Temperature-modulated permeation of hydroxy urea through thermotropic liquid crystals embedded in poly-HEMA

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 °C, 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 (9.85×10−6 cm/s) at temperatures above the phase-transition temperature of liquid crystals than that (0.69×10−6 cm/s) at temperatures below the phase-transition temperature of liquid crystals.