Specialty & fine chemicals
Cosmetic peptides
The stratum corneum as a size-selective barrier and the 500-dalton rule, the three functional classes of cosmetic peptide and their proposed mechanisms, and why growth factors are the weakest case.
Cosmetic peptides are short amino-acid sequences applied topically with claims about wrinkles, firmness and pigmentation. Several have coherent proposed mechanisms. All of them face the same obstacle first, and it is worth taking that in order.
The barrier is the point
The stratum corneum is the outermost skin layer: flattened dead corneocytes filled with keratin, embedded in a lamellar matrix of ceramides, cholesterol and free fatty acids. It exists to stop things crossing in either direction.
Passive permeation through it is strongly size-limited. The widely used 500-dalton rule — that molecules much above 500 Da do not appreciably cross intact stratum corneum — is a rule of thumb rather than a sharp cut-off, but it is well supported by the observation that essentially all common contact allergens and effective topical drugs fall below it.
A tripeptide is around 300–400 Da. A pentapeptide is around 500–600. Anything larger, and anything strongly hydrophilic or charged, faces a steeply rising barrier — which is why cosmetic peptides are short, and why they are usually lipidated, given a palmitoyl or myristoyl tail. That tail is not decoration: it raises lipophilicity so the molecule partitions into the lipid matrix instead of being excluded by it.
Three classes, three proposed mechanisms
Signal peptides are fragments of matrix proteins — most often collagen — that the skin is proposed to interpret as evidence of matrix breakdown, prompting fibroblasts to synthesise more. Palmitoyl pentapeptide and related sequences work on this logic. The premise is reasonable, since collagen fragments do act as signals in wound biology.
Carrier peptides deliver a metal ion. The best characterised is the tripeptide glycyl-histidyl-lysine complexed with copper, GHK-Cu, which occurs naturally in plasma and declines with age. Copper is a required cofactor for lysyl oxidase, the enzyme that cross-links collagen and elastin, and GHK-Cu has genuine documented activity in wound healing — the strongest evidence base in the category, though largely from wound rather than cosmetic contexts.
Neurotransmitter-inhibiting peptides are the most mechanistically specific, and they connect directly to botulinum toxin. Acetyl hexapeptide-8 is a fragment mimicking the N-terminal region of SNAP-25, the SNARE protein that botulinum type A cleaves. The proposed action is competitive: the peptide interferes with SNARE complex assembly, reducing acetylcholine release and so reducing muscle contraction.
The mechanism is coherent in vitro. Whether it operates in use is a delivery question — the target is the neuromuscular junction, well beneath the epidermis, and a hydrophilic hexapeptide reaching it in meaningful concentration through intact skin is precisely what the barrier argues against.
Where the claims are weakest
Growth factors and conditioned media. EGF is about 6 kDa; most growth factors are several kilodaltons and hydrophilic. These are an order of magnitude above the size at which passive permeation effectively stops, so a topically applied growth factor is very unlikely to reach living cells in the dermis at any relevant concentration. Effects reported for such products are more plausibly attributable to the formulation’s other components, or to the barrier being compromised.
The honest position for the category: short lipidated peptides can plausibly cross, the mechanisms proposed for them are specific and testable, and the clinical trials are mostly small, short, manufacturer-conducted and rarely compared against an active control such as retinol.