Short amino-acid chains meant to signal skin to make collagen or to relax expression lines. Small, mostly manufacturer-run trials; effects are modest.
Cosmetic peptides are short protein fragments, often with a fatty acid (palmitoyl) attached to help them into skin. Signal peptides (palmitoyl pentapeptide-4, palmitoyl tripeptide-1, palmitoyl tetrapeptide-7) mimic collagen fragments and prompt fibroblasts to make more matrix in lab tests. Neurotransmitter-inhibiting peptides (acetyl hexapeptide-8, pentapeptide-18) interfere with the nerve signal that contracts facial muscles — in a lab dish — and are sold as a topical alternative to wrinkle-relaxing injections. Copper tripeptide-1 carries copper and is linked to wound healing.
Most are near or above the roughly 500-dalton size that passes easily through skin, which limits how much reaches its target, and they are used at tiny concentrations.
Controlled trials are few, small and usually run by ingredient or product makers: a 12-week split-face trial in 93 women found 3 ppm palmitoyl pentapeptide-4 reduced fine lines versus the same moisturizer without it. Trials of the wrinkle-relaxing peptides are small, and none shows effects comparable to injections. Independent replication is scarce.
93 women aged 35-55 used a moisturizer with 3 ppm palmitoyl pentapeptide on one side of the face and the same moisturizer without it on the other for 12 weeks; the peptide side showed significantly fewer wrinkles and fine lines. Run by a skincare manufacturer.
Reviews controlled ex vivo and in vivo studies of topical peptides and proteins for skin aging, grouping them as signal, enzyme-inhibitor, neurotransmitter-inhibitor and carrier peptides.
Argues that a compound must weigh under about 500 daltons to pass through the skin's outer layer, based on the size of known contact allergens and of the drugs that work topically or through transdermal patches.