The Concept of Optical Tweezers In Cell Biology

Because of the different refractive index of a cell membrane compared to the bathing solution, it will tend to refract a laser beam impinging on it. Consider the illustration below, in the left side of the figure, with a parallel laser beam impinging on the left side of the cell, the cell is seen to gain a lateral component of momentum. It will tend to move to the left, because the light will gain an overall momentum to the right because of the way the different paths are refracted. In the same way, when a focused beam impinges centrally on the cell, the refraction of the cell gives a net downward momentum to the laser beam and therefore the cell will tend to move up. This describes the idea behind optical tweezers.

These techniques generally work well for particles around 10 µm in size. As the size increases, so does the laser power required, leading to greater cell damage. Typically, by altering the focusing of the beam and the direction at which it impinges on the cell, the cell can be maneuvered at will. Because the momentum imparted represents a force on the cell if it is attached to other cells, the laser beam can be used to detach particular cells and hence act as ‘scissors’. By employing a ring-shaped laser beam (referred to as Laguerre-Gaussian profile, with a helical wavefront), a twisting motion can be applied to the cells. This ‘optical spanner’ makes use of the fact if the phase of the wavefront changes by 360° m times per rotation, the beam will have angular momentum of mh/(2 x 3.142) per photon. A quarter-wave plate can control the rate of rotation.

You may also read: What is Electroporation?