Medium chain length polyhydroxyalkanoates, promising new biomedical materials for the future

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Abstract

Medium chain length polyhydroxyalkanoates, mcl-PHAs (C6–C14 carbon atoms), are polyesters of hydroxyalkanoates produced mainly by fluorescent Pseudomonads under unbalanced growth conditions. These mcl-PHAs which can be produced using renewable resources are biocompatible, biodegradable and thermoprocessable. They have low crystallinity, low glass transition temperature, low tensile strength and high elongation to break, making them elastomeric polymers. Mcl-PHAs and their copolymers are suitable for a range of biomedical applications where flexible biomaterials are required, such as heart valves and other cardiovascular applications as well as matrices for controlled drug delivery. Mcl-PHAs are more structurally diverse than short chain length PHAs and hence can be more readily tailored for specific applications. Composites have also been fabricated using mcl-PHAs and their copolymers, such as poly (3-hydroxyoctanoate) [P(3HO)] combined with single walled carbon nanotubes and poly(3-hydroxbutyrate-co-3-hydroxyhexanoate) [P(3HB-co-3HHx)] combined with hydroxyapatite. Because of these attractive properties of biodegradability, biocompatibility and tailorability, Mcl-PHAs and their composites are being increasingly used for biomedical applications. However, studies remain limited mainly to P(3HO) and the copolymer P(3HB-co-3HHx), which are the only mcl-PHAs available in large quantities. In this review we have consolidated current knowledge on the properties and biomedical applications of these elastomeric mcl-PHAs, their copolymers and their composites.

Introduction

Polyhydroxyalkanoates (PHAs) are polyesters of 3, 4, 5, and 6-hydroxyalkanoic acids which have the general structure shown in Fig. 1. PHAs are synthesized by numerous Gram positive and Gram negative bacteria through fermentation of carbon and serve as intracellular carbon and energy storage compounds [1], [2]. These are accumulated as cytoplasmic inclusions, the number per cell and size of which varies among different species. Usually, bacteria produce these granules during the stationary phase of growth, when subjected to an unbalanced growth condition, with excess carbon source and simultaneous limitation of nutrients such as oxygen, nitrogen, sulphur, magnesium and phosphorous [3], [4], [5]. Some bacteria however produce them without being subjected to any kind of nutritional constraints, for example, Alcaligenes latus. Depending on the number of carbon atoms in the monomeric unit, PHAs are classified as short chain length PHAs, scl-PHAs, that contain 3–5 carbon atoms, for example poly(3-hydroxybutyrate), P(3HB), poly(4-hydroxybutyrate), P(4HB), and medium chain length PHAs, mcl-PHAs, that contain 6–14 carbon atoms, for example poly(3-hydroxyhexanoate), P(3HHx), and poly(3-hydroxyoctanoate), P(3HO). Also, depending on the kind of monomer present, PHAs can be a homopolymer containing only one type of hydroxyalkanoate as the monomer unit, e.g., P(3HB), P(3HHx), or a heteropolymer containing more than one kind of hydroxyalkanoate as monomer units, e.g., poly(3-hydroxybutyrate-co-3-hydroxyvalerate), P(3HB-co-3HV), poly(3-hydroxyhexanoate-co-3-hydroxyoctanoate), P(3HHx-co-3HO), and poly-3-hydroxybutyrate-co-3-hydroxyhexanoate, P(3HB-co-3HHx) [6], [7].

In terms of the physical properties exhibited by scl and mcl-PHAs, mcl-PHAs have Tm values ranging between 40 and 60 °C and Tg values between −50 and −25 °C. These polymers are thermo elastomers, have low crystallinity, low tensile strength and high elongation to break [8]. Scl-PHAs exhibit a broader range of properties depending on the monomeric composition. For example, P(3HB) with a Tm of 180 °C and a Tg of 4 °C, is highly crystalline, brittle and stiff and has tensile strength comparable with that of polypropylene. On the other hand, P(4HB) with a Tm of 54 °C and a Tg of −49 °C is a malleable thermoplastic material whose tensile strength is comparable to that of polyethylene [9]. Introduction of a comonomer into the polymer backbone, as in the case of heteropolymers, greatly affects the polymer properties by increasing its flexibility, toughness and decreasing its stiffness [10]. For example, a copolymer like poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), P(3HB-co-3HHx), has a lower melting temperature, crystallinity and is more malleable than P(3HB). In fact P(3HB-co-3HHx) has similar mechanical properties to those of representative commercial polymers such as low density polyethylene (LDPE), which are used for making articles that require low temperature flexibility, toughness and durability such as all purpose plastic containers and plastic bags [11], as discussed below. The properties of PHAs vary considerably depending on their monomer content and hence can be tailored by controlling their compositions. For example, when P. putida GPO1 was grown on octanoate and varying mole percent of 10-undecenoate, the organism accumulated copolymers of mcl-PHAs with varying mole percent of individual monomers, all of which exhibited different thermal and molecular properties [12].

PHAs are biodegradable, biocompatible, exhibit piezoelectricity (which stimulates bone growth and aids in wound healing) and exhibit wide ranging physical and mechanical properties that arise from the diversity in their chemical structures. It is because of these properties that PHAs are gaining attention as the biomaterial of choice for various applications, particularly medical applications. In fact, a major milestone in the medical application of PHAs has been the approval of P(4HB) by the US Food and Drug Administration (FDA) as a biomaterial for use as absorbable suture [13]. Clearly the biomedical applications of PHAs depend on their physical properties. For instance short chain length PHAs like P(3HB), are hard and brittle in nature and hence they can be considered more suitable for hard tissue engineering or bone replacement material. In contrast, for applications in contact with soft tissue, more elastomeric and flexible materials are needed and hence mcl-PHAs and its copolymers are more attractive for applications such as heart valves, cardiac patches and other vascular applications, skin tissue engineering, wound healing and controlled drug delivery. In addition, due to the fact that mcl-PHAs are more structurally diverse than scl-PHAs, this structural diversity provides more flexibility in tailoring the physical and mechanical properties of mcl-PHAs to meet the requirement of the engineered tissue. In this context mcl-PHAs have been also investigated for bone tissue engineering as discussed below in this review. Because of these important properties, mcl-PHAs such as P(3HO), P(3HHx), copolymers like P(3HB-co-3HHx), P(3HO-co-3HHx) are being increasingly studied to develop osteosynthetic materials, surgical sutures, stents, scaffolds for tissue engineering and matrices for drug delivery [14], [15], [16]. However, in spite of their important and varied applications, studies on mcl-PHAs in general still remain limited mainly because of the lack of availability of these polymers in large quantities. Hence, most studies have concentrated mainly on the copolymer P(3HB-co-3HHx) and on P(3HO) containing different mol% of other monomers. Industrial production of P(3HB-co-3HHx) was carried out in 2001, albeit the large scale production of P(3HO) containing 96 mol% of 3-hydroxyoctanoic acid was carried out as recently as 2009 [17]. P(3HB-co-3HHx) with different 3HHx concentrations ranging from 0% to 20% are now available for biomedical investigations [18]. Thus, mcl-PHAs, because of their characteristic properties of biodegradability, biocompatibility and elasticity, are being increasingly studied and used as biomaterial of choice for several medical applications. Due to their emerging relevance, in this review we consolidate present knowledge on the properties and medical applications of this elastomeric class of PHAs, their copolymers and their composites.

Section snippets

Microrganisms involved in the production of mcl-PHAs and their copolymers

Mcl-PHAs were first discovered in 1983 when P. oleovorans was grown in octane [19]. Since then many fluorescent Pseudomonas sp., belonging to the rRNA homology group I have been used for their production (Table 1). To date more than 150 units of mcl-PHA monomers have been produced by culturing various Pseudomonas strains on different carbon substrates [20]. The versatility of Pseudomonas sp. in using a range of carbon sources and low substrate specificity of the mcl-PHA synthase, the key enzyme

Chemical structure

The physical and material properties of PHAs are greatly influenced by their monomer composition and chemical structure i.e. the length of the pendant groups which extend from the polymer backbone, the chemical nature of this pendant group and the distance between the ester linkages in the polymer backbone [25], [30]. The type of monomer incorporated in the growing polymer chain is in turn affected by the organism used, the culture conditions and the carbon source provided [1], [2]. For example

Fabrication of mcl-PHAs to form composites

As indicated above, interest in PHAs as biomaterials for various biomedical applications has increased in recent years because of their structural diversity, useful mechanical properties, biodegradibility, piezoelectricity and biocompatibility [30], [76], [77]. These properties of PHAs determine their potential applications in particular; scl-PHAs, because of their intrinsic mechanical properties are being considered for hard tissue regeneration. Kostopoulus and Karring [78] used P(3HB-co-3HV)

Medical applications of elastomeric PHAs

P(3HB) and P(3HB-co-3HV) are the most studied PHAs for medical applications. However, these scl-PHAs are brittle and stiff and some medical applications require more elastomeric polymers. Hence, research on the relatively more elastomeric mcl-PHAs and its copolymers has accelerated in the past few years. However, studies on mcl-PHAs have been limited because of the lack of availability of these polymers in large quantities, hence most of the current research has concentrated on the copolymer,

Biocompatibility

Biocompatibility is an essential requirement for a biomaterial. The biocompatibility of PHAs, like for any other biomaterial, is dependent on factors such as shape, surface porosity, surface hydrophilicity, surface energy, chemistry of the material, the environment where it is incorporated and its degradation products [73], [75], [128]. In tissue engineering, it is important that the cellular behaviour affected by the degradation products be considered for a comprehensive biocompatibility

Conclusions

Mcl-PHAs and their copolymers are elastomeric biodegradable and biocompatible polymers with low crystallinity and high elongation to break. The properties of the polymers can be tailored by controlling their compositions. Owing to their amenable properties, mcl-PHAs and their copolymers are increasingly being studied for biomedical applications. The wide range of applications of these polymers in biomedical implants, tissue engineering and drug delivery were reviewed. Studies on mcl-PHAs are

Acknowledgement

RR would like to thank Quintin Hogg Foundation of the University of Westminster (London, UK) for the financial support. Support from Dr. Ian Locke for the HaCaT cell culture work (University of Westminster, London, UK) and from Dr. Vehid Salih and Dr. Nicola Mordon (UCL Eastman Dental Institute, London, UK) for the SEM analysis of the HaCaT cells is also greatly appreciated.

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