Hydrogels for dermal applications

This thesis is concerned with the development of hydrogels that adhere to skin and can be used for topical or trans dermal release of active compounds for therapeutic or cosmetic use. The suitability of a range of monomers and initiator systems for the production of skin adhesive hydro gels by photopolymerisation was explored and an approximate order of monomer reactivity in aqueous solution was determined. Most notably, the increased reactivity of N-vinyl pyrrolidone within an aqueous system, as compared to its low rate of polymerisation in organic solvents, was observed. The efficacy of a series of photoinitiator systems for the preparation of sheet hydrogels was investigated. Supplementary redox and thermal initiators were also examined. The most successful initiator system was found to be Irgacure 184, which is commonly used in commercial moving web production systems that employ photopolymerisation. The influence of ionic and non-ionic monomers, crosslinking systems, water and glycerol on the adhesive and dynamic mechanical behaviour of partially hydrated hydrogel systems was examined. The aim was to manipulate hydrogel behaviour to modify topical and transdermal delivery capability and investigated the possibility of using monomer combinations that would influence the release characteristics of gels by modifying their hydrophobic and ionic nature. The copolymerisation of neutral monomers (N-vinyl pyrrolidone, N,N-dimethyl acrylamide and N-acryloyl morpholine) with ionic monomers (2-acrylamido-2-methylpropane sulphonic acid; sodium salt, and the potassium salt of 3-sulphopropyl acrylate) formed the basis of the study. Release from fully and partially hydrated hydrogels was studied, using model compounds and a non-steroidal anti-inflammatory drug, Ibuprofen. Release followed a common 3-stage kinetic profile that includes an initial burst phase, a secondary phase of approximate first order release and a final stage of infinitesimally slow release such that the compound is effectively retained within the hydrogel. Use of partition coefficients, the pKa of the active and a knowledge of charge-based and polar interactions of polymer and drug were complementary in interpreting experimental results. In summary, drug ionisation, hydrogel composition and external release medium characteristics interact to influence release behaviour. The information generated provides the basis for the optimal design of hydrogels for specific dermal release applications and some understanding of the limitations of these systems for controlled release applications.