Measurement of forces in optical tweezers with applications in biological systems

Abstract

Optical tweezers, a tool for contactless manipulation of micro- and nano-particles, are widely used to apply and measure forces. This thesis investigates various force measurement methods and their applications for force measurements in biological systems. Unlike position-based methods, the direct optical force measurement technique does not require calibration of the trap stiffness. The direct force measurement method utilises the determination of the change in the momentum of the trapping light to measure the optical forces acting on a trapped object. This thesis has developed a method for the measurement of the calibration constant for the force detector based on simultaneous detection of the position of the trapped particle and the optical force. Rigorous tests of the calibration technique and the direct force measurement method using different particles (red blood cell, vaterite, silica spheres), a variety of trapping media (water, plasma, ethanol), and trapping beams (HG00 and HG01) have shown its robustness and accuracy. These unique properties of the method make it highly beneficial for force measurements in biological systems. This thesis has developed a position sensitive masked detector for the high-speed measurement of the optical force including the measurements of the axial forces (in the direction of the beam propagation). When combined with a position sensitive detector, which is typically used for the radial force measurements, it allows full three-dimensional measurements of the optical force. Finally, the direct force measurement method has been applied to study biological systems. In the first experiment, the aging during storage of the red blood cells (RBC) is investigated using the stretching of the cells in two optical traps. The results of the stiffness measurements show that the stiffness of the RBCs does not change within the same morphological type. The previously observed increase in the stiffness is linked with an increase in the number of echinocytes - a type of RBC with higher stiffness. In the second experiment, I demonstrate the measurements of the swimming force generated by a trapped Escherichia coli. As these bacteria are cylindrically shaped, they orient in the trap along the beam propagation direction. A position sensitive masked detector (with the mask for axial force measurements) measures the swimming force directly and does not require any beam-shaping techniques to trap the bacteria horizontally.