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

Frederic Català,Arnau Farré,Estela Martín-Badosa,Ferran Marsà,Mario Montes-Usategui

doi:10.1038/srep42960

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.

Highlights

Optical trapping has become an optimal choice for biological research at the microscale due to its noninvasive performance and accessibility for quantitative studies, especially on the forces involved in biological processes
One solution that is widely applied in bacterial swimming studies consists of monitoring the trapping laser light with a photodiode and inferring information after complex processing of the electric signals obtained
Force measurement based on beam momentum detection is independent of any local parameter present in the experiments, e.g., the laser power, beam structure, objective numerical aperture (NA), and sample geometry and refractive indices

Summary

Introduction

Optical trapping has become an optimal choice for biological research at the microscale due to its noninvasive performance and accessibility for quantitative studies, especially on the forces involved in biological processes. 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. This method can be applied to non-spherical specimens, with synthetic microbeads bound to the sample of interest used as force probes This has enabled numerous investigations, such as those into biopolymer stretching[2], the assembly dynamics of microtubules[3], cell membrane mechanics[4] and parasite flagellar forces[5]. The force-position relationship, i.e. the trap stiffness k, has been measured for Escherichia coli using Stokes’ drag force calibration[14] and equipartition theorem[15]

Methods

Results

Conclusion