Automatic droplet transportation on a plastic microfluidic device having wettability gradient surface Y. Nakashima, Y. Nakanishi, and T. Yasuda Citation: Review of Scientific Instruments 86, 015001 (2015); doi: 10.1063/1.4905530 View online: http://dx.doi.org/10.1063/1.4905530 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/86/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Spreading of droplet with insoluble surfactant on corrugated topography Phys. Fluids 26, 092103 (2014); 10.1063/1.4895064 Handling of artificial membranes using electrowetting-actuated droplets on a microfluidic device combined with integrated pA-measurements Biomicrofluidics 6, 012813 (2012); 10.1063/1.3665719 Spontaneous high-speed transport of subnanoliter water droplet on gradient nanotextured surfaces Appl. Phys. Lett. 95, 063108 (2009); 10.1063/1.3197574 Transport and solidification phenomena in molten microdroplet pileup J. Appl. Phys. 92, 1675 (2002); 10.1063/1.1492019 Electrowetting-based actuation of liquid droplets for microfluidic applications Appl. Phys. Lett. 77, 1725 (2000); 10.1063/1.1308534
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REVIEW OF SCIENTIFIC INSTRUMENTS 86, 015001 (2015)
Automatic droplet transportation on a plastic microfluidic device having wettability gradient surface Y. Nakashima,1,a) Y. Nakanishi,1 and T. Yasuda2
1
Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 096-8555, Japan 2 Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4 Hibikino, Kitakyushu 808-0196, Japan
(Received 23 June 2014; accepted 22 December 2014; published online 12 January 2015) This paper presents a microfluidic device that can automatically transport a droplet on a plastic plate. This device consists of a Cyclo Olefin Polymer (COP) plate and a SiO2 membrane and has wettability gradient surface. Lithographic patterns of hydrophilic SiO2 permitted wettability modification of a hydrophobic COP surface. A series of alternate hydrophobic and hydrophilic wedge-shaped patterns generated a required gradient in wettability. When we dropped a droplet on the wettability gradient surface, it moved along the wettability gradient due to an imbalance between surface tension forces acting on the opposite sides of the droplet edge. The droplet transportation test was carried out using water of 5 µl. As a result, we succeeded in automatically transporting the droplet on the SiO2/COP wettability gradient pattern. We also carried out droplet transportation in an enclosed microchannel for preventing droplet evaporation using DI (Deionized) water of 5 µl. In this case, the droplet was automatically transported by forming the wettability gradient pattern at the top and bottom in an enclosed microchannel without evaporation. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4905530]
I. INTRODUCTION
Droplet transportation techniques on the substrate are required for various droplet-based micro total analysis system (µTAS) applications such as medical diagnosis, drug discovery, and micro assay. Many techniques for droplet transportation have been developed using pneumatic pressure,1 dielectrophoresis,2,3 electro static,4,5 electrowetting-ondielectric,6–9 electroosmosis,10–12 thermal,13,14 optical controls of surface tension forces,15,16 etc. These devices require external power sources such as a pressure source or electric power supply for inducing a droplet motion. They also require tube connections and electric wiring between devices and power sources. The need of this peripheral equipment will greatly increase the overall dimension of a microfluidic system, and makes device setting and replacement very cumbersome and complicated. Moreover, tube connections will increase dead volume in a microfluidic system and need a large volume of redundant expensive sample reagents. This will result in increased system complexity and fabrication difficulty and acts as a huge limitation for applying them to medical diagnoses and environmental analyses, where an inexpensive and disposable device is a must. Previously, the authors and other researchers demonstrated simple techniques for automatically transporting a droplet on a wettability gradient surface.17,18 In these techniques, a droplet can be transported in a single direction along the wettability gradient without any power source or a)Author to whom correspondence should be addressed. Electronic mail:
[email protected]
tube connection. These techniques have been applied to a droplet dispensing device for drug discovery.19–22 The device presented in previous paper will satisfy the request of medical diagnoses and environmental analyses because it uses a very simple mechanism and does not require any power source. However, these presented devices were fabricated on Si substrate or quartz glass, and device components consisted of Au, self-assembled monolayer (SAM), fluoropolymer, etc. Because these materials are expensive, disposable, low cost microfluidic devices have not yet been fabricated. In this paper, we present an automatic droplet transportation device using a plastic microfluidic device made by a hydrophilic SiO2 layer and a hydrophobic plastic plate that is fabricated by low cost injection molding. We demonstrate the device fabrication and observe the transportation behaviour of a droplet on the fabricated device. The device presented in this paper can be applied to various disposable µTAS applications because it has a very simple mechanism, does not require any power source, and is made by inexpensive materials.
II. MATERIALS AND METHODS
A transparency substrate is required for various µTAS applications such as drug discovery, medical diagnosis, and microarray assay because the response of cells and the detection of reagents are measured by optical microscopes. In this research, the cyclo olefin polymer (COP) using injection molding, which allows for mass production and disposable use, is used as substrate material. COP is transparent, which is a chemically stable plastic, and its glass transition temperature (T g = 140 ◦C) is higher than PMMA and PP (Polypropy-
0034-6748/2015/86(1)/015001/6/$30.00 86, 015001-1 © 2015 AIP Publishing LLC This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.174.255.116 On: Wed, 11 Mar 2015 05:57:32
015001-2
Nakashima, Nakanishi, and Yasuda
Rev. Sci. Instrum. 86, 015001 (2015)
FIG. 2. Series of wedge-shaped patterns for wettability gradient. The base of the triangle is 100 µm and height of the triangle is 15 mm. FIG. 1. The contact angle of a DI water droplet on the COP surface. COP is a hydrophobic material. The contact angle between COP and DI water is about 100◦.
lene).23 Because the water absorption coefficient of COP is less than 0.01%, the surface stability is better than other plastic materials. Fig. 1 shows the contact angle of a DI water droplet on the COP surface. The COP is a hydrophobic material and the contact angle of a droplet on the COP surface was about 100◦. A series of alternate hydrophobic and hydrophilic wedgeshaped patterns is shown in Fig. 2. For application in µTAS devices, such as droplet dispensing devices, a droplet needs to be transported 5 mm or more. We previously demonstrated the 5-10 µl droplet transportation on a wettability gradient surface made of SiO2 and hydrophobic SAM on the Si substrate or quartz glass. When the h was constant and d got smaller, the droplet was not able to move along the wettability gradient surface because the creation of the wettability gradient became difficult. A droplet was successfully transported over 5 mm distance using d = 100 µm and h = 15 mm.20 For this reason, we designed the surface with a pattern of d = 100 µm and h = 15 mm. The 75 wedge-shaped patterns made of SiO2 were created on a COP plate. A droplet placed using micro pipette on the hydrophobic (COP) area of the device moves along the wettability gradient toward the hydrophilic (SiO2) area automatically. In this case, a driving force for droplet movement, F, that will act on a cross section of the droplet is given by dF = γLG(cosθ a − cosθ r)dy,
(1)
where γLG is the surface free energy of the liquid-vapor interface, dy is the thickness of a cross-sectional droplet, and θ a and θ r are contact angles in the front and rear edges of a droplet, respectively (Fig. 3).24 A net force, F, can be obtained by integrating Eq. (1) over the entire width of a droplet. This means that a difference of the contact angles created by a wettability gradient surface will generate a driving force for droplet transportation. The device is achieved by a very simple fabrication process (Fig. 4). First, the photoresist ZPN-1150 90cp (ZEON Corp.) was spin-coated and exposed on the COP plate (Fig. 4(a)). Next, hydrophilic SiO2 of 300 nm in thickness was deposited by sputtering (Fig. 4(b)). Finally, a hydrophilic surface was formed by lift-off technique (Fig. 4(c)). The series of alternate hydrophobic and hydrophilic wedge-shaped patterns as shown in Fig. 5 was made on the COP plate, in order to create a wettability gradient.
III. RESULTS AND DISCUSSION A. Droplet transportation on the COP plate
Droplets need to be transported 5 mm or more for application in µTAS devices. Judging by the results of previous work, we expected that transportation more than 5 mm distance was realizable, when using a 5 µl droplet. Thus, the droplet transportation test was carried out using 5 µl of DI water to demonstrate droplet transportation on a wettability gradient surface made by SiO2 and COP. The experimental setup of
FIG. 3. Automatic droplet transportation on a wettability gradient surface. A droplet is transported from the right side (COP area) to the left side (SiO2 area). This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.174.255.116 On: Wed, 11 Mar 2015 05:57:32
015001-3
Nakashima, Nakanishi, and Yasuda
Rev. Sci. Instrum. 86, 015001 (2015)
FIG. 4. Fabrication of the droplet transportation device. (a) The photoresist is patterned by spin-coating on the COP plate. (b) The SiO2 layer is created by sputtering. (c) The SiO2 is patterned by lift-off process.
the droplet transportation is shown in Fig. 6. The motion of a droplet on the gradient surface (d = 100 µm, h = 15 mm) is observed using a CCD camera. Fig. 7 shows the time-lapse image of the automatic droplet transportation. Note that the shapes like ladder have no impact on the droplet transportation test because its pattern is reflected in the background of the COP plate. When the droplet was dropped on the hydrophobic area by a micro pipette (Fig. 7(a)), the droplet was immediately set into motion toward the hydrophilic area (Fig. 7(b)). During the transportation, the droplet became long and thin (Fig. 7(c)). A time history of droplet positions was plotted in Fig. 8. The time is defined as 0 s when the droplet makes contact with the device surface. Also, the rear edge position of the droplet is defined as the origin position. The droplet center is the average of the front and rear edge coordinates. The front edge of the droplet was quickly moved toward the hydrophilic surface when the droplet makes contact with the device surface, and the droplet was transported with constant velocity until the end of the wettability gradient pattern. The droplet was transported in about 400 ms to the end point of the wettability gradient pattern from the start point. On the other hand, the rear edge of droplet was not moved until 150 ms and
FIG. 6. Experimental set up. The motion of the droplet is observed by CCD camera and recorded by a HDD (Hard disk) recorder.
was gradually moved after a lapse of 150 ms. When the front edge of the droplet reached the end of the wettability gradient pattern, the rear edge of the droplet was immediately accelerated and reached the end of wettability gradient pattern. This means that the driving force which acts on a front edge of droplet is larger than it, which acts on a rear edge of droplet. Moreover, the transportation distance of the proposed technique and previous work,20 in the case of experiments using a 5 µl droplet, was compared to verify the effectiveness of transportation performance. A transportation distance was defined as the length from the starting point to the end of the hydrophobic patterns (Fig. 9). As a result, a droplet was transported 8.7 mm. On the other hand, droplets of the same volume in the previous work were transported 6.0 mm. These results mean that the proposed technique is more advantageous when comparing transportation distance and cost with previous work. B. Automatic droplet transportation in an enclosed microchannel
We demonstrated automatic droplet transportation without Au, Si, and fluoropolymer by using SiO2 patterning on a COP surface. This device will be an effective low cost µTAS device. However, a droplet is quickly vaporized by atmosphere because of the large droplet surface area as against droplet volume. This is a disadvantage for use in µTAS applications. Therefore, a droplet transportation test intended to prevent droplet evaporation is carried out by using a newly fabricated plastic microfluidic device that is an enclosed microchannel having a wettability gradient pattern at top and bottom surfaces. The schematic of a new plastic microfluidic device is shown in Fig. 10. The enclosed microchannel consists of three parts of the following: two COP plates having wettability gradient pattern, and a pair of PDMS (Polydimethylsiloxane) spacers. These parts are bonded with each other by the selfadhesiveness of PDMS. When the droplet is injected into
FIG. 5. Photograph of the wedge-shaped pattern created on a COP plate surface. The right side is hydrophobic area (COP area) and the left side is hydrophilic area (SiO2 area). This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:
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015001-4
Nakashima, Nakanishi, and Yasuda
Rev. Sci. Instrum. 86, 015001 (2015)
FIG. 7. Experiment of automatic droplet transportation on the SiO2/COP wettability gradient pattern. (a) The droplet was dropped on the hydrophobic area. (b)-(d) The droplet was automatically transported from the right side to the left side on the wettability gradient surface.
the microchannel by a micro pipette, a droplet is transported toward the hydrophilic surface by driving force received from top and bottom surfaces. The base of triangle is 100 µm and the height of the triangle is 15 mm. The result of the automatic droplet transportation test in an enclosed channel is shown in Fig. 11. Note that the shapes like ladder have no impact on the droplet transportation test
because their pattern is reflected in the background of the COP plate. When the droplet of 5 µl was injected into the entrance of the microchannel (Fig. 11(a)), the droplet was flattened at the top and bottom surfaces, and became a horizontally long shape (Fig. 11(b)). The horizontally long droplet moved along the channel (Fig. 11(c)) and was automatically transported to the end point of the wettability gradient pattern (Fig. 11(d)).
FIG. 8. Time history of the front edge, center, and rear edge positions of a droplet. The front edge of the droplet was transported with constant velocity until the end of the wettability gradient pattern. The rear edge of the droplet was gradually moved during 200-400 ms and was accelerated after 400 ms. FIG. 9. Definition of transportation distance of 5 µl droplet. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:
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015001-5
Nakashima, Nakanishi, and Yasuda
Rev. Sci. Instrum. 86, 015001 (2015)
FIG. 10. Schematic of a plastic microfluidic device for automatic droplet transportation in an enclosed region. (a) Each layer constitutes the enclosed microchannel. The PDMS spacer connects the top and bottom COP plate having the wettability gradient surface. (b) Top view of the enclosed microchannel. (c) Close up view of the enclosed microchannel.
FIG. 11. Experiment of automatic droplet transportation in an enclosed region. (a) The droplet is injected into the enclosed microchannel by a pipette. (b) The injected droplet was formed horizontally long by being flattened from the top and bottom plates. (c) and (d) The droplet was automatically transported by wettability gradient surface at top and bottom surfaces. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:
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015001-6
Nakashima, Nakanishi, and Yasuda
Rev. Sci. Instrum. 86, 015001 (2015)
about 12 mm in 8 s. This device is effective for droplet transportation method of disposable medical diagnostic devices and drug discovery devices. Because the presented device does not require any external power source such as a pneumatic pressure source or electronic power supply, it will be easy to set and replace devices and to miniaturize the total system including peripheral equipment. In the near future, we will discuss the details of the relationship between the various droplet volumes and distance/velocity of transportation. 1K.
FIG. 12. Time history of the front edge, center, and rear edge positions of a droplet in an enclosed channel. The front and rear edges were moved at the uniform velocity at about 7 s. When the rear edge of the droplet begins to accelerate, the front edge of droplet was gradually accelerating.
A time history of the droplet positions was plotted in Fig. 12. The time is defined as 0 s when the droplet makes contact with the device surface. Also, the rear edge position of the droplet is defined as the origin position. The droplet center is the average of the front and rear edge coordinates. The front edge of droplet gradually moved faster than the rear edge toward the hydrophilic surface at constant velocity, during about 7 s. The rear edge of droplet moved very little until 4.5 s, gradually moved after about 5 s, and was finally transported with constant velocity. When the rear edge of the droplet began to accelerate at 7.5 s, the front edge of droplet gradually accelerated and reached the end of the wettability gradient pattern. The velocity of droplet transportation in the enclosed channel was slower than on the plane surface. This means that the drive of front edge is prevented by the rear edge because the surface area of rear edge is in a hydrophobic area and a driving force did not work on the rear edge. After the rear edge was dragged by the front edge, the rear edge obtained the driving force through the wettability gradient surface and accelerated rapidly.
IV. CONCLUSION
We demonstrated the automatic droplet transportation on the SiO2/COP wettability gradient pattern without external power sources, tube connecting, and expensive materials. The device fabrication is very simple and allows for mass production and disposable use. The presented device can transport a droplet of 5 µl about 8.7 mm at 600 ms. This result shows that the presented device is more effective than previous work for droplet transportation. We also demonstrated automatic droplet transportation in an enclosed channel for preventing the droplet evaporation. The wettability gradient pattern was fabricated on the top and bottom surface of the enclosed channel. A droplet in the enclosed channel can be transported
Hosokawa, T. Fujii, and I. Endo, in Proceedings of the IEEE MEMS 1999, Orland, FL, 1999, edited by K. J. Gabriel and K. Najafi (The Institute of Electrical and Electronics Engineers, Inc., Piscataway, NJ, 1999), p. 388. 2M. Gunji, T. B. Jones, and M. Washizu, in Proceedings of the IEEE MEMS 2001, Interlaken, Switzerland, 2001, edited by H. Baltes and S. Bouwstra (The Institute of Electrical and Electronics Engineers, Inc., Piscataway, NJ, 2001), p. 385. 3T. Taniguchi, T. Torii, and T. Higuchi, Lab Chip 2(1), 19 (2002). 4N. N. Jetha and A. Marziali, BioTechniques 40, 148 (2006). 5W.-K. Choi, E. Lebrasseur, M. I. Al-Haq, H. Tsuchiya, T. Torii, H. Yamazaki, E. Shinohara, and T. Higuchi, Sens. Actuators, A 136, 484 (2007). 6K. Imamura and T. Yasuda, IEEJ Trans. Sens. Micromach. 130(1), 1 (2010). 7C. G. Cooney, C. Y. Chen, M. R. Emerling, A. Nadim, and J. D. Sterling, Microfluid. Nanofluid. 2(5), 435 (2006). 8J. Lee, H. Moon, J. Fowler, T. Schoellhammer, and C. J. Kim, Sens. Actuators, A 95, 259 (2002). 9S. K. Cho, H. Moon, and C. J. Kim, J. Microelectromech. Syst. 12(1), 70 (2003). 10G. R. Eykholt, J. Hazard. Mater. 55, 171 (1997). 11D. Kim, J. D. Posner, and J. G. Santiago, Sens. Actuators, A 141, 201 (2007). 12M. Yairi and C. Richter, Sens. Actuators, A 137, 350 (2007). 13K. Takahashi, K. Yoshino, K. Nagayama, and T. Asano, in Proceedings of the 4th JSME-KSME Thermal Engineering Conference, Kobe, Japan, 2000, edited by T. Nakajima and H. D. Shin (The Japan Society of Mechanical Engineers, Shinjuku, Tokyo, 2000), p. 367. 14C.-C. Hong, S. Murugesan, S. Kim, G. Beaucage, J.-W. Choi, and C. H. Ahn, Lab Chip 3, 281 (2003). 15S. K. Cho, S.-K. Fan, H. Moon, and C.-J. Kim, in Proceedings of the IEEE MEMS 2002, Las Vegas, NV, 2002, edited by Y. B. Gianchandani and Y.-C. Tai (The Institute of Electrical and Electronics Engineers, Inc., Piscataway, NJ, 2001), p. 32. 16K. Ichimura, S.-K. Oh, and M. Nakagawa, Science 288, 1624 (2000). 17T. Yasuda, K. Suzuki, and I. Shimoyama, in Proceedings of the Micro Total Analysis Systems Symposium 2003, Squaw Valley, CA, 2003, edited by M. Allen Northrup, K. F. Jensen, and D. J. Harrison (The Printing House, Inc., Stoughton, WI, 2003), p. 1129. 18T. Yasuda and S. Harada, in Proceedings of the Micro Total Analysis Systems Symposium 2008, San Diego, CA, 2008, edited by L. E. Locascio, M. Gaitan, B. M. Paegel, D. J. Ross, and W. N. Vreeland (The Printing House, Inc., Stoughton, WI, 2008), p. 1396. 19H. S. Khoo and F.-G. Tseng, in Proceedings of 2006 International Conference on Microtechnologies in Medicine and Biology, Okinawa, Japan, 2006 (IEEE, Piscataway, NJ, 2006), p. 273. 20T. Yasuda, S. Harada, and K. Daimon, IEEJ Trans. Sens. Micromach. 128(3), 75 (2008). 21Y. Nakashima and T. Yasuda, in Proceedings of the Micro Total Analysis Systems Symposium 2009, Jeju, Korea, 2009, edited by T. S. Kim, Y.-S. Lee, T.-D. Chung, N. L. Jeon, S.-H. Lee, K.-Y. Suh, J. Choo, and Y.-K. Kim (The Printing House, Inc., Stoughton, WI, 2009), p. 725. 22T. Yasuda, J. Nakamura, K. Nakayama, and M. Yamanaka, in Proceedings of the Micro Total Analysis Systems Symposium 2012, Okinawa, Japan, 2012, edited by T. Fujii, A. Hibara, S. Takeuchi, and T. Fukuba (The Printing House, Inc., Stoughton, WI, 2012), p. 1456. 23M. Yamazaki, J. Mol. Catal. A: Chem. 213, 81 (2004). 24M. K. Chaudhury and G. M. Whitesides, Science 256, 1535 (1992).
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.174.255.116 On: Wed, 11 Mar 2015 05:57:32
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.174.255.116 On: Wed, 11 Mar 2015 05:57:32
REVIEW OF SCIENTIFIC INSTRUMENTS 86, 015001 (2015)
Automatic droplet transportation on a plastic microfluidic device having wettability gradient surface Y. Nakashima,1,a) Y. Nakanishi,1 and T. Yasuda2
1
Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 096-8555, Japan 2 Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4 Hibikino, Kitakyushu 808-0196, Japan
(Received 23 June 2014; accepted 22 December 2014; published online 12 January 2015) This paper presents a microfluidic device that can automatically transport a droplet on a plastic plate. This device consists of a Cyclo Olefin Polymer (COP) plate and a SiO2 membrane and has wettability gradient surface. Lithographic patterns of hydrophilic SiO2 permitted wettability modification of a hydrophobic COP surface. A series of alternate hydrophobic and hydrophilic wedge-shaped patterns generated a required gradient in wettability. When we dropped a droplet on the wettability gradient surface, it moved along the wettability gradient due to an imbalance between surface tension forces acting on the opposite sides of the droplet edge. The droplet transportation test was carried out using water of 5 µl. As a result, we succeeded in automatically transporting the droplet on the SiO2/COP wettability gradient pattern. We also carried out droplet transportation in an enclosed microchannel for preventing droplet evaporation using DI (Deionized) water of 5 µl. In this case, the droplet was automatically transported by forming the wettability gradient pattern at the top and bottom in an enclosed microchannel without evaporation. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4905530]
I. INTRODUCTION
Droplet transportation techniques on the substrate are required for various droplet-based micro total analysis system (µTAS) applications such as medical diagnosis, drug discovery, and micro assay. Many techniques for droplet transportation have been developed using pneumatic pressure,1 dielectrophoresis,2,3 electro static,4,5 electrowetting-ondielectric,6–9 electroosmosis,10–12 thermal,13,14 optical controls of surface tension forces,15,16 etc. These devices require external power sources such as a pressure source or electric power supply for inducing a droplet motion. They also require tube connections and electric wiring between devices and power sources. The need of this peripheral equipment will greatly increase the overall dimension of a microfluidic system, and makes device setting and replacement very cumbersome and complicated. Moreover, tube connections will increase dead volume in a microfluidic system and need a large volume of redundant expensive sample reagents. This will result in increased system complexity and fabrication difficulty and acts as a huge limitation for applying them to medical diagnoses and environmental analyses, where an inexpensive and disposable device is a must. Previously, the authors and other researchers demonstrated simple techniques for automatically transporting a droplet on a wettability gradient surface.17,18 In these techniques, a droplet can be transported in a single direction along the wettability gradient without any power source or a)Author to whom correspondence should be addressed. Electronic mail:
[email protected]
tube connection. These techniques have been applied to a droplet dispensing device for drug discovery.19–22 The device presented in previous paper will satisfy the request of medical diagnoses and environmental analyses because it uses a very simple mechanism and does not require any power source. However, these presented devices were fabricated on Si substrate or quartz glass, and device components consisted of Au, self-assembled monolayer (SAM), fluoropolymer, etc. Because these materials are expensive, disposable, low cost microfluidic devices have not yet been fabricated. In this paper, we present an automatic droplet transportation device using a plastic microfluidic device made by a hydrophilic SiO2 layer and a hydrophobic plastic plate that is fabricated by low cost injection molding. We demonstrate the device fabrication and observe the transportation behaviour of a droplet on the fabricated device. The device presented in this paper can be applied to various disposable µTAS applications because it has a very simple mechanism, does not require any power source, and is made by inexpensive materials.
II. MATERIALS AND METHODS
A transparency substrate is required for various µTAS applications such as drug discovery, medical diagnosis, and microarray assay because the response of cells and the detection of reagents are measured by optical microscopes. In this research, the cyclo olefin polymer (COP) using injection molding, which allows for mass production and disposable use, is used as substrate material. COP is transparent, which is a chemically stable plastic, and its glass transition temperature (T g = 140 ◦C) is higher than PMMA and PP (Polypropy-
0034-6748/2015/86(1)/015001/6/$30.00 86, 015001-1 © 2015 AIP Publishing LLC This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.174.255.116 On: Wed, 11 Mar 2015 05:57:32
015001-2
Nakashima, Nakanishi, and Yasuda
Rev. Sci. Instrum. 86, 015001 (2015)
FIG. 2. Series of wedge-shaped patterns for wettability gradient. The base of the triangle is 100 µm and height of the triangle is 15 mm. FIG. 1. The contact angle of a DI water droplet on the COP surface. COP is a hydrophobic material. The contact angle between COP and DI water is about 100◦.
lene).23 Because the water absorption coefficient of COP is less than 0.01%, the surface stability is better than other plastic materials. Fig. 1 shows the contact angle of a DI water droplet on the COP surface. The COP is a hydrophobic material and the contact angle of a droplet on the COP surface was about 100◦. A series of alternate hydrophobic and hydrophilic wedgeshaped patterns is shown in Fig. 2. For application in µTAS devices, such as droplet dispensing devices, a droplet needs to be transported 5 mm or more. We previously demonstrated the 5-10 µl droplet transportation on a wettability gradient surface made of SiO2 and hydrophobic SAM on the Si substrate or quartz glass. When the h was constant and d got smaller, the droplet was not able to move along the wettability gradient surface because the creation of the wettability gradient became difficult. A droplet was successfully transported over 5 mm distance using d = 100 µm and h = 15 mm.20 For this reason, we designed the surface with a pattern of d = 100 µm and h = 15 mm. The 75 wedge-shaped patterns made of SiO2 were created on a COP plate. A droplet placed using micro pipette on the hydrophobic (COP) area of the device moves along the wettability gradient toward the hydrophilic (SiO2) area automatically. In this case, a driving force for droplet movement, F, that will act on a cross section of the droplet is given by dF = γLG(cosθ a − cosθ r)dy,
(1)
where γLG is the surface free energy of the liquid-vapor interface, dy is the thickness of a cross-sectional droplet, and θ a and θ r are contact angles in the front and rear edges of a droplet, respectively (Fig. 3).24 A net force, F, can be obtained by integrating Eq. (1) over the entire width of a droplet. This means that a difference of the contact angles created by a wettability gradient surface will generate a driving force for droplet transportation. The device is achieved by a very simple fabrication process (Fig. 4). First, the photoresist ZPN-1150 90cp (ZEON Corp.) was spin-coated and exposed on the COP plate (Fig. 4(a)). Next, hydrophilic SiO2 of 300 nm in thickness was deposited by sputtering (Fig. 4(b)). Finally, a hydrophilic surface was formed by lift-off technique (Fig. 4(c)). The series of alternate hydrophobic and hydrophilic wedge-shaped patterns as shown in Fig. 5 was made on the COP plate, in order to create a wettability gradient.
III. RESULTS AND DISCUSSION A. Droplet transportation on the COP plate
Droplets need to be transported 5 mm or more for application in µTAS devices. Judging by the results of previous work, we expected that transportation more than 5 mm distance was realizable, when using a 5 µl droplet. Thus, the droplet transportation test was carried out using 5 µl of DI water to demonstrate droplet transportation on a wettability gradient surface made by SiO2 and COP. The experimental setup of
FIG. 3. Automatic droplet transportation on a wettability gradient surface. A droplet is transported from the right side (COP area) to the left side (SiO2 area). This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.174.255.116 On: Wed, 11 Mar 2015 05:57:32
015001-3
Nakashima, Nakanishi, and Yasuda
Rev. Sci. Instrum. 86, 015001 (2015)
FIG. 4. Fabrication of the droplet transportation device. (a) The photoresist is patterned by spin-coating on the COP plate. (b) The SiO2 layer is created by sputtering. (c) The SiO2 is patterned by lift-off process.
the droplet transportation is shown in Fig. 6. The motion of a droplet on the gradient surface (d = 100 µm, h = 15 mm) is observed using a CCD camera. Fig. 7 shows the time-lapse image of the automatic droplet transportation. Note that the shapes like ladder have no impact on the droplet transportation test because its pattern is reflected in the background of the COP plate. When the droplet was dropped on the hydrophobic area by a micro pipette (Fig. 7(a)), the droplet was immediately set into motion toward the hydrophilic area (Fig. 7(b)). During the transportation, the droplet became long and thin (Fig. 7(c)). A time history of droplet positions was plotted in Fig. 8. The time is defined as 0 s when the droplet makes contact with the device surface. Also, the rear edge position of the droplet is defined as the origin position. The droplet center is the average of the front and rear edge coordinates. The front edge of the droplet was quickly moved toward the hydrophilic surface when the droplet makes contact with the device surface, and the droplet was transported with constant velocity until the end of the wettability gradient pattern. The droplet was transported in about 400 ms to the end point of the wettability gradient pattern from the start point. On the other hand, the rear edge of droplet was not moved until 150 ms and
FIG. 6. Experimental set up. The motion of the droplet is observed by CCD camera and recorded by a HDD (Hard disk) recorder.
was gradually moved after a lapse of 150 ms. When the front edge of the droplet reached the end of the wettability gradient pattern, the rear edge of the droplet was immediately accelerated and reached the end of wettability gradient pattern. This means that the driving force which acts on a front edge of droplet is larger than it, which acts on a rear edge of droplet. Moreover, the transportation distance of the proposed technique and previous work,20 in the case of experiments using a 5 µl droplet, was compared to verify the effectiveness of transportation performance. A transportation distance was defined as the length from the starting point to the end of the hydrophobic patterns (Fig. 9). As a result, a droplet was transported 8.7 mm. On the other hand, droplets of the same volume in the previous work were transported 6.0 mm. These results mean that the proposed technique is more advantageous when comparing transportation distance and cost with previous work. B. Automatic droplet transportation in an enclosed microchannel
We demonstrated automatic droplet transportation without Au, Si, and fluoropolymer by using SiO2 patterning on a COP surface. This device will be an effective low cost µTAS device. However, a droplet is quickly vaporized by atmosphere because of the large droplet surface area as against droplet volume. This is a disadvantage for use in µTAS applications. Therefore, a droplet transportation test intended to prevent droplet evaporation is carried out by using a newly fabricated plastic microfluidic device that is an enclosed microchannel having a wettability gradient pattern at top and bottom surfaces. The schematic of a new plastic microfluidic device is shown in Fig. 10. The enclosed microchannel consists of three parts of the following: two COP plates having wettability gradient pattern, and a pair of PDMS (Polydimethylsiloxane) spacers. These parts are bonded with each other by the selfadhesiveness of PDMS. When the droplet is injected into
FIG. 5. Photograph of the wedge-shaped pattern created on a COP plate surface. The right side is hydrophobic area (COP area) and the left side is hydrophilic area (SiO2 area). This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:
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Rev. Sci. Instrum. 86, 015001 (2015)
FIG. 7. Experiment of automatic droplet transportation on the SiO2/COP wettability gradient pattern. (a) The droplet was dropped on the hydrophobic area. (b)-(d) The droplet was automatically transported from the right side to the left side on the wettability gradient surface.
the microchannel by a micro pipette, a droplet is transported toward the hydrophilic surface by driving force received from top and bottom surfaces. The base of triangle is 100 µm and the height of the triangle is 15 mm. The result of the automatic droplet transportation test in an enclosed channel is shown in Fig. 11. Note that the shapes like ladder have no impact on the droplet transportation test
because their pattern is reflected in the background of the COP plate. When the droplet of 5 µl was injected into the entrance of the microchannel (Fig. 11(a)), the droplet was flattened at the top and bottom surfaces, and became a horizontally long shape (Fig. 11(b)). The horizontally long droplet moved along the channel (Fig. 11(c)) and was automatically transported to the end point of the wettability gradient pattern (Fig. 11(d)).
FIG. 8. Time history of the front edge, center, and rear edge positions of a droplet. The front edge of the droplet was transported with constant velocity until the end of the wettability gradient pattern. The rear edge of the droplet was gradually moved during 200-400 ms and was accelerated after 400 ms. FIG. 9. Definition of transportation distance of 5 µl droplet. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:
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Nakashima, Nakanishi, and Yasuda
Rev. Sci. Instrum. 86, 015001 (2015)
FIG. 10. Schematic of a plastic microfluidic device for automatic droplet transportation in an enclosed region. (a) Each layer constitutes the enclosed microchannel. The PDMS spacer connects the top and bottom COP plate having the wettability gradient surface. (b) Top view of the enclosed microchannel. (c) Close up view of the enclosed microchannel.
FIG. 11. Experiment of automatic droplet transportation in an enclosed region. (a) The droplet is injected into the enclosed microchannel by a pipette. (b) The injected droplet was formed horizontally long by being flattened from the top and bottom plates. (c) and (d) The droplet was automatically transported by wettability gradient surface at top and bottom surfaces. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:
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Nakashima, Nakanishi, and Yasuda
Rev. Sci. Instrum. 86, 015001 (2015)
about 12 mm in 8 s. This device is effective for droplet transportation method of disposable medical diagnostic devices and drug discovery devices. Because the presented device does not require any external power source such as a pneumatic pressure source or electronic power supply, it will be easy to set and replace devices and to miniaturize the total system including peripheral equipment. In the near future, we will discuss the details of the relationship between the various droplet volumes and distance/velocity of transportation. 1K.
FIG. 12. Time history of the front edge, center, and rear edge positions of a droplet in an enclosed channel. The front and rear edges were moved at the uniform velocity at about 7 s. When the rear edge of the droplet begins to accelerate, the front edge of droplet was gradually accelerating.
A time history of the droplet positions was plotted in Fig. 12. The time is defined as 0 s when the droplet makes contact with the device surface. Also, the rear edge position of the droplet is defined as the origin position. The droplet center is the average of the front and rear edge coordinates. The front edge of droplet gradually moved faster than the rear edge toward the hydrophilic surface at constant velocity, during about 7 s. The rear edge of droplet moved very little until 4.5 s, gradually moved after about 5 s, and was finally transported with constant velocity. When the rear edge of the droplet began to accelerate at 7.5 s, the front edge of droplet gradually accelerated and reached the end of the wettability gradient pattern. The velocity of droplet transportation in the enclosed channel was slower than on the plane surface. This means that the drive of front edge is prevented by the rear edge because the surface area of rear edge is in a hydrophobic area and a driving force did not work on the rear edge. After the rear edge was dragged by the front edge, the rear edge obtained the driving force through the wettability gradient surface and accelerated rapidly.
IV. CONCLUSION
We demonstrated the automatic droplet transportation on the SiO2/COP wettability gradient pattern without external power sources, tube connecting, and expensive materials. The device fabrication is very simple and allows for mass production and disposable use. The presented device can transport a droplet of 5 µl about 8.7 mm at 600 ms. This result shows that the presented device is more effective than previous work for droplet transportation. We also demonstrated automatic droplet transportation in an enclosed channel for preventing the droplet evaporation. The wettability gradient pattern was fabricated on the top and bottom surface of the enclosed channel. A droplet in the enclosed channel can be transported
Hosokawa, T. Fujii, and I. Endo, in Proceedings of the IEEE MEMS 1999, Orland, FL, 1999, edited by K. J. Gabriel and K. Najafi (The Institute of Electrical and Electronics Engineers, Inc., Piscataway, NJ, 1999), p. 388. 2M. Gunji, T. B. Jones, and M. Washizu, in Proceedings of the IEEE MEMS 2001, Interlaken, Switzerland, 2001, edited by H. Baltes and S. Bouwstra (The Institute of Electrical and Electronics Engineers, Inc., Piscataway, NJ, 2001), p. 385. 3T. Taniguchi, T. Torii, and T. Higuchi, Lab Chip 2(1), 19 (2002). 4N. N. Jetha and A. Marziali, BioTechniques 40, 148 (2006). 5W.-K. Choi, E. Lebrasseur, M. I. Al-Haq, H. Tsuchiya, T. Torii, H. Yamazaki, E. Shinohara, and T. Higuchi, Sens. Actuators, A 136, 484 (2007). 6K. Imamura and T. Yasuda, IEEJ Trans. Sens. Micromach. 130(1), 1 (2010). 7C. G. Cooney, C. Y. Chen, M. R. Emerling, A. Nadim, and J. D. Sterling, Microfluid. Nanofluid. 2(5), 435 (2006). 8J. Lee, H. Moon, J. Fowler, T. Schoellhammer, and C. J. Kim, Sens. Actuators, A 95, 259 (2002). 9S. K. Cho, H. Moon, and C. J. Kim, J. Microelectromech. Syst. 12(1), 70 (2003). 10G. R. Eykholt, J. Hazard. Mater. 55, 171 (1997). 11D. Kim, J. D. Posner, and J. G. Santiago, Sens. Actuators, A 141, 201 (2007). 12M. Yairi and C. Richter, Sens. Actuators, A 137, 350 (2007). 13K. Takahashi, K. Yoshino, K. Nagayama, and T. Asano, in Proceedings of the 4th JSME-KSME Thermal Engineering Conference, Kobe, Japan, 2000, edited by T. Nakajima and H. D. Shin (The Japan Society of Mechanical Engineers, Shinjuku, Tokyo, 2000), p. 367. 14C.-C. Hong, S. Murugesan, S. Kim, G. Beaucage, J.-W. Choi, and C. H. Ahn, Lab Chip 3, 281 (2003). 15S. K. Cho, S.-K. Fan, H. Moon, and C.-J. Kim, in Proceedings of the IEEE MEMS 2002, Las Vegas, NV, 2002, edited by Y. B. Gianchandani and Y.-C. Tai (The Institute of Electrical and Electronics Engineers, Inc., Piscataway, NJ, 2001), p. 32. 16K. Ichimura, S.-K. Oh, and M. Nakagawa, Science 288, 1624 (2000). 17T. Yasuda, K. Suzuki, and I. Shimoyama, in Proceedings of the Micro Total Analysis Systems Symposium 2003, Squaw Valley, CA, 2003, edited by M. Allen Northrup, K. F. Jensen, and D. J. Harrison (The Printing House, Inc., Stoughton, WI, 2003), p. 1129. 18T. Yasuda and S. Harada, in Proceedings of the Micro Total Analysis Systems Symposium 2008, San Diego, CA, 2008, edited by L. E. Locascio, M. Gaitan, B. M. Paegel, D. J. Ross, and W. N. Vreeland (The Printing House, Inc., Stoughton, WI, 2008), p. 1396. 19H. S. Khoo and F.-G. Tseng, in Proceedings of 2006 International Conference on Microtechnologies in Medicine and Biology, Okinawa, Japan, 2006 (IEEE, Piscataway, NJ, 2006), p. 273. 20T. Yasuda, S. Harada, and K. Daimon, IEEJ Trans. Sens. Micromach. 128(3), 75 (2008). 21Y. Nakashima and T. Yasuda, in Proceedings of the Micro Total Analysis Systems Symposium 2009, Jeju, Korea, 2009, edited by T. S. Kim, Y.-S. Lee, T.-D. Chung, N. L. Jeon, S.-H. Lee, K.-Y. Suh, J. Choo, and Y.-K. Kim (The Printing House, Inc., Stoughton, WI, 2009), p. 725. 22T. Yasuda, J. Nakamura, K. Nakayama, and M. Yamanaka, in Proceedings of the Micro Total Analysis Systems Symposium 2012, Okinawa, Japan, 2012, edited by T. Fujii, A. Hibara, S. Takeuchi, and T. Fukuba (The Printing House, Inc., Stoughton, WI, 2012), p. 1456. 23M. Yamazaki, J. Mol. Catal. A: Chem. 213, 81 (2004). 24M. K. Chaudhury and G. M. Whitesides, Science 256, 1535 (1992).
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