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Pillar Array Microtraps with Negative Dielectrophoresis Hai-Hang Cui*,† and Kian-Meng Lim*,†,‡ Singapore-MIT Alliance and Department of Mechanical Engineering, National UniVersity of Singapore, Singapore, 119260 ReceiVed NoVember 12, 2008. ReVised Manuscript ReceiVed January 22, 2009 We present a microfluidic particle-trap array that utilizes negative dielectrophoresis (nDEP) force and hydrodynamic force. The traps are located at the stagnation points of cylindrical pillars arranged in a regular array, and they can function as both single-particle traps (capable of discriminating particles based on size) and multiparticle traps (capable of controlling the number of particles trapped). By adjusting the relative strength of the nDEP and hydrodynamic forces, we are able to control the number of trapped particles accurately. We have used 5 µm polystyrene beads to validate and demonstrate the capability of this new particle-trap design. Pulsed nDEP was used to increase the selectivity and stability. Good correlation between simulation and the experimental results was obtained.
Trapping of single particles in microfluidic devices is very important because of its potential application in single-cell analysis.1-4 DEP provides a simple and inexpensive way to trap and manipulate particles by varying the voltage, frequency, or phase of the electrical signal applied to the electrodes in such devices.5-7 So far, a few research groups8-16 have demonstrated the ability to trap a single particle in a large array in their devices. However, there are still some drawbacks, such as the unadjustable trap,9 neglecting the particle-particle interaction,10-15 the complicated fabrication,8,12,14,15 blocking of the device,17 high trans-membrane electric potential due to positive DEP,14,15 and the strength of trapping.13 In this letter, we put forward a method that utilizes the negative dielectrophoresis (nDEP) force generated by interdigital electrodes and the hydrodynamic force generated by an array of cylindrical pillars to control the number of trapped polystyrene (PS) beads accurately, as shown in Figure 1. Different from the conventional method where the trap is often designed on the basis of the matching geometry to the particle (such as a well or dam), the present trap is located in the region close to the downstream stagnation point of the pillar, where the beads experience a nDEP force that pushes the particle back against the pillar to overcome * Corresponding author. E-mail:
[email protected]. † Singapore-MIT Alliance. ‡ Department of Mechanical Engineering.
(1) Voldman, J. Annu. ReV. Biomed. Eng. 2006, 8, 425–454. (2) Voldman, J. Curr. Opin. Biotechnol. 2006, 17, 532–537. (3) Lim, C. T.; Zhou, E. H.; Li, A.; et al. Mater. Sci. Eng. C 2006, 26, 1278– 1288. (4) Muller, T.; Pfennig, A.; Klein, P.; et al. Eng. Med. Biol. Mag., IEEE 2003, 22, 51–61. (5) Pohl, H. A. Dielectrophoresis; Cambridge University Press: Cambridge, UK, 1978. (6) Jones, T. B. Electromechanics of Particles; Cambridge University Press: Cambridge, UK, 1995. (7) Hughes, M. P. Nanoelectromechanics in Engineering and Biology; CRC Press: Boca Raton, FL, 2003. (8) Hunt, T. P.; Westervelt, R. M. Biomed. MicrodeV. 2006, 8, 227–230. (9) Di Carlo, D.; Wu, L. Y.; Lee, L. P. Lab Chip 2006, 6, 1445–1449. (10) Kim, B. G.; Yun, K. S.; Yoon, E. Proceeding IEEE Micro Electro Mechanical Systems Workshop, 2005; pp 702-705. (11) Voldman, J.; Braff, R. A.; Toner, M.; et al. Biophys. J. 2001, 80, 531–541. (12) Voldman, J.; Toner, M.; Gray, M. L.; et al. J. Electrost. 2003, 57, 69–90. (13) Rosenthal, A.; Voldman, J. Biophys. J. 2005, 88, 2193–2205. (14) Taff, B. M.; Voldman, J. Anal. Chem. 2005, 77, 7976–7983. (15) Hunt, T. P.; Lee, H.; Westervelt, R. M. Appl. Phys. Lett. 2004, 85, 6421– 6423. (16) Aubry, N.; Singh, P. Europhys. Lett. 2006, 74, 623–629. (17) Skelley, A. M.; Kirak, O.; Jaenisch, R.; Voldman, J. Micro Total Analysis Systems ’07, Paris, 2007.
the hydrodynamic force. The region of the trap that holds the PS beads is controllable through the adjustment of the relative strength of the nDEP force and hydrodynamic force, allowing it to function both as a single-bead trap for various diameter beads or a trap for multiple beads. Because of the low flow velocity downstream from the stagnation point, high-strength nDEP is no longer required, which is useful for the cell application where a very low shear rate is desired. This design also provides active loading and unloading capabilities. Furthermore, a pulsed nDEP force is used in place of continuous nDEP to improve the selectivity and stability of the single-bead trap and to reduce the blocking of the system in a large-scale pillar array. Figure 1 shows a schematic drawing of a representative section in a pillar array structure. The height of the channel is about 30 µm. Because the gap (δ ≈ O (1 µm)) between the pillar and the substrate is small, it is not crucial for the operation of the device. The combination of the nDEP force and fluid flow field in the pillar array generates a small region at the stagnation point behind the pillar for trapping the microbeads. For a spherical particle (with radius r, relative permittivity εp, and conductivity σp) suspended in a medium (with relative permittivity εf and conductivity σf), the expression of DEP force is given by
FDEP ) 2πε0εfr3Re[K*(ω)] ∇ Erms2
(1)
where Erms is the root-mean-square of electrical field strength and ε0 is the permittivity in vacuum. The direction of DEP depends on the sign of Re(K*(ω)). In this work, commercial finite element software (COMSOL, Inc.) is used to analyze the steady-state electric field and fluid flow field. The parameters and boundary conditions used in the model are listed in Table 1. After obtaining the distributions of the electrical field and flow field separately, the region where a particle can be trapped can be found. Countered by the hydrodynamic force, the sphere reaches a terminal velocity Up given by
Up )
FDEP + Uf 6πrµ
(2)
where µ is the viscosity of the fluid and Uf is the fluid velocity. The region where particles will be trapped is given by that where the x component of the particle velocity is less than zero (as given by eq 2). The region is highlighted by the bold contour in Figure 3. By adjusting the relative strength between the Stokes force and nDEP, a controllable trap (enclosed by the bold contour
10.1021/la803761f CCC: $40.75 2009 American Chemical Society Published on Web 02/11/2009
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Figure 1. Schematic drawing of the pillar array. (a) Top view of the device with fluid flowing from left to right. The diameter of the pillar is 45 µm; the distance between two pillars in the y direction is 70 µm. The pillar centers are located at the upstream edge of the intedigital electrodes (IDEs), which are 30 µm wide and 30 µm apart. An array of 30 (in the x direction) × 20 (in the y direction) pillars is constructed in the middle region of a 3-mm-wide fluid channel. (b) Motion of a microbead is illustrated by the dashed lines. A microbead coming from upstream flows over the pillar according to the flow streamline. In the presence of the nDEP force (in the negative x direction), it is pushed against the pillar. (c) Side view of the device. The ITO electrodes, with a thickness of approximately 100 nm, are embedded at the bottom of the channel. The resultant vertical component of the nDEP force pushes the particle to the top surface of the device. Table 1. Property Parameters Used in Simulation pillar array system: 5 µm PS bead suspended in water, with all geometrical dimensions available in Figure 1 flow field property
electric field value
viscosity of water density of water applied pressure resultant flow rate
1e-3 Pa · s 1e3 kg/m3 1 Pa 19.6e-14 m3/s for Figure 1b
boundary condition
symmetry, nonslip, pressure, neutral and periodical BCs
property
value
CM factor at 10 MHz dielectric constant of particle dielectric constant of water dielectric constant of SU-8 dielectric constant of glass peak-to-peak voltage boundary condition
-0.47 2.55 78 3.0 3.50 1 Vpp electric potential, ground, electric displacement ) 0, and periodical BCs
Table 2. Number of Trapped Beads for Various Combinations of Fluid Flow Rate and Applied Voltagea average ( standard deviation (simulation result)
flow rate (mL/h)
a
voltage (Vpp)
10
9
8
7
6
0.12 0.11 0.10 0.09 0.08 0.07
1.6 ( 1.1 (2) 2.3 ( 1.1 (2) 4.4 ( 1.1 (3) 5.3 ( 1.3 (5) 6.5 ( 1.4 (5) 7.2 ( 1.8 (6)
0.2 ( 0.4 (2) 2.0 ( 1.3 (2) 2.8 ( 1.0 (3) 4.4 ( 1.4 (3) 5.4 ( 1.4 (5) 6.3 ( 1.2 (5)
0 (1) 1.4 ( 0.8 (1) 2.2 ( 1.2 (2) 2.3 ( 1.1 (2) 2.0 ( 1.7 (3) 4.7 ( 1.3 (3)
0(0) 0.6 ( 1.5 (1) 0.5 ( 0.5 (1) 0.8 ( 1.1 (1) 1.1 ( 1.1 (2) 2.6 ( 1.3 (2)
0 (0) 0 (0) 0 (0) 0 (0) 0 (1) 0.4 ( 0.7 (1)
5 0 0 0 0 0 0
(0) (0) (0) (0) (0) (0)
Averages and standard deviations from 10 sets of experiments and simulation results (indicated in parentheses) are given.
line) can be obtained to trap the different number of beads as desired. ITO-coated glass is used because of its high transparency and good electrical conductivity. Standard photolithography is used to pattern the interdigital electrodes. Two SU-8 layers were then fabricated: one for the pillar array and the other for the rectangular channel. PS beads were mixed with water, and a little surfactant was added to reduce the nonspecific adhesion. A 10 MHz sinusoidal voltage of up to 20 Vpp generated by signal generator (Agilent 33220A) is applied to the electrodes. The built-in amplitude-modulating function of the signal generator is also used to produce the pulsed DEP. We first tested the behavior in continuous nDEP mode. The number of beads that are trapped is studied as the applied voltage and the flow rate are varied. The experiments are repeated many times using 5 µm beads. Table 2 shows the average number of beads trapped for 10 sets of experiments at a single pillar in the device under the same operating conditions. The standard deviation for each set of data is also included. Simulation results for each case are also given in parenthesis for comparison. From the data set, it can be seen that the number of trapped beads increases with the higher applied voltage and lower flow rate. The very important phenomenon is that the single-bead trap can be realized with several combinations of voltage and flow rate conditions. At the same time, it is also possible to trap a specific number of beads if the relative strength of the nDEP force and Stokes force is controlled properly. The number of particles
trapped (as given in Table 2) is very sensitive to the position of the electrodes, so it requires accurate microfabrication to obtain a repeatable data set. A comparison between the simulation and the experiment was conducted. After normalizing the data by the minimum flow rate and the minimum voltage in Table 2, the simulation result shows the same trend as the experimental result, and it is capable of predicting the number of particles trapped for various flow rates and voltages. However, there are slight discrepancies in predicting the number of beads trapped (about 1 to 2 beads). These could be due to factors such as the uncertainty in measurement of the channel height, the near-wall effect of the Stokes force and friction force at the particle/solid interface, and the dipole approximation used in calculating the nDEP force. To improve the selectivity and stability of this single-bead trap, we have used pulsed nDEP in place of continuous nDEP. By turning the DEP force on and off repeatedly, we introduce an additional time scale comparable to that for fluid/particle motion. This gives us an additional parameter for controlling the motion of the particles and hence the performance of the trap. The typical time scale of particle motion in a microchannel is given by L/Uf, where L is the characteristic dimension of length. The channel used in our device has a width of 3 mm and a height of 30 µm. The average linear velocity of the fluid at the inlet is about 300 µm/s for a flow rate of 0.1 mL/h. For typical values of L)30 µm and Uf ) 300 µm/s in our experimental microchannel, the time scale is about 0.1-1 s. In the present design, we used
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Figure 4. Unstable equilibrium points are defined uniquely by the distribution of total force from the sum of external flow and nDEP. After an off period of nDEP force, two 5 µm particles are released from a pillar and reach different sides of the separation point. As a result, the left one is pushed backward and the right one is pushed forward when nDEP is recovered.
Figure 2. Simulation results for the electric field and flow velocity. (a) Spatial distribution of the electric field norm when 1 Vpp is applied at one electrode and 0 V is applied at the other electrode. (b) Spatial distribution of the x component of fluid velocity when a 1 Pa pressure drop is applied across the structure of 70 µm. A very low velocity zone is observed downstream of the pillar. (c) Electric field norm and x component of the fluid velocity along the x axis downstream of the pillar. The results in plots a and b are given at a distance 2.5 µm below the top surface of the channel. This corresponds to the locations of the centers of 5 µm beads when they are pushed against the top surface.
a pulsed DEP force by applying a 50% duty-cycle ratio rectanglar wave signal with a frequency of less than 10 Hz, corresponding to the fluid flow time scale, to modulate the amplitude of the 10 MHz DEP signal. Pulsed DEP works by turning off the nDEP force for a short period of time to release the particles downstream. Because the velocity field shaped by the pillar array is highly nonuniform, the released particles move at different speeds on the basis of their locations. After a short period of time, the particles will spread over a larger region than when held in the stagnation region of the pillar. Some of the particles would have moved past the electrode, but some of them may not have. When the nDEP force is turned on again, some of the particles will be pushed back to the pillar, and the others will continue to flow downstream. The critical point that separates these two groups of particles is known as the separation point (sp), which is decided by the distribution of total force as shown in Figure 4. By carefully controlling the time period of switching the nDEP on/off, we can keep only one bead in the trap while releasing the extra particles to downstream trap units. Generally, a higher amplitude of the AC voltage is needed (compared to that of continuous DEP) to trap an equivalent number of particles as a result of the effect of time-averaged strength. The lower and upper bounds of time period for turning off the DEP are illustrated in cases 1 and 2, respectively, in Figure 4. In case 1, the leading particle just crosses the separation point; hence, if the DEP force is turned off for a time period shorter than this, then the particles will not be separated. In case 2, the trailing particle just arrives at the separation point; hence, if the
Figure 3. Simulation results showing the patterns of different numbers of trapped beads with fixed voltage at 10 V and various flow rates from 0.07 to ∼0.12 mL/h (top panel) and with a fixed flow rate at 0.07 mL/h and various voltages from 5 to 10 V (bottom panel).
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gap ) D
Figure 5. (a) Series of snapshots depicting the process of isolating a single particle in the trap when two particles are trapped initially. Fluid flows from top to bottom in this case. (b) Fluorescence image of the experimental result under pulsed nDEP to trap single 5 µm beads in the pillar array. Fluid flows from left to right in this case.
DEP force is turned off for a time period longer than this, then both particles will be released. The pulsing frequency can be chosen between the range of time given by the above two extreme cases. To obtain a very stable trap, the gap between the two particles should be as large as possible. The velocity gradient of the nonuniform flow field plays a key role. When the DEP force is turned off, the gap between two particles with a diameter D is given by
j P P V 1 2 jP P V 1 2
or gap ) D
j PP V 1 2 jP P V 1 2
(3)
jP P , V j P′ P′ , and V j P′′ P′ represent the average velocities for Here, V 1 2 1 2 1 2 segments P1P2, P1′ P2′, and P1′′P2′′, respectively. Because of the nonslip boundary condition at the pillar array, the gap will increase drastically when the particles move away from the pillar. The pulsed DEP particle trap is tested using 5 µm PS beads. Figure 5a shows a series of snapshots depicting the process of isolating a single particle in the trap when two particles are trapped initially. The experimental videos are included in the Supporting Information. The number of trapped particles in the pulsed DEP particle trap can be controlled by varying not only the flow rate and amplitude of ac voltage but also the pulsed frequency. The pulsed DEP trap has an additional advantage of being able to flush out large clusters of particles that tend to block the microchannel and causing conventional continuous DEP single-particle traps to fail. Generally, the size of a cluster of particles is much larger than a single particle. Hence, the cluster will move faster than a single particle after nDEP is off. Under the right operating parameters, the cluster will be in an unstable oscillation and will move downstream beyond the separation point. After several cycles, these clusters will be flushed away, as shown in the Supporting Information. Figure 5b shows a snapshot of an array of 40 pillars. For a flow rate of 0.1 mL/h and 20 Vpp with a pulsing frequency of 0.8 Hz, it is observed that 70% of the pillars trap a single bead, 27.5% of the pillars trap two beads, and 2.5% of the pillars trap three beads. In summary, a microfluidic particle-trap array was developed on the basis of the interplay of two sets of forces: fluid flow and nDEP. The number of beads trapped is controlled by the voltage and flow rate applied. Pulsing the nDEP force helps to obtain a more robust single-particle trap. Experimental results obtained with 5 µm polystyrene beads are in rough agreement with the prediction from a simulation model. Acknowledgment. This work is sponsored by the SingaporeMIT Alliance (SMA) Computational Engineering Program flagship research project. We thank Joel Voldman for helpful discussions. Supporting Information Available: Videos of cluster motion. This material is available free of charge via the Internet at http://pubs.acs.org. LA803761F
