Water and solute active transport through human epidermis: Contribution of electromigration

A triphasic, coarse-grained model of mass transport through the human epidermis is developed, consisting of free extracellular water, live cells (keratinocytes), and inert extracellular matrix. The model accounts for the superposition of active transport of Na⁺, K⁺ and Cl^- ions across the membrane of keratinocytes, and electromigration driven by an externally imposed electrostatic potential difference. Local cell volume is regulated by the transmembrane fluxes of water and ions according to a time-delay scheme which aims to keep the volume between certain thresholds. Numerical simulations reveal that either weak hyposmotic shocks or negative potential gradients smaller than one millivolt/micrometer across the epidermis can generate travelling waves in extracellular ion concentration. By monitoring the transmembrane (Na⁺-K⁺ -ATPase) pump flux, we have found that maintaining a higher transepidermal potential gradient requires faster active transport through the cells.