Measuring local properties inside a cell‐mimicking structure using rotating optical tweezers

Abstract

Exploring the rheological properties of intracellular materials is essential for understanding cellular and subcellular processes. Optical traps have been widely used for physical manipulation of micro and nano objects within fluids enabling studies of biological systems. However, experiments remain challenging as it is unclear how the probe particle's mobility is influenced by the nearby membranes and organelles. We use liposomes (unilamellar lipid vesicles) as a simple biomimetic model of living cells, together with a trapped particle rotated by optical tweezers to study mechanical and rheological properties inside a liposome both theoretically and experimentally. Here, we demonstrate that this system has the capacity to predict the hydrodynamic interaction between three‐dimensional spatial membranes and internal probe particles within submicron distances, and it has the potential to aid in the design of high resolution optical micro/nanorheology techniques to be used inside living cells.

Optical trapping in vivo: theory, practice, and applications

“…OTs have now been widely employed in biology as a tool to actively probe, manipulate, and position biological systems or to measure mechanical properties from the nanoscale to microscale [10][11][12][13][14][15]. Highly focused laser light is able to deliver forces of the order of piconewtons and torques of the order of tens of piconewtons nanometers, which can be significant in the microscopic world.…”

Hydrodynamic forces in optical tweezers

“…The success of the trained model using a relatively small network means that arbitrary motion of a sphere moving within another sphere can be efficiently modelled using easy-to-implement code. Zhang's et al [4] results are a good example how the model can be applicable in real optical tweezers experiments.…”

“…Reynolds numbers for smaller particles at slower speeds would be even smaller. 4 The stress on the surface of the particle acts in the opposite direction to the stress acting on the surface of the fluid.…”

“…These results are then used to train a feed-forward artificial neural network allowing the wall effects of an outer spherical boundary on the arbitrary motion of an internal sphere for all experimentally achievable configurations to be conveniently and efficiently determined. Furthermore, some experimental results by Zhang et al [4] are presented in section 4.4 to give an indication of how applicable this theory is in real biological systems.…”

“…The cytoplasm of living cells is a crowded yet dynamic environment [94,95]. So to investigate wall effects and the effectiveness of rotational particle tracking microrheology inside cells, Zhang et al [4] replaced the real cell with artificial unilamellar lipid vesicles, known as liposomes, to serve as a simple model of a cell. The Liposomes were prepared following a protocol described in Yamada et al [96] that was modified so that the liposomes encapsulated vaterite particles.…”