Manipulation of magnetic particles on chip by magnetophoretic actuationand dielectrophoretic levitationChengxun Liu, Liesbet Lagae, and Gustaaf Borghs

Citation: Appl. Phys. Lett. 90, 184109 (2007); doi: 10.1063/1.2736278 View online: http://dx.doi.org/10.1063/1.2736278 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v90/i18 Published by the American Institute of Physics.

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Manipulation of magnetic particles on chip by magnetophoretic actuationand dielectrophoretic levitation

Chengxun LiuaInteruniversity Microelectronics Center (IMEC), Kapeldreef 75, Heverlee 3001, Belgium and Department ofElectrical Engineering (ESAT), Katholieke Universiteit Leuven, Leuven 3000, BelgiumLiesbet Lagae and Gustaaf BorghsInteruniversity Microelectronics Center (IMEC), Kapeldreef 75, Heverlee 3001, BelgiumReceived 22 January 2007; accepted 10 April 2007; published online 4 May 2007

The prospect of using magnetic particles for biomedical purposes in lab-on-a-chip systems compelsaccurate and flexible particle manipulation. Toward such a goal we designed a microdevicecomprising a pair of meander-shaped current carrying conductors, which enable simultaneousmagnetophoresis and dielectrophoresis by generating both a traveling magnetic field and an acelectric field. Therefore, both the in-plane and out-of-plane movements of magnetic particles can beelectrically controlled. A transport speed of tens of m/s was achieved with actuation forces atpiconewton scale. The enhanced control of particle movement avoids the contact and nonspecificadhesion between the particle and device. 2007 American Institute of Physics.DOI: 10.1063/1.2736278

The past decade has witnessed growing applications ofmagnetic particles in lab-on-a-chip LOC systems for bio-analyte separation,1 magnetic bioassay,2 etc. The target bio-molecules or cells are captured by functionalized magneticparticles and can then be attracted35 or transported69 byelectromagnetic fields i.e., magnetophoresis. Various in-plane transport modes of magnetic particles have beenachieved by on-chip electromagnetic fields. However, theout-of-plane movement is still primarily determined by theDerjaguin-Landav-Verwey-Overbeek DLVO force FDLVObetween the particle and the device surface, and thus difficultto control. The DLVO interaction is the combination of thevan der Waals force and the electrostatic force between thecharged surfaces of magnetic particles and the device. It ishighly dependent on pH, ionic strength, and the surfaceproperties of particles and device. In common physiologicalbuffers with neutral pH and high ionic strength, the DLVOforce is very often attractive or weakly repulsive, making itimpossible anymore to balance other attractive forces, e.g.,the out-of-plane component of the magnetic force Fig. 1a.Consequently, the magnetic particles contact and then adhereto the device surface, resulting a failure in transporting thebiomolecules or cells as carriers.9,10 This problem has be-come one of the major obstacles for the use of magneticparticles in LOC systems. In order to circumvent the uncon-trollability of FDLVO, we designed a device for the manipu-lation of magnetic particles effectively by a simultaneousmagnetophoresis and dielectrophoresis. The device excelsprior art11 by the capability of controlling both the in-planeand out-of-plane movements of the particles.

Magnetophoresis MAP represents the movement of amagnetic particle that is actuated by magnetic forces in amedium. One-dimensional magnetophoresis can be ex-pressed by Eq. 1, where Fmag and FD are the magnetic forceEq. 2 and fluidic drag force Eq. 3, respectively. Fmag,xis the component force of Fmag in the x direction.

Fmag,x

+ FD = md2xdt2

, 1

Fmag =V 20

B2, 2

aElectronic mail: chengxun.liu@imec.be

FIG. 1. Color online Continuous transport scheme with combined MAPand DEP. a shows the force diagram for a magnetic particle with Idc carriedby conductor A. The in-plane actuation force is the combination of thein-plane magnetic force component Fmag,x and the fluidic drag force FDcaused by particle movement. The out-of-plane separation distance z is de-termined by the interactions of the DEP force FDEP, out-of-plane magneticforce component Fmag,z, DLVO force FDLVO, and the particle gravity G.The particle movement in the transport plane is determined by the totalbalanced force F. In step a, with a constant B0, the magnetic particle isattracted from the current position over conductor B to the neighbor stripof conductor A to the right, where the total in-plane magnetic field is stron-ger. In b and c, the current is switched between conductors A and B asshown with an alternate current direction. Following this scheme, the mag-netic particle could be transported continuously. The transport direction canbe simply reversed by changing the step sequences or the direction of B0.

APPLIED PHYSICS LETTERS 90, 184109 2007

0003-6951/2007/9018/184109/3/$23.00 2007 American Institute of Physics90, 184109-1Downloaded 22 Oct 2012 to 132.181.2.66. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

FD = 3Ddxdt

fD, 3

where m is the mass of the particle, V the volume of theparticle, 0 the magnetic permeability in free space, thedifference of volume magnetic susceptibility between theparticle and the medium, D the hydrodynamic diameter ofthe particle, the viscosity of the fluid, fD the fluidic dragforce coefficient,10 and B the magnetic flux density.

Dielectrophoresis DEP is the movement of a particlewhen it is subjected to an inhomogeneous alternating electricfield and polarized with respect to the medium. Both thepolarity and amplitude of the DEP force FDEP are deter-mined by the particle size, permittivity of the particle andmedium, and particularly the amplitude and frequency of theelectric field.12

The device comprises two meander-shaped current-carrying conductors Figs. 1 and Fig. 2a. The two conduc-

tors are electrically insulated and can be operated indepen-dently. When a dc Idc is sent to one conductor, a magneticfield is built around it. The symmetric structure of the con-ductor layout leads to a zero in-plane component of B2 andconsequently a zero net in-plane magnetic force exerted onmagnetic particles in the middle of two neighbor meanderstripes according to Eq. 2. Based on this fact, we applied aconstant homogeneous field B0 in the +x direction so that thein-plane field is biased and the in-plane force is no longerzero Fig. 1a. In this way, the magnetic particle can bemoved by one step from conductor A to conductor B in the+x direction. For continuous actuation, both conductors arefed with an alternate and periodic dc, accompanied by alter-nate current directions for every conductor. Consequently, atraveling in-plane magnetic field is produced, which actuatesthe magnetic particles step by step. In comparison with priorart,69 the current device does not induce any magnetic fieldgradient in the y direction, which allows for a group of singleparticles aligning in the y direction to move in the x directionwithout aggregation. In addition to the magnetophoresis, inorder to control the out-of-plane particle positioning, a highfrequency ac sinusoidal signal Vac is applied across the twoconductors to generate an inhomogeneous ac electric fieldnear the device surface. The frequency was carefully selectedin order to achieve a repulsive negative DEP force. Bysimultaneously applying the alternate dc and the high fre-quency ac signal, the magnetic particles can be transported inthe x direction and kept at a controllable distance to the chipsurface in the z direction.

The device was fabricated using standard optical lithog-raphy. On a silicon wafer with SiO2 on top, Ti 10 nm/Au100 nm was sputtered and patterned as the first metal layerlight gray in Fig. 2a. Afterward 450 nm Si3N4 was depos-ited and the contacts between the first and second metal lay-ers were opened by reactive ion etching brown in Fig. 2a.Finally, the second metal layer Ti 10 nm/Au 1.2 m wassputtered and patterned by ion milling dark gray in Fig.2a. The manipulation experiment was performed withDynabead CD45 D=4.5 m, =0.1; Invitrogen in theEagles minimum essential medium MEM cell culture me-dium pH=7.4.

We first compared the movement of magnetic particleswith and without FDEP. The solution with magnetic particleswas added on the device and the traveling magnetic field wasactivated without applying Vac. With a constant B0=0.6 mT,when a 10 mA current was applied, the magnetic particleswere quickly attracted to the conductors by the traveling

FIG. 4. Dependency of the in-plane actuation velocity of the particle on Idcand Vac. a Magnetic particle transport velocity at different actuation cur-rents. Vac=2 Vp-p at 1 MHz and B0=0.6 mT were applied. b Maximumactuation current and transport velocity at different Vac amplitudes. Vac wasalways at 1 MHz.

FIG. 3. Color online Continuous actuation of magnetic particles. Stepsac correspond to ac in Fig. 1, respectively. Both the width andspacing of the conductors were 5 m. Idc=15 mA, switching frequency of0.6 Hz, Vac=2 Vp-p at 1 MHz and B0=0.6 mT.

FIG. 2. Color online Demonstration of the DEP levitation. a a magneticparticle on the device schematic; b when no ac signal was applied acrossthe conductors, the Dynabead CD45 particle simply sat on the devicesurface; c when the conductors were fed with the ac signal Vac=2 Vp-p at1 MHz, the negative DEP force levitated the particle from the device sur-face and hence the particle went off focus of the microscope while thedevice remains in the focus. Pictures b and c were taken from micro-scope with Idc=0 mA.

184109-2 Liu, Lagae, and Borghs Appl. Phys. Lett. 90, 184109 2007

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magnetic field and sticked to the device permanently dataincluded in Fig. 4b as Vac=0 V. This experiment indicatedthat in the MEM medium FDLVO cannot be used to balancethe out-of-plane component of the magnetic force Fmag,z.The necessity of the DEP levitation is thus proven. In thesecond experiment, the dielectrophoretic property of themagnetic particle was studied. With Idc=0, Vac is appliedacross the conductors to induce FDEP. Figure 2b clearlyshows that the magnetic particle was levitated by FDEP fromthe device surface. By observing the out-of-plane particlepositioning with a microscope while sweeping the Vac fre-quency, the greatest negative DEP was found to occur around1 MHz.

With the DEP levitation, a continuous in-plane magneticactuation became feasible Fig. 3. The magnetic particle be-ing actuated by the traveling magnetic field, the transportvelocity is controlled by modulating Idc amplitude andswitching frequency. When the switching frequency is suffi-ciently low with a fixed Idc amplitude, the magnetic particlecan always follow the traveling field. The highest velocity isachieved at the cutting frequency, above which the particlestarts to lag behind the traveling field. The highest velocity isplotted in Fig. 4a at different Idc. The maximum velocityincreases monotonously as Idc increases from 0 to 20 mA.Afterward when Idc continues to increase, the repulsive FDEPis not strong enough to balance the attractive Fmag,z. Hencethe particle adheres to the device and stops moving. Thisfinding shows that the maximum velocity is restricted byFDEP. Therefore, we investigated the maximum velocity ofthe particle at different Vac amplitudes Fig. 4b. The analy-ses show that the velocity of the magnetic particles can bemonotonously increased by a larger in-plane magnetic force,which requires a larger in-plane bias magnetic field B0 or ahigher current-induced traveling magnetic field gradient.However, as Fmag,x and Fmag,z are positively correlated, FDEPis to be increased as well in order to keep the separationdistance and sufficient particle velocity.

The forces were calculated using finite element analysisin ANSYS based on Eq. 2. The maximum Fmag,x is

3.7 pN at 20 mA, and the maximum FDEP is 5.5 pN at 2 V,1 MHz. The heat dissipation induced by Joule heating wasalso evaluated in ANSYS. The result showed a temperatureincrease by only 10 C from 20 C without active cooling,which is acceptable for most bioapplications.

In conclusion, we presented a method to transport mag-netic particles in a microdevice. The scheme used a travelingmagnetic field for in-plane magnetic particle actuation. Anadditional dielectrophoresis was applied on the same pair ofmeandering conductors in order to balance the DLVO force,the particle gravity, and the out-of-plane component of themagnetic force. With the device magnetic particles can bemoved bidirectionally up to 36 m/s with a controllableout-of-plane position of a few micrometers. This methodprovides more flexibility to the transport of magnetic par-ticles in lab-on-a-chip systems.

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184109-3 Liu, Lagae, and Borghs Appl. Phys. Lett. 90, 184109 2007

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