- Email: [email protected]
On-chip micromanipulation by AC-EWOD driven twin bubbles
Sensors and Actuators A 195 (2013) 167–174
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
On-chip micromanipulation by AC-EWOD driven twin bubbles Jeong Hyun Lee, Kyung Ho Lee, Jeong Byung Chae, Kyehan Rhee, Sang Kug Chung ∗ Department of Mechanical Engineering, Myongji University, Yongin 449-728, South Korea
a r t i c l e
i n f o
Article history: Received 14 April 2012 Received in revised form 19 July 2012 Accepted 19 July 2012 Available online 27 July 2012 Keywords: Microfluidics Alternating current electrowetting-on-dielectric (AC-EWOD) Microbubbles Microstreaming
a b s t r a c t A novel on-chip micromanipulation method has been developed by which the streaming generated by oscillating twin bubbles actuated by alternating current electrowetting-on-dielectric (AC-EWOD) was used to manipulate micro/bio-objects in an aqueous medium. First, the behavior of a bubble placed on a plain EWOD electrode submerged in a water chamber was investigated under different AC-EWOD actuation conditions such as various frequencies and voltages using high-speed images. The results showed that the bubble oscillation amplitude highly depended on the applied frequency and was proportional to the strength of the bubble-induced streaming. To investigate the effect of the streaming, the forces generated by the oscillating bubble-induced streaming were indirectly measured using Stokes’ drag approximation under the different AC-EWOD actuation conditions at various distances between the bubble and a fish egg. The maximum force was calculated as 9 N. To improve the controllability for manipulating objects, a system of twin bubbles was proposed, and twin-bubble-induced streaming patterns at different distances between the twin bubbles were experimentally studied on microfabricated AC-EWOD chips to determine the optimum distance. To transport twin bubbles on microfluidic chips, the arrays of EWOD electrodes were microfabricated, and EWOD signals were controlled by a digital I/O board using custom-programmed Labview code. The transportation of twin bubbles was successfully demonstrated by sequentially activating the arrays of EWOD electrodes. Finally, as proof of the feasibility of the proposed method, the manipulation of a fish egg by AC-EWOD driven twin bubbles was experimentally achieved. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The development of miniaturized systems for manipulating micro/bio-objects in an aqueous medium is becoming a great technological challenge for micro device assembly and biomedical applications such as cell manipulation and microsurgery [1–3]. An effective micromanipulation system should have the ability to grasp micro-objects of different shapes with high manipulating (transporting and positioning) accuracy to avoid any damage to the manipulated objects [4,5]. Various microgrippers operated by different actuation schemes such as electrostatic actuators, piezoelectric actuators, shape-memory alloy actuators, pneumatic actuators, and thermal actuators have been developed for some of the most popular micromanipulation tools to grasp and transport micro/bio-objects to a desired location [6–9]. However, most of these microgrippers cannot be operated in an aqueous environment compatible with living conditions for cells [10,11]. Particularly, these tools cause damage to manipulated objects, because the solid parts of the tools must physically contact the objects during micromanipulation [12–15].
∗ Corresponding author. Tel.: +82 31 330 6346. E-mail address: [email protected] (S.K. Chung). 0924-4247/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2012.07.019
To minimize contact damage, various non-invasive techniques have been developed, which include optical tweezers, optoelectronic manipulators, bubble tweezers, acoustic tweezers, and electrical-force-assisted manipulators such as electrophoresis and dielectrophoresis [16–22]. Chung and Cho [23] developed microbubble tweezers with an acoustically oscillating mobile bubble that manipulates microobjects on microfluidic chips. When an air bubble is excited by an acoustic wave at its resonant frequency, it captures neighboring objects by means of the Bjerknes force generated by the acoustically excited bubble [24,25]. The captured objects can then be carried to a desired place by the mobile oscillating bubble via electrowettingon-dielectric (EWOD) actuation [26,27]. However, this technique may also damage manipulated objects by shear stresses induced by the cavitational microstreaming generated by the acoustically excited bubble as well as the Bjerknes force [28,29]. To grasp objects using the Bjerknes force, which is inversely proportional to the fifth power of the distance, the operating distance between the bubble and object must be small enough so the effects of the microstreaming induced-shear stresses are sufficient to manipulate the object [25,28]. Most recently, our research group demonstrated a non-invasive micromanipulation method using cavitational microstreaming generated from acoustically oscillating twin bubbles attached to
168
J.H. Lee et al. / Sensors and Actuators A 195 (2013) 167–174
Fig. 1. Micro-object manipulation by AC-EWOD-driven twin bubbles: (a) initial state; (b) when twin bubbles are actuated by AC-EWOD, they simultaneously oscillate and generate streaming, pushing an object to the next EWOD electrode; (c) the object is carried by oscillating twin bubbles and moves to the right end of EWOD electrodes and (d) when the twin bubbles are transported to the original position without streaming, the object is released from the bubbles and remains in place.
the tips of a U-shaped rod [30,31]. To minimize the microstreaminginduced damage to manipulated objects, the effects of the oscillating twin bubbles were reduced by controlling the intensity of the applied acoustic signals as well as the operation distances between the bubbles and objects using a three-dimensional (3-D) traverse system combined with the U-shaped rod. In this configuration, however, two different actuation systems were required: (1) a piezoactuator for bubble oscillation and (2) a 3-D traverse system for bubble transportation, which makes the total system bulky and integration with micro total analysis systems (TAS) difficult. In contrast to our previous work, we propose a novel approach in which AC-EWOD is solely used for both bubble oscillation and transportation to manipulate a micro/bio-object on a microfluidic chip, as shown in Fig. 1 [32]. When twin bubbles are actuated by AC-EWOD at around 100 Hz, they oscillate and simultaneously generate microstreaming, pushing and relocating a neighboring object to the next EWOD electrode, as shown in Fig. 1(b). The twin bubbles can be also transported near to the object by AC-EWOD. By repeating these processes, the object can be carried by sequential AC-EWOD operations on the twin bubbles to the desired location on the microfluidic chip. When twin bubbles actuated by AC-EWOD at 1 Hz are transported to the original position without microstreaming, the object is not affected by the twin bubbles and remains stationary, as shown in Fig. 1(d). The proposed method is simple yet efficient, and allows non-contact on-chip micromanipulation between the bubbles and objects while minimizing physical damage.
applied between the electrode and the droplet, the dielectric layer behaved as a capacitor preventing the current flow, and electrical charges in the droplet accumulated around a triple contact line (TCL) resulting in a change in the contact angle of the droplet due to the modification of the interfacial tension, as shown in Fig. 2. This is the phenomenon known as EWOD. Berge et al. [34,35] also derived a Lippmann–Young equation to explain the relationship between the modified contact angle and the applied electrical potential as follows: cos = cos e +
εV 2 2t
(1)
2. Theoretical background
where is the contact angle under the externally applied electrical potential V, e is the equilibrium contact angle at V = 0 V, ε is the permittivity of the dielectric layer, is the interfacial tension between the droplet and the surrounding insulating fluid, and t is the thickness of the dielectric layer. The EWOD principle has several outstanding features such as the use of small fluid volumes, low power consumption, and fast response time [26,27]. Hence, it has been used in various applications such as lab-on-a-chip systems, electrical switches, liquid lenses, microprism arrays, electrowetting displays, and others [27,36]. And it can also be applied to a bubble in a conducting medium which has a configuration opposite to that of the above case [37]. Zhao and Cho [38] demonstrated an EWOD experiment for a sessile air bubble in a deionized (D.I.) water chamber and proved that the modification of its contact angle due to applied voltages was also in good agreement with the Lippmann–Young equation.
2.1. Electrowetting-on-dielectric (EWOD) principle
2.2. Oscillating bubble-induced microstreaming
Electrowetting-on-dielectric (EWOD) is one of the methods for manipulating the interfacial tension between two fluids using an external electrical potential [27,33]. In the early 1990s, Berge et al. [34,35] deposited a thin dielectric material on top of a metal electrode to prevent direct contact between the electrode and a conducting droplet. Hence, when an electrical potential was
When a gas bubble is exposed to external disturbances, it easily deforms due to its compressibility, and it sometimes oscillates in harmony with periodic disturbances and generates microstreaming around it. Many research groups have investigated acoustically oscillating microbubble-induced microstreaming [28,29]. Marmottant and Hilgenfeldt [39] analytically derived the stream function
J.H. Lee et al. / Sensors and Actuators A 195 (2013) 167–174
169
Fig. 2. Schematic diagram of electrowetting-on-dielectric (EWOD).
of the cavitational microstreaming for an oscillating bubble located on a solid wall as follows: a = −3ε2 a3 ω sin() cos2 sin2 + O(ε2 r −2 ) (2) r
velocity as the boundary condition, and it showed good agreement with the experimentally results. In this work, this streaming was used for on-chip micromanipulation.
here ε is the amplitude of the bubble oscillation normalized by the bubble radius a, ω is the angular frequency of the applied acoustic wave, r is the distance from the bubble center, is the angle coordinate with respect to the translation axis, and is the phase difference between volume and translational oscillations. Tho et al. [40] experimentally studied bubble oscillation modes under various acoustic conditions in single and twin oscillating bubble-induced microstreaming using microparticle image velocimetry (PIV) systems. However, Ko et al. [41] used AC-EWOD to actuate and oscillate a bubble in an aqueous medium instead of acoustic excitation and observed similar streaming of a so-called synthetic jet, which was the same as in our case. They also derived analytically the velocity potential by the small-amplitude axially symmetric oscillation of a sessile bubble in a weakly viscous medium using the domain perturbation method and obtained a steady tangential velocity at the bubble surface. Then they numerically calculated the streaming by solving the Navier–Stokes equation using the obtained tangential
3. Fabrication of testing devices and experimental setups To oscillate and transport twin bubbles in a controlled manner, testing devices, mainly consisting of two parallel plates (top and bottom), were microfabricated using the standard lithography processes, as shown in Fig. 3. The top plate contained the arrays of EWOD electrodes whereas the bottom plate had an EWOD ground electrode. The fabrication processes for the top plate are given as follows. For the metal layer, a chromium layer with a thickness of 300 A˚ was deposited on a glass slide by sputtering, as shown in Fig. 3(a). Further, the metal layer was patterned by lithography processes (Fig. 3(b) and (c)) and etched by a Cr etchant (Fig. 3(d)). The remaining photo resist (PR) layer was completely removed in acetone and rinsed in isopropyl alcohol (IPA) and D.I. water, respectively. Each EWOD actuating electrode was square in shape with side lengths of 700 m. For the EWOD dielectric layer, a 1.6-mthick polyimide (Durimide 7505, Fujifilm Co.) layer was spin-coated onto the entire surface of the top plate, as shown in Fig. 3(e). Finally,
Fig. 3. Microfabrication processes for the chips.
170
J.H. Lee et al. / Sensors and Actuators A 195 (2013) 167–174
Fig. 4. Schematic diagram of experimental setups.
the top plate was coated with a hydrophobic Teflon layer (AF Teflon 1600, Dupont Co.). For the bottom plate, an indium tin oxide (ITO) layer was deposited onto a glass slide by sputtering and coated with a hydrophobic Teflon layer over the entire surface. The last step was to integrate the 2 plates, as shown in Fig. 3(f). After putting a large water drop and then injecting air bubbles and objects using a micro-syringe (600 series MICROLITER Syringe model 62, Hamilton Co.) onto the top plate, the bottom plate was gently pressed against the walls that were already placed on the top plate. Multiple layers of double-sided tape (each thickness of approximately 100 m) were used for the walls to confine the water in place and served as spacers. For tests, the integrated plates were turned over; however, the thin chrome layer of the top plate was sufficiently transparent so that the motions of bubbles and objects beneath the plate observable. The schematic of the experimental setups is shown in Fig. 4. The setups mainly consisted of a function generator, a voltage amplifier, a digital I/O board with custom-programmed Labview code, microfabricated chips, and vision systems. For AC-EWOD actuation, a sinusoidal voltage was generated by a function generator (33210A, Agilent Co.). Then the voltage was amplified sufficiently for EWOD actuation (up to 120 V) by a voltage amplifier (PZD700, Trek Co.). To achieve sequential activation of the patterned arrays of EWOD electrodes, the amplified signal was transmitted through a series of photocoupled relays (PhotoMos® , AQW614EH, Aromat Co.), which were controlled by a digital I/O board (DAQpad-6229 BNC, NI Co.). Custom-programmed Labview code was used to control the digital I/O board and transmit the amplified voltage selectively to the desired EWOD electrodes. To obtain visual results, a charge coupled device (CCD, EO-1312C, Edmund Optics) or a high-speed camera (Phantom Miro eX4, Vision Research Inc.) with a zoom lens (VZMTM 450i eo, Edmund Optics) were selectively used, and the image data were saved on a personal computer. For image analysis and measurement of bubble oscillation amplitude, the public-domainlicensed software Image J (NIH, USA) was used.
Fig. 6. Bubble oscillation amplitudes at different frequencies and voltages.
4. Experiment results and discussion Oscillating bubble-induced streaming was visualized in a water chamber. When a bubble (1 mm diameter) which was injected by a syringe onto the plain EWOD plate attached to the bottom of the water chamber [2(W) × 3(L) × 2(H) cm3 ] was actuated by AC-EWOD at 100 Hz, it oscillated in a harmony with the applied frequency and simultaneously generated large upward circulation flow patterns around itself, as shown in Fig. 5. For flow visualization, polymer particles (10 m diameter) were seeded and suspended in the water, and the white lines traced by the particles showed the flow patterns. To investigate streaming generated by an AC-EWOD-driven oscillating bubble, the oscillation behavior of a bubble under a wide range of AC-EWOD actuation conditions such as different frequencies and voltages was studied using high-speed images, as shown in Fig. 6. When a bubble (1 mm diameter) was actuated by AC-EWOD in the same experimental setups as shown in Fig. 5, side images of the oscillating bubble were captured by the high-speed camera. The oscillation amplitude of the bubble was then characterized as the dimensionless oscillation amplitude (ε = /D, where is the peak-to-peak oscillation amplitude at the bubble apex, and D is the diameter of the bubble). The results showed that the oscillation amplitude highly depended on the applied frequency and was proportional to the applied voltage. The maximum amplitude of the bubble oscillation occurred at 100 Hz. It was also found from simultaneous flow visualization experiments that the strength of the streaming generated from an AC-EWOD-driven bubble was closely related to the bubble oscillation amplitude, i.e., the larger the bubble oscillation amplitude, the stronger the streaming.
Fig. 5. Microstreaming patterns around an AC-EWOD-driven oscillating bubble.
J.H. Lee et al. / Sensors and Actuators A 195 (2013) 167–174
171
Fig. 7. Sequential snapshots of a fish egg pushed by an AC-EWOD-driven oscillating bubble.
The effect of oscillating bubble-induced streaming on objects near the bubble was investigated using a fish egg. When a bubble (1 mm diameter) was injected by a syringe and attached to the bottom surface of the top EWOD plate and actuated by AC-EWOD at 100 Hz and 100 V, an adjacent fish egg (800 m diameter) placed on the bottom plate was pushed away from the bubble by the oscillating bubble-induced streaming, as shown in Fig. 7. The speeds of the fish egg being pushed at different frequencies and voltages were measured by image analysis, and the streaming-induced forces were calculated by Stokes’ drag assumption (F = 6 Dv, where , D, and v are the fluid viscosity, the fish egg diameter, and the fish egg speed, respectively) and plotted in Fig. 8. The maximum bubble-induced force (9 N) was also generated at 100 Hz when the oscillation amplitude of the bubble was the largest, as shown in Fig. 6. Note that the speed of the fish egg being pushed at 100 Hz was measured at 57 mm/s. The effect of the distance between a bubble and an object was also investigated to optimize the height of the chips. The experiment used the same setups as Fig. 7 and was conducted for different heights and voltages at a fixed frequency of 100 Hz using a fish egg, as shown in Fig. 9. As expected, the oscillating bubble-induced force decreased as the height increased. The magnitude of the generated force at a height of 1.5 mm was less than half that at a height of 1 mm. Hence, a height of 1 mm was selected and used for further experiments after considering the size of the manipulated objects.
Fig. 8. Streaming-induced forces at different frequencies and voltages.
We learned from the previous tests that although singlebubble-induced streaming was strong enough to push and move neighboring objects, it was difficult to control the direction of the pushed objects. To improve the controllability of manipulated objects and increase the strength of the streaming, twin bubbles of the same size were proposed for the envisioned on-chip micromanipulation. To find the optimal distance between the twin bubbles, a flow visualization experiment at 4 different distances (0.5D, 0.75D, 1.0D, and 2.0D, where D is the bubble diameter) was conducted. We injected two bubbles (800 m diameter) with the same volume at each designed spot on a microfabricated EWOD chip consisting of two circular-shaped EWOD electrodes with the bubbles sitting on and surrounding ground electrodes to prevent the bubbles from moving and merging during AC-EWOD actuation. Polymer particles (10 m diameter) were seeded in water for visualization. When the twin bubbles at different distances were simultaneously actuated by AC-EWOD, they generated similar streaming patterns, as shown in Fig. 10. Note that the twin bubbles shown in Fig. 10 appear smaller than their actual size due to their oscillation. As the distance decreased, the effect of streaming on the manipulated objects increased; however, the distance was so small that the probability of the twin bubbles merging during AC-EWOD actuation increased. After considering the strength of the streaming and the risk of the
Fig. 9. Streaming-induced forces at different heights and voltages at a fixed frequency of 100 Hz.
172
J.H. Lee et al. / Sensors and Actuators A 195 (2013) 167–174
Fig. 10. Flow visualization at different distances of the twin bubbles actuated by AC-EWOD.
twin bubbles merging, a distance of 600 m was suggested though trial and error. For the development of the envisioned on-chip micromanipulation system, one of the most important operations is the reliable transportation of twin bubbles. Fig. 11 shows a top view of the sequential images of the transportation of twin bubbles on the arrays of EWOD electrodes. The same sized twin bubbles (800 m diameter) were initially injected by a syringe on the right end of the EWOD electrodes, as shown in Fig. 11(a). When all electrodes were activated at 1 Hz and 100 V except the adjacent left electrodes, the twin bubbles were immediately transported to the inactivated adjacent electrodes. Based on the EWOD principle, an initially hydrophobic surface property was changed to hydrophilic under EWOD actuation. Hence, a bubble tries to move to an adjacent hydrophobic surface, because it prefers to stay on the hydrophobic surface. By shifting and repeating this procedure to the next electrodes, the twin bubbles were successfully transported to the left
end electrodes as shown in Fig. 11(b) and (c) and back to the original position as shown in Fig. 11(d)–(f). The total movement of the twin bubbles was accomplished in 12 steps, which shows the reliability of the operation. Note that the EWOD electrodes were activated for 1-s period in each step, and the instantaneous moving speed of the bubble was measured at 30 mm/s through image analysis. Finally, as proof of the feasibility of the proposed micro-object manipulation method, manipulation of a fish egg (1 mm diameter) by AC-EWOD-driven twin bubbles was experimentally achieved, as shown in Fig. 12. Twin bubbles were initially attached to the top plate, and a fish egg was placed on the bottom plate as described in Fig. 3(f). When the stationary bubbles were actuated by AC-EWOD at 100 Hz, they oscillated in place and simultaneously generated streaming, resulting in the fish egg being pushed to the next EWOD electrode, as shown in Fig. 12(b). Then the bubbles were transported to the next EWOD electrode by AC-EWOD actuation at 1 Hz, as shown in Fig. 12(c). During the twin bubble transportation, the
Fig. 11. Sequential snapshots of the transportation of twin bubbles.
J.H. Lee et al. / Sensors and Actuators A 195 (2013) 167–174
173
Fig. 12. Sequential top-view images of the manipulation of a fish egg using AC-EWOD-driven oscillating twin bubbles.
applied frequency was shifted to 1 Hz to remove an unexpected effect on the manipulated fish egg without microstreaming. When the stationary bubbles were again actuated by AC-EWOD at 100 Hz, the fish egg was pushed until it finally reached the reference point, as shown in Fig. 12(d). These results show the high feasibility of the concept of the proposed micromanipulation method. 5. Conclusion This paper describes a novel non-invasive on-chip micromanipulation method. Single AC-EWOD actuation accomplished both bubble oscillation and transportation on a microfluidic chip. First, the bubble oscillation behavior and bubble-induced streaming under various AC-EWOD actuation conditions were experimentally investigated. The strength of the streaming was proportional to the bubble oscillation amplitude, and the maximum oscillation amplitude occurred at 100 Hz. Second, the streaming-induced forces with different AC-EWOD actuation conditions and distances between the bubble and manipulated object were calculated by measuring the speed of the pushed object based on Stokes’ drag assumption. The maximum force for the largest bubble oscillation at a small distance (1 mm) was calculated as 9 N. Third, the streaming patterns from AC-EWOD-driven twin bubbles for different distances between the twin bubbles were investigated using microfabricated chips. Fourth, the transportation of twin bubbles was demonstrated on microfabricated arrays of EWOD electrodes via AC-EWOD signal controls using a digital I/O board and custom-programmed Labview code. Finally, the manipulation of a fish egg was successfully achieved solely by AC-EWOD actuation as proof of the envisioned method. This micromanipulation method may provide an efficient tool for handling micro/bio-objects such as biological cells. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0025039). References [1] E.W.H. Jager, O. Inganäs, I. Lundström, Microrobots for micrometer-size objects in aqueous media: potential tools for single-cell manipulation, Science 288 (2000) 2335–2338. [2] J.P. Desai, A. Pillarisetti, A.D. Brooks, Engineering approaches to biomanipulation, Annual Review of Biomedical Engineering 9 (2007) 35–53.
[3] K.F. Böhringer, R.S. Fearing, K.Y. Goldberg, Microassembly, in: S. Nof (Ed.), The Handbook of Industrial Robotics, 2nd ed., John Wiley & Sons, 1999, pp. 1045–1066. [4] J. Cecil, D. Powell, D. Vasquez, Assembly and manipulation of micro devices – a state of the art survey, Robotics and Computer Integrated Manufacturing Systems 23 (2007) 580–588. [5] S.K. Nah, Z.W. Zhong, A microgripper using piezoelectric actuation for microobject manipulation, Sensors and Actuators A: Physical 133 (2007) 218–224. [6] J. Cecil, D. Vasquez, D. Powell, A review of gripping and manipulation techniques for micro-assembly applications, International Journal of Production Research 43 (2005) 819–828. [7] Y.-W. Lu, C.-J. Kim, Microhand for biological applications, Applied Physics Letters 89 (2006) 164101. [8] A. Vasudev, A. Jagtiani, L. Du, J. Zhe, A low-voltage droplet microgripper for micro-object manipulation, Journal of Micromechanics and Microengineering 19 (2009) 075005. [9] C.-J. Kim, A. Pisano, R.S. Muller, Silicon processed overhanging microgripper, Journal of Microelectronic Systems 1 (1992) 31–36. [10] N. Chronis, L.P. Lee, Electrothermally activated SU-8 microgripper for single cell manipulation in solution, Journal of Microelectronic Systems 14 (2005) 857–863. [11] H. Andersson, A.V.D. Berg, Microfluidic devices for cellomics: a review, Sensors and Actuators B: Chemical 92 (2003) 315–325. [12] B.J. Nelson, I.K. Kaliakatsos, J.J. Abbott, Microrobots for minimally invasive medicine, Annual Review of Biomedical Engineering 12 (2010) 55–85. [13] J. Park, S.-H. Jung, Y.-H. Kim, B. Kim, S.-K. Lee, J.-O. Park, Design and fabrication of an integrated cell processor for single embryo cell manipulation, Lab Chip 5 (2005) 91–96. [14] K.K. Tan, H.X. Zhou, Computer controlled piezo micromanipulation system for biomedical applications, Engineering Science and Education Journal 10 (2001) 249–256. [15] Y. Zhao, S.K. Cho, Micro bubble manipulation towards single cell handling tool, in: Proceedings of IEEE International Conference on Robotics and Biomimetics (ROBIO), Hong Kong, 2005, pp. 269–273. [16] A. Ashkin, J.M. Dziedzic, T. Yamane, Optical trapping and manipulation of single cells using infrared laser beams, Nature 330 (1987) 769–771. [17] J.E. Curtis, B.A. Koss, D.G. Grier, Dynamic holographic optical tweezers, Optics Communication 207 (2002) 169–175. [18] S.K. Chung, S.K. Cho, Oscillating mobile bubbles for microfluidic mixing enhancement, in: The 11th International Conference on Miniaturized Systems for Chemistry and Life Sciences (TAS 2007), Paris, France, 2007, pp. 913–915. [19] J.O. Kwon, J.S. Yang, S.J. Lee, K. Rhee, S.K. Chung, Electromagnetically actuated micromanipulator using an acoustically oscillating bubble, Journal of Micromechanics and Microengineering 21 (2011) 115023. [20] J. Hu, J. Yang, J. Xu, Ultrasonic trapping of small particles by sharp edges vibrating in a flexural mode, Applied Physics Letters 85 (2004) 6042–6044. [21] B.R. Lutz, J. Chen, D.T. Schwartz, Hydrodynamic tweezers: 1. Noncontact trapping of single cells using steady streaming microeddies, Analytical Chemistry 78 (2006) 5429–5435. [22] J. Voldman, Electrical forces for microscale cell manipulation, Annual Review of Biomedical Engineering 8 (2006) 425–454. [23] S.K. Chung, S.K. Cho, On-chip manipulation of objects using mobile oscillating bubbles, Journal of Micromechanics and Microengineering 18 (2008) 125024. [24] S.K. Chung, S.K. Cho, 3-D manipulation of millimeter- and micro-sized objects using an acoustically-excited oscillating bubble, Microfluidics and Nanofluidics 6 (2008) 261–265. [25] D.L. Miller, Particle gathering and microstreaming near ultrasonically activated gas-filled micropores, Journal of the Acoustical Society of America 84 (1988) 1378–1387.
174
J.H. Lee et al. / Sensors and Actuators A 195 (2013) 167–174
[26] R.B. Fair, Digital microfluidics: is a true lab-on-a-chip possible? Microfluidics and Nanofluidics 3 (2007) 245–281. [27] F. Mugele, J.C. Baret, Electrowetting, From basics to applications, Journal of Physics: Condensed Matter 17 (2005) R705–R774. [28] S.A. Elder, Cavitation microstreaming, Journal of the Acoustical Society of America 31 (1958) 54–64. [29] T.G. Leighton, The Acoustic Bubble, Academic Press, 1997. [30] K.H. Lee, J.H. Lee, J.M. Won, K. Rhee, S.K. Chung, Micromanipulation using cavitational microstreaming generated by acoustically oscillating twin bubbles, Sensors and Actuators A: Physical (2011), http://dx.doi.org/10.1016/j.sna.2011.11.037. [31] K.H. Lee, J.H. Lee, J.M. Won, S.K. Chung, Novel tweezers using acoustically oscillating twin bubbles, in: The 16th International Conference on Solid-State Sensors, Actuators and Microsystems (Transducers 2011), Beijing China, 2011, pp. 1713–1716. [32] J.H. Lee, K.H. Lee, J.B. Chae, K. Rhee, S.K. Chung, Bubble-induced streaming for micro-object manipulation, in: The 25th International Conference on Micro Electro Mechanical Systems (MEMS 2012), Paris, France, 2012, pp. 1013–1016. [33] S.K. Chung, Y. Zhao, S.K. Cho, Electrowetting-on-dielectric (EWOD) microfluidic devices, in: K.E. Herold, A. Rasooly (Eds.), Lab on a Chip (LOC) Technologies and Applications, Horizon Scientific Press/Caister Academic Press/Caister Academic Press, 2009. [34] C. Quilliet, B. Berge, Electrowetting: a recent outbreak, Current Opinion in Colloid and Interface Science 6 (2001) 34–39. [35] M. Vallet, M. Vallade, B. Berge, Limiting phenomena for the spreading of water on polymer films by electrowetting, European Physical Journal B (1999) 1–9. [36] S.K. Chung, K. Ryu, S.K. Cho, Electrowetting propulsion of water-floating objects, Applied Physics Letters 95 (2009) 014107. [37] S.K. Chung, K. Rhee, S.K. Cho, Bubble actuation by electrowetting-on-dielectric (EWOD) and its applications: a review, International Journal of Precision Engineering and Manufacturing 11 (2010) 991–1006. [38] Y. Zhao, S.K. Cho, Micro bubble manipulation by electrowetting on dielectric: transporting, splitting, merging and eliminating of bubbles, Lab Chip 7 (2007) 273–280. [39] P. Marmottant, S. Hilgenfeldt, Controlled vesicle deformation and lysis by single oscillating bubbles, Nature 423 (2003) 153–156. [40] P. Tho, R. Manasseh, A. Ooi, Cavitation microstreaming patterns in single and multiple bubble systems, Journal of Fluid Mechanics 576 (2007) 191–233. [41] S.H. Ko, S.J. Lee, K.H. Kang, A synthetic jet produced by electrowetting-driven bubble oscillations in aqueous solution, Applied Physics Letters 94 (2009).
Biographies Jeong Hyun Lee received the Bachelor’s degree of Mechanical Engineering from Myongji University in 2011. He is currently a graduate student in Myongji University and his research interests lie on the development of microfluidic applications based on an electrowetting-on-dielectric principle. Kyung Ho Lee received the Bachelor’s degree of Mechanical Engineering from Myongji University in 2011. He is currently a graduate student in Myongji University and his research interests lie on the development of micromanipulator using acoustically oscillating microbubbles. Jeong Byung Chae received the Bachelor’s degree of Mechanical Engineering from Myongji University in 2012. He is currently a graduate student in Myongji University and his research interests lie on the optimization of EWOD (electrowetting-ondielectric) systems. Kyehan Rhee is a professor of the Department of Mechanical Engineering at the Myongji University in Korea. He received his Ph.D. degree from the University of Minnesota, Minneapolis, U.S.A and worked as a Post-Doctoral Fellow in the Pennsylvania State University, State College, U.S.A. His research interests include hemodynamics, microfluidics, polymer smart material actuators and their application in biomedical devices. He serves as an editor of the International Journal of Precision Engineering and Manufacturing and the Journal of Biomechanical Science and Engineering. Sang Kug Chung is an assistant professor of the Department of Mechanical Engineering at the Myongji University in Korea. He received the Ph.D. degree in Mechanical Engineering and Materials Science from the University of Pittsburgh in 2009 along with the Graduate Research Excellence Award. He received the M.S. degree from Pohang University of Science and Technology (POSTECH) and B.S. from Myongji University. He had worked for the development of the world first Liquid Lens at Samsung Electro-Mechanics from 2003 to 2009 and researched oil spills at Advanced Fluids Engineering Research Center from 2000 and 2001. Upon joining the faculty at Myongji University in 2009, he has directed the Microsystems Laboratory. His research is in microfluidics and MEMS, including design and fabrication of micro/nano actuators and systems.
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
On-chip micromanipulation by AC-EWOD driven twin bubbles Jeong Hyun Lee, Kyung Ho Lee, Jeong Byung Chae, Kyehan Rhee, Sang Kug Chung ∗ Department of Mechanical Engineering, Myongji University, Yongin 449-728, South Korea
a r t i c l e
i n f o
Article history: Received 14 April 2012 Received in revised form 19 July 2012 Accepted 19 July 2012 Available online 27 July 2012 Keywords: Microfluidics Alternating current electrowetting-on-dielectric (AC-EWOD) Microbubbles Microstreaming
a b s t r a c t A novel on-chip micromanipulation method has been developed by which the streaming generated by oscillating twin bubbles actuated by alternating current electrowetting-on-dielectric (AC-EWOD) was used to manipulate micro/bio-objects in an aqueous medium. First, the behavior of a bubble placed on a plain EWOD electrode submerged in a water chamber was investigated under different AC-EWOD actuation conditions such as various frequencies and voltages using high-speed images. The results showed that the bubble oscillation amplitude highly depended on the applied frequency and was proportional to the strength of the bubble-induced streaming. To investigate the effect of the streaming, the forces generated by the oscillating bubble-induced streaming were indirectly measured using Stokes’ drag approximation under the different AC-EWOD actuation conditions at various distances between the bubble and a fish egg. The maximum force was calculated as 9 N. To improve the controllability for manipulating objects, a system of twin bubbles was proposed, and twin-bubble-induced streaming patterns at different distances between the twin bubbles were experimentally studied on microfabricated AC-EWOD chips to determine the optimum distance. To transport twin bubbles on microfluidic chips, the arrays of EWOD electrodes were microfabricated, and EWOD signals were controlled by a digital I/O board using custom-programmed Labview code. The transportation of twin bubbles was successfully demonstrated by sequentially activating the arrays of EWOD electrodes. Finally, as proof of the feasibility of the proposed method, the manipulation of a fish egg by AC-EWOD driven twin bubbles was experimentally achieved. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The development of miniaturized systems for manipulating micro/bio-objects in an aqueous medium is becoming a great technological challenge for micro device assembly and biomedical applications such as cell manipulation and microsurgery [1–3]. An effective micromanipulation system should have the ability to grasp micro-objects of different shapes with high manipulating (transporting and positioning) accuracy to avoid any damage to the manipulated objects [4,5]. Various microgrippers operated by different actuation schemes such as electrostatic actuators, piezoelectric actuators, shape-memory alloy actuators, pneumatic actuators, and thermal actuators have been developed for some of the most popular micromanipulation tools to grasp and transport micro/bio-objects to a desired location [6–9]. However, most of these microgrippers cannot be operated in an aqueous environment compatible with living conditions for cells [10,11]. Particularly, these tools cause damage to manipulated objects, because the solid parts of the tools must physically contact the objects during micromanipulation [12–15].
∗ Corresponding author. Tel.: +82 31 330 6346. E-mail address: [email protected] (S.K. Chung). 0924-4247/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2012.07.019
To minimize contact damage, various non-invasive techniques have been developed, which include optical tweezers, optoelectronic manipulators, bubble tweezers, acoustic tweezers, and electrical-force-assisted manipulators such as electrophoresis and dielectrophoresis [16–22]. Chung and Cho [23] developed microbubble tweezers with an acoustically oscillating mobile bubble that manipulates microobjects on microfluidic chips. When an air bubble is excited by an acoustic wave at its resonant frequency, it captures neighboring objects by means of the Bjerknes force generated by the acoustically excited bubble [24,25]. The captured objects can then be carried to a desired place by the mobile oscillating bubble via electrowettingon-dielectric (EWOD) actuation [26,27]. However, this technique may also damage manipulated objects by shear stresses induced by the cavitational microstreaming generated by the acoustically excited bubble as well as the Bjerknes force [28,29]. To grasp objects using the Bjerknes force, which is inversely proportional to the fifth power of the distance, the operating distance between the bubble and object must be small enough so the effects of the microstreaming induced-shear stresses are sufficient to manipulate the object [25,28]. Most recently, our research group demonstrated a non-invasive micromanipulation method using cavitational microstreaming generated from acoustically oscillating twin bubbles attached to
168
J.H. Lee et al. / Sensors and Actuators A 195 (2013) 167–174
Fig. 1. Micro-object manipulation by AC-EWOD-driven twin bubbles: (a) initial state; (b) when twin bubbles are actuated by AC-EWOD, they simultaneously oscillate and generate streaming, pushing an object to the next EWOD electrode; (c) the object is carried by oscillating twin bubbles and moves to the right end of EWOD electrodes and (d) when the twin bubbles are transported to the original position without streaming, the object is released from the bubbles and remains in place.
the tips of a U-shaped rod [30,31]. To minimize the microstreaminginduced damage to manipulated objects, the effects of the oscillating twin bubbles were reduced by controlling the intensity of the applied acoustic signals as well as the operation distances between the bubbles and objects using a three-dimensional (3-D) traverse system combined with the U-shaped rod. In this configuration, however, two different actuation systems were required: (1) a piezoactuator for bubble oscillation and (2) a 3-D traverse system for bubble transportation, which makes the total system bulky and integration with micro total analysis systems (TAS) difficult. In contrast to our previous work, we propose a novel approach in which AC-EWOD is solely used for both bubble oscillation and transportation to manipulate a micro/bio-object on a microfluidic chip, as shown in Fig. 1 [32]. When twin bubbles are actuated by AC-EWOD at around 100 Hz, they oscillate and simultaneously generate microstreaming, pushing and relocating a neighboring object to the next EWOD electrode, as shown in Fig. 1(b). The twin bubbles can be also transported near to the object by AC-EWOD. By repeating these processes, the object can be carried by sequential AC-EWOD operations on the twin bubbles to the desired location on the microfluidic chip. When twin bubbles actuated by AC-EWOD at 1 Hz are transported to the original position without microstreaming, the object is not affected by the twin bubbles and remains stationary, as shown in Fig. 1(d). The proposed method is simple yet efficient, and allows non-contact on-chip micromanipulation between the bubbles and objects while minimizing physical damage.
applied between the electrode and the droplet, the dielectric layer behaved as a capacitor preventing the current flow, and electrical charges in the droplet accumulated around a triple contact line (TCL) resulting in a change in the contact angle of the droplet due to the modification of the interfacial tension, as shown in Fig. 2. This is the phenomenon known as EWOD. Berge et al. [34,35] also derived a Lippmann–Young equation to explain the relationship between the modified contact angle and the applied electrical potential as follows: cos = cos e +
εV 2 2t
(1)
2. Theoretical background
where is the contact angle under the externally applied electrical potential V, e is the equilibrium contact angle at V = 0 V, ε is the permittivity of the dielectric layer, is the interfacial tension between the droplet and the surrounding insulating fluid, and t is the thickness of the dielectric layer. The EWOD principle has several outstanding features such as the use of small fluid volumes, low power consumption, and fast response time [26,27]. Hence, it has been used in various applications such as lab-on-a-chip systems, electrical switches, liquid lenses, microprism arrays, electrowetting displays, and others [27,36]. And it can also be applied to a bubble in a conducting medium which has a configuration opposite to that of the above case [37]. Zhao and Cho [38] demonstrated an EWOD experiment for a sessile air bubble in a deionized (D.I.) water chamber and proved that the modification of its contact angle due to applied voltages was also in good agreement with the Lippmann–Young equation.
2.1. Electrowetting-on-dielectric (EWOD) principle
2.2. Oscillating bubble-induced microstreaming
Electrowetting-on-dielectric (EWOD) is one of the methods for manipulating the interfacial tension between two fluids using an external electrical potential [27,33]. In the early 1990s, Berge et al. [34,35] deposited a thin dielectric material on top of a metal electrode to prevent direct contact between the electrode and a conducting droplet. Hence, when an electrical potential was
When a gas bubble is exposed to external disturbances, it easily deforms due to its compressibility, and it sometimes oscillates in harmony with periodic disturbances and generates microstreaming around it. Many research groups have investigated acoustically oscillating microbubble-induced microstreaming [28,29]. Marmottant and Hilgenfeldt [39] analytically derived the stream function
J.H. Lee et al. / Sensors and Actuators A 195 (2013) 167–174
169
Fig. 2. Schematic diagram of electrowetting-on-dielectric (EWOD).
of the cavitational microstreaming for an oscillating bubble located on a solid wall as follows: a = −3ε2 a3 ω sin() cos2 sin2 + O(ε2 r −2 ) (2) r
velocity as the boundary condition, and it showed good agreement with the experimentally results. In this work, this streaming was used for on-chip micromanipulation.
here ε is the amplitude of the bubble oscillation normalized by the bubble radius a, ω is the angular frequency of the applied acoustic wave, r is the distance from the bubble center, is the angle coordinate with respect to the translation axis, and is the phase difference between volume and translational oscillations. Tho et al. [40] experimentally studied bubble oscillation modes under various acoustic conditions in single and twin oscillating bubble-induced microstreaming using microparticle image velocimetry (PIV) systems. However, Ko et al. [41] used AC-EWOD to actuate and oscillate a bubble in an aqueous medium instead of acoustic excitation and observed similar streaming of a so-called synthetic jet, which was the same as in our case. They also derived analytically the velocity potential by the small-amplitude axially symmetric oscillation of a sessile bubble in a weakly viscous medium using the domain perturbation method and obtained a steady tangential velocity at the bubble surface. Then they numerically calculated the streaming by solving the Navier–Stokes equation using the obtained tangential
3. Fabrication of testing devices and experimental setups To oscillate and transport twin bubbles in a controlled manner, testing devices, mainly consisting of two parallel plates (top and bottom), were microfabricated using the standard lithography processes, as shown in Fig. 3. The top plate contained the arrays of EWOD electrodes whereas the bottom plate had an EWOD ground electrode. The fabrication processes for the top plate are given as follows. For the metal layer, a chromium layer with a thickness of 300 A˚ was deposited on a glass slide by sputtering, as shown in Fig. 3(a). Further, the metal layer was patterned by lithography processes (Fig. 3(b) and (c)) and etched by a Cr etchant (Fig. 3(d)). The remaining photo resist (PR) layer was completely removed in acetone and rinsed in isopropyl alcohol (IPA) and D.I. water, respectively. Each EWOD actuating electrode was square in shape with side lengths of 700 m. For the EWOD dielectric layer, a 1.6-mthick polyimide (Durimide 7505, Fujifilm Co.) layer was spin-coated onto the entire surface of the top plate, as shown in Fig. 3(e). Finally,
Fig. 3. Microfabrication processes for the chips.
170
J.H. Lee et al. / Sensors and Actuators A 195 (2013) 167–174
Fig. 4. Schematic diagram of experimental setups.
the top plate was coated with a hydrophobic Teflon layer (AF Teflon 1600, Dupont Co.). For the bottom plate, an indium tin oxide (ITO) layer was deposited onto a glass slide by sputtering and coated with a hydrophobic Teflon layer over the entire surface. The last step was to integrate the 2 plates, as shown in Fig. 3(f). After putting a large water drop and then injecting air bubbles and objects using a micro-syringe (600 series MICROLITER Syringe model 62, Hamilton Co.) onto the top plate, the bottom plate was gently pressed against the walls that were already placed on the top plate. Multiple layers of double-sided tape (each thickness of approximately 100 m) were used for the walls to confine the water in place and served as spacers. For tests, the integrated plates were turned over; however, the thin chrome layer of the top plate was sufficiently transparent so that the motions of bubbles and objects beneath the plate observable. The schematic of the experimental setups is shown in Fig. 4. The setups mainly consisted of a function generator, a voltage amplifier, a digital I/O board with custom-programmed Labview code, microfabricated chips, and vision systems. For AC-EWOD actuation, a sinusoidal voltage was generated by a function generator (33210A, Agilent Co.). Then the voltage was amplified sufficiently for EWOD actuation (up to 120 V) by a voltage amplifier (PZD700, Trek Co.). To achieve sequential activation of the patterned arrays of EWOD electrodes, the amplified signal was transmitted through a series of photocoupled relays (PhotoMos® , AQW614EH, Aromat Co.), which were controlled by a digital I/O board (DAQpad-6229 BNC, NI Co.). Custom-programmed Labview code was used to control the digital I/O board and transmit the amplified voltage selectively to the desired EWOD electrodes. To obtain visual results, a charge coupled device (CCD, EO-1312C, Edmund Optics) or a high-speed camera (Phantom Miro eX4, Vision Research Inc.) with a zoom lens (VZMTM 450i eo, Edmund Optics) were selectively used, and the image data were saved on a personal computer. For image analysis and measurement of bubble oscillation amplitude, the public-domainlicensed software Image J (NIH, USA) was used.
Fig. 6. Bubble oscillation amplitudes at different frequencies and voltages.
4. Experiment results and discussion Oscillating bubble-induced streaming was visualized in a water chamber. When a bubble (1 mm diameter) which was injected by a syringe onto the plain EWOD plate attached to the bottom of the water chamber [2(W) × 3(L) × 2(H) cm3 ] was actuated by AC-EWOD at 100 Hz, it oscillated in a harmony with the applied frequency and simultaneously generated large upward circulation flow patterns around itself, as shown in Fig. 5. For flow visualization, polymer particles (10 m diameter) were seeded and suspended in the water, and the white lines traced by the particles showed the flow patterns. To investigate streaming generated by an AC-EWOD-driven oscillating bubble, the oscillation behavior of a bubble under a wide range of AC-EWOD actuation conditions such as different frequencies and voltages was studied using high-speed images, as shown in Fig. 6. When a bubble (1 mm diameter) was actuated by AC-EWOD in the same experimental setups as shown in Fig. 5, side images of the oscillating bubble were captured by the high-speed camera. The oscillation amplitude of the bubble was then characterized as the dimensionless oscillation amplitude (ε = /D, where is the peak-to-peak oscillation amplitude at the bubble apex, and D is the diameter of the bubble). The results showed that the oscillation amplitude highly depended on the applied frequency and was proportional to the applied voltage. The maximum amplitude of the bubble oscillation occurred at 100 Hz. It was also found from simultaneous flow visualization experiments that the strength of the streaming generated from an AC-EWOD-driven bubble was closely related to the bubble oscillation amplitude, i.e., the larger the bubble oscillation amplitude, the stronger the streaming.
Fig. 5. Microstreaming patterns around an AC-EWOD-driven oscillating bubble.
J.H. Lee et al. / Sensors and Actuators A 195 (2013) 167–174
171
Fig. 7. Sequential snapshots of a fish egg pushed by an AC-EWOD-driven oscillating bubble.
The effect of oscillating bubble-induced streaming on objects near the bubble was investigated using a fish egg. When a bubble (1 mm diameter) was injected by a syringe and attached to the bottom surface of the top EWOD plate and actuated by AC-EWOD at 100 Hz and 100 V, an adjacent fish egg (800 m diameter) placed on the bottom plate was pushed away from the bubble by the oscillating bubble-induced streaming, as shown in Fig. 7. The speeds of the fish egg being pushed at different frequencies and voltages were measured by image analysis, and the streaming-induced forces were calculated by Stokes’ drag assumption (F = 6 Dv, where , D, and v are the fluid viscosity, the fish egg diameter, and the fish egg speed, respectively) and plotted in Fig. 8. The maximum bubble-induced force (9 N) was also generated at 100 Hz when the oscillation amplitude of the bubble was the largest, as shown in Fig. 6. Note that the speed of the fish egg being pushed at 100 Hz was measured at 57 mm/s. The effect of the distance between a bubble and an object was also investigated to optimize the height of the chips. The experiment used the same setups as Fig. 7 and was conducted for different heights and voltages at a fixed frequency of 100 Hz using a fish egg, as shown in Fig. 9. As expected, the oscillating bubble-induced force decreased as the height increased. The magnitude of the generated force at a height of 1.5 mm was less than half that at a height of 1 mm. Hence, a height of 1 mm was selected and used for further experiments after considering the size of the manipulated objects.
Fig. 8. Streaming-induced forces at different frequencies and voltages.
We learned from the previous tests that although singlebubble-induced streaming was strong enough to push and move neighboring objects, it was difficult to control the direction of the pushed objects. To improve the controllability of manipulated objects and increase the strength of the streaming, twin bubbles of the same size were proposed for the envisioned on-chip micromanipulation. To find the optimal distance between the twin bubbles, a flow visualization experiment at 4 different distances (0.5D, 0.75D, 1.0D, and 2.0D, where D is the bubble diameter) was conducted. We injected two bubbles (800 m diameter) with the same volume at each designed spot on a microfabricated EWOD chip consisting of two circular-shaped EWOD electrodes with the bubbles sitting on and surrounding ground electrodes to prevent the bubbles from moving and merging during AC-EWOD actuation. Polymer particles (10 m diameter) were seeded in water for visualization. When the twin bubbles at different distances were simultaneously actuated by AC-EWOD, they generated similar streaming patterns, as shown in Fig. 10. Note that the twin bubbles shown in Fig. 10 appear smaller than their actual size due to their oscillation. As the distance decreased, the effect of streaming on the manipulated objects increased; however, the distance was so small that the probability of the twin bubbles merging during AC-EWOD actuation increased. After considering the strength of the streaming and the risk of the
Fig. 9. Streaming-induced forces at different heights and voltages at a fixed frequency of 100 Hz.
172
J.H. Lee et al. / Sensors and Actuators A 195 (2013) 167–174
Fig. 10. Flow visualization at different distances of the twin bubbles actuated by AC-EWOD.
twin bubbles merging, a distance of 600 m was suggested though trial and error. For the development of the envisioned on-chip micromanipulation system, one of the most important operations is the reliable transportation of twin bubbles. Fig. 11 shows a top view of the sequential images of the transportation of twin bubbles on the arrays of EWOD electrodes. The same sized twin bubbles (800 m diameter) were initially injected by a syringe on the right end of the EWOD electrodes, as shown in Fig. 11(a). When all electrodes were activated at 1 Hz and 100 V except the adjacent left electrodes, the twin bubbles were immediately transported to the inactivated adjacent electrodes. Based on the EWOD principle, an initially hydrophobic surface property was changed to hydrophilic under EWOD actuation. Hence, a bubble tries to move to an adjacent hydrophobic surface, because it prefers to stay on the hydrophobic surface. By shifting and repeating this procedure to the next electrodes, the twin bubbles were successfully transported to the left
end electrodes as shown in Fig. 11(b) and (c) and back to the original position as shown in Fig. 11(d)–(f). The total movement of the twin bubbles was accomplished in 12 steps, which shows the reliability of the operation. Note that the EWOD electrodes were activated for 1-s period in each step, and the instantaneous moving speed of the bubble was measured at 30 mm/s through image analysis. Finally, as proof of the feasibility of the proposed micro-object manipulation method, manipulation of a fish egg (1 mm diameter) by AC-EWOD-driven twin bubbles was experimentally achieved, as shown in Fig. 12. Twin bubbles were initially attached to the top plate, and a fish egg was placed on the bottom plate as described in Fig. 3(f). When the stationary bubbles were actuated by AC-EWOD at 100 Hz, they oscillated in place and simultaneously generated streaming, resulting in the fish egg being pushed to the next EWOD electrode, as shown in Fig. 12(b). Then the bubbles were transported to the next EWOD electrode by AC-EWOD actuation at 1 Hz, as shown in Fig. 12(c). During the twin bubble transportation, the
Fig. 11. Sequential snapshots of the transportation of twin bubbles.
J.H. Lee et al. / Sensors and Actuators A 195 (2013) 167–174
173
Fig. 12. Sequential top-view images of the manipulation of a fish egg using AC-EWOD-driven oscillating twin bubbles.
applied frequency was shifted to 1 Hz to remove an unexpected effect on the manipulated fish egg without microstreaming. When the stationary bubbles were again actuated by AC-EWOD at 100 Hz, the fish egg was pushed until it finally reached the reference point, as shown in Fig. 12(d). These results show the high feasibility of the concept of the proposed micromanipulation method. 5. Conclusion This paper describes a novel non-invasive on-chip micromanipulation method. Single AC-EWOD actuation accomplished both bubble oscillation and transportation on a microfluidic chip. First, the bubble oscillation behavior and bubble-induced streaming under various AC-EWOD actuation conditions were experimentally investigated. The strength of the streaming was proportional to the bubble oscillation amplitude, and the maximum oscillation amplitude occurred at 100 Hz. Second, the streaming-induced forces with different AC-EWOD actuation conditions and distances between the bubble and manipulated object were calculated by measuring the speed of the pushed object based on Stokes’ drag assumption. The maximum force for the largest bubble oscillation at a small distance (1 mm) was calculated as 9 N. Third, the streaming patterns from AC-EWOD-driven twin bubbles for different distances between the twin bubbles were investigated using microfabricated chips. Fourth, the transportation of twin bubbles was demonstrated on microfabricated arrays of EWOD electrodes via AC-EWOD signal controls using a digital I/O board and custom-programmed Labview code. Finally, the manipulation of a fish egg was successfully achieved solely by AC-EWOD actuation as proof of the envisioned method. This micromanipulation method may provide an efficient tool for handling micro/bio-objects such as biological cells. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0025039). References [1] E.W.H. Jager, O. Inganäs, I. Lundström, Microrobots for micrometer-size objects in aqueous media: potential tools for single-cell manipulation, Science 288 (2000) 2335–2338. [2] J.P. Desai, A. Pillarisetti, A.D. Brooks, Engineering approaches to biomanipulation, Annual Review of Biomedical Engineering 9 (2007) 35–53.
[3] K.F. Böhringer, R.S. Fearing, K.Y. Goldberg, Microassembly, in: S. Nof (Ed.), The Handbook of Industrial Robotics, 2nd ed., John Wiley & Sons, 1999, pp. 1045–1066. [4] J. Cecil, D. Powell, D. Vasquez, Assembly and manipulation of micro devices – a state of the art survey, Robotics and Computer Integrated Manufacturing Systems 23 (2007) 580–588. [5] S.K. Nah, Z.W. Zhong, A microgripper using piezoelectric actuation for microobject manipulation, Sensors and Actuators A: Physical 133 (2007) 218–224. [6] J. Cecil, D. Vasquez, D. Powell, A review of gripping and manipulation techniques for micro-assembly applications, International Journal of Production Research 43 (2005) 819–828. [7] Y.-W. Lu, C.-J. Kim, Microhand for biological applications, Applied Physics Letters 89 (2006) 164101. [8] A. Vasudev, A. Jagtiani, L. Du, J. Zhe, A low-voltage droplet microgripper for micro-object manipulation, Journal of Micromechanics and Microengineering 19 (2009) 075005. [9] C.-J. Kim, A. Pisano, R.S. Muller, Silicon processed overhanging microgripper, Journal of Microelectronic Systems 1 (1992) 31–36. [10] N. Chronis, L.P. Lee, Electrothermally activated SU-8 microgripper for single cell manipulation in solution, Journal of Microelectronic Systems 14 (2005) 857–863. [11] H. Andersson, A.V.D. Berg, Microfluidic devices for cellomics: a review, Sensors and Actuators B: Chemical 92 (2003) 315–325. [12] B.J. Nelson, I.K. Kaliakatsos, J.J. Abbott, Microrobots for minimally invasive medicine, Annual Review of Biomedical Engineering 12 (2010) 55–85. [13] J. Park, S.-H. Jung, Y.-H. Kim, B. Kim, S.-K. Lee, J.-O. Park, Design and fabrication of an integrated cell processor for single embryo cell manipulation, Lab Chip 5 (2005) 91–96. [14] K.K. Tan, H.X. Zhou, Computer controlled piezo micromanipulation system for biomedical applications, Engineering Science and Education Journal 10 (2001) 249–256. [15] Y. Zhao, S.K. Cho, Micro bubble manipulation towards single cell handling tool, in: Proceedings of IEEE International Conference on Robotics and Biomimetics (ROBIO), Hong Kong, 2005, pp. 269–273. [16] A. Ashkin, J.M. Dziedzic, T. Yamane, Optical trapping and manipulation of single cells using infrared laser beams, Nature 330 (1987) 769–771. [17] J.E. Curtis, B.A. Koss, D.G. Grier, Dynamic holographic optical tweezers, Optics Communication 207 (2002) 169–175. [18] S.K. Chung, S.K. Cho, Oscillating mobile bubbles for microfluidic mixing enhancement, in: The 11th International Conference on Miniaturized Systems for Chemistry and Life Sciences (TAS 2007), Paris, France, 2007, pp. 913–915. [19] J.O. Kwon, J.S. Yang, S.J. Lee, K. Rhee, S.K. Chung, Electromagnetically actuated micromanipulator using an acoustically oscillating bubble, Journal of Micromechanics and Microengineering 21 (2011) 115023. [20] J. Hu, J. Yang, J. Xu, Ultrasonic trapping of small particles by sharp edges vibrating in a flexural mode, Applied Physics Letters 85 (2004) 6042–6044. [21] B.R. Lutz, J. Chen, D.T. Schwartz, Hydrodynamic tweezers: 1. Noncontact trapping of single cells using steady streaming microeddies, Analytical Chemistry 78 (2006) 5429–5435. [22] J. Voldman, Electrical forces for microscale cell manipulation, Annual Review of Biomedical Engineering 8 (2006) 425–454. [23] S.K. Chung, S.K. Cho, On-chip manipulation of objects using mobile oscillating bubbles, Journal of Micromechanics and Microengineering 18 (2008) 125024. [24] S.K. Chung, S.K. Cho, 3-D manipulation of millimeter- and micro-sized objects using an acoustically-excited oscillating bubble, Microfluidics and Nanofluidics 6 (2008) 261–265. [25] D.L. Miller, Particle gathering and microstreaming near ultrasonically activated gas-filled micropores, Journal of the Acoustical Society of America 84 (1988) 1378–1387.
174
J.H. Lee et al. / Sensors and Actuators A 195 (2013) 167–174
[26] R.B. Fair, Digital microfluidics: is a true lab-on-a-chip possible? Microfluidics and Nanofluidics 3 (2007) 245–281. [27] F. Mugele, J.C. Baret, Electrowetting, From basics to applications, Journal of Physics: Condensed Matter 17 (2005) R705–R774. [28] S.A. Elder, Cavitation microstreaming, Journal of the Acoustical Society of America 31 (1958) 54–64. [29] T.G. Leighton, The Acoustic Bubble, Academic Press, 1997. [30] K.H. Lee, J.H. Lee, J.M. Won, K. Rhee, S.K. Chung, Micromanipulation using cavitational microstreaming generated by acoustically oscillating twin bubbles, Sensors and Actuators A: Physical (2011), http://dx.doi.org/10.1016/j.sna.2011.11.037. [31] K.H. Lee, J.H. Lee, J.M. Won, S.K. Chung, Novel tweezers using acoustically oscillating twin bubbles, in: The 16th International Conference on Solid-State Sensors, Actuators and Microsystems (Transducers 2011), Beijing China, 2011, pp. 1713–1716. [32] J.H. Lee, K.H. Lee, J.B. Chae, K. Rhee, S.K. Chung, Bubble-induced streaming for micro-object manipulation, in: The 25th International Conference on Micro Electro Mechanical Systems (MEMS 2012), Paris, France, 2012, pp. 1013–1016. [33] S.K. Chung, Y. Zhao, S.K. Cho, Electrowetting-on-dielectric (EWOD) microfluidic devices, in: K.E. Herold, A. Rasooly (Eds.), Lab on a Chip (LOC) Technologies and Applications, Horizon Scientific Press/Caister Academic Press/Caister Academic Press, 2009. [34] C. Quilliet, B. Berge, Electrowetting: a recent outbreak, Current Opinion in Colloid and Interface Science 6 (2001) 34–39. [35] M. Vallet, M. Vallade, B. Berge, Limiting phenomena for the spreading of water on polymer films by electrowetting, European Physical Journal B (1999) 1–9. [36] S.K. Chung, K. Ryu, S.K. Cho, Electrowetting propulsion of water-floating objects, Applied Physics Letters 95 (2009) 014107. [37] S.K. Chung, K. Rhee, S.K. Cho, Bubble actuation by electrowetting-on-dielectric (EWOD) and its applications: a review, International Journal of Precision Engineering and Manufacturing 11 (2010) 991–1006. [38] Y. Zhao, S.K. Cho, Micro bubble manipulation by electrowetting on dielectric: transporting, splitting, merging and eliminating of bubbles, Lab Chip 7 (2007) 273–280. [39] P. Marmottant, S. Hilgenfeldt, Controlled vesicle deformation and lysis by single oscillating bubbles, Nature 423 (2003) 153–156. [40] P. Tho, R. Manasseh, A. Ooi, Cavitation microstreaming patterns in single and multiple bubble systems, Journal of Fluid Mechanics 576 (2007) 191–233. [41] S.H. Ko, S.J. Lee, K.H. Kang, A synthetic jet produced by electrowetting-driven bubble oscillations in aqueous solution, Applied Physics Letters 94 (2009).
Biographies Jeong Hyun Lee received the Bachelor’s degree of Mechanical Engineering from Myongji University in 2011. He is currently a graduate student in Myongji University and his research interests lie on the development of microfluidic applications based on an electrowetting-on-dielectric principle. Kyung Ho Lee received the Bachelor’s degree of Mechanical Engineering from Myongji University in 2011. He is currently a graduate student in Myongji University and his research interests lie on the development of micromanipulator using acoustically oscillating microbubbles. Jeong Byung Chae received the Bachelor’s degree of Mechanical Engineering from Myongji University in 2012. He is currently a graduate student in Myongji University and his research interests lie on the optimization of EWOD (electrowetting-ondielectric) systems. Kyehan Rhee is a professor of the Department of Mechanical Engineering at the Myongji University in Korea. He received his Ph.D. degree from the University of Minnesota, Minneapolis, U.S.A and worked as a Post-Doctoral Fellow in the Pennsylvania State University, State College, U.S.A. His research interests include hemodynamics, microfluidics, polymer smart material actuators and their application in biomedical devices. He serves as an editor of the International Journal of Precision Engineering and Manufacturing and the Journal of Biomechanical Science and Engineering. Sang Kug Chung is an assistant professor of the Department of Mechanical Engineering at the Myongji University in Korea. He received the Ph.D. degree in Mechanical Engineering and Materials Science from the University of Pittsburgh in 2009 along with the Graduate Research Excellence Award. He received the M.S. degree from Pohang University of Science and Technology (POSTECH) and B.S. from Myongji University. He had worked for the development of the world first Liquid Lens at Samsung Electro-Mechanics from 2003 to 2009 and researched oil spills at Advanced Fluids Engineering Research Center from 2000 and 2001. Upon joining the faculty at Myongji University in 2009, he has directed the Microsystems Laboratory. His research is in microfluidics and MEMS, including design and fabrication of micro/nano actuators and systems.