Optical and magnetic tweezers

Optical and magnetic tweezers manipulate a handle in the form of a bead attached to the end of a molecule such as DNA. Optical tweezers and traps exploit the restoring force that can be exerted on a dielectric microbead by the electric-field gradients at the focus of a laser beam. In the case of magnetic tweezers, a magnetic bead is manipulated between magnetic poles. 

Molecular probes, such as optical or magnetic tweezers,64-71 micropipets,72 and microfibers,73-74 have been developed to manipulate single molecules and to measure their response to mechanical actions such as stretching, torsion, and compression. A force resolution down to 0.1 pN enabled quantitative measurement of the molecular forces and provided novel information on the basic principles of folding, motion, and interactions of individual molecules. Complementary to the local mechanical probes, actions of external fields were monitored on individual polymer molecules.75 77... 

The rheology of networks of cross-linked and bundled F-actin has been shown to exhibit exceptional elastic behaviour that reflects the mechanical properties of individual filaments. The local viscoelasticity has been investigated by microrheology techniques using embedded colloidal probe particles and optical interferometry, or magnetic tweezers. The elastic modulus is a strong function of the actin concentration, thus cross-link density. Actin can be cross-linked to form a network coating the surface of a vesicle. The viscoelastic and deformation properties of actin-coated vesicles has also been examined. 

Particles can be manipulated in suspension using strongly focused laser beams ( optical tweezers ) [25] or magnetic fields [26] and by collecting statistics on tire particle movements using video microscopy, infonnation on the particle interactions can be obtained. 

This effect can be used to position and manipulate single cells or large viruses in solution. For other biophysical studies, individual macromolecules can be tethered to one or more latex or plastic beads and manipulated using optical tweezers. Other techniques combine similar approaches using small magnetic beads. 

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