Application of gradient laser fields in biology and medicine: physical principles and prospects

In 1986 Ashkin and coworkers have for the first time demonstrated the trapping of microparticles by a sharply focused laser beam [1]. Since then the optical tweezers have be- come indispensable tool for selective capture and translation of dielectric microparticles. The tool proved to be especially efficient in biological studies. Biological objects such as cells or cell organelles as well as small probes inside a cell can be held and moved by exertion of forces as low as several piconewtons without damaging the cell. A vast number of biological applications, such as micromanipulation of viruses and bacteria [2], quantitative measure- ments of forces in molecular motors [3-7], induced cell fusion [8], tweezing and cutting in immunology and molecular genetics [9,10], chromosome movement [1 1], motility of human spermatozoa [12], mobility of transmembrane proteins [13,14], mechanics of kinesin mole- cules [15], and others have been successfully demonstrated. Mechanism of particle trapping in laser tweezers is based on the influence of light pressure and dipole gradient forces on a particle in a strongly focused light beam. The theory of the interaction of a microparticle with a focused light beam and methods of calculating trapping forces have been presented in a number of publications [16-23]. At the same time more general problem — the interaction of spatially modulated light fields with ensemble of particles in liquid and the possible influence of such fields on the on functioning biological systems, in particular, human organism — remains unexplored. In this paper we are discussing the possible effects of spatially modulated laser light at nonresonance interaction with heterogeneous media, including biological objects. In particular we are frying to show that, contrary to popular beliefs, the effect of the highly coherent laser radiation on biological structures, and on the living organism as a whole, can differ radically from that of incoherent radiation with the same intensity, duration, and wavelength. The main difference between coherent and incoherent illumination is the formation of speckles in the first case. Upon illumination of heterogeneous medium by laser radiation, a speckle structure of light field is created both on a surface and in a bulk of the body due to the interference of scattered components with each other and with the incident light beam. This structure is charrn acterized by a sharp small-scale spatial modulation ofradiation intensity, i.e. by strong spatial gradients ofthe light field. Apart from the speckles, which appear in a body automatically, the gradient laser field may be created artificially by using interference of laser beams. It allows to obtain one-, two-, or three-dimensional modulation of radiation intensity with the controllable spatial period across a biological object. Interaction of such spatially modulated laser fields with heteroge— neous system will be accompanied with the local action of gradient forces. Basically the gra— client forces can cause two effects: 1) change of the local concentration of certain biological components of a micrometer-scale (enzymes, erythrocytes, leukocytes, etc.) and 2) stimulation of conformational changes in various biological structures both within the cell and on the supracellular level. As a result, this can lead to changes in the character of the cell metabolism, and possibly to changes of its genetic properties. First we shall discuss the physics of gradient laser fields interaction with particles and then very briefly their possible applications in biology.