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The assembly of complex structures out of simple colloidal building blocks is of practical interest for building materials with unique optical properties (for example photonic crystals (1) and DNA biosensors (2)) and is of fundamental importance in improving our understanding of self-assembly processes occurring on molecular to macroscopic length scales (3-5). Here we demonstrate a self-assembly principle that is capable of organizing a diverse set of colloidal particles into highly reproducible, rotationally symmetric arrangements. The structures are assembled using the magnetostatic interaction between effectively diamagnetic and paramagnetic particles within a magnetized ferrofluid. The resulting multipolar geometries resemble electrostatic charge configurations such as axial quadrupoles ('Saturn rings'), axial octupoles ('flowers'), linear quadrupoles (poles) and mixed multipole arrangements ('two tone'), which represent just a few examples of the type of structure that can be built using this technique., Colloidal particle dispersions feature a wide variety of attractive and repulsive interactions and mechanical packing constraints that have been exploited to assemble a diverse set of structures, including Pickering emulsions [...]

The drug-eluting stent's increasingly frequent occurrence late stage thrombosis have created a need for new strategies for intervention in coronary artery disease. This paper demonstrates further development of our minimally invasive, targeted drug delivery system that uses induced magnetism to administer repeatable and patient specific dosages of therapeutic agents to specific sites in the human body. Our first aim is the use of magnetizable stents for the prevention and treatment of coronary restenosis; however, future applications include the targeting of tumors, vascular defects, and other localized pathologies. Future doses can be administered to the same site by intravenous injection. This implant-based drug delivery system functions by placement of a weakly magnetizable stent or implant at precise locations in the cardiovascular system, followed by the delivery of magnetically susceptible drug carriers. The stents are capable of applying high local magnetic field gradients within the body, while only exposing the body to a modest external field. The local gradients created within the blood vessel create the forces needed to attract and hold drug-containing magnetic nanoparticles at the implant site. Once these particles are captured, they are capable of delivering therapeutic agents such as antineoplastics, radioactivity, or biological cells. Index Terms--Magnetic drug delivery, magnetic separation cardiovascular disease, stents.

We review our recent work on the manipulation and assembly of nonmagnetic colloidal materials above magnetically programmable surface templates. The nonmagnetic materials are manipulated by a fluid dispersion of magnetic nanoparticles, known as ferrofluid. Particle motion is guided by a program of magnetic information stored in a substrate in the form of a lithographically patterned template of micromagnets. We show how dynamic control over the motion of nonmagnetic particles can be accomplished by applying rotating external magnetic field. This unexpectedly large degree of control over particle motion can be used to manipulate large ensembles of particles in parallel, potentially with local control over particle trajectory. Index Terms--Assembly, colloid, inverse ferrofluid, patterned magnets.

We demonstrate a dynamically tunable elastic metamaterial, which employs remote magnetic force to adjust its vibration absorption properties. The 1D metamaterial is constructed from a flat aluminum beam milled with a linear array of cylindrical holes. The beam is backed by a thin elastic membrane, on which thin disk-shaped permanent magnets are mounted. When excited by a shaker, the beam motion is tracked by a Laser Doppler Vibrometer, which conducts point by point scanning of the vibrating element. Elastic waves are unable to propagate through the beam when the driving frequency excites the first elastic bending mode in the unit cell. At these frequencies, the effective mass density of the unit cell becomes negative, which induces an exponentially decaying evanescent wave. Due to the non-linear elastic properties of the membrane, the effective stiffness of the unit cell can be tuned with an external magnetic force from nearby solenoids. Measurements of the linear and cubic static stiffness terms of the membrane are in excellent agreement with experimental measurements of the bandgap shift as a function of the applied force. In this implementation, bandgap shifts by as much as 40% can be achieved with ~30?mN of applied magnetic force. This structure has potential for extension in 2D and 3D, providing a general approach for building dynamically tunable elastic metamaterials for applications in lensing and guiding elastic waves. [ABSTRACT FROM AUTHOR]

We investigate the non-linear dynamics of superparamagnetic beads moving around the periphery of patterned magnetic disks in the presence of an in-plane rotating magnetic field. Three different dynamical regimes are observed in experiments, including (1) phase-locked motion at low driving frequencies, (2) phase-slipping motion above the first critical frequency fc1, and (3) phase-insulated motion above the second critical frequency fc2. Experiments with Janus particles were used to confirm that the beads move by sliding rather than rolling. The rest of the experiments were conducted on spherical, isotropic magnetic beads, in which automated particle position tracking algorithms were used to analyze the bead dynamics. Experimental results in the phase-locked and phaseslipping regimes correlate well with numerical simulations. Additional assumptions are required to predict the onset of the phase-insulated regime, in which the beads are trapped in closed orbits; however, the origin of the phase-insulated state appears to result from local magnetization defects. These results indicate that these three dynamical states are universal properties of bead motion in non-uniform oscillators. [ABSTRACT FROM AUTHOR]

A magnetic tracking system for monitoring the performance and activity of mechanical heart valve prostheses has been developed. Physicians may find the system valuable in detecting thrombosis (blood clots), scar tissue, or other complications that may indicate life-threatening medical conditions, such as heart attacks, strokes, or aneurysms. In this paper, a disk-shaped NdFeB magnet comparable in size to the floating disk of a single-leaflet tilting disk valve was employed as a magnetic marker. The orientation of the marker was tracked using standard least-squares localization algorithms with data from two sets of three mutually orthogonai magnetic field sensors fixed in an imaging plane. The best fitting magnetic dipole moment of the magnetic marker was determined by 360 sensor measurements. The orientation of the magnet was analyzed by rotating the magnet about a single axis through a range of 90[degrees] at three imaging planes (3-6 in. from the sensors). The results show that the estimation error between the physical measurements and field-based predictions of marker orientation is within 2[degrees] over all the measured angles for distances up to 6 in. from the imaging plane. This finding suggests that with optimization, the motion of magnetized prosthetic heart valves can be determined with high accuracy by an array of magnetic field sensors placed on the surface of the skin. Index Terms--Biomedical implant, heart valve, magnetic tracking, noninvasive imaging.

We propose a novel method for detecting large bioparticles (e.g., viruses, cells, bacteria) using solid-state magnetic field sensors and utilizing magnetic fluid (e.g., ferrofluid) to provide magnetic susceptibility contrast for the nonmagnetic bioparticle. The proposed sensing approach measures the volume of the bioparticle directly for known particle/substrate separation, effectively overcoming deficiencies with conventional magnetic tagging. An exact analytical solution was developed in bispherical coordinates for the bioparticle's field near the interface, and we compare this solution both with the first-order approximation of a dipole and its induced image, and with a numerical finite-element solution. The results indicate that the dipole-image model works well for most of the particle/substrate separations considered, however, for very small separation or large fluid permeability the dipole-image model can deviate from the exact solution. Index Terms--Biosensor, colloid, ferrofluid, Hall effect, virus.

The ability to manipulate an ensemble of single particles and cells is a key aim of lab-on-a-chip research; however, the control mechanisms must be optimized for minimal power consumption to enable future large-scale implementation. Recently, we demonstrated a matter transport platform, which uses overlaid patterns of magnetic films and metallic current lines to control magnetic particles and magnetic-nanoparticle-labeled cells; however, we have made no prior attempts to optimize the device geometry and power consumption. Here, we provide an optimization analysis of particle-switching devices based on stochastic variation in the particle's size and magnetic content. These results are immediately applicable to the design of robust, multiplexed platforms capable of transporting, sorting, and storing single cells in large arrays with low power and high efficiency. [ABSTRACT FROM AUTHOR]

We theoretically study particle motion on a periodic array of positive and negative magnetization domains having non-equal widths driven by a multi-frequency magnetic field input signal. Colloidal particles move across the substrate with positive, zero, or negative time-averaged velocity depending on the frequency ratio, the phase difference, and the ratio of the external field strength to the substrate magnetization. Simulations show that particles of a certain size range can either be transported or locally trapped, thereby providing a mechanism for implementing tunable colloidal band-pass and colloidal band-gap filters. © 2014 AIP Publishing LLC. [ABSTRACT FROM AUTHOR]

The capability to deliver high effective dosages to specific sites in the human body has become the holy grail of drug delivery research. Drugs with proven effectiveness under in vitro investigation often reach a major roadblock under in vivo testing due to a lack of an effective delivery strategy. In addition, many clinical scenarios require delivery of agents that are therapeutic at the desired delivery point, but otherwise systemically toxic. We propose a method for targeted drug delivery by applying uniform magnetic fields to an injected superparamagnetic colloidal fluid carrying a drug. The experimental and theoretical models presented give insight into the use of magnetic microspheres for site-specific delivery of therapeutic agents and blood flow occlusion for embolotherapy. Index Terms--Embolotherapy, magnetic particles, magnetic targeting, targeted drug delivery.

Chains of superparamagnetic colloidal particles interacting in uniform fields superimposed on top of fields created by magnetized templates are numerically modeled to analyze the possibility of controlling the self-assembly process. It is seen that by tuning the uniform field bias, the number of particles deposited on templates can be controlled well for up to a few particles (0-3, depending on particle size).

An approach for studying steady-state nanoparticle concentration gradients arising in magnetic nanoparticle suspensions in response to strong magnetic field gradient is presented. The experimental approach makes use of microscopic optical absorption measurements of ferrofluid interacting with arrays of patterned magnets. Experimental results are found to be consistent with a simple theoretical description that predicts the local nanoparticle concentration over a wide range of magnetic field conditions and ferrofluid volume fractions. [ABSTRACT FROM AUTHOR]

An approximate analytic model is used for describing the equilibrium concentration profile within mixed suspensions of magnetic and nonmagnetic colloidal particles. The results have shown that the smallest particle that can be manipulated by ferrofluid and magnetic field gradients is on the order of 50-100 nm.

The controllability of orienting colloidal nanorods suspended in ferrofluid is examined as a function of the external magnetic field intensity and the concentration of conventional ferrofluid. The studies have shown that local magnetic susceptibility surrounding the nanorod is equivalent to the bulk fluid susceptibility, which has indicated that ferrofluid composed of 12 nm iron oxide particles can be treated as a continuum on the 100-1000 nm length scale.

The controllability of orienting colloidal nanorods suspended in ferrofluid is investigated as a function of the external magnetic field intensity and the concentration of conventional ferrofluid. The expected value for the nanorod orientation was derived from the competition between the potential energy variation of an ellipsoid in a uniform magnetic field and randomizing rotational thermal energy. The agreement between experiment and theory is found to be quite accurate when the measured nanorod volume and vendor-supplied values for the ferrofluid’s magnetic susceptibility are used as inputs in the model. Our investigations confirm that local magnetic susceptibility surrounding the nanorod is equivalent to the bulk fluid susceptibility, indicating that ferrofluid composed of 12 nm iron oxide particles can be treated as a continuum on the 100–1000 nm length scale. [ABSTRACT FROM AUTHOR]

Magnetophoretic conductor tracks are used to transport single magnetized beads and magnetically labeled single cells in a 3D time-varying magnetic field. The vertical field bias, in addition to the in-plane rotating field, has the advantage of reducing the attraction between particles, which inhibits the formation of particle clusters. However, the inclusion of a vertical field requires the re-design of magnetic track geometries, which can transport magnetized objects across the substrate. Following insights from magnetic bubble technology, it is found that successful magnetic conductor geometries defined in soft magnetic materials must be composed of alternating sections of positive (convex) and negative (concave) curvature. In addition to the previously studied magnetic tracks from the magnetic bubble literature, a drop-shape pattern is found to be even more adept at transporting small magnetic beads and single cells. Symmetric patterns are shown to achieve bi-directional conduction, whereas asymmetric patterns achieve unidirectional conduction. These designs represent the electrical circuit corollaries of the conductor and diode, respectively. Finally, biological applications are demonstrated in the transport of single cells and in the size-based separation of magnetic particles. [ABSTRACT FROM AUTHOR]