Abstract
The incorporation of drugs into phosphorylcholine (PC) polymers coated onto coronary stent surfaces is one potential method of treatment for reducing restenosis, the reclosure of the artery after angioplasty treatment. This work on the characterisation of the swelling performance of thin PC polymer films represents a further extension of the study on biocompatible polymers. The broad aim of this work is to relate the PC polymer structure and film processing conditions to their swelling, drug loading and release kinetics. As the two highly sensitive and powerful techniques in film structure determination, both ellipsometry and neutron reflection have proved to be useful in characterising PC polymer films and drug release processes. Following an established ellipsometry measurement method, a two stage process consisting of diffusion and relaxation has been observed during the PC film swelling: this suggests an anomalous mechanism, and this performance is well described by the coupled diffusion and relaxation model developed by Berens and Hopfenberg. Furthermore, the swelling of PC polymer films was investigated as a function of cross-linking, annealing temperature, chemical composition, hydrophilic/hydrophobic ratios, film thickness and environmental conditions by ellipsometry measurements, and their effects on swelling kinetics well quantified. The structures of the PC polymer films (PC100B) with cross-linking groups have been further characterised by neutron reflection. The segregation of hydrophobic and hydrophilic domains was found in the PC films with different dimensions. The PC100B with deuterated dodecyl chains was used to highlight the interfacial structures of the PC films. The hydrophilic segments, including phosphorylcholine groups and hydroxypropyl groups, are preferentially adsorbed at the polymer/substrate interface. The hydrophobic dodecyl chains are expelled away from the silicon oxide surface. The main part of PC100B films is the middle uniform region with 40% of water in the sample annealed at 150°C, and 55% of water in the sample annealed at 50°C. The combination of the ellipsometry results and the drug release profiles from UV measurements indicates that the drug release pattern is strongly affected by the film swelling kinetics when the drug molecules and polymer matrix interact weakly. Otherwise, a strong interaction between the drug and the polymer will dominate the drug release behaviour from the PC polymer films.