Table 2.

Summary of features of the different theoretical SC LM models

Model	Type	Methodology	Lipid source	Repeating unit	Chain packing	Permeability	Layers	Peculiarities
Brick and mortar	Basic	eM and FF	hSkin/pSkin	na	na	LM	na	COR impermeable
Asymmetry	Molecular	eM, XRD, IRS, and MDS	hSkin/Syn	4–6.5 nm	Ort/Hex	−	Asymetric bilayers	Asymmetric
Domain mosaic	Microscopic	XRD	pSkin/Syn	na	Ort/Hex/Liq	Liq phase	Bilayers	Permeable liq phase
Sandwich	Molecular	eM, XRD, and ND	hSkin/pSkin/Syn	12−13 nm	Ort/Hex	−	Triple layer	Liquid middle section
Single-gel phase	Microscopic	eM, XRD, ND, and MDS	hSkin/pSkin/Syn	na	Ort/Hex	−	Bilayers	None
Armature reinforcement	Molecular	ND	Syn	Var. 3–4.55 nm	Ort/Hex	Hydrated	Bilayers	Swelling/conformation change
Tri-layer	Molecular	ND and NMR	Syn	5–6 nm	Ort/Hex	−	Bilayers	Fluid middle layer
Unified	Molecular	Mixed	Mixed	4–4.5 and 5–6 nm	Ort/Hex	−	Mixed bilayers	Mixed features

Bold face indicates different; roman type indicates similar; na indicates not determinable. COR, corneocyte; SC, stratum corneum; LM, lipid matrix; eM, electron microscopy; FF, freeze fracture microscopy; XRD, X-ray diffraction; IRS, infrared spectroscopy; MDS, molecular dynamics simulations; ND, neutron diffraction; NRM, nuclear magnetic resonance; pSkin, porcine skin; hSkin, human skin; Syn, synthetic lipids; Ort, orthorhombic; Hex, hexagonal; Liq, liquid.