ИСТИНА

Optical tweezers (OT) is a novel tool that allows for noncontact trapping and manipulating single micro- and nanosized particles using tightly focused laser beam. The most perspective feature of OT is that they can measure forces ranging from several pN to almost hundred pN. Such forces characterize the interactions between biological cells and macromolecules. [1-3] Multi-channel laser tweezers or other modifications allow for trapping and manipulating multiple particles (or cells) simultaneously.It makes the OT method a powerful tool for assessing the mechanisms of biological structures’ interaction at single cell level.[2,4,5] Thesekind of studies were not feasible before. Nowadays the OT is one of most efficient methods used in biophysics and has an immensely wide range of applications. [2,4,5] In order to make quantitative measurements, OT must be calibrated. Shortly speaking, calibration for optical tweezers means determining the trap stiffness and trapping force magnitudefor certain intensities of the laser beam. Consequently, the force exertedonto an object as a function of the measured displacement of the object in the trap is then known. Figure 1 shows the main steps of calibration. The power spectrum analysis of the Brownian motion of a trapped particle is usually considered to be the most reliable option to accomplish this goal [6]. The main idea behind is that the frequency content of the particle motion is related to the strength of the trap: as the stiffness increases, the high frequency components start to dominate the movement of the object. Different methods, such as quadrant photo detector (QPD) or fast video camera [1,7], can be used for detecting the particle during the calibration. We used ahigh-speed video camera for recording theBrownian motion of the trapped particles and Matlab for signal processing. Contrast of the video was adjusted by changing the sample illumination light. We explain the calibration method, discuss thevideo quality optimization, and show some examples of power spectrum analysis at different trapping power levels. In addition, we show some results where red blood cell disaggregation forces are measured with optical tweezers.