Osmotic pressure enables high yield assembly of giant vesicles in solutions of physiological ionic strengths

Giant unilamellar vesicles (GUVs) are micrometer scale minimal cellular mimics that are useful for synthetic biology and drug delivery. However, their assembly in solutions with ionic concentrations of 100-150 mM Na/KCl (salty solutions) is challenging. Here, we investigate quantitatively the effects of temperature and chemical identity of small molecule compounds and macromolecular polymers on the molar yields of GUVs using high resolution confocal microscopy and large dataset image analysis. The macromolecular polymer low gelling temperature agarose is the singular compound that is effective in producing high yields of GUVs in salty solutions. We propose a free-energy model of budding to explain the effects of polymers in assisting the assembly of GUVs. Experimental data obtained by modulating the ionic strength of the solution support our model’s prediction that the osmotic pressure of dissolving polymers results in a net reduction in the free energy for bud formation. The uncovered mechanistic insight provides a quantitative experimental and theoretical framework to guide future studies and shows a facile means for obtaining GUVs in solutions of physiological ionic strengths.