Magneto-mechanical mixing and manipulation of picoliter volumes in vesicles

The relevance of curvature-induced quadrupolar interactions in dipolar chain aggregation

The aggregation of dipolar chains driven by thermal fluctuations in an external strong (electric or magnetic) field is investigated theoretically. We discover a new simple electrostatic mechanism that rationalizes the counter-intuitive lateral coalescence of dipolar chains. There, we first demonstrate that two bent dipolar chains can either attract or repel each other depending if they possess similar or opposite curvatures, respectively. Upon bending, dipolar chains become the siege of polarization-induced local charges that in turn lead to quadrupolar couplings. This striking feature is then exploited to understand our conducted Monte Carlo simulations at finite temperature where thermal fluctuations cause local curvatures in the formed dipolar chains. The related quadrupolar attractive mode with correlated chain-curvatures is clearly identified in the simulation snapshots. Our findings shed new light on a longstanding problem in soft matter and related areas.

Magnetically Enhanced Intracellular Uptake of Superparamagnetic Iron Oxide Nanoparticles for Antitumor Therapy

Superparamagnetic iron oxide nanoparticles (SPIONs) have been widely employed in biomedical fields, including targeted delivery of antitumor therapy. Conventional magnetic tumor targeting has used simple static magnetic fields (SMFs), which cause SPIONs to linearly aggregate into a long chain-like shape. Such agglomeration greatly hinders the intracellular targeting of SPIONs into tumors, thus reducing the therapeutic efficacy. In this study, we investigated the enhancement of the intracellular uptake of SPIONs through the application of rotating magnetic fields (RMFs). Based on the physical principles of SPION chain disassembly, we investigated physical parameters to predict the chain length favorable for intracellular uptake. Our prediction was validated by clear visualization of the intracellular distributions of SPIONs in tumor cells at both cellular and three-dimensional microtissue levels. To identify the potential therapeutic effects of enhanced intracellular uptake, magnetic hyperthermia as antitumor therapy was investigated under varying conditions of magnetic hyperthermia and RMFs. The results showed that enhanced intracellular uptake reduced magnetic hyperthermia time and strength as well as particle concentration. The proposed method will be useful in the development of techniques to determine the optimized physical conditions for the enhanced intracellular uptake of SPIONs in antitumor therapy.

Development of a Simple Fabrication Method for Magnetic Micro Stir Bars and Induction of Rotational Motion in Chlamydomonas reinhardtii

A magnetic micro stirrer bar (MMSB) is used in the mixing operation of microfluidic devices. We have established a low-cost and easy method to make MMSBs using magnetic (neodymium magnets, magnet sheets) or non-magnetic powders (SUS304) as materials. We demonstrated three kinds of MMSB have respective advantages. To confirm the practical use of this MMSB, a cell suspension of the motile unicellular green alga Chlamydomonas reinhardtii was stirred in microwells. As a result, the number of rotating cells increased with only one of the two flagella mechanically removed by the shear force of the rotating bar, which facilitates the kinetic analysis of the flagellar motion of the cell. The rotational motion of the monoflagellate cell was modeled as translational (orbital) + spinning motion of a sphere in a viscous fluid and the driving force per flagellum was confirmed to be consistent with previous literature. Since the present method does not use genetic manipulations or chemicals to remove a flagellum, it is possible to obtain cells in a more naturally viable state quickly and easily than before. However, since the components eluted from the powder material harm the health of cells, it was suggested that MMSB coated with resin for long-term use would be suitable for more diverse applications.

Superparamagnetic colloids in a rotating field: Transition state from chains to disks

When exposed to an external magnetic field, 2D layers of spherical superparamagnetic colloids form specific structures which depend on the features of the external field. If the magnetic field is constant along time, superparamagnetic colloids self-organize into chains oriented in the direction of the field. If the magnetic field is rotating in the plane of the suspension, below a critical frequency, the superparamagnetic beads still aggregate into chains, but these chains rotate with the magnetic field. When the rotation reaches a certain speed, the colloids aggregate in rotating disklike clusters. In this work, we focused on the early stages of the disklike clusters' aggregation and the dynamics of this process. In particular, we observed experimentally that before clustering into disklike structures, the colloids were aggregating into rotating chains, just as they did in suspensions submitted to a magnetic field rotating at a lower rate. Over time, the chains interact with one another and aggregate into disklike clusters, resulting in a mixture of chains and disks in the sample. Finally, we propose a model to characterize the suspension over time in terms of the proportion of chains and disklike clusters, and report its deduced temporal evolution for different frequencies and volume fractions.

Dynamics and rheology of a suspension of super-paramagnetic chains under the combined effect of a shear flow and a rotating magnetic field

This study presents an analysis of the dynamics of single and multiple chains of spherical super-paramagnetic beads suspended in a Newtonian fluid under the combined effect of an external rotating magnetic field and a shear flow. Viscosity results depend on two main non-dimensional numbers: the ratio between the shear rate and the magnetic rotation frequency and the ratio between the hydrodynamic and magnetostatic interactions (the Mason number). When the shear rate is smaller than the magnetic field frequency, the chain rotation accelerates the surrounding fluid, reducing the value of the measured suspension viscosity even below that of the solvent. In this regime, shear-thickening is observed. For values of the shear rates comparable to the rotation magnetic frequency, the viscosity reaches a maximum and non-linear coupling effects come up. If the shear rate is increased to values above the rotation frequency, the viscosity decreases and a mild shear-thinning is observed. In terms of the Mason number, the suspension viscosity reduces in line with the literature results reported for fixed magnetic fields, whereas the shear-rate/magnetic-frequency ratio parameters induce a shift of the viscosity curve towards larger values. Results at larger concentrations and multiple chains amplify the observed effects.

Hydrodynamic and physicochemical phenomena in liquid droplets under the action of nanosecond spark discharges: A review

This review presents experimental studies of phenomena occurring in droplets of various liquids under the effect of nanosecond spark discharges. Inorganic liquids and liquids of biological origin are considered here. Attention is payed to hydrodynamic and physico-chemical phenomena in droplets, including a movement of sessile droplets on a substrate under the effect of the discharges, internal flows in droplets (excited by the discharges), plasma capillary phenomena, features of the droplets drying under the effect of the discharges, traces (patterns) left by the droplets, exposed to the discharges, on the substrates etc.

Enhanced diffusion and magnetophoresis of paramagnetic colloidal particles in rotating magnetic fields

Dispersions of paramagnetic colloids can be manipulated with external magnetic fields to assemble structures via dipolar assembly and control transport via magnetophoresis. For fields held steady in time, the dispersion structure and dynamic properties are coupled. This coupling can be problematic when designing processes involving field-induced forces, as particle aggregation competes against and hinders particle transport. Time-varying fields drive dispersions out-of-equilibrium, allowing the structure and dynamics to be tuned independently. Rotating the magnetic field direction using two biaxial fields is a particularly effective mode of time-variation and has been used experimentally to enhance particle transport. Fundamental transport properties, like the diffusivity and magnetophoretic mobility, dictate dispersions' out-of-equilibrium responses to such time-varying fields, and are therefore crucial to understand to effectively design processes utilizing rotating fields. However, a systematic study of these dynamic quantities in rotating fields has not been performed. Here, we investigate the transport properties of dispersions of paramagnetic colloids in rotating magnetic fields using dynamic simulations. We find that self-diffusion of particles is enhanced in rotating fields compared to steady fields, and that the self-diffusivity in the plane of rotation reaches a maximum value at intermediate rotation frequencies that is larger than the Stokes-Einstein diffusivity of an isolated particle. We also show that, while the magnetophoretic velocity of particles through the bulk in a field gradient decreases with increasing rotation frequency, the enhanced in-plane diffusion allows for faster magnetophoretic transport through porous materials in rotating fields. We examine the effect of porous confinement on the transport properties in rotating fields and find enhanced diffusion at all pore sizes. The confined and bulk values of the transport properties are leveraged in simple models of magnetophoresis through tortuous porous media.

Dynamic Magnetic Nanomixers for Improved Microarray Assays by Eliminating Diffusion Limitation

Microarrays are widely used in high-throughput analysis of DNA, protein, and small molecules. However, the majority of microarray assays need improved assay speed and sensitivity due to the slow molecular diffusion from bulk solutions to probe surfaces. Here, a new class of magnetic nanomixers in DNA and protein microarray assays is reported to eliminate the diffusion constraint through dynamic mixing. It is demonstrated that the dynamic nanomixers can improve the assay kinetics at least by a factor of 4 and 2 for DNA and protein microarray assays, respectively. By using the dynamic nanomixers, the sensitivities of detecting Escherichia coli O157:H7 DNA and prostate specific antigen increase by more than four-fold. The dynamic mixing also greatly reduces the spot-to-spot variation to below 10% across a broad concentration range, providing more accurate assay results. In comparison with existing methods, this magnetic nanomixer-based approach offers rapid turnaround, improved sensitivity, good accuracy, low cost, simple operation, and excellent compatibility with commercial microarrays.

Magnetic Biohybrid Vesicles Transported by an Internal Propulsion Mechanism

Some biological microorganisms can crawl or swim due to coordinated motions of their cytoskeleton or the flagella located inside their bodies, which push the cells forward through intracellular forces. To date, there is no demonstration of synthetic systems propelling at low Reynolds number via the precise actuation of the material confined within an enclosing lipid membrane. Here, we report lipid vesicles and other more complex self-assembled biohybrid structures able to propel due to the advection flows generated by the actuated rotation of the superparamagnetic particles they contain. The proposed swimming and release strategies, based on cooperative hydrodynamic mechanisms and near-infrared laser pulse-triggered destabilization of the phospholipid membranes, open new possibilities for the on-command transport of minute quantities of drugs, fluid or nano-objects. The lipid membranes protect the confined substances from the outside environment during transportation, thus enabling them to work in physiological conditions.

On-chip density mixer enhanced by air chamber

This paper proposes an on-chip density mixer that can achieve even density in a target chamber with a swirling flow enhanced by an air chamber. The system is composed of a main channel, a target chamber where two liquids with different densities are included, an isolated air chamber, and an external vibration pump driven by a piezo actuator at the entrance of the main channel. The air chamber is expected to amplify the vibration owing to structure softening. The amplification would be more pronounced at the resonance frequencies of the structure. We developed the system and conducted experiments. We showed that the swirling motion in the target chamber with an air chamber is stronger than that without an air chamber. We also confirmed that the time resulting in even density is shorter when the pump is driven at a resonance frequency. An air-based virtual valve is introduced for maintaining a constant density in the target chamber.

Anti-IgG-anchored liquid crystal microdroplets for label free detection of IgG

AIgG anchored LC microdroplets showing configurational transition from radial (a) to bipolar (b) upon interaction with IgG.

Strong vortical flows generated by the collective motion of magnetic particle chains rotating in a fluid cell

Magnetic microparticles, assembled into chains that are actuated with rotating magnetic fields, can be used as microstirrers to promote fluid transport and biochemical reactions in microfluidic systems. We show that, within a certain range of magnetic field rotation frequency, the microstirrers exhibit a coherent collective motion: the rotating magnetic particle chains move throughout the volume of a flat fluid cell and generate very strong (~1 mm s(-1)) and global (9 mm) vortical fluid flows, with many eddy-type substructures that fluctuate continuously in time, resembling turbulent flow. The collective motion makes the microstirrers not only defy gravity, but also move against magnetic field gradients. The induced fluid flow is directly related to the stirring rate and the amount of magnetic particle chains. The observed behavior is caused by the magnetic and hydrodynamic interactions between the magnetic microparticles and the fluid. We utilized the phenomenon of swarming particles to enhance biochemical assays with magnetic capture particles (4000 μL(-1)) and IgG targets (500 pM). When compared to a reference system of sedimented magnetic capture particles, magnetic actuation leads to both a ~9 times increase in the initial assay kinetics as well as a ~7 times increase of target capture signal after 30 minutes.

Syringe-pump-induced fluctuation in all-aqueous microfluidic system implications for flow rate accuracy

We report a new method to display the minute fluctuations induced by syringe pumps on microfluidic flows by using a liquid-liquid system with an ultralow interfacial tension. We demonstrate that the stepper motor inside the pump is a source of fluctuations in microfluidic flows by comparing the frequencies of the ripples observed at the interface to that of the pulsation of the stepper motor. We also quantify the fluctuations induced at different flow rates, using syringes of different diameters, and using different syringe pumps with different advancing distances per step. Our work provides a way to predict the frequency of the fluctuation that the driving syringe pump induces on a microfluidic system and suggests that syringe pumps can be a source of fluctuations in microfluidic flows, thus contributing to the polydispersity of the resulting droplets.

Disaggregation of microparticle clusters by induced magnetic dipole-dipole repulsion near a surface

Ensembles of magnetic particles are known to align and aggregate into multi-particle clusters in an applied magnetic field, and the physical laws governing these processes are well described in literature. However, it has been elusive how to achieve the opposite process, i.e. the disaggregation of particle clusters in a magnetic field. We report a novel method to disaggregate clusters of superparamagnetic microparticles using time-dependent magnetic fields. The disaggregating field is designed to generate repulsive dipole-dipole forces between the particles and to stabilize the disaggregated particles on a physical surface. We demonstrate the disaggregation of large clusters of several tens of particles, within about one minute, using fields generated by a multipole electromagnet. After the disaggregation process the particles are uniformly distributed over the surface and ready for further lab-on-chip processing. Our results represent a novel methodology to disaggregate magnetic particle clusters and thereby improve the effectiveness and reproducibility of biological assays based on magnetic microparticles.

Continuous-flow particle guiding based on dipolar coupled magnetic superstructures in rotating magnetic fields

Under the influence of homogeneous, rotating magnetic fields, superparamagnetic beads can be assembled into one- and two-dimensional superstructures on demand and used as dynamic components in microfluidic systems for colloidal separation. In this paper, the influence of the magnetic field strength and the rotation frequency on the device efficiency is studied. The optimum region is found to be between 100 and 200 rpm for a magnetic field strength of 330 Oe, while the highest value for separated mass per time (28 pg s(-1)) is achieved for a flow velocity of 370 μm s(-1) at a magnetic field strength of 690 Oe. Furthermore, the employment of superparamagnetic beads as a continuous-flow bioseparation device is shown in a proof-of-principle study.

Numerical and experimental study of a rotating magnetic particle chain in a viscous fluid

A simple and fast numerical method is developed capable of accurately determining the 3D rotational dynamics of a magnetic particle chain in an infinite fluid domain. The focus is to control the alternating breakup and reformation of the bead chain which we believe is essential to achieve effective fluid mixing at small scales. The numerical scheme makes use of magnetic dipole moments and extended forms of the Oseen-Burgers tensor to account for both the magnetic and hydrodynamic interactions between the particles. It is shown that the inclusion of hydrodynamic interaction between the particles is crucial to obtain a good description of the particle dynamics. Only a small error of deviation is observed when benchmarking the numerical scheme against a more computationally intensive method, the direct simulation method. The numerical results are compared with experiments and the simulated rotational dynamics correspond well with those obtained from video-microscopy experiments qualitatively and quantitatively. In addition, a dimensionless number (R(T)) is derived as the sole control parameter for the rotational bead chain dynamics. Numerically and experimentally, R(T)≈ 1 is the boundary between rigid "rod" and dynamic "breaking and reformation" behaviors.

Accelerated immunoassays based on magnetic particle dynamics in a rotating capillary tube with stationary magnetic field

A rapid and simple magnetic particle-based immunoassay has been demonstrated in a capillary mixing system. Antibody-coated micrometer size superparamagnetic polystyrene (SPP) particles were used in an assay for rabbit IgG in a sandwich (noncompetitive) format. The kinetics of the assay was compared between a plate-based system and a single capillary tube. The interaction between the antigen (R-IgG) and the antibody (anti-R-IgG) that was carried by the SPP particles in a rotating capillary was tested under a stationary magnetic field. Competing magnetic and viscous drag forces helped to enhance the interaction between the analyte and the capture antibodies on the particles. The dimensionless Mason number (Mn) was employed to characterize the magnetic particle dynamics; a previously determined critical Mason number (Mn(c)) was employed as a guide to the appropriate experimental conditions of magnetic field strength and rotational speed of the capillary. The advantage of the rotating capillary system included a short assay time and a reduced reactive volume (20 μL). The results show that the immunoassay kinetics were improved by the formation of chains of the SPP particles for the conditions that corresponded to the critical Mason number.

Superparamagnetic particle dynamics and mixing in a rotating capillary tube with a stationary magnetic field

The dynamics of superparamagnetic particles subject to competing magnetic and viscous drag forces have been examined with a uniform, stationary, external magnetic field. In this approach, competing drag and magnetic forces were created in a fluid suspension of superparamagnetic particles that was confined in a capillary tube; competing viscous drag and magnetic forces were established by rotating the tube. A critical Mason number was determined for conditions under which the rotation of the capillary prevents the formation of chains from individual particles. The statistics of chain length were investigated by image analysis while varying parameters such as the rotation speed and the viscosity of the liquid. The measurements showed that the rate of particle chain formation was decreased with increased viscosity and rotation speed ; the particle dynamics could be quantified by the same dimensionless Mason number that has been demonstrated for rotating magnetic fields. The potential for enhancement of mixing in a bioassay was assessed using a fast chemical reaction that was diffusion-limited. Reducing the Mason below the critical value, so that chains were formed in the fluid, gave rise to a modest improvement in the time to completion of the reaction.

Droplet based microfluidics

Droplet based microfluidics is a rapidly growing interdisciplinary field of research combining soft matter physics, biochemistry and microsystems engineering. Its applications range from fast analytical systems or the synthesis of advanced materials to protein crystallization and biological assays for living cells. Precise control of droplet volumes and reliable manipulation of individual droplets such as coalescence, mixing of their contents, and sorting in combination with fast analysis tools allow us to perform chemical reactions inside the droplets under defined conditions. In this paper, we will review available drop generation and manipulation techniques. The main focus of this review is not to be comprehensive and explain all techniques in great detail but to identify and shed light on similarities and underlying physical principles. Since geometry and wetting properties of the microfluidic channels are crucial factors for droplet generation, we also briefly describe typical device fabrication methods in droplet based microfluidics. Examples of applications and reaction schemes which rely on the discussed manipulation techniques are also presented, such as the fabrication of special materials and biophysical experiments.

Manipulating particle trajectories with phase-control in surface acoustic wave microfluidics

We present a 91 MHz surface acoustic wave resonator with integrated microfluidics that includes a flow focus, an expansion region, and a binning region in order to manipulate particle trajectories. We demonstrate the ability to change the position of the acoustic nodes by varying the electronic phase of one of the transducers relative to the other in a pseudo-static manner. The measurements were performed at room temperature with 3 μm diameter latex beads dispersed in a water-based solution. We demonstrate the dependence of nodal position on pseudo-static phase and show simultaneous control of 9 bead streams with spatial control of -0.058 μm/deg ± 0.001 μm/deg. As a consequence of changing the position of bead streams perpendicular to their flow direction, we also show that the integrated acoustic-microfluidic device can be used to change the trajectory of a bead stream towards a selected bin with an angular control of 0.008 deg/deg ± 0.000(2) deg/deg.

Ordered complex structures formed by paramagnetic particles via self-assembly under an ac/dc combined magnetic field

We apply ac and dc magnetic fields simultaneously in orthogonal directions to each other to a solution, in which paramagnetic microparticles are dispersed, and show that complex secondary structures composed of oscillating chain clusters, that is, long linear clusters interconnected by T-, L-, and criss-cross-junctions, are self-assembled. Disklike clusters are formed at some junctions and the number of disklike clusters increases as the frequency of the ac magnetic field increases. We finally show that the angle between long linear clusters can be altered by changing the ratio of the intensities of the ac and dc magnetic fields.

Cluster-cluster aggregations of superparamagnetic particles in a rotational magnetic field

We investigate the cluster-cluster aggregations of superparamagnetic particles in a rotational magnetic field numerically by the Brownian dynamics method, focusing on the cases of ϕ = 0.01 and 0.03 and Ma = 0, 0.001, 0.01, and 0.1, where ϕ is the area fraction of superparamagnetic particles and Ma is the Mason number, i.e., the ratio of viscous drag to magnetic force acting on a magnetic particle. We clarify the effect of ϕ and Ma on the cluster-cluster aggregation process from the point of view of dynamic scaling law.

Magnetically activated micromixers for separation membranes

Presented here is a radically novel approach to reduce concentration polarization and, potentially, also fouling by colloids present in aqueous feeds: magnetically responsive micromixing membranes. Hydrophilic polymer chains, poly(2-hydroxyethyl methacrylate) (PHEMA), were grafted via controlled surface-initiated atom transfer radical polymerization (SI-ATRP) on the surface of polyamide composite nanofiltration (NF) membranes and then end-capped with superparamagnetic iron oxide magnetite (Fe(3)O(4)) nanoparticles. The results of all functionalization steps, that is, bromide ATRP initiator immobilization, SI-ATRP, conversion of PHEMA end groups from bromide to amine, and carboxyl-functional Fe(3)O(4) nanoparticle immobilization via peptide coupling, have been confirmed by X-ray photoelectron spectroscopy (XPS) and field emission scanning electron microscopy (FESEM). These nanoparticles experience a magnetic force as well as a torque under an oscillating external magnetic field. It has been shown, using particle image velocimetry (PIV), that the resulting movement of the polymer brushes at certain magnetic field frequencies induces mixing directly above the membrane surface. Furthermore, it was demonstrated that with such membranes the NF performance could significantly be improved (increase of flux and salt rejection) by an oscillating magnetic field, which can be explained by a reduced concentration polarization in the boundary layer. However, the proof-of-concept presented here for the active alteration of macroscopic flow via surface-anchored micromixers based on polymer-nanoparticle conjugates has much broader implications.

A microfluidic microprocessor: controlling biomimetic containers and cells using hybrid integrated circuit/microfluidic chips

We present an integrated platform for performing biological and chemical experiments on a chip based on standard CMOS technology. We have developed a hybrid integrated circuit (IC)/microfluidic chip that can simultaneously control thousands of living cells and pL volumes of fluid, enabling a wide variety of chemical and biological tasks. Taking inspiration from cellular biology, phospholipid bilayer vesicles are used as robust picolitre containers for reagents on the chip. The hybrid chip can be programmed to trap, move, and porate individual living cells and vesicles and fuse and deform vesicles using electric fields. The IC spatially patterns electric fields in a microfluidic chamber using 128 × 256 (32,768) 11 × 11 μm(2) metal pixels, each of which can be individually driven with a radio frequency (RF) voltage. The chip's basic functions can be combined in series to perform complex biological and chemical tasks and can be performed in parallel on the chip's many pixels for high-throughput operations. The hybrid chip operates in two distinct modes, defined by the frequency of the RF voltage applied to the pixels: Voltages at MHz frequencies are used to trap, move, and deform objects using dielectrophoresis and voltages at frequencies below 1 kHz are used for electroporation and electrofusion. This work represents an important step towards miniaturizing the complex chemical and biological experiments used for diagnostics and research onto automated and inexpensive chips.

High Voltage Dielectrophoretic and Magnetophoretic Hybrid Integrated Circuit / Microfluidic Chip

A hybrid integrated circuit (IC) / microfluidic chip is presented that independently and simultaneously traps and moves microscopic objects suspended in fluid using both electric and magnetic fields. This hybrid chip controls the location of dielectric objects, such as living cells and drops of fluid, on a 60 × 61 array of pixels that are 30 × 38 μm(2) in size, each of which can be individually addressed with a 50 V peak-to-peak, DC to 10 MHz radio frequency voltage. These high voltage pixels produce electric fields above the chip's surface with a magnitude , resulting in strong dielectrophoresis (DEP) forces . Underneath the array of DEP pixels there is a magnetic matrix that consists of two perpendicular sets of 60 metal wires running across the chip. Each wire can be sourced with 120 mA to trap and move magnetically susceptible objects using magnetophoresis (MP). The DEP pixel array and magnetic matrix can be used simultaneously to apply forces to microscopic objects, such as living cells or lipid vesicles, that are tagged with magnetic nanoparticles. The capabilities of the hybrid IC / microfluidic chip demonstrated in this paper provide important building blocks for a platform for biological and chemical applications.