Peptides (Matrixyl)
In plain terms
The signal peptides in anti-aging products, Matrixyl being the best known. They nudge skin to make more collagen and are about as gentle as actives get: non-irritating, non-sensitizing, and fine for sensitive or retinol-shy skin. The honest catch is strength of effect, not safety.
What the evidence shows
Cosmetic signal peptides are short amino-acid chains modeled on fragments of skin's own collagen. Matrixyl (palmitoyl pentapeptide-4, or pal-KTTKS) is the best-studied: a fragment of type I collagen with a fatty tail that helps it cross the surface, where it signals fibroblasts to build collagen. In the benchmark double-blind, placebo-controlled trial, it measurably softened wrinkles at just a few parts per million, and across studies these peptides come back as non-irritating and non-sensitizing, tolerated by all skin types including oily and acne-prone. They are large, gentle molecules that work gradually. The CIR panel's review of the pentapeptides is still in progress, with requests for more irritation data on a couple of specific sequences, but the marketed signal peptides have a clean tolerability record in the trials run so far. The honest limit here is effect size, not safety: peptides are a gentle supporting active, not a retinoid-strength one, and "peptides" is a broad family, so this read covers the common collagen-signaling type rather than every peptide on the market.
On the label
Listed as Matrixyl or by INCI names like palmitoyl pentapeptide-4 and palmitoyl tetrapeptide-7. Concentration percentages tell you little here, since these work at parts-per-million; a peptide complex sitting well up the ingredient list matters more. Copper peptides and neuropeptide types are their own ingredients with their own profiles.