Dielectrophoretic manipulation and separation of particles in an S-shaped microchannel with hurdles

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1 University of Wollongong Research Online Faculty of Engineering and Inforation Sciences - Paers: Part A Faculty of Engineering and Inforation Sciences 2013 Dielectrohoretic aniulation and searation of articles in an S-shaed icrochannel with hurdles Ming Li University Of Wollongong, l433@uowail.edu.au Shunbo Li Hong Kong University of Science and Technology Weihua Li University of Wollongong, weihuali@uow.edu.au Weijia Wen Hong Kong University of Science and Technology Gursel Alici University of Wollongong, gursel@uow.edu.au Publication Details Li, M., Li, S., Li, W., Wen, W. & Alici, G. (2013). Dielectrohoretic aniulation and searation of articles in an S-shaed icrochannel with hurdles IEEE/ASME International Conference on Advanced Intelligent Mechatronics (AIM) ( ). United States: IEEE. Research Online is the oen access institutional reository for the University of Wollongong. For further inforation contact the UOW Library: research-ubs@uow.edu.au

2 Dielectrohoretic aniulation and searation of articles in an S-shaed icrochannel with hurdles Abstract This aer resents a novel dielectrohoresis ()-based icrofluidic device, which incororates ultile round hurdles within an S-shaed curved icrochannel for continuous aniulation and searation of icroarticles. Local nonunifor electric fields are induced by eans of both constricted gas fored between hurdles and outer channel wall, and variable current lengths in curved sections with equal width. Under the effect of negative, articles will be directed away fro either inner wall or hurdle edge, as they transort throughout the icrochannel electrokinetically. Both exerient and nuerical siulation were conducted, the results of which showed that fix-sized (i.e. 10 or 15 P) olystyrene (PS) articles could be successfully switched, directed and focused by adjusting alied voltages at inlet and outlets, and size-based searation of 10 and 15 P articles was achieved with a careful selection of alied voltages. Coared to other icrochannel designs that ake use of either obstacle or curvature individually for inhoogeneous electric fields, this design offers advantages such as iroved controllability over article otion, lower requireent of alied voltage, reduced fouling and article adhesion, etc. Keywords shaed, icrochannel, hurdles, dielectrohoretic, aniulation, articles, searation Discilines Engineering Science and Technology Studies Publication Details Li, M., Li, S., Li, W., Wen, W. & Alici, G. (2013). Dielectrohoretic aniulation and searation of articles in an S-shaed icrochannel with hurdles IEEE/ASME International Conference on Advanced Intelligent Mechatronics (AIM) ( ). United States: IEEE. This conference aer is available at Research Online: htt://ro.uow.edu.au/eisaers/1247

3 Dielectrohoretic aniulation and searation of articles in an S-shaed icrochannel with hurdles* Ming Li, Shunbo Li, Weihua Li, Weijia Wen, and Gursel Alici Abstract This aer resents a novel dielectrohoresis ()-based icrofluidic device, which incororates ultile round hurdles within an S-shaed curved icrochannel for continuous aniulation and searation of icroarticles. Local nonunifor electric fields are induced by eans of both constricted gas fored between hurdles and outer channel wall, and variable current lengths in curved sections with equal width. Under the effect of negative, articles will be directed away fro either inner wall or hurdle edge, as they transort throughout the icrochannel electrokinetically. Both exerient and nuerical siulation were conducted, the results of which showed that fix-sized (i.e. 10 or 15 µ) olystyrene (PS) articles could be successfully switched, directed and focused by adjusting alied voltages at inlet and outlets, and size-based searation of 10 and 15 µ articles was achieved with a careful selection of alied voltages. Coared to other icrochannel designs that ake use of either obstacle or curvature individually for inhoogeneous electric fields, this design offers advantages such as iroved controllability over article otion, lower requireent of alied voltage, reduced fouling and article adhesion, etc. I. INTRODUCTION Dielectrohoresis (), first adoted by Pohl [1], is a henoenon that occurs due to a translational force exerted on a dielectric article in a nonunifor electric field. With the raid develoent of lab-on-a-chi (LOC) devices in recent years, it has been widely used as one of the ost oular ethods to aniulate various icro/nano scale bioarticles (i.e. DNA, rotein, bacteria, virus, aalian and yeast cells) in icrofluidic systes [2-3]. Coared to other (i.e. echanical, theral, agnetic, acoustic, otical, cheical and electrical) techniques alied within icrofluidics [4], offers a nuber of significant advantages: label-free nature, favourable scaling effects, silicity of the instruentation, ability to aniulate neutral bioarticles, and analysis of high selectivity and sensitivity [3, 5-6]. Traditionally, the satial nonuniforities required for effect are generated by alying alternative current (AC) electric fields to the icroelectrodes atterned within icrochannels [3, 7], however, such electrode-based icrodevices suffer fro fabrication colexities due to electrode construction, electrode fouling [8], and *Research suorted by University of Wollongong through an international links roject. M. Li, W. Li, and G. Alici are with School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, Wollongong, NSW 2522, Australia (corresonding author: Prof. Weihua Li; hone: ; fax: ; e-ail: weihuali@uow.edu.au). S. Li and W. Wen are with Deartent of Physics, The Hong Kong University of Science of Technology, Clear Water Bay, Kowloon, Hong Kong (e-ail: hwen@ust. hk). electrocheical reactions on the electrode surface [9]. These robles are avoided in insulator-based icrodevices, where direct current (DC) or DC-biased AC electric fields are alied via external electrodes suberged in inlet and outlet reservoirs, and electric field gradients are induced around insulating objects. In such devices, two ain aroaches have been used to generate required nonunifor electric fields [10-11]: obstacles and icrochannel curvature. However, icrodevices with electrically insulated obstacles (i.e. including osts, rectangular/triangular hurdles, ridge, oil drolet, and oil enisci) ebedded in straight icrochannels, have such liitations as locally alified electric fields, large trans-ebrane voltages and shear stresses on cells, Joule heating, and fouling due to article clogging or adhesion [12]. Although the highly-intensify local electric fields can be avoided in curved insulating icrochannels, including sawtooth, serentine, circular, siral, waved icrochannels, this curvature-based ethod requires sufficiently large alied DC voltage and/or long curved section for effective erforance of the device, inducing colexities in ters of oeration and fabrication. In addition, the device is ore sensitive to containation (i.e. article adhesion on channel wall), as long channel increases the ossibility of surface inhoogeneity. In this work, we develoed a design cobing the effects of obstacle and curvature to generate electric field gradient required for the effect, where ultile round hurdles are ebedded within an S-shaed curved icrochannel to achieve continuous article aniulation and searation. The aforeentioned adverse effects of using each aroach individually, such as locally alied electric fields, article clogging, large alied voltage, etc., have been significantly reduced. Moreover, the desired functionality of the icrofluidic device can be achieved ore easily due to the increase of aroach for article aniulation. The aniulation functioning of the design was deonstrated by controlling the trajectories of 10 or 15 µ PS articles by adjusting alied voltages. In addition, the searation functioning was verified by continuously searating 10 and 15 µ PS articles according to their differences in size. Both exeriental and nuerical results were resented, which showed a reasonable agreeent. II. THEORY Particles susended in an electrically conducting liquid under the influence of external electric field are subjected to electrohoretic, electroosotic and dielectrohoretic effect. The cobination of fluid electroososis (EO) and article electrohoresis (EP) is tered electrohoretic (EK) flow, resulting in the electrokinetic velocity of articles written as [13]

4 u = µ E = u + u = µ ) E EK EK EO EP ( EO µ EP (1) µ, = ε ζ η and ε ζ η µ EO w µ EP where EK = are electrokinetic, electroosotic, and electrohoretic obility, resectively. ε and η are the erittivity and dynaic viscosity of the susending ediu, resectively. ζ w and ζ reresent, resectively, the zeta otential of the channel and the article. E is the electric field vector. Equation (1) shows that the electrokinetic velocity of the article is linearly roortional to the local electric field, leading to the article transort along the electric lines. Using the diole oent ethod, the tie-average force and the induced dielectrohoretic velocity acting on a dielectric sherical article in a nonunifor DC electric field are given by [14] and u F 3 (1 2) πε d f ( E E) (2) = 2 = µ ( E E) = ( ε d f 6η ) ( E E) (3) where d is the article diaeter, µ is the dielectrohoretic obility, f = ( σ σ ) ( σ + 2σ ) is known as the Clausius-Mossotti () factor, σ and σ are the electric conductivities of article and the susending ediu, resectively. If the article is less conductive than the susending ediu ( σ < σ ), factor will be negative ( f < 0 ), inducing a negative force that reels articles away fro the region of higher electric field. u EK F F F F Figure 1. Illustration of the negative dielectrohoretic aniuulation and searation of articles in a curved icrochannel ebedded with a round hurdle. Distribution of the electric-field lines and contours of the electric-field strength (E) within the icrochannel are shown (darker region has a stronger electric field). Particles oving through the icrochannel electrokinetically are subjected to negative forces (indicated by dark blue arrows). Herein, we utilized the effect of both obstacle and curvature to generate local electric field gradient throughout the icrochannel, which contributes to a novel technique for the continuous control of article oveent in a icrofluidic chi by effect. The echanis of the roosed design is illustrated scheatically in Figure (1), where a sei-circular icrochannel ebedded with a round hurdle are resented along with the electric field lines (or equivalently the strealines with black arrows indicating the direction) and contours of the electric field strength (the darker the stronger). The overall distribution of electric field is as follows: a relatively weaker and slightly nonunifor electric field is generated in the width direction of the curved channel with unifor cross sections; whereas, a stronger and highly nonunifor electric field is created near the edge of the hurdle (electric field obtained axiu value at the hurdle edge and decayed as increasing distance away fro the hurdle). Consider a article subjected to negative effect assing though the icrochannel under the cobined effect of EOF and EP, reulsive forces (dark blue arrows, relatively weak in the curved section, while strong in the constricted region) are exerted on the article all along its oveent. III. MATERIALS AND METHODS A. Microchannel Fabrication and Layout Figure 2. Photograh of the -based icrofluidic chi for continuous article aniulation and searation. The inset indicates the structrues and diensions of the design. The olydiethylsiloxane (PDMS) icrofluidic channel was fabricated using standard soft-lithograhy technique, and a detailed fabrication rocess can be found in our revious work [15]. As shown in Figure (2), the icrofluidic chi is coosed of two sei-circular channels that each integrates with three round hurdles fro inner wall, one inlet (A) and two outlet (B and C) reservoirs, and three straight connecting icrochannels. Both sei-circular channels are 300 µ in width, having a large curvature of 600 µ, while a sall curvature of 300 µ. The round hurdles in the first and second curved channel have radius of, resectively, 150 µ and 170 µ, creating 40 µ and 60 µ-wide gas between hurdle and outer wall, resectively. All three straight connecting channels have a width of 300 µ and a length of 1. The entire icrochannel has the sae deth of 40 µ. All three reservoirs at the inlet and outlets are 3 in height and 6 in diaeter.

5 Figure 3. Scheatic diagra of the S-shaed icrochannel ebedded with ultile round hurdles. As indicated in Figure (3), the roosed icrochannel taking advantage of curvature and hurdle for continuous article aniulation and searation consists of two units. The first (or clockwise) unit connects with an inlet, while the second (or counterclockwise) unit connects with two outlets lacing with an intersection angle of 90 degrees. Both units incororate a sei-circular icrochannel with three round hurdles fro the inner wall. Multile round hurdles were chosen to enhance effect and irove device erforance for article aniulation and searation [16]. Moreover, it has been stated in the revious work of Kang et al [17] that incoing osition of articles could affect the article trajectory shift after assing the block, hence the first curved section of hurdle-ebedded sei-circular icrochannel for re-rocessing was designed. The first and second curved section are for article re-focusing and ost-aniulation (or searation), resectively. More iortantly, the first curved channel with ebedded round hurdles was designed to re-focus articles close to the hurdle region within the second curved unit for subsequent aniulation, therefore, it allows for increased efficiency of switching and sorting articles. B. Sale Prearation In this study, two tyes of articles with varied sizes (Fluosheres, Invotrogen, CA, USA) were utilized: yellow-green fluorescent 10 µ and blue fluorescent 15 µ olystyrene (PS) icrosheres. Both original article solutions were diluted by deionized (DI) water 15 ties. For searation exerient, the diluted 10 and 15 µ article solutions were ixed at a volue ratio of 1:1. The article solutions were gently vibrated rior to be introduced into the inlet reservoir using a iette. Moreover, the outlet reservoirs were filled with the corresonding working solution, and the liquid level in each reservoir was carefully balanced before alying DC voltages via suberged latinu (Pt) electrodes. C. Exeriental Setu The electric field was generated by a DC ower suly (SL10P300/200, Sellan High Voltage Electronics Cor., Hauauge, NY). The otion of article through the icrochannel was onitored and recorded by an inverted icroscoe (Olyus IX71, Tokyo, Jaan) equied with a CCD caera (DP 70, Olyus, Tokyo, Jaan). The caera was run in the video ode at the seed of 15 fraes er second and the acquired digital iages had a resolution of 680 X 512 ixels. All the videos and iages were ost-rocessed by MATLAB (Mathworks Inc., Natick, MA), and the article trajectories were obtained by sueriosing consecutive iages converted fro videos. D. Nuerical Modeling In the siulation, we used a two-diensional (2D) odel that first develoed by Kang et al [18], and has also validated by other researchers in various icrofluidic channel structures [15, 19-21] to redict the article otion throughout the icrochannel. By introducing a correction factor, c, to account for the effects of article size, article-article interaction, etc. on the dielectrohoretic velocity, the velocity of article can be written as u = u EK + c u = µ EKE + cµ ( E E) (4) The above equation was erfored in COMSOL 4.0 to redict the article trajectory, where the electrokinetic obility and the dielectrohoretic obility were calculated by Equation (1) and Equation (3), resectively. The zeta otential values of PS articles and channel wall in 10 M NaCl solution were set to be -33 and -54 V, resectively 3 [22-23]. The dynaic viscosity, η = kg ( s), and 10 erittivity, ε = C ( v ), for ure water at 25 C were also used. As the electric conductivity of olystyrene article in DC electric field is uch saller than that of susending ediu (i.e. 10 M NaCl solution) used in our exerients, the factor, f, which deends on electrical conductivities of both the article and the susending ediu was set to be IV. RESULTS AND DISCUSSION A. Continuous Maniulation of Particles (a1) (b1) (a2) (b2) Figure 4. Exeriental (left colun: sueriosed iages) and nuerical (right colun) deonstration of aniulating 10 µ articles. The alied voltage at the inlet was increased fro (a) 18 to (b) 32, while alied voltages at outlet B and outlet C were fixed at 0 and 2, resectively. The red arrows indicate the flow direction.

6 Figure (4) shows the coarison between exerientally obtained (left colun) and nuerically redicted (right colun) results of 10 µ article trajectories under varied inlet voltages. It can be found that at lower inlet voltage (a: 18), all articles were oved out of the icrochannel fro outlet B, however, articles were directed to a narrower strea in outlet C at higher alied voltage (b: 32). According to Equation (2), force deends on the gradient of the square of the electric field, larger alied voltage can induce larger reulsive effect, which in turn causes articles to be deflected further away fro the hurdle and inner wall. The right colun of Figure (4) reresents the nuerically redicted 10 µ article trajectories in the sae conditions as those in the exerients. The correction factor was set to be 0.5 by atching the siulated results to those of obtained sueriosed iages, which reained constant for articles of fixed size in all the cases. (a1) 1 (a2) outlet C voltage to 3, but fixed outlet B to be grounded, articles were diverted to ove fro outlet B again due to the re-distribution of the flow at the bifurcation. It could be found that alied voltages at both inlet and outlet could affect article trajectory, and 15 µ articles can be directed into either outlet B or outlet C deending on the alied voltages. In addition, with the increase of inlet and/or outlet voltage, 15 µ articles were observed to obtain a better focusing effect (forced into a narrower strea), which corresonds to our revious founding that the erforance of article focusing could be iroved with the rise of alied electric field [15]. By setting the correction factor to be 0.4 for 15 µ articles, the nuerically redicted results (right colun) coincides accetably with the exerientally observed sueriosed iages (left colun). B. Continuous Size-based Searation of Particles (a) 5 V (b) 15 µ articles (b1) (c1) (b2) (c2) Figure 5. Exeriental (left colun: sueriosed iages) and nuerical (right colun) deonstration of aniulating 15 µ articles. The alied voltage to inlet and outlet C were, resectively: (a) 90 and 1; (b) 180 and 1; (c) 180 and 3, while the outlet B was grounded in all cases. In siilar analysis, we studied exerientally and nuerically how to control the otion of 15 µ articles by adjusting alied voltages at inlet and outlet. Figure (5) reresents the exerientally observed (left colun: sueriosed iages) trajectories of 15 µ articles at the exit region of the icrochannel under different cobinations of inlet and outlet voltages: (a) when alied voltages at inlet, outlet B and outlet C were, resectively, 90, 0 and 1, articles oved out of the icrochannel in a confined strea fro outlet B; (b) by increasing the inlet voltage to 18 but fixing outlet voltages, articles were ushed further fro round hurdles and directed to outlet C in a narrower strea due to larger reulsive force; (c) further increased the 10 µ articles Figure 6. Continuous searation of 10 (yellow) and 15 µ (blue) articles in the roosed design, when the alied voltages at inlet A and outlet C were 160 and 5 V, resectively, and outlet B was grounded: (a) exerientally obtained sueriosed iage, and (b) nuerically redicted result. The red arrows indicate the flow direction. According to Equation (2), the force is roortional to the cube of article diaeter, therefore, larger articles are subjected to larger forces, and hence tend to be deflected further fro the hurdle edge and inner wall coared to saller ones. This echanis is utilized in the roosed icrochannel to erfor continuous searation of articles based on size. A tyical case of the searation of ixed 10 (yellow) and 15 µ (blue) PS articles with alied voltages at inlet, outlet B and outlet C of 160, 0 and 5 V, resectively, is shown in Figure (6). It can be seen that there were two distinct article streas at the bifurcation, as 10 and 15 µ articles were sorted and oved in a focused strea fro outlet B and outlet C, resectively. The searation erforance was also reasonably redicted by nuerical siulation (see right colun of Figure 6), in which the trajectories of 10 and 15 µ articles were reresented by yellow and blue lines, resectively, and the correction factors were fixed to be 0.4 and 0.5, resectively, for 10 and 15 µ articles. V. CONCLUSION In this work, an S-shaed curved icrochannel ebedded with ultile round hurdles for continuous aniulation and searation of icroarticles using negative was resented. In such design, the effect of obstacle and curvature are cobined to induce nonunifor electric fields, generating controllable local electric fields all along the icrochannel. Both exeriental and nuerical results indicated that by adjusting alied voltages at the inlet and outlets,

7 icroarticles that can be directed into distinct outlets, ositioned at different locations along the outlet width direction, and focused into streas of varied widths. Moreover, size-deendent searation was achieved for sorting 10 and 15 µ PS articles siultaneously. In general, a good agreeent for the articles trajectories is found between the exeriental observation and the nuerical rediction. The roosed hurdle-cobined curved icrochannel has great advantages: (1) it rovides ore araeters (i.e. ga width, configuration of hurdle and curved channel) to be otiized, leading to increased controllability of the article otion; (2) it relives or eliinates the robles that occur in the insulator-based icrodevices that taking advantage of either obstacle or curvature effect individually for electric field gradients, allowing the iroveent of device erforance; (3) the ustrea curved unit, a sei-circular icrochannel cobined with round hurdle, was designed to re-focus articles into a strea close to the hurdle region, thus facilitates subsequent rocess of article aniulation and searation. The novel design with deonstrated integrated functionality of focusing, switching and sorting icroarticles is exected to be widely used in LOC devices for biological, cheical and edical alications. ACKNOWLEDGMENT The author (Miss Ming Li) wishes to thank the China Scholarshi Council and the University of Wollongong for offering joint PhD scholarshis. [13] Raos, A., et al., Ac electrokinetics: a review of forces in icroelectrode structures. Journal of Physics D-Alied Physics, (18): [14] Jones, T.B., Electroechanics of Particles. 1995, Cabridge: Cabridge Unviersity Press. [15] Li, M., et al., Continuous article focusing in a waved icrochannel using negative dc dielectrohoresis. Journal of Microechanics and Microengineering, (9). [16] Lewiriyawong, N., C. Yang, and Y.C. La, Dielectrohoretic aniulation of articles in a odified icrofluidic H filter with ulti-insulating blocks. Bioicrofluidics, (3). [17] Kang, K.H., et al., Effects of dc-dielectrohoretic force on article trajectories in icrochannels. Journal of Alied Physics, (6). [18] Kang, K.H., et al., Continuous searation of icroarticies by size with direct current-dielectrohoresis. Electrohoresis, (3): [19] Zhu, J., et al., DC dielectrohoretic focusing of articles in a serentine icrochannel. Microfluidics and Nanofluidics, (6): [20] Zhu, J. and X. Xuan, Particle electrohoresis and dielectrohoresis in curved icrochannels. Journal of Colloid and Interface Science, (2): [21] Zhu, J. and X. Xuan, Dielectrohoretic focusing of articles in a icrochannel constriction using DC-biased AC flectric fields. Electrohoresis, (15): [22] Takahashi, N., et al., Proosal and exeriental validation of the electrohoretic Coulter ethod for analyzing icroarticles and biological cells. Sensors and Actuators B-Cheical, (2): [23] Venditti, R., X. Xuan, and D. Li, Exeriental characterization of the teerature deendence of zeta otential and its effect on electroosotic flow velocity in icrochannels. Microfluidics and Nanofluidics, (6): REFERENCES [1] Pohl, H.A., Dielectrohoresis : the behavior of neutral atter in nonunifor electric fields. 1978, Cabridge: Cabridge University Press. [2] Pethig, R., Review Article-Dielectrohoresis: Status of the theory, technology, and alications. Bioicrofluidics, (2). [3] Zhang, C., et al., Dielectrohoresis for aniulation of icro/nano articles in icrofluidic systes. Analytical and Bioanalytical Cheistry, (1): [4] Gossett, D.R., et al., Label-free cell searation and sorting in icrofluidic systes. Analytical and Bioanalytical Cheistry, (8): [5] Gascoyne, P.R.C. and J. Vykoukal, Particle searation by dielectrohoresis. Electrohoresis, (13): [6] Hughes, M.P., Strategies for dielectrohoretic searation in laboratory-on-a-chi systes. Electrohoresis, (16): [7] Tay, F.E.H., L. Yu, and C. Iliescu, Particle Maniulation by Miniaturised Dielectrohoretic Devices. Defence Science Journal, (6): [8] Voldan, J., Electrical forces for icroscale cell aniulation, in Annual Review of Bioedical Engineering [9] Gencoglu, A. and A. Minerick, Cheical and orhological changes on latinu icroelectrode surfaces in AC and DC fields with biological buffer solutions. Lab on a Chi, (13): [10] Regteier, J., et al., Electrodeless dielectrohoresis for bioanalysis: Theory, devices and alications. Electrohoresis, (17): [11] Srivastava, S.K., A. Gencoglu, and A.R. Minerick, DC insulator dielectrohoretic alications in icrodevice technology: a review. Analytical and Bioanalytical Cheistry, (1): [12] Xuan, X., J. Zhu, and C. Church, Particle focusing in icrofluidic devices. Microfluidics and Nanofluidics, (1):