Daniel H. Reich

Magnetic microposts as an approach to apply forces to living cells

Cells respond to mechanical forces whether applied externally or generated internally via the cytoskeleton. To study the cellular response to forces separately, we applied external forces to cells via microfabricated magnetic posts containing cobalt nanowires interspersed among an array of elastomeric posts, which acted as independent sensors to cellular traction forces. A magnetic field induced torque in the nanowires, which deflected the magnetic posts and imparted force to individual adhesions of cells attached to the array. Using this system, we examined the cellular reaction to applied forces and found that applying a step force led to an increase in local focal adhesion size at the site of application but not at nearby nonmagnetic posts. Focal adhesion recruitment was enhanced further when cells were subjected to multiple force actuations within the same time interval. Recording the traction forces in response to such force stimulation revealed two responses: a sudden loss in contractility that occurred within the first minute of stimulation or a gradual decay in contractility over several minutes. For both types of responses, the subcellular distribution of loss in traction forces was not confined to locations near the actuated micropost, nor uniformly across the whole cell, but instead occurred at discrete locations along the cell periphery. Together, these data reveal an important dynamic biological relationship between external and internal forces and demonstrate the utility of this microfabricated system to explore this interaction.

Magnetic trapping and self-assembly of multicomponent nanowires

Magnetic nanowires suspended in fluid solutions can be assembled and ordered by taking advantage of their large shape anisotropy. Magnetic manipulation and assembly techniques are demonstrated, using electrodeposited Ni nanowires, with diameter 350 nm and length 12 μm. Orienting suspended nanowires in a small magnetic field H≈10 G promotes self-assembly of continuous chains that can extend over several hundred μm. The dynamics of this process can be described quantitatively in terms of the interplay of magnetic forces and fluid drag at low Reynolds number. In addition, a new technique of magnetic trapping is described, by which a single magnetic nanowire can be captured between lithographically patterned magnetic microelectrodes. The use of three-segment Pt–Ni–Pt nanowires yields low resistance, Ohmic electrical contacts between the nanowires and the electrodes. This technique has potential for use in the fabrication and measurement of nanoscale magnetic devices.

Cell manipulation using magnetic nanowires

The use of magnetic nanowires is demonstrated as a method for the application of force to mammalian cells. Magnetic separations were carried out on populations of NIH-3T3 mouse fibroblast cells using ferromagnetic Ni wires 350 nm in diameter and 35 μm long. Separation purities in excess of 90% and yields of 49% are obtained. The nanowires are shown to outperform magnetic beads of comparable volume.

Biological applications of multifunctional magnetic nanowires (invited)

Magnetic particles that can be bound to cells and biomolecules have become an important tool for the application of force in biology and biotechnology. Multifunctional magnetic nanowires fabricated by electrochemical deposition in nanoporous templates are a type of magnetic carrier that offers significant potential advantages over commercially available magnetic particles. Recent experimental work aimed at developing these wires for this purpose is reviewed. Results on chemical functionalization of Au and Au/Ni wires and magnetic manipulation of wires in suspension are described. Fluorescence microscopy was used to demonstrate the covalent binding of thiol-terminated porphyrins to Au nanowires, and to optimize functionalization of two-segment gold–nickel nanowires for selectivity and stability of the nanowire–molecule linkages. Magnetic trapping is a technique where single nanowires are captured from fluid suspension using lithographically patterned micromagnets. The influence of an external magnetic fiel...

Mechanical Coupling Between Myofibroblasts and Cardiomyocytes Slows Electric Conduction in Fibrotic Cell Monolayers

Background— After cardiac injury, activated cardiac myofibroblasts can influence tissue electrophysiology. Because mechanical coupling through adherens junctions provides a route for intercellular communication, we tested the hypothesis that myofibroblasts exert tonic contractile forces on the cardiomyocytes and affect electric propagation via a process of mechanoelectric feedback. Methods and Results— The role of mechanoelectric feedback was examined in transforming growth factor-&bgr;–treated monolayers of cocultured myofibroblasts and neonatal rat ventricular cells by inhibiting myofibroblast contraction and blocking mechanosensitive channels. Untreated (control) and transforming growth factor-&bgr;–treated (fibrotic) anisotropic monolayers were optically mapped for electrophysiological comparison. Longitudinal conduction velocity, transverse conduction velocity, and normalized action potential upstroke velocity (dV/dtmax) significantly decreased in fibrotic monolayers (14.4±0.7 cm/s [mean±SEM], 4.1±0.3 cm/s [n=53], and 3.1±0.2% per ms [n=14], respectively) compared with control monolayers (27.2±0.8 cm/s, 8.5±0.4 cm/s [n=40], and 4.9±0.1% per ms [n=12], respectively). Application of the excitation-contraction uncoupler blebbistatin or the mechanosensitive channel blocker gadolinium or streptomycin dramatically increased longitudinal conduction velocity, transverse conduction velocity, and dV/dtmax in fibrotic monolayers (35.9±1.5 cm/s, 10.3±0.6 cm/s [n=17], and 4.5±0.1% per ms [n=14], respectively). Similar results were observed with connexin43–silenced cardiac myofibroblasts. Spiral-wave induction in fibrotic monolayers also decreased after the aforementioned treatments. Finally, traction force measurements of individual myofibroblasts showed a significant increase with transforming growth factor-&bgr;, a decrease with blebbistatin, and no change with mechanosensitive channel blockers. Conclusions— These observations suggest that myofibroblast-myocyte mechanical interactions develop during cardiac injury, and that cardiac conduction may be impaired as a result of increased mechanosensitive channel activation owing to tension applied to the myocyte by the myofibroblast.

Direct observation of field-induced incommensurate fluctuations in a one-dimensional S=1/2 antiferromagnet

Neutron scattering from copper benzoate, Cu(C6D5COO)(2) . 3D(2)O, provides the first direct experimental evidence for field-dependent incommensurate low energy modes in a one-dimensional spin S = 1/2 antiferromagnet. Soft modes occur for wave vectors (q) over tilde = pi +/- delta (q) over tilde(H), where delta (q) over tilde(H) approximate to 2 pi M(H)/g mu(B) as predicted by Bethe ansatz and spinon descriptions of the S = 1/2 chain. Unexpected was a field-induced energy gap Delta(H) alpha H-alpha, where alpha = 0.65(3) as determined from specific heat measurements. At H = 7 T (g mu(B)H/J = 0.52), the magnitude of the gap varies from 0.06J to 0.3J depending on the orientation of the applied field.

Optimization of yield in magnetic cell separations using nickel nanowires of different lengths.

Ferromagnetic nanowires are shown to perform both high yield and high purity single‐step cell separations on cultures of NIH‐3T3 mouse fibroblast cells. The nanowires are made by electrochemical deposition in nanoporous templates, permitting detailed control of their chemical and physical properties. When added to fibroblast cell cultures, the nanowires are internalized by the cells via the integrin‐mediated adhesion pathway. The effectiveness of magnetic cell separations using Ni nanowires 350 nm in diameter and 5–35 micrometers long in field gradients of 40 T/m was compared to commercially available superparamagnetic beads. The percent yield of the separated populations is found to be optimized when the length of the nanowire is matched to the diameter of the cells in the culture. Magnetic cell separations performed under these conditions achieve 80% purity and 85% yield, a 4‐fold increase over the beads. This effect is shown to be robust when the diameter of the cell is changed within the same cell line using mitomycin‐C.

Characterization of a quasi-one-dimensional spin-1/2 magnet which is gapless and paramagnetic for g μ B H ≲ J and k B T ≪ J

High field mangetization, field-dependent specific heat measurements, and zero field inelastic magnetic neutron scattering have been used to explore the magnetic properties of copper pyrazine dinitrate (Cu(C4H4N2)(NO3)2). The material is an ideal one-dimensional spin-1/2 Heisenberg antiferromagnet with nearest neighbor exchange constant J=0.90(1) meV and chains extending along the orthorhombic a-direction. As opposed to previosly studied molecular-based spin-1/2 magnetic systems, coppyer pyrazine dinitrate remains gapless and paramagnetic for g mu_B H/J at least up to 1.4 and for k_B T/J at least down to 0.03 this makes the material an excellent model system for exploring the T=0 critical line which is expected in the H - T phase diagram on the one-dimensional spin-1/2 Heisenberg antiferromagnet. As a first example of such a study we present accurate measurements of the Sommerfeld constant of the spinon gas versus g mu_B H/J < 1.4 which reveal a decrease of the averages spinon velocity by 32% in that field range. The results are in excellent agreement with numerical calculations based on the Bethe ansatz with no adjustable parameters.

High-yield cell separations using magnetic nanowires

Ferromagnetic nanowires are demonstrated as a new tool in performing high-yield, single step cell separations on cultures of mammalian cells. The nanowires are made by electrochemical deposition in nanoporous templates, and when added to cultures of 3T3 mouse fibroblast cells, the nanowires can readily bind to the cells. The effectiveness in cell separations of Ni nanowires 350 nm in diameter and 5-35 /spl mu/m long in field gradients of 40 T/m were compared to commercially available superparamagnetic beads. The percentage yield of the separated populations is found to be optimized when the length of the nanowire is matched to the diameter of the cells in the culture. Magnetic cell separations performed under these conditions achieve 80% purity and 85% yield, a four-fold increase over the beads.

Giant positive magnetoresistance of Bi nanowire arrays in high magnetic fields

We have studied the magnetoresistance of electrodeposited Bi wires with diameters between 200 nm and 2 μm in magnetic fields up to B=55 T. In zero field, the resistance increases with decreasing temperature, indicating that the mean free path is strongly influenced by the nanowire geometry. The high-field magnetoresistance shows strong dependence on field orientation; typically 200% for B parallel to the wires, and 600%–800% for B perpendicular to the wires. The perpendicular magnetoresistance is well described by a modified two-current model which suggests that the high-field response of the arrays is fairly insensitive to the wire diameter, and is dominated by bulk properties of Bi.