In-vivo characterization of optically trapped Brownianprobes at a glance

TL;DR

This study introduces a novel in-vivo calibration method for optically trapped particles using an extended viscoelastic model, enabling accurate measurement of cytoplasmic viscosity and relaxation times without probe size dependence.

Contribution

It applies a recent viscoelastic model extension to in-vivo particle calibration, improving accuracy and reliability in measuring cellular cytoplasmic properties.

Findings

Viscosity in MCF7 cells aligns with literature values (2-16 mPa sec).

Relaxation time measured at about 0.1 seconds matches magnetic tweezers data.

Calibration technique effectively studies in-vivo translational motion.

Abstract

Calibration of optically trapped particles in-vivo has been complicated given the frequency dependence and spatial inhomogeneity of the cytoplasmic viscosity, and the requirement of accurate knowledge of the medium refractive index. Further, it has been demonstrated that the medium viscosity is dependent upon the measurement probe leading to reliability issues for measurements with even micrometer sized particles. Here, we employ a recent extension of Jeffery's model of viscoelasticity in the microscopic domain to fit the passive motional power spectra of micrometer-sized optically trapped particles embedded in a viscoelastic medium. We find excellent agreement between the 0 Hz viscosity in MCF7 cells and the typical values of viscosity in literature, between 2 to 16 mPa sec expected for the typical concentration of proteins inside the cytoplasmic solvent. This bypasses the dependence on probe size by relying upon small thermal displacements. Our measurements of the relaxation time also match values reported with magnetic tweezers, at about 0.1 sec. Finally, we calibrate the optical tweezers and demonstrate the efficacy of the technique to the study of in-vivo translational motion.