Penetration of different molecular weight hydrolysed keratins into hair fibres and their effects on the physical properties of textured hair E. Malinauskyte* , R. Shrestha*, P. A. Cornwell* , S. Gourion-Arsiquaud* and M. Hindley† *TRI Princeton, 601 Prospect Avenue, Princeton NJ, 08540, USA and † Croda Europe Ltd, Foundry Lane, Widnes WA8 8UB, UK Received 8 February 2020, Revised 22 August 2020, Accepted 9 September 2020 Keywords: hair treatment, hydrolysed protein, keratin, penetration, peptide, tensile properties, textured hair Abstract OBJECTIVE: To investigate the effects of different molecular weight (MW), wool derived hydrolysed keratins (i.e. peptides) on the physi- cal properties of relaxed textured hair. METHODS: Very curly hair of African origin was relaxed using sodium hydroxide-based treatment. Relaxed hair was treated with different MW peptides derived from keratin protein and an amino acid, L-Leucine. The low-MW keratin peptides were 221 Da, the mid-MW keratin peptides were approximately 2577 Da, and the high-MW keratin peptides were approximately 75 440 Da. The penetration of these different peptides into relaxed hair was evalu- ated using a laser scanning micrometre and by fluorescence micro- scopy. The effect of these compounds on single-fibre mechanical properties and thermal properties was evaluated using tensile and DSC testing, respectively. RESULTS: Low- and mid-MW compounds were able to penetrate deep into the hair cortex. High-MW peptide adsorbed onto the hair surface and possibly slightly penetrated into the outer layers of the fibre surface. Both mid- and high-MW keratin peptides, increased Young’s modulus and reduced hair breakage at 20% and 80% rela- tive humidity. With the exception of mid-MW peptide, other pep- tides and amino acid were not able to modify thermal properties of relaxed textured hair. CONCLUSIONS: Our data suggest that low-MW compounds may increase hair volume, and high-MW peptides may repair damage on freshly relaxed textured hair. Resume OBJECTIF: Etudier les effets des keratines hydrolysees, issues de la laine, de differents poids moleculaires (PM) (par ex : peptides) sur les proprietes physiques des cheveux textures defrises. METHODES: Les cheveux tres boucles d’origine africaine ont ete defrises a l’aide d’un traitement a base d’hydroxyde de sodium. Les cheveux defrises ont ete traites avec des peptides de differents PM derives de la proteine de keratine et un acide amine, la L-leucine. Les peptides de keratine de PM faible etaient de 221 Da, les peptides de keratine de PM moyen etaient d’environ 2 577 Da et les pepti- des de keratine de PM eleve etaient d’environ 75 440 Da. La penetration de ces differents peptides dans les cheveux defrises a ete evaluee a l’aide d’un micrometre a balayage laser et par microsco- pie a fluorescence. L’effet de ces composes sur les proprietes meca- niques a fibre unique et les proprietes thermiques a ete evalue a l’aide de tests de traction et de l’analyse calorimetrique differentielle (ACD), respectivement. RESULTATS: Les composes de PM faible et moyen ont pu penetrer en profondeur dans le cortex des cheveux. Les peptides de PM eleve ont ete adsorbes sur la surface des cheveux et ont eventuellement penetre, legerement, dans les couches externes de la surface des fibres. Les peptides de keratine de PM moyen et eleve ont augmente le module de Young et reduit la casse des cheveux a 20 % et 80 % d’humidite relative. A l’exception des peptides de PM moyen, d’autres peptides et acides amines n’ont pas pu modifier les pro- prietes thermiques des cheveux textures defrises. CONCLUSIONS: Nos donnees suggerent que les composes de PM faible peuvent augmenter le volume des cheveux et que les peptides de PM eleve peuvent reparer les dommages sur les cheveux tex- tures fra^ ıchement defrises. Introduction Human hair is a unique bio-substrate, ideally adapted to fulfil its function as a protective and insulating shield for our skin. Hair fibres consist of an inner cortex, encased in a protective cuticle layer, which makes up most of the hair mass and provides much of hair’s mechanical strength as well. The key structural proteins in the hair cortex are (a) the keratins that constitute the intermediate filaments, and (b) keratin-associated proteins that form the matrix surrounding intermediate filaments. Both protein types are respon- sible for the hair’s tensile strength. Matrix proteins help provide flexibility to the hair as well. Although hair is naturally a very flexible and strong material, consumers choose to chemically modify, wash, treat and style their hair to change their appearance and look their best. Unfortu- nately, these practices can induce severe alterations in the hair. Furthermore, hair condition is made worse daily through expo- sure to chlorine from swimming pool water and environmental factors such as sunlight and pollution. To address these problems, a multitude of materials are available for the cosmetic formulator to develop various cosmetic treatments. Several approaches were designed to restore damaged hair condition, including hair lipid repair, cuticle surface repair and internal hair protein repair. Internal protein repair technologies that recently piqued interest Correspondence: Ernesta Malinauskyte, TRI Princeton, 601 Prospect Avenue, Princeton NJ 08540, USA. Tel.: +1 609 4304838 fax: +1 609 6837149 e-mail: emalinauskyte@triprinceton.org © 2020 The Authors. International Journal of Cosmetic Science published by John Wiley & Sons Ltd on behalf of Society of Cosmetic Scientists and Societe Francaise de Cosmetologie 1 This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. International Journal of Cosmetic Science, 2020, 1–12 doi: 10.1111/ics.12663 International Journal of Cosmetic Science Check for updates