The skin of humans protects from water loss in part due to fibrous structural proteins (keratins) and cross-linking.

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“The vertebrate integument represents an evolutionary compromise between the needs for mechanical protection and those of sensing the environment and regulating the exchange of materials and energy. Fibrous keratins evolved as a means of strengthening the integument while simultaneously providing a structural support for lipids, which comprise the principal barrier to cutaneous water efflux in terrestrial taxa…How do the structural features of keratin influence its resistance to water movement? Generally, structural features that alter the free volume (equivalent to pores or channels) should alter the permeation of water molecules accordingly. Resistance to diffusion is affected by the molecular mass of side chains and tends to increase with cross-linking beyond certain critical levels (Lieberman et al., 1972)…The stability of cross-linkages is dependent on a large number of intermolecular forces, including covalent, ionic, and hydrogen bonding in addition to van der Waals attractive forces between non-polar amino acid side chains. All of these act to influence the mobility and free volumes of the structure…Studies of human skin have indeed demonstrated that gradients of water exist in the stratum corneum (Warner et al., 1988; Bommannan et al., 1990; Caspers et al., 2001; Bouwstra et al., 2003a). Fourier transform infrared spectroscopy has demonstrated that free water content in stratum corneum is greater in central regions relative to superficial and deeper cell layers at moderate levels of hydration (57%–87%, w/w), whereas at higher levels of hydration (300% w/w) water swells corneocytes in a direction perpendicular to the skin surface except for the deepest cell layers adjacent to the viable epidermis (Bouwstra et al., 2003a). While the mechanism excluding free water from the deeper cell layers of stratum corneum is not understood, it is speculated to play a role in preventing dehydration of the viable epidermis. In relatively dry conditions (18%–26% w/w), only bound water is present in the stratum corneum (Bulgin and Vinson, 1967; Hansen and Yellin, 1972; Bouwstra et al., 2003a).” (Lillywhite 2006:202, 212, 213)

Journal article
Water relations of tetrapod integumentLillywhite, H. B.

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