Silicone-based Hydrogels for Use in Biomedical Applications

Abstract

Hydrogels can be explained in a variety of descriptions, but they are overarchingly defined as threedimensional polymer structures with the ability to absorb and retain large amounts of water. This project aimed to synthesise variations of silicone-based hydrogels with formulations that were inspired by commercially available contact lenses, with N-vinyl-pyrrolidone (NVP), N,N-dimethylacrylamide (DMA) and tris(trimethylsiloxy)-3-methylacryloxpropyl silane (TRIS) as monomers, and determine whether changes in reaction conditions (the use of a diluent and/or post-cure) could trigger changes in their fundamental characteristics including water content, permeation, partitioning and morphology. These characteristics are important when determining the suitability of a hydrogel in biomedical applications, particularly those that rely on the ability of a hydrogel membrane to absorb and release substances. The equilibrium water content (EWC) of the hydrogels was determined and differential scanning calorimetry (DSC) was utilised to determine the freezing water content (FWC), ice-like water content (ILWC) and polymer-associated water content (PAWC) whilst non-freezing water content was extrapolated. Permeation studies were conducted on a single-salt and multiple-salt basis, making use of calcium, sodium, potassium and magnesium chloride salts within a hydrated ion radius of 0.31 to 0.39 nm. Tests were carried out to determine partition coefficients, using the same salts as those used in permeation studies. Comparative elemental analysis studies of the hydrogels were conducted using energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) whilst environmental scanning electron microscopy (ESEM) captured images of the hydrogels to determine the occurrence of phase separation. The different water types within the hydrogels were identified and their quantities varied depending on their formulation. Permeability wise, the hydrogels showed an ability to elute salts within a mixture on the basis of size/charge, which was not explicitly seen on a single-salt basis – the order of salt permeation did not follow their size/charge trends. Relationships between the established parameters were considered and discussed where applicable. Opportunities for further study were identified and suggestions for the suitability of the hydrogels in biomedical applications were considered.