Design of magnetic tweezers for DNA manipulation

We study different configurations of permanent magnets and ferromagnetic circuit, in order to optimize the magnetic field for the so-called ``magnetic tweezers'' technique, for studing mechanical properties of DNA molecules. The magnetic field is used to pull and twist a micron-sized superparamagnetic bead,tethered to a microscope slide surface by a DNA molecule. The force applied to the bead must be vertical, pointing upwards, being as strong as possible, and it must decrease smoothly as the magnets are moved away from the bead. In order to rotate the bead around the vertical axis, the field must be horizontal. Moreover, the volume occupied by the magnets is limited by the optical system. We simulate different configurations by solving the equations for the static magnetic field; then, we test some of the configurations by measuring the force acting on a bead tethered by a DNA molecule. One of the configurations is able to generate a magnetic field ten times stronger than usually reported.