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Title: Degradation behaviour of PLLA-Calcium carbonate composites for orthopaedic applications
Author: Fee, Kathryn M.
ISNI:       0000 0004 5372 7094
Awarding Body: Queen's University Belfast
Current Institution: Queen's University Belfast
Date of Award: 2015
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Bioresorbable polymers are used extensively in a wide range of medical applications, including orthopaedic fixation devices and bone scaffolds. They have excellent mechanical properties and the fact they do not· require removal roves advantageous over other materials. However, a relatively slow degradation rate has been observed and furthermore, undesirable acidic products are produced late in its degradation process. Incorporating bioactive fillers such as, calcium carbonate (CaC03) into bioresorbable polymers has the potential benefit of: (1) neutralising these acidic products and (2) releasing calcium (Ca) into the local environment to enhance bone formation (osteogenesis). The long term use of such composites has proven problematic due to the unpredictability of their degradation mechanisms. Therefore, a need exists to optimise the incorporation of bioactive fillers, with the ability to evaluate the long term resorption of the polymer and release mechanism of the bioactive fillers. The aim of this research was to evaluate the effect of incorporating CaC03 as a bioactive filler, into a bioresorbable polymer via twin screw extrusion, looking in particular at the Ca release mechanism, what influence particle size and shape has on this and the overall degra~ation profile. The accelerated methodology presented seems appropriate for rapid evaluation of slow-degrading polymer-composites, although caution is advised in terms of precise interpretation of results in relation to in vivo behaviour. It was found that minimal mass loss occurred at 37.5°C, which correlated with h low level of Ca release within the same period. Accelerating the study, using increased temperature, saw the extent of mass loss increase as a function of time. The increase in mass loss also correlated with the amount of filler material, with the smallest particle size relating to the largest mass loss. This work clearly demonstrates that CaC03 characteristics (particle size, shape and morphology) influence mechanical properties, degradation behaviour and calcium release from the polymer. Results also offered important insight into the mechanism by which CaC03 is released from PLLA, clearly showing there is minimal diffusion-controlled release (typical for many drug-release systems), with release instead being strongly dependent on the stage of polymer degradation. This offers exciting prospects for controlled bioactive release (including calcium phosphates) via manipulation of the polymer degradation profile. It is believed, once the correct levels of Ca required for optimising bone remodelling have been established, devices with improved release efficacy can be developed to fully aid osteogenesis.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available