Electrospinning of locust bean gum and polysulfides: towards natural and responsive scaffolds for biomedical applications

Abstract

In this PhD thesis, investigations into the feasibility of using locust bean gum and polysulfides as biomaterials were conducted. With a growing uncertainty of future petroleum supplies and the need to reduce the quantity of polymer waste, the use of materials from natural sources is ever more important. Locust bean gum (LBG) is one potential natural resource that is currently underutilised. As a plant-based polysaccharide, locust bean gum is considered to be non-toxic, non-mutagenic, and non-teratogenic, indicating the suitability of the polymer for biomedical applications. Polysulfides are another class of polymers that have potential in biomedical applications due to their REDOX-responsive nature and antioxidant activity. The next generation of biomaterials will adapt to their environment and responsive polymers provide the opportunity to do this. Polysulfides such as poly(propylene sulfide) are oxidation-responsive, making them particularly suited to applications dealing with inflammation, where there is a high oxidative potential. The investigation consists of solubility studies and functionalisation of LBG, followed by electrospinning as a blend with poly(ethylene oxide) (PEO). Subsequently, the electrospinning of polysulfides as blends with PEO and poly(e-caprolactone) (PCL) was explored, followed by biocompatibility studies. LBG was shown to exhibit a ‘partial solubility’ in water, this ‘partial solubility’ led to a sigmoidal solubility curve forming as the solubilisation temperature increased from room temperature to 80°C. The sigmoidal nature of the curve is theorised to arise due to chain to chain variation in mannose:galactose ratio seen in the polymer. The soluble fraction of LBG was increased from 50% up to over 90% through increasing the solubilisation temperature, purification, and functionalisation to form carboxymethylated LBG (CMLBG). To carboxymethylate the polymer, it was heated with chloroacetic acid in the presence of an alkali. Thin films of LBG were produced by polymer solution casting, and thin fibre mats were produced by electrospinning. Crude LBG was formed into thin films without plasticisers for the first time. Both crude LBG and CMLBG were electrospun as a blend with PEO and the fibres were analysed using optical and SEM microscopy. The latter of these fibres were homogenous, with a unimodal fibre diameter distribution around 100-200 nm in diameter. To produce stimuli-responsive fibres, PEO was electrospun with a suspension of oxidation-sensitive poly(propylene sulfide) nanoparticles (PPS-NPs). After SEM, EDS, DLS, and an enzyme-linked immunosorbent assay (ELISA), these particles were shown to have no adverse effect on the homogenous nature of the electrospun fibres and retain their size and anti-inflammatory character after electrospinning. The anti-inflammatory PPS-NPs were shown to be cytocompatible for 48 hours, therefore these fibres have potential for use in wound dressings since inflamed sites in the body have a high oxidative potential. Electrospun poly(propylene sulfide-co-ethylene sulfide) (PPS-ES) fibres were also produced by electrospinning with PCL. These PCL/PPS-ES fibres had diameters ranging from 500 nm (when containing 50wt% PPS-ES) and 1500 nm (when containing 10wt% PPS-ES). The fibres produced from PCL/PPS-ES blends had a significantly higher UTS compared to pure PCL fibres (5.3 MPa v 1.2 MPa). The degradation of PCL/PPS-ES electrospun fibre mats incubated in an aqueous sodium acetate buffer solution with a pH of 7.2 at 37°C was investigated for 24 weeks. During this time, the tensile properties of the fibres did not decrease. The PPS-ES showed no signs of degradation using SEC, whereas the PCL lost roughly 10% of its peak molecular weight over the 24-week period. No visible signs of fibre degradation were seen using SEM after 24 weeks, suggesting the polymer would be suitable for long term applications, such as hernia meshes. In this work, both locust bean gum and polysulfides have shown promise in biomedical applications and have been processed to form homogenous fibres via electrospinning. The polymers can be combined as early work shows that PPS-NPs can be incorporated into CMLBG/PEO electrospun fibres, utilising both natural and responsive materials together. The potential applications of PPS-NP loaded CMLBG/PEO fibres include topical patches and wound dressings. PCL/PPS-ES is more suited to longer-term applications such as tissue engineering scaffolds and hernia meshes. However, further biocompatibility studies of LBG and PPS-ES are required to confirm the preliminary results shown here.