Encapsulation is a wide spread industrial technique. The use of biopolymers to form capsules has an obvious advantage for food, biomedical or personal care applications, as they can be made of food grade and biodegradable materials. Polyelectrolyte complexes (PEC) have been widely used to form capsules by the multilayer technique, usually coating a solid core with a layer by layer deposition technique involving various steps of deposition of polyelectrolyte of opposite charge. In this thesis we focused on capsules made with a one step technique where a liquid drop containing a concentrated solution of the polycation is dropped into a dilute solution of the polyanionic solution. Chitosan was the natural polycation used, due to its interesting properties such as biodegradability, biocompatibility or muco-adhesion, and because it is one of the rare cationic polysaccharides which is safe to be used in food applications. The simple one step technique used to form these capsules brings an advantage in terms of manufacturing. The drop deformation due to the impact of droplets at the surface of a liquid bath was studied and the conditions for these droplets to detach from the surface and disperse into the solution were examined. It was found that a balance between the kinetic energy and the absorbed viscous energy of the drop during impact is necessary for the formation of capsules by this technique. Initial work was performed on capsules made with chitosan and polyanionic biopolymers such as alginate or pectins. However, these biopolymers are usually provided with a polydispersed molecular weight distribution and impurities. Polyacrylic acid was thus chosen to study the effect of molecular weight, charge density, and pH on the microstructure, mechanical, swelling and release behaviour of these capsules. The microstructure of the capsules was observed by cryo-scanning electron microscopy, and the effect of molecular weight charge density of the polyelectrolytes on the shell thickness were shown and related to the mechanical and release properties of the capsules. It was shown that the molecular weight has an important role as it determines the thickness of the shell. The charge density of the polyelectrolytes, which can be controlled by the pH and ionic strength of the solution, dictates the density of the shell. The study of the release properties of these capsules showed that they could be completely impermeable to molecules with a molecular weight higher than 2000 g/mol, and surprisingly, the capsules could withstand a wide range of pH extending beyond the pH where electrostatic interactions occur, as they only dissolved at extreme pHs of 2 and 12. Chitosan was also used to make swellable hydrogels microparticles, by cross-linking with glutaraldehyde. The swelling behaviour in gastro-intestinal environment was studied in vitro and in vivo using Magnetic Resonance Imaging techniques.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:553343 |
| Date | January 2012 |
| Creators | Pregent, Stive |
| Contributors | Waigh, Thomas |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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