The connection between in vitro water uptake and in vivo skin moisturization

Background/purpose: Humectancy or hygroscopy is the water absorption tendency of a substance from the surroundings. Our interest, from the clinical point of view, consists of correlating this tendency in vitro and its effect in vivo for the development of drugs and formulations for the treatment of dry skin syndrome or diseases accompanied by dry skin. Method: In vitro, water absorption was measured using the comparative isopiestic method. This method is based on bringing the vapor of the water to isothermal equilibrium between a reference system and the material to be studied. The in vivo model on guinea‐pigs for the dry skin syndrome tested the therapeutic ability of mono‐, di‐ and tri‐glycerols to provide moisture to dry skin leading to healing. The moisture content in the stratum corneum was measured with a Corneometer CM 825^® PC that measures merely the presence of high dielectric material (humectant or water), whereas the Mexameter MX 16^® measures a pathological parameter – the erythema. Results: Adding hydroxyl groups to a consecutive set of polyhydroxyalkanes increases the humectancy of the polyols in vitro. This elevation was found to be linear at low relative humidities (Relative humidity=31.9% and 37°C). In vivo, moisture was returned to normal within a week in all three groups. However, only glycerol managed to abolish the erythema within 7 days. Conclusion: A rise in water absorption ability in vitro, at a rate of about 0.25 mol water per hydroxyl group was revealed in a consecutive set of glycerols (mono‐, di‐ and tri‐glycerols). One would expect that the better humectant is a material, i.e. in which the higher its physical ability to hold water in vitro the more effective it will be in recovering skin dryness. We have found, however, that glycerol, which has the lowest humectant activity in vitro, from the set of glycerols, di‐ and tri‐glycerol, has been proven to be the best for eliminating the signs of skin dryness. Accordingly, we propose to distinguish between the in vitro humectancy (i.e. the water uptake of a material), and its in vivo moisturizing effect, i.e. its ability to cure skin dryness and erythema. This finding supports our conclusion that the connection between in vitro humectancy and in vivo moisturization is not a simple correlation.