from the inside by creating salt-linkages, this peptide may also have created covalent bonds to some extent via -S-S- -SH inter- change at several locations inside the hair. Small increases in denaturation temperature in comparison with the control supports this proposed mechanism of action. Literature also agrees that peptides containing disulphide bonds have a high affinity to hair keratins [14]. High-MW peptides, because of size restriction, could not pene- trate deeper than the cuticle layer or cracked cortex. Alternatively, high-MW peptides can unfold and adsorb on the surface forming a cohesive film and filling the cracks. The film was visible at 10 0009 images, and the peptides were not rinsed from the cracks as effectively as from the surface. We postulate that the strength of the thin proteinaceous film was able to suppress the premature breakage (reduced crack formation and delayed the crack propaga- tion) resulting in higher break stresses than if fibre would have broken prematurely. This was indirectly confirmed by the data pre- sented in Table 1 (lower amount of premature breakage observed). To confirm or disprove these hypotheses, additional experiments must be designed and executed. Our findings showing partial hair strength recovery after treat- ment with mid- and high-MW peptides are in agreement with the trends observed by research groups who studied (i) mid-MW peptide effect on relaxed African hair [34], (ii) mid-MW peptide effect on bleached Caucasian hair [15] and (iii) high-MW protein effect on over-bleached Caucasian hair [35]. Similar trends of mid-MW peptide effects on bleached hair tensile strength were observed by Barba et all however, the significance of these results was compromised by the sample size being very small (N = 10) [10]. Conclusions To the best of our knowledge, ours is the first study that compares the effect of low-, mid- and high-MW peptides on relaxed hair using the same test conditions. The results from this study show that the high-MW keratin peptides only penetrated into the surface layers of relaxed textured hair, whereas the mid-range keratin pep- tides were able to penetrate deeper into the cortex. However, both the mid-range and high-MW keratin peptides were able to improve breakage parameters: increase in break stress and reduce the num- ber of premature fractures. Although fibre dimension measure- ments suggest significant penetration of the low-MW keratin peptides and leucine (significantly increased cross-sectional area of the hair), it appears that these compounds did not have any pro- tein stabilizing effect. Our data, therefore, suggest that low-MW compounds may have a volume increase effect, and mid- and high- MW peptides may indeed have a damage repair effect on freshly relaxed textured hair. Acknowledgements We wish to acknowledge the technical assistance by Katerin Mateo of TRI Princeton and Oluwaseun (Seun) Okimi, an undergrad stu- dent of Duke University (DSC and tensile data collection). This work was supported by funding from Croda and TRI Princeton. Conflicts of interest The authors report no conflicts of interest. References 1. Song, K., Xu, H. and Xie, K. Effects of chem- ical structures of polycarboxylic acids on molecular and performance manipulation of hair keratin. RSC Adv. 6, 58594–58603 (2016). 2. Pressly, E.D. and Hawker, C.J. Keratin treat- ment formulations and methods. 9 498 419 B2, Washington DC (2016). 3. Itou, T., Nojiri, M., Ootsuka, Y. et al. Study of the interaction between hair protein and organic acid that improves hair-set durabil- ity by near-infrared spectroscopy. J. Cosmet. Sci. 57, 139–151 (2006). 4. Schulze Zur Wiesche E. Powerful color-pro- tecting hair treatment agent having a dipeptide World patent WO 2016/091784 AI, Evonik Degussa (2014). 5. Wortmann, F.J., Schulze Zur Wiesche, E., Oberkobusch, D. et al. Influencing absorption and desorption of ions in human hair Presented at HairS’15 Conference, Trier, Germany, September 2–4th (2015). 6. Everaert, E.P.J.M., Tran, D., Zhang, S. et al. Strengthening the hair fiber from within: repairing the cortex of damaged hair. IFSCC Mag. 3, 111–117 (2015). 7. Pressly, E.D. and Hawker, C.J. Methods for fixing hair and skin. 9 095 518, Washington DC (2015). 8. Gargano, E.M., Mangiatordi, G.F., Weber, I. et al. 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International Journal of Cosmetic Science published by John Wiley & Sons Ltd on behalf of Society of Cosmetic Scientists and Societe Francaise de Cosmetologie 11 International Journal of Cosmetic Science, 1–12 Larger hydrolyzed keratins reduce hair breakage E. Malinauskyte et al.