Shaping the Skin: The Interplay of Mesoscale Geometry and Corneocyte Swelling

The stratum corneum, the outer layer of mammalian skin, provides a remarkable barrier to the external environment, yet it has highly variable permeability properties where it actively mediates between inside and out. On prolonged exposure to water, swelling of the corneocytes (skin cells composed of keratin intermediate filaments) is the key process by which the stratum corneum controls permeability and mechanics. As for many biological systems with intricate function, the mesoscale geometry is optimized to provide functionality from basic physical principles. Here we show that a key mechanism of corneocyte swelling is the interplay of mesoscale geometry and thermodynamics: given helical tubes with woven geometry equivalent to the keratin intermediate filament arrangement, the balance of solvation free energy and elasticity induces swelling of the system, importantly with complete reversibility. Our result remarkably replicates macroscopic experimental data of native through to fully hydrated corneocytes. This finding not only highlights the importance of patterns and morphology in nature but also gives valuable insight into the functionality of skin.

Figure 1

(Left) The periodic ${\mathrm{\Sigma }}^{+}$ rod packing composed of straight rods. (Right upper) One unit cell of the structure with slightly helical filaments. (Right lower) A single helix of the structure shown with the variable parameters: $L$ is the unit cell edge length, $R$ the radius of the helix. The tube radial thickness $r$ is 3.7 nm, the radius of keratin IFs.

Figure 2

The excess surface free energy per area $f_{\text{surf}}(R)$ as function of the inverse solute sphere radius $1/R$. Symbols denote numerical values obtained from DFT calculations, while the full line represents the morphometric form with fitted thermodynamic coefficients $\gamma$, $\kappa$, and $\overline{\kappa }$. The agreement of the numerical DFT data with the morphometric form is excellent for all values of $R$.

Figure 3

Total free energy $\mathrm{\Omega }+E$ (per unit cell) of the helical ${\mathrm{\Sigma }}^{+}$ rod packing (Fig. 1), in units of $k_{B}T\times {10}^4$ as a function of unit cell size $L$ and helix radius $R$. The lower plot shows a smaller region of the upper plot. A valley of low total free energy configurations, whose extremes are indicated on the lower plot, traces a pathway for the swelling of corneocytes and subsequent drying. The packing fractions range from 11% to 38%, comparable to volume fractions (15%–35% [7]) that can be inferred from experimental hydration data [4, 5].