A 1-M stock solution of glucose is prepared by dissolving glucose powder in high-performance liquid chromatography (HPLC) grade water (Fisher). Glucose solution is then spiked into the Franz cells in 200L aliquots, up to 600 L, following removal of the same volume of receptor phase solution. Each injection of concentrated glucose solution thus increases the glucose concentration in the Franz cell by approximately 28 mM, based on a cell volume of approximately 21 mL. After each injection, the experiment is left for 30 min to equilibrate in temperature and concentration.