PMC

Schematic of process of cell chain formation in ferrofluid. (a) Suspension of cells in ferrofluid assumes a random orientation in the absence of a magnetic field. (b) The suspended cells form linear arrangements in ferrofluid in presence of magnetic field (arrows), where the ferrofluid particles shepherd the cells into chains due to their induced magnetic dipoles. (c) Linear arrangement of cells adherent to cell-adhesive surface survive and grow upon removal of ferrofluid and magnetic field. (d) Schematic of BSA-passivated nanoparticle synthesis.

(a) Schematic representation of preparation of magnetic phthalocyanines: step (1) ferrofluid produced with a mixture of Fe₃O₄ and oleic acid; step (2) reaction of ferrofluid with phthalocyanines. (b) Influence of the magnetic field under the suspension of Fe₃O₄@OA/Pc in ethanol. Reproduced from ref. [], MDPI, 2019.

(a) Schematic representation of preparation of magnetic phthalocyanines: Step 1) ferrofluid produced with the mixture of Fe₃O₄ and oleic acid; Step 2) reaction of ferrofluid with phthalocyanines. (b) Influence of the magnetic field under the suspension of Fe₃O₄@OA/Pc in ethanol.

Flow rate effect on diamagnetic yeast cell fractionation by morphology in 0.1× EMG 408 ferrofluid. The flow rate ratio between the sheath ferrofluid and the yeast cell suspension is fixed at 20 when the latter is varied from 5 μl/h to 13 μl/h. The left and right plots at each flow rate show the experimental cell image and the numerically predicted cell trajectories, respectively.

Depletion force profiles are shown for a suspension of non-magnetic, , and magnetic, , non-linked colloidal particles, . In the magnetic case, it corresponds to a simple ferrofluid suspension. The plot also shows the force profiles for suspensions of colloidal polymers of length . All cases plotted in this figure correspond to zero-field. The size ratio of the non-magnetic colloidal particles to monomers is set to . The density of the suspension (see Equation ()) is fixed to in all cases. The largest error in the force profiles shown in this figure is at for () case.

(a) Néel and Brown relaxation times calculated over a range of particle sizes for a water-based magnetite ferrofluid []; (b) Imaginary part of susceptibility of maghemite based aqueous suspension in comparison to the identical particles immobilized in the gel. Reprinted with permission [,]. Copyright 2021, Elsevier.

a) Commercial ferrofluid and its behavior when exposed to a magnetic field provided by a cubical magnet; b) Ferrofluid behavior inside the channel used for the experiments when placed next to the cubical magnet; c) Greyscale measurements along the channel height before and after the SPION suspension was inserted in QMS_A for 2 hr at RT; d) Greyscale measurements across the channel width after placing the sample inside QMS_A; e) Concentration ratio (C_t=2hr/C_t=0) of SPIONs as a function of h for C₀ = 25 g·L⁻¹; f) Concentration ratio (C_t=2hr/C_t=0) of SPIONs as a function of h for C₀ = 2.5 g·L⁻¹.

Release kinetics of MTO from SEON^LA-BSA*^MTO in PBS at 37°C.
Notes: No burst release was observed; MTO is slowly released with linear kinetics. After 72 hours, only 8.01%±0.24% of the original amount of MTO is located outside the membrane. In the lysate of particle suspension after 72 hours, we detected 93.38%±2.72% of the original amount of MTO. All measurements were performed in triplicate.
Abbreviations: MTO, mitoxantrone; SEON^LA-BSA*MTO, bovine serum albumin/lauric acid-coated ferrofluid loaded with MTO; PBS, phosphate-buffered saline.

Cell viability in the presence of ferrofluid. (a) Cell viability of HUVECs is high after culture with 30 mg/mL magnetic nanoparticles for 2 hours as visualized by LIVE/DEAD staining [green = all cells; red = dead cells]. The 2 dead cells in field of view are shown with arrows. Scale bar represents 100 μm. (b) Ferrofluid concentration does not affect cell viability.

Demonstration of magnetic fractionation of yeast cells in the flow of 0.1× EMG 408 ferrofluid through a T-shaped microchannel: (a) experimental image at the T-junction; (b) experimental image at the expansion of the main branch; (c) PDF plots of the four groups of yeast cells at the expansion of the main branch [obtained from the cell distribution in (b)]; (d) predicted cell trajectories (only four cells are illustrated for each group in the horizontal plane for clarity) at the T-junction, and (e) predicted cell trajectories at the expansion of the main branch. The flow rates of the sheath ferrofluid and yeast cell suspension are 180 μl/h and 9 μl/h, respectively, in both the experiment and the simulation. The block arrows in (a) and (b) indicate the flow directions.

Yeast cell fractionation in 0.05× EMG 408 ferrofluid at various flow rates of the cell suspension: (a) Superimposed images of four groups of yeast cells at the expansion of the main branch; (b) Experimentally measured (symbols with error bars to cover the span of the cell stream) positions of each cell group at the expansion of the main branch. The flow rate ratio between the sheath fluid and yeast cell suspension is fixed at 20. The upright arrow In the right-most image of (a) indicates the reference point to which the position of each group of cells in (b) was measured.

Effect of the flow rate ratio between the sheath ferrofluid (fixed at 180 μl/h) and cell suspension (varied from 36 μl/h to 4.5 μl/h) on yeast cell fractionation by morphology in 0.1× EMG 408 ferrofluid: (a) Superimposed images of four groups of yeast cells at the expansion of the main branch; (b) Comparison of the experimentally measured (symbols with error bars to cover the span of the cell stream) and numerically predicted (colored band) positions of each cell group at the expansion of the main branch. The upright arrow in the right-most image of (a) indicates the reference point to which the position of each group of cells in (b) was measured.

Characterization of the used nanoparticles.
Notes: (A) Core sizes were stated as given in the datasheet of the supplier. The hydrodynamic diameter (n=5) and the ζ-potential (n=3) were analyzed in double distilled water. Standard deviations are given in brackets. (B) Depiction of the clustered particle cores as observed by high resolution transmission electron microscopy. (C) SAR of the used MNPs (n=3) measured as water suspension (ferrofluid) or after immobilization in 10% PVA.
Abbreviations: MNPs, magnetic nanoparticles; MTX, methotrexate; PDI, polydispersity index; PVA, polyvinyl alcohol; SAR, specific absorption rate.

(a) X-ray diffraction (XRD) pattern, (b) transmission electron microscopy (TEM) image, (c) high-resolution (HR)-TEM images and the corresponding fast Fourier transform (FFT), (d) hydrodynamic radius of the as-prepared ferrofluid, (e) ζ-potential, (f) thermogravimetric analysis (TGA) and (g) UV-Vis-near-infrared (NIR) absorption spectrum of a suspension of 4.5 mM iron oxide nanoparticles (IONs) in water and after oxidation (maghemite at the same concentration).

(a) Structure map of the supraparticles obtained with the variation of the magnetic strength and initial concentration of ferrofluid. The regime in gray, red, blue, and green indicates supraparticle with deflated ball, cone, barrel, two-tower shape, respectively. These supraparticles were obtained by drying of 5 μL NPs suspension. b) Two-tower shape supraparticles with different tower orientations by drying the 21 wt % of ferrofluid under different directions of the magnetic field (left one with the vertical direction and the right one with non-vertical direction, both were under a magnetic field of 160 kA/m). (c) Microscopic images of supraparticles obtained by drying CoFe₂O₄ NPs suspension on superamphiphobic surfaces with or without magnetic field. (d) Aspect ratio of binary supraparticles as a function of weight fraction of mgPS NPs. (e) Binary supraparticles fabricated by drying a cosuspension of mgPS NPs with titanium dioxide NPs (TiO₂ /mgPS, upper panel) or polystyrene NPs (PS/mgPS, bottom panel). The weight fraction of mgPS NPS was varied from 0%, 50%, 83% to 91%. The total initial NPs concentration remained constant at 6 wt %, and the magnetic field was 160 kA/m. Scale bars are 0.5 mm.

Experimental measurement and analytical approximation of critical operating parameters in iFCS. (A) Normalized experimental magnetization curve and fitted Langevin function of a maghemite ferrofluid used in this study. Fitted data at high field magnetization yielded a saturation magnetization of 1,085 A/m, which corresponded to a volume fraction of 0.029 of magnetic materials in this ferrofluid. Goodness of fit (R²) was 0.999. Full measurement data were in the . (B) Fitted data of the magnetization curve with a log-normal distribution of particle diameters yielded a volume-weighted median magnetic nanoparticle diameter of 10.8 nm, and a geometric standard deviation of the magnetic nanoparticle diameter distribution of 0.44. Temperature was 298 K, bulk magnetization of maghemite was 370,000 A/m, density of the ferrofluid was 1060.6 kg/m³, demagnetization factor due to the sample holder of the vibrating sampling magneto-meter was 0.211. (C) Experimentally measured ferrofluid viscosity and its fitted curve under no external magnetic field. The ferrofluid was a suspension of maghemite nanoparticles in a mixture of water and HBSS buffer with Atlox 4913 (Croda, Inc., Edison, NJ) graft copolymer as surfactants. Goodness of fit (R²) was 0.979. (D) Comparison of the measured magnetic flux density (error bar was the standard deviation of 3 measurements) to the analytical expressions in –). The x-axis label is the distance between the active area of the sensor and the magnet surface. The dimensions of the neodymium magnet were (L×W×H, 50.8 mm×12.7 mm×12.7 mm). The remnant magnetization from this fit was determined to be 1,055,693 A/m (residual magnetic flux density 1.33 T). Goodness of fit (normalized mean square error) was 0.997 (1 was perfect fit).

(A) Scanning electron microscopy (SEM) and (B) fluorescence light microscopy images of freestanding Janus particle. Fe and PS represent the magnetic iron patch and nonmagnetic polystyrene, respectively. Scale bar, 2 μm. (C) Representation of the Helmholtz coil setup used for assembly on microscope stage. The particle dispersion is placed at the center of the electromagnet to guarantee a uniform magnetic field over the suspension. (D) Schematic representation and corresponding brightfield images of model assembly system: Under magnetic field, the iron patch (blue) aligns ferromagnetically, while the polystyrene microsphere (green) and the core of the patchy particle (red) align diamagnetically due to the ferrofluid suspension. Brightfield images demonstrating the spontaneous assembly and disassembly of the cluster by cycling the magnetic field on and off. Scale bar, 5 μm. (E to I) Sequence of multistep supraparticle growth as four isotropic satellite particles assemble on the patchy particle, one microsphere at a time. Scale bar, 5 μm. (J to N) Fluorescence micrographs showing the dynamics of the assembly process in bulk ferrofluid at τ = 2. The supraparticle structures approach a near-equilibrium state after 35 min of field application (2500 A m⁻¹). Red hemispheres are Fe-patched Janus particles (red region, polystyrene; dark region, Fe), and green particles are isotropic polystyrene satellite particles. Scale bar, 40 μm.

Overview of an integrated ferrohydrodynamic cell separation (iFCS) system and its working principle. a Top: schematic of an unlabeled circulating tumor cell (CTC) experiencing “diamagnetophoresis” in a colloidal magnetic nanoparticle suspension (ferrofluids) and moving towards the minima of a non-uniform magnetic field. Magnetization of the unlabeled CTCs is near zero and less than its surrounding ferrofluids. The diamagnetic body force on the cell is generated from magnetic nanoparticle induced pressure imbalance on the cell surface, and is proportional to cell volume. Bottom: schematic of a magnetic bead labeled white blood cell (WBC) experiencing both “diamagnetophoresis” from its cell surface and “magnetophoresis” from its attached beads in a ferrofluid and moving towards the maxima of a non-uniform magnetic field due to the fact that “magnetophoresis” outweighs “diamagnetophoresis”. Magnetization of the WBC-bead conjugates is larger than its surrounding ferrofluid medium. Color bar indicates relative amplitude of the magnetic field. Red arrows show the direction of cell movement, small black arrows on cell surface show the direction of magnetic nanoparticle induced surface pressure on cells, while white arrows show the magnetophoretic force on magnetic beads. b Two enrichment stages were integrated into a single iFCS device to achieve cell size variation-inclusive and tumor antigen-independent enrichment of viable CTCs, and simultaneous depletion of contaminating WBCs. Prior to device processing, WBCs in blood were labeled with magnetic microbeads through leukocyte surface biomarkers so that the overall magnetization of the WBC-bead conjugates was larger than surrounding ferrofluids. Magnetization of the unlabeled CTCs was less than ferrofluids. In the first stage, a magnetic field gradient was generated to push unlabeled and sheath-focused CTCs to remain at the upper boundary of a microchannel, while attract unbound magnetic microbeads and WBCs labeled with ≥ 3 microbeads towards a waste outlet. A significant percentage of magnetic beads and WBCs were depleted before the second stage to alleviate potential bead aggregation. In the second stage, a symmetric magnetic field with its maximum at the middle of the channel was used to attract remaining WBC-bead conjugates towards to the channel center for fast depletion, and direct unlabeled CTCs towards the upper and lower boundaries for collection. Green arrows with gradients indicate the distribution of magnetic fields in each stage. c Top-view of the iFCS microchannel. The microchannel consists of a filter that removes large than ~;50 μm debris, a first and second stage for CTC enrichment and WBC depletion. d A photo of prototype microchannel (left) and assembled iFCS device with four permanent magnets in quadrupole configuration inside a holder (right). The microfluidic device and permanent magnets were placed within an aluminum manifold during its operation. Scale bars: 1 cm.

Overview of the label-free ferrohydrodynamic nanoparticle separation principle and the FerroChip device design. a (top panel) Schematic of an extracellular vesicle (EV, 30 – 1000 nm in diameter) experiencing both diffusion and “particle ferrohydrodynamics” in a colloidally-stable magnetic nanoparticle (~10 nm particle diameter) suspension (i.e., ferrofluids). The magnetization of the unlabeled EV M_EV is near zero and much less than its surrounding ferrofluids M_ferrofluid. The ferrohydrodynamic force on the EV is generated from magnetic nanoparticle-induced pressure imbalance on the vesicle’s surface, which is proportional to the EV’s volume. The color bar indicates the relative amplitude of the magnetic field strength. Red arrows show the direction of vesicle movement, small black arrows on the EV’s surface show the direction of magnetic nanoparticle-induced surface pressure. (bottom panel) The relationship between characteristic migration time of a diamagnetic nanoparticle in ferrofluids and the diameter of the nanoparticle. The migration of nanoparticles is affected by both diffusion and “particle ferrohydrodynamics”, and determined by the faster process out of the two. For nanoparticles with a diameter of less than ~30 nm, the diffusive process dominates the migration; for nanoparticles with a diameter of larger than ~30 nm, the ferrohydrodynamic process dominates the migration. b Work principle of a label-free continuous-flow EV focusing/separation device, termed as FerroChip. In the focusing mode of device operation, samples of EVs were premixed with a dilute ferrofluid and entered a straight microchannel at a relatively slow flow rate and with a uniform distribution across the channel width. A symmetric magnetic field with its minimum in the middle of the microchannel was used to direct EVs towards the center of the microchannel, effectively focusing them into a narrow stream. In the separation mode, EVs entered the channel predominately through the regions close to the channel wall due to a sheath flow, with a relatively fast flow rate. The same magnetic field was used to direct unlabeled EVs from the sidewall region towards the channel center for continuous collection. EVs of various sizes migrated towards the center of the microchannel, with varying speeds that depended on their sizes. Large EVs migrated to the channel center at a faster speed than smaller exosomes. Yellow arrows with gradients indicate the distribution of magnetic fields in the microchannel. c Top-view schematic drawing of the FerroChip’s microchannel. Initial samples of nanoparticles and/or EVs were injected into the channel from inlet 1. The samples, after first going through a debris filter that removed large debris, entered a straight channel (labeled as focusing/separation region) which focused or separated nanoparticles and/or EVs based on their sizes. Processed samples were collected from middle and side outlets for characterizations. When the FerroChip operates in focusing mode, inlet 2 (sheath flow) is not used. When the FerroChip operates in separation mode, inlet 2 provides a ferrofluid sheath flow so that nanoparticles and/or EVs entered from the top and bottom walls of the channel. The width, height and length of the microchannel are 1200 μm, 150 μm and 53 mm, respectively. d A photo of a microchannel (left, blue ink indicating the channel geometry) and assembled FerroChip with four permanent magnets in quadrupole configuration inside a holder (right). The microfluidic device and permanent magnets were placed within an aluminum manifold during its operation. Scale bars: 1 cm.