1. INTRODUCTION

Hydrogels can be seen as intermediates between solids and liquids, exhibiting a rather complex mixture of properties of these two limiting states.1 Moreover, hydrogels are of great importance in living systems (e.g. the cytoskeleton is a hydrogel) and are of broad interest for a large variety of industrial products (e.g. as storage and separation media).

In recent years a large number of works ^{2 6} and also reviews7 ^,8 on the subject of so called intelligent macroscopic hydrogels were published. This class of hydrogels is of special interest, due to the volume phase transition encountered in these systems i.e. the collapse of the gel when a certain temperature (low critical solution temperature (LCST))6 ^{,9 11} or ionic strength12 is reached. Most of these intelligent gels are made of N-isopropyl acrylamide (NIPAM). In addition to the already mentioned applications NIPAM based gels exhibit bio-adhesive properties. This may allow for applications as drug delivery systems.

Copolymerisation with charged9 and uncharged co-monomers13 can be used to tune the transition temperature and other properties of these network materials.

However, the major disadvantage of these macroscopic gels is their rather slow response to the change of an external parameter. It may take several days until the equilibrium is reached.2 ^,3 Therefore, since the pioneering works of Pelton and Chibante14 there is growing interest in NIPAM based so-called microgel particles. Microgels are colloidal particles (diameter typically ranging from 100 nm to 1 ?m), which internally have the same...