Introduction
Nail as a part of the body offers not only a protective function, but is also a barrier for drugs intended for nail diseases’ treatment. Despite of the need of human nails for studying ungual drug preparation, unfortunately, their availability is somehow limited. Bovine hoof is up to now an accepted model for human nail but significant differences between them have been reported [1]. These differences could be related to hooves’ ability in retaining more water and thus making them more permeable, although human nail, hair and bovine hoof are composed of similar keratin proteins (α−form). Therefore a novel nail model made of human hair was developed and its physical properties were then compared with bovine hooves. An early investigation showed that this model resembled bovine hooves in terms of permeation and binding of three tested marker substances, i.e. sodium fluorescein (SF), fluorescein isothiocyanate-dextran MW 4000 (FD4) and rhodamine B (RB) [2]. Based on this finding, a further comprehensive investigation was conducted to see the effect of three nail penetration enhancers (PEs) with different mechanisms (urea/ UR, thioglycolic acid/ TA and papain from papaya latex/ PA) on both materials using SF, FD4 and RB as markers, which represented a water soluble, a lipid soluble and a large molecule drug model, respectively.