Sonochemically-generated Stimuli-responsive Polymeric Microspheres for Controlled Release

Abstract

Ultrasonic irradiation at the interface between a protein solution and a gas or non-aqueous liquid facilitates the formation of protein-shelled microspheres by a phenomenon of simultaneous emulsification and encapsulation. Sonochemically-generated hollow proteinaceous microspheres have been widely reported in the literature, with a range of current and potential applications including ultrasound contrast agents, drug delivery vehicles and nutrient carriers in the food processing industry. This project builds upon preliminary investigations conducted in the field into the use of synthetic polymers as alternative shell species by developing sonochemically-generated microspheres, employing synthesised polymeric and novel stimuli-responsive block copolymeric shell species that are capable of releasing their payload in response to changes in the external environment. Biocompatible poly(methacrylic acid) (PMAA) and PMAA-based di- and triblock copolymers, containing thermoresponsive poly(N-isopropylacrylamide) (PNIPAAM), were synthesised by reversible addition-fragmentation (RAFT) polymerisation with a measured lower critical solution temperature (LCST) of 31 °C. LCST-modified polymers and block copolymers were also successfully synthesised by copolymerisation of the PNIPAAM block with hydrophobic methyl methacrylate (MMA), with an LCST of 28 °C. Functionalisation via a carbodiimide crosslinking mechanism yielded thiol-functionalised polymers, capable of undergoing radically-initiated crosslinking to form disulphide-stabilised microsphere shells. Both thiolated and non-thiolated polymers were successfully employed in the synthesis of sonochemically-generated polymeric microspheres with comparable morphologies, supporting recent literature describing the synthesis of proteinaceous and polymeric microspheres in the absence of thiol-functionalities. Hydrophobic species, including tetradecane and naturally-occurring oils, and aqueous sodium chloride (NaCl(aq)) within water-in-oil emulsions were successfully encapsulated. Optical microscopy was employed to measure the size and stability of the microspheres with time, whilst the encapsulation efficiency of Sudan III-labelled tetradecane-filled microspheres was characterised by Ultraviolet-Visible (UV/Vis) spectroscopy. Laser scanning confocal microscopy (LSCM) was also employed to observe the successful encapsulation of non-aqueous Nile Red-labelled tetradecane and aqueous 5,6-carboxyfluorescein-labelled 1M NaCl aq-in-oil emulsions within the polymeric microspheres. The release behaviour of fluorescently-labelled tetradecane from polymeric microspheres was monitored by optical microscopy, LSCM and UV/Vis spectroscopy. A range of release mechanisms were utilised, including sonochemical disruption, extreme pH and the specific release from thermoresponsive polymeric microspheres in response to an increase in temperature beyond the elevated LCST of 36-38 °C and 32-33 °C for LCST-modified microspheres. In addition to optical and confocal microscopy, the thermally-induced release of NaCl, quantified by a change in sample conductivity, was also investigated. The work conducted during the course of this project forms the foundation for further investigation into the optimisation of thermoresponsive and stimuli-responsive microspheres, with an aim to tailor the release mechanisms of encapsulants for use as smart delivery vehicles with potential applications in the field of therapeutics, food processing, and agrochemicals.

Date of Award	8 Jan 2018
Original language	English
Awarding Institution	University of Bath
Supervisor	Gareth Price (Supervisor)