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Ionic Liquids for Microfluidic Actuation Multiplexed Hydraulic Valve Actuation Using Ionic Liquid Filled Soft Channels and Braille Displays Yi-Chung Tung and Shuichi Takayama Department of Biomedical Engineering, University of Michigan, Ann Arbor, USA
Use of ionic liquids has grown in recent years, particularly as environmentally friendly chemical solvents, in part because they have no detectable vapour pressure. Microfluidics has also been obtaining great attention for biological and chemical research; its actuation scheme plays a critical role to manipulate small amount of samples within channels. However, the existing microfluidic actuation mechanisms have drawbacks, such as complicated fabrication, tedious operation, and difficulty for scaling up. Here, several representative microfluidic actuation schemes are reviewed based on whether they require channel deformation. This is followed by a demonstration of a newly developed microfluidic actuation scheme based on ionic liquid filled poly(dimethylsiloxane) (PDMS) hydraulic microchannels actuated by Braille displays. Exploiting ionic liquids in a microfluidic device helped to solve the evaporation problem commonly faced in the PDMS microfluidic systems. Thus, the developed actuation scheme allows multiplexed hydraulic actuation while preserving all the associated advantages of using PDMS microchannels. Such integration will be useful for future development of new types of microfluidic systems.
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Introduction of Microfluidics The manipulation of fluids in channels with dimensions of tens of micrometers, microfluidics, has emerged as a distinct new field. Due to the unique characteristics of microfluidics, such as small size and laminar flow, it offers a number of useful capabilities compared to macro-scale fluidics: the ability to use very small quantities of samples and reagents, to perform analysis within short times, and to carry out separations and detections with high resolution and sensitivity. It also offers fundamentally new capabilities in the control of concentrations of molecules in space and time. Moreover, the microfluidic systems can be low cost and have small footprints for analytical devices. Consequently, microfluidics has the potential to influence subject areas from chemical synthesis and biological analysis to optics and information technology (1). In order to precisely manipulate the small amount of flow within the channels, fluidic actuation schemes play essential roles in all microfluidic systems. To facilitate better integration, easier fabrication and operation for practical use, various microscale fluidic pumps and valves have been developed over the past decades (2). Research into ionic liquids is also expanding rapidly due to their unique and advantageous materials properties. Useful properties of ionic liquids include no detectable vapour pressure, high conductivity, thermal and oxidative stability, air and moisture stability, nonflammability, and excellent heat transfer properties (3-5). To take advantage of these features, in recent years, researchers have been actively working to utilise ionic liquids in microfluidic systems (6-9). In this article, an overview of existing microfluidic actuators is presented, followed by introduction of a new microfluidic actuation scheme, exploiting ionic liquids for their ability to serve as low volume (nanolitres to microlitres) hydraulic fluids that do not evaporate in deformationbased microfluidic actuation schemes. We divide the microfluidic pumps and valves into two sections, depending on whether or not they utilise deformable structures comprised of poly(dimethylsiloxane). Then finally, we describe the details of how ionic liquids can play a critical role in poly(dimethylsiloxane) deformation based microfluidic actuation schemes.
Microfluidic Actuation Schemes The most common way to drive microfluidic flow is to use syringe pumps attached through tubes and fluidic interconnections to a microfluidic device. Although syringe pumps are well-designed and straightforward to implement, they have some disadvantages that retard their exploitation in microfluidic
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Figure 1. (A) Schematic drawing and experimental photographs of the dropletbased passive pumping process. (Reproduced from (14). Copyright 2007 Royal Society of Chemistry.) (B) An array of 192 microfluidic channels each with two access ports positioned according microtitre plate standards. Inset: Preadipocytes (3T3-L1) on day 5 in culture using the developed device (scalebar: 1 mm.) (Reproduced from (15). Copyright 2008 Royal Society of Chemistry.) systems (10). For example, they are quite bulky, expensive, not scalable, and are not amenable to miniaturisation, especially for point-of-care, biometric and biowarfare applications. Moreover, syringe pump-based systems do not allow recirculation of fluid within the microfluidic chip, a desirable requirement for certain applications in cell biology and sensing. The syringe pump on its own lacks on-chip valves, further limiting microscale fluid manipulation. In order to alleviate these problems, a large number of microfluidic actuation schemes have been developed based on actuation mechanisms, such as electroosmosis pumps (11,12), bimorph piezoelectric actuators (13), surface tension-driven flows (14,15), and external magnetic fields (16-18). Below are just a few representative examples. Droplet-Based Passive Pumping To eliminate complex external connections and equipment, Beebe’s group has developed droplet-based passive pumping systems (14,15). In this passive pumping system, the surface energy presented in a small drop of liquid is used to pump the liquid through a microchannel. The amount of pressure present within a drop of liquid at an air/liquid interface is given by the Young-Laplace equation. A consequence of the Young-Laplace equation is that smaller drops have a higher internal pressure than larger drops. Therefore, if two drops of different size are connected via a fluid-filled microfluidic channel, the smaller drop will shrink while the larger one grows as shown in Figure 1(A). The group demonstrated a microfluidic platform based on the method, and the only skill needed to operate the platform is the use of the pipette to touch drops onto a
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Figure 2. (A) Schematic diagram of a single micro stir-bar in microfluidic channel. An external magnet was used to drive the magnetic bar. (Reproduced from (17). Copyright 2004 Royal Society of Chemistry.) (B) Schematic illustration of the integrated magnetic stir-bar driven microfluidic device for long-term perfusion cell culture. The semipermeable membrane on which the cells are cultivated is sandwiched between two PDMS chips. (Reproduced from (18). Copyright 2008 Royal Society of Chemistry.) surface. They also demonstrate a high degree of parallelisation (96-192 channels per array) while retaining basic microfluidic operations including routing, compartmentalisation, and laminar flow, Figure 1(B). Although limited in the types of fluid manipulation that can be performed, this method is elegant in its simplicity and scalability. Actuation by External Magnetic Field Active magnetic microfluidic components have been embedded within microfluidic devices to create flow. Inspired by large-scale magnetic barstirrers, various micro magnetic stir-bar mixers and pumps have been developed. For instance, Liu’s group has developed a micro stir-bar in surfacemicromachined Parylene housings and channels for rapid mixing. They also demonstrated the feasibility of pumping using a similar device, Figure 2(A) (16,17).
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Exploiting the same concept, Kimura et al. developed an integrated microfluidic device with fluidic pumping and optical detection system for longterm perfusion cell culture, Figure 2(B) (18). The fluidic pumping is achieved by rotating stir-bars embedded within the microfluidic devices using an external magnetic field. Another interesting example uses ferroelectric plugs in microfluidic channels as pistons. Hatch et al. reported the first micropump using ferrofluid plugs as pistons, which are driven by external magnets as shown in Figure 3(A) (19). Recently, Sun et al. developed a circular ferrofluid driven PCR microfluidic device as shown in Figure 3(B) (20).
Figure 3. (A) Schematic of the operating principle for the circular ferrofluidic pump. One plug of ferrofluid, held in place by a stationary magnet, acts as a closed valve between the inlet and outlet chambers. A second plug translates through the channel, pulling fluid in the inlet and pushing it out the outlet. (Reproduced from (19). Copyright 2001 Institute of Electrical and Electronics Engineers, Inc.) (B) Photograph of the circular micro PCR device fabricated in PMMA, and its operation concept. The PCR reaction mixture is pushed around the circular channel by a ferrofluid plug and flows continuously through the three temperature zones. (Reproduced from (20). Copyright 2007 Royal Society of Chemistry.)
Polydimethylsiloxane (PDMS) Microfluidic System and Its Actuation One of the most common ways to drive fluidic flows, other than by using a syringe pump, is to use peristaltic pumps. These types of pumps move fluids by sequentially squeezing liquid through deformable tubing. This concept of fluid actuation by channel deformation has been adapted to the microscale to achieve highly versatile, scalable, and complex microfluidic actuation. Currently, the elastomer material, polydimethylsiloxane (PDMS), is one of the most popular materials for constructing such deformable microfluidic devices because of its unique material properties and mouldability suited for low-cost rapid prototyping based on a fabrication technique called soft lithography. PDMS is inexpensive, easily moulded, mechanically robust, disposable, chemically inert, non-toxic, optically transparent, and the properties of the surface is readily modified chemically. These properties result from the presence of an inorganic siloxane backbone and organic methyl groups which branch off of the silicon in the polymer backbone.
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Rapid Prototyping of Microfluidic Channels in PDMS A fabrication technique called soft lithography has been widely adapted to design and fabricate PDMS microfluidic systems in a relative short time (less than 24 h). Soft lithography is a non-photolithographic microfabrication technique that is based upon replica moulding. It provides a convenient, effective, and low-cost method for the manufacturing of microfluidic systems (21,22). In a typical procedure, a network of microfluidic channels is designed using a computer-aided design (CAD) program that is then converted into a transparency by a high-resolution printer. The transparency can be used as a photomask in photolithography to create a mould with positive relief patterns. The use of a thick photoresist called SU-8 as the mould material is a very attractive option for PDMS microfluidic devices since it allows the micromoulds to easily be patterned using conventional photolithography (22-24). Figure 4 shows the basic process steps of soft lithographic PDMS fabrication using an SU-8 mould created on a silicon wafer. First, an SU-8 photoresist is spun onto a silicon wafer to form a film of specific thickness, using a highly controlled spinning speed. Then, the SU-8 photoresist is soft baked to evaporate the solvent. Afterwards, the SU-8 is exposed to ultraviolet light using optical lithography and photomasks. A post-exposure bake is then applied on the wafer to increase the cross-linking of the exposed SU-8 parts followed by developing which removes uncross-linked resist.
Figure 4. PDMS microfluidic device fabrication process. After producing the SU-8 mould on the substrate, the wafer is silanised to promote parting of the PDMS structure from the mould after curing. Then, the PDMS precursor liquid is well mixed with the curing agent before pouring it onto the wafer mould. The cured PDMS can be easily separated from the master moulds and attached onto another substrate, which could be another PDMS layer, glass, or silicone substrate, to form enclosed microchannels. An oxygen-
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163 plasma surface treatment can be applied to form a permanent PDMS bonding to a variety of substrate materials (25). PDMS possesses great mechanical flexibility; therefore, many actuation schemes have been developed for PDMS microfluidic systems based on channel deformation. Below, this article will briefly review some of the channel deformation-based actuation methods utilised for PDMS microfluidic devices.
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Multilayer Soft Lithography with Pneumatic Actuation By bonding multiple layers of elastomer, Quake’s group has developed a fabrication technique, Multilayer Soft Lithography (MSL), to build active microfluidic systems containing on-off valves, switching valves, and pumps using pneumatic actuation, Figures 5(A) and (B) (26). Moreover, exploiting the same technique, a large-scale integration of microfluidic components onto a single chip (containing more than 2000 microvalves), which is comparable to integrated circuit (IC) architecture, has been demonstrated, Figure 5(C) (27).
Figure 5. (A) Process flow for multilayer soft lithography. (B) A 3D diagram of an elastomeric peristaltic pump. Peristalsis was typically actuated by the pattern 101, 100, 110, 010, 011, 001, where 0 and 1 indicate “valve open” and “valve closed,” respectively. (Reproduced from (26). Copyright 2000 American Association for the Advancement of Science.) (C) Optical micrograph of a microfluidic large-scale integration example device: a microfluidic comparator chip. The various inputs have been loaded with food dyes to visualise the channels and sub-elements of the fluidic logic. (Reproduced from (27). Copyright 2002 American Association for the Advancement of Science.)
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Thermally Actuated Phase-Change Microfluidic Components Taking advantage of materials that exhibit large volumetric change during phase change (e.g. polyethylene glycol (PEG), and paraffin), temperature regulated active microfluidic components have been developed. Carlen et al. first introduced a microvalve using a paraffin microactuator as the active component as shown in Figure 6(A) (28). The device was fabricated by simple surface micromachining techniques at low temperatures, which allows other electronic components to be fabricated on the same chip beforehand. Kaigala et al. developed an integrated polymerase chain reaction (PCR) and capillary electrophoresis chip using electrically controlled phase-change microvalves (29). The actuation is achieved by the volumetric expansion of PEG during its phase transition. The device consists of three layers: the glass control layer, the PDMS flexible membrane and a glass fluidic layer as shown in Figure 6(B).
Figure 6. (A) Cross-sectional view of a surface micromachined paraffinactuated microvalve, and photograph of and actual paraffin actuator part. (Reproduced from (28). Copyright 2002 Institute of Electrical and Electronics Engineers, Inc.) (B) Design of the glass-PDMS-glass integrated RT-PCR-CEmicrovalves chip, and cross-sectional and top views of the microvalve structure. (Reproduced from (29). Copyright 2008 Royal Society of Chemistry.) Shape memory Alloy Actuators A shape memory alloy (SMA) is an alloy that “remembers” its shape, and can be returned to that shape after being deformed, by applying heat to the alloy. The most effective and widely used alloys include NiTi, CuZnAl, and CuAlNi. These unusual properties are being applied to a wide variety of applications in a number of different fields, including microfluidic actuation. Vyawahare et al. report a microfluidic device that is actuated using SMA. The devices combine multi-layer soft-lithography with SMA wires on printed circuit boards (PCBs) as control element for microfluidic manipulation
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(Figure 7) (30). The electronically activated microfluidic components such as valves, pumps, latches and multiplexers were demonstrated.
Figure 7. (A) Design of an SMA valve. An SMA wire is looped around a rounded channel, soldered to a PCB and embedded in PDMS. (B) Operation of an SMA wire squeezing two microfluidic channels shut, and a photograph of a PDMS chip with several SMA valves on a PCB. (Reproduced from (30). Copyright 2008 Royal Society of Chemistry.)
Figure 8. Schematics of a planar peristaltic pump integrated with a microfluidic device. Cross-sectional photographs of a microfluidic channel in open and close states where stainless-steel ball bearing with various diameters are used to deform the channel. The bearing is attracted to the magnet positioned below the bottom substrate. (Reproduced from (31). Copyright 2008 Royal Society of Chemistry.) Peristaltic Pumping by Magnetic Actuation Yobas et al. took advantage of the magnetic actuation method and used readily-available stainless-steel ball bearings coupled with rotating/translating rare-earth magnets as reconfigurable actuating elements on soft-state microfluidic devices (Figure 8) (31). They demonstrated a disposable planar peristaltic pump suitable for microfluidic integration into lab-on-a-chip using the developed actuation scheme. The pump is capable of delivering a relatively large range of flow rates (up to ml min-1). The utilisation of non-contact magnetic force preserves the simplicity, reliability, and stability of the mechanical hardware.
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Braille Display Microfluidics with Ionic Liquid Filled Hydraulic Channels The existing actuation methods provide great potential for constructing practical microfluidic devices; however, they do have some disadvantages depending on the application. For example, the MSL system has capability for large-scale integration of valves and pumps, which provides versatile functions for microscale fluid manipulation. Conversely, the requirement of an external pneumatic source, and the often large numbers of tubing connections for each chip, makes the system bulky and the operation tedious. In addition, the pressurised air may dry up the liquid in the fluidic channels and cause serious problems in long term experiments. Magnetic and thermal actuation methods have poor spatial resolution due to interference from another actuation source when placing two valves too close. The integration of on-chip actuation mechanism such as SMA wires and stainless steel components makes the fabrication process more complicated, and also makes the entire system less disposable. Braille Display Microfluidics Braille display microfluidics is a method to precisely control fluid flow inside elastomeric microfluidic channels by using multiple (tens to hundreds) computer-controlled, piezoelectric, movable pins. These pins are positioned as a grid on a refreshable Braille display, which is a tactile device used by the visually impaired to read computer text. Each pin can act as a valve and be shifted upward to push against channels contained in silicone rubber and completely shut the channel. Utilising synchronous control of multiple pins through Braille display software, in situ, integrated microfluidic pumps and valves can be achieved. In these systems, microfluidic channels have a deformable PDMS membrane (thickness: 100~200 µm) at the bottom and channels with a smooth curved wall. The channel acts as a valve when the membrane is pressed by a pin of a Braille display and deformed. Also, three (or more) pins on a channel placed in series act as a pump by a peristaltic actuation sequence of the Braille pins (Figure 9) (32).
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Figure 9. Schematic representation of Braille display-based microfluidic systems. (A) A typical microfluidic channel design for use with Braille displays. The channel is constructed by a PDMS slab with channel features bonded with a PDMS deformable membrane at the bottom. (B) Overview of the typical experimental setup. The microfluidic device is positioned on top of the Braille display so that channels are aligned about Braille pins, facing down. (Reproduced from (32). Copyright 2004 National Academy of Science.)
Figure 10. Photographs of Braille display-based microfluidic devices: (A) recirculation microfluidic system for long-term culture and imaging of cells (Reproduced from (34). Copyright 2006 Royal Society of Chemistry), (B) microfluidic flow cytometry system designed for efficient and non-damaging cell analysis (Reproduced from (35). Copyright 2007 Royal Society of Chemistry), and (C) individually programmable cell stretching microwell array system (Reproduced from (36). Copyright 2008 Elsevier Ltd.) Various microfluidic devices have been developed using Braille Display microfluidics. For instance, cell culture systems using commercially-available refreshable Braille displays with microfluidic circulation architectures that eliminate the need for external media pumps and tanks have been developed as shown in Figure 10(A) (32-34). A microflow cytometer system utilising actuation of Braille-display pins for microscale fluid manipulation has also been developed as shown in Figure 10(B) (35). The system was designed for efficient and non-damaging analysis of samples with small numbers of precious cells. Moreover, a device consisting of twenty-four miniature cell stretching chambers with flexible bottom membranes that are deformed using the computercontrolled, piezoelectrically actuated pins of a Braille display has been demonstrated as shown in Figure 10(C) (36). The system is a fast and easy-tofabricate device that is capable of simultaneously exposing various cells to cyclical stretch at different frequencies.
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Using Braille displays as active fluidic controllers allows the basic function of microfluidic systems to be readily accessible and programmable without the need for large and expensive laboratory facilities. The total fluid actuation system is compact in size and can be powered by batteries or computer interfaces such as Universal Serial Bus (USB), which makes Braille display microfluidics portable. The Braille display microfluidics completely eliminates the requirement of fluidic interconnections between the microfluidic chips and external fluid manipulation systems. Furthermore, functions of microfluidic systems can be easily modified by reprogramming the software that controls a Braille display and/or the microfluidic channel design. As a result, this method of fluidic control is versatile and cost-effective. Ionic Liquid Filled Hydraulic Valve Although convenient, versatile, and overall compact, there are limitations in the usage of Braille-display actuators for microfluidic manipulation. Since Braille pins have a constant contact diameter on PDMS deformable membranes and are arranged in a regular grid, there is limited flexibility in the organised placement of pins as active microfluidic components (e.g. valves and pumps). The arrangement lacks multiplexing capability as each pin operates one valve, thus limiting the number of valves on a chip to the number of Braille pins. The Braille pin valve strategy also limits imaging capabilities, such as transmitted light phase contrast microscopy because the actuators are opaque. Device Design In order to alleviate the drawbacks of the Braille Display Microfluidics, Gu et al. developed hydraulic valves that are analogous to the MSL pneumatic valves but are instead pressurised mechanically by movable Braille pins rather than by externally delivered and gated high pressure gas, Figure 11(A-C) (37). Each pin movement compresses an on-chip piston and pressurises the connected control channel. Due to the reversible pressurisation of the control channel, it can close and reopen regions of fluidic channels directly below. A typical Braille pin is ~0.49 mm2 in contact area, and delivers 0.18 N of force to a piston (KGS Co., Saitama, Japan). Pistons compressed by the mechanical pins are approximately 0.83 mm2 in area and 150 µm in height. Typical cross-sectional dimensions are approximately 16 µm high and 95 µm wide for control channels and 8.5 µm high and 95 µm wide for fluidic channels making valve intersections approximately 100 x 100 µm2. Similar to MSL pneumatic valves, the hydraulic valves can act on multiple fluidic channels in parallel and are able to skip fluidic channels by decreasing the width of the control channel from 100 µm to 40 µm, Fig. 11(D). For full optical accessibility, the location of hydraulic valves can be removed away from the site of the Braille pins and pistons, Figure 11(E). Thus, the multiplexing ability and flexibility in high density valve placement that MSL pneumatic actuation schemes provide is retained, while the advantages of
In Ionic Liquids: From Knowledge to Application; Plechkova, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.
169 interconnect-free hydraulic actuation using Braille pin actuation are realised as shown in Figure 11(F). Each device is composed of three bonded PDMS layers with the top control layer serving as a mould with features for both pistons and control channels Figure 11(A). A middle layer serves as a mould for the fluidic channels as well as the membrane separating the control and fluidic channels. A bottom sheet closes the fourth side of the fluidic channels and along with the middle layer serves as the separation between the pistons and their corresponding Braille pins.
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Ionic Liquid Critical for Sustained Stable Actuation Because the volume of the fluid in the hydraulic channels is small (nanolitres to microlitres), it is crucial to eliminate any losses due to evaporation and permeation of the hydraulic fluid through the PDMS. Air and water are not
Figure 11. Schematic representation of Braille display-based microfluidic with ionic liquid filled hydraulic valves. (A) The arrangement of a microfluidic device with a hydraulic valve actuated by a Braille pin. (B) and (C) The operation of hydraulic valves. At the normal (not-actuated) situation, the hydraulic valve will maintain its open status (B) to allow fluid flowing through the fluidic channel. While the Braille pin is actuated, the control channel will be pressurised. As a result, the membrane between the control and fluidic channels will be deformed and pushed down to seal the fluidic channel creating close status (C). (D) A top-down view of the intersections of pressurised control (red) and fluidic (green) channels. (E) A top-down view of the Braille pins aligned underneath pistons (left) and microfluidic valves (right). (F) Photograph of a PDMS device with multiple hydraulic valves mounted on a computer-controlled Braille display with 64 pins. (Reproduced from (37). Copyright 2007 American Institue of Physics.)
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well-suited as actuating fluids, because gas permeation through the PDMS or direct evaporation of water causes rapid loss of pressure in the actuation channel, Figure 12(A). To overcome this fluid loss problem, an ionic liquid (1butyl-3-methylimidazolium tetrafluoroborate) was used as an incompressible piston fluid. Unlike gases such as air and vaporisable water, no decrease in ionic liquid volume was observed within the microfluidic channels, or when open to the atmosphere for more than ten days. In contrast to air and water, the ionic liquid was the only hydraulic fluid that was able to keep valves closed over a significant time period, Figure 12(A). Figure 12(B) shows repetitive opening and closing of an ionic-liquid filled hydraulic valve. Despite the successful demonstration of the actuation mechanism, the ionic liquid used contains an expensive cation and is known to hydrolyse to form hydrogen fluoride in the presence of water, which may chemically react with PDMS. Therefore, the compatibility of the ionic liquid and PDMS may need to be tested, and more stable and less expensive ionic liquids may need to be chosen.
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Figure 12. (A) Graph of the abilities of valves to stay closed when actuated by different control channel fluids at zero time. The ionic liquid filled control channels consistently sustain valve closure over all observed periods. (B) Graph of a valve filled with ionic liquid repeatedly actuated. (Reproduced from (37). Copyright 2007 American Institue of Physics.) Device Fabrication and Preparation A schematic of the fabrication process is shown in Figure 13. Silicon moulds (i) and (iii) are fabricated through photolithography techniques (1). The photoresist AZ 9260 (Microchem Co., Newton, MA) for control and fluidic channels are spun on silicon wafers at 2000 and 3500 rpm respectively for 35 s and cured. The final control layer mould (ii) is made by punching holes in a thin (~150 um thick) replica (a) of the original control channels on silicon (i) and then incorporating the space of the punched hole as a piston. Photocurable epoxy (Epoxy Technology, Billerica, MA) was used as the second mould (ii)
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and this mould was cast while the original PDMS replica (b) was placed upside down. With the 3 layers (c) derived from the final control mould (ii), the fluidic mould (iii), and another flat substrate (glass or outside of a Petri dish), all three are sequentially bonded together with plasma oxidation (50 W, 300 mTorr, 30 seconds). In (d), a side-view showing three layers: top layer for control piston and channel, middle for the fluidic channels, and a last layer to seal the fluidic channels. The control channels are primed with hydraulic liquid immediately after plasma oxidation then sealed with superglue. The liquid enters spontaneously but can also be introduced with positive pressure before permanently sealing the channel input with superglue or other suitable sealants (silicone).
Figure 13. Fabrication process of a microfluidic device with ionic liquid filled channels for Braille display actuation. (Reproduced from (37). Copyright 2007 American Institue of Physics.)
Conclusion A novel microfluidic actuation scheme based on ionic liquid filled PDMS hydraulic microchannels actuated by Braille displays is presented here. The key enabling breakthrough is the use of non-volatile ionic liquids as the hydraulic fluid. Use of any other type of gas or liquid leads quickly to volatilisation and escape of fluid from the actuation microchannels because of the porosity of PDMS. By using ionic liquids, hydraulic microchannel actuation is realised even in PDMS materials, preserving all the associated advantages of using PDMS microchannels. The system overcomes the need for potentially numerous leak-proof interconnections found on various other microfluidic actuation methods. The described hydraulic valve control strategy has the shared advantages of both MSL pneumatic and mechanical Braille valves through the availability of parallel, arbitrarily arranged valves that are powered by potentially many independent actuators available in a portable format. Like MSL pneumatic valves, ionic liquid hydraulic control microchannels can control
In Ionic Liquids: From Knowledge to Application; Plechkova, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.
172 multiple fluidic channels and yet skip others. The functionality of Braille valves and pumps can be readily used in conjunction with hydraulic control valves. In addition, the separation between the actuation mechanism and microfluidic channels makes the microfluidic device fully disposable, eliminates the cross contamination between chips, and reduces the device fabrication complexity.
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