John A. Killion

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.

A rapid crosslinking injectable hydrogel for stem cell delivery, from multifunctional hyperbranched polymers via RAFT homopolymerization of PEGDA

Stem cell therapies have attracted much attention for the last few decades in the field of regenerative medicine and tissue engineering. The 3-dimensional (3D) microenvironment surrounding the transplanted stem cells plays an essential role that influences the cell fate and behaviors. Thus advanced functional biomaterials and extracellular matrix (ECM) replacements with adjustable chemical, mechanical and bioactive properties are requisites in this field. In this study, PEG-based hyperbranched multifunctional homopolymers were developed via RAFT homopolymerization of the divinyl monomer of poly(ethylene glycol) diacrylate (PEGDA). Due to its high degree of multi-acrylate functionality, the hyperbranched polyPEGDA can rapidly crosslink with a thiolated hyaluronic acid under physiological conditions and form an injectable hydrogel for cell delivery. In addition, by simply varying the synthesis conditions such as the reaction time and the ratio of the monomer to the chain transfer agent (CTA), the polymer molecular weight, acrylate functionality degree and the cyclized/hyperbranched polymeric architecture can be finely controlled in a one-step reaction. The gelation speed and the mechanical properties of this hydrogel can be easily adjusted by altering the crosslinking conditions. Rat adipose-derived stem cells (rASCs) were embedded into the in situ crosslinked hydrogels, and their cellular behavior such as the morphology, viability, metabolic activity and proliferation were fully evaluated. The results suggested that the hydrogel maintained good cell viability and it can be easily modified with other bioactive signals, which provide this injectable hydrogel delivery system with good potential for polymeric biomaterials and tissue regeneration applications.

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.

Development of novel chitosan-poly(N,N-diethylacrylamide) IPN films for potential wound dressing and biomedical applications

Novel interactive and thermoresponsive interpenetrating polymer network (IPN) films, which are transparent, permeable to oxygen, and have the potential to be easily stripped from a wound bed, were synthesised using rapid photopolymerisation and crosslinking of DEAAm in the presence of chitosan. This study provides the first evaluation and optimisation of a UV-polymerised chitosan–PDEAAm IPN composite film for application in wound dressings. FTIR spectroscopy and DSC analysis were used to initially characterise the resulting films. Modulated differential scanning calorimetry results showed that the dressings exhibited lower critical solution temperatures in the desired range, while the samples were also observed to undergo temperature-dependent swelling behaviour. This thermosensitive property would potentially allow the dressings to be easily detachable, which would enable frequent dressing changes if desired without causing further injury to healing tissues. Furthermore, the water content values recorded are in the typical and desired ranges for commercial wound dressings.

Compressive Strength and Bioactivity Properties of Photopolymerizable Hybrid Composite Hydrogels for Bone Tissue Engineering

The drug release capabilities of synthetic bone scaffolds have often been overlooked. In this study novel poly(ethylene) glycol and beta-tricalcium phosphate hydrogel composites were photopolymerized and evaluated in terms of mechanical strength, bioactivity, antimicrobial release profile, and the efficacy of released antimicrobials. Youngs modulus values ranged between 4.36 and 8.70 MPa. This increase was associated with the physical bonding interaction between polymer and bioceramic. Bioactivity was confirmed by the formation of globular crystals. Drug release studies showed the diffusion of vancomycin from hydrogel composites can be controlled by the hydrogels’ three-dimensional structure. Moreover, vancomycin loaded samples showed activity against Staphylococcus aureus.

Evaluation of Novel Antibiotic-Eluting Thermoresponsive Chitosan-PDEAAm Based Wound Dressings

Thermoresponsive interpenetrating polymer networks were synthesized via rapid photo-polymerization and crosslinking of DEAAm in the presence of chitosan. The dressings are transparent, permeable to moisture vapor but impermeable to bacteria. They possessed negative temperature-dependent swelling properties, hence, swelled and became less adhesive at low temperatures, as demonstrated using peel adhesion tests. Water vapor permeability values of the chitosan-PDEAAm hydrogel films were in the typical and desired range for commercial wound dressings. Antibacterial activity studies demonstrated the ability of the dressings to control the release of incorporated antibiotics, which can enhance wound healing by enabling bacterial inhibition.

The effect of processing conditions for polylactic acid based fibre composites via twin-screw extrusion

Hemp, jute and lyocell fibres were incorporated into polylactic acid via twin-screw extrusion using three screw configurations, with varying lengths of mixing sections, in order to reduce the levels of shear and fibre attrition. When mixing zones were reduced, the measured fibre lengths increased and as a result the tensile properties of polylactic acid composites were improved. Similarly impact properties were observed to improve as fibre length increased. However, by increasing the fibre length in polylactic acid composites, fibre surface area within the composite was reduced and subsequently the rate of biodegradation decreased. Composites prepared using different extrusion temperature profiles were shown to have vastly different mechanical properties and in all cases composites produced using low temperature profiles exhibited superior properties to those produced at higher temperatures, indicating thermal degradation at the more elevated temperatures. For example, 50 wt% jute composites exhibited increases of 20.9% and 199% in tensile strength and flexural modulus, the greatest improvement of all composite types at that loading.

Melt Extruded Bioresorbable Polymer Composites for Potential Regenerative Medicine Applications

ABSTRACT Biodegradable polymers—polyethylene oxide and poly (ϵ-caprolactone)—were melt extruded with β-tricalcium phosphate. Breakdown analysis revealed that the percentage increase in bioceramic caused a prolonged degradation rate, with samples containing 20 wt% β-tricalcium phosphate losing significantly less weight over time in comparison to the control sample. Compression testing of samples following submission in aqueous environments revealed the composites exhibited enhanced strength with increasing bioceramic loading. The mechanical properties were significantly reduced over a period of 5 weeks. It was found that hot-melt extrusion of β-tricalcium phosphate is a viable and effective method of producing novel composite scaffolds with potential for regenerative medicine applications. GRAPHICAL ABSTRACT

Effect of Compatibilizer Content on the Mechanical Properties of Bioplastic Composites via Hot Melt Extrusion

Polyhydroxybutyrate fiber biocomposites were prepared via twin screw extrusion. Results show mechanical properties of short natural fiber composites can be greatly improved with the addition of a compatibilizer. Compatibilizer composites exhibited an increased resistance to water absorption at low temperatures. A considerable increase in flexural modulus was also observed, in particular for the jute fiber composites. The dispersion of fibers was visibly improved as was observed using optical microscopy which allowed for a more even transfer of stress throughout the composite matrix. Lyocell composites however continued to display fiber agglomerates and although these were visibly reduced as PHB-g-MA content increased.