Michael J.D. Nugent

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.

Extraction Method Plays Critical Role in Antibacterial Activity of Propolis-Loaded Hydrogels

Extracted propolis has been used for a long time as a remedy. However, if the release rate of propolis is not controlled, the efficacy is reduced. To overcome this issue, extracted propolis was added to a cryogel system. Propolis collected from southern Brazil was extracted using different methods and loaded at different concentrations into polyvinyl alcohol (PVA) and polyacrylic acid hydrogels as carrier systems. The material properties were investigated with a focus on the propolis release profiles and the cryogel antibacterial properties against 4 different bacteria, namely: Staphylococcus aureus, Escherichia coli, Salmonella typhimurium, and Pseudomonas putida. Swelling studies indicated that the swelling of the hydrogel was inversely related to propolis content. In addition, propolis release studies indicated a decreased release rate with increased propolis loading. PVA and PVA/polyacrylic acid-loaded propolis were effective against all 4 bacteria studied. These results indicate that the efficacy of propolis can be enhanced by incorporation into hydrogel carrier systems and that hydrogels with higher concentrations of propolis can be considered for use as bactericide dressing.

The Effect Acetic Acid has on Poly(N-Vinylcaprolactam) LCST for Biomedical Applications

ABSTRACT The aim of this research was to synthesize Poly(N-vinylcaprolactam) (PNVCL) with the incorporation of hydrophilic acetic acid with thermosensitive N–vinylcaprolactam for potential drug delivery applications. Preparation of the heterogeneous mixture involved photopolymerization of a combination of N–vinylcaprolactam and acetic acid at different ratios. By altering the feed ratio, hydrogels were synthesized to have lower critical solution temperature close to physiological temperature. This ability to shift the phase transition temperature of PNVCL provides excellent flexibility in tailoring transitions to suit physiological temperature, inheriting great potential in drug delivery. GRAPHICAL ABSTRACT

Electrospinning of Hydrogels for Biomedical Applications

The field of biomedical applications for hydrogels requires the development of nanostructures with specific controlled diameter and mechanical properties. Nanofibers are ideal candidates for these advanced requirements, and one of the easiest techniques that can produce one-dimensional nanostructured materials in fibrous form is the electrospinning process. This technique provides extremely thin fibers with controlled diameter and highly porous microstructure with interconnected pores. Electrospinning demonstrates extreme versatility allowing the use of different polymers for tailoring properties and applications. It is a simple cost-effective method for the preparation of scaffolds. In this section, we will discuss recent and specific applications with a focus on their mechanisms. As such, we conclude this section with a discussion on perspectives and future possibilities on this field.

Structure Response for Cellulose-Based Hydrogels Via Characterization Techniques

Hydrogels are three-dimensional cross-linked polymeric networks capable of imbibing substantial amounts of water or biological fluids and are widely used in biomedical applications, especially in pharmaceutical industry as drug delivery systems. Although their solvent content can be over 99%, hydrogels still retain the appearance and properties of solid materials, and the structural response can include a smart response to environmental stimuli (pH, temp, ionic strength, electric field, presence of enzyme, etc.) These responses can include shrinkage or swelling. Cellulose-based hydrogels are one of the most commonly used materials and extensively investigated due to the widespread availability of cellulose in nature. Cellulose is the most abundant renewable resource on earth that is intrinsically degradable. Additionally, the presence of hydroxyl groups results in fascinating structures and properties. Also, cellulose-based hydrogels with specific properties can be obtained by combining it with synthetic or natural polymers. This chapter surveys different characterization for cellulose hydrogels and the structure-response relationship. As such we would describe the techniques involved for characterizing cellulose-based hydrogels and their response in terms of their morphology such as polarized optical microscopy (POM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), their stability by thermal properties (often with differential scanning calorimetry, DSC), and structure response such as Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR). In addition, we give a focus on measuring the mechanical properties of superabsorbent hydrogels giving examples with cellulose where applicable. Finally, we describe the techniques for analyzing biological techniques and the applications with cellulose.