- Magnetic tweezers
A magnetic tweezer is a scientific instrument for exerting and measuring forces on magnetic particles using a magnetic field gradient. Typical applications are single-molecule micromanipulation, rheology of soft matter, and studies of force-regulated processes in living cells. Forces are typically on the order of pico- to nanonewtons. Due to their simple architecture, magnetic tweezers are one of the most popular and widespread biophysical techniques.
Types of magnetic tweezers
Depending on the implementation, magnetic tweezers can be divided into several categories including translational or rotational, unipolar or multipolar.
The most simple setup is the unipolar translational magnetic tweezer. It consists of an electromagnet with a paramagnetic core material and a tip-shaped end. This results in a high field gradient around the tip. Any paramagnetic material within that gradient is magnetized and pulled towards the tip. The force magnitude depends on the magnitude and gradient of the magnetic field. While the magnitude can be controlled by the current that drives the electromagnet, the gradient depends on the distance to the tip of the core. Alternatively, the electromagnet can be replaced by a permanent magnetic needle.
Several electromagnets can be combined into a multipolar magnetic tweezer setup, allowing for three-dimensional translation, rotation and trapping of magnetic particles.
When permanently magnetized particles are subjected to a magnetic field, their internal magnetic moment will try to align with the external field, resulting in a rotational or twisting force. Magnetic twisting has been used to study the viscoelastic properties of living cells and biological materials.
While early experiments used magnetic powder as probes and measured the resulting change of the magnetic field, nowadays magnetic microbeads of varying sizes and well-known magnetic properties are commercially available, and microscope image processing can be used to detect the displacement of the probes.
Other biophysical techniques used to apply and measure microscopic forces are Atomic Force Microscopy and Optical Tweezers.
Examples of research groups using magnetic tweezers
 Ingber Lab (Harvard Medical School)
 Saleh Lab (Material science department UC Santa Barbara)
 Mechanobiology Group (MIT, Roger Kamm)
 Nanosystems Measurement & Controls Lab (Department of Mechanical Engineering & Material Science, University of Pittsburgh)
 So Lab (MIT)
 Forgacs Lab (Department of Physics & Astronomy, University of Missouri)
 Wirtz Lab (Johns Hopkins University)
 Bustamante Lab (University of California, Berkeley)
 Marko Lab (Northwestern University)
 Precision Measurement & Control Lab ([The Ohio State University])
 Forde Lab (Simon Fraser University, British Columbia)
 Kazuhiko Kinosita (Waseda University, Tokyo)
 Jie YAN (National University of Singapore, Singapore)
 Biological Physics Research Group (University of Manchester, Uk)
 Single Molecule Nanomanipulation (Institut Jacques Monod, Paris, France)
 Molecular and Nanoscale Physics Group (University of Leeds, Leeds, England)
 Erich Sackmann (Technical University of Munich, Germany)
 Biophysical Engineering Group (University of Twente, The Netherlands)
 Fabry Lab (University of Erlangen-Nuremberg, Germany)
 Joachim Spatz (University of Heidelberg, Germany)
 Cécile Sykes (Curie Institute, Paris, France)
 David Bensimon and Vincent Croquette (École Normale Supérieure, Paris, France)
 Nynke Dekker - Molecular Biophysics (Technical University Delft, The Netherlands)
 Ralf Seidel - DNA Motors group (Technical University of Dresden, Germany)
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