Today I am going to introduce you to an instrument that is widely used in biophysics and biomaterial research: magnetic tweezers.

-An illustration of a magnetic tweezer

In my project, we are studying the mechanical properties of microtubules and my particular area of focus is on the stiffness of microtubules. So far we simply rely on the thermal fluctuation of of microtubules to study the stiffness of microtubules. However, sometimes it might be in our interest to be able to control the exact magnitude and direction of an applied force as we wish. A magnetic tweezer allows us to do exactly so.

A magnetic tweezer usually consists of magnets and a magnetic bead that is attached to the biological entity, such as a DNA molecule. Magnetic field created by magnets exerts a force on a magnetic bead, which in turns exerts a force on biological entities. In order to track magnetic beads and study biological entities’ deformations, a microscope and a CCD-camera is usually implemented.

-A flurorecent tagged magnetic bead under the influence of external magnetic field causing the deformation of an entangled microtubule network

The magnitude and the direction of a force is usually controlled by the position of magnetic beads, since magnetic field created by magnets are not uniformly distributed throughout the space. Magnetic beads are usually in the micrometer range and only move by a few micrometers, therefore, the applied magnetic force can be well approximated as constant. Besides linear motions, magnetic field can also exert torque on magnetic beads. Magnetic beads often consist of magnetic nanoparticles, and the net magnetic moments of magnetic beads are not perfectly aligned with external magnetic field. Therefore, external magnetic field will exert a torque on magnetic beads and causing it to align the magnetic moment with the net external field.

-Equation of the torque on magnetic beads,  where m is the magnetic moment and B is the external field

The advantages of allowing users to control the magnitude and the direction of an applied force precisely and the ability of producing rotational motion make magnetic tweezers great instruments in single molecules and soft matter studies. Considering their broad applications in various projects, Dr. Valentine’s group has done work on optimizing magnetic tweezers and making them more portable. Great work is being done in this area and the advances in instrument development will certainly help researchers make progress in various areas in biophysics and biomaterial at a faster pace.

-Resources:

Neuman, Keir C; Nagy, Attila (June 2008). “Single-molecule force spectroscopy: optical tweezers, magnetic tweezers and atomic force microscopy”. Nature Methods

Yali Yang*, Jun Lin*, Ryan Meschewski, Erin Watson, and Megan T. Valentine. “Portable magnetic tweezers device enables visualization of the three-dimensional microscale deformation of soft biological materials,” BioTechniques 51 29-34 (2011)