An Atomistic First-Principles Density Functional Theory Model for Single Layer Dry \textit{Stratum Corneum}

Abstract: Many questions concerning the biophysical and physiological properties of skin are still open. Skin aging, permeability, dermal absorption, hydration and drug transdermal delivery, are few examples of processes with its underlying mechanisms unveiled. In this work we present a first-principles density functional quantum atomistic model for single layer stratum corneum (SC) in order to contribute to unveil the molecular interactions behind the skin properties at this scale. The molecular structure of SC was modeled by an archetype of its hygroscopic proteic portion inside of the corneocytes, the natural moisturizing factor (NMF), coupled to glycerol molecules which represent the lipid fraction of SC. The vibrational spectra was calculated and compared to Fourier-Transform Infrared Absorption spectroscopy (FTIR) experimental data obtained on animal model of SC. We noticed that bands in the fingerprint region (800-1800 cm$^{-1}$) were correctly assigned. Moreover, our calculations revealed the existence of two coupled vibration between hydroxyl group of lipid and NMF methylene (1120 and 1160 cm$^{-1}$), which are of special interest since they probe the lipid-amino acid coupling. The model was also able to predict the shear modulus of dry SC in excellent agreement with the reported value on literature. Others physical/chemical properties could be calculated exploring the chemical accuracy and molecular resolution of our model. Research in dermatology, cosmetology, and biomedical engineering in the specific topics of drug delivery and/or mechanical properties of skin are examples of fields that would potentially take advantage of our approach.