K18 peptide (sh-Oligopeptide-78)

What it is

• K18 says its peptide technology was developed by a research group led by Prof. Artur Cavaco-Paulo at the University of Minho in Portugal. It is covered by a US patent granted in 2023.
• That patent claims keratin-binding peptides of 6-12 amino acids containing 2-5 cysteines.
• The INCI name on the K18 mask is sh-Oligopeptide-78. The mask also contains alcohol denat., cetearyl alcohol, behentrimonium chloride, hydrolysed wheat protein and hydrolysed wheat starch. Martins 2024 Those conditioning ingredients alone would improve feel and combing.
• The exact commercial sequence is not public.

How it works

• Company claim: the peptide binds "covalently" to keratin chains and takes part in "disulfide reconformation". No data are shown.
• The inventors' published science: screening 1,235 keratin-derived peptides showed that binding to hair keratin rose with cysteine and hydrophobic content, pointing to disulfide formation and hydrophobic interactions. Cruz 2017 A keratin peptide improved mechanical and thermal properties of relaxed hair in vitro and was not toxic to cells in water. Fernandes 2012 A 2026 paper from the same group showed that a small peptide penetrates hair and improves mechanics without a reducing agent, while larger cysteine-rich proteins mostly bind the cuticle. Carvalho 2026
• Penetration depends on damage: charged peptides penetrate only damaged hair, which carries more negative charge. Silva 2007
• Independent view: electron and atomic force microscopy suggested the K18 mask "acts at a superficial level without a significant effect on the core of the hair fiber", forming an external layer. Martins 2024

What the studies show

• Independent test (3 tresses per condition): one leave-in application of the K18 mask raised tensile strength of bleached hair by 19.5% (not significant), versus +47.6% for Olaplex No. 0 (significant). It left a smoother surface and brighter colour. Martins 2024
• Inventor patent examples on 8-times-bleached and relaxed hair showed stiffness with the peptide about 12-24% above the vehicle alone, and relaxed hair stiffness up about 40% in one example. These are inventor data without statistics on fibre numbers.
• Peptide screening and relaxed-hair work by the inventors are listed above. Cruz 2017; Fernandes 2012
• A 2025 paper from the same group fused the peptide to elastic protein motifs and found one fusion best for stiffness and denaturation temperature. Carvalho 2025
• No human (on-head) controlled study of K18 has been published. The company's "reverse damage in 4 minutes" line is product copy with no cited study.

How it's used in studies

• The patent describes 0.001-20% of the peptide, with examples at 0.01%.
• The independent test used the commercial mask at 10% of tress weight, left in. Martins 2024

Side effects and safety

• No specific safety problems have been reported. The keratin peptide in water was not toxic to skin cells in the lab, though an organic-solvent formulation was. Fernandes 2012
• The K18 mask contains hydrolysed wheat protein, so people with wheat allergy should check the label (see hydrolyzed wheat protein).
• It is a cosmetic in the US, EU and UK.

Bottom line

It suits people with bleached, coloured, relaxed or heat-damaged hair who want a leave-in treatment and accept paying for an ingredient with mainly inventor-generated evidence. Use it on clean, towel-dried hair before styling. Using it together with Olaplex "for complementary mechanisms" has not been studied. A cheaper route is a leave-in with hydrolysed keratin plus a cationic conditioner (see proteins and hair). We rate the evidence D: plausible chemistry and a real research lineage, but independent evidence of internal repair is weak, and benefits on heads are likely mostly conditioning. See bond builders.