There are several other uses of PHAs apart from those discussed above. Some of these are briefly described below.
1.12.1 Active Drug Compounds
In recent times, the demand for the use of chiral compounds as drugs has inspired research into industrial production of enantiomerically pure compounds from PHAs. Being a family of polyesters consisting of over 140 chiral R- Hydroxycarboxylic acids (R-HAs), PHAs are considered a promising renewable source of obtaining chiral compounds that may be used in the synthesis of active drug compounds. P(3HB) is among the few commercially available PHAs. At present P(3HB) is known to be well tolerated due to their biocompatibility and hence have recently been employed as chiral products in the treatment of traumatic injuries like haemorrhagic shock, severe burns, myocardial damage and ischemia, anoxia and cerebral hypoxia (Massieu et al., 2003). Kashiwaya et al., further reported that these compounds were capable of reducing the death rate associated with human neuronal cell model culture for Alzheimer’s and Parkinson’s disease and also reduce apoptosis that brings about corneal epithelial erosion (Kashiwaya et al., 2000). The antimicrobial activities of R-3HB are also well documented (Sandoval et al., 2005; Chen and Wu, 2005; Shiraki et al., 2006; Ruth et al., 2007).
1.12.2 Health food and Nutrition
3HB oligomers are known to have a good penetration and rapid diffusion into peripheral tissues and hence could be utilised as emergency energy substrates for injured patients (Tasaki et al., 1999). These oligomers, when evaluated in vivo were found to release ketone bodies over prolonged periods. For this reason, several potential uses of these compounds including parenteral nutrition, management of diabetes and insulin resistance states, seizure control, reduction of protein catabolism, appetite suppression and control of some metabolic diseases (Williams et al., 2003)
1.12.3 Orthopaedics
Some studies have shown that scaffolds made up of PHAs result in consistent and favourable bone tissue adaptation response without any undesirable chronic inflammatory evidence after implantation periods of up to 12 months (Doyle et al., 1991). PHA compression-moulded T-plates prepared from P(3HB) reinforced with 7% carbon fibre were used to fix osteotomies of tibial diaphysis in rabbits. The implants, fixed to the tibia by absorbable sutures were compared to implants made from vicryl. After 12 weeks, better results were obtained from the reinforced PHA implants whereas implants from the vicryl plates resulted in frequent breakages and angulation (Bostman et al., 1987). The possibility of manufacturing several composite materials from among different PHAs and with other polymers underlines the potential of designing materials with superior qualities such as suitable mechanical strength, biocompatibility and optimal degradation time for use as implants in orthopaedics (Vainionpaa et al., 1986).
1.12.4 Uses in urology
The possibility of PHAs being used to repair a ureter was first proposed by Baptist and Ziegler in 1965. Another study in this area was carried out by Bowald and Johansson in 1990 where a vicryl tube coated with a solution of P(3HB-co-3HV) copolymer were implanted to replace the ureter in dogs. A fully functional ureter was claimed to have been formed within all model animals within a period of nine months (Baptist and Ziegler, 1965; Bowald and Johansson, 1990).
1.12.5 Wound management
The use of PHAs in the manufacture of sutures with superior quality has been suggested as far back as 1965 (Baptist and Ziegler, 1965). The biocompatibility and biodegradability of these materials, among other suitable qualities could explain the use of PHAs as clinical sutures. Looking into the wider context of controlled drug delivery by PHA materials, the prospects of designing wound dressings with these biological polymers to deliver such agents as antibiotics, minerals and vitamins, proteins and other wound healing mediators, in a controlled manner, would be worth considering.
1.12.6 Dusting powders
Small particle powders from P(3HB) have been produced and have been proposed to be used as dusting powders, particularly on surgical gloves (Holmes, 1985)
Indeed, several other medical and pharmaceutical applications of PHAs have been suggested, especially in recent times. Williams et al., have listed ligament and tendon grafts, spinal fusion cages, surgical mesh and repair patches, ocular cell implants, bulking and filling agents, vein valves and haemostats are among the many possible medical uses of PHAs (Williams et al., 2003).
AIMS AND OBJECTIVES
The aim of this research project was to carry out the production of PHAs from Pseudomonas mendocina and use the PHAs produced for biomedical applications.
Specific objectives of this project are:
1. Investigation of the use of various renewable non-expensive carbon sources such as sugarcane molasses, biodiesel waste and glycerol for the economical production of MCL-PHAs by P. mendocina. The polymer produced will be characterised for their chemical properties using Gas Chromatography Mass Spectrometry (GC-MS), structural properties using Fourier Transform Infrared Spectroscopy (FTIR) and thermal properties using Differential Scanning Calorimetry (DSC).
2. Development of novel, biocompatible and biodegradable P(3HO)/bacterial cellulose composites. These composites will be characterised with respect to their mechanical properties using a Dynamic Mechanical Analyser (DMA), microstructural properties using Scanning Electron Microscopy (SEM) and thermal properties using DSC. In addition, the effect of the addition of cellulose microcrystals on the degradability of P(3HO) will be investigated, focusing on their potential use as scaffolds for tissue engineering. Biocompatibility of these P(3HO)/bacterial cellulose composites will be tested using Human dermal microvascular endothelial cells (HMEC-1).
3. Development of novel, biocompatible and biodegradable multifunctional 2D P(3HO)/P(3HB) blend films with varying percentages of P(3HO) and P(3HB) such as P(3HO)/P(3HB) 80:20, P(3HO)/P(3HB) 50:50 and P(3HO)/P(3HB) 20:80. These blend films will be characterised with respect to their mechanical properties using DMA, thermal properties using DSC and microstructural properties using SEM, White Light Interferometry and contact angle measurements. They will also be characterised for their bioactivity using HMEC-1 cells and their
degradation properties to assess their suitability in the development of biodegradable stents and scaffolds in tissue engineering.
4. (1) Surface micropatterning of the P(3HO)/P(3HB) 50:50 blend films using the laser micropatterning technique. Bearing in mind, the potential application of the P(3HO)/P(3HB) 50:50 blend films as a platform material for the development of a biodegradable stent and as a tissue engineering scaffold, HMEC-1 will be grown on the micropatterned P(3HO)/P(3HB) 50:50 blend films to assess the influence of micropatterning on cell growth and proliferation. The effect of the laser micropatterning on the surface and mechanical properties of the P(3HO)/P(3HB) 50:50 blend films will be examined using SEM, AFM and water contact angle measurements and DMA respectively. (2) Development of micropatterned and non- micropatterned stent prototypes.
5. Investigation of the use of P(3HO)/P(3HB) 50:50 2D blend films with aspirin as the base material for the development of a drug eluting biodegradable stent and as a drug delivery vehicle for the controlled release of aspirin. P(3HO)/P(3HB) 50:50 blend film with aspirin will be characterised for its mechanical properties using DMA, thermal properties using DSC and surface properties using SEM, AFM and water contact angle measurements to investigate the effect of the incorporation of aspirin on the blend film. Biocompatibility of these blend films containing aspirin will be investigated using HMEC-1 and their degradation properties will also be characterised to determine the effect of drug loading. Drug release studies will be carried out using High performance Liquid Chromatography (HPLC).
6. Microbial production of P(3HB) and their utilisation in the synthesis of microspheres and films to investigate their potential as a drug delivery tool for the controlled delivery of aspirin. P(3HB) microspheres and films containing aspirin will be characterised. Drug release rates from the microspheres and the films will be measured using HPLC.
The ultimate aim of this research project is to use PHAs, their composites and blends for various biomedical applications such as scaffolds for tissue engineering, development of biodegradable stents and drug delivery.