Extending calibration-free force measurements to optically-trapped rod-shaped samples

Abstract

Optical trapping has become an optimal choice for biological research at the microscale due to its non-invasive performance and accessibility for quantitative studies, especially on the forces involved in biological processes. However, reliable force measurements depend on the calibration of the optical traps, which is different for each experiment and hence requires high control of the local variables, especially of the trapped object geometry. Many biological samples have an elongated, rod-like shape, such as chromosomes, intracellular organelles (e.g., peroxisomes), membrane tubules, certain microalgae, and a wide variety of bacteria and parasites. This type of samples often requires several optical traps to stabilize and orient them in the correct spatial direction, making it more difficult to determine the total force applied. Here, we manipulate glass microcylinders with holographic optical tweezers and show the accurate measurement of drag forces by calibration-free direct detection of beam momentum. The agreement between our results and slender-body hydrodynamic theoretical calculations indicates potential for this force-sensing method in studying protracted, rod-shaped specimens.

Influence of experimental parameters on the laser heating of an optical trap

“…As described in Methods and in the Supplementary Information, drag forces are assessed from light-momentum measurements, such that: F drag = − α detector ·S x , where S x is the position sensitive detector (PSD) signal. The light-momentum calibration parameter, α detector , has been demonstrated to be independent of the geometry of the trapped object and of the structure of the trapping beam 14 – 17 , and importantly, it is not dependent on the laser power or sample temperature. In this way, changes in the measured drag force when incrementing the trap power, and therefore increasing the sample temperature, are directly caused by the variation in the medium viscosity, given as follows: where R is the radius of the trapped microsphere, v is the flow velocity and b is the Faxén correction due to a sphere-to-surface interaction 18 .…”

Optical Tweezers Exploring Neuroscience

“…Methods for calculating optical forces can be approximately grouped into two categories: inference based methods, which often involve calculating the force as a function of position in the trap using a particle of a known size; and direct force measurement methods, which involve estimating the optical force directly from the scattered light distribution (Fällman et al, 2004;Farré and Montes-Usategui, 2010;Jun et al, 2014;Thalhammer et al, 2015;Català et al, 2017;Bui et al, 2018). While both methods need to be calibrated, when and how these difference methods are calibrated can vary significantly.…”

Optical trapping in vivo: theory, practice, and applications

“…If we assume that the microscope imaging the light scattered by a transparent object obeys the Abbe sine condition, then its centroid is proportional to the angular displacement of the light scattered by the particle and transmitted by the microscope, and this observation permits force quantification. This method is very powerful, as it can be used to determine forces on nonspherical and deformable objects [50,87]. It has some strict limitations of applicability due to the fact that the light collection angle of microscopes is limited and the light emitted and absorbed by the particle needs to be completely resolved for an exact determination of the force.…”