Micromagnet structures for magnetic positioning and alignment L. F. Zanini, O. Osman, M. Frenea-Robin, N. Haddour, N. M. Dempsey et al. Citation: J. Appl. Phys. 111, 07B312 (2012); doi: 10.1063/1.3675067 View online: http://dx.doi.org/10.1063/1.3675067 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v111/i7 Published by the American Institute of Physics.
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JOURNAL OF APPLIED PHYSICS 111, 07B312 (2012)
Micromagnet structures for magnetic positioning and alignment L. F. Zanini,1,2 O. Osman,3 M. Frenea-Robin,3 N. Haddour,3 N. M. Dempsey,1 G. Reyne,2 and F. Dumas-Bouchiat1,a)
1 Institut Ne´el, Centre National de la Recherche Scientifique & Universite´ Joseph Fourier, 25 rue des Martyrs, 38042 Grenoble, France 2 G2Elab, Grenoble Electrical Engineering Laboratory, 38402 St Martin d’He`res, France 3 Laboratoire Ampe`re, Ecole Centrale de Lyon - Universite´ Claude Bernard Lyon 69134, Ecully, France
(Presented 2 November 2011; received 23 September 2011; accepted 2 November 2011; published online 29 February 2012) High performance hard magnetic films (NdFeB, SmCo) have been patterned at the micron scale using thermo-magnetic patterning. Both out-of-plane and in-plane magnetized structures have been prepared. These micromagnet arrays have been used for the precise positioning and alignment of superparamagnetic nano- and microparticles. The specific spatial arrangement achieved is shown to depend on both the particle size and the size and orientation of the micromagnets. These micromagnet arrays were used to trap cells magnetically functionalized by endocytosis of 100 nm superparamagnetic particles. These simple, compact, and autonomous structures, which need neither an external magnetic field source nor a power supply, have much C 2012 American Institute of Physics. potential for use in a wide range of biological applications. V [doi:10.1063/1.3675067]
I. INTRODUCTION
Magnetic micro- and nanoparticles are used extensively in the biomedical field, for the manipulation of biological elements (cells, molecules…) using magnetic fields.1 Handling and positioning such particles and elements functionalized with these particles has greatly benefited from advances in microfabrication. Magnetic devices developed to date for manipulation at the micron scale require either an external source of magnetic field to polarize soft magnetic microelements,2–6 or else a power supply to control microelectromagnets.7–10 Here we demonstrate the potential to develop autonomous devices for controlled particle positioning using permanent micromagnets. The micromagnets are prepared with techniques compatible with standard silicon fabrication methods, allowing their integration into more complex microdevices. The fields (1 T) and field gradients (up to 106 T/m) produced by the permanent micromagnet arrays11 are comparable to those achievable with soft magnets and greatly superior to those achievable with microelectromagnets. Their autonomous nature (no need for external magnetic field or power supply) renders permanent micromagnets particularly interesting for applications where device size and autonomy are important. II. EXPERIMENTAL DETAILS
Triode sputtering was used to produce 5 lm thick high performance hard magnetic films (NdFeB and SmCo) on 4-in. silicon substrates at high deposition rates (up to 20 lm/h).12,13 Buffer and capping layers of Ta (100 nm) were included to prevent interdiffusion with the substrates and oxidation, respectively. While the NdFeB films are out-of-plane (oop) textured (l0HC ¼ 2.0 T, BR ¼ 1.2 T), the SmCo films are inplane (ip) textured (l0HC ¼ 1.3 T, BR ¼ 0.8 T). These hard a)
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magnetic films were magnetically structured using the recently developed thermo-magnetic patterning (TMP) method.14 Stripe-, chessboard- and grid-like magnetic patterns with lateral dimensions ranging from 100 down to 7 lm were prepared. Magnetooptical indicator films (MOIFs) were used to characterize the stray magnetic field patterns produced above the hard magnetic micropatterns. While uniaxial MOIF (U-MOIF) simply reveal the direction of the oop component of the stray field (“up” or “down”), planar MOIF (P-MOIF) are sensitive to both the sign and the intensity of this field component. The spatial distribution of the magnetic field and the magnetic field gradients of idealized structures (assuming the thermomagnetically reversed sections to be parallelepipeds of depth 1.3 lm)14 were calculated analytically. Commercial superparamagnetic fluorescent nano/microparticles (Chemicell: ø 200 nm; Micro Particles GmbH: ø 1.4 and ø 4.9 lm) and nonfluorescent microparticles (Micro Particles GmbH: ø 10.3 lm), which are dispersed in an aqueous solution, were imaged using scanning electron microscopy (SEM). The microparticles (1.4 10.3 lm) consist of superparamagnetic iron-oxide nanoparticles dispersed in a polystyrene matrix. Small volumes of microparticle solutions were dropped onto micromagnet arrays and their positioning into precise patterns was imaged using conventional and fluorescence optical microscopy. Human embryonic kidney cells (HEK293, ATCC No. CRL-1573, ø 10 lm) were magnetically functionalized with 100 nm superparamagnetic particles (Ademtech) by endocytosis. Endocytosis is an essential biological process by which cells internalize macromolecules and particles into transport vesicles invaginated from the plasma membrane. In non phagocytic cells (such as HEK 293 cells), the uptake of particles is driven by an endocytic mechanism called pinocytosis, or fluid-phase uptake, which itself encompasses different pathways, depending on cell and particle type.15 The positioning
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FIG. 1. (Color online) U-MOIF images of (a) stripe and (b) chessboard oop patterns in NdFeB; (c) P-MOIF image of an ip chessboard pattern in SmCo; (d) NdFeB film patterned oop with a mask that consists of square array (7 7 lm2) motifs. The insets represent the modulus of the magnetic field gradient @B/@z.
of magnetically functionalized cells above micromagnet arrays was observed using a Nikon eclipse LV150 microscope with a 20 objective. III. RESULTS AND DISCUSSION
Stripe- and chessboard-like oop magnetized patterns produced in NdFeB films are revealed by magneto-optical (MO) imaging of an overlying U-MOIF [Figs. 1(a) and 1(b)]. These binary images (dark/light contrast) confirm that the direction of magnetization is reversed during the TMP process. A MO image of a P-MOIF overlaid on an in-plane magnetized SmCo film patterned with a chessboard-like mask is shown in Fig. 1(c). The image is coded in 256 colors and the zones of intense dark/light contrast which appear at the edges of the squares, are perpendicular to the axis of magnetization, where the oop component of the stray magnetic field is highest. Much smaller oop magnetized patterns produced in a NdFeB film using a grid with holes of size 7 7 lm2, spaced by 5 lm, revealed using a U-MOIF, are shown in Fig. 1(d). The magnetic patterns appear rounded because of the limited spatial resolution of the imaging technique. The modulus of the magnetic field gradient along the z-axis (@B/@z) calculated 1 lm above the patterned films are shown in the insets of Fig. 1 (this distance corresponds to the estimated gap between the magnetic film and the active layer of the MOIF). The most intense gradients are found on the edges of the magnetic patterns. Figure 2(a) show SEM images of the superparamagnetic particles used here. The smallest particles, of nominal size 200 nm, are in fact agglomerates of particles of much smaller size (10 nm). The coarser polystyrene based particles (1.4, 4.9, and 10.3 lm) are spherical in shape and show a narrow size distribution. The attractive magnetic force between the micromagnets and a superparamagnetic particle is given by Fm ¼ l0VmagM(H)rH, where l0 is the permeability of vacuum, Vmag the magnetic volume of the particle, and H the magnetic field acting on the particle. M is the magnetization of the nanometric superparamagnetic inclusions in the polystyrene matrix of the particles, which is determined by the magnetic field and follows a Langevin-type function. Such particles show a very high suceptibility in low fields (up to a
few hundreds of mT) followed by saturation in higher fields. The force is thus calculated using M(H), as fitted to the experimental magnetization curves of each type of particle. As a result, when an aqueous solution containing superparamagnetic particles is dropped onto a micromagnet array, the particles are magnetically attracted to the regions of highest field
FIG. 2. (Color online) (a) SEM images of the superparamagnetic particles; (b)–(f): Fluorescence images of the superparamagnetic particles trapped by micromagnets [(b): 200 nm above oop magnetized NdFeB; (c): 1.4 lm above oop magnetized NdFeB; (d): 1.4 lm above ip magnetized SmCo; (e), (f): 4.9 lm above oop magnetized NdFeB]; (g) 10.3 lm particles individually positioned in a square lattice above an oop magnetized NdFeB film (the reversed magnets are 7 7 lm2 in surface area, separated by 5 lm). The insets in (b)–(g) present a zoom on the particles positioned above each magnetic configuration.
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buffer solution (PSB)] and resuspended in PBS. A drop of solution was deposited on an oop magnetized NdFeB film (50 50 lm2 squares). The cells were observed to align along the edges of the magnetic patterns (Fig. 3). Due to biological cellular adhesion, some cell clusters also formed. The trapping and precise positioning of cells, together with the possibility of making the micromagnet structures biocompatible by the deposition of an appropriate protective layer (polymer, oxide…) opens the possibility for a vast range of new biological applications. IV. CONCLUSIONS
FIG. 3. (Color online) Cells (HEK293) functionalized with magnetic nanoparticles trapped on a chessboard-like magnetic pattern (50 50 lm2 squares).
gradient, i.e., to all the interfaces between oop magnetized structures and at the interfaces perpendicular to the direction of magnetization for ip magnetized structures [Figs. 2(b)–2(f)]. The attractive forces are estimated to be in the nN range. For instance, the estimated force acting on a ø 1.4 lm bead, in which the superparamagnetic inclusions account for 30% of its mass, in direct contact with the film is 13.6 nN. Note that the liquid solution was maintained in place by positioning the micromagnet array in a small container and deionized water was flushed through the solution to displace particles that were are not rigidly trapped by the micromagnets. The smaller particles [200 nm, 1.4 lm] were found to form chains which may be a few particles wide (Figs. 2(b), 2(c), and 2(d)]. It is also observed that these particles may be pinned at sites away from the micromagnet interfaces. This can be explained by local variations of the magnetic field gradient caused by inhomogeneities in the magnetic film (these variations have been characterized by magnetic force microscopy and will be reported elsewhere). The very localized nature of these variations explains the fact that only the smaller particles are trapped at these zones. The particles of size 4.9 lm form relatively straight chains [Figs. 2(e) and 2(f)]. In this case the lateral extent of the field gradient is insufficient to form chains more than one particle wide. A NdFeB film oop patterned using a mask with holes of size 7 7 lm2, separated by 5 lm, serves to individually position the coarsest particles of size 10.3 lm in a relatively regular square lattice [Fig. 2(g)]. These simple experiments demonstrate the possibility to accurately position/align superparamagnetic nano- and microparticles using arrays of permanent micromagnets. The type of pattern achieved can be tailored by varying the size of the particles and the size/orientation of the micromagnets. To demonstrate the interest of such structures for biological applications, we have used them to trap HEK293 cells which have internalized superparamagnetic nanoparticles via endocytosis. At present, the average number of internalized particle per cell and, thus, the corresponding magnetic force acting on the cell are not precisely known. More extensive studies would be needed to clarify the endocytotic pathway and to estimate the average number of particles taken up by such cells. The cells having taken up ø 100 nm particles were purified by centrifugation [washed twice in phosphate
High performance micromagnet arrays have been developed using triode sputtering and thermo-magnetic patterning. These micromagnet arrays have been used for the precise positioning and alignment of superparamagnetic particles ranging in size from hundreds of nm to a few lm. The particle distribution above the magnets depends on the particle size and the size and orientation of the micromagnets. Individual particle positioning is achieved when the size of the magnet is comparable to the size of the particle. The trapping of magnetically functionalized cells has been demonstrated. The compatibility of the fabrication method used with standard microfabrication techniques will facilitate the integration of these structures into more complex devices, in particular those concerning micro-total-analysis-systems (lTAS). The small size and autonomous nature of the structures are of particular interest for point-of-care testing (POCT) and for collecting, storing and transporting samples. ACKNOWLEDGMENTS
The authors acknowledge the financial support from the Re´gion Rhoˆne-Alpes (cluster Micro Nano) and the Agence Nationale de la Recherche (ANR-08-CESA-013-01, EMERGENT Project). 1
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