James E. Kennedy

Cytotoxic effects induced by unmodified and organically modified nanoclays in the human hepatic HepG2 cell line

The term ‘nanoclay’ generically refers to the natural clay mineral, montmorillonite, with silica and alumina as the dominant constituents. The incorporation of nanoclays into polymeric systems dramatically enhances their barrier properties as well as their thermal and mechanical resistance. Consequently, nanoclays are employed in a wide range of industrial applications with recent studies reporting potential use in the modulation of drug release. With the increase in manufacturing of nanoclay‐containing products, information on the toxicological and health effects of nanoclay exposure is warranted. Thus, the objective of the present study was to evaluate the cytotoxicity of two different nanoclays: the unmodified nanoclay, Cloisite Na+®, and the organically modified nanoclay, Cloisite 93A®, in human hepatoma HepG2 cells. Following 24 h exposure the nanoclays significantly decreased cell viability. Cloisite Na+ induced intracellular reactive oxygen species (ROS) formation which coincided with increased cell membrane damage, whilst ROS generation did not play a role in Cloisite 93A‐induced cell death. Neither of the nanoclays induced caspase‐3/7 activation. Moreover, in the cell culture medium the nanoclays aggregated differently and this appeared to have an effect on their mechanisms of toxicity. Taken together, our data demonstrate that nanoclays are highly cytotoxic and as a result pose a possible risk to human health. Copyright

Effects of gamma ray and electron beam irradiation on the mechanical, thermal, structural and physicochemical properties of poly (ether-block-amide) thermoplastic elastomers.

Both gamma ray and electron beam irradiation are widely used as a means of medical device sterilisation. However, it is known that the radiation produced by both processes can lead to undesirable changes within biomedical polymers. The main objective of this research was to conduct a comparative study on the two key radiosterilisation methods (gamma ray and electron beam) in order to identify the more detrimental process in terms of the mechanical, structural, chemical and thermal properties of a common biomedical grade polymer. Poly (ether-block-amide) (PEBA) was prepared by injection moulding ASTM testing specimens and these were exposed to an extensive range of irradiation doses (5-200 kGy) in an air atmosphere. The effect of varying the irradiation dose concentration on the resultant PEBA properties was apparent. For instance, the tensile strength, percentage elongation at break and shore D hardness can be increased/decreased by controlling the aforementioned criteria. In addition, it was observed that the stiffness of the material increased with incremental irradiation doses as anticipated. Melt flow index demonstrated a dramatic increase in the melting strength of the material indicating a sharp increase in molecular weight. Conversely, modulated differential scanning calorimetry established that there were no significant alterations to the thermal transitions. Noteworthy trends were observed for the dynamic frequency sweeps of the material, where the crosslink density increased according to an increase in electron beam irradiation dose. Trans-vinylene unsaturations and the carbonyl group concentration increased with an increment in irradiation dose for both processes when observed by FTIR. The relationship between the irradiation dose rate, mechanical properties and the subsequent surface properties of PEBA material is further elucidated throughout this paper. This study revealed that the gamma irradiation process produced more adverse effects in the PEBA material in contrast to the electron beam irradiation process.

An evaluation of the thermal and mechanical properties of a salt-modified polyvinyl alcohol hydrogel for a knee meniscus application

The treatment of irreparable knee meniscus tears remains a major challenge for the orthopaedic community. The main purpose of this research was to analyse the mechanical properties and thermal behaviour of a salt-modified polyvinyl alcohol hydrogel, in order to assess its potential for use as an artificial meniscal implant. Aqueous poly vinyl alcohol was treated with a sodium sulphate solution to precipitate out the polyvinyl alcohol resulting in a pliable hydrogel. The freeze-thaw process, a strictly physical method of crosslinking, was employed to crosslink the hydrogel. Physical crosslinks in the form of crystalline regions were induced within the hydrogel structure which resulted in a large increase in mechanical resistance. Results showed that the optimal sodium sulphate addition of 6.6% (w/v) Na2SO4 in 8.33% (w/v) PVA causes the PVA to precipitate out of its solution. The effect of multiple freeze thaw cycles was also investigated. Investigation comprised of a variety of well-established characterisation techniques such as differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), mechanical analysis, rheometry and swelling studies. DSC analysis showed that samples cross-linked using the freeze thaw process display a thermal shift due to increased crosslink density. FTIR analysis confirmed crystallisation is present at 1142cm(-1) and also showed that no chemical alteration occurs when PVA is treated with sodium sulphate. Swelling studies indicated that that PVA/sodium sulphate hydrogels absorb less water than untreated hydrogels due to increased amounts of PVA present. Compressive strength analysis of PVA/sodium sulphate hydrogels prepared at -80°C displayed average maximum loads of 2472N, 2482.4N and 2476N of over 1, 3 and 5 freeze thaw cycles respectively. Mechanical analysis of the hydrogel indicated that the material is thermally stable and resistant to breakdown by compressive force. These properties are crucial for potential use as a meniscus or cartilage replacement. As such, the results of this study indicate that polyvinyl alcohol modified with sodium sulphate may be a suitable material for the construction of an artificial knee meniscus.

The rheological and thermal characteristics of freeze-thawed hydrogels containing hydrogen peroxide for potential wound healing applications.

The current study involves the development of a hydrogel carrier for a H(2)O(2) delivery system. In this work poly (vinyl alcohol) (PVA) and poly (acrylic acid) (PAA) based hydrogels were prepared, and their mechanical and physical properties examined. The novel aspect of this research is the differing functionality created by varying the concentration of H(2)O(2). The mechanical and thermal properties were determined by parallel plate rheometry and modulated differential scanning calorimetry (MDSC) respectively. The results indicated that the hydrogels containing H(2)O(2) are significantly weaker than those synthesised using water alone at test temperatures of 30 and 45 degrees C. MDSC analysis suggested that thermal transitions occur at temperatures that may make these hydrogels useful as temperature sensitive drug delivery systems. The chemical structure of the hydrogels was confirmed by means of attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), while swelling experiments in distilled water indicate that the swelling of the gels is temperature dependent.

Cell encapsulation and cryostorage in PVA–gelatin cryogels: incorporation of carboxylated ε‐poly‐L‐lysine as cryoprotectant

It is desirable to produce cryopreservable cell‐laden tissue‐engineering scaffolds whose final properties can be adjusted during the thawing process immediately prior to use. Polyvinyl alcohol (PVA)‐based solutions provide platforms in which cryoprotected cell suspensions can be turned into a ready‐to‐use, cell‐laden scaffold by a process of cryogelation. In this study, such a PVA system, with DMSO as the cryoprotectant, was successfully developed. Vascular smooth muscle cell (vSMC)‐encapsulated cryogels were investigated under conditions of cyclic strain and in co‐culture with vascular endothelial cells to mimic the environment these cells experience in vivo in a vascular tissue‐engineering setting. In view of the cytotoxicity DMSO imposes with respect to the production procedure, carboxylated poly‐L‐lysine (COOH–PLL) was substituted as a non‐cytotoxic cryoprotectant to allow longer, slower thawing periods to generate more stable cryogels. Encapsulated vSMC with DMSO as a cryoprotectant responded to 10% cyclic strain with increased alignment and proliferation. Cells were stored frozen for 1 month without loss of viability compared to immediate thawing. SMC‐encapsulated cryogels also successfully supported functional endothelial cell co‐culture. Substitution of COOH–PLL in place of DMSO resulted in a significant increase in cell viability in encapsulated cryogels for a range of thawing periods. We conclude that incorporation of COOH–PLL during cryogelation preserved cell functionality while retaining fundamental cryogel physical properties, thereby making it a promising platform for tissue‐engineering scaffolds, particularly for vascular tissue engineering, or cell preservation within microgels. Copyright

Synthesis and Characterisation of Styrene Butadiene Styrene Based Grafted Copolymers for Use in Potential Biomedical Applications

In the annals of history the evolution of the synthetic rubber industry can be traced to the early 1930s where the first emulsion polymerised styrene butadiene rubber known as Buna S was prepared by I. G. Farbenindustrie in Germany. But it was not until the US Government in 1940 established the Rubber Reserve Company, a stockpile of natural rubber and the development of a synthetic rubber program came into full fruition. However, when the United States entered World War II, the synthetic rubber plants owned by the US Government were either closed or sold to private industry between the years 1946 and 1955, and from this the development of this formidable technology began. In the early 1960’s one primary objective prevailed and that was the economical polymerisation of polyisoprene with a high cis–1,4 structure, which is the synthetic version of natural rubber(Holden & Hansen, 2004). Around this time, workers at Shell investigated lithium metal initiators for isoprene polymerisation and found that alkyllithiums yielded some interesting results. In particular, there was no chain termination or chain transfer steps present. Thus, when all of the original monomer was consumed, the polymer chain still remained active and could initiate further polymerisation if more monomer, either of the same or different species, were added (Holden & Hansen, 2004). Parallel with these developments, tri-block copolymers using difunctional initiators were also reported in the literature (Szwarc et al., 1956; Szwarc, 1956). These block copolymers were produced under conditions that gave polydiene segments a relatively low 1,4 content(Holden & Hansen, 2002). However, poor elastomeric properties were acknowledged whereby the rheological properties of both polybutadiene (PB)(Gruver, 1964) and isoprene(Holden, 1965) resulted in the materials exhibiting Newtonian behaviour and the viscosities of the pure polymers approach constant values as the shear rate approaches zero. This behaviour resulted in bales of these elastomers appearing to be solid but in fact behaved as viscous liquids which hindered both their storage and commercial attractiveness. In light of this, Shell chemical research polymerised polydiene elastomers with various molecular weights to combat this problem (Holden & Hansen, 2004). Later studies included work on block copolymers resulting in the formation of a material which contained short blocks of polystyrene on either end of the elastomeric chain to form a styrene butadiene styrene (SBS), as illustrated in Figure 1. In contrast to the diene homopolymer, these block copolymers demonstrated, non-Newtonian

Development of a novel porous cryo-foam for potential wound healing applications

This body of work describes the development of a porous hydrogel for wound healing applications. In the present study poly (vinyl alcohol) (PVA) and poly (acrylic acid) (PAA) based hydrogels were prepared, and their properties were examined. Varying concentrations of the polymers and distilled water were used to prepare the hydrogels. The use of a high shear mixer, for foaming the PVA and PVA/PAA gels, and how this physical change can affect the structure and porosity of the hydrogel in question, represents a key feature of this work. The mechanical and thermal properties were determined by parallel plate rheometry and modulated differential scanning calorimetry (MDSC) respectively. The results indicated that the hydrogels containing low concentration of PVA and high volume of H2O are significantly weaker than those synthesised with higher concentrations of PVA. The thermal analysis shows distinct endotherms and provides evidence of crystallisation. The chemical structure of the hydrogels was confirmed by means of attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR).

An evaluation of the biocompatibility properties of a salt-modified polyvinyl alcohol hydrogel for a knee meniscus application

The treatment of irreparable knee meniscus tears remains a major challenge for the orthopaedic community. The main purpose of this research was to analyse the biocompatibility properties of a salt-modified polyvinyl alcohol hydrogel, in order to assess its potential for use as an artificial meniscal implant. Aqueous polyvinyl alcohol was treated with a sodium sulphate solution to precipitate out the polyvinyl alcohol resulting in a pliable hydrogel. Cytotoxicological analysis indicates that PVA/sodium sulphate hydrogels display a non-toxic disposition and were found to be compatible with the L929 fibroblast cell line.

Biomechanical analysis of a salt-modified polyvinyl alcohol hydrogel for knee meniscus applications, including comparison with human donor samples

The primary objective of this research was the biomechanical analysis of a salt-modified polyvinyl alcohol hydrogel, in order to assess its potential for use as an artificial meniscal implant. Aqueous polyvinyl alcohol (PVA) was treated with a sodium sulphate (Na2SO4) solution to precipitate out the polyvinyl alcohol resulting in a pliable hydrogel. The freeze-thaw process, a strictly physical method of crosslinking, was employed to crosslink the hydrogel. Development of a meniscal shaped mould and sample housing unit allowed the production of meniscal shaped hydrogels for direct comparison to human meniscal tissue. Results obtained show that compressive responses were slightly higher in PVA/Na2SO4 menisci, displaying maximum compressive loads of 2472N, 2482N and 2476N for samples having undergone 1, 3 and 5 freeze-thaw cycles respectively. When compared to the human meniscal tissue tested under the same conditions, an average maximum load of 2467.5N was observed. This suggests that the PVA/Na2SO4 menisci are mechanically comparable to the human meniscus. Biocompatibility analysis of PVA/Na2SO4 hydrogels revealed no acute cytotoxicity. The work described herein has innovative potential in load bearing applications, specifically as an alternative to meniscectomy as replacement of critically damaged meniscal tissue in the knee joint where repair is not viable.

Characterisation of polyamide 11/copper antimicrobial composites for medical device applications

Direct incorporation of antimicrobial additive into the polymer matrix is a cost effective approach for the development of polymer/metal antimicrobial composites. Application of these antimicrobial composite systems for manufacturing medical devices addresses the issue of device related infections. In the present study, commercially available inorganic copper based additive, Plasticopper, was incorporated into a Polyamide 11(PA 11) matrix during the polymer processing stage. These polymer composites were evaluated for their morphological, mechanical, antimicrobial and ion release properties. Isothermal crystallisation studies showed that the copper additive acted as a nucleating agent and promoted faster crystallisation. Short term mechanical studies confirmed that the incorporation of copper has reinforcing effect on the composites with 5 and 10% copper loadings and did not adversely affect the short-term mechanical performance of the polymer composites. These composite systems were shown to be active against Escherichia coli ATCC 8739 with >99.99% reduction in bacterial population. Corresponding ion release profiles for these composites indicated long term antimicrobial activity.