magnetic track array for efficient bead capture in microchannels

ORIGINAL PAPER

Magnetic track array for efficient bead capturein microchannels

Mlanie Abonnenc & Anne-Laure Gassner &Jacques Morandini & Jacques Josserand &Hubert H. Girault

Received: 30 April 2009 /Revised: 20 July 2009 /Accepted: 23 July 2009 /Published online: 16 August 2009# Springer-Verlag 2009

Abstract Magnetism-based microsystems, as those dedi-cated to immunoaffinity separations or (bio)chemicalreactions, take benefit of the large surface area-to-volumeratio provided by the immobilized magnetic beads, thusincreasing the sensitivity of the analysis. As the sensitivityis directly linked to the efficiency of the magnetic beadcapture, this paper presents a simple method to enhance thecapture in a microchannel. Considering a microchannelsurrounded by two rectangular permanent magnets ofdifferent length (Lm=2, 5, 10 mm) placed in attraction, itis shown that the amount of trapped beads is limited by themagnetic forces mainly located at the magnet edges. Toovercome this limitation, a polyethylene terephthalate(PET) microchip with an integrated magnetic track arrayhas been prototyped by laser photo-ablation. The magneticforce is therefore distributed all along the magnet length. Itresults in a multi-plug bead capture, observed by micro-scope imaging, with a magnetic force value locallyenhanced. The relative amount of beads, and so the specificbinding surface for further immunoassays, presents asignificant increase of 300% for the largest magnets. Theinfluence of the track geometry and relative permeability on

the magnetic force was studied by numerical simulations,for the microchip operating with 2-mm-long magnets.

Keywords Microfluidics . Magnetism . Permanent magnet .

Magnetic bead . Ink .Magnetic track array . Photo-ablation .

Polymer . Numerical simulation

Introduction

Target analytes in biological samples are often present inlow concentrations relative to the surrounding matrixcomponents, necessitating effective separation techniques.Nowadays, immunoaffinity separation is a widespread toolin clinical testing, diagnostic, and bio-chemical sampleanalysis. Whereas enzyme-linked immunosorbent assay(ELISA) tests require at best few minutes for easy-to-useformats and hours for classical microtiter well tests,immunoassays in microfluidic format present the advan-tages of reduced reagent consumption and fast time ofanalysis [1]. Proteins may be either attached directly to thewalls of a capillary column or microchip channel, or tosolid-phase supports and packed into columns or channels.Solid support may consist of particles or beads made fromplastic, silica or glass, and magnetic materials. Proteinadsorption in polymer microchannels [2] and its dynamicunder stopped flow and continuous flow have beenpreviously investigated in our lab [3, 4]. Alternatively,magnetic beads were also used as solid-phase support todevelop magnetic bead-based immunoassay both in micro-chip for biosensing and in capillary for immunoaffinitycapillary electrophoresis (IA-CE) [5-7]. As the sensitivityof the analysis is highly dependent on the magnetic bead-trapping efficiency, we propose here a novel design ofphoto-ablated polymer microchip integrating a magnetic

Electronic supplementary material The online version of this article(doi:10.1007/s00216-009-3006-3) contains supplementary material,which is available to authorized users.

M. Abonnenc :A.-L. Gassner : J. Josserand :H. H. Girault (*)Laboratoire dElectrochimie Physique et Analytique,Ecole Polytechnique Fdrale de Lausanne (EPFL)-SB-ISIC-LEPA, Station 6,1015 Lausanne, Switzerlande-mail: [email protected]

J. MorandiniAstek Rhne-Alpes,1 place du Verseau,38130 Echirolles, France

Anal Bioanal Chem (2009) 395:747757DOI 10.1007/s00216-009-3006-3

track array to enhance the magnetic bead capture in amicrochannel.

Magnetic beads are usually superparamagnetic particlesthat exhibit a high magnetization in presence of a magneticfield, allowing an easy manipulation of the particles. Oncethe external magnetic field removed, they present noresidual magnetism and can be freely re-suspended [8]. Awide range of bioreactive molecules can be adsorbed orcoupled to the bead-surface, which provides a high-bindingsurface area. Yet, they have proven to be efficient tools inmedical applications such as drug targeting [9, 10] and bio-separations including protein/peptide isolation, bioassays,or cell sorting [11, 12]. Typically, isolation of bio-molecules,such as peptides and proteins, is usually performed using avariety of chromatography, electrophoretic, ultrafiltration,precipitation techniques. The use of magnetic particles forthis purpose can simplify considerably the purification steps,limiting as well the volume of eluting or precipitation buffers[13]. Whereas the magnetic beads are nowadays commonlyused at laboratory-scale (i.e., in tube), the development ofmethodologies in microfluidic format has recently emergeddecreasing sample/reagent consumption, cost and timeconsumption [14-17]. Such magnetism-based devices havefound great applications in biology, as for cell manipulation[18-23], (bio)chemical reaction as proteolysis [24, 25],bioassay [26-28], DNA or RNA hybridization [29, 30].Magnetic beads appeared as well in microsystems to designmixers, valves, or switches [31-37].

The microfabrication complexity of such magnetism-based devices is highly dependent to whether the magneticfield actuation is integrated or not to the device. Activemagnetic microsystems rely on the use of on-chip micro-electromagnets capable of a local addressability [15].However, the complex microfabrication processes and thelimited field strength (0100 mT) are the weak points ofthese devices. In contrast, passive magnetic microsystemsuse external macro-sized permanent magnets or electro-magnets [25, 27, 38]. The process is simplified and largermagnetic fields (>0.5 T) and forces can be reached. In arecent study, the distribution of magnetic field and force ina microchannel surrounded by permanent magnets wassimulated [39]. It has been shown that the magnetic fieldand force is restricted to a specific location defined by themagnet geometry and pole orientation (attraction, repulsion,single magnet). In order to control the magnetic fieldaddressing towards specific locations, magnetic elementscan be integrated to the structure, both in active and passivemicrosystems [40, 41]. Smistrup et al. introduced an on-chip magnetic bead microarray in which an externalmagnetic field (21 mT) magnetizes soft magnetic elementsplaced along the microchannel [29]. In combination withhydrodynamic focusing, the beads from different incomingstreams are captured at the sidewalls and functionalized

with two types of magnetic beads carrying different probesto assess DNA hybridization. Alternatively to their work inwhich the magnetic field value limits the bead trapping atthe sidewalls, the present study uses a higher field tomagnetize the elements leading to full multi-plugs of beads.The fabrication process is also simpler and based onpolymer laser ablation. Indeed, in contrast with intensiveclean-room and multi-steps fabrication processes as silicon-based techniques, fast prototyping such as soft lithography[42] and polymer laser ablation [43, 44] are quite simple,cheap, and in most cases, do not require a heavy clean-room environment. Several groups have used soft lithogra-phy to design magnetism-based devices [42, 45]. Viovyet al. introduced a PDMS microchip, based on softlithography and fast prototyping, with embedded permanentmagnets oriented in repulsion from 20 with respect to themicrochannel axis [25]. The chip was demonstrated to bean efficient reactor to perform protein digestion thanks tothe high surface area-to-volume ratio provided by the beadsthat are organized in columns parallel to the flow, thuslimiting the flow resistance.

Taking advantage of our lab experience in polymer laserphoto-ablation [46-48], a microchip, including a magnetictrack array, was designed in order to get an efficient beadcapture along the microchannel for further use in immu-noaffinity separation (Fig. 1). The actuation is made by twoface-to-face permanent magnets placed in attraction andfixed on both microchannel sides to magnetize the tracksfilled with a home-prepared magnetic ink. The magneticfield is therefore addressed through these tracks, symmetri-cally located as an array perpendicular to the microchannel,resulting in a multi-plug bead capture. The study includesmicroscope imaging showing the multi-plug bead organiza-tion in microchips including a magnetic track array. Bycomparison with a microchip without array, the gain of usingmulti-plugs vs. a single plug of beads is evaluated, whenmagnets of 2-, 5-, and 10-mm long are used. Numericalsimulations of the inter-track gap, channel-track gap, and inkpermeability provide an orientation to improve this gain.

Theory

Magnetic particle transport in a microfluidic system

The present investigation describes the physical phenomenaoccurring in a microfluidic system constituted of a micro-channel surrounded by two face-to-face permanent magnetsin attraction (Fig. 1). The superparamagnetic beads insolution are infused under pressure-driven flow from themicrochannel left side.

The main forces experienced by the particles are themagnetic force, Fmag and the hydrodynamic drag force,

748 M. Abonnenc et al.

Fdrag. The gravitational force, particle/fluid interactions andinter-particle effects are neglected.

In a material submitted to a magnetic field H generatedby a magnetic source, the local flux density (or induction)B is the sum of the induction 0H given by the source inthe vacuum and of the local induction 0M due to themagnetization M of the studied medium (M = H in thelinear magnetization zone of the material):

B m0 H M m0 1 # H m0mrH mH 1where 0 and are the respective vacuum and bead

absolute permeability, r the relative bead permeability and the magnetic susceptibility of the bead material. For asuperparamagnetic bead of volume V immersed in asolution, the magnetic moment is given by mbead VM V#H where # # # water is the relative susceptibil-ity of the bead compared to the one of the solution. Themagnetic force applied on these particles can be expressedas the derivative of the magnetic potential energy:

Fmag r mbead I B mbead Ir B 2

which results in Eq. 3 by taking into account Eq. 1 and themoment expression under the saturation conditions of thebeads (mbeadmsat):

Fmag V#m0B Ir B 3

In addition to the magnetic force, the particles experience ahydrodynamic drag force, defined by the Stokes equation,

Fdrag 6phRv 4

with the assumption that the magnetic beads have theapproximate shape of a sphere of radius R, is the fluidviscosity and v the velocity of the bead compared to thesurrounding fluid velocity.

Numerical simulations

Numerical simulations of the magnetic field and forcedistribution, based on the finite elements (FE) method, werecarried out in a 2D geometry, in magnetostatic conditions.The simulations were performed without any flow in themicrochannel. The FE formulation was implemented in thecommercial software Flux-Expert (Astek, Rhne-Alpes,Grenoble, France) on a Mac Pro with Ubuntu Linux 7.10operating system.

Numerical model and assumptions

As the different study domains are nonconductive (withoutany free electrical current), the first Maxwell equation writesr ^H 0 and consequently the magnetic field can bederived from the magnetic scalar potential f H rf .From the magnetic induction conservation and Eq. 1 inwhich the permanent induction Bmag imposed by themagnet is added, we have:

r I B r I mrf Bmag 0 5

which is solved by using a Galerkin formulation (Support-ing information S1). In any point of the domain, the fluxdensity vector B is deduced from the value.

The following assumptions are made: (a) magnetostaticconditions (@B=@t 0), (b) 2D Cartesian form: the thirddimension of the system (i.e., the magnet channel and trackthickness), is supposed to be at least ten times larger than itsheight (i.e., magnet gap), so it can be neglected), (c) nomagnetic elements around the magnets, (d) homogeneousmedia ( uniform in each of the different domains), (e)stopped-flow conditions (i.e. static bead solution in themicrochannel), (f) the particlefluid or inter-particle interac-tion are not considered. (g) The effect of the beads on thesolution permeability (solution) is neglected. (h) The mag-netic beads are assumed to be ideal (msat>>Bmax) as there isno quantitative comparison with experimental results.

This numerical model has been previously validatedshowing a good correlation with published results [39,49].

Geometry and numerical parameters

The reference geometry considers two face-to-face perma-nent macro-magnets (Lm=2 mm) placed in attraction with

Magnets Capillary junction

Magnetic tracks

Microchannel

Microchip holder

Lm

g

Fig. 1 Top-view of the microchip with magnetic tracks. The micro-channel (100 m wide, 50 m deep, 4 cm long) is positioned in aholder with a capillary connection fittings. The magnetic track array iscomposed of 34 tracks spaced by 200 m and is surrounded by twopermanent magnets of 10 mm long fixed in attraction on a glass slide.Lm is the magnet length and g the inter-magnet gap. The inset pictureshows eight magnetic tracks spaced by 200 m designed for 2 mm-long magnets, which is the study case in the numerical simulations

Magnetic track array for efficient bead capture in microchannels 749

an inter-magnet gap g=2 mm, as illustrated in Fig. 1.Consequently, the magnet length over inter-magnet gapratio Lm/g=1. The microchip presents a thinner width of1.3 mm in the magnet region and includes at its center a100-m-wide microchannel. The following parameters areapplied on the studied geometry: relative permeability ofthe materials r=1 in the entire domain, except in themagnetic tracks where r=4. Imposed flux density in themagnets: Bmag 1:3 T. The beads parameters are fixed atunity: =1, V=1.41020m3 (for a bead diameter of0.3 m). An air box is meshed around the magnets togive enough space for the magnetic field rotation from onepole to the other. The size of the mesh elements and of theair box has been systematically calibrated.

The numerical investigations concerning the microchipwith magnetic tracks were restricted to the design with2-mm-long magnets as the number of tracks to mesh wouldbe too high for larger magnets.

Experimental

Chemicals

Three different magnets were used for this study: arectangular NdFeB 228 mm3 magnet (magnetic rema-nence of 1.3 T at the pole and a polarization in the longestdimension, from Chen Yang, Finsing, Germany), tworectangular 52.52 mm3 and 1032 mm3 magnets(magnetic remanence of 1.3 T at the pole, with apolarization in the smallest dimension, from Supermagnet,Switzerland). Fused-silica capillaries (100/375 m i.d./o.d.)were obtained from BGB Analytik AG (Bckten, Switzer-land). Protein A-coated superparamagnetic beads of uni-form size (mean diameter of 300 nm, binding capacity100 g/mL) were purchased from Ademtech (Pessac,France). The bead suspensions were sonicated and dilutedten times in water produced by an alpha Q MilliporeSystem (Zug, Switzerland).

Magnetic ink preparation and characterization

The home-prepared magnetic ink is made of 1 g of carbonink (Electrador, Electra Polymer & Chemicals Ltd., UK)mixed with 200 mg of 10 m iron particles (Merck,Darmstadt, Germany). The density and magnetic perme-ability of the ink were determined. The density of4.9679 g/cm3 was measured with a helium pycnometer onthe cured ink. The mass magnetization of the ink wasmeasured with a SQUID as a function of the field at 300 K.(Supporting information S2) The resulting value of dimen-sionless magnetic permeability was 4.07. This value wasused in the model.

Device microfabrication

The microfabrication process is based on polymer photo-ablation and is illustrated in Fig. 2.

Step 1. The main microchannel (100 m wide, 50 mdeep and 4 cm long) is drilled on the topside of apolyethylene-terephthalate sheet of 100 m thick-ness (Melinex sheet from Dupont, Wilmington,DE, USA) by photo-ablation with an ArF excimerlaser (193 nm, Lambda Physik, Gttingen,Germany). Spaced by 650 m from the micro-channel x axis, a parallel channel (50 m wide,100 m deep and 1 cm long) is drilled to furtherfacilitate the microchip lateral sides cutting. Then,rectangular tracks (100500100 m3) perpen-dicular to the main microchannel are drilled bystatic laser shoots. The reference design is definedby a channel-track gap of 10 m and an inter-track gap of 200 m. Only the photo-ablation stepis made in a clean-room.

Step 2. The chip is cleaned with MeOH and the lateralsides of the chip are removed to allow the laterpermanent magnet positioning close to the mag-netic tracks. The chip is laminated and cured 1 hat 80 C.

Step 3. On the PET chip backside, the microchannel inletsare protected and the tracks are filled with thehome-prepared magnetic ink.

Step 4. The backside of the chip is laminated and the chipis cured 30 min at 80 C.

The entire process is achieved in a single day witharound ten microchips per batch.

Microfluidic system operation and cleaning

The permanent magnets are fixed on a microscope slide inattraction with an inter-magnet gap of 2 mm. The microchip isthen positioned in the inter-magnet gap, as illustrated in Fig. 2.

A PACE MDQ system (Beckman-Coulter, Nyon, Swit-zerland) embedding an autosampler and UV detector isused for the sample delivery, in pressure mode. Theinfluence of the capillary connections to the microchipand the magnetic beads on the pressure drop was evaluated.(Supporting information S3)

The beads are captured at a flow rate of 100 nL/min,optimum to avoid beads sedimentation and unspecificbinding during the filling. The time of injection was variedaccording to the experiments. For cleaning, the beads areflushed out after magnet removing. The chip is washed withacidic solutions or solvents, and if necessary immersed inultra-sonic bath.


Imaging of magnetic bead capture/release

The magnetic beads were observed with a microscopeAxiovert 200 (Carl Zeiss, Gottingen, Germany) and aCCD-IRIS camera (Sony, Tokyo, Japan). Images wereprocessed with Igor 6.0 software (WaveMetrics, Portland,OR, USA).

Results and discussion

Forces and magnet length

Figure 3a illustrates the variation of the magnetic force (xcomponent), along a microchannel surrounded by twopermanent magnets of length Lm=2, 5, or 10 mm. Themagnets, in attractive configuration, present a symmetryaxis at x=0. Fmag is positive at the magnet left-edge andnegative at the right-edge. For small magnets (Lm=2 mm),the zero-Fmag is restricted to a point located at thesymmetry axis. When Lm>2 mm, the zero-Fmag regionbecomes larger.

The main forces experienced by a magnetic bead in sucha microfluidic system with large magnets (Lm=10 mm) aresummarized in Fig. 3b. The microchannel region betweenthe magnets can be subdivided by three where: (1) Fmagand Fdrag are in the same direction resulting in the

acceleration of the beads; (2) Fmag=0, leading to the beadsslow down; (3) Fmag is opposite to Fdrag. Consequently, ifFmag>Fdrag, the beads will be trapped in this area. Instopped-flow conditions, the beads move back to themiddle of the magnets, as the negative Fmag x componentrepels them.

The bead capture, in a microchannel surrounded bytwo 10 mm-long magnets in attraction, was followed bymicroscope imaging. The next conditions were fixed forthe entire study: (a) the flow velocity was kept constantfor the bead delivery and only the bead injection timewas changed according to the experiment; (b) thedistance between the two magnets was kept constant(g=2 mm) to allow the microscopic imaging of the beads.In agreement with the force distribution, the beads aretrapped from the magnet end-edge, as illustrated inFig. 3c.

As the major magnet length is not optimally exploited insuch conditions, our approach was to integrate a magnetictrack array in order to distribute the magnetic field all alongthe microchannel.

Magnetic track integration

The microchip design and fabrication process are displayedin Figs. 1 and 2, and detailed in the Experimental section.Briefly, the microchip is constituted of a main microchannel

Fig. 2 Microchip fabricationprocess


with an array of magnetic tracks integrated perpendicularlyand on both side of it in order to address the magnetic fieldin successive locations. The number of tracks is adjusted tofit the magnet length. In the present work, microchips with8, 17, and 34 tracks were processed to fit with 2-, 5-, and10-mm-long magnets, respectively. As illustrated in Fig. 1,the microchip is positioned in a holder with capillaryconnection fittings for sample delivery. The magnetic trackregion is therefore positioned between the magnets fixed inattraction.

Magnetic field addressing

Figure 4 presents the isovalues of the magnetic fielddistribution (y component) obtained by numerical simula-tions. In the gap between the two magnets in attraction andwithout magnetic tracks (Fig. 4a), the magnetic field goesstraight from one magnet towards the other and its

magnitude along the microchannel (y=0) presents amaximum at the magnet vertical symmetry axis (x=0).Along the microchannel (y=0), the magnitude decreasesnear the magnet edges due to the enlarging of the field linesat the magnet extremities. The field line curvatures are atthe origin of the Fmag force maxima in these regions (due tothe BrB term in Eq. 3) [8, 39].

In the presence of magnetic tracks (Fig. 4b), themagnetic field lines are focused through the tracks andamplified because of the field line concentration. Thisconfiguration is analogous to one with successive micro-magnets positioned along a microchannel where eachtrack is comparable to a single magnet. However, whenthe external magnetic field is generated by permanentmagnets, the magnetic field is not equally distributedbetween the tracks (as shown by the force in Fig. 5c).Then, the inner tracks experience a higher magnetic fieldthan the outer ones. However, compared to on-chip micro-

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F x / p

N

6420-2-4

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Magnet symmetrical axis

a

b

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S

N

Fig. 3 a Variation of the mag-netic force (x component) infunction of the magnet length(Lm=2, 5 and 10 mm).b Scheme summarizing the mainforces, Fmag and Fdrag, experi-enced by the magnetic beads,with large magnets (Lm>2 mm).c Microscope picture of themagnetic bead in a microchan-nel surrounded by two magnetsof Lm=10 mm


electromagnets, the use of macro-sized permanent mag-nets combined with magnetic tracks enables to reachhigher magnetic field values (>0.5 T).

Magnetic force mapping and resulting magnetic beadcapture

Figure 5 shows the magnetic force distribution and the beadcapture along the microchannel. Without tracks (Fig. 5ab),the negative value of Fmag at the magnet end-edge counter-balances the Fdrag magnitude and enables the magneticbead trapping (assuming that Fmag>Fdrag). For low flowrates (low value of Fdrag) the starting position of the plug(S) is located approximately at the middle of the magnet, asqualitatively confirmed on Fig. 5b. The beads are organizedin a single plug growing with time until reaching the pointwhere Fmag is not sufficient to retain them. It also confirmsthe partial bead release outside the end-side (E) of the plug.For the flow velocity value applied here, the region whereFmag>Fdrag (defined as the plug length LP in dynamicconditions) represents 80% of the magnet length Lm(Fig. 5ab).

Using the magnetic track configuration (Fig. 5cd), themagnetic field distribution generates a pattern of successivepositive-negative peaks of Fmag along the microchannel.The negative peaks located at each track allow to locallytrap the beads, with a Fmag x component amplitude tentimes higher. The first plug is growing from the firstmagnetic track. When saturation occurs, the beads move tofill the next plug, and so on.

Design considerations

The influence of the channel-track gap, inter-track gap andmagnetic ink permeability values on the magnetic force ispresented in Fig. 6. The results are evaluated as a gain,calculated from the ratio of magnetic forces between thechip embedding tracks and not. The position of thepermanent magnets with respect to the microchannelremains unchanged. The force is considered at the maximalvalue at the center of the microchannel.

First, the channel-track vertical gap was increased from 1to 25 m. The gain, in terms of magnetic force, wasevaluated for the x- and y components of the force, asillustrated in Fig. 6a and b, respectively. The firstobservation from both figures is that closer the magnetictracks from the microchannel, larger are the local magneticforce value. In the presence of the magnetic tracks, theminimal Fx gain is 10 for a gap of 25 m and can reach 19for a gap of 1 m (Fig. 6a). Because of the fixed permanentmagnets position, the addition of tracks has a major effecton the y component of the force with values reaching threeorders of magnitude. The Fy gain is of 2510

3 for a gap of1 m and rapidly decreases to 1103 for a 25-m gap(Fig. 6b). To illustrate the effect of the track verticalposition on the y component of the force, a chip wasdesigned with an asymmetrical channel-track distance, asshown on the microscope image in Fig. 6b. When one ofthe magnetic tracks is closer to the microchannel, the beadsstart to form a cluster towards its sidewall resulting in anasymmetrical plug. From a microfabrication point of view,the reference design value was fixed at 10 m to be surethat the lamination layer resists to the pressure-driven flow.

The second studied parameter was the inter-track gapvalue, which was varied from 50 to 300 m (Fig. 6c). Thenumber of tracks was adjusted to fit the magnet length. Thetracks were positioned symmetrically from the magnetcentre. Therefore, when the gap is small, the number oftracks in the area between the two permanent magnetsbecomes higher. When the gap is larger, the Fx gain alsoincreases from five to 17 and approaches a plateau for a gapvalue higher than 300 m. For a gap of 50 m, the positionof the tracks are so close that they can be assimilated to aglobal magnet without changing significantly the magneticfield lines distribution. On the other hand, with a larger

0

1.3

By

0

2.1

By

a

b

Fig. 4 Isovalues of the magnetic field distribution (y component).Microchips a without and b with eight magnetic tracks are simulatedwith 2-mm-long permanent magnets. The inter-magnet gap is 2 mmand channel width 100 m. The white lines underline the curvatures ofthe magnetic field lines


inter-track gap, each track adds as individual small magnetpresenting a negative Fmag value. Obviously, a compromisebetween the number of tracks (i.e., the number of plugs andby the way the amount of trapped beads) and the intensity ofthe repelling magnetic force has to be found. It is the reasonwhy, with the flow velocities applied, an inter-track gap of200 m appeared as a good compromise for the chip.

In order to extend the present study to other kind ofmagnetic inks, the effect of the ink properties on the Fx gainwas evaluated. The dimensionless relative permeability valuewas varied from 1 to 3,000 and it shows a linear gainincrease from 1 to about 20, for r=1 to 10, respectively. Forr values higher than 100, the gain reaches a plateau. Thepermeability value of the home-prepared magnetic ink wasdetermined to be around 4 that correspond to a gain of 12.Working on the optimization of the ink composition, threetimes higher gain could be reached. In some publications,permalloys are used as magnetic elements [40]. They areoften made of nickel (80%) and iron (20%), which typicallypresent a relative permeability close to 100, which wouldprovide a gain of 35.

The last parameter to be investigated was the length ofthe track array (data not shown). If the tracks are locatedoutside of the inter-magnet region, the magnetic field lines

will also be attracted to these outer tracks, decreasing themagnetic field and force of the inner tracks. In addition,these tracks will not efficiently trap beads because of theirlimited magnetic force amplitude.

In the reference design, the channel-track gap is 10 mand inter-track gap is 200 m, and the magnetic tracks fitthe permanent magnet length ending with 8, 17, and 34magnetic tracks for 2-, 5-, and 10-mm-long magnet,respectively.

Relative quantification of the magnetic bead capture

As the available binding surface for bioassays is directlycorrelated to the amount of trapped beads, relativequantification was done comparing a multi-plug vs. asingle-plug bead capture (Fig. 7). First, the bead capturewas imaged within three microchips with 8, 17, and 34tracks used with 2-, 5-, and 10-mm-long magnets, respec-tively. The magnetic bead injection time was increased (theflow velocity was kept constant). From the microscopeimages, the surface occupied by the beads was extracted.The resulting surfaces obtained from each experimentalcondition were then normalized by the maximal occupancyvalue (i.e., value at the plateau) obtained with the 34-track

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Fig. 5 Fluctuation of magnetic force (x component) along themicrochannel and microscope imaging of the magnetic bead capture.ab Microchips without, and cd with magnetic tracks are studied.

The vertical dashed lines on the pictures show the position of thepermanent magnets extremities respect to microchannel. S Start, E endof the plug of beads. LP plug length


microchip. When the relative occupancy reaches a plateau(Fig. 7), all the plugs are saturated with beads. As expected,this saturation value increases with the number of tracks.

Afterwards, the same amounts of beads corresponding tothe multi-plug saturation were injected in the respective

microchips without magnetic tracks, at the same flowvelocity (horizontal lines with gray markers on Fig. 7).The relative occupancy then represents 10%, 29%, 31% ofthe maximal occupancy for 2-, 5-, and 10-mm-longmagnets, respectively, showing a relevant impact of the

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Fig. 7 Quantification of themagnetic bead capture with 2-,5-, and 10-mm-long magnetsand comparison with microchipwithout tracks. The magneticbead injection time is increased,with the injection protocol set at250 nL/min for injection and100 nL/min for the bead flowingand capture

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Fig. 6 Influence of the geometrical parameters and magnetic trackpermeability on the magnetic force. The gain is expressed as the ratioof magnetic forces between the microchip embedding tracks and not.ab Gain (x- and y components) in function of the channel-track gap

value. cd Gain (x component) in function of the inter-track gap value.d Generalization of the study to other ink permeability values. Thepicture tags are microscopic images done in parallel to thesimulations. The gray region shows the value of the reference design


magnetic track incorporation. The difference is actually dueto a loss of beads without tracks. For the magnets 5- and10-mm-long without tracks, at a given flow rate, approx-imately 60% of the beads are lost during the capturingprotocol. It is partly due to the magnetic force value, i.e.,the magnetic force over drag force ratio, which is lower inthe absence of magnetic tracks, as shown in Figs. 5 and 6.For the 2-mm-long magnets, the small magnet length limitsthe amount of trapped beads as the area where Fmagcompetes with Fdrag has a length which order of magnitudeis half of the magnet length (in stopped-flow conditions).

The use of magnetic tracks allows a significant increasein the amount of trapped beads and potentially, theavailable binding surface for bio-chemical assays. Theincrease reaches 200% for the 2- and 5-mm-long magnetsand goes up to 300% for the largest tested. Of course, thisvalue depends on the flow rate and amount of injectedbeads, as well as the track position (i.e., the number oftracks and consequently the number of plugs).

Conclusions

This paper introduces a simple way to enhance themagnetic bead capture in a microchannel, and so theavailable specific binding surface for bio-chemical assays,by locally addressing the magnetic field through magnetictrack arrays. Microscope imaging and 2D finite elementssimulations were performed considering a microchannelsurrounded by two permanent magnets in attraction.Microscope imaging has shown that, for the given flowvelocity conditions, the bead capture remains almostconstant for a magnet length higher than 5 mm. To enhancethe bead capture, 100-m-wide magnetic tracks perpendic-ular to the microchannel axis, were integrated to drive themagnetic field towards specific locations. Therefore, in thegap between each opposite track, a repelling magnetic forceretains the beads leading to a multi-plug capture.

The influence of geometrical parameters on the magneticforce, such as the channel-track and inter-track gap values,as well as the magnetic track permeability, were evaluatedby numerical simulations, with a microchip operating with2-mm-long magnets. When the inter-track gap is large, thex component of the force is mainly concerned, leading to anincrease of its amplitude (Fx multiplied by three for a inter-track gap increasing from 50 to 300 m). On the otherhand, changing the vertical position of the tracks mostlyaffects the y component of the force (Fy increased by afactor 25 for a channel-track gap decreasing from 25 to1 m). The closer to the microchannel the tracks are, thelarger is the y component force magnitude.

Thanks to the multi-plug capture, the amount of trappedbeads can be increased from 200% for the 8- and 17-track

to 300% for the 34-track reference designs. The increase ofthe bead amount considerably enhances the binding surfaceavailable for in vitro applications, such as immunoaffinityseparation. Moreover, the negative force retaining the beadsis amplified by passing through the tracks (ten times higherfor the inner tracks) enabling a capture at higher flow rates.Finally, the precise positioning of the tracks can overcomethe difficult symmetrical positioning of the chip in betweenthe magnets.

It would be interesting to further investigate the growthof the plugs in order to understand more deeply thedependence of the plug length with the fluidic and magneticparameters, and in particular with the Fmag over Fdrag ratio.

Further developments on the present microchip, will beto integrate on-chip electrochemical detection to performenzyme-based immunoassays, or to embed carbon electro-des to use it as electrospray emitter in order to followon-line reactions directly by mass spectrometry.

Acknowledgments Prof. Jean-Philippe Ansermet and Dr. SimonGranville from the Laboratory of the Physics of NanostructuredMaterials (EPFL, Switzerland) are thanked for the magnetic inksusceptibility measurements. Prof. Heinrich Hofmann and CarlosMorais from the Powder technology laboratory (EPFL, Switzerland)are thanked for their contribution in the magnetic ink densitymeasurements and fruitful discussions. This work was supported bythe Fond National Suisse pour la Recherche Scientifique (FN 200020-113413 Analytical tools for fast phosphoproteome analysis, FN-PNR 404740-117324 Supramolecular phases for protein adsorption,CTI 9215.1 Microfluidic plateform for fast immunoassays).

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Magnetic track array for efficient bead capture in microchannelsAbstractIntroductionTheoryMagnetic particle transport in a microfluidic system

Numerical simulationsNumerical model and assumptionsGeometry and numerical parameters

ExperimentalChemicalsMagnetic ink preparation and characterizationDevice microfabricationMicrofluidic system operation and cleaningImaging of magnetic bead capture/release

Results and discussionForces and magnet lengthMagnetic track integrationMagnetic field addressingMagnetic force mapping and resulting magnetic bead captureDesign considerationsRelative quantification of the magnetic bead capture

ConclusionsReferences

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magnetic track array for efficient bead capture in microchannels

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