Clement L. Higginbotham

The synthesis of novel pH-sensitive poly(vinyl alcohol) composite hydrogels using a freeze/thaw process for biomedical applications

Physically cross-linked hydrogels composed of 75% poly(vinyl alcohol) PVA and 25% poly(acrylic acid) were prepared by a freeze/thaw treatment of aqueous solutions. Between 0.5 and 1wt% of aspirin was incorporated into the systems. The purpose of the research was the development of a novel pH-sensitive hydrogel composite for the delivery of aspirin to wounds. Extensive research has being conducted on freeze/thaw poly(vinyl alcohol) hydrogels for use in active pharmaceutical ingredient (API) delivery. However very little research has been reported on the effects of an API on the overall properties of a freeze/thaw hydrogel. From the rheological analysis undertaken it was apparent that aspirin has a limiting effect on the formation of hydrogen bonding leading to hydrogels with reduced mechanical strength. To counteract this, a novel hydrogel system was developed encompassing a reinforcing film in the centre of the hydrogels. Freezing profiles were obtained to gain a better knowledge of the freezing behaviour of the hydrogels during the formation stage. Thermograms obtained from modulated differential scanning calorimetry (MDSC) indicated that the aspirin lowered the glass transition temperatures (T(g)) of the constituent polymers. The pH-sensitive nature of the hydrogels was apparent from solvent uptake studies carried out. Increasing alkaline media led to a greater degree of swelling due to increased ionisation of PAA. The hydrogels exhibited non-Fickian release kinetics. The release rates were relatively slow with total release achieved at between 30 and 40 h. The quantity of drug incorporated was found to influence the release rates considerably.

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

Hydrogel/bioactive glass composites for bone regeneration applications: Synthesis and characterisation

Due to the deficiencies of current commercially available biological bone grafts, alternative bone graft substitutes have come to the forefront of tissue engineering in recent times. The main challenge for scientists in manufacturing bone graft substitutes is to obtain a scaffold that has sufficient mechanical strength and bioactive properties to promote formation of new tissue. The ability to synthesise hydrogel based composite scaffolds using photopolymerisation has been demonstrated in this study. The prepared hydrogel based composites were characterised using techniques including Fourier Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), Energy-dispersive X-ray spectrometry (EDX), rheological studies and compression testing. In addition, gel fraction, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), porosity and swelling studies of the composites were carried out. It was found that these novel hydrogel bioglass composite formulations did not display the inherent brittleness that is typically associated with bioactive glass based bone graft materials and exhibited enhanced biomechanical properties compared to the polyethylene glycol hydrogel scaffolds along. Together, the combination of enhanced mechanical properties and the deposition of apatite on the surface of these hydrogel based composites make them an ideal candidate as bone graft substitutes in cancellous bone defects or low load bearing applications.

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.

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.

Effect of serum concentration on the cytotoxicity of clay particles

Nanoparticle cytotoxicity testing based on in vitro methods frequently lack consistency. Even the inclusion of the commonly employed growth supplement, FCS (fetal calf serum), generates variable results. Thus, our object was to investigate the effect of FCS concentration on the cytotoxic behaviour of the unmodified nanoclay, Cloisite® Na+. Human monocytic U937 cells in medium supplemented with 5% FCS, 2.5% FCS or serum‐free medium were treated with 1 mg/ml Cloisite Na+. Cell growth in 2.5% FCS was significantly inhibited by Cloisite Na+ within 48 h, whereas little effect was seen with a supplement of 5% FCS. Without serum, cell growth was inhibited and Cloisite Na+ had a detrimental effect on these cells. In media supplemented with FCS, the nanoclays agglomerated together to form large bundles, whereas they were evenly dispersed throughout the medium in the absence of serum. Clay particles, therefore, have cytotoxic properties that may be linked to their dispersion pattern. These adverse effects seem to be masked by 5% FCS. Serum supplementation is an important consideration in the toxicological assessments of nanomaterials on cells, which needs to be addressed in the standardization of in vitro testing methods.

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

In vitro fibroblast and pre-osteoblastic cellular responses on laser surface modified Ti-6Al-4V.

The success of any implant, dental or orthopaedic, is driven by the interaction of implant material with the surrounding tissue. In this context, the nature of the implant surface plays a direct role in determining the long term stability as physico-chemical properties of the surface affect cellular attachment, expression of proteins, and finally osseointegration. Thus to enhance the degree of integration of the implant into the host tissue, various surface modification techniques are employed. In this work, laser surface melting of titanium alloy Ti-6Al-4V was carried out using a CO2 laser with an argon gas atmosphere. Investigations were carried out to study the influence of laser surface modification on the biocompatibility of Ti-6Al-4V alloy implant material. Surface roughness, microhardness, and phase development were recorded. Initial knowledge of these effects on biocompatibility was gained from examination of the response of fibroblast cell lines, which was followed by examination of the response of osteoblast cell lines which is relevant to the applications of this material in bone repair. Biocompatibility with these cell lines was analysed via Resazurin cell viability assay, DNA cell attachment assay, and alamarBlue metabolic activity assay. Laser treated surfaces were found to preferentially promote cell attachment, higher levels of proliferation, and enhanced bioactivity when compared to untreated control samples. These results demonstrate the tremendous potential of this laser surface melting treatment to significantly improve the biocompatibility of titanium implants in vivo.

Review of Multifarious Applications of Poly (Lactic Acid)

ABSTRACT Poly (lactic acid) is considered to be a promising alternative to petroleum-based polymers due to its renewability, biodegradability, biocompatibility, and good mechanical properties. Because of the high cost, the applications of poly (lactic acid) were limited to the medical field. Over the past decade, improvements in polymerization allow the economical mass production of high molecular weight poly (lactic acid). Therefore, the applications of poly (lactic acid) have recently spread to domestic, commercial packaging, and textile applications. This review outlines the chemical, thermal characteristics of poly (lactic acid) and discusses the use of poly (lactic acid) in medical applications such as sutures, stents, drug carrier, orthopaedic devices, scaffolds, as well as commercial applications in textile and packaging fields with superior properties such as high wicking performance, good dyeability, antibacterial feature, good ultraviolet resistance, high water vapor transmission rates, shrink wrapping, and dead fold property. While the drawbacks of poly (lactic acid) utilized in these fields are also discussed. It is clear that the advantages of using poly (lactic acid) outlined in this review will ensure that the market for poly (lactic acid) products will continue to expand. GRAPHICAL ABSTRACT

Fabrication and in vitro biological evaluation of photopolymerisable hydroxyapatite hydrogel composites for bone regeneration

The aim of this study was to improve the bioactive and compressive properties of photopolymerisable polyethylene glycol hydrogels with the incorporation of hydroxyapatite at different loadings. The synthesis of pure hydroxyapatite was verified through Fourier transform infrared spectroscopy (FTIR) analysis by the complete reaction of all constituents. The formation of a bioactive layer of the hydrogel based composites was confirmed through the formation of carbonate hydroxyapatite after soaking the samples in simulated body fluid. The incorporation of hydroxyapatite into the system resulted in an increase in Young’s modulus from 4.36 to 12.73 MPa and an increase in the stress at limit value from 1.20 to 4.42 MPa. This was due to the hydroxyapatite absorbing the compressive load, the polymer matrix distributing the load, a reduction in swelling and the presence of physical crosslinking between both components. Drug dissolution testing showed that the release rate of a drug from the hydrogels was dependent on the molecular weight of the polymer and the type of drug used.