Skin barrierStratum corneum · Moisture barrier

The skin barrier is the outermost layer of the skin — corneocytes held in a lipid matrix, often pictured as bricks and mortar — that limits water loss and keeps irritants and microbes out.

It matters because when the barrier is disrupted, skin loses water faster and becomes dry, reactive and prone to stinging; much of what people treat as 'sensitive skin' is a barrier problem, often made worse by over-cleansing and stacking too many actives.

In practice, barrier care is unglamorous and effective: gentle cleansing, moisturisers containing humectants and lipids, and introducing actives such as retinoids slowly. Ingredients including niacinamide and hyaluronic acid are studied for barrier support and hydration.

Sources

• Mechanistic Basis and Clinical Evidence for the Applications of Nicotinamide (Niacinamide) to Control Skin Aging and Pigmentation (barrier function); PMID 34439563
• 24-hour skin hydration and barrier function effects of a hyaluronic acid, glycerin and Centella asiatica moisturising fluid (randomised, assessor-blinded); PMC5560567