A Simplified Microfluidic Device for Particle Separation with Two

Subscriber access provided by UNIVERSITY OF ADELAIDE LIBRARIES

Article

A simplified microfluidic device for particle separation with two consecutive steps# induced charge electroosmotic prefocusing and dielectrophoretic separation Xiaoming Chen, Yukun Ren, Weiyu Liu, Xiangsong Feng, Yankai Jia, Ye Tao, and Hongyuan Jiang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b02892 • Publication Date (Web): 07 Aug 2017 Downloaded from http://pubs.acs.org on August 7, 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 8

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

A simplified microfluidic device for particle separation with two consecutive steps： induced charge electroosmotic prefocusing and dielectrophoretic separation Xiaoming Chen1, Yukun Ren*1,2, Weiyu Liu1, Xiangsong Feng1, Yankai Jia1, Ye Tao1 and Hongyuan Jiang*1,2 1. School of Mechatronics Engineering, Harbin Institute of Technology, Harbin 150001, PR China 2. State Key Laboratory of Robotics and System, Harbin Institute of Technology, Harbin 150001, PR China

ABSTRACT: Continuous dielectrophoretic separation is recognized as a powerful technique for a large number of applications including early-stage cancer diagnosis, water quality analysis, and stem cell-based therapy. Generally, the prefocusing of a particle mixture into a stream is an essential process to ensure all particles are subjected to the same electric field geometry in the separation region. However, accomplishing this focusing process either requires hydrodynamic squeezing, which requires an encumbering peripheral system and a complicated operation to drive and control the fluid motion, or depends on dielectrophoretic forces, which are highly sensitive to the dielectric characterization of particles. An alternative focusing technigue, induced charge electroosmosis (ICEO), has been demonstrated to be effective in focusing an incoming mixture into a particle stream as well as non-selective regarding the particles of interest. Encouraged by these aspects, we propose a hybrid method for micro-particle separation based on a delicate combination of ICEO focusing and dielectrophoretic deflection. This method involves two steps: focusing the mixture into a thin particle stream via ICEO vortex flow and separating the particles of differing dielectic properties through dielectrophoresis. To demonstrate the feasibility of the method proposed, we designed and fabricated a microfluidic chip and separated a mixture consisting of yeast cells and silica particles with an efficiency exceeding 96%. This method has good potential for flexible integration into other microfluidic chips in future.

The separation of constitutents is one of the crucial processes in tackling major issues1–5 such as detecting viable pathogenic bacteria for water quality analysis, isolating rare circulating tumor cells for the diagnosis of early-stage cancer6-10, and extracting stem cells from their differentiated derivatives for stem cellbased therapies.11-16 Various interesting on-chip particle separation methods have been explored, namely electroosmosis, electrophoresis, dielectrophoresis (DEP)17-20, optical interference, acoustic standing waves, splitting laminar flows, mechanical obstacles21-22, restrictions and centrifugation.23-27 Remarkably, continuous DEP separation offers a gentle but effective approach with several advantages over other techniques, including no moving mechanical parts, label-free manipulation, no invasion and limited interaction with surface.28-37 Indeed, it is pivotal for continuous DEP separation to focus the particles into a narrow stream before dielectrophoretic separation, because the particle mixture flows along similar streamlines and enters the separation region at approximately the same position guaranteeing higher separation efficiency. For this reason, many researchers endeavored to develop separation devices with particle prefocusing functionality. One popular technique to accomplish focusing employs hydrodynamic squeezing.12,22,38-45 The preferred focusing method usually includes the following main processes: feeding the solution containing the particles into one inlet, injecting the buffer solution into other inlets, controlling the flow rates by adjusting the pump speed or liquid level differences with great care, and finally squeezing the mixed particles into a narrow particle stream in a steady

state.44 Although this squeezing process has been used for the separation of various types of particles with high efficiency, easy-integration into other microfluidic chips remains a major challenge because the flow rates of the two or more fluids need to be controlled carefully aided by an often impeding peripheral system.1,3,46 To simplify the focusing of particles in samples for dielectrophoretic separation, a DEP-based method has been employed.47 Some groups achieved continuous particle focusing through a negative DEP force from a complex geometric arrangement of lateral metal electrodes and a patterned insulator17 or a trapezoidal electrode48. Obviously, this dielectrophoretic focusing is highly sensitive to the dielectric characterizations of particles and also requires complex geometric electrode structures, which hinders the practical application of these separation methods. The current situation has motivated us to seek an alternative method for prefocusing the particle mixture. Fortunately, induced charge electroosmosis (ICEO) can artfully address the issues existing with the above-mentioned separation methods.4953 In brief, ICEO refers to flows possessing a vortex pair actuated around an oppositely charged electrode that arises from the interaction of the tangential electric field component from mobile ions within the induced double-layer (IDL) on a polarizable surface.51,54,55 Under this ICEO action, the sample can be transported to the center of a floating electrode. Specifically, on the floating electrode, the pair of counter-rotating vortices cancel each another out producing a flow stagnation line (FSL); the pa-

ACS Paragon Plus Environment


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

Page 2 of 8

Figure 1. The configuration of the device. (a)-(e) A graphic description of the device structure used in the experiment. (a) A three-dimensional schematic of the device. (b) The illustration of the novel separation method: a mixture is focused by ICEOF in the focusing region, next passes through the transition structure, and then enters the separation region. This mixture is separated thoroughly under the DEP effect in the separation region. (c) A top view of the structures in the focusing region, transition region and separation region. (d) and (e) The crosssection views of structures in the focusing and separation region (cross sections A-A', B-B' defined in the top view respectively). (f) The photograph of the device.

rticles are then gathered into a stream around the FSL. This promising strategy, i.e., ICEO focusing (ICEOF) of micro-particles, first introduced by Ren and coworkers, successfully trapped different kind of particles into streams within the microfluidic systems.56-59 Certain properties of the ICEO foucusing (ICEOF), such as the arrangement of floating electrodes free from external connecting wires, large effective actuating range, and efficient trapping capability, especially its non-selectivity to the particles of interest, have made it an attractive candidate for prefocusing in dielectrophoretic separation. Herein, we propose a novel separation method for a microfluidic device, where ICEO is exploited for prefocusing particles. This device comprises three regions: an upstream particle focusing region, a downstream particle separation region, with an interconnecting transition region. The experimental sample mixture, consisting of silica particles and yeast cells, was focused into an ideally thin particle stream under the effect of ICEOF in the focusing region. We characterized the performance of the ICEOF by investigating the voltage dependence of the width of the particle stream and the focusing efficiency, to determine the optimum voltage for prefocusing. The influence of the width of the side branches on the motion and the deflection of the particle stream in the transition region were investigated. To explore the DEP characterization of the binary mixture and determine the appropriate settings for the separation parameters, we studied the trajectories of the silica particles and yeast cells in the separation region. Supported by these results, integral experiments were performed for which the separation efficiency surpassed 96%. The main advantages of our device are non-selectivity to the target particles, high throughput, and minimal complicated control of fluids, making it easily-integrable with other chips that perform other more comprehensive tasks, such as facilitating disease diagnosis, environmental monitoring, cell sorting, and chemical analysis.

MATERIALS AND METHODS Device design and fabrication. We begin with the particle focusing region. To enhance focusing efficiency in this region, a horn-shaped structure is adopted at the entrance to guarantee that nearly all particles can be focused at the center of the floating electrode surface by the ICEO micro-vortex pair. The transition region contains three branches. By varying the widths of the two side branches, one can adjust the velocity and trajectory of the particle stream within the middle branch. This branch is connected to the separation region, in which a wall facing opposite to the electrodes is inclined at angle of 3o relative to the baseline (Figure 1c). This provides sufficient space for particle separation. The general structure of the device and the principle of this method are depicted in Figure 1a, b. The characteristic dimensions of the channel and electrodes are indicated in Figure 1c, d, e. The specific dimensions of the chip employed in the study are listed in Table 1. The glass slide is coated with a thinfilm of transparent indium tin oxide (ITO) material and serves as the substrate in the device. Fabricated using photolithography technology, the patterned conductive ITO film also features predesigned electrodes. There are two steps in the fabrication of the channel structure: the first is to form a mold 70 μm in height by soft lithography with a negative dry-film resist (Riston SD238, Dupont, USA); the second is to cast the PDMS-based channel. The ITO electrode and the PDMS-based channel are integrated employing plasma bonding technology. Details of the processing steps of the chip are given in previous reports.40,60-62 In the focusing region, the electric potential imposed on the left driving electrode is 1 =A1 cos t    , the right driving electrode is electrically grounded ( 1 =0 ) (Figure 1d). In separation region, the first, third and fifth coplanar interdigitated electrodes have a given electric potential 2 =A 2 cos   t    , the


Page 3 of 8

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


second, fourth and sixth ones have an electric potential  2 = 0 (form the left, in Figure 1e). The optical photograph of the device is displayed in Figure 1f. Table 1. Specific dimensions of the channel and electrode of the device Parameter

W1

W2

W3

W4

W5

Value (µm)

150

600

350

200

350

Parameter

W6

W7

W8

F1

L1

Value (µm)

100

300

50

4000

300

Parameter

L2

L3

L4

L5

H

Value (µm)

100

280

200

200

70

Sample preparation and system setup. To prepare yeast cells, 100 mg dry yeast was stood in 50 mL DI water at 50 ℃ for 2 h. To obtain relatively pure yeast cells, we pipetted 2 mL cultured yeast solution for further processing. The solution was centrifuged for 30 s, after which the culture solution was replaced by 10 μS/cm KCl solution. The yeast cells in the KCl solution were then washed and resuspended by performing an ultrasonic treatment for 30 s. Moreover, this procedure was implemented 6 times. To prepare silica particles, a 20 μL suspending solution containing 5% particles (diameter 4 μm) was diluted with the 10 μS/cm KCl solution into 2 mL. Prior to every experiment, the internal structure of this device, including channel, inlets and outlets, needed to be soaked with Tween 20 solution, which was diluted five times with alcohol to slow down the adhesion of yeast cells. The optical microscope (CKX41, Olympus, Japan) makes it easy to access the behaviors of particles in our microfluidic chip. A high-speed CCD camera (DP27, Olympus, Japan) can record the movement of the particles. The AC signal energized on the electrode, magnified by an amplifier and monitored by an oscilloscope (TDS2024, Tekronix, USA) was generated by a function generator (TGA12104). We conducted the numerical calculations in this work by using a FEM software, COMSOL Multiphysics 5.2. To quantify the ICEOF process in the experiment, we introduced two parameters, the focusing efficiency F and width ratio WR to assess focusing performance. The former is given by

 F  1  RL / RA   100%

(1)

where RA is the number of particles entering the focusing region (counted per 5 s), and R L the number of particles leaking into the exterior of the floating electrode. The latter is given by

WR  R / W7

(2)

where R is the width of the focused particle stream, and W7 is width of the floating electrode (Figure 1d). The calculation of the separation efficiency in the separation region has been given elsewhere.45

THEORY BACKGROUND Particle focusing based on ICEOF. To investigate the particle focusing behavior, we used a floating bipolar electrode which has a geometrical configuration that facilitates the validation of the improved standard ICEO model. Under the normal component of the electric field generated initially by the driving electrode, counter ions follow the field lines and migrate to the

floating electrode (Figure 2a). Consequently, after a characterictic RC charge relaxation time, an IDL at the interface of the floating electrode and the electrolyte comes into play at a steady state through a balance between an electrostatic attraction and thermal diffusion, which repels the bulk field vectors, leaving only the tangential electric field component to promote the IDL into the ICEO streaming flow (Figure 2b). By decoupling the electrostatics from the hydrodynamics by appropriately ignoring the convection current in front of the conduction current, a series of numerical simulations of ICEO flow field based on the condition listed in Table S-1 was performed to predict the motion of particles. At low frequency, the induced charges in the IDL turn the floating electrode into an ideal insulator by completely screening the normal electric field. Therefore, the motion of the particles induced through DEP from the uniform field is negligibly small, and is thus dominated by the ICEOF, irrespective of floating or driving electrode (Figure 2c). Figure 2c presents the voltage dependence of the velocity of the D-ICEO, F-ICEO, D-DEP, and F-DEP at f=200 Hz (prefixes D and F refer to driving and focusing electrodes). Note that the particle motion due to DEP on the floating electrode almost vanishes. When the driving electrode is energized by a AC signal at f=200 Hz and A1=6 V, a relatively fast slip velocity at the edges of the floating electrode transports particles to the FSL enabling particle focusing to take place. Moreover, the low slip velocity and acceleration on the central area of the floating electrode surface gives rise to a region of effective stagnant flow, thereby producing ultimately stable focusing of the particles (Figure 2d, e). On increasing the voltage to 9 V, a larger slip velocity and acceleration on the central surface area of the floating electrode exert a negative influence on particle focusing. With a decrease in voltage to 3 V, the slip velocity at the edges of the floating electrode is too low to achieve particle focusing. Figure 2f, g indicates that a pair of symmetric micro-vortices emerges above the floating electrode, and a region of stagnant flow forms on the surface of the floating electrode. Subject to a balance of forces between upward ICEO fluidic drag and downward buoyancy, the particle stays in the region of stagnant flow. Negligible DEP arises on the surface of floating electrode as revealed by the simulation of the DEP-induced velocity distribution for silica particles (Figure 2h). We also studied the over-potential on the floating electrode as Faradaic reaction may lead to a negative effect on particle focusing.63-66 The maximum zeta potential at the electrode edge (x=W8+W6+W7) is obtained from (3)  max  W7 A 1 cos  t  2 d  1.5 cos  t  [V] . Therefore, the effective over-potential of the floating electrode in a low-frequency AC field is 1.5 / 2  1.06  2 V and is below the threshold voltage producing appreciable Faradaic (redox) reactions. A detailed physical demonstration is presented in Supporting Information. Particle separation based on DEP. The appropriate location and precise velocity of particle stream at the entrance to the separation region are prerequisites to good particle separation. Hence numerical simulations of the transition region based on equations T 2.2, T 2.4, T 2.6, and T 2.8 in Table S-2 were performed to assess the influence of branch widths on the flow rate and deflection of the particle stream (silica particles and yeast cells). Taking into account the above results, a series of simulations using the Lagrangian tracking method were conducted



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

Page 4 of 8

Figure 2. Simulation in the focusing region. (a) and (b) Sketches of IDL and ICEO: (a) The ions migrate to the surface of the floating electrode under the action of the normal component of the electric field; (b) Bulk electric field lines are repelled from the IDL, reaching a steady state. (c) The velocity of particles induced by ICEO and DEP, D-ICEO, the ICEO on the driving electrode; F-ICEO, the ICEO on the floating electrode; D-DEP, the DEP on the driving electrode; F-DEP, the DEP on the floating electrode. (d) and (e) The slip velocity and the rate of velocity change along the transverse direction of the floating electrode under different voltages. (f) A pair of counter-rotating micro vortexes are formed on the floating electrode. (g) The transverse flow field in the focusing region. (h) DEP velocity distribution of silica particle on the floating and driving electrode.

main channel furcates into three branches, labeled Branches 1– 3 (Figure 3a). With different designs of transition region, the trajectory of the particle stream can be controlled by varying the size of Branches 1 and 3. Figure 3b presents the deflection of the particle stream relative to the baseline as a function of the dimension of Branch 1 at the inlet flow rate of 150 μm/s, the width of Branch 3 is fixed at 350 μm. The fluid rates in these branches vary roughly linearly with the width of Branch 1 (Figure 3c); moreover, the fluid rates in Branches 1 and 3 are on the whole larger than that in Branch 2. When Branches 1 and 3 have the same width of 350 μm, the particle stream moves along the baseline and enters the separation region maintaining straightline motion (Figure 3d) at a flow rate of 88.89 μm/s (Figure 3c). In such situations, for AC signal of frequency 1 MHz at 225 V in the separation region, particles separate ideally (Figure 3e). When the width of Branch 1 is 380 µm, the particle stream is shifted upward by 47.17 μm (Figure 3b) and the flow rate of the particle stream is 81.28 μm/s (Figure 3c), commencing to move into Branch 2. Under this experimental condition, the component of the electric field gradient in the Z direction near the electrode is sufficiently strong that the silica particles interfere at the top of the channel, hindering their motion (Figure S-1a, b). When the width of Branch 1 is 320 μm, the particle stream shifts downward by 45.35 μm and the flow rate of the particle stream is 96.72 μm/s. In this instance, a weak electric field gradient arises in the yeast cells, which are subjected to a weak positive DEP force and are diverted to the wrong outlet (Figure S-1c, d). Therefore, at f=1 MHz and A2=225 V, with Branches 1 and 3

having the same width of 350 μm, the mixture enters the separation region along the baseline and bifurcates.

RESULTS AND DISCUSSION With high-efficient separation of particles being the ultimate goal, the designed chip has to meet three requirements: (i) under the effect of the ICEOF, the particles introduced into the device need to be focused into a particle stream with high focusing efficiency and small width ratio in the focusing region; (ii) the desired flow rate and the entry location of the particle stream entering the separation region are acquired from the transition region; and (iii) in the separation region, the different particles must flow into distinct outlets. ICEOF characterization. To avoid particle leakage outside of the floating electrode into the side branches, the entrance to the focusing region is designed to be narrower than the floating electrode. For high-efficiency separation, the particles need to enter the separation region along with the same flow streamline. Therefore, we require a slender particle stream with a small width ratio to ensure all particles enter the separation region at approximately the same position. We used the silica particle and yeast cell as an experimental sample to study the nature of the ICEOF convergence in the focusing region at an inlet flow rate of 150 μm/s. Figure 4a illustrates the behaviors of particles under the ICEOF. Setting A1=6 V and f=200 Hz, the focusing of silica, yeast, and their mixture in the experiments is evident in Figure 4b, c, d. The width ratios and the focusing efficiencies of


Page 5 of 8

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


(Figure 4f). On the other hand, when the voltage is 6 V, the focusing efficiency also reaches maximum values with 100.0% for silica particles, 94.0% for yeast cells, and 96.1% for the mixture. Therefore, we conclude that the desired results from particle focusing have been achieved with F  95% and WR  0.2 for the mixture, when the applied voltage is 6 V at 200 Hz in the focusing region.

Figure 3. Numerical simulation results in the transition and separation regions. (a) Diagram of the transition region connecting the focusing and separation regions. (b) Deflection of the particle stream from the baseline as a function of the dimension of Branch 1 with the inlet flow rate set at 150 μm/s with a Branch 3 width of 350 μm. (c) Fluid rates in the three branches as functions of Branch 1 width. (d) Motion of the particle stream setting the width of Branches 1 and 3 at 350 μm. (e) Perfect particle separation in the separation region along the baseline at 88.89 μm/s flow rate.

silica, yeast and their mixture at frequencies ranging from 100 Hz to 600 Hz share the same varying trend at 6 V, which is also reflected in Figure 4e, f. For a particular field intensity (here we apply 6 V), the width ratios increase monotonically as frequency increases. In particular, the peak values of the focusing efficiency occur at 200 Hz (Figure 4e). One reasonable explanation for this result is that the frequency, 200 Hz, is much closer to the surface-averaged RC charging frequency arising from the relaxation process; i.e., fRC averge   1+  / 2 Cd  0.195L  175 Hz 67 for the equivalent circuit of the electrolyte resistance coupled to the double-layer capacitance at the electrode surface. In this instance, we obtain an optimal width ratio for the silica of 0.1167, 0.2693 for the yeast, and 0.1917 for the mixture. Moreover, particle focusing based on ICEOF occurs in a particular voltage range. When the voltage is very weak (less than 4 V), the ICEO fluidic drag is not sufficiently strong to overcome the downward buoyancy force of the particle. ICEO becomes saturated at voltages in excess of 10 V due to several nonlinear effects, and therefore we cannot achieve favorable focusing results. Figure 4g, h presents the functional relationships between the focusing indexes (width ratios and focusing efficiency of particles) and field voltage intensity. On the one hand, the width ratios rapidly fall as the voltage increases from 4 V to 6 V, whereas this index decreases marginally as the voltage increases from 6 V to 10 V

Figure 4. Characterization of continuous particle focusing by ICEOF in the focusing region with an inlet flow rate 150 μm/s. (a) Diagram of particle focusing based on ICEOF. (b)–(d) ICEOF characterization of silica and yeast particles at A1=6 V and f=200 Hz: (b) focusing of silica particles, (c) focusing of yeast cells, (d) focusing of the mixture. (e) Frequency dependence of the width ratios of silica, yeast, and their mixture at fixed voltage A1=6 V. (f) Frequency dependence of the focusing efficiencies of silica, yeast, and their mixture at A1=6 V. (g) Voltage dependence of the width ratios of silica, yeast and their mixture at f=200 Hz. (h) Voltage dependence of the focusing efficiencies of silica, yeast, and their mixture at f=200 Hz.

DEP characterization. Prior to the separation of silica particle and yeast cell, we studied their DEP characterization. An induced dipole is formed when particles suspended in a medium are subjected to an AC field generated by the interdigitating coplanar electrode array. Figure 5a provides schematically the focusing performances of the particles in the AC electric field. The Clausius–Mossotti (CM) factor is a key predictor in determining the DEP force.68 The curve in Figure 5b depicts numerically the real part of the CM factor as a function of field frequency for silica particles and yeast cells suspended in KCl solution with conductivity of 10 μS/cm. Note that the difference in Re  fCM  between these two particles reaches a maximum

value at frequency 1 MHz. Under the different DEP forces, silica particle and yeast cell move in opposite directions and hence



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

particle separation is achieved. Hence, this frequency is chosen in separating the particle mixture.

Figure 5. DEP characterization of particles: (a) Schematic for the focusing performances of particles in the AC electric field; (b) Frequency dependence of the real part of the CM factors for silica particles and yeast cells for the electric field in a 10 μS/cm KCl solution; (c) Offset displacements of the yeast and silica particles under the DEP force are defined as h1 and h2; (d) Offset displacement of silica and yeast particles at different voltage settings; (e) and (f) State of silica and yeast in the separation region with the AC voltage switched off (A2=0); (g) and (h) Simulation results for silica and yeast particles with driving voltage amplitude set at A2=225 V at frequency f=1 MHz; (i) and (j) Experimental results corresponding to the simulations given in (g) and (h). To obtain a suitable voltage amplitude for an effective particle separation, we performed a simulation analysis and experimental study of the trajectories of the silica and yeast in the separation region. Here, we introduce the deviations from the baseline, h1 and h2, corresponding to yeast and silica (Figure 5c), which are studied in a series of experiments and simulations for voltages from 0 to 250 V. From such data, the DEP characterization of particles can be quantitatively evaluated. Figure 5d shows a comparison of the simulation and experimental results. In both simulations and experiments, the silica particle stream reaches a limit position with a deviation of 181 μm at A2=225 V. In the experiment, the deflection of the yeast particle stream reaches its maximum value 70 μm at 150 V. The simulation shows that the maximum deflection for yeast cells is 75 μm at 150 V. Therefore, silica and yeast particles both reach their limit position at 225 V. The states of the silica particles and yeast cells in the absence of the DEP force in the separation region are shown in Figure 5e, f; Figure 5g, i gives the results of simulations and experiments for silica particles, at 1 MHz and 225 V. The simulation and experiment results for yeast cells are presented in Figure 5h, j for the same conditions.

Page 6 of 8

Integral experiment. With the initial few steps of our study, the practicability of the device structure has been demonstrated and optimal experimental parameters have been found. The outcomes of the integral experiment are shown in Figure 6. To understand the precise behavior of the particles in each section of the channel in the experiment, the main structure of the channel was separated into five sections (labeled A through E in Figure 6a). An image of the silica and yeast particles captured using a 20X objective lens clearly displays the mixture being focused into a fine particle stream under the ICEOF (Figure 6b). The particle stream moves over the surface of the floating electrode while maintaining a compact flow state (Figure 6c). Figure 6d shows the structure of the transition region, and the profile formed by the particle stream; almost all the particles were focused and transported downstream into Branch 2. Once the mixed particles enter the separation region, the different components of the particle stream split into distinct branches under the influence of the DEP force. The silica particles move away from the electrodes, whereas the yeast cells are drawn towards them. Consequently, the particle stream bifurcates into an upward and a downward branch, thereby achieving the separation of the mixture (Figure 6e; a superimposed image of several consecutive separation images produced using software Image J). To confirm the specific phenomenon under which the silica particles and yeast cells move towards the desired outlets in section E, we observed their behavior under a 20X objective lens; From the image (Figure 6f), we see both silica particles and yeast cells have entered their prescribed outlet. The inlet flow rate is another pivotal factor affecting the separation of particles that requires an analysis. When no AC signal is imposed on the interdigitating coplanar electrode array, the mixture in the focusing region was continuously focused into a single particle stream and flowed into Outlet C along an approximately straight trajectory because diffusion is negligible (Figure 7a, b). Imposing an AC signal of 1 MHz and 225 V, a separation of silica and yeast particles occurs in the separation region at an inlet flow rate of 250 μm/s (Figure 7c, d). From these results of simulation and experiment, we find that the silica particles receive a smaller deflection under a negative DEP force and enter Outlet D. Meanwhile, the streamline of the yeast cells curves towards the electrode surface under a positive DEP force and finally disappears into Outlet C subject to slight fluctuations. This interesting phenomenon is speculated to derive from a larger electric field gradient in the Y direction near the electrode that causes a greater DEP displacement. When the axial Poiseuille flow velocity is reduced to 150 μm/s, thereby providing more time for the DEP force to exert an effect, deflections of the trajectories for silica and yeast are enhanced (Figure 7e, f). When the flow rate is reduced to 100 μm/s, silica particles move along the periphery of the separation region; wall friction then suppresses the axial silica motion. Meanwhile, the yeast particle stream disperses because of larger transverse fluctuations from a larger electric field gradient in Y direction given sufficient time. Nevertheless, the silica particles and yeast cells are transported to the desired outlet.We counted manually the number of the silica particles and yeast cells reaching Outlets C and D to quantitatively evaluate the separation performance of the device under the optimum parameter settings (Figure 8).44 The indices giving the percentage of each particle species for each branch outlet exceed 96% at a moderate flow rate of 150 μm/s.


Page 7 of 8

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


Figure 6. Images from the integral experiment: (a) internal channel structure and electrode configuration of the device; (b) silica and yeast particles are focused into a single particle stream in the focusing region; (c) particle stream maintains a slender flow on the surface of the floating electrode; (d) channel structure in the transition region and streaming of the mixed particles; (e) particle stream bifurcates under DEP force; (f) silica and yeast particles enter the desired outlets.

Figure 7. Theoretical and experimental demonstrations of the separation of silica and yeast at different inlet flow rate for a specific AC signal. (a) and (b) A2 = 0, u=250 μm/s. (c) and (d) A2 = 225 V, f=1 MHz, u=250 μm/s. (e) and (f) A2 = 225 V, f=1 MHz, u=150 μm/s. (g) and (h) A2 = 225 V, f=1 MHz, u=100 μm/s.

of 150 μm/s. We demonstrated that the velocity and trajectory of the particle stream can be controlled by adjusting the width of branches in the transition region. Depending on the DEP characterization of the binary particles, the streamlines of silica particles and yeast cells both reach their limit position at 225 V and 1 MHz at an inlet flow rate of 150 μm/s in the separation region. Using the parameter settings obtained in the three steps, the separation efficiency of this device exceeded 96% in the integral experiment. Moreover, this method performs well over inlet flow rates ranging from 100 to 250 μm/s. The prominent advantages of our device, i.e., non-selectivity to target particles, and no need for complicated control of the flow rate and encumbering peripheral systems, enable it to be directly integrated with other microfluidic components in a complete laboratoryon-chip platform of comprehensive functions, facilitating disease diagnosis, environmental monitoring, cell sorting, and chemical analysis.

ASSOCIATED CONTENT Supporting Information A movie recording the performances of particles. (AVI) The conditions employed in numerical simulations of the ICEO prefocusing and DEP separation; The theoretical analysis of the width of Branch 1 and 3 affecting the particles separation. (PDF)

AUTHOR INFORMATION Figure 8. Separation efficiency of yeast cells and silica particles

CONCLUSIONS We presented a novel separation method that effectively combined particle prefocusing by ICEO and separation by DEP. A binary mixture of yeast cells and silica particles was used as samples in a microfluidic device to test the feasibility of the method. In the focusing region, this binary mixture has excellent ICEOF characterization with focusing efficiency exceeding 95% and focusing width ratios below 0.2 employing voltage and frequency settings of A1= 6 V and f=200 Hz with an inlet flow rate

Corresponding Author * Fax: +86-451-86402658; Yukun Ren: [email protected]; Hongyuan Jiang: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (No. 11672095 and 11372093), Self-Planned Task



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

(No. 201510B and SKLRS201606C) of State Key Laboratory of Robotics and System (HIT).

REFERENCES (1) Feng, X.; Du, W.; Luo, Q.; Liu, B. F. Anal Chim Acta 2009, 650, 83-97. (2) Wu, H.; Wheeler, A.; Zare, R. N. Proc Natl Acad Sci U S A 2004, 101, 12809-12813. (3) Gao, J.; Yin, X. F.; Fang, Z. L. Lab Chip 2004, 4, 47-52. (4) Kwon, K. W.; Choi, S. S.; Lee, S. H.; Kim, B.; Lee, S. N.; Park, M. C.; Kim, P.; Hwang, S. Y.; Suh, K. Y. Lab Chip 2007, 7, 1461-1468. (5) Tao, Y.; Rotem, A.; Zhang, H.; Chang, C. B.; Basu, A.; Kolawole, A. O.; Koehler, S. A.; Ren, Y.; Lin, J. S.; Pipas, J. M. Lab on a Chip 2015, 15, 3934-3940. (6) Song, S.; Choi, S. Applied Physics Letters 2014, 104, 074106. (7) Yang, F.; Yang, X.; Jiang, H.; Bulkhaults, P.; Wood, P.; Hrushesky, W.; Wang, G. Biomicrofluidics 2010, 4, 13204. (8) Holmes, D.; Green, N. G.; Morgan, H. IEEE Engineering in Medicine and Biology Magazine 2003, 22, 85-90. (9) Sun, J.; Liu, C.; Li, M.; Wang, J.; Xianyu, Y.; Hu, G.; Jiang, X. Biomicrofluidics 2013, 7, 011802. (10) Sun, J.; Li, M.; Liu, C.; Zhang, Y.; Liu, D.; Liu, W.; Hu, G.; Jiang, X. Lab on a chip 2012, 12, 3952-3960. (11) Adekanmbi, E. O.; Srivastava, S. K. Lab Chip 2016, 16, 2148-2167. (12) Song, H.; Rosano, J. M.; Wang, Y.; Garson, C. J.; Prabhakarpandian, B.; Pant, K.; Klarmann, G. J.; Perantoni, A.; Alvarez, L. M.; Lai, E. Lab Chip 2015, 15, 1320-1328. (13) Shafiee, H.; Sano, M. B.; Henslee, E. A.; Caldwell, J. L.; Davalos, R. V. Lab Chip 2010, 10, 438-445. (14) Tao, Y.; Rotem, A.; Zhang, H.; Cockrell, S. K.; Koehler, S. A.; Chang, C. B.; Ung, L. W.; Cantalupo, P. G.; Ren, Y.; Lin, J. S. ChemBioChem 2015, 16, 2167-2171. (15) Leinweber, F. C.; Eijkel, J. C.; Bomer, J. G.; van den Berg, A. Analytical chemistry 2006, 78, 1425-1434. (16) Srivastava, S. K.; Gencoglu, A.; Minerick, A. R. Analytical and bioanalytical chemistry 2011, 399, 301-321. (17) Demierre, N.; Braschler, T.; Muller, R.; Renaud, P. Sensors and Actuators B: Chemical 2008, 132, 388-396. (18) Huang, Y.; Joo, S.; Duhon, M.; Heller, M.; Wallace, B.; Xu, X. Analytical chemistry 2002, 74, 3362-3371. (19) Hawkins, B. G.; Smith, A. E.; Syed, Y. A.; Kirby, B. J. Analytical chemistry 2007, 79, 7291-7300. (20) Hughes, M. P.; Morgan, H. Analytical Chemistry 1999, 71, 34413445. (21) Lewpiriyawong, N.; Yang, C.; Lam, Y. C. Biomicrofluidics 2008, 2, 34105. (22) Kang, Y.; Cetin, B.; Wu, Z.; Li, D. Electrochimica Acta 2009, 54, 1715-1720. (23) Bhagat, A. A.; Bow, H.; Hou, H. W.; Tan, S. J.; Han, J.; Lim, C. T. Med Biol Eng Comput 2010, 48, 999-1014. (24) Arosio, P.; Muller, T.; Mahadevan, L.; Knowles, T. P. Nano Lett 2014, 14, 2365-2371. (25) Green, N. G.; Morgan, H. Journal of Physics D: Applied Physics 1998, 31, L25. (26) Li, D.; Lu, X.; Xuan, X. Analytical Chemistry 2016, 88, 1230312309. (27) Lu, X.; Xuan, X. Analytical chemistry 2015, 87, 11523-11530. (28) Braschler, T.; Demierre, N.; Nascimento, E.; Silva, T.; Oliva, A. G.; Renaud, P. Lab Chip 2008, 8, 280-286. (29) Dash, S.; Mohanty, S. Electrophoresis 2014, 35, 2656-2672. (30) Doh, I.; Cho, Y.-H. Sensors and Actuators A: Physical 2005, 121, 59-65. (31) Lapizco-Encinas, B. H.; Simmons, B. A.; Cummings, E. B.; Fintschenko, Y. Electrophoresis 2004, 25, 1695-1704. (32) Morgan, H.; Hughes, M. P.; Green, N. G. Biophysical journal 1999, 77, 516-525. (33) Wang, X.; Yang, J.; Huang, Y.; Vykoukal, J.; Becker, F. F.; Gascoyne, P. R. Analytical chemistry 2000, 72, 832-839. (34) Yang, J.; Huang, Y.; Wang, X.-B.; Becker, F. F.; Gascoyne, P. R. Analytical chemistry 1999, 71, 911-918.

Page 8 of 8

(35) Yang, J.; Huang, Y.; Wang, X.; Wang, X.-B.; Becker, F. F.; Gascoyne, P. R. Biophysical journal 1999, 76, 3307-3314. (36) Yang, J.; Huang, Y.; Wang, X.-B.; Becker, F. F.; Gascoyne, P. R. Biophysical journal 2000, 78, 2680-2689. (37) Huang, Y.; Wang, X.-B.; Becker, F. F.; Gascoyne, P. Biophysical journal 1997, 73, 1118-1129. (38) Park, S.; Zhang, Y.; Wang, T. H.; Yang, S. Lab Chip 2011, 11, 2893-2900. (39) Song, Y.; Yang, J.; Shi, X.; Jiang, H.; Wu, Y.; Peng, R.; Wang, Q.; Gong, N.; Pan, X.; Sun, Y.; Li, D. Science China Chemistry 2012, 55, 524-530. (40) Jia, Y.; Ren, Y.; Jiang, H. Electrophoresis 2015, 36, 1744-1753. (41) Li, Y.; Dalton, C.; Crabtree, H. J.; Nilsson, G.; Kaler, K. V. Lab Chip 2007, 7, 239-248. (42) Hu, X.; Bessette, P. H.; Qian, J.; Meinhart, C. D.; Daugherty, P. S.; Soh, H. T. Proc Natl Acad Sci U S A 2005, 102, 15757-15761. (43) Sun, J.; Gao, Y.; Isaacs, R. J.; Boelte, K. C.; Lin, P. C.; Boczko, E. M.; Li, D. Anal Chem 2012, 84, 2017-2024. (44) Lewpiriyawong, N.; Kandaswamy, K.; Yang, C.; Ivanov, V.; Stocker, R. Anal Chem 2011, 83, 9579-9585. (45) Zhao, K.; Peng, R.; Li, D. Nanoscale 2016, 8, 18945-18955. (46) Moon, H. S.; Kwon, K.; Kim, S. I.; Han, H.; Sohn, J.; Lee, S.; Jung, H. I. Lab Chip 2011, 11, 1118-1125. (47) Sackmann, E. K.; Fulton, A. L.; Beebe, D. J. Nature 2014, 507, 181-189. (48) Choi, S.; Park, J.-K. Lab on a Chip 2005, 5, 1161-1167. (49) Bazant, M. Z.; Squires, T. M. Current Opinion in Colloid & Interface Science 2010, 15, 203-213. (50) Lazo, I.; Peng, C.; Xiang, J.; Shiyanovskii, S. V.; Lavrentovich, O. D. Nat Commun 2014, 5, 5033. (51) Squires, T. M.; Bazant, M. Z. Journal of Fluid Mechanics 2004, 509, 217-252. (52) Bazant, M. Z.; Squires, T. M. Phys Rev Lett 2004, 92, 066101. (53) Davidson, S. M.; Andersen, M. B.; Mani, A. Phys Rev Lett 2014, 112, 128302. (54) Pascall, A. J.; Squires, T. M. Phys Rev Lett 2010, 104, 088301. (55) Leinweber, F. C.; Tallarek, U. The Journal of Physical Chemistry B 2005, 109, 21481-21485. (56) Ren, Y.; Liu, W.; Jia, Y.; Tao, Y.; Shao, J.; Ding, Y.; Jiang, H. Lab Chip 2015, 15, 2181-2191. (57) Liu, W.; Shao, J.; Jia, Y.; Tao, Y.; Ding, Y.; Jiang, H.; Ren, Y. Soft Matter 2015, 11, 8105-8112. (58) Jia, Y.; Ren, Y.; Jiang, H. RSC Adv. 2015, 5, 66602-66610. (59) Liu, W.; Shao, J.; Ren, Y.; Liu, J.; Tao, Y.; Jiang, H.; Ding, Y. Biomicrofluidics 2016, 10, 034105. (60) Feng, X.; Ren, Y.; Jiang, H. Biomicrofluidics 2013, 7, 54121. (61) Anderson, J. R.; Chiu, D. T.; Jackman, R. J.; Cherniavskaya, O.; McDonald, J. C.; Wu, H.; Whitesides, S. H.; Whitesides, G. M. Analytical chemistry 2000, 72, 3158-3164. (62) Feng, X.; Ren, Y.; Jiang, H. Biomicrofluidics 2014, 8, 034106. (63) Park, S.; and Yossifon, G. Phys Rev E 2016, 93, 062614. (64) Loget, G.; Zigah, D.; Bouffier, L.; Sojic, N.; and Kuhn, A. Accounts of chemical research 2013, 46, 2513-2523. (65) Olesen, L. H.; Bruus, H.; and Ajdari, A. Phys Rev E Stat Nonlin Soft Matter Phys 2006, 73, 056313. (66) Liu, W.; Shao, J.; Ren, Y.; Wu, Y.; Wang, C.; and Ding, H.; Jiang, H.; and Ding, Y. Journal of Micromechanics and Microengineering 2016, 26, 095003. (67) Tao, Y.; Ren, Y.; Liu, W.; Wu, Y.; Jia, Y.; Lang, Q.; Jiang, H. Electrophoresis 2016, 37, 1326-1336. (68) Jiang, H.; Ren, Y.; Tao, Y. Chinese physics B 2011, 20, 057701.

for TOC only


A Simplified Microfluidic Device for Particle Separation with Two

Recommend Documents