Swelling and Deswelling Kinetics of Thermo-Responsive Microcapsules with Ultrathin Membrane

Abstract

Microcapsules with a stimuli-responsive hydrogel membrane are promising as drug carriers and microreactors as they can regulate transmembrane transport. Thinner membranes are usually preferred to facilitate molecular transport and maximize payload. However, swelling behaviors of ultrathin hydrogel membrane have not been systematically investigated. Here, swelling and deswelling kinetics of ultrathin membranes made of thermo-responsive hydrogels with a lower critical solution temperature are studied. The thinness of hydrogel membrane dramatically alters the response of the microcapsules. The membrane rapidly responds to temperature changes by ejecting or accommodating a small amount of water. By contrast, the microcapsules require the transmembrane transport of a large volume of water for shrinkage or expansion, which takes a relatively long time. The difference in the timescales causes the microcapsules to behave unexpectedly. For example, the thickness of membrane decreases to be even thinner than the equilibrium for sudden increases of temperature. The shrinkage of the microcapsules is slower for higher temperature even though the deviation from equilibrium is greater. For sudden decreases of temperature, the fast expansion of membranes causes the microcapsules to buckle. Understanding the two different response timescales helps predict the peculiar responses and design microcapsules for the drug carriers and microreactors.