ABSTRACT
This article is a summary of work the author had the privilege to be involved in at the laboratories of Professors Η. E. Junginger and J. C. Leyte, at the University of Leiden, The Netherlands. The data presented in this article have been reported in earlier publications [1-4], but not in a single, comprehensive overview like the present one. The article seeks to make a contribution to a lively discussion between two schools of thought about water structures in hydrogels. One school favors the opinion that based on calorimetric experiements one can distinguish between thermodynamically different classes of water in hydrogels, a concept that has always had great appeal in areas of pharmaceutical technology. The other school teaches that the irregular behavior of many hydrogels in calorimetric experiments is a consequence of the development of non-equilibrium conditions, and are, by them selves, not an indication for the existence of different classes of water in these systems. Hydrogels are often described as polymeric materials, that swell in water, but do not dissolve, having a water content of about 20% or more. This definition is rather broad, and a wide variety of hydrogels has been studied for many different purposes. This is illustrated in an impressive way by the work of Allan Hoffman, whose group over the years has investigated such aspects as hydrogel synthesis and characterization, thermal reversibility of swelling, protein delivery, hydrogel grafting for hemocompatibility and enzyme immobilization [5-10].
