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

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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.