Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES
Technical Note
On-demand droplet capture and release using microwell-assisted surface acoustic waves Jin Ho Jung, Ghulam Destgeer, Jinsoo Park, Husnain Ahmed, Kwangseok Park, and Hyung Jin Sung Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b04542 • Publication Date (Web): 27 Jan 2017 Downloaded from http://pubs.acs.org on January 31, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Analytical Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 6
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
On-demand droplet capture and release using microwell-assisted surface acoustic waves Jin Ho Jung, Ghulam Destgeer, Jinsoo Park, Husnain Ahmed, Kwangseok Park and Hyung Jin Sung* Department of Mechanical Engineering, KAIST, Daejeon 34141, Korea. E-mail:
[email protected] ABSTRACT: We demonstrate an acoustofluidic platform that uses surface acoustic waves (SAWs) for the facile capture of droplets inside microwells and their on-demand release. When the AC signal applied to the device is tuned to modulate the location of the SAW, the SAW-based acoustic radiation force retracts or pushes the droplets into or out of one of three microwells fabricated inside a microchannel to selectively capture or release the droplet.
Introduction In droplet microfluidics, a single droplet that is isolated from a continuous phase fluid can be used as an individual experimental platform. Droplets are useful as biological or chemical micro-reactors because of their advantages such as the prevention of cross contamination and reagent evaporation, the accurate control of experimental conditions, and the possibility of single cell monitoring.1–3 In a microfluidic channel, droplets can be produced at a rate of up to a few kHz with uniform size,4 separated,5 coalesced,6 split,7 detected,8 mixed,9 and captured.10 The facile control of such droplets and the inherent advantages of their high surface to volume ratio mean that diverse applications of droplet microfluidics have been demonstrated, such as polymer chain reactions,11 drug delivery,12 nanomaterial syntheses,13 single cell culture,14 and cell sorting.15 In order to monitor a single cell or cell to cell interactions over a long period, the capture of a cell-laden droplet at a specific location within a microchannel is very important. Cell-laden droplets can be captured by using geometrical traps or constraints inside a microchannel. However, such methods lack the control required to capture target droplets with a passive technique.14 Wang et al. demonstrated droplet anchoring at specific microwells in a microchannel by applying a DC electric field.10 Although this active method can provide on-demand manipulation of droplets, it requires high voltages in excess of 1 kV. Moreover, droplets in multiple microwells cannot be manipulated individually. In this study, we developed a platform based on surface acoustic waves (SAWs) for on-demand droplet capture and release at specific microwells in a microfluidic channel. SAWs have been integrated in microfluidic channels for manipulating the micro-objects (particles, droplets and cells) due to their label-free, contact-less, non-invasive, biocompatible nature.16–19 Especially, they have been extensively utilized for droplet merging,20 size control,21 sorting,22 single droplet production,23 droplet steering,24 pipetting from moving droplets25 and splitting.26 Previous-
ly, we demonstrated a droplet splitting device utilizing the acoustic radiation force (ARF) from a high gradient of the SAW beam.26 We adapted a slanted finger interdigitated transducer (SF-IDT) to focus the SAW energy by decreasing the effective aperture of the beam to less than 100 µm. In addition, the position of the SAW beam center can be controlled by tuning the corresponding frequency. The droplets can then be captured in or released from the microwells by shifting the location of the SAW beam.
Experimental section A schematic diagram of the acoustofluidic device for droplet capture and release is shown in Figure 1. The device was fabricated by attaching a PDMS channel (Sylgard 184, Dow corning) to a piezoelectric substrate (Lithium niobate (LN), LiNbO3, 128˚ Y-cut, MTI Korea) with oxygen
Figure 1. Schematic diagram of the acoustomicrofluidic droplet capture and release device composed of a PDMS channel and a piezoelectric substrate patterned with slanted finger interdigitated transducers (SF-IDT). The droplets can be captured or released through application of an acoustic radiation force (ARF) and the location of the SAW beam is shifted by tuning the actuation frequency.
plasma bonding.27 The PDMS channel was fabricated
ACS Paragon Plus Environment
Analytical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
a
Capture
0 ms
Page 2 of 6
50 ms
60 ms
340 ms
620 ms
SAW on 220 ms
SAW off
b Release
0 ms
50 ms
70 ms
130 ms
190 ms
SAW on 90 ms
SAW off
with a soft lithography technique. The microchannel height and width are 40 µm and 100 µm respectively. The microwell area is 100 µm ⅹ 100 µm while that of the microwell neck is 50 µm ⅹ 70 µm. The entrance to the microwell has a bottleneck shape so that captured droplets can be locked in the microwell and resist any disturbance from the continuous phase fluid flow or pressure fluctuations. In order to produce water-in-oil droplets, the inner surface of the channel was rendered hydrophobic by flowing EGC-1720 solution (3M) into the microfluidic channel.28 DI-water and HFE-7500 (3M) are used as the dispersed and continuous phase fluids respectively. Droplet coalescence is prevented by dissolving 1 wt% surfactant (Pico-SurfTM 1) in the continuous phase fluid. The droplets are produced at the T-junction geometry and the ratio of the flow rates of the continuous and dispersed phase fluids is maintained constant to produce droplets with a uniform size. A syringe pump (neMESYS Cetoni GmbH) is used to control the continuous and dispersed phase flow rates. The experimental images were recorded using a high speed camera (pco.1200 hs PCO camera) attached to a microscope (Olympus IX71).29, 30 The piezoelectric substrate was patterned with bimetallic comb-like electrodes (Cr/Au, 300 Å / 1000 Å, E-beam evaporation process) that form the SF-IDT. The LN substrate with the SF-IDT was covered with a SiO2 layer (2000 Å, plasma-enhanced chemical deposition) to enhance the O2 plasma bonding
200 μm
with the PDMS microchannel. AC electrical signals are produced with an RF signal generator (N5171B, Keysight Technologies) after amplification (UP-3015, Unicorn Tech.) and the SAW voltage is measured with an oscilloscope (DSO-X 2022A, Keysight Technologies). The number of finger pairs is 40 and the aperture of the SFIDT is 1 mm. The pitch of the fingers (λ) ranges from 28 to 36 µm, which corresponds to a range of SAW frequencies (fSAW = cs/λ, cs is the speed of sound in LiNbO3) from 109 to 141 MHz. The effective aperture of the SF-IDT, Ai, can be estimated as31 ܣ୧ ≅
(ౄ ିై )
A
(1)
where fi is the working frequency, N is the number of finger pairs, fH and fL are the highest and lowest frequencies generated from the SF-IDT, and A0 is the total aperture. The effective aperture of the SAW beam (~ 100 µm) is much smaller than the entire SF-IDT aperture (1 mm). Moreover, as the aperture of the SAW is decreased, the center of the SAW beam is easily shifted transversely by tuning the corresponding SAW frequency. The displacement of the center of the SAW beam, ∆x, can be estimated as Δ= ݔ
బ ౄ ିై
Δ݂୧ .
ACS Paragon Plus Environment
(2)
Page 3 of 6
b 150
6.06 Vrms
i) Fail 9.09 Vrms
Droplet velocity, u (mm/s)
a Droplet displacement, x (μm)
ii) Capture 10.60 Vrms
100
12.11 Vrms 15.14 Vrms
t=0
50
i 0 x
ii
0
i) Fail
10
6.06 Vrms
8 6
9.09 Vrms
4 2
ii) Capture 10.60 Vrms 12.11 Vrms
0 15.14 Vrms
0
20
40
60
80
0
20
40
t (ms) Time, t (ms)
60
80
t (ms) Time, t (ms)
Figure 3. The droplet displacements and velocity in the x-direction. The flow rates of the continuous and dispersed phase fluids were 197 µL/hr and 3 µL/hr respectively. The voltages applied to the SF-IDT were 6.06, 9.09, 10.60, 12.11, and 15.14 Vrms. The SAW frequency was set at 117 MHz in order to locate the SAW beam at the trailing end of the entrance of the microwell. (a) The droplet displacement was measured with the program ImageJ at 1 ms intervals. A schematic diagram of the parameters used to characterize the droplet displacement is shown in the inset in Figure 3(a). The symbols ‘Δ’ and ‘O’ represent i) failed and ii) successful droplet capture respectively. (b) The droplet velocity was estimated by dividing Δx by Δt.
Results and discussion Experimental images of droplet capture and release by the SF-IDT are shown in Figure 2 (see also ESI movie I). The flow rates of the continuous and dispersed phase fluids were 197 µL/hr and 3 µL/hr respectively. This flow rate ratio results in a droplet diameter of 100 µm. The droplet capture sequence is depicted in Figure 2(a). When the SAW beam is carefully aligned at the end of the microwell (fSAW = 117 MHz, VSAW = 12.11 Vrms), the velocity of the droplet decreases rapidly to a standstill at t = 50 ms. When the acoustic impedance of the droplet is higher than that of the continuous phase fluid, there exists an ARF pushing the droplet away from the acoustic beam center because of the acoustic field gradient.32 The transverse directional ARF arises from the Gor’kov potential associated with the gradient of the SAW beam. The ARF is larger than the drag force acting on the droplet so it slows the droplet (t = 60 ms). After t = 60 ms, the droplet enters the microwell within the next 300 ms because the fluidic resistance of the microwell is smaller than that of the other main fluidic path. The droplet can be stored stably inside the microwell after the capture process because the length of the microwell orifice (70 µm) is less than the droplet diameter (100 µm), which prevents the droplet from escaping the microwell on its own. On-demand droplet release is demonstrated in Figure 2(b) (see ESI movie II). Prior to the droplet release, the microwells were filled with droplets by using the SAW beam in the capture technique described above. Releasing the droplets from the microwells can easily be performed by tuning the appropriate frequency. The location of the SAW beam is carefully aligned so that it is positioned at the middle of the microwell (fSAW = 114.5 MHz, VSAW = 5.17 Vrms). As soon as the signal is turned on, the SAW pushes the droplet into the main fluid stream. At 90 ms and 130 ms, the other droplets in the first and third microwells are slightly disturbed. However, the ARFs acting on those droplets are insignificant so only the targeted droplet in the central microwell is released.
In order to characterize the droplet capture sequence, the droplet displacement (Figure 3(a)) and velocity (Figure 3(b)) were measured at 1 ms intervals. The flow rates of the continuous and dispersed phase fluids were kept constant at 197 µL/hr and 3 µL/hr respectively. The peakto-peak voltages applied to the SF-IDT were 6.06, 9.09, 10.60, 12.11, and 15.14 Vrms. Without SAW actuation, the droplet displacement increases linearly as the droplet moves from left to right in the microchannel. For a SAW voltage of 9.09 Vrms, the displacement slope decreases and momentarily approaches zero as the droplet enters the SAW activation zone (t = 10 ms) (Figure 3(a)). However, the input voltage is not sufficient to completely halt the droplet so the droplet regains its velocity after 30 ms. The droplet velocity decreases below 1 mm/s (t = 20–40 ms), but it soon recovers before it is dominated by the SAW (Figure 3(b), VSAW = 400
Flow rate, Q (μl/hr)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
X
X
X
X X X X
Fail
300
Capture X X X
200
X
100
X X X X
X
X
X X
6
8
10
12
14
SAW voltage, VSAW (Vrms)
Figure 4. A regime diagram of the droplet capture sequence. The total flow rate was ranged from 100 µl/hr to 400 µl/hr while the SAW voltage was varied from 3.03 Vrms to 15.14 Vrms. The symbol ‘x’ and ‘o’ in Figure 4 represent the failure and success of the droplet capture sequence. The transition line between failure and capture was roughly estimated.
9.09 Vrms). In contrast, at higher input voltages (10.60, 12.11, and 15.14 Vrms), the droplets are completely over-
ACS Paragon Plus Environment
Analytical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
a
Capture
0s
3s
111 MHz
b
Release
0s
7s
117 MHz
10 s
123.2 MHz
3s
109 MHz
Page 4 of 6
7s
114.5 MHz
10 s
120.4 MHz
100 μm
Figure 5. Experimental images of sequential droplet capture (a) and release (b) as a demonstration of multiple droplet manipulation with a single SF-IDT. The orange arrows represent the location and direction of the SAW beam. (a) The droplets were captured in three microwells in consecutive order by applying operation frequencies of 111 MHz, 117 MHz, and 123.2 MHz. (b) Selective droplet release was demonstrated by applying operation frequencies of 109 MHz, 114.5 MHz, and 120.4 MHz. Droplet capture and release were achieved with a single SF-IDT.
powered by the ARF and captured by the microwell as their velocities decrease to zero. In Figure 3(a), it can be seen that the droplet displacements converge at x = 100 µm, which is the middle of the microwell. In Figure 3(b), the droplet velocities decelerate (t = 10–20 ms) and converge at u = 0 mm/s, which means that these droplets are captured in the microwell and are no longer affected by the continuous phase fluid. A regime diagram of the droplet capture is shown in Figure 4 for various flow rates and SAW voltages. The total flow rate was varied from 100 µL/hr to 400 µL/hr while the SAW voltage was varied from 3.03 Vrms to 15.14 Vrms. The symbols ‘x’ and ‘o’ in Figure 4 represent the failure and success of the droplet capture sequence. The transition line between failure and capture is a rough estimate The acoustic radiation force (ARF) acting on the droplet was proportional to the square of the SAW voltage while the drag force resisting the ARF was proportional to the flow velocity. The transition line was estimated as a secondorder polynomial of the input voltage (Q = 1.07 VSAW2 + 36.48 VSAW – 219). As the flow rate increases, the drag force acting on the droplet also increases, so a higher SAW voltage is required to achieve droplet capture. Experimental demonstrations of sequential droplet capture and release are shown in Figure 5(a) and (b) respectively (ESI movie III). The droplets were captured in the three microwells in consecutive order by applying operation frequencies of 111 MHz, 117 MHz, and 123.2 MHz (Figure 5(a)). Even if the effective aperture of the SAW beam affects the adjacent droplets, these droplets are not released from the microwell. Selective droplet release was demonstrated by applying operation frequencies of 109 MHz, 114.5 MHz, and 120.4 MHz (Figure 5(b)). The SAW beam produced by the SF-IDT has a narrow working zone, so it is able to manipulate each droplet individually. Shifting the SAW beam location can easily be achieved by tuning the SAW frequency, so this approach can be implemented in an automated system. In this platform, the droplet size should be smaller than the microwell to hold the droplets inside the microwell stably. Otherwise, the droplets may split at the microwell neck into the two daughter droplets. Also, the droplet diameter should be larger than the microwell neck so that it does not escape the well on its own. The small sized droplet could be captured but it easily escaped
the microwell due to the flow from the continuous phase fluid when the SAW voltage was off.
Conclusion In summary, we have demonstrated a droplet capture and release platform based on SAWs. The SF-IDT and the PDMS channel are implemented on a piezoelectric substrate. The SF-IDT can produce a narrow SAW beam with a location that is readily controlled by tuning the operating frequency. The droplets can be captured or released at the microwells simply by controlling the SAW frequency with the single SF-IDT. Our study demonstrated the simple and precise control of droplets in a microchannel. These features provide a simple and robust method of implementing a droplet monitoring platform.
ASSOCIATED CONTENT Supporting Information Details about additional results. Movie I: Droplet capture by the SF-IDT. Movie II: Droplet release by the SF-IDT. Movie III: Sequential droplet capture and release at the microwells by the SF-IDT. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author * Email:
[email protected] Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT This work was supported by the Creative Research Initiatives (no. 2016-004749) program of the National Research Foundation of Korea (MSIP) and the KUSTAR-KAIST Institute.
REFERENCES (1) (2)
Teh, S. Y.; Lin, R.; Hung, L. H.; Lee, A. P. Lab Chip 2008, 8 (2), 198–220. Theberge, A. B.; Courtois, F.; Schaerli, Y.; Fischlechner, M.; Abell, C.; Hollfelder, F.; Huck, W. T. Angew Chem Int Ed Engl 2010, 49 (34), 5846–5868.
ACS Paragon Plus Environment
Page 5 of 6
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
(3) (4) (5)
(6) (7) (8)
(9) (10) (11)
(12) (13) (14) (15)
(16)
(17)
(18)
Analytical Chemistry Joensson, H. N.; Andersson Svahn, H. Angew Chem Int Ed Engl 2012, 51 (49), 12176–12192. Garstecki, P.; Fuerstman, M. J.; Stone, H. A.; Whitesides, G. M. Lab Chip 2006, 6 (3), 437–446. Agresti, J. J.; Antipov, E.; Abate, A. R.; Ahn, K.; Rowat, A. C.; Baret, J. C.; Marquez, M.; Klibanov, A. M.; Griffiths, A. D.; Weitz, D. A. Proc Natl Acad Sci U S A 2010, 107 (9), 4004–4009. Niu, X.; Gulati, S.; Edel, J. B.; deMello, A. J. Lab Chip 2008, 8 (11), 1837–1841. Link, D. R.; Anna, S. L.; Weitz, D. A.; Stone, H. A. Phys. Rev. Lett. 2004, 92 (5), 54503. Guan, Z.; Zou, Y.; Zhang, M.; Lv, J.; Shen, H.; Yang, P.; Zhang, H.; Zhu, Z.; James Yang, C. Biomicrofluidics 2014, 8 (1), 14110. Paik, P.; Pamula, V. K.; Fair, R. B. Lab Chip 2003, 3 (4), 253–259. Wang, W.; Yang, C.; Liu, Y.; Li, C. M. Lab Chip 2010, 10 (5), 559–562. Tewhey, R.; Warner, J. B.; Nakano, M.; Libby, B.; Medkova, M.; David, P. H.; Kotsopoulos, S. K.; Samuels, M. L.; Hutchison, J. B.; Larson, J. W.; Topol, E. J.; Weiner, M. P.; Harismendy, O.; Olson, J.; Link, D. R.; Frazer, K. A. Nat Biotechnol 2009, 27 (11), 1025– 1031. Zhao, C. X. Adv Drug Deliv Rev 2013, 65 (11–12), 1420–1446. Hoang, P. H.; Park, H.; Kim, D. P. J. Am. Chem. Soc. 2011, 133 (37), 14765–14770. Shi, W.; Qin, J.; Ye, N.; Lin, B. Lab Chip 2008, 8 (9), 1432–1435. Baret, J. C.; Miller, O. J.; Taly, V.; Ryckelynck, M.; ElHarrak, A.; Frenz, L.; Rick, C.; Samuels, M. L.; Hutchison, J. B.; Agresti, J. J.; Link, D. R.; Weitz, D. A.; Griffiths, A. D. Lab Chip 2009, 9 (13), 1850–1858. Ding, X.; Li, P.; Lin, S.-C. S.; Stratton, Z. S.; Nama, N.; Guo, F.; Slotcavage, D.; Mao, X.; Shi, J.; Costanzo, F.; Huang, T. J. Lab Chip 2013, 13 (18), 3626–3649. Guo, F.; Li, P.; French, J. B.; Mao, Z.; Zhao, H.; Li, S.; Nama, N.; Fick, J. R.; Benkovic, S. J.; Huang, T. J. Proc. Natl. Acad. Sci. U. S. A. 2015, 112 (1), 43–48. Lin, S.-C. S.; Mao, X.; Huang, T. J. Lab Chip 2012, 12 (16), 2766.
(19) (20) (21) (22) (23) (24) (25) (26) (27) (28)
(29) (30)
(31)
(32)
Destgeer, G.; Sung, H. J. Lab Chip 2015, 15 (13), 2722– 2738. Sesen, M.; Alan, T.; Neild, A. Lab Chip 2014, 14 (17), 3325–3333. Schmid, L.; Franke, T. Lab Chip 2013, 13 (9), 1691– 1694. Franke, T.; Abate, A. R.; Weitz, D. A.; Wixforth, A. Lab Chip 2009, 9 (18), 2625–2627. Collins, D. J.; Alan, T.; Helmerson, K.; Neild, A. Lab Chip 2013, 13 (16), 3225–3231. Sesen, M.; Alan, T.; Neild, A. Lab Chip 2015. Sesen, M.; Devendran, C.; Malikides, S.; Alan, T.; Neild, A. Lab Chip 2016. Jung, J. H.; Destgeer, G.; Ha, B.; Park, J.; Sung, H. J. Lab Chip 2016, 16 (17), 3235–3243. Destgeer, G.; Ha, B. H.; Jung, J. H.; Sung, H. J. Lab Chip 2014, 14 (24), 4665–4672. Jung, J. H.; Lee, K. H.; Destgeer, G.; Lee, K. S.; Cho, H.; Ha, B. H.; Sung, H. J. Microfluid. Nanofluidics 2015, 18 (5–6), 1247–1254. Kim, S.; Kang, H. W.; Lee, K. H.; Sung, H. J. J. Micromechanics Microengineering 2011, 21 (9), 95026. Kang, H. W.; Sung, H. J.; Lee, T.-M.; Kim, D.-S.; Kim, C.-J. J. Micromechanics Microengineering 2009, 19 (1), 15025. Yatsuda, H.; Noguchi, K.; Yamanouchi, K. Japanese J. Appl. Phys. Part 1-Regular Pap. Short Notes Rev. Pap. 2000, 39 (5b), 3041–3044. Lee, J.; Shung, K. K. J. Acoust. Soc. Am. 2006, 120 (2), 1084–1094.
ACS Paragon Plus Environment
Analytical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Table of Content Graphic
ACS Paragon Plus Environment
Page 6 of 6
