|Title||Biosynthesis of polyhydroxyalkanoates, their novel blends and composites for biomedical applications|
Polyhydroxyalkanoates (PHAs) are a family of polyhydroxyesters of 3-, 4-, 5- and 6- hydroxyalkanoic acids produced by bacterial fermentation in a nutrient limiting conditions with excess carbon. They can be produced easily using renewable carbon sources. They are biodegradable and biocompatible in nature. Their physical properties are highly tailorable and a range of desired properties can be achieved based on the type of application. Owing to these properties, there has been a considerable interest in the commercial exploitation of PHAs, particularly for biomedical applications.
The main aim of this research project was to produce MCL-PHAs from Pseudomonas mendocina and use them for biomedical applications. In this study, an economical production of MCL-PHAs using renewable and cheap carbon sources such as sugarcane molasses, biodiesel waste and pure glycerol was carried out. Maximum PHA yield of 43.2% dcw was obtained in the media containing biodiesel waste. The results demonstrated the successful utilisation of these cheap carbon sources by P. mendocina for the economical production of MCL-PHAs.
One of the main objectives of this project was to utilize the PHAs produced for biomedical applications. Multifunctional novel 2D P(3HO)/bacterial cellulose composite films were developed for their potential use in tissue engineering applications. Chemically modified bacterial cellulose microcrystals were used as the reinforcing agent to improve the properties of P(3HO). Mechanical properties such as the Young’s modulus and tensile strength values of the P(3HO)/bacterial cellulose composite films were significantly higher in comparison to the neat P(3HO) film. Also, the composite film had a rougher and more hydrophilic surface compared to the neat P(3HO) film. It is known from literature that surface roughness and hydrophilicity affects protein adsorption on the surface of the biomaterial. Protein adsorption, in turn, plays an important role in determining the biocompatibility of a material being used for medical applications (Das et al., 2007). In this study, protein adsorption was higher in the P(3HO)/25% bacterial cellulose composite film compared to the neat P(3HO) film. In vitro biocompatibility studies using Human microvascular endothelial cells (HMEC-1) was carried out. Both neat and composite films were able to support the proliferation of HMEC-1 cells. However, the biocompatibility of the P(3HO)/25% bacterial cellulose composite films had increased. The cell proliferation significantly higher on the P(3HO)/25%
bacterial cellulose composite film as compared to the neat P(3HO) film on day 7.
In addition, multifunctional 2D P(3HO)/P(3HB) blend films with varying percentages of P(3HO) and P(3HB) were developed and assessed for their suitability in the development of biodegradable stents. Mechanical, thermal and microstructural properties of the P(3HO)/P(3HB) blends were characterised. The results highlighted the role of P(3HB) in enhancing the mechanical properties and thermal stability of the blend films compared to the neat P(3HO) films. However, the results suggested that the mechanical properties of the P(3HO)/P(3HB) had to be further improved to meet the desired values required for the development of a biodegradable stent. The overall protein adsorption and % cell viability was significantly higher in the blend films compared to the neat P(3HO) film. Hydrolytic degradation was faster in the blend films and the degradation rate could potentially be tailored to achieve the optimum rate required for a particular medical application.
From the literature, it is known that the surface topography determines the compatibility of a biomaterial by governing important processes such as wettability, protein adsorption, cell adhesion and proliferation (Duncan et al., 2007). In this part of the study, P(3HO)/P(3HB) 50:50 blend films were micropatterned using the laser micropatterning technique to improve their biocompatibility. The results demonstrated an increase in hydrophilicity and protein adsorption on the micropatterned blend films compared to the plain P(3HO)/P(3HB) 50:50 blend films. Cell attachment, proliferation and alignment was significantly higher on the micropatterned blend films compared to the P(3HO)/P(3HB) 50:50 blend films which was a desirable outcome.
Furthermore, an investigation of the P(3HO)/P(3HB) 50:50 2D films as the base material for the development of a drug eluting biodegradable stent was carried out by incorporating aspirin within the film. The percentage viability of the HMEC-1 cells was higher in the blend films with aspirin compared to the blend films without aspirin indicating an increased biocompatibility of the P(3HO)/P(3HB) 50:50 blend film containing aspirin. Controlled release of aspirin was observed without any burst release and 96.6% release was achieved within 25 days, ideal for the development of biodegradable drug eluting stents.
Finally, a drug delivery system for the controlled delivery of aspirin was successfully developed. In this part of the study, 2D solvent cast films and microspheres (average size=30 μm) were developed using P(3HB). Drug release pattern from P(3HB) films as well as P(3HB) microspheres were monitored. The results demonstrated that the P(3HB) films with aspirin were suitable for sustained long term drug release whereas P(3HB) microspheres with aspirin were more suitable for fast release.
In conclusion, this project has led to the successful production of PHAs, and their utilisation in the development of a range of composites, blends and drug elution structures with promising potential medical applications.