What Skin Lets In
The outer layer of skin is designed to keep molecules out. Peptides in creams face the same wall as drugs do.
Stop 3 of 10 on the Repair Line.
Skin is a barrier first and an organ second. Its top layer, the stratum corneum, is a few dozen cell-widths of flattened dead cells packed in lipids, often compared to bricks and mortar. It keeps water in and most things out.
The size rule
Studies of how molecules cross skin suggest that passive penetration falls sharply as size rises. A frequently cited guide, called the "500 dalton rule", says molecules above roughly that mass cross intact skin poorly. Most peptides are heavier. A small peptide of three or four amino acids sits near the limit, and a larger one is well above it.
Why cosmetic peptides still exist
Some small peptides are used in cosmetics, often fatty-acid-linked to improve fat solubility. Typical claims involve signaling skin cells to make more collagen, or relaxing wrinkle-causing muscles. Evidence usually comes from:
- Cell cultures, where the peptide touches cells directly.
- Small studies that measure skin appearance, sometimes without a proper vehicle control.
Lab results on isolated cells do not show that enough peptide reaches living cells under the stratum corneum.
Changing the barrier
Methods that improve penetration, such as microneedles, certain solvents and electrical methods, are active research areas. Microneedling at home with peptide products raises safety questions, including infection and irritation.
Where the skin barrier is damaged
Broken skin, as in a wound (see the first Repair stop), is a different situation. Absorption is higher and so is the risk of irritation and infection.
A sensible reading
Moisturizers and sunscreen have far stronger evidence for visible skin benefit than most peptide creams. A pleasant cream can be perfectly fine, but a mechanism claim should come with proof that the active reaches its target.
End of this stop
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Education only, not medical advice. See the service notice.