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Optical tweezers have become a popular manipulation and force measurement tool in cellular and molecular biology. However, there is still a lack of a sophisticated model for optical tweezers on trapping cells. In this paper, we present a novel model for optical tweezers to calculate the stiffness of trapping force upon a spherically symmetric Rayleigh sphere, which stimulates a common biological cell. A numerical simulation of this model shows that the stiffness of an optical tweezers system in trapping a cell is significantly smaller than that in trapping a polystyrene bead of the same size. Furthermore, under a small variant condition of the refractive index, the proposed model provides an approximate method which requires only the radial distribution of the trapped cell’s refractive index for calculating the stiffness.

In optical tweezers applications, tracking a trapped particle is essential for force measurement. One of the most popular techniques for single-particle tracking is achieved by analyzing the forward and backward light pattern, scattered by the target particle trapped by a trap laser beam, of an additional probe-laser beam with different wavelength whose focus is slightly apart from the trapping center. However, the optimized focal offset has never been discussed. In this paper, we investigate the tracking range and sensitivity as a function of the focal offset between the trapping and the probe-laser beams. As a result, the optimized focal offsets are a 3.3-fold radius ahead and a 2.0-fold radius behind the trapping laser focus in the forward tracking and the backward tracking, respectively. The experimental result agrees well with a theoretical prediction using the Mie scattering theory.

Since their invention in 1986, optical tweezers have become a popular manipulation and force measurement tool in cellular and molecular biology. However, until recently there has not been a sophisticated model for optical tweezers on trapping cells in the ray-optics regime. We present a model for optical tweezers to calculate the optical force upon a spherically symmetric multilayer sphere representing a common biological cell. A numerical simulation of this model shows that not only is the magnitude of the optical force upon a Chinese hamster ovary cell significantly three times smaller than that upon a polystyrene bead of the same size, but the distribution of the optical force upon a cell is also much different from that upon a uniform particle, and there is a 30% difference in the optical trapping stiffness of these two cases. Furthermore, under a small variant condition for the refractive indices of any adjacent layers of the sphere, this model provides a simple approximation to calculate the optical force and the stiffness of an optical tweezers system.