Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Home Search Collections Journals About Contact us My IOPscience Electrostatic actuator with liquid metal–elastomer compliant electrodes used for on-chip microvalving This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2012 J. Micromech. Microeng. 22 097001 (http://iopscience.iop.org/0960-1317/22/9/097001) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 142.157.144.63 The article was downloaded on 27/07/2012 at 15:02 Please note that terms and conditions apply. 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