Oscillatory magnetic tweezers based on ferromagnetic beads and simple coaxial coils

Abstract

We report the design and validation of simple magnetic tweezers for oscillating ferromagnetic beads in the piconewton and nanometer scales. The system is based on a single pair of coaxial coils operating in two sequential modes: permanent magnetization of the beads through a large and brief pulse of magnetic field and generation of magnetic gradients to produce uniaxial oscillatory forces. By using this two step method, the magnetic moment of the beads remains constant during measurements. Therefore, the applied force can be computed and varies linearly with the driving signal. No feedback control is required to produce well defined force oscillations over a wide bandwidth. The design of the coils was optimized to obtain high magnetic fields ͑280 mT͒ and gradients ͑2 T/m͒ with high homogeneity ͑5% variation͒ within the sample. The magnetic tweezers were implemented in an inverted optical microscope with a videomicroscopy-based multiparticle tracking system. The apparatus was validated with 4.5 m magnetite beads obtaining forces up to ϳ2 pN and subnanometer resolution. The applicability of the device includes microrheology of biopolymer and cell cytoplasm, molecular mechanics, and mechanotransduction in living cells.

Stiffening and Contraction Induced by Dexamethasone in Alveolar Epithelial Cells

“…This technique is based on measuring the complex shear modulus of cells by applying a torque on beads firmly attached on the cytoskeleton, and on measuring the displacement induced on the beads. Recent theoretical and experimental studies have been focused on characterizing the most complex aspects of the technique, mainly the detection of the oscillatory bead displacement with nanometer resolution [19] and the issues concerning the attachment and embedment of the beads [18,28,29]. Unlike other techniques employed to assess cell viscoelasticity, such as atomic force microscopy [30] or optical tweezers [31], OMTC does not allow the measurement of the absolute value of G* given that the degree of embedment of each bead is unknown.…”

“…1). Then, the position of each bead was computed in the subsequent images with nanometer resolu- tion using a centroid algorithm [19]. A moving average filter with a time window of one oscillation period was applied to the recording.…”

Effect of stretch on structural integrity and micromechanics of human alveolar epithelial cell monolayers exposed to thrombin

“…The beads are permanently magnetized with a brief (20 ms) and strong (120 mT) pulse of magnetic field in the horizontal direction of the cell monolayer and subsequently twisted in a weak sinusoidal magnetic field applied in the vertical direction. The resulting lateral bead displacement is measured with nanometer resolution using videomicroscopy (33). Images were obtained with a progressive scan black-and-white charge-coupled device camera (CV-M10 BX; JAI, Glostrup, Denmark) and digitized by an eight-bit resolution frame grabber (PC Eye4; Eltec, Mainz, Germany).…”

“…Image analysis was performed with a multiple particle tracking software detailed elsewhere (33). This software computed the position of each individual bead (ϳ100 -150 beads/ well) throughout the experiments using a centroid algorithm.…”

Mechanobiology in Lung Epithelial Cells: Measurements, Perturbations, and Responses

“…In this inverse configuration, the coils generate a local homogeneous magnetic gradient while the field in the central plane vanishes. When applied to ferrimagnetic beads of 5 μm this configuration induces forces up to ∼2 pN with fN resolution (254). Larger magnetic gradients can be obtained with one-pole microneedle permanent magnet (164) or with electromagnets with a sharpened soft iron core (17,18, 139).…”