Measured skin levels give a qualified answer: in part, and essentially only when the peptide carries a lipid chain. In full-thickness mouse skin the pentapeptide KTTKS was undetectable in every layer, while its palmitoylated form reached 4.2 ± 0.7 µg/cm² in the stratum corneum and 0.3 ± 0.1 µg/cm² in the dermis. Neither compound crossed into the receptor fluid.
Published 21 October 2026. Last reviewed 21 October 2026.
The terms, defined so they stand alone
Stratum corneum. The outermost layer of the epidermis, composed of flattened corneocytes embedded in an intercellular lipid matrix. It is the principal barrier to any molecule applied to the skin surface.
Receptor fluid. In a diffusion-cell experiment, the compartment beneath the skin sample. A substance detected there has crossed the full thickness of the specimen; a substance absent from it has not, however much of it is found inside the tissue.
Palmitoylation. The covalent attachment of palmitic acid, a sixteen-carbon saturated fatty acid, to a molecule. In cosmetic chemistry it is used to raise lipophilicity — affinity for lipid environments — and so to increase partitioning into the lipid matrix of the stratum corneum.
KTTKS. A pentapeptide with the sequence lysine–threonine–threonine–lysine–serine, derived from a fragment of type I procollagen. Its palmitoylated derivative is the ingredient listed as palmitoyl pentapeptide-4. The question of what such sequences do once inside the skin is separate from the question addressed here, and we have reviewed the controlled-trial evidence on that point in a companion article.
Where the size assumption comes from, and what it omits
The conventional starting point is a molecular-size threshold. Gautam and colleagues, writing in Scientific Reports in 2016, state the premise plainly: the stratum corneum is impermeable to most compounds above 500 Da. We have examined the origin and the limits of that rule in a separate piece on molecular weight and lipophilicity.
A 2021 review in Pharmaceuticals applies the premise to cosmetic ingredients, asserting that "peptides used in cosmetic products present a molecular weight lower than 500 Da and hydrophilic properties", and adding that "chemical modifications such as esterification with alkyl chains, are usually required" to address penetration. The two statements sit awkwardly together: if size alone were the operative variable, no modification would be required. The measurements below indicate which of the two propositions the data support.
What was measured in mouse skin
Choi and colleagues (Biomolecules & Therapeutics, 2014) quantified both the free pentapeptide and its palmitoylated form by liquid chromatography–tandem mass spectrometry, using full-thickness hairless mouse skin and, separately, skin extracts and homogenates for stability work. Three results matter.
First, the unmodified pentapeptide was not detected in any skin layer. Not reduced — absent from the stratum corneum, the epidermis and the dermis alike. Second, the palmitoylated form was detected in all three, at 4.2 ± 0.7 µg/cm² in the stratum corneum, 2.8 ± 0.5 µg/cm² in the epidermis and 0.3 ± 0.1 µg/cm² in the dermis. The dermal figure is roughly an order of magnitude below the corneal one, which is the expected shape of a concentration gradient away from the application site. Third, neither compound appeared in the receptor solution, so neither traversed the full thickness of the specimen.
The same study reports that both peptides were rapidly degraded in skin extracts and homogenates, that the palmitoylated form was the more stable of the two, and that adding protease inhibitors improved the stability of both. Delivery and survival are therefore two distinct constraints, and a molecule can satisfy the first while failing the second.
What was measured in human skin
Ligorio and colleagues (ACS Applied Bio Materials, 2025) applied an infinite dose of 100 µL of a 1% solution of palmitoyl hexapeptide-12 per cm² of human skin in a Franz diffusion cell, for 8 hours at 36.5 °C, and profiled the specimen by OrbiSIMS — secondary-ion mass spectrometry with a mass resolving power of 240,000 at m/z 200, which identifies a molecule by its exact mass rather than by a label attached to it.
Peptide monomers were found to have penetrated more than approximately 100 µm into the epidermal thickness. Calcium-bound assemblies of the same peptide were located up to 80 µm below the stratum corneum, which the authors took to be approximately 20 µm deep. The depth figures were estimated from a uniform profiling rate, which is a stated assumption rather than a direct measurement, and the dermal compartment was not quantified. On the methodological caution that applies to all such figures, see our note on how penetration is actually measured.
| Compound | Skin model | Method | Detected | Reached receptor fluid | Source |
|---|---|---|---|---|---|
| KTTKS (unmodified pentapeptide) | Full-thickness hairless mouse skin | LC–MS/MS | Not detected in stratum corneum, epidermis or dermis | No | Choi et al., 2014 |
| Palmitoyl-KTTKS | Full-thickness hairless mouse skin | LC–MS/MS | Stratum corneum 4.2 ± 0.7 µg/cm²; epidermis 2.8 ± 0.5 µg/cm²; dermis 0.3 ± 0.1 µg/cm² | No | Choi et al., 2014 |
| Palmitoyl hexapeptide-12 | Human skin, Franz cell, 1% solution, 100 µL/cm², 8 h at 36.5 °C | OrbiSIMS depth profiling | Monomers > ~100 µm into epidermal thickness; calcium-bound assemblies up to 80 µm below a stratum corneum of ~20 µm | Not assessed | Ligorio et al., 2025 |
What follows, and what does not
The lipid chain, not the modest size, is what distinguishes a sequence that was measurable inside the tissue from one that was not. That is a statement about partitioning: a lipophilic conjugate distributes into the lipid matrix of the stratum corneum, which a small hydrophilic peptide does not, irrespective of its nominal molecular weight. The widely repeated claim that cosmetic peptides penetrate because they fall below 500 Da is not supported by the comparison above; the peptide that fell below the threshold was the one that could not be found.
Three limits should be stated with the same precision as the figures. The quantitative layer-by-layer data come from hairless mouse skin, which is more permeable than human skin, so the absolute amounts are not transferable to a human forearm. The human study used an infinite dose over eight hours — conditions that maximise flux and do not correspond to a thin film applied once. And in neither experiment did a peptide traverse the full specimen, so "reaches the dermis" is defensible only in the narrow sense of being detectable within dermal tissue, at amounts an order of magnitude below those at the surface.
What the evidence does support is narrower and more useful than the marketing formulation of it: that for a peptide, the vehicle and the conjugation chemistry determine whether anything arrives at all, and that arrival is necessary but not sufficient, because proteolytic degradation in the tissue operates on whatever does arrive.
Scope
This article describes findings published in the peer-reviewed literature and the conditions under which they were obtained. It does not constitute a claim about any product, nor a statement of the performance of any formulation. Cosmetic products are not intended to diagnose, treat, cure or prevent disease.
References
- Choi, Y. L., Park, E. J., Kim, E., Na, D. H. & Shin, Y.-H. (2014). Dermal stability and in vitro skin permeation of collagen pentapeptides (KTTKS and palmitoyl-KTTKS). Biomolecules & Therapeutics, 22(4), 321–327. DOI 10.4062/biomolther.2014.053. PMID 25143811.
- Ligorio, C., Tavasoli, E., Karaman-Jurukovska, N., Ittycheri, A., Kotowska, A. M., Khan, M. H., Scurr, D. J., Gupta, S. A., Moogan, L. V., Emmetsberger, J., Lu, F., German, G. K., Mammone, T. & Mata, A. (2025). Noninvasive monitoring of palmitoyl hexapeptide-12 in human skin layers: mechanical interaction with skin components and its potential skincare benefits. ACS Applied Bio Materials. DOI 10.1021/acsabm.4c01816. PMID 39964201.
- Gautam, A., Nanda, J. S., Samuel, J. S. et al. (2016). Topical delivery of protein and peptide using novel cell penetrating peptide IMT-P8. Scientific Reports, 6, 26278. DOI 10.1038/srep26278.
- Usage of synthetic peptides in cosmetics for sensitive skin (2021). Pharmaceuticals, 14(8), 702. DOI 10.3390/ph14080702. PMID 34451799.