ELSEVIER Journal of Controlled Release 31 (1994) 9-19 journal of controlled release Radiation formation of hydrogels for drug delivery Janusz M. Rosiak Institute of Applied Radiation Chemistry, Technical University of Lodz, Wroblewskiego 15, 93-590 Lodz, Poland Received 15 April 1993; accepted 8 September 1993 Abstract Using N-vinylpyrrolidone as polymer of choice, it has been pointed out that radiation formation of hydrogels in aqueous solutions proceeds according to the radical mechanism. Both in the processes of polymerization and crosslinking, the main role is played by indirect effect, i.e., these processes are initiated by intermediate products of radiolysis of water. In the presence of oxygen in the irradiated solutions, the process of gel formation is accompanied by the reactions leading to the scission of polymer main chain. Analysing the course of gel formation the use of modified Chadesby equation was suggested. This equation has an universal character. In order to calculate the values of the yield of radiation crosslinking a new method was used. It allows to obtain results without previous determination of the molecular weights of polymer to be crosslinked. 1. Introduction The field of biomaterials has advanced rapidly in the twentieth century, mainly as a result of attempts to replace body tissues with natural and man-made mate- rials. One of the most promising classes of materials for biomedical applications seems to be the hydrogels. They usually show good biocompatibility in contact with blood, body fluids and tissues. Hence, they are often used as contact lenses, burn wound dressings, artificial cartilages or membranes. They are often used to coat the materials being applied in the contact with the living organism, e.g., coating of the surface of cath- eters, electrodes, vascular prostheses, etc. Because of the ability of hydrogels to swell as well as their ability to release trapped particles into surrounding medium they are often used as drug delivery systems [ 1,2]. Hydrogels are most often defined as two-component systems where one of the components is a hydrophilic polymer, insoluble in water because of existing three- dimensional network joining its chains, and the second one is water. These systems may swell in water up to a 0168-3659/94/$07.00 © 1994 Elsevier Science B.V. All fights reserved SSDI0168-3659 (94)00027-R certain equilibrium state and retain their original shape. The interactions bringing about water sorption by hydrogels include the processes of hydration (it is con- nected with the presence of such chemical groups as - OH, -COOH, -CONH2, -CONH-, -SO3H), and those related to the existence of capillary areas and differ- ences in osmotic pressure. The forces which make hydrogel dissolution impossible are connected with the existence of covalent bonds between individual poly- mer chains although they may also have a character of electrostatic or hydrophobic interactions. Since the pioneer work of Wichterle and Lim [ 3], the directions of the research devoted to the radiation formation of hydrogels were presented in a number of revues [4-7]. These investigations are also included in research and technical progral:nmes co-ordinated by the International Atomic Energy Agency in Vienna [8]. Drugs are most often administered orally by the ingestion of tablets or capsules or via subcutaneous, intramuscular or intravenous injection. Although these methods of drug administration are well documented