Biosynthesis of polyhydroxyalkanoates and their medical applications

Biomaterials have gained significant importance in the field of tissue engineering. For example, these biomaterials have been considered for the reconstruction of tissues for structural applications where the tissue morphology is of paramount importance such as bone, cartilage, blood vessels and skin etc. (Ameer et al., 2002). Often these biomaterials are used for tissue regeneration when the surrounding defective tissue exhibits the inherent potential for tissue regeneration. However, in situations where the tissue lacks this ability of regeneration, relevant cells as well as growth factors have been used to accelerate tissue regeneration. In addition, these biomaterials have also been combined with drugs and used as drug delivery systems, thereby reducing the microbial infections while maximising tissue regeneration (Gomes et al., 2004).

There are several factors that make polyhydroxyalkanoates (PHAs) excellent materials for use in TE, however, when used as scaffolds in bone TE, PHAs fail to actively bind to the living tissue by means of a biologically active apatite layer or meet the mechanical demands in load-bearing applications. Therefore, these polymers are combined with inorganic bioactive materials, such as bioactive glass, to increase their applicability in bone TE. Also, there is a growing need to apply these scaffolds for applications such as drug delivery.

For the first part of this study, a solid-in-oil-water (s/o/w) technique was used in an attempt to produce tailored poly(3-hydroxybutyrate) P(3HB) microspheres. The effects of different parameters used for microsphere production on the microsphere size, porosity, and drug distribution were investigated. The P(3HB) microspheres were encapsulated with the drugs such as gentamicin and tetracycline and their in vitro release kinetics studied.

A multifunctional P(3HB)/45S5Bioglass® composite system for bone tissue engineering was also developed which could exhibit topographical features to increase cell attachment and can also act as a carrier for controlled drug delivery via the immobilization of P(3HB) microspheres on the scaffold surfaces. Thus a microsphere coating was created on the 45S5Bioglass® scaffolds resulting in a composite scaffold with increased compressive strength, surface nanotopography and bioactivity as compared to the original 45S5Bioglass® scaffold. The multifunctional scaffold was also successfully used as a drug delivery vehicle. Hence, this multifunctional scaffold can be used to deliver drugs, proteins or growth factors to treat bone related diseases. This is the first time such a multifunctional scaffold has been developed. P(3HB) were also used to prepare composite films with a combination of nanoscale bioactive glass (n-BG) for wound healing applications. The various analyses done showed that the addition of n-BG particles had increased the surface roughness of the films and improved the surface wettability. Surface mediated reactions such as clot formation was also found to decrease linearly with the increase in the amount of n-BG particles.

The microspheres were encapsulated with three model proteins BSA, Lucentis® and RNase A to understand the effect of morphology, drug distribution on the in vitro release profiles. The microspheres produced were of an average size of 2μm. P(3HB) microspheres investigated for use as drug delivery vehicles in the intraocular environment exhibited a gradual release of BSA over a period of time and the encapsulation process did not lead to any degradation of BSA. The % cumulative release of Lucentis® from both PBS and contact lens solution was for an extended period of 75 days. The microencapsulation process also did not lead to any degradation of Lucentis®. Finally the effects of solvent treatment encountered by the model protein Ribonuclease A (RNase A) during microencapsulation in P(3HB) microspheres were investigated. From the results, structural integrity of RNase A was maintained. A gradual decrease in the enzyme activity of RNase A was observed over a period of time.