Abstract
Polyhydroxyalkanoates (PHAs) are a family of biodegradable, biocompatible polymers produced by several species microorganisms. Different types of PHA polymers possess favorable mechanical properties (e.g., strength and elongation properties) and have been examined for use in medical applications, such as sutures, scaffolds and implants. The polymer poly(3-hydroxybutyrate) [PHB] and the copolymer poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) [P(HB-co-HHx)] were extensively examined in this study because of these promising properties. In this study, copolymers varying in HHx monomer content (0 - 30mol%) were recovered from different Ralstonia eutropha strains. Mechanical properties of solvent-cast samples of PHB, P(HB-co-17mol%HHx), P(HB-co-23mol%HHx), and P(HB-co-30mol%HHx) were investigated. Our results show that with higher content of HHx monomer in the material, the more flexible and tougher the polymer, and the greater the reduction of the polymer's crystallinity. One of the major under-addressed issues associated with the use of biodegradable PHA polymers in resorbable medical products is the correlation between the mechanical properties and in vivo degradation over time. Therefore, one of the major requirements is a thorough understanding of PHA degradation kinetics of polymer in in vitro and in vivo environments. In this study, P(HB-co-17mol% HHx) matrices were either incubated in cultures of human embryonic kidney cells (HEK) for in vitro degradation studies for up to 4 weeks, or inserted into Danio rerio (zebrafish) tissues for in vivo degradation studies for up to 7 weeks. Also, PHB was subcutaneously implanted in mice and incubated for up to 16 weeks. After removal from the animal tissue or cell culture environment at different intermittent time points, the strength, elongation, mass loss, and enthalpy of the polymer were tested, and scanning electron microscopy images were taken. Our results show that Young's modulus of P(HB-co-17mol%HHx) during in vitro studies decreased gradually within 4 weeks, and in vivo breakdown resulted in a significant decrease in Young's modulus and a mass loss of 59% within 7 weeks. While PHB incubated in mice exhibited slower degradation due to the strength and dense structure of the material. From these data, a mathematical model was generated by Rayleigh's method of dimensional analysis. It was found that the developed model was aligned with experimental results and could predict the strength of the polymer when in contact with cells, and that strength predicted by the model follows the trend of our gathered experimental data. Lastly, P(HB-co-HHx) membranes were prepared by solvent-casting or electrospinning methods to be loaded with antibiotics and used as an anti-biofilm sheet. Our results have shown that these loaded sheets are effective for inhibiting biofilm formation, and eletrospun sheets were observed to more effectively inhibit biofilm formation. It was concluded that P(HB-co-HHx) sheets display potential as raw material for fabrication of wound dressings to be used in anti-biofilm treatments. This dissertation contributes to PHA studies by developing a novel mathematical model that describes its degradation and variables, by determining the porosity and flexibility of the PHB polymer and the copolymer P(HB-co-HHx), and by employing the P(HB-co-HHx) porosity and its ability to bind with proteins to become an anti-biofilm sheet loaded with antibiotics.