Dryness

Skin hydration is the product of three biological systems working in concert. When all three perform well, the surface is supple, smooth, and reflective. When any one of them fails, the visible signs of dryness emerge. Understanding the system that has slipped clarifies which moisturiser ingredients earn space in the routine, and why the same product can transform one person's skin and barely affect another's.

Transepidermal water loss (TEWL)

The body holds approximately 60% of its water by mass. The stratum corneum is the gatekeeper that decides how much of that water diffuses out into the air. Healthy skin maintains TEWL at roughly 5–10 grams per square metre per hour. Dry, compromised skin commonly measures 15–30 g/m²/h, sometimes higher. The water that exits is not replaced as fast as it leaves, and the stratum corneum dehydrates below the 10% water content threshold required for normal enzymatic desquamation.

When stratum-corneum hydration drops below this threshold, corneocytes fail to detach individually and instead shed in visible clumps: the flaking, scaling, and roughness that define clinical dryness. The intervention is structural: occlusive ingredients reduce TEWL by 30–50% within hours of application. Petrolatum remains the most studied occlusive (cuts TEWL by ~99% over twelve hours); dimethicone, squalane, and shea butter offer lighter alternatives. Layering occlusive over humectant is the biophysical reason a moisturiser applied to damp skin outperforms the same product applied to dry skin.

Lipid bilayer architecture

The mortar between corneocytes is a precisely structured lipid mix: 50% ceramides, 25% cholesterol, and 15% free fatty acids by mass. This ratio is not arbitrary: it forms a lamellar bilayer that water cannot easily traverse. When any component drops, the bilayer reorganises into a less ordered structure, and the barrier loses its water-retention capacity. Ceramide synthesis falls with age, with frequent exposure to surfactants, and in inflammatory dermatoses such as eczema and atopic dermatitis.

Topical replacement is the targeted intervention. Ceramide NP, ceramide AP, and ceramide EOP (the three most abundant endogenous forms) incorporate into the lipid bilayer when applied at physiological concentrations. The ratio matters: published studies show that 3:1:1 cholesterol-to-ceramide-to-fatty-acid mixtures restore barrier function in two to four weeks, whereas individual ceramide application alone is significantly slower. BIOSAR formulations follow this stoichiometry, supplemented with squalane and phytosterols to mimic the full sebaceous lipid profile that aging skin no longer produces in sufficient quantity.

Natural moisturising factor (NMF)

Inside each corneocyte sits a pool of small, water-binding molecules collectively called the natural moisturising factor: free amino acids, pyrrolidone carboxylic acid (PCA), urea, lactate, and various salts. NMF accounts for roughly 20–30% of corneocyte dry weight in healthy skin and is the molecular reason skin holds water osmotically against a drying environment. NMF is generated by enzymatic breakdown of filaggrin during keratinocyte differentiation, and its concentration falls when differentiation is disturbed by inflammation, age, or genetic filaggrin loss-of-function variants (common in atopic dermatitis populations).

Topical humectants supplement the depleted NMF pool. Glycerin at 5–20% concentration draws water from the dermis and atmosphere into the stratum corneum and is the single most studied humectant in dermatology. Hyaluronic acid binds up to 1,000 times its weight in water; low-molecular-weight fragments penetrate the upper stratum corneum while high-molecular-weight forms remain on the surface as a hydrating film. Sodium PCA, urea (at 5–10%), and panthenol each contribute. The clinical pattern is reliable: humectant-only routines in low-humidity environments can paradoxically worsen dryness. The humectant pulls water from the dermis upward when there is no atmospheric moisture to draw from. This is why an occlusive layer applied over a humectant in dry climates is non-negotiable, and why the BIOSAR Hydraderm sequence layers humectant essence under ceramide cream rather than relying on either alone.