Synthesis and Unusual Swelling Behavior of Combined Cationic/Non-Ionic Hydrogels Based on Chitosan Mehrdad Yazdani-Pedram,* 1 Cristian Tapia, 1 Jaime Retuert, 2 Jose ´ Luis Arias 3 1 Facultad de Ciencias Quı ´micas y Farmace ´uticas y Centro para la Investigacio ´n Interdisciplinaria Avanzada en Ciencia de los Materiales (CIMAT), Universidad de Chile, casilla 233, Santiago, Chile Fax: þ56 2 678 2868; E-mail: myazdani@ciq.uchile.cl 2 Facultad de Ciencias Fı ´sicas y Matema ´ticas y Centro para la Investigacio ´n Interdisciplinaria Avanzada en Ciencias de los Materiales (CIMAT), Universidad de Chile, casilla 2777, Santiago, Chile 3 Facultad de Ciencias Veterinarias y Pecuarias y Centro para la Investigacio ´n Interdisciplinaria Avanzada en Ciencias de los Materiales (CIMAT), Universidad de Chile, casilla 2, correo 15, Santiago, Chile Keywords: chitosan; drug delivery systems; grafting; hydrogels; swelling Introduction Hydrogels are water-swollen networks based on hydro- philic homopolymers or copolymers. Synthetic hydrogel applications began to grow rapidly from the mid 1980s. [1] The most common synthetic route is the free radical poly- merization of vinyl monomers in the presence of a difunc- tional crosslinking agent and a swelling agent. However, considerable efforts have also been made to synthesize hydrogels based on the chemical modification of natural polymers. [2–7] Currently they are being used widely in a variety of applications such as thickening agents in food, moisture releaser to plants, fluid uptake and retention in the sanitary area, hydrophilic coatings for textile applications, separation and diffusion gel in chromatography and electro- phoresis, contact lenses, and in pharmaceutical applica- tions, principally as drug-delivery matrixes. [8–15] On the other hand, it is well-known that hydrogels based on cationic polyelectrolytes have higher swelling capacity under acidic pH while those of an anionic nature have this maximum capacity at basic pH. [1] In the case of polymers with non-ionic functionalities, such as those with amido 577