A method of determining a molecular descriptor of absorption for a candidate compound is disclosed. The method includes: providing a test solution comprising one or more candidate compounds; contacting the test solution with a simulated biological membrane to partition the one or more candidate compounds into the membrane detecting the presence or amount of the one or more candidate compound in the membrane at one or more permeation times; and determining a molecular descriptor of absorption using the presence or amount of the one or more candidate compound in the membrane at the one or more permeation times. In one aspect the invention provides a skin-imitating membrane, and one or more percutaneous absorption parameters are determined. In another aspect the membrane is disposed on a fiber. In another aspect the test solution is contacted with two or more simulated biological membranes to partition the one or more candidate compounds into the membranes.