# Using holographic microscopy to measure the effect of confinement on crowding agents in lipid vesicles

## TL;DR

This study introduces a digital holographic microscopy method to measure how confinement within lipid vesicles influences the diffusion of crowding agents, revealing effects dependent on solute type and concentration.

## Contribution

The paper presents a novel holographic microscopy technique for quantifying diffusion of microspheres in confined lipid vesicles, highlighting how crowding agent behavior varies with solute properties.

## Key findings

- Diffusion of sucrose and dextran is unaffected by confinement at low concentrations.
- Polyethylene glycol shows reduced diffusion inside vesicles at high concentrations.
- Confinement effects depend on the solute's critical overlap concentration.

## Abstract

The hydrodynamic effects of macromolecular crowding inside cells is often studied in vitro by using polymers as crowding reagents. Confinement of polymers inside cell-sized droplets has been shown to affect the diffusion of small molecules. Here we develop a method, based on digital holographic microscopy, to measure the diffusion of polystyrene microspheres that are confined within lipid vesicles containing a high concentration of solute. We apply the method to three solutes of varying complexity: sucrose, dextran, and PEG, prepared at ~7 % (w/w). We find that diffusion inside and outside the vesicles is the same when the solute is sucrose or dextran that is prepared below the critical overlap concentration. For polyethylene glycol, which is present at a concentration higher than the critical overlap concentration, the diffusion of microspheres inside vesicles is slower, hinting at the potential effects of confinement on crowding agents.