Technical Note pubs.acs.org/ac

Microchannel Anechoic Corner for Size-Selective Separation and Medium Exchange via Traveling Surface Acoustic Waves Ghulam Destgeer,† Byung Hang Ha,† Jinsoo Park,† Jin Ho Jung,† Anas Alazzam,‡ and Hyung Jin Sung*,† †

Flow Control Laboratory, Department of Mechanical Engineering, KAIST, Daejeon, 305-701, South Korea Department of Mechanical Engineering, Khalifa University, Abu Dhabi, 127788, United Arab Emirates



S Supporting Information *

ABSTRACT: We demonstrate a miniaturized acoustofluidic device composed of a pair of slanted interdigitated transducers (SIDTs) and a polydimethylsiloxane microchannel for achieving size-selective separation and exchange of medium around polystyrene particles in a continuous, label-free, and contactless fashion. The SIDTs, deposited parallel to each other, produce tunable traveling surface acoustic waves (TSAWs) at desired locations, which, in turn, yield an anechoic corner inside the microchannel that is used to selectively deflect particles of choice from their streamlines. The TSAWs with frequency f R originating from the right SIDT and propagating left toward the microchannel normal to the fluid flow direction, laterally deflect larger particles with diameter d1 from the hydrodynamically focused sample fluid that carries other particles as well with diameters d2 and d3, such that d1 > d2 > d3. The deflected particles (d1) are pushed into the top-left corner of the microchannel. Downstream, the TSAWs with frequency f L, such that f L > f R, disseminating from the left SIDT, deflect the medium-sized particles (d2) rightward, leaving behind the larger particles (d1) unaffected in the top-left anechoic corner and the smaller particles (d3) in the middle of the microchannel, thereby achieving particle separation. A particle not present in the anechoic corner could be deflected rightward to realize twice the medium exchange. In this work, the three-way separation of polystyrene particles with diameters of 3, 4.2, and 5 μm and 3, 5, and 7 μm is achieved using two separate devices. Moreover, these devices are used to demonstrate multimedium exchange around polystyrene particles ∼5 μm and 7 μm in diameter. incremental studies proposing the “size-selective” separation of micro-objects19,20 lacked versatility in their operation as the conventional separation mechanism was employed to separate the smaller or larger particles from the other. To the best of authors’ knowledge, the successful separation of particles, irrespective of size, from the entire population has yet to be demonstrated, where a medium size particle could be easily singled out from the other smaller- and larger-sized particles. In this work, we propose a novel size-selective separation technique based on TSAWs to segregate three polystyrene (PS) microspheres of different sizes within a polydimethylsiloxane (PDMS) microchannel. The proposed miniaturized acoustofluidic device is comprised of a pair of slanted interdigitated transducers (SIDTs) and a microchannel (Figure 1a). The SIDTs can be actuated at a range of frequencies, f1−f 2 (right) and f 2−f 3 (left), such that f1 < f 2 < f 3. The triple separation of hydrodynamically focused PS particles with diameters of 3, 4.2, and 5 μm is achieved; first, by actuating both SIDTs simultaneously at the desired locations, and second, by

M

icrofluidic technology developments have paved the way for practical fluid actuation and micro-object manipulation techniques. In this regard, miniaturized surface acoustic wave-based devices, in particular, have made remarkable progress recently.1−5 The present study demonstrates the use of traveling surface acoustic waves (TSAWs) to efficiently manipulate microparticles and achieve size-selective separation and medium exchange. Size-Selective Separation. The isolation of selected particles that are not necessarily the smallest or largest in the population is termed “size-selective separation”. Numerous microfluidic separation techniques have been developed to segregate micro-objects (particles, cells, droplets) based on their sizes in an orderly manner.6,7 Active separation techniques (dielectrophoresis (DEP), 8 magnetophoresis, 9 optofluidics,10−12 or acoustofluidics13−15) incorporate external force fields proportional to the micro-object’s radius, such that the highest excitation force is applied to the largest micro-object and the force decreases with the size of the micro-object. Similarly, passive separation techniques (deterministic lateral displacement,16 inertial migration,17 or hydrodynamic filtration18) can easily single out the largest or the smallest size population of the micro-objects from the whole. A handful of © XXXX American Chemical Society

Received: February 9, 2015 Accepted: March 24, 2015

A

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Figure 1. (a) Schematic diagram showing a size-selective separator. (b) Triple separation of the PS particles via actuation of both SIDTs at distinct locations. (c) Separation realized by single SIDT actuation. (d) Schematic diagram illustrating the medium exchange around particles. (e) The prefocused PS particles (5 μm) are deflected from the primary carrier fluid leftward into the secondary fluid and then into the tertiary fluid downstream. (f) A side-view (section A-A′ in panel (a)) of the device shows the interaction between the TSAW and the fluid inside the PDMS microchannel. The TSAW (Rayleigh wave), coupled with the fluid, is turned into a leaky TSAW that radiates at the Rayleigh angle θt. The contour plot depicts the approximate acoustic pressure field inside the microchannel. The microchannel corner is indicated as a “region void of ARF” (where ARF = acoustic radiation force), because the particles present here do not experience any significant force in the lateral rightward direction, because of the relative weak acoustic field in this region and the direction of wave propagation.

mechanism.25 In a passive technique, two-step carrier medium exchange was applied to a cell suspension using a hydrodynamic filtration approach.26,27 Similarly, micro-object medium exchange was demonstrated through the inertial migration of particles28 and the presence of slanted microobstacles.19 The passive techniques usually have limitations in their microchannel design and mostly the fluid flow rates are adjusted to tune the operation, which also suffer from high time lags. The TSAW-based medium exchange device, which is the first of its kind, could overcome the limitations of a passive microfluidic system and offer a refined alternative to the previously used active microfluidic devices. Along with the size-selective separation, the miniaturized acoustofluidic device proposed here is also capable of continuous medium exchange around PS microspheres using TSAWs. The particle deflection mechanism of TSAWs1,13 differs from that of taSSAWs;25,29 however, both techniques share the advantages of continuous, contactless, and label-free manipulation of micro-objects. In the present device, the TSAWs, originating from the right SIDT, deflect the prefocused particles into the secondary medium (Figure 1d). Later, the TSAWs from the left SIDT deflect the particles into a tertiary medium downstream. The medium exchange around 5 μm PS particles is demonstrated in Figure 1e. The particles with a diameter of 7 μm are also subjected to medium exchange in a device operated at lower frequencies.

actuating only one SIDT (see Figures 1b and 1c). The separation of a medium size (4.2 μm) particles from relatively smaller (3 μm) and larger (5 μm) particles in a single step using one SIDT is made possible through the unintuitive response of PS particles when exposed to the TSAWs with a range of different frequencies. The size-selective separation mechanisms are explained in the subsequent section (enttiled “Working Mechanism”) with a focus on the “corner effect” and the acoustic radiation force factor. In another experimental demonstration, we separated PS particles 3, 5, and 7 μm in diameter, using a second device that worked at a lower range of frequencies. Medium Exchange. The transfer of suspended microobjects in a microchannel from one reagent-bearing medium to another without mixing the two media is termed “medium exchange”. A variety of microfluidic active as well as passive techniques have been developed to carry out this task. A DEPbased cell immersion and dipping technique was used to replace the cellular medium and wash the cells using a buffer.21 However, it required precise control of the electrical properties of the suspension media and the cells. To circumvent the limitation of DEP-based techniques, standing bulk acoustic waves were used to exchange the medium surrounding microparticles and cells via an ultrasonic particle switching mechanism.22−24 The bulk acoustic waves propagate as the whole of the substrate vibrates and generally require a high input power for their operation. An energy-efficient alternative is found in the surface acoustic wave technology where acoustic energy is concentrated only on the surface of the piezoelectric substrate. Recently, tilted-angle standing surface acoustic waves (taSSAWs) were used to wash cells using a similar switching



WORKING MECHANISM The operation of the miniaturized acoustofluidic devices used in this study can be understood in terms of the TSAW generation, acoustic wave−fluid coupling, the wave propagation B

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Analytical Chemistry inside the microchannel, and finally, the wave−particle interaction. The following subsections describe the specialized propagation mechanism by which the TSAWs moved across the substrate surface and leaked inside the microchannel, as well as the effects of the acoustic radiation force (ARF) acting on the microparticles. Corner Effect. A side-view of the miniaturized acoustofluidic device (Figure 1a) is shown in Figure 1f. The SIDTs are positioned on the sides with the PDMS microchannel in the middle. When actuated by a high-frequency ( f L) AC signal, the left SIDT produces a TSAW (Rayleigh wave) that couples with the fluid inside the microchannel, while a longitudinal leaky TSAW is radiated at the Rayleigh angle θt (Figure 1f). The simulated acoustic pressure field inside the microchannel determined the direction of wave propagation at the Rayleigh angle, in agreement with the Snell’s law (for details, see Figure S1 and Movie I in the Supporting Information). The inclined propagation of acoustic waves produce corner regions in which the net ARF does not deflect the suspended particles (Figure 1f). The microchannel corner (top-left corner in this case) is termed the “anechoic corner” (analogous to the microscale anechoic architecture30), because of its property to harbor particles without being affected by the acoustic waves. A rightpropagating TSAW (moving in the rightward direction) with adequate frequency (f L) would push the suspended particles situated outside of the top-left corner zone, first, toward the microchannel ceiling and, second, toward the top-right corner of the microchannel (a similar behavior is recently observed by Collins et al.31). Similarly, a left-propagating TSAW (f R) would push the suspended particles into the top-left corner of the microchannel. A schematic diagram of the size-selective separator (Figure 1a) reveals a sample fluid carrying particles of various sizes (red, blue, black) sandwiched between two sheath fluids (see Section I in Figure 1a). A left-propagating TSAW pushes the largest particles (red) into the top-left corner of the microchannel, leaving behind the smaller particles (see Section II in Figure 1a). Downstream of the microchannel, a right-propagating TSAW drives the intermediate-sized particles (blue) toward the top-right corner. The largest particles (red) are not influenced by the TSAW, because of their location (anechoic corner), and the smallest particles (black) are too small to be affected by the ARF (see Section III in Figure 1a). The propagation angle of the leaky TSAW appears to produce an anechoic corner region that is not subjected to an effective ARF. In this region, even larger particles are not influenced by the acoustic waves. This phenomenon is called the “corner effect” in this text and is used to dexterously manipulate the particles to achieve size-selective separation and medium exchange. A schematic illustration of a similar device used for medium exchange is shown in Figure 1d. Particles of uniform size (red), suspended in a central fluid (yellow) sandwiched by sheath fluids (green and blue), flow through the microchannel (see Section IV in Figure 1d). The left-propagating TSAW pushes the particles from the primary fluid (yellow) to the secondary fluid (green) in a controlled manner such that the particles do not enter the top-left corner region, which lacks an effective ARF (see Section V in Figure 1d). Downstream of the microchannel, the right-propagating TSAW pushes the particle through the primary fluid into the tertiary fluid (blue), where the medium around the particle is changed again (see Section VI in Figure 1d). Acoustic Radiation Force Factor. The acoustic radiation force factor (FF) is a dimensionless parameter defined as the

ARF [N] acting on a particle per unit acoustic energy density [J/m3] per unit cross-sectional area of the particle [m2]. The FF may be estimated using the elastic theory of Hasegawa et al.,32 which we applied recently to realize the submicrometer separation of PS microspheres.1 FF =

4 κ2



∑ {(n + 1)(Pn′+ 1Q n′ − Pn′Q n′+ 1)κ 2 n=0

− n(n + 1)(n + 2)(Pn + 1Q n − PnQ n + 1) + [n(n + 1)(Pn + 1Q n′+ 1 − Pn′+ 1Q n) − (n + 1)(n + 2)(Pn′Q n + 1 − Pn + 1Q n′)]κ + (n + 1)(Pn + 1Q n − PnQ n + 1)κ 2}

(1)

where Pn = (1 + Mn)Jn + NnYn

(2a)

Q n = NnJn − MnYn

(2b)

and Mn = −

Nn = −

[R nJn − κJn′]2 [R nJn − κJn′]2 + [R nYn − κY n′]2

(3)

[R nJn − κJn′][R nYn − κY n′] [R nJn − κJn′]2 + [R nYn − κY n′]2

(4)

and Rn =

χκ2 2 2 ⎧ ⎫ κ1Jn′ (κ1) 2n(n + 1)Jn (κ2) − ⎪ ⎪ ⎪ ⎪ κ1Jn′ (κ1) − Jn (κ1) (n + 2)(n − 1)Jn (κ2) + κ2 2Jn′′(κ2) ⎬ ×⎨ 2 ′′ ⎪ κ1 [(σ / (1 − 2σ ))Jn(κ1) − Jn (κ1)] − 2n(n + 1)[Jn(κ2) − κ2Jn′(κ2)] ⎪ ⎪ ⎪ κ1Jn′ (κ1) − Jn (κ1) (n + 2)(n − 1)Jn (κ2) + κ2 2Jn′′(κ2) ⎭ ⎩ (5)

⎛ 2πf ⎞ κ1 = k1a = ⎜ ⎟a ⎝ c1 ⎠

(6a)

⎛ 2πf ⎞ κ2 = k 2a = ⎜ ⎟a ⎝ c2 ⎠

(6b)

χ=

ρf ρp

(6c)

Here, the wavenumbers (k1 and k2) and sound velocities in an elastic sphere (c1 and c2) correspond to the longitudinal and shear waves propagating inside the particle with radius a, respectively. ρf and ρp are densities of the fluid and the elastic PS particle, respectively. σ is the Poisson ratio of PS, whereas Jn and Yn are the spherical Bessel functions of the first and second kind with order n, respectively. The dependence of FF on the particle diameter and the acoustic wave (here, the TSAW) frequency is illustrated in Figure 2 (for details, see Figure S2 in the Supporting Information). The solid lines corresponding to the κ factors indicate the regions in which the FF values are most prominent (κ > 1) or least significant (κ < 1). The dimensionless parameter κ is a function of the wave frequency (f) and the particle diameter (d), according to the equation C

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wavelengths of the right and left SIDTs varied from top to bottom over the respective ranges of 30−25 μm and 25−20 μm. Similar values for the second device were 40−35 μm and 35−30 μm, respectively. The approximate TSAW actuation frequencies were 128−154 MHz (device 1, right), 154−192 MHz (device 1, left), 96−110 MHz (device 2, right), and 110− 128 MHz (device 2, left). The SIDTs in both devices each had aperture lengths of 5 mm and 60 finger pairs. Experimental Setup. The PS microparticles used in this study (from Thermo Scientific, Inc. and Polysciences, Inc.) were 3, 4.2, 5, and 7 μm in diameter (see the Supporting Information for details). The sample fluid mixture was prepared by suspending the particles in DI water (a drop of particle suspension per milliliter of water) to a sufficient dilution level where the effect of acoustic scattering on the adjoining particles can be ignored. The sample and sheath fluids were pumped through separate inlet ports using a syringe pump (neMESYS, Cetoni GmbH). An RF signal generator (Agilent, Model N5181A) produced high-frequency signals that were amplified (Mini Circuits LZT-22+) prior to delivery to the SIDTs. A twostep sweep signal (dwell time = 2 ms) was produced using two different frequencies and amplitudes to actuate both of the SIDTs simultaneously. In this simple setup, a signal generation apparatus was used to produce left- or right-propagating TSAWs with variable frequencies at specific positions relative to the microchannel. For a lone SIDT actuation, a singlefrequency signal was delivered to the targeted SIDT.

Figure 2. A contour plot showing FF plotted against the acoustic wave frequency and the PS particle diameter. The solid line corresponds to the factor κ.

κ=

πfd cf

where cf is the speed of wave propagation in the particlecarrying fluid. The plot shown in Figure 2 suggests that the ARF acting on a suspended particle may be readily manipulated by adjusting the TSAW frequency or varying the particle diameter. For example, a 6 μm PS particle exposed gradually to TSAWs with frequencies in the range of 100−130 MHz displays initially low FF values that reach a maximum value at 112 MHz and subsequently fall off. Similarly, a TSAW frequency of 112 MHz applied to a mixture of 5, 6, and 7 μm PS particles produces a maximum FF value on the 6 μm particles, which facilitates the isolation of the 6 μm particles from the others. These effects provide a mechanism for achieving on-demand deflection of chosen particles, irrespective of their size, to realize size-selective separation and medium exchange around the particles.



RESULTS AND DISCUSSION We used two miniaturized acoustofluidic devices to achieve size-selective separation and medium exchange of PS particles of various sizes at different flow rates. The first device was composed of a pair of SIDTs (right: 128−154 MHz, left: 154− 192 MHz) to manipulate particles with diameters of 3, 4.2, and 5 μm (see Figures 1b, 1c, and 1e and Movie II in the Supporting Information). In Figure 1b, a hydrodynamically focused mixture of PS particles (3, 4.2, 5 μm) suspended in deionized (DI) water was pumped through the microchannel at a net flow rate of 75 μL/h (sample: 25 μL/h; sheath: 25 μL/h each). A left-propagating TSAW (135 MHz, 0.85 W) deflected the 5-μm particles into the top-left corner of the microchannel. The points corresponding to the 3-, 4.2-, and 5-μm particles and the 135 MHz frequency could be identified in the FF contour plot shown in Figure 2. The 5-μm particles experienced a maximum force (highest FF value) relative to the force experienced by the other particles. Downstream of the microchannel (Figure 1b), the left SIDT was actuated at 175 MHz, 0.85 W to radiate a right-propagating TSAW and deflect 4.2 μm particles. It is important to note that the 5-μm particles were not deflected, because of their presence in the top-left corner of the microchannel, where acoustic waves did not affect them. The mapping of points in Figure 2 corresponding to the 3-, 4.2-, and 5-μm particles and a 175 MHz frequency revealed that the 3-μm particles did not experience a significant force, whereas the 5-μm particles were subjected to the “corner effect”, even though the FF value was reasonable. The corresponding values of κ were consistent with the values predicted based on a recently conducted study (i.e., the ARF on the PS particles was most significant for κ ≈ 1.4 and least effective for κ < 1).1,39 Recently, Skowronek et al.39 realized a particle band-pass filter using a similar particle deflection mechanism; however, a theoretical explanation of the phenomena was lacking. The triple separation of PS particles



EXPERIMENTAL SECTION Device Design and Fabrication. The acoustofluidic miniaturized device was prepared using a PDMS microchannel attached to a piezoelectric substrate (lithium niobate (LiNbO3), 128° Y−X cut, MTI Korea) bearing interdigitated metal electrodes (SIDT, Cr/Au, 300 Å/1000 Å) deposited on top using an e-beam evaporation process. The device fabrication process was quite similar to the process described in Destgeer et al.;1,2,13 however, an additional SiO2 layer (2000 Å, plasmaenhanced chemical vapor deposition) was deposited onto the substrate to protect the SIDTs from damage and enhance the PDMS bonding to the substrate.33−35 The effect of SiO2 layer on wave mode is discussed elsewhere.36,37 The PDMS microchannel (w × h: 200 μm × 40 μm), molded using conventional soft lithography processes, was bonded to the substrate using O2 plasma bonding. A narrow microchannel ensured a weak acoustic streaming flow (measured using micro PIV technique), suppressed by a higher bulk fluid velocity, that would be of significance in a wider microchannel.2,13,38 This feature prevented undesired mixing of fluids during the medium exchange experiments. In this study, we used two different device designs corresponding to a variable range of TSAW actuation frequencies. In the first device, the nominal D

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Analytical Chemistry was demonstrated in a continuous flow by actuating two SIDTs at the desired locations, with variable frequencies (Figure 1b). Figure 1c shows that, by actuating only a single SIDT at a frequency of 165 MHz, the right-propagating TSAW could be utilized to separate particles such that the 4.2-μm particles underwent a large deflection, compared to the deflection distances of the 3- and 5-μm particles. In this way, the 4.2-μm particles could be easily isolated. The particles’ deflection distances were confirmed, as the points corresponding to particle diameters of 3, 4.2, and 5 μm and the 165 MHz frequency were identified (Figure 2). The TSAW-based device achieved the single-step size-selective separation of particles with a given diameter from a population of particles having smaller and larger diameters. This device was used to exchange the medium around PS particles ∼5 μm in diameter (Figure 1e). The particle-carrying sample flow (25 μL/h) was sandwiched between two sheath flows (50 μL/h each) and exposed to a left-propagating TSAW (135 MHz, 0.85 W) to deflect the 5-μm particles from the primary medium (yellow) into the secondary medium (green). The power input was precisely controlled to push the particles in the secondary medium while avoiding the top-left anechoic corner of the microchannel. Downstream of the microchannel, the right-propagating TSAW (175 MHz, 1.33 W) deflected the particles from the secondary medium to the tertiary medium (blue) through the primary medium. The input power was relatively high at this stage, to further deflect the particles. A second device with a pair of SIDTs (right: 96−110 MHz, left: 110−128 MHz) was used to manipulate the 3-, 5-, and 7μm PS particles (see Figure 3 and Movie II in the Supporting

successfully applied to the 7-μm particles, using left- and rightpropagating TSAWs (97 and 122 MHz, respectively) at various powers and at different flow rates (see Figures 3c and 3d). Additional separation and medium exchange results are reported in Figure S2 in the Supporting Information. Fluorescence imaging was also used to further explain the separation behavior under the corner effect and TSAWs (see Movie III and Figure S3 in the Supporting Information for details).



CONCLUSIONS We designed two TSAW-based devices that could be operated over a range of frequencies (96−192 MHz). The devices were used to manipulate PS particles of various sizes (3, 4.2, 5, 7 μm). A contour plot of the acoustic radiation force factor mapped against the frequency and particle diameter further supported the experimental results. A simulation of the acoustic pressure field inside the microchannel demonstrated acoustic wave propagation. The “corner effect” permitted realization of the size-selective particle separation: first, by actuation of both SIDTs and the corner effect, and second, by a single SIDT actuation. The medium around the different particles was exchanged using a variety of flow rates, and the input power was controlled to work around the corner effect.



ASSOCIATED CONTENT

S Supporting Information *

Details about simulation and additional results. Movie I: simulated acoustic pressure field. Movie II: size-selective separation and medium exchange of polystyrene particles. Movie III: “corner effect” for microparticle manipulation via TSAW. This material is available free of charge via the Internet at http://pubs.acs.org.



AUTHOR INFORMATION

Corresponding Author

*Tel.: +82-42-350-3027. Fax: +82-42-350-5027. E-mail: [email protected]. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was supported by the KUSTAR-KAIST Institute and the Creative Research Initiatives (No. 2014-001493) program of the National Research Foundation of Korea (MSIP).



Figure 3. Triple separation of PS particles with diameters of 3, 5, and 7 μm under continuous flow rates of (a) 75 μL/h or (b) 300 μL/h. The medium around the 7-μm PS particles was successfully changed (c) from the primary to the secondary and then tertiary fluids (sample, 25 μL/h; sheath, 50 μL/h each), or (d) from the primary to the secondary and then back to the primary fluid (sample, 50 μL/h; sheath, 100 μL/h each). A series of images were stacked together to obtain the particles’ trajectories, while sample and sheath fluids were highlighted with different pseudo-colors for the sake of clarity.

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NOTE ADDED AFTER ASAP PUBLICATION This manuscript was published ASAP on April 7, 2015. The ACKNOWLEDGMENTS had been deleted in error. The revised version was reposted on April 16, 2015.

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DOI: 10.1021/acs.analchem.5b00525 Anal. Chem. XXXX, XXX, XXX−XXX

Microchannel anechoic corner for size-selective separation and medium exchange via traveling surface acoustic waves.

We demonstrate a miniaturized acoustofluidic device composed of a pair of slanted interdigitated transducers (SIDTs) and a polydimethylsiloxane microc...
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