Metal-based antiperspirants have been in use for centuries; however, there is an increasing consumer demand for a metal-free alternative that works effectively. Here, we develop an artificial sweat duct rig and demonstrate an alternative, metal-free approach to antiperspiration. Instead of clogging sweat ducts with metal salts, we use a hygroscopic material to induce the evaporation of sweat as it approaches the outlet (i.e. pore) of the sweat duct. As a result, the sweat dehydrates almost completely while still being inside of the duct, forming a natural gel-like salt plug that halts the flow. We show that the critical pressure gradient within the duct (∼3 kPa), beneath which clogging occurs, can be rationalized by balancing the mass flow rates of the liquid (Poiseuille’s law) and the evaporative vapor (Fick’s law).

Aluminum hydrolysis chemistry is an important part of modern society because of the dominance of Al(III) as a highly effective antiperspirant active. However, the century-old chemistry centered on aluminum chloride (ACL) is not comprehensive enough to address all of the in vivo events associated with current commercial antiperspirants and their mechanism of action. The present study aims to address the knowledge gap among extensively studied benchmark ACL, its modified version aluminum chlorohydrate (ACH), and a more complex but less explored group of aluminum zirconium chlorohydrate glycine complexes (ZAG salts) toward understanding the mechanism of action under consumer-relevant conditions. ACH, which is the Al source used in the manufacture of ZAG salts, provides a bridge between ACL and ZAG chemistry. High viscosity and gel formation driven by pH and a specific Al(III) salt upon hydrolysis are considered the criteria for building an in vivo occlusive mass to retard or stop the flow of sweat to the skin surface, thus providing an antiperspirant effect. Rheological studies indicated that ACL and aluminum zirconium tetrachlorohydrex glycine (TETRA) were the most efficacious salt actives. Spectroscopic studies, diffraction studies, and elemental analysis suggested that small metal oxide and hydroxide species with coparticipating glycine as well as various polynuclear and oligomeric species are the key to gel formation. At a given pH, the key ingredients (NaCl, urea, bovine serum albumin, and lactic acid) in artificial sweat were found to have little influence on Al(III) salt hydrolysis. The effects of the sweat components were mostly limited to local complex formation and kinetic modification. The in vitro comparative experiments with various Al(III) and ZAG salt systems offer unprecedented insights into the chemistry of different salt types, thus paving the way for engineering more efficacious antiperspirant systems.

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