 8]. In compliant electrodes built from solids, elastic and
electrical properties are intimately coupled: improving the
conductivity by depositing thicker metal films or by increasing
the concentration of the nanoparticle dopant leads to an often
undesirable increase in the stiffness of the system. In addition,
laminate and composite solids are prone to mechanical failure
under repetitive stress.
While a broad range of functionalities and applications
have been developed within the prevailing solid-state
Electrostatic actuation is commonly used in microelectromechanical systems (MEMS) such as micromirror arrays and
accelerometers. Owing to low power consumption, direct
transduction of electrical signals into mechanical action and
compatibility with established microfabrication processes,
electrostatic actuation is also being considered as a means
of valving for highly integrated and parallel microfluidic
processors [1]. Presently, state-of-the-art microfluidic systems
rely on passive membrane valves that are actuated by
pressurized gas lines controlled by off-chip solenoid valves [2].
Electrostatic microvalves may offer an alternative to solenoids
that is amenable to full integration and requires significantly
4 Present address: National Institute for Nanotechnology, 11421 Saskatchewan
Drive, Edmonton, AB T6G 2M9, Canada.
5 Present address: Department of Electrical and Computer Engineering,
McGill University, 3480 University Street, Montréal, Quebec H3A 2A7,
Canada.
0960-1317/12/097001+06$33.00
1
© 2012 IOP Publishing Ltd
Printed in the UK & the USA
J. Micromech. Microeng. 22 (2012) 097001
Note
paradigm, inclusion of liquids as active components may
in certain cases open new possibilities or improve the
performance of MEMS. Mercury is a well-known liquid metal,
and electrostatic [9] and electrocapillary [10] actuation of
mercury electrodes has been described previously. However,
when injected into microchannels, mercury spontaneously
recedes to minimize its surface energy [11]. Galliumbased liquid metal alloys (LMAs) have recently been
introduced as passive, electrically conductive components
to realize stretchable interconnects [12], compliant electrical
probes [13], stretchable antennas [14], aligned electrodes in
microfluidic systems [15] and pressure sensors [16]. Active
heat-management systems based on movement of droplets
of LMAs by electrowetting have also been proposed [17].
Gallium–indium eutectic (75.5% Ga by weight) melts at
15.6 ◦ C, and the addition of tin or other elements may further
lower the melting point. Unlike mercury, LMAs are nonvolatile and can readily be filled into microchannels to form
stable structures. The moldability of LMAs is attributed to the
presence of a thin skin of native oxides of Ga under ambient
conditions [11, 13]. The oxide layer is typically 2–3 nm thick
and acts as a barrier to oxidation of the bulk alloy [18]. Here,
we extend possible applications of LMAs into the realm of
active components for on-chip valving. We used GaInSn alloy
(68.5%, 21.5% and 10% by weight, respectively) with the
reported melting point of −18 ◦ C. The electrical resistivity of
this LMA of 0.4 × 10−6 m is lower than that of mercury,
and the viscosity is reported as 2.4 mPa s—comparable to
isopropanol. The use of a conductive liquid sealed by a
thin elastomeric membrane as a compliant electrode provides
a flowable functional element that, unlike solid electrodes,
does not crack or delaminate under stress. Importantly, the
mechanical properties of the LMA/elastomer electrode are
dominated by the elastomeric membrane, and are therefore
tunable without compromising the electrical properties.
(a)
(b)
2. Design and fabrication
Figure 1. (a) Exploded schematic of the electrostatic microvalve,
and (b) the fabrication process flow. A 2 μm thin membrane
contains the LMA while offering minimal mechanical resistance to
electrostatic actuation.
Design and fabrication of the microvalve are outlined in
figure 1. The device was assembled from two dies and a thin
elastomeric membrane sandwiched between them. The top
die with channels for LMA was made from PDMS (Sylgard
184, Dow Corning) using standard rapid prototyping methods
[19]. A ∼2.0 μm thick membrane was fabricated from a
1:4 mixture by weight of PDMS and toluene by spin coating
it onto a 150 mm silicon wafer at 3000 rpm for 60 s. In order
to ease the subsequent release of the membrane, the wafer
underwent anti-adhesion treatment in CHF3 plasma before spin
coating. The top die and the cured membrane were bonded
together after a brief activation in oxygen plasma, and the
assembly was gently peeled off from the carrier wafer. The
bottom die was fabricated from a 100 mm borosilicate glass
wafer (Borofloat, Montco Silicon Technologies). First, 15 μm
deep channels of trapezoidal cross-section were wet etched
using the receding-mask method that we developed to this
end [20]. The bottom portion of the channels was 200 μm
wide with a sidewall angle of 14◦ from the horizontal plane,
resulting in the total channel width of 320 μm at the top.
Next, a lift-off process was used to pattern fixed electrodes
in a sputter-deposited, 300 nm thick film of indium-tin oxide
(ITO). Finally, the PDMS die was aligned and attached to the
bottom die by adhesive bonding [21]. The top die included
microchannels that cross over the ITO electrodes, and also
holes for gas access that were aligned with the ends of the
channels in the bottom die. PDMS channels were 2.5 mm
long, 150 μm wide and 50 μm deep. The alloy was injected
into PDMS microchannels from a prefilled teflon capillary
(360 μm o.d., 100 μm i.d., Upchurch Scientific) connected to
a plastic syringe. Electrical connections were established by
inserting a short piece of Pt wire into the access holes of filled
channels, and sealing the holes with silicone adhesive (Dow
Corning 732). The resistance between the contacts was below
1 , without noticeable increase over a period of months.
2
J. Micromech. Microeng. 22 (2012) 097001
Note
(a)
(a)
(b)
(b)
Figure 3. Characterization of electrostatic valving. (a) Flow rates of
nitrogen when a pressure of 14 kPa is applied after closing the valve
by various voltages. The error bars represent two standard deviations
derived from three separate measurements. (b) Proportional valving
of nitrogen at 14 kPa by pulse-width modulation. A 1 kHz pulse
wave of 250 V was applied at variable duty cycles.
Figure 2. Reflected light micrographs taken through the bottom
glass die from below of the valve in the open (a) and closed state
(b). The fixed electrode is fabricated in ITO and is therefore
transparent, allowing visualization of the liquid metal electrode. In
the open state, the membrane is deflected by the applied pressure,
resulting in the apparent discontinuity in the liquid metal in (a). The
valve closes under the applied voltage of 200 V between LMA and
ITO; however, a small bulge forming in the middle of the channel
(b) prevents the complete closure.
times larger than typical gaps in parallel-plate configurations
that operate at comparable dc voltages [6, 26, 27]. When the
applied voltage is removed the valve opens due to the applied
pressure of 14 kPa in about 300 ms—comparably slower than
the closing because of the adhesion of PDMS to ITO. The
opening speed in membrane gas valves can be increased by
surface treatment to reduce the adhesion of the membrane to
the valve seat [28].
The possibility of electrical breakdown of the thin
membrane needs to be considered. The dielectric strength
of a material, expressed as the average breakdown voltage
divided by the thickness of the test specimen, does not scale
linearly with thickness [29]. The dielectric strength of thin
membranes often exceeds the values reported for the bulk
material. Limited data available for thin PDMS membranes in
contact with aqueous electrolytes indicate a dielectric strength
of about 50 V μm−1 [30], while for PDMS with gold-sputtered
contacts, a value of 100 V μm−1 is reported [7]. Leakage
current in our setup was below the detection limit of 70 nA,
indicating that no electrical breakdown occurred for fields of
up to 125 V μm−1. The absence of any visual evidence of
breakdown further confirmed the conclusion that the integrity
of the membrane was preserved.
Results of preliminary valving tests are shown in
figure 3. In the open state, a flow rate of 9 μL s−1 was
measured for a pressure difference of 14 kPa. The pressurehold tests of the valve in the closed state were performed by
3. Results and discussion
Supplying nitrogen under pressure to the gas channel detached
the PDMS membrane and opened the channel to the flow.
Conversely, when no pressure was applied, the membrane
spontaneously adhered to the ITO and closed the channel. At
potentials above 100 V between the LMA and ITO electrodes,
the LMA exerted sufficient electrostatic pressure on the PDMS
membrane to collapse and close the valve against the applied
gas pressure (see figure 2). The estimated displacement volume
based on the geometry of the gas channel was 0.8 nL. Video
capture at 60 frames s–1 (see electronic supplementary material
available at stacks.iop.org/JMM/22/000000/mmedia) revealed
that the valve closed in about 30 ms. The initial closing times
were somewhat longer than average and then decreased after
several tens of close/open cycles. This observation may be
attributed to the conditioning of the oxide skin [22]. Referring
back to figure 1, the trapezoidal cross section of the gas channel
ensures that two electrodes are in close proximity along the
edges of the gas channel even when the valve is in an open state.
Since the electrostatic force scales with the inverse square of
the distance between the electrodes, a trapezoidal geometry
leads to a ‘zipping’ closure of the valve [1, 23–25] and allows
the maximum channel depth in our system to be two to three
3
Reference
Type
Diaphragm
material
Maximum
normal
displacement
(max. gap)
Polla et al [32]
Peristaltic pump
Polysilicon/Si3N4
1.2–4.0 μm
100–400 μm
Shikida et al [23]
Gas valve
S-shaped FeNi sheet
2.5 mm
50 000 μm
Bifano et al [27]
Quake et al [6]
Gas valve array
Gas valve,
indirectly liquid
valving
Gas valve with
optional pressure
balancing
Peristaltic
micropump for
fluids
Direct valving of
liquids (ac drive)
Polysilicon/Si3N4
PDMS/Cu
5 μm
10 μm
300–500 μm
100 μm
Silicon
6 μm at 10 kPa
3000 μm
Parylene/Cr/Au
3 μm
100 μm per
valve
PDMS/Au/PDMS
5–10 μm
500 μm
Gas valve with
pressure balancing
Gas valve
Polyimide/Cr/Au/Cr/
polyimide
PDMS doped with
conductive nanoparticles
10 μm
5 μm
5000 μm
(estimate)
500 μm
Gas valves with
water-filled gap,
hydraulic coupling
Gas valve
Parylene/Cr/Au
6.5 μm
2000 μm
PDMS/liquid metal
15 μm
300 μm
Stemme et al [25]
4
Xie at al [5]
Maharbiz et al [4]
Shannon et al [33]
Kenis et al [8]
Najafi et al [34]
This work
Width of
the diaphragm
Pressure
difference
Operating
voltage
Actuation
time
No valving or
pumping reported,
only actuation
10 kPa at 100 V
50–200 V to
actuate
membranes
100–200 V
–
10 kPa at 210 V
No valving
reported, only
actuation
10 kPa at 170 V
(without pressure
balancing)
0.5 kPa per valve at
140 Vp ac
(estimate)
–
150–250 V
1200 V
J. Micromech. Microeng. 22 (2012) 097001
Table 1. Overview of electrostatically actuated microvalves for gases.
12 ms at 150 V
(estimate)
–
100 ms to close,
seconds to open
170 V
–
100–140 Vac
–
15–30 Vac
80 kPa at 140 V
50–140 V
1000 ms to
close, 5 s to
open
0.03 ms
No valving
reported, only
actuation
50 kPa at 120 Vac;
14 kPa at
100–180 Vdc
14 kPa at 170 V
40 V
–
60–140 Vac
100 ms
150–250 V
30 ms to close,
300 ms to open
Note
J. Micromech. Microeng. 22 (2012) 097001
Note
supplying a specified potential between the LMA and ITO
electrodes to close the valve, and then applying a pressure
of 14 kPa to the gas inlet and monitoring the flow rate.
The measured flow rate sharply dropped for potentials above
150 V, and maximum nine-fold reduction was achieved at
voltages above 175 V. Alternatively, closing the valve against
a steady pressure of 14 kPa reduced the flow rate by half. We
attribute less-than-perfect valving to the wrinkled membrane
apparent in figure 2. The PDMS membrane in our system is
somewhat flimsy, presumably due to plastic deformation [31]
during the separation of membrane from the carrier wafer. We
note, however, that many applications do not require a perfect
closure of the valve [39]: peristaltic pumping, for example,
depends mainly on the volume displacement during the valving
cycle. Investigation of the influence of the shape and aspect
ratio of the membrane on the formation of the wrinkle will be
the subject of a future study.
Proportional valving was demonstrated on the same
device by applying a 250 V, 1 kHz pulse wave with variable
duty cycle (see figure 3(b)). The flow rate scaled linearly with
pulse widths of up to 50% of the period, whereas pulse widths
of 60% and above resulted in the full closure of the valve
with leak rates similar to those in the pressure-hold tests
shown in figure 3(a). Table 1 compares the performance of
various electrostatic valves described to date. The LMA valve
presented here performs comparably to devices based on solid
electrodes, while the liquid nature of the alloy eliminates the
possibility of fatigue and cracking. Liquid metal electrodes
were subject to tens of thousands of actuation cycles without
any noticeable change in performance.
The presence of oxide at the surface of LMA is likely
to influence the mechanical properties of the electrovalve.
Reported surface moduli of the oxide skin are in the range
of 1–10 N m−1, depending on the strain history and time
[11, 22]. Assuming that the average value after the initial cycles
is closer to the ‘soft’ limit of ∼1 N m−1 [22], this would be
a mechanical equivalent of additional 2.5 μm of PDMS with
shear modulus of 400 kPa.
microvalves, for example, parylene [5, 34, 37], fluoropolymers
[38] and polyimide [33].
Acknowledgments
We thank the Natural Sciences and Engineering Research
Council (NSERC) and the Canada Foundation for Innovation
(CFI) for financial support. DJ also thanks the CRC for
a Canada Research Chair. We would like to acknowledge
the assistance of the McGill Nanotools Microfab Laboratory
(funded by CFI, NSERC and VRQ).
References
[1] Nguyen N-T and Wereley S T 2002 Microfluidics for internal
flow control: microvalves Fundamentals and Applications
of Microfluidics (Boston, MA: Artech House) pp 211–54
[2] Melin J and Quake S R 2007 Microfluidic large-scale
integration: the evolution of design rules for biological
automation Annu. Rev. Biophys. Biomol. Struct. 36 213–31
[3] Udrea F 2007 State-of-the-art technologies and devices for
high-voltage integrated circuits IET Circuits Devices Syst.
1 357–65
[4] Bansal T, Chang M-P and Maharbiz M M 2007 A class of low
voltage, elastomer-metal ‘wet’ actuators for use in
high-density microfluidics Lab Chip 7 164–6
[5] Xie J, Shih J, Lin Q A, Yang B Z and Tai Y C 2004 Surface
micromachined electrostatically actuated micro peristaltic
pump Lab Chip 4 495–501
[6] Driggs B S, Enzelberger M M and Quake S R (inventors) 2002
Electrostatic valves with elastomeric flow channel for
microfluidic devices Application: US patent 2002109114
20011023 (assignee: California Institute of Technology,
USA)
[7] Rosset S, Niklaus M, Stojanov V, Felber A, Dubois P
and Shea H R 2008 Ion-implanted compliant and
patternable electrodes for miniaturized dielectric elastomer
actuators Electroactive Polymer Actuators and Devices
(EAPAD 2008) vol 6927 p 69270W
[8] Tice J, Desai A, Apblett C, Ten Eyck G, Givler R and Kenis P
2008 Electrostatic microvalves for integrated microfluidics
ECS Meeting Abstracts vol 802 p 3167
[9] Shen W J, Edwards R T and Kim C J 2006 Electrostatically
actuated metal-droplet microswitches integrated on CMOS
chip J. Microelectromech. Syst. 15 879–89
[10] Ni J, Zhong C J, Coldiron S J and Porter M D 2001
Electrochemically actuated mercury pump for fluid flow
and delivery Anal. Chem. 73 103–10
[11] Dickey M D, Chiechi R C, Larsen R J, Weiss E A, Weitz D A
and Whitesides G M 2008 Eutectic gallium-indium
(EGaIn): a liquid metal alloy for the formation of stable
structures in microchannels at room temperature Adv.
Funct. Mater. 18 1097–104
[12] Hu H, Shaikh K and Liu C 2007 Super flexible sensor skin
using liquid metal as interconnect IEEE Sensors
1–3 815–7
Kim H J, Son C and Ziaie B A 2008 Multiaxial stretchable
interconnect using liquid-alloy-filled elastomeric
microchannels Appl. Phys. Lett. 92 011904
[13] Chiechi R C, Weiss E A, Dickey M D and Whitesides G M
2008 Eutectic gallium-indium (EGaIn): a moldable liquid
metal for electrical characterization of self-assembled
monolayers Angew. Chem. Int. Ed. 47 142–4
[14] Cheng S, Rydberg A, Hjort K and Wu Z G 2009 Liquid metal
stretchable unbalanced loop antenna Appl. Phys. Lett.
94 144103
4. Conclusions
While preliminary, our results indicate that LMA contained
in microchannels can act as a compliant electrode in onchip electrostatic actuators. Gallium-based LMAs exhibit
negligible vapor pressure at room temperature, mechanical
stability in microchannels and excellent electrical conductivity,
thereby enabling fabrication of robust, non-volatile liquid
functional components in MEMS. The liquid nature of the
alloy also opens the possibility of scaling and distributing the
actuation force by virtue of simple principles of hydraulics
[34, 35]. The concept is also applicable to direct valving of
liquids by applying an ac bias and adding a passivation layer
to the fixed electrode [4, 5, 36]. For practical use in certain
cases, it will be important to design a valve using a material
and geometry that minimize the formation of the bulge in the
membrane. While we chose PDMS as a convenient prototyping
material, the architecture is compatible with a broad range
of other materials previously used to fabricate membrane
5
J. Micromech. Microeng. 22 (2012) 097001
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
Note
So J H, Thelen J, Qusba A, Hayes G J, Lazzi G
and Dickey M D 2009 Reversibly deformable and
mechanically tunable fluidic antennas Adv. Funct. Mater.
19 3632–7
So J-H and Dickey M D 2011 Inherently aligned
microfluidic electrodes composed of liquid metal Lab Chip
11 905–11
Park Y L, Majidi C, Kramer R, Berard P and Wood R J 2010
Hyperelastic pressure sensing with a liquid-embedded
elastomer J. Micromech. Microeng. 20 125029
Mohseni K and Baird E S 2007 Digitized heat transfer using
electrowetting on dielectric Nanoscale Microscale
Thermophys. Eng. 11 99–108
Scharmann F et al 2004 Viscosity effect on GaInSn studied by
XPS Surf. Interface Anal. 36 981–5
Duffy D C, McDonald J C, Schueller O J A
and Whitesides G M 1998 Rapid prototyping of
microfluidic systems in poly(dimethylsiloxane) Anal.
Chem. 70 4974–84
Pekas N, Zhang Q, Nannini M and Juncker D 2010
Wet-etching of structures with straight facets and adjustable
taper into glass substrates Lab Chip 10 494–8
Wu H K, Huang B and Zare R N 2005 Construction of
microfluidic chips using polydimethylsiloxane for adhesive
bonding Lab Chip 5 1393–8
Larsen R J, Dickey M D, Whitesides G M and Weitz D A 2009
Viscoelastic properties of oxide-coated liquid metals
J. Rheol. 53 1305–26
Sato K and Shikida M 1994 An electrostatically actuated gas
valve with an S-Shaped film element J. Micromech.
Microeng. 4 205–9
Legtenberg R, Gilbert J, Senturia S D and Elwenspoek M 1997
Electrostatic curved electrode actuators
J. Microelectromech. Syst. 6 257–65
van der Wijngaart W, Ask H, Enoksson P and Stemme G 2002
A high-stroke, high-pressure electrostatic actuator for valve
applications Sensors Actuators A 100 264–71
Collier J, Wroblewski D and Bifano T 2004 Development of a
rapid-response flow-control system using MEMS
microvalve arrays J. Microelectromech. Syst. 13 912–22
Vandelli N, Wroblewski D, Velonis M and Bifano T 1998
Development of a MEMS microvalve array for fluid flow
control J. Microelectromech. Syst. 7 395–403
Han J, Flachsbart B, Masel R I and Shannon M A 2008
Micro-fabricated membrane gas valves with a non-stiction
coating deposited by C4F8/Ar plasma J. Micromech.
Microeng. 18 095015
ASTMD 2004 Standard Test Method for Dielectric
Breakdown Voltage and Dielectric Strength of Solid
Electrical Insulating Materials at Commercial Power
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
6
Frequencies ASTM D149-97a(2004) (American Society for
Testing Materials)
Feng J T and Zhao Y P 2008 Experimental observation of
electrical instability of droplets on dielectric layer J. Phys.
D 41 052004
McDonald J C, Metallo S J and Whitesides G M 2001
Fabrication of a configurable, single-use microfluidic device
Anal. Chem. 73 5645–50
Werber A and Zappe H 2006 Tunable, membrane-based,
pneumatic micro-mirrors J. Opt. A 8 S313–S7
Judy J W, Tamagawa T and Polla D L 1991 Surface-machined
micromechanical membrane pump Proc. IEEE Micro
Electro Mech. Syst. vol 182-6 p 288
Bae B, Han J, Masel R I and Shannon M A 2007 A
bidirectional electrostatic microvalve with microsecond
switching performance J. Microelectromech. Syst.
16 1461–71
Najafi K and Kim H (inventors) 2009 Liquid-gap electrostatic
hydraulic micro actuators Application: WO patent
2009048952 20081008 (assignee: The Regents of the
University of Michigan, USA)
Landers J P, Begley M R, Ferrance J P, Jones M H
and Monahan-Dian J (inventors) 2006 Electrostatic
actuation for management of flow in micro-total analysis
systems (μ-TAS) and related method thereof Application:
WO patent 2006044458 20051013 (assignee: University of
Virginia Patent Foundation, USA)
Gu W, Chen H, Tung Y C, Meiners JC and Takayama S 2007
Multiplexed hydraulic valve actuation using ionic liquid
filled soft channels and Braille displays Appl. Phys. Lett.
90 033505
Mukundan V, Ponce P, Butterfield H E and Pruitt B L 2009
Modeling and characterization of electrostatic comb-drive
actuators in conducting liquid media J. Micromech.
Microeng. 19 065008
Hua Z, Srivannavit O, Xia Y M and Gulari E 2004 A compact
chemical-resistant microvalve array using parylene
membrane and pneumatic actuation Proc. Int. Conf. on
Mems, Nano and Smart Systems pp 72–6
Grover W H, von Muhlen M G and Manalis S R 2008 Teflon
films for chemically-inert microfluidic valves and pumps
Lab Chip 8 913–8
Willis P A et al 2008 Monolithic photolithographically
patterned fluorocur (TM) PFPE membrane valves and
pumps for in situ planetary exploration Lab Chip 8 1024–6
Willis P A et al 2007 Monolithic teflon (R) membrane valves
and pumps for harsh chemical and low-temperature use Lab
Chip 7 1469–74
Pandolfi A and Ortiz M 2007 Improved design of low-pressure
fluidic microvalves J. Micromech. Microeng. 17 1487–93