A magnetic manipulator for studying local rheology and micromechanical properties of biological systems

Abstract

A magnetic micromanipulator capable of generating two-dimensional translational and rotational motions on a microscope stage is described. With 3 μm-diam paramagnetic beads, forces in the piconewton range and torques on the order of 10−14 N m are obtained and can be modulated in time at moderate frequencies (<5 Hz). Typical magnetic fields between 0.1 and 0.2 T, and gradients between 10 and 20 T m−1 are created by four independent feedback-controlled electromagnets. Video microscopy and computerized image analysis are used to locate the beads on each image with a resolution of 0.1 pixel (20 nm). The device is primarily designed to study, at a microscopic scale, the local mechanical properties of biological polymers such as actin in solution, and of cell cytoplasm. Possible applications include the in situ manipulation of intracellular organelles.

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…Mechanical properties of single cells have been successfully probed with a variety of other techniques as cell poking [905], the micropipette aspiration technique [791,906,907], optical [908] and magnetic tweezers [909][910][911], and ultrasonic microscopy [912,913]. A first measurement of the mechanical properties of animal tissue by AFM force-distance curves was done by Tao Usually values of the Young's modulus are obtained by using a Hertzian contact model with different geometries (see Table 2 and Eq.…”

Oscillatory magnetic tweezers based on ferromagnetic beads and simple coaxial coils

“…[20][21][22] These beads are homogeneous in size and shape and do not show any remanent magnetization after removal of a magnetizing field. However, their magnetic moment is low and, consequently, relatively high gradients are required to reach forces in the pN range.…”

“…21 The optimization of the shape, dimensions and separation of a pair of coaxial coils has been largely studied for a wide range of applications. [31][32][33][34] These designs typically focus on maximizing the field 35,36 or the gradient 24 homogeneity either in the direct or the inverse configurations.…”

“…Second, the hysteretic effects related to the use of soft magnetic materials demand a feedback loop to control the applied field in a relatively low bandwidth. 21 Third, as the magnetization of the beads is dependent on the applied magnetic field, their magnetic moment varies during experiments that involve time-varying forces. Moreover, this dependence is nonlinear and, therefore, the force generated on the beads is not proportional to the driving signal.…”

“…For instance, Amblard and co-workers developed a system based on four sets of independent electromagnets normal to the optical axis of a microscope that allowed the rotation and biaxial translation of magnetic beads in a controlled way. 21 More recently, Gosse and Croquette designed a magnetic setup built with six electromagnets coupled to a video-tracking system to apply vertical and lateral forces. 22 The abovementioned magnetic tweezers based either on permanent magnets or electromagnets with soft iron cores suffer from a number of limitations.…”