Learning from biology to design stimuli-responsive capsules

Abstract

As well as being model cell membranes, lipid vesicles are widely used as an encapsulation technology due to their impermeable membrane. Hydrogels are similarly useful due to their biocompatibility, mechanical strength, and potential for stimulus-responsive behaviour.
By combining these two structures, the benefits of both can be reaped, giving a structure with both mechanical strength and the possibility to encapsulate actives within an impermeable membrane.
However, the interactions between the gel and membrane, and their implications, are not well understood. This thesis considers two different composite structures of lipid vesicles and hydrogels as potential systems for encapsulation and controlled release. These structures are hydrogel-embedded vesicles, and Gel-Filled Vesicles (GFVs). The hydrogel-embedded vesicles are subjected to different types of mechanical stresses. Osmotic shocks are used to apply a uniform pressure on the lipid bilayer, and compression of the hydrogel by a micromanipulator is used to cause a uni-directional force.

Agarose-embedded vesicles are shown to experience an adhesive interaction between the membrane and the gel, causing vesicle behaviours to be altered in comparison to free-floating vesicles. Of particular note is the formation of a buckled morphology for embedded vesicles subjected to hyperosmotic shocks.

Additionally, the formation of GFVs demonstrating the poration mechanism of controlled release is attempted. A suitable gel core of poly(acrylamide-co-acrylic acid) is synthesised and characterised for Upper Critical Solution Temperature behaviour.

In summary, this thesis demonstrates that interactions between the lipid bilayer and a hydrogel can strongly affect membrane behaviours, and therefore their uses for either encapsulation systems or for biophysical models.