Properties and Applications of Temperature Sensitive Gels

Abstract: The work presented deals with the characterisation and application of critical gels. In order to provide a theoretical foundation, the thermodynamics of the reversible swelling and shrinking behaviour of such gels is outlined. The temperature-sensitive poly(N-isopropylacrylamide) gel (NiPAAm) was chosen for experimental investigation. Dif ferent methods of producing the gel are described, and the problems encountered in the various production techniques are discussed in some detail. A simple new method for the determination of the pore-size distribution in gels, based on a combination of in-diffusion and gel filtration, is presented. The method was used for the determination of the pore-size distribution in NiPAAm gels at 10, 20 and 30'C, i.e. at different degrees of swelling. It was found that more spherical molecules, such as dextran, could more easily penetrate the gel network than more linear molecules, such as polyethylene glycol. The effective diffusivity of glucose and insulin in NiPAAm gel at different degrees of swelling was studied using a method based on out-diffusion from gel pieces. The swelling and shrinking had no effect on the small glucose molecule, while the insulin experienced an increased resistance against mass transport when the gel was in its shrunken state. For glucose, the temperature was found to have the dominating influence on the diffusion rate. The swelling/shrinking kinetics between different temperatures was determined. The swelling kinetics for the NiPAAm gel follows a relatively simple model based on the swelling theory by Tanaka and Fillmore (1979). Simulations of the gel extraction process show that the diffusion and the pore-size distribution determine the total separation efficiency. The size of the gel bodies, the temperature interval and the period of the swelling-shrinking cycles will also influence the efficiency of the gel extraction process. Future applications of critical gels in slow-release and controlled-release pharmaceuticals are briefly discussed.