Stimuli-responsive materials have become very important in scientific research, allowing for "smart" control over material properties when triggered by external signals, such as changes in temperature or pH. This control has enabled ground-breaking scientific advances in fields such as tissue engineering, soft robotics, healthcare and diagnostics. One reported class of smart materials are "engineered emulsions", which use branched copolymer surfactants (BCSs) to stabilise emulsion droplets. These emulsions respond to changes in pH by solidifying into gels due to a change in the interactions between copolymers on adjacent emulsion droplets becoming attractive, leading to the self-assembly of the droplets into a hierarchical network structure. These "smart" materials are highly attractive, displaying stimulus-responsiveness combined with the availability of large hydrophobic and aqueous domains, which could be used as a reservoir or solubilisation locus for large payloads, released on demand. While the use of pH may be of interest for specific applications, temperature as a trigger offers wider applicability, particularly in biomaterials and food.
We have recently demonstrated that engineered emulsions stabilised by poly(ethylene glycol) - poly(N-isopropyl acrylamide) BCSs exhibit "thermothickening" behaviour, in other words, they respond to temperature as a stimulus and their viscosity dramatically increases upon warming. These new materials have many potential applications in advanced therapeutics, tissue engineering, and in emerging fields such as 3D printing. However, poly(N-isopropyl acrylamide), which is the most widely used polymers for thermothickening applications, is cytotoxic against some cell types, so a more biocompatible alternative must be found. In this project, candidate temperature-responsive materials with promising safety profiles have been identified, namely poly(N-vinylcaprolactam), poly(2-dimethylaminoethylmethacrylate), and poly(N,N-diethyl acrylamide), which could replace poly(N-isopropylacrylamide). In addition, the relationship between polymer block composition and thermothickening behaviour must be established to inform the design of future smart gelling materials. Finally, a better understanding of the mechanisms behind the thickening need to be achieved.
This project hypothesises that BCSs containing PEGMA and a temperature-responsive component may be used to engineer emulsions which thicken upon warming to body temperature, leading to the design of advanced functional materials. This project will explore the relationship between BCS structure and supracolloidal assembly in BCS-stabilised emulsions, to generate optimised materials with smart thermoresponsive thickening. Cutting-edge neutron scattering and reflectometry techniques will be used to understand morphology at the nanoscale relates to the gelling properties of the hierarchical assemblies, informing the design of future advanced materials.
Once developed, thermothickening BCS-stabilised emulsions have numerous potential applications, enhancing existing technologies and providing a platform for future advanced materials. Thermothickening materials could be used in mucosal drug delivery to sites such as the eye, vagina, and rectum, where a fluid containing drug may pass through an applicator, before forming a viscous gel at the site of administration, enhancing retention at sites where rapid clearance leads to poor therapeutic effect and low patient compliance. In tissue engineering, cellular medicines may be administered within a thickening material which forms a scaffold in situ in which the cells may grow and either replace damaged tissue or act as bioreactors. In 3D printing, these materials could be solidified using heat as a stimulus, creating scaffolds with microscale patterning. The temperature-responsive BCSs could also be impactful in areas such as cosmetics, chemically-enhanced oil recovery, and as flocculants.
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