An optical stretcher

An optical stretcher, a laser tool for studying the elastic properties of cells, has been developed. When light enters a transparent object with an index of refraction higher than that of the surrounding medium, it gains momentum and therefore exerts a force on the object. A group at the University of Texas at Austin, led by Josef Käs, showed that if a laser beam is defocused so as to encompass an entire biological cell, the force acts backward where the light enters the cell and forward where it exits the cell. The result is that the cell gets stretched by an amount that depends on the power in the beam. The difference between the front and back forces is the much smaller total scattering force that acts at the cell’s center of gravity and tends to push the cell in the direction of the light propagation. A second divergent laser beam in the opposite direction keeps the cell stationary—and doubles the stretching. The researchers used the technique to study very soft human red blood cells (shown here) and much stiffer mammalian cells that contain a cytoskeleton. The tool might be used to screen cell populations for changes in elasticity due to diseases such as cancer. (An early discussion is J. Guck et al., Phys. Rev. Lett. 84, 5451, 2000 http://dx.doi.org/10.1103/PhysRevLett.84.5451 . J. Guck et al., Biophys. J. 81, 767, 2001 http://dx.doi.org/10.1016/S0006-3495(01)75740-2 .)