Lau, Chi Ching; (2018) A Comprehensive Study on Synthesis and Bioapplication of Calcium Phosphates, Poly(glycerol sebacate) and the Biocomposite by Microwave Approaches. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Microwave synthesis capable of improving reaction rate, yield and purity is highly desirable in preparing a wide range of biomaterials. Although existing conventional methods have been widely used to fabricate the biomaterial, these methods have various limitations especially time and energy consuming. In this thesis, the project aims is to fabricate two interesting biomaterials, calcium phosphates (CaP) and poly(glycerol sebacate) [PGS], with controlled properties by unique heating mechanism of microwave. Firstly, three interesting CaP phases [i.e. hydroxyapatite (HA), β-tricalcium phosphate (β-TCP) and dicalcium phosphate anhydrate (DCPA)] with improved reaction rate and purity were first synthesised under identical microwave conditions. These CaP particles with varied morphology, specific surface area and pore size were produced after a short reaction time (5 mins) in microwave. However, the yields of these CaPs were low and the reaction time was extended to one hour. In this work, it was found that the solvent species with different solubility and microwave efficiency are crucial to control the phase and morphology of CaP. For instance, the highest solubility of calcium precursors in water produced HA particles which are the highest Ca/P ratio among these phases, and these particles grew in one-dimensional due to its slower heating rate in microwave. On the other hand, mesopores of β-TCP and DCPA particles were produced in methanol and ethanol, respectively. These particles with higher specific surface area demonstrated higher BSA loading than the nonporous HA particles. The pore size of the CaP particles also affected the release profile; larger pore size in β-TCP particles allowed faster release of BSA compared to DCPA. The phase and morphology-controlled of these CaP materials provide a platform to readily control both the drug delivery profile and loading capacity of proteins. Secondly, pre-polymer of PGS (pre-PGS) were prepared successfully using a modified microwave approach. This approach allowed better temperature control in microwave and minimized the overheating of the monomers. In additions, water was collected to tailor the degree of esterification (DE) of the pre-PGS and this DE strongly affected the mechanical properties and degradation rate of the PGS. This method also sped up the pre-polymerisation and curing process by at least a factor of six. More interestingly, the highly branched pre-PGSs formed in microwave due to the special heating mechanism. In microwave, both primary and secondary hydroxyl groups of glycerol reacted simultaneously with sebacic acids. On the contrary, pre-polymerisation by conventional heating was mainly reacted the primary hydroxyl group. The PGS samples prepared by microwave showed a wider range (59%) of degradation rate than the conventional heating method owing to the highly branched of polymer structure. Apart from that, a stiffer polymer obtained after 2 h of curing time as compared to the viscous polymer by conventional heating. The Young’s modulus of PGS prepared after 8 h was similar with the one prepared by conventional heating after 48 h, owing to the branched structure. This microwave approach can synthesise PGS with higher degree of freedom to tune mechanical properties and degradation rate can meet demands of various applications. Thirdly, PGS/β-TCP biocomposites were prepared by microwave method. The purpose of introducing the β-TCP particles on the polymer surface was to enhance the biocompatibility of PGS since the fast degraded sebacic acids always create cytotoxicity issue. The presence of the β-TCP particles not only increased the degree of crosslinking, but also improved the hydrophilicity which could not be achieved by single material of PGS. On the other hand, the mass loss of the biocomposites was also reduced significantly. Owing to these improved properties, the cell viability in these biocomposites also showed enhancement as compared to PGS. Finally, the microwave synthesis of PGS was further modified without adding any solvent or catalyst. PGS was prepared successfully by single step microwave synthesis and no curing step was needed. To the best of my knowledge, this is the first study to get the biopolymer synthesised by a means of one step. It was found that a high microwave power with short reaction time is crucial for the formation of PGS. The cooling intervals minimised the monomer loss caused by the overheating issues. The prepared PGS were characterised with IR spectra, 1H NMR spectra and thermal analysis, and these properties are comparable with the reported literature. The ester linkage of the PGS was confirmed by IR spectra. Surprisingly, as indicated in 1H NMR spectra, both primary and secondary hydroxyl groups were activated by microwave to form linear and branched polymer, simultaneously. Both crystallisation and melting temperature were reduced with increment in reaction time, owing to limited mobility of the chains.

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