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Exploration and evaluation of embedded shape memory alloy (SMA) microvalves for high aspect ratio microchannels
Sensors and Actuators A 168 (2011) 155–161
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Exploration and evaluation of embedded shape memory alloy (SMA) microvalves for high aspect ratio microchannels Lin Gui, Carolyn L. Ren ∗ Department of Mechanical and Mechatronics Engineering, University of Waterloo, 200 University Ave West, Waterloo, Ontario, N2L3G1, Canada
a r t i c l e
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Article history: Received 17 December 2010 Received in revised form 18 March 2011 Accepted 20 March 2011 Available online 12 April 2011 Keywords: Shape memory alloy Microvalve Microfluidic Thermomicrovalve
a b s t r a c t This paper presents a microvalve actuated by shape memory alloy (SMA). This microvalve is designed particularly for controlling high aspect ratio polydimethylsiloxane (PDMS) channels and can be embedded into PDMS based microfluidic chips. This SMA microvalve can be designed either as a normally closed or normally open microvalve. Both designs utilize SMA wires for thermal actuation and operation to open or close microchannel flow. In this work, detailed parametric studies have been performed. The response open time of the normally open microvalve could be as low as 1 s at a flow rate of 4 L/min, which is very short for a thermomicrovalve. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Microfluidic platforms have found a wide range of applications including chemical/pathogenic detection, polymerase chain reaction for DNA amplification, drug delivery, on-chip immunoassay, and cell sorting [1,2]. One of the main tasks for designing and operating microfluidic chips is to control fluid flow in microchannels using microvalves. Off-chip microvalves are difficult to operate which has prompted the development of on-chip microvalves. For integration purposes, such valves are desired to be small and fast in response. PDMS has been widely used as chip material for microfluidic applications because of its low cost of fabrication using standard soft lithography technology and its superior optical properties. Its elastic property not only makes it a good choice of valve material because it can deform under external pressure, but also allows other microvalves to be integrated in an easy manner. The SMA valve presented here is designed to work with PDMS based microfluidic chips. Different mechanisms have been reported for actuating PDMS based microvalves, which include electrostatic [3], piezo [4], bimetallic [5,6], phase change [7,8], electromagnetic [9], thermopneumatic [10] and SMA [11]. The most successful example of integrating a large number of microvalves onto one single chip is Quake’s valve [12] which was first presented in 2000. In Quake’s microvalve, the microfluidic chip consists of three layers, the top,
∗ Corresponding author. Tel.: +1 519 888 4567x33030; fax: +1 519 885 5862. E-mail address: [email protected] (C.L. Ren). 0924-4247/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2011.03.038
middle and bottom layer. Both the top and bottom layers contain a microchannel network while one channel network is filled with sample fluids and the other is filled with gas serving as a control layer. The channels in these two layers are generally aligned perpendicularly at the valve locations. These two layers are separated by the middle layer, which is a very thin PDMS film (typically 30 m thick). When pressure is applied to the control fluid (i.e., gas), the middle PDMS film will deflect to block the microchannel filled with the sample fluid. In one of their studies [13], thousands of these microvalves are integrated onto one single chip. Because of its pneumatic actuation mechanism, Quake’s microvalve is well suited for low aspect ratio (height to width) microchannels. Similar to Quake’s valves, most other PDMS based microvalves are designed to close or open a microchannel in the height direction (vertically) which requires the channel to have a low aspect ratio so that it can be completely closed or opened. However, in many other applications, high aspect ratio microchannels are preferred. For example, UV absorbance based protein detection methods prefer microchannels to be as high as possible for better detection sensitivity provided that the total volume of the liquid remains the same. This is because the detection sensitivity is proportional to the light path (often is the channel depth), as reported by Liu et al. [14] where the highest aspect ratio used was slightly above 6 (250 mhigh, 40 m-wide). In some of the 3D particle focusing applications where the two focusing streams flow into the main channel from the top and bottom separately, the focusing microchannel is desired to have a high aspect ratio for better focusing effects [15]. It is very challenging to close a high aspect ratio channel using the above mentioned valves which prompted the exploration of new SMA microvalves for high aspect ratio microchannels.
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Fig. 1. Sketch of SMA actuator.
SMA, also known as the memory metal, is able to return to its memorized original shape after it is deformed. Generally speaking, a SMA wire shrinks when it is heated, and returns to its original length when cooled down to its original temperature, which is the actuation mechanism of SMA wire [16]. Because of its large recoverable strain and large work-to-volume ratio (∼107 J m−3 ), SMA wire is ideal for microfluidic applications [17]. Another advantage of using SMA wire as an actuator is that its actuation does not require significant amount of heat. For example, a SMA wire can be heated by its joule heating generated by applying an electrical current through it, which greatly simplifies the fabrication of microvalves. SMA wire has been used as valves to switch the flow in deformable channels in late 1990s. For example, SMA wire was first used to pinch deformable rubber tubes in an implantable drug delivery system in 1997 by Reynaerts et al. [18]. Later, in 1999, Pemble and Towe [19] presented another normally closed SMA pinch valve that switches the flow in silicon tubing by tightening or relaxing the tubing. In 2006, Piccini and Towe [11] presented another SMA wire based pinch valve where they used SMA wire to stretch the cross section of the silicon tubing to allow the fluid to flow. However, none of the above is small enough for microfluidic chip applications. Until 2008, Vyawahare et al. [20] presented the first SMA wire based microvalve for microfluidic chip applications. In their study, the PDMS microfluidic chip was fabricated using standard soft-lithography technology with a printed circuit board (PCB) integrated beneath it and Ni–Ti alloy wire was used as SMA material. In their design, the SMA wire was bent over the top of the channel and put through two holes that were punched very close to each side of the channel wall. Two ends of the wire were then soldered on the PCB board beneath the chip. When an electrical current is applied to the SMA wire, its temperature increases due to joule heating. When the temperature of the wire reaches its transition temperature, the wire shrinks in its length and thus squeezes the microchannel from the top to bottom. The typical size of the microchannel reported was 100 m in width and 10 m in height. This structure was attempted at the beginning of this study for high aspect ratio channels; unfortunately, little success was received because leakage happened in most cases. This prompted the exploration of other structures for SMA wire based microvalves for high aspect ratio PDMS microchannels. The proposed structure can be designed as a normally closed or a normally open microvalve. 2. Principle of SMA valve After several failures of attempting to control high aspect ratio channels in their height direction, an idea of controlling them in
Fig. 2. Top view of (a) normally open microvalve and (b) normally closed microvalve.
their width direction, for example, squeezing a microchannel from its side walls, was proposed. Fig. 1 shows the schematic of a SMA wire based microvalve for applying to high aspect ratio PDMS microchannels. Basically the valve consists of a main 14 mm-long SMA wire (the black wire) which is referred to as A-wire in the following. The Awire is coiled by a long aluminum wire (the grey wire) which is referred to as B-wire in the following. The B-wire works as an electrical heater to heat up the A-wire for actuation when needed. The two ends of the A-wire are welded with a 1 mm-long SMA wire (referred to as C-tip) and a 5 mm-long SMA wire (referred to as Dhandle) separately as shown in Fig. 1. Both the C-tip and D-handle are aligned perpendicularly to the A-wire and they are perpendicular to each other as well to help prevent the D-handle from moving when the C-tip is actuated to move. The entire structure is embedded in the PDMS substrate that contains the microchannel while keeping the A-wire aligned perpendicularly to the channel at the valve location. The D-handle is relatively large and difficult to move in the substrate due to large resistance serving as a position holder so that only one end of the A-wire (C-tip) moves when actuated. As shown in Fig. 1, there is a thin layer of SU-8 wrapping the whole C-tip which is designed to help close the microchannel because the surface of SU-8 layer is softer, smoother and less sticky to PDMS than SMA and thus is easier to move in cured PDMS. Details of the fabrication will be introduced later. When an electrical current is applied to the B-wire, joule heat generated within it will increase the temperature of itself and the coiled A-wire. When the temperature of the A-wire is higher than its transition temperature (i.e., 70 ◦ C), it will shrink in length pulling the C-tip and D-handle towards each other. Since the D-handle barely moves, the C-tip will move causing the motion of the channel that is close to the C-tip achieving the purpose of valving. This SMA valve can be designed as normally closed or normally open microvalve for high aspect ratio microchannels as shown in Fig. 2. For a normally open microvalve, the A-wire is placed on the top of the channel while the inner wall of the C-tip is close to one side of the channel walls. When the A-wire is thermally actuated and then shrinks, the C-tip will squeeze the microchannel to close the flow as shown in Fig. 2(a). For a normally closed microvalve, there is no overlap between the channel and the A-wire while the outer wall of the C-tip is touching one of the channel side walls. In this design, there is a SU-8 block placed inside the channel at the valve location to close flow in the normal stage. When the valve
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Fig. 4. Operation of normally open SMA valve without SU-8 layer. The channel width is 60 m.
Fig. 3. SMA wires were welded together by using a spot welding machine.
is actuated, the C-tip will pull the channel side wall away from the block to open flow, as shown in Fig. 2(b). When the electrical current is turned off, the generation of joule heating is stopped causing the SMA wire to cool down to room temperature and return to its normal length. The C-tip will move back to its origin position. 3. Materials and methods 3.1. Materials Sylgard 184 silicone elastomer base and curing agent (Dow Corning Corporation) were used to make PDMS chips. SU-8 2005 (Micro Chem) was used to fabricate the valve tip. SU-8-2075 (Micro Chem) was used to fabricate the straight channel. FlexinolTM , 0.015 in. diameter SMA wires with 70 ◦ C transition temperature were obtained from Dynalloy, Inc. The SMA wires were conditioned by the supplier Dynalloy and were provided in a prestrained state for the convenience of actuator construction (Precise details of the thermomechanical treatment during conditioning are unavailable, since it is proprietary to Dynalloy Inc.). Aluminum wire was extracted from an electrical wire, but covered by an electrical insulation layer to ensure safety and avoid short circuit. T type thermocouple TT-T-40-SLE (0.017 in. × 0.026 in.) was obtained from OMEGATM . 3.2. Fabrication To characterize the SMA microvalve, simple straight high aspect ratio channels were used. All the microchannels had the same channel height of 150 m. The width of the microchannel varied from 50 m to 60 m. The microfluidic chips were fabricated using standard soft lithography technology. Briefly SU-8 2075 was spincoated on a 4 in. silicon wafer at 1200 rpm. The coated 150 m-thick SU8 layer was then soft baked at 65 ◦ C for 5 min and then 95 ◦ C for 30 min. The straight channel was patterned onto the coated silicon wafer via UV light with an exposure dosage of 1000 mJ/cm2 through a single transparency mask. The exposed wafer was baked again at 65 ◦ C for 5 min and then at 95 ◦ C for 12 min. After cooling the wafer down to room temperature, the wafer was developed in SU-8 developer for 15 min which was used as the master for making PDMS channels. The A-wire was carefully coiled 50 loops of the B-wire without distance in between the loops. One end of A-wire was welded with C-tip and the other end was welded with the center of D-handle as shown in Fig. 1. Fig. 3 shows how the wires were welded together by using a spot welding machine. A 20× microscope was used to observe and control the welding. The C-tip was then grinded until
its length was only 1 mm long. The grinding was also performed under a 20× microscope by hands with great care in the similar way as welding. For better connection and easy of fabrication, the C-tip and D-handle were also made of the same material as the A-wire. When actuated, the A-wire, C-tip and D-wire were all heated up and then shrunk. However, the shrinkage of the C-tip is negligible due to its small length and the shrinkage of D-handle along its length will not affect the valve performance since it is welded with the A-wire at its center and it is placed far away from the channel to be controlled. At the beginning, the SMA-made C-tip was used directly to open or close microchannels, which worked well to operate as a normally open valve as shown in Fig. 4. However, this design has very low repeatability and reliability. Leakage was often observed in experiments. This is because the rough surface of the C-tip makes it difficult to move when embedded in cured PDMS. Therefore, an improvement was proposed aiming to address the leakage problem. To make the tip surface smoother and softer so that it can move when the valve is actuated, a SU-8 layer was used to wrap the tip. This structure did resolve the leakage problem almost completely (see no-leakage movie in supplemental information), however, it tends to deform the channel due to repeated heating and cooling required in fabrication which will be explained below. Therefore, precaution should be paid when using this structure for the flow which requires very accurate channel dimension throughout testing. Fig. 5 shows the procedure of fabricating the SU-8 layer. First, the C-tip is dipped into SU-8 2005 (Fig. 5(a)) and then the entire valve is placed on a silicon wafer with the bottom of the tip touching the wafer surface (Fig. 5(b)). The entire structure including the wafer and valve is baked in a vacuum oven at 65 ◦ C for 20 min and then 95 ◦ C for 30 min. Then it is left in room condition for 20 min before exposed to UV light at a dosage of 3000 mJ/cm2 to harden the SU8 layer (Fig. 5(c)). After UV exposure, the entire structure is baked again at 65 ◦ C for 10 min and then 95 ◦ C for 20 min in the oven. After removed from the silicon wafer, the SMA-made C-tip has a smooth layer of SU-8 2005 which allows the tip to move smoothly when actuated. Finally, the tip is polished using a grinder (Fig. 5(d)). To ensure the channel can be closed or opened completely, the C-tip end should be at the same level of the channel bottom wall so that the entire channel height can be covered and controlled by the C-tip. This has proven to be not trivial. The first attempt was making a PDMS chip integrated with a SMA valve at the same time by placing the valve in the desired valving location on the silicon wafer containing the channel feature. In this attempt, the C-tip end was made to touch the silicon surface and then standard casting procedures were followed. Little success was received from this attempt and leakage happened all the time because the valve floated away from the silicon wafer surface after PDMS prepolymer mixture was poured onto the wafer. As a result, the C-tip end is not aligned with the channel bottom wall and thus cannot close or open the channel completely. To solve the floating problem, a half-curing technique was introduced.
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Fig. 5. Fabrication procedures of the C-tip coated with SU-8: (a) dipping the SMA-made C-tip into SU-8 2005, (b) placing the valve on a silicon wafer with the bottom of the C-tip touching the wafer surface and then baking it at 65 ◦ C for 20 min and 95 ◦ C for 30 min, (c) undergoing UV exposure and post-exposure baking at 65 ◦ C for 10 min and at 95 ◦ C for 20 min, and (d) polishing the SU-8 surface using a grinder.
Fig. 6. Images of (a) normally closed and (b) normally open microvalve.
Basically, the chip with integrated SMA valve was fabricated in the following two steps. The first step is to ensure the C-tip end to be at the same level as the channel bottom wall after the chip is fabricated. To do so, a small drop of PDMS was first applied near the tip while the tip is touching the surface of the wafer containing the
channel feature and a small paper clip was placed on top of the SMA wire to avoid any possible floating due to this small drop of PDMS. The entire structure was put in a vacuum oven (ISO temp Vacuum Oven Model 280A) for 30 min to degas the PDMS, then baked at 80 ◦ C for 30 min to make the PDMS to be half-cured and finally left
Fig. 7. Open time (a) and close time (b) of the normally closed microvalve with different channel width and flow rate.
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Fig. 8. Influence of actuation current on (a) the open time for normally closed microvalve and (b) the close time for normally open microvalve.
in room temperature for 10 min to cool down. The paper clip was then removed leaving the silicon wafer with the valve where the C-tip end is fixed at the wafer surface. In the second step, 30 g of PDMS prepolymer mixture was poured onto the master to make the PDMS substrate with the designed high aspect ratio channel, however, the curing procedure was different for normally closed and normally open microvalves. To make normally open microvalves, the PDMS substrate was cured at room temperature for 72 h. Curing PDMS in room temperature was to avoid the motion of the SMA valve caused by the temperature change during the curing process at a relatively high temperature (i.e., 50 ◦ C) and then the cooling in room temperature after curing. This motion tends to deform the fabricated microchannel. This was not a serious concern for normally closed microvalves which will be explained below. To make a normally closed microvalve, the PDMS substrate was first fully cured in an oven at 50 ◦ C for 12 h and then a small drop of SU-8 was deposited into the microchannel at the location aligned with the C-tip of the valve to block the channel. This step was completed under microscope. Then the channel with liquid SU-8 drop was exposed to UV light (3000 mJ/cm2 ) to solidify the SU-8 block. As mentioned above, when the PDMS substrate is cured in oven and then cooled in room temperature, the SMA valve will shrink and then return to its original shape. This motion may cause deformation in the adjacent channel (like valving the channel). However, the region with possible deformation is filled with solidified SU-8 for normally closed valve and thus the deformation is not a serious concern. The cured PDMS substrate with the channel and embedded SMA valve was bonded with a glass slide via oxygen plasma treatment at 300 mTorr for 12 s (29.6 W, Harrick Scientific, Ithaca, NY, USA). Fig. 6 shows the image of normally closed microvalve and normally open microvalve taken through a microscope (Olympus GX71). The operation of the normally open microvalve can be visualized from video clip available from the supplemental information. The use of SU-8 layer to wrap the C-tip resolved the leakage problem successfully. However, it makes the fabrication more tedious and often causes the adjacent microchannel deformed to a certain extent due to repeated heating and cooling even extra attention is paid, as shown in Fig. 6. Since it fully solved the leakage problem, the valve with the C-tip wrapped by SU-8 was used for the rest of testing. 4. Results and discussions To test the performance of the valve, a syringe pump (Harvard Apparatus, PicoPlus) was used to pump DI water through the microchannel at a constant flow rate. To show the status of the flow, 1 mM fluorescent particles were used to visualize the flow. The electrical current applied at the beginning was 1.8 A and then dropped
to 1.5 A once the actuation occurs to prevent the microvalve from overheating. The actuation was observed through a microscope (Olympus GX71) during the experiments. Fig. 7 shows the open and close time of the normally closed microvalve with different channel width versus the flow rate at the open state (set by the syringe pump). The response time of the normally closed valve were evaluated at four flow rates which were 0.05 L/min, 1.0 L/min, 2.0 L/min and 4.0 L/min. Two sets of microchannels (50 m and 60 m in width) were chosen and both of them were 150 m high. There were two reasons choosing these two widths for testing. First, the exploration of SMA microvalves were motivated by the need of microvalves for controlling high aspect ratio channels in other projects which are often between 50 and 60 m in width. The second reason is due to the difficulty in fabricating SMA valves adjacent to small channels. Any deformation to the channels caused during the fabrication of the valve (C-tip coated with SU-8) will affect the testing significantly. Each experiment for either open time or close time of the microvalve was repeated five times and the results shown in Fig. 7 are the average value of five experimentally measured data. As shown in Fig. 7, when the flow rate increases, the open time decreases and the close time increases which is easy to understand because higher flow rates make the flow more difficult to close and easier to open. It can be seen that the channel width does not have significant influence on the open time for normally closed microvalves. This is because the working distance for both cases is almost the same, around 20 m (i.e., from 50 m to 70 m and from 60 m to 80 m). However, it does have more influence on the close time although the trend is similar for both channels for the tested flow rate range (0–4 L/min). There is a large discrepancy at the flow rate of 2 L/min which is more likely attributed by experimental uncertainties. The close time depends on the cooling time of the SMA actuator which is influenced by the surrounding PDMS, and experimental environment (i.e., room temperature or heat convection conditions near the chip). The actuation current influences the operation time as well. Fig. 8 shows the open time of the normally closed microvalve (Fig. 8(a)) and the close time of the normally open microvalve (Fig. 8(b)) versus the applied actuation current at the flow rate of 4 L/min. The highest flow rate was chosen among the tested flow rate range (0–4 L/min) for this testing in order to consider the worst scenario. The reported open or close time here could be lower when for lower flow rates. One can see that when the actuation current is increased, the response time is shorter. This is because the generated joule heat is increased with current which leads to larger shrinkage of the A-wire and shorter response time. In both cases, the current dropped to 1.5 A once the actuation starts. The close time.
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Fig. 9. Close time (a) and open time (b) of the normally open microvalve with different channel width and flow rate.
Normally open microvalves were also performed in two channel dimensions: 50 m and 60 m wide. Fig. 9 shows the close time and open time of the normally open microvalve. The performance of normally open microvalves was quite different from that of normally closed microvalves. Because the C-tip of the normally open microvalve has to move across the whole microchannel width to close the flow, the working distance for the microvalve to close the flow is the channel width. In addition, the flow is open as long as the C-tip detached from the other side wall of microchannel. The working distance for the microvalve to open the flow is much shorter compared with closing the flow. Therefore the open time (Fig. 9(b)) of the normally open valve is much shorter than the close time (Fig. 9(a)). As shown in Fig. 9, the shortest open time of the microvalve could be less than 2 s, while the close time could be longer than 10 s. For the same reason mentioned above, higher flow rates also delay the microvalve to close which results in longer close time and shorter open time for the normally open microvalve. The close time varied from 17.09 s to 49.67 s when the flow rate increased from 0.05 L/min to 4.0 L/min for 60 m-wide microchannels. Because the channel width affects the working distance of the microvalve, both the close time and open time increases when the channel width increases from 50 m to 60 m.
5. Conclusions This study explored the possibility of using SMA valves for high aspect ratio channels. Two aspect ratios were tested which were 3 (150 m high and 50 m wide) and 2.5 (150 m high and 60 m wide), respectively. The presented design that closes or opens microchannels from their side walls works well with these two channel dimensions. This design can be employed as a normally closed microvalve or a normally open microvalve, both of which were evaluated in this study. It was found that channel width has less influence on the response time of normally closed microvalve than normally open microvalve. Therefore, normally closed microvalve is more suitable for wider microchannels. The design of the presented microvalve is separated from the microchannel design and thus there is more flexibility in changing the valve design. Although the presented SMA microvalve works well for high aspect ratio channels, it tends to have a long response time due to the material property. The higher the actuation current is applied, the shorter the response time is. However, it is limited that how high the current can be applied because of slow heat dissipation through surrounding PDMS which may also affect the liquid temperature as well. It is recommended to drop the applied current after actuation starts to avoid overheating.
Acknowledgements The authors gratefully acknowledge the support of Natural Sciences and Engineering Research Council (NSERC) of Canada through a grant to Carolyn L. Ren and Ontario Ministry of Research and Innovation through the Early Research Award to Carolyn L. Ren.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.sna.2011.03.038.
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Biographies Lin Gui is a Professor of the Chinese Academy of Sciences, Beijing, China. He received a B.E. degree in Refrigeration and Cryogenics Engineering in July, 2000, from the
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Chongqing University, Chongqing, China. Then he received a Ph. D. degree in June, 2005, from Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing China. He was a post-doctoral Research Associate at University of Waterloo, Canada. His current research interests include microfluidics, bioheat and mass transfer, MEMS, as well as thermally enabled biotechnology and medical instrumentation. He is a chapter author of “Encyclopedia of Microfluidics and Nanofluidics”. He holds 7 China patents. Dr. Ren received her Ph.D. in Mechanical Engineering from the University of Toronto in 2004 and Master’s and Bachelor’s degrees in Thermal Engineering from Harbin Institute of Technology in 1995 and 1992, separately. She joined the University of Waterloo (UW) in May 2004 as an Assistant Professor and promoted to Associate Professor with tenure in July 2010. She has received many research awards in recognition of her research excellence since she started at UW, which include Research Excellence Award from UW in 2010, the prestigious Canada Research Chair in Lab-on-a-Chip Technology in 2009, and Early Research Award from the Ministry of Research and Innovation of Ontario in 2007.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Exploration and evaluation of embedded shape memory alloy (SMA) microvalves for high aspect ratio microchannels Lin Gui, Carolyn L. Ren ∗ Department of Mechanical and Mechatronics Engineering, University of Waterloo, 200 University Ave West, Waterloo, Ontario, N2L3G1, Canada
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Article history: Received 17 December 2010 Received in revised form 18 March 2011 Accepted 20 March 2011 Available online 12 April 2011 Keywords: Shape memory alloy Microvalve Microfluidic Thermomicrovalve
a b s t r a c t This paper presents a microvalve actuated by shape memory alloy (SMA). This microvalve is designed particularly for controlling high aspect ratio polydimethylsiloxane (PDMS) channels and can be embedded into PDMS based microfluidic chips. This SMA microvalve can be designed either as a normally closed or normally open microvalve. Both designs utilize SMA wires for thermal actuation and operation to open or close microchannel flow. In this work, detailed parametric studies have been performed. The response open time of the normally open microvalve could be as low as 1 s at a flow rate of 4 L/min, which is very short for a thermomicrovalve. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Microfluidic platforms have found a wide range of applications including chemical/pathogenic detection, polymerase chain reaction for DNA amplification, drug delivery, on-chip immunoassay, and cell sorting [1,2]. One of the main tasks for designing and operating microfluidic chips is to control fluid flow in microchannels using microvalves. Off-chip microvalves are difficult to operate which has prompted the development of on-chip microvalves. For integration purposes, such valves are desired to be small and fast in response. PDMS has been widely used as chip material for microfluidic applications because of its low cost of fabrication using standard soft lithography technology and its superior optical properties. Its elastic property not only makes it a good choice of valve material because it can deform under external pressure, but also allows other microvalves to be integrated in an easy manner. The SMA valve presented here is designed to work with PDMS based microfluidic chips. Different mechanisms have been reported for actuating PDMS based microvalves, which include electrostatic [3], piezo [4], bimetallic [5,6], phase change [7,8], electromagnetic [9], thermopneumatic [10] and SMA [11]. The most successful example of integrating a large number of microvalves onto one single chip is Quake’s valve [12] which was first presented in 2000. In Quake’s microvalve, the microfluidic chip consists of three layers, the top,
∗ Corresponding author. Tel.: +1 519 888 4567x33030; fax: +1 519 885 5862. E-mail address: [email protected] (C.L. Ren). 0924-4247/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2011.03.038
middle and bottom layer. Both the top and bottom layers contain a microchannel network while one channel network is filled with sample fluids and the other is filled with gas serving as a control layer. The channels in these two layers are generally aligned perpendicularly at the valve locations. These two layers are separated by the middle layer, which is a very thin PDMS film (typically 30 m thick). When pressure is applied to the control fluid (i.e., gas), the middle PDMS film will deflect to block the microchannel filled with the sample fluid. In one of their studies [13], thousands of these microvalves are integrated onto one single chip. Because of its pneumatic actuation mechanism, Quake’s microvalve is well suited for low aspect ratio (height to width) microchannels. Similar to Quake’s valves, most other PDMS based microvalves are designed to close or open a microchannel in the height direction (vertically) which requires the channel to have a low aspect ratio so that it can be completely closed or opened. However, in many other applications, high aspect ratio microchannels are preferred. For example, UV absorbance based protein detection methods prefer microchannels to be as high as possible for better detection sensitivity provided that the total volume of the liquid remains the same. This is because the detection sensitivity is proportional to the light path (often is the channel depth), as reported by Liu et al. [14] where the highest aspect ratio used was slightly above 6 (250 mhigh, 40 m-wide). In some of the 3D particle focusing applications where the two focusing streams flow into the main channel from the top and bottom separately, the focusing microchannel is desired to have a high aspect ratio for better focusing effects [15]. It is very challenging to close a high aspect ratio channel using the above mentioned valves which prompted the exploration of new SMA microvalves for high aspect ratio microchannels.
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Fig. 1. Sketch of SMA actuator.
SMA, also known as the memory metal, is able to return to its memorized original shape after it is deformed. Generally speaking, a SMA wire shrinks when it is heated, and returns to its original length when cooled down to its original temperature, which is the actuation mechanism of SMA wire [16]. Because of its large recoverable strain and large work-to-volume ratio (∼107 J m−3 ), SMA wire is ideal for microfluidic applications [17]. Another advantage of using SMA wire as an actuator is that its actuation does not require significant amount of heat. For example, a SMA wire can be heated by its joule heating generated by applying an electrical current through it, which greatly simplifies the fabrication of microvalves. SMA wire has been used as valves to switch the flow in deformable channels in late 1990s. For example, SMA wire was first used to pinch deformable rubber tubes in an implantable drug delivery system in 1997 by Reynaerts et al. [18]. Later, in 1999, Pemble and Towe [19] presented another normally closed SMA pinch valve that switches the flow in silicon tubing by tightening or relaxing the tubing. In 2006, Piccini and Towe [11] presented another SMA wire based pinch valve where they used SMA wire to stretch the cross section of the silicon tubing to allow the fluid to flow. However, none of the above is small enough for microfluidic chip applications. Until 2008, Vyawahare et al. [20] presented the first SMA wire based microvalve for microfluidic chip applications. In their study, the PDMS microfluidic chip was fabricated using standard soft-lithography technology with a printed circuit board (PCB) integrated beneath it and Ni–Ti alloy wire was used as SMA material. In their design, the SMA wire was bent over the top of the channel and put through two holes that were punched very close to each side of the channel wall. Two ends of the wire were then soldered on the PCB board beneath the chip. When an electrical current is applied to the SMA wire, its temperature increases due to joule heating. When the temperature of the wire reaches its transition temperature, the wire shrinks in its length and thus squeezes the microchannel from the top to bottom. The typical size of the microchannel reported was 100 m in width and 10 m in height. This structure was attempted at the beginning of this study for high aspect ratio channels; unfortunately, little success was received because leakage happened in most cases. This prompted the exploration of other structures for SMA wire based microvalves for high aspect ratio PDMS microchannels. The proposed structure can be designed as a normally closed or a normally open microvalve. 2. Principle of SMA valve After several failures of attempting to control high aspect ratio channels in their height direction, an idea of controlling them in
Fig. 2. Top view of (a) normally open microvalve and (b) normally closed microvalve.
their width direction, for example, squeezing a microchannel from its side walls, was proposed. Fig. 1 shows the schematic of a SMA wire based microvalve for applying to high aspect ratio PDMS microchannels. Basically the valve consists of a main 14 mm-long SMA wire (the black wire) which is referred to as A-wire in the following. The Awire is coiled by a long aluminum wire (the grey wire) which is referred to as B-wire in the following. The B-wire works as an electrical heater to heat up the A-wire for actuation when needed. The two ends of the A-wire are welded with a 1 mm-long SMA wire (referred to as C-tip) and a 5 mm-long SMA wire (referred to as Dhandle) separately as shown in Fig. 1. Both the C-tip and D-handle are aligned perpendicularly to the A-wire and they are perpendicular to each other as well to help prevent the D-handle from moving when the C-tip is actuated to move. The entire structure is embedded in the PDMS substrate that contains the microchannel while keeping the A-wire aligned perpendicularly to the channel at the valve location. The D-handle is relatively large and difficult to move in the substrate due to large resistance serving as a position holder so that only one end of the A-wire (C-tip) moves when actuated. As shown in Fig. 1, there is a thin layer of SU-8 wrapping the whole C-tip which is designed to help close the microchannel because the surface of SU-8 layer is softer, smoother and less sticky to PDMS than SMA and thus is easier to move in cured PDMS. Details of the fabrication will be introduced later. When an electrical current is applied to the B-wire, joule heat generated within it will increase the temperature of itself and the coiled A-wire. When the temperature of the A-wire is higher than its transition temperature (i.e., 70 ◦ C), it will shrink in length pulling the C-tip and D-handle towards each other. Since the D-handle barely moves, the C-tip will move causing the motion of the channel that is close to the C-tip achieving the purpose of valving. This SMA valve can be designed as normally closed or normally open microvalve for high aspect ratio microchannels as shown in Fig. 2. For a normally open microvalve, the A-wire is placed on the top of the channel while the inner wall of the C-tip is close to one side of the channel walls. When the A-wire is thermally actuated and then shrinks, the C-tip will squeeze the microchannel to close the flow as shown in Fig. 2(a). For a normally closed microvalve, there is no overlap between the channel and the A-wire while the outer wall of the C-tip is touching one of the channel side walls. In this design, there is a SU-8 block placed inside the channel at the valve location to close flow in the normal stage. When the valve
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Fig. 4. Operation of normally open SMA valve without SU-8 layer. The channel width is 60 m.
Fig. 3. SMA wires were welded together by using a spot welding machine.
is actuated, the C-tip will pull the channel side wall away from the block to open flow, as shown in Fig. 2(b). When the electrical current is turned off, the generation of joule heating is stopped causing the SMA wire to cool down to room temperature and return to its normal length. The C-tip will move back to its origin position. 3. Materials and methods 3.1. Materials Sylgard 184 silicone elastomer base and curing agent (Dow Corning Corporation) were used to make PDMS chips. SU-8 2005 (Micro Chem) was used to fabricate the valve tip. SU-8-2075 (Micro Chem) was used to fabricate the straight channel. FlexinolTM , 0.015 in. diameter SMA wires with 70 ◦ C transition temperature were obtained from Dynalloy, Inc. The SMA wires were conditioned by the supplier Dynalloy and were provided in a prestrained state for the convenience of actuator construction (Precise details of the thermomechanical treatment during conditioning are unavailable, since it is proprietary to Dynalloy Inc.). Aluminum wire was extracted from an electrical wire, but covered by an electrical insulation layer to ensure safety and avoid short circuit. T type thermocouple TT-T-40-SLE (0.017 in. × 0.026 in.) was obtained from OMEGATM . 3.2. Fabrication To characterize the SMA microvalve, simple straight high aspect ratio channels were used. All the microchannels had the same channel height of 150 m. The width of the microchannel varied from 50 m to 60 m. The microfluidic chips were fabricated using standard soft lithography technology. Briefly SU-8 2075 was spincoated on a 4 in. silicon wafer at 1200 rpm. The coated 150 m-thick SU8 layer was then soft baked at 65 ◦ C for 5 min and then 95 ◦ C for 30 min. The straight channel was patterned onto the coated silicon wafer via UV light with an exposure dosage of 1000 mJ/cm2 through a single transparency mask. The exposed wafer was baked again at 65 ◦ C for 5 min and then at 95 ◦ C for 12 min. After cooling the wafer down to room temperature, the wafer was developed in SU-8 developer for 15 min which was used as the master for making PDMS channels. The A-wire was carefully coiled 50 loops of the B-wire without distance in between the loops. One end of A-wire was welded with C-tip and the other end was welded with the center of D-handle as shown in Fig. 1. Fig. 3 shows how the wires were welded together by using a spot welding machine. A 20× microscope was used to observe and control the welding. The C-tip was then grinded until
its length was only 1 mm long. The grinding was also performed under a 20× microscope by hands with great care in the similar way as welding. For better connection and easy of fabrication, the C-tip and D-handle were also made of the same material as the A-wire. When actuated, the A-wire, C-tip and D-wire were all heated up and then shrunk. However, the shrinkage of the C-tip is negligible due to its small length and the shrinkage of D-handle along its length will not affect the valve performance since it is welded with the A-wire at its center and it is placed far away from the channel to be controlled. At the beginning, the SMA-made C-tip was used directly to open or close microchannels, which worked well to operate as a normally open valve as shown in Fig. 4. However, this design has very low repeatability and reliability. Leakage was often observed in experiments. This is because the rough surface of the C-tip makes it difficult to move when embedded in cured PDMS. Therefore, an improvement was proposed aiming to address the leakage problem. To make the tip surface smoother and softer so that it can move when the valve is actuated, a SU-8 layer was used to wrap the tip. This structure did resolve the leakage problem almost completely (see no-leakage movie in supplemental information), however, it tends to deform the channel due to repeated heating and cooling required in fabrication which will be explained below. Therefore, precaution should be paid when using this structure for the flow which requires very accurate channel dimension throughout testing. Fig. 5 shows the procedure of fabricating the SU-8 layer. First, the C-tip is dipped into SU-8 2005 (Fig. 5(a)) and then the entire valve is placed on a silicon wafer with the bottom of the tip touching the wafer surface (Fig. 5(b)). The entire structure including the wafer and valve is baked in a vacuum oven at 65 ◦ C for 20 min and then 95 ◦ C for 30 min. Then it is left in room condition for 20 min before exposed to UV light at a dosage of 3000 mJ/cm2 to harden the SU8 layer (Fig. 5(c)). After UV exposure, the entire structure is baked again at 65 ◦ C for 10 min and then 95 ◦ C for 20 min in the oven. After removed from the silicon wafer, the SMA-made C-tip has a smooth layer of SU-8 2005 which allows the tip to move smoothly when actuated. Finally, the tip is polished using a grinder (Fig. 5(d)). To ensure the channel can be closed or opened completely, the C-tip end should be at the same level of the channel bottom wall so that the entire channel height can be covered and controlled by the C-tip. This has proven to be not trivial. The first attempt was making a PDMS chip integrated with a SMA valve at the same time by placing the valve in the desired valving location on the silicon wafer containing the channel feature. In this attempt, the C-tip end was made to touch the silicon surface and then standard casting procedures were followed. Little success was received from this attempt and leakage happened all the time because the valve floated away from the silicon wafer surface after PDMS prepolymer mixture was poured onto the wafer. As a result, the C-tip end is not aligned with the channel bottom wall and thus cannot close or open the channel completely. To solve the floating problem, a half-curing technique was introduced.
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Fig. 5. Fabrication procedures of the C-tip coated with SU-8: (a) dipping the SMA-made C-tip into SU-8 2005, (b) placing the valve on a silicon wafer with the bottom of the C-tip touching the wafer surface and then baking it at 65 ◦ C for 20 min and 95 ◦ C for 30 min, (c) undergoing UV exposure and post-exposure baking at 65 ◦ C for 10 min and at 95 ◦ C for 20 min, and (d) polishing the SU-8 surface using a grinder.
Fig. 6. Images of (a) normally closed and (b) normally open microvalve.
Basically, the chip with integrated SMA valve was fabricated in the following two steps. The first step is to ensure the C-tip end to be at the same level as the channel bottom wall after the chip is fabricated. To do so, a small drop of PDMS was first applied near the tip while the tip is touching the surface of the wafer containing the
channel feature and a small paper clip was placed on top of the SMA wire to avoid any possible floating due to this small drop of PDMS. The entire structure was put in a vacuum oven (ISO temp Vacuum Oven Model 280A) for 30 min to degas the PDMS, then baked at 80 ◦ C for 30 min to make the PDMS to be half-cured and finally left
Fig. 7. Open time (a) and close time (b) of the normally closed microvalve with different channel width and flow rate.
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Fig. 8. Influence of actuation current on (a) the open time for normally closed microvalve and (b) the close time for normally open microvalve.
in room temperature for 10 min to cool down. The paper clip was then removed leaving the silicon wafer with the valve where the C-tip end is fixed at the wafer surface. In the second step, 30 g of PDMS prepolymer mixture was poured onto the master to make the PDMS substrate with the designed high aspect ratio channel, however, the curing procedure was different for normally closed and normally open microvalves. To make normally open microvalves, the PDMS substrate was cured at room temperature for 72 h. Curing PDMS in room temperature was to avoid the motion of the SMA valve caused by the temperature change during the curing process at a relatively high temperature (i.e., 50 ◦ C) and then the cooling in room temperature after curing. This motion tends to deform the fabricated microchannel. This was not a serious concern for normally closed microvalves which will be explained below. To make a normally closed microvalve, the PDMS substrate was first fully cured in an oven at 50 ◦ C for 12 h and then a small drop of SU-8 was deposited into the microchannel at the location aligned with the C-tip of the valve to block the channel. This step was completed under microscope. Then the channel with liquid SU-8 drop was exposed to UV light (3000 mJ/cm2 ) to solidify the SU-8 block. As mentioned above, when the PDMS substrate is cured in oven and then cooled in room temperature, the SMA valve will shrink and then return to its original shape. This motion may cause deformation in the adjacent channel (like valving the channel). However, the region with possible deformation is filled with solidified SU-8 for normally closed valve and thus the deformation is not a serious concern. The cured PDMS substrate with the channel and embedded SMA valve was bonded with a glass slide via oxygen plasma treatment at 300 mTorr for 12 s (29.6 W, Harrick Scientific, Ithaca, NY, USA). Fig. 6 shows the image of normally closed microvalve and normally open microvalve taken through a microscope (Olympus GX71). The operation of the normally open microvalve can be visualized from video clip available from the supplemental information. The use of SU-8 layer to wrap the C-tip resolved the leakage problem successfully. However, it makes the fabrication more tedious and often causes the adjacent microchannel deformed to a certain extent due to repeated heating and cooling even extra attention is paid, as shown in Fig. 6. Since it fully solved the leakage problem, the valve with the C-tip wrapped by SU-8 was used for the rest of testing. 4. Results and discussions To test the performance of the valve, a syringe pump (Harvard Apparatus, PicoPlus) was used to pump DI water through the microchannel at a constant flow rate. To show the status of the flow, 1 mM fluorescent particles were used to visualize the flow. The electrical current applied at the beginning was 1.8 A and then dropped
to 1.5 A once the actuation occurs to prevent the microvalve from overheating. The actuation was observed through a microscope (Olympus GX71) during the experiments. Fig. 7 shows the open and close time of the normally closed microvalve with different channel width versus the flow rate at the open state (set by the syringe pump). The response time of the normally closed valve were evaluated at four flow rates which were 0.05 L/min, 1.0 L/min, 2.0 L/min and 4.0 L/min. Two sets of microchannels (50 m and 60 m in width) were chosen and both of them were 150 m high. There were two reasons choosing these two widths for testing. First, the exploration of SMA microvalves were motivated by the need of microvalves for controlling high aspect ratio channels in other projects which are often between 50 and 60 m in width. The second reason is due to the difficulty in fabricating SMA valves adjacent to small channels. Any deformation to the channels caused during the fabrication of the valve (C-tip coated with SU-8) will affect the testing significantly. Each experiment for either open time or close time of the microvalve was repeated five times and the results shown in Fig. 7 are the average value of five experimentally measured data. As shown in Fig. 7, when the flow rate increases, the open time decreases and the close time increases which is easy to understand because higher flow rates make the flow more difficult to close and easier to open. It can be seen that the channel width does not have significant influence on the open time for normally closed microvalves. This is because the working distance for both cases is almost the same, around 20 m (i.e., from 50 m to 70 m and from 60 m to 80 m). However, it does have more influence on the close time although the trend is similar for both channels for the tested flow rate range (0–4 L/min). There is a large discrepancy at the flow rate of 2 L/min which is more likely attributed by experimental uncertainties. The close time depends on the cooling time of the SMA actuator which is influenced by the surrounding PDMS, and experimental environment (i.e., room temperature or heat convection conditions near the chip). The actuation current influences the operation time as well. Fig. 8 shows the open time of the normally closed microvalve (Fig. 8(a)) and the close time of the normally open microvalve (Fig. 8(b)) versus the applied actuation current at the flow rate of 4 L/min. The highest flow rate was chosen among the tested flow rate range (0–4 L/min) for this testing in order to consider the worst scenario. The reported open or close time here could be lower when for lower flow rates. One can see that when the actuation current is increased, the response time is shorter. This is because the generated joule heat is increased with current which leads to larger shrinkage of the A-wire and shorter response time. In both cases, the current dropped to 1.5 A once the actuation starts. The close time.
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Fig. 9. Close time (a) and open time (b) of the normally open microvalve with different channel width and flow rate.
Normally open microvalves were also performed in two channel dimensions: 50 m and 60 m wide. Fig. 9 shows the close time and open time of the normally open microvalve. The performance of normally open microvalves was quite different from that of normally closed microvalves. Because the C-tip of the normally open microvalve has to move across the whole microchannel width to close the flow, the working distance for the microvalve to close the flow is the channel width. In addition, the flow is open as long as the C-tip detached from the other side wall of microchannel. The working distance for the microvalve to open the flow is much shorter compared with closing the flow. Therefore the open time (Fig. 9(b)) of the normally open valve is much shorter than the close time (Fig. 9(a)). As shown in Fig. 9, the shortest open time of the microvalve could be less than 2 s, while the close time could be longer than 10 s. For the same reason mentioned above, higher flow rates also delay the microvalve to close which results in longer close time and shorter open time for the normally open microvalve. The close time varied from 17.09 s to 49.67 s when the flow rate increased from 0.05 L/min to 4.0 L/min for 60 m-wide microchannels. Because the channel width affects the working distance of the microvalve, both the close time and open time increases when the channel width increases from 50 m to 60 m.
5. Conclusions This study explored the possibility of using SMA valves for high aspect ratio channels. Two aspect ratios were tested which were 3 (150 m high and 50 m wide) and 2.5 (150 m high and 60 m wide), respectively. The presented design that closes or opens microchannels from their side walls works well with these two channel dimensions. This design can be employed as a normally closed microvalve or a normally open microvalve, both of which were evaluated in this study. It was found that channel width has less influence on the response time of normally closed microvalve than normally open microvalve. Therefore, normally closed microvalve is more suitable for wider microchannels. The design of the presented microvalve is separated from the microchannel design and thus there is more flexibility in changing the valve design. Although the presented SMA microvalve works well for high aspect ratio channels, it tends to have a long response time due to the material property. The higher the actuation current is applied, the shorter the response time is. However, it is limited that how high the current can be applied because of slow heat dissipation through surrounding PDMS which may also affect the liquid temperature as well. It is recommended to drop the applied current after actuation starts to avoid overheating.
Acknowledgements The authors gratefully acknowledge the support of Natural Sciences and Engineering Research Council (NSERC) of Canada through a grant to Carolyn L. Ren and Ontario Ministry of Research and Innovation through the Early Research Award to Carolyn L. Ren.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.sna.2011.03.038.
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Biographies Lin Gui is a Professor of the Chinese Academy of Sciences, Beijing, China. He received a B.E. degree in Refrigeration and Cryogenics Engineering in July, 2000, from the
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Chongqing University, Chongqing, China. Then he received a Ph. D. degree in June, 2005, from Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing China. He was a post-doctoral Research Associate at University of Waterloo, Canada. His current research interests include microfluidics, bioheat and mass transfer, MEMS, as well as thermally enabled biotechnology and medical instrumentation. He is a chapter author of “Encyclopedia of Microfluidics and Nanofluidics”. He holds 7 China patents. Dr. Ren received her Ph.D. in Mechanical Engineering from the University of Toronto in 2004 and Master’s and Bachelor’s degrees in Thermal Engineering from Harbin Institute of Technology in 1995 and 1992, separately. She joined the University of Waterloo (UW) in May 2004 as an Assistant Professor and promoted to Associate Professor with tenure in July 2010. She has received many research awards in recognition of her research excellence since she started at UW, which include Research Excellence Award from UW in 2010, the prestigious Canada Research Chair in Lab-on-a-Chip Technology in 2009, and Early Research Award from the Ministry of Research and Innovation of Ontario in 2007.