A magnetic tweezer is a scientific instrument for exerting and measuring forces on magnetic particles using a magnetic field

Magnetic field

A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

 gradient. Typical applications are single-molecule micromanipulation, rheology

Rheology

Rheology is the study of the flow of matter, primarily in the liquid state, but also as 'soft solids' or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force....

 of soft matter

Soft matter

Soft matter is a subfield of condensed matter comprising a variety of physical states that are easily deformed by thermal stresses or thermal fluctuations. They include liquids, colloids, polymers, foams, gels, granular materials, and a number of biological materials...

, 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

Electromagnet

An electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current is turned off...

 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

Microscope image processing

Microscope image processing is a broad term that covers the use of digital image processing techniques to process, analyze and present images obtained from a microscope. Such processing is now commonplace in a number of diverse fields such as medicine, biological research, cancer research, drug...

 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

Optical tweezers

Optical tweezers are scientific instruments that use a highly focused laser beam to provide an attractive or repulsive force , depending on the refractive index mismatch to physically hold and move microscopic dielectric objects...

.

Examples of research groups using magnetic tweezers

USA:


http://www.childrenshospital.org/research/ingber/
Ingber Lab (Harvard Medical School

Harvard Medical School

Harvard Medical School is the graduate medical school of Harvard University. It is located in the Longwood Medical Area of the Mission Hill neighborhood of Boston, Massachusetts....

)


http://www1.engr.ucsb.edu/~saleh/
Saleh Lab (Material science department UC Santa Barbara)


http://web.mit.edu/meche/mb/
Mechanobiology Group (MIT, Roger Kamm)


http://www.engr.pitt.edu/mems/
Nanosystems Measurement & Controls Lab (Department of Mechanical Engineering & Material Science, University of Pittsburgh

University of Pittsburgh

The University of Pittsburgh, commonly referred to as Pitt, is a state-related research university located in Pittsburgh, Pennsylvania, United States. Founded as Pittsburgh Academy in 1787 on what was then the American frontier, Pitt is one of the oldest continuously chartered institutions of...

)


http://web.mit.edu/solab/
So Lab (MIT)


http://forgacslab.missouri.edu/tweezers.html
Forgacs Lab (Department of Physics & Astronomy, University of Missouri

University of Missouri

The University of Missouri System is a state university system providing centralized administration for four universities, a health care system, an extension program, five research and technology parks, and a publishing press. More than 64,000 students are currently enrolled at its four campuses...

)


http://www.jhu.edu/~cheme/wirtz/
Wirtz Lab (Johns Hopkins University

Johns Hopkins University

The Johns Hopkins University, commonly referred to as Johns Hopkins, JHU, or simply Hopkins, is a private research university based in Baltimore, Maryland, United States...

)


http://alice.berkeley.edu/
Bustamante Lab (University of California, Berkeley

University of California, Berkeley

The University of California, Berkeley , is a teaching and research university established in 1868 and located in Berkeley, California, USA...

)


http://markolab.bmbcb.northwestern.edu/marko/
Marko Lab (Northwestern University

Northwestern University

Northwestern University is a private research university in Evanston and Chicago, Illinois, USA. Northwestern has eleven undergraduate, graduate, and professional schools offering 124 undergraduate degrees and 145 graduate and professional degrees....

)


http://www.mecheng.osu.edu/pmcl/
Precision Measurement & Control Lab ([The Ohio State University])


Canada:


http://www.sfu.ca/fordelab
Forde Lab (Simon Fraser University

Simon Fraser University

Simon Fraser University is a Canadian public research university in British Columbia with its main campus on Burnaby Mountain in Burnaby, and satellite campuses in Vancouver and Surrey. The main campus in Burnaby, located from downtown Vancouver, was established in 1965 and has more than 34,000...

, British Columbia)


Asia:


http://www.k2.phys.waseda.ac.jp/index.html
Kazuhiko Kinosita (Waseda University

Waseda University

, abbreviated as , is one of the most prestigious private universities in Japan and Asia. Its main campuses are located in the northern part of Shinjuku, Tokyo. Founded in 1882 as Tokyo Senmon Gakko, the institution was renamed "Waseda University" in 1902. It is known for its liberal climate...

, Tokyo)


http://www.physics.nus.edu.sg/~biosmm/index.html
Jie YAN (National University of Singapore

National University of Singapore

The National University of Singapore is Singapore's oldest university. It is the largest university in the country in terms of student enrollment and curriculum offered....

, Singapore)


Europe:


http://www.biological.physics.manchester.ac.uk/
Biological Physics Research Group (University of Manchester, Uk)


http://www.ijm.fr/en/ijm/research/research-groups/single-molecule-nanomanipulation/
Single Molecule Nanomanipulation (Institut Jacques Monod, Paris, France)


http://www.mnp.leeds.ac.uk/
Molecular and Nanoscale Physics Group (University of Leeds, Leeds, England)


http://cell.e22.physik.tu-muenchen.de
Erich Sackmann (Technical University of Munich

Technical University of Munich

The Technische Universität München is a research university with campuses in Munich, Garching, and Weihenstephan...

, Germany)


http://bpe.tnw.utwente.nl/
Biophysical Engineering Group (University of Twente, The Netherlands)


http://www.biomed.uni-erlangen.de/lpmt
Fabry Lab (University of Erlangen-Nuremberg, Germany)


http://www.pci.uni-heidelberg.de/bpc/biophysik.html
Joachim Spatz (University of Heidelberg, Germany)


http://www.curie.fr/recherche/themes/detail_equipe.cfm/lang/_gb/id_equipe/221.htm
Cécile Sykes (Curie Institute

Curie Institute

* the Curie Institute in Paris, a research foundation.* the Curie Institute in Warsaw, a cancer research and treatment center...

, Paris, France)


http://www.phys.ens.fr/~biolps/
David Bensimon and Vincent Croquette (École Normale Supérieure

École Normale Supérieure

The École normale supérieure is one of the most prestigious French grandes écoles...

, Paris, France)


http://www.bn.tudelft.nl
Nynke Dekker - Molecular Biophysics (Technical University Delft, The Netherlands)


http://www.biotec.tu-dresden.de/seidel
Ralf Seidel - DNA Motors group (Technical University of Dresden, Germany)