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Biophys J, June 2002, p. 3314-3329, Vol. 82, No. 6
Laboratoire de Physique Statistique, École Normale Supérieure, Unité de Recherche 8550 associée au Centre National de la Recherche Scientifique et aux Universités Paris VI et VII, 75231 Paris, France
Cantilevers and optical tweezers are widely used
for micromanipulating cells or biomolecules for measuring their
mechanical properties. However, they do not allow easy rotary motion
and can sometimes damage the handled material. We present here a system of magnetic tweezers that overcomes those drawbacks while retaining most of the previous dynamometers properties. Electromagnets are coupled to a microscope-based particle tracking system through a
digital feedback loop. Magnetic beads are first trapped in a potential
well of stiffness ~10
7 N/m. Thus, they can be
manipulated in three dimensions at a speed of ~10 µm/s and rotated
along the optical axis at a frequency of 10 Hz. In addition, our
apparatus can work as a dynamometer relying on either usual calibration
against the viscous drag or complete calibration using Brownian
fluctuations. By stretching a DNA molecule between a magnetic particle
and a glass surface, we applied and measured vertical forces ranging
from 50 fN to 20 pN. Similarly, nearly horizontal forces up to 5 pN
were obtained. From those experiments, we conclude that magnetic
tweezers represent a low-cost and biocompatible setup that could become
a suitable alternative to the other available micromanipulators.
Biophys J, June 2002, p. 3314-3329, Vol. 82, No. 6
© 2002 by the Biophysical Society 0006-3495/02/06/3314/16 $2.00
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