Year

2018

Degree Name

Doctor of Philosophy

Department

School of Chemistry

Abstract

Proteins have attracted considerable attention in the treatment of various chronic diseases due to their high potency and specificity. However, physiochemical properties of proteins and complex physiology of the non-invasive routes pose significant challenges for site specific delivery of these macromolecules. Various polymeric delivery systems have been developed to include bioactive cues by encapsulating proteins into the structure to form a protein-polymer composite system. Developmental work in the field of protein delivery structures has revealed a number of characteristics that promise great benefits to their possible use in a broad range of devices and applications including tissue scaffolds. There has, therefore, been much interest and many attempts to produce various structures with the ability to release the proteins in a sustained approach. The main aim of this thesis is to establish fabrication processes to develop three-dimensional structures encapsulating protein with a sustained release profile.

In systemic delivery of proteins, biodegradable microspheres as parenteral depot formulation occupy an important place because of several aspects like protection of sensitive proteins from degradation, prolonged or modified release, pulsatile release patterns. In this study, two types of protein-loaded microspheres composed of alginate (Alg) and poly(lactic-co-glycolic acid) (PLGA) were prepared (Chapter 2). Emulsion and double-emulsion fabrication methods were applied to produce Alg and PLGA microspheres, respectively and have been systematically characterized to show their ability to control the release profiles of a small protein Fluorescein isothiocyanate-bovine serum albumin (FITC-BSA). The mean sizes of the FITC-BSA loaded Alg and PLGA microspheres, loaded with low (type α) and high (type ß) levels of protein, were 11, 26 and 12 micrometres, respectively. FITC-BSA releases from all types of microspheres showed the classic biphasic profile, which were governed by resolving or degradation (diffusion and polymer erosion) reasons for Alg and PLGA microspheres, respectively. In short, PLGA microspheres loaded with higher levels of protein provided an efficient system to achieve almost linearly controlled protein release at a rate suitable for further applications.

In addition, wet-spun microfibres have gained considerable interest as scaffolding substrates over the past decade. From a protein delivery perspective, wet-spinning is most similar to conventional microspheres-based protein encapsulation techniques and avoids the potential for thermal denaturation of therapeutics, unlike melt spinning and dry spinning. In addition, the high surface area-to-volume ratios of fibres help them to provide local and sustained delivery of proteins to the site of injury. With the aim of developing structures for long-term local delivery of proteins, a two-phase hybrid structure was devised by incorporation of protein-loaded microspheres into polymeric wet-spun fibrous structures. These hybrid fibres not only possess a long-term protein release profile, but also act as a promising candidate choice for longitudinal protein release applications. The structure was optimized to achieve a uniform fibre with sustained release profile (Chapter 3). The fibres were further morphologically, chemically, mechanically and thermally studied. The two-phase delivery matrices display retarded FITC-BSA release significantly in both initial and late stages compared to release from the PLGA microspheres or alginate fibre alone. It was found that fibres fabricated from Alg material (low viscosity type) with higher concentration, compared with Alg (medium viscosity type) with lower concentrations, can provide a lower level of burst release as well as a slower release profile over the observation time.

In recent years, there have been some attempts to develop concentration gradient structures with the ability of distributing proteins in a spatially graded manner. Despite all the efforts, there is still a challenge to develop a gradient structure which can be easily fabricated and programmed in terms of protein concentration and also be able to present a long-term release profile. As a result of this research, the production of protein-loaded microspheres concentration gradient fibre has been successfully achieved. This research presents a simple technique to develop programmable structures that have good potential for promotion and direction of guided tissue regeneration in “linear” systems such as muscle and nerve. Since in this study, Fluorescein isothiocyanate-bovine serum albumin was applied as a model protein which can be detected easily, the further studies can be done by optimizing the presented fabrication techniques by loading the growth factors.

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