The use of thermotropic liquid crystals (LCs) embedded in poly-2-hydroxy ethyl methacrylate (HEMA) membranes as a temperature-controlled drug release system was studied. The aim of the study was to examine the application of LCs as on-off switches for drug transport through membranes. n-Pentyl-cyanobiphenyl (K15) and n-heptyl-cyanobiphenyl (K21) liquid crystals with nematic-to-isotropic phase-transition temperatures (Tn-i) of 36.3 and 43.1 degreesC, respectively, were used to modulate the permeation of hydroxy urea (HU) as a low molecular weight model drug. The permeability of HU through thermpotropic liquid-crystal embedded poly-HEMA membranes was found to be directly related to temperature changes. HU permeability was markedly higher at temperatures above the Tn-i of the LCs compared to that at lower temperatures. HU permeability through LC membranes at both temperatures below and above the phase-transition temperature followed zero-order kinetics. However, HU permeability through the membranes was much higher (9.85 x 10(-6) cm/s) at temperatures above the phase-transition temperature of liquid crystals than that (0.69 X 10(-6) cm/s) at temperatures below the phase-transition temperature of liquid crystals. (C) 2003 Elsevier B.V. All rights reserved.