Analytical solutions were derived for the time lag and steady-state transdermal flux of drugs across a heat-aided drug-delivery device. The expressions incorporate thermodynamic and physical properties of the solvent/medicament and membrane system, making the approach amenable to in silico evaluation of process performance in a spreadsheet-like environment. Methods and concepts from classical control theory were applied to predict the onset of the steady-state flux. The methodology was based on the system's time constant, computed by taking the inverse of the first eigenvalue of a Sturm-Liouville problem. This framework does not require a solution to the transient heat-enhanced diffusion problem and relaxes the assumption of a constant diffusion coefficient throughout the membrane. The results match published data, partially explain some clinical trial observations, and suggest a novel method to control the plasma drug concentration. An optimal control strategy was proposed to keep the delivery rate as close as possible to 9.07 mu g cm(-2) h(-1) over a 30-min period by adjusting the skin surface temperature. The integral of the absolute value of the error was 1138.19 compared to 1217.08 when the surface temperature was fixed at 37 degrees C. (c) 2007 Elsevier Ltd. All rights reserved.