INTERNAL LIPIDS IN HAIR HEALTH 355 These data illustrate that it is feasible for lipids, such as FaOHs, to penetrate hair and increase hair strength. The exact nature of how FaOHs increase fatigue strength is not completely understood but it is hypothesized that these actives partition into the lipid- rich cortical CMC and rebuild its structural order, preventing the formation and prop- agation of fl aws when hair is subjected to repeated extensions. CONCLUSIONS Optical microscopy has been used to show how lipid structures in hair form a continuous linked structure throughout the cortex playing a major role in maintaining fatigue strength. Measurement of lipid levels has shown how these lipids are lost after washing and exposure to coloring treatments and UV. A mechanism for oxidation of unsaturated fatty acids was proposed and evidence provided by the measurement of LPOs. Replenish- ment of these lipids was achieved via incorporation into a gel network structure that ef- fectively deposits on hair and releases the lipids into hair during rinsing. This penetration of internal lipids with FaOH, increasing internal strength was measured by fatigue breakage testing. Table II Log P and molecular volume properties of FaOHs versus fatty acids Lipid Log P (octanol:water) Molecular volume (Å3) Cetyl alcohol (C16) 6.45 200.3 Palmitic acid (C16) 6.39 205.8 Stearyl alcohol (C18) 7.36 221.2 Stearic acid (C18) 7.31 228.8 Calculated by ACD/Labs Software (http://www.acdlabs.com/home/). Figure 8. FaOH surface deposition and penetration from a shampoo formulated with a FaOH gel network and from shampoo where FaOH was added straight into shampoo (2.3% total FaOH for both shampoos).

JOURNAL OF COSMETIC SCIENCE 356 ACKNOWLEDGMENTS The authors would like to thank The Procter & Gamble Company for support of this work. REFERENCES (1) J. R. Schwartz, J. P. Henry, K. M. Kerr, H. Mizoguchi, and L. Li, The role of oxidative damage in poor scalp health: ramifi cations to causality and associated hair growth, Int. J. Cosmet. Sci., 37(S2) 9–15 (2015). (2) C. R. Robbins, The cell membrane complex: three related but different cellular cohesion components of mammalian hair fi bers, J. Cosmet. Sci., 60, 437–465 (2009). (3) Y. Masukawa, H. Tsujimura, H. Tanamachi, H. Narita, and G. Imokawa, Damage to human hair caused by repeated bleaching combined with daily weathering during daily life activities, Exog. Dermatol., 3, 273–281 (2004). (4) C. F. Cruz, M. M. Fernandes, A. C. Gomes, L. Coderch, M. Mart, S. Mendez, L. Gales, N. G. Azoia, U. Shimanovich, and A. Cavaco-Paulo, Keratins and lipids in ethnic hair, Int. J. Cosmet. Sci., 35, 244–249 (2013). (5) J. Koch, K. Altzetmüller, G. Bittorf, and J. Waibel, Hair lipids and their contribution to the perception of oiliness: parts I and II, J. Soc. Cosmet. Chem., 33, 317–343 (1982). (6) Y. Masukawa, H. Narita, and G. Imokawa, Characterization of the lipid composition at the proximal root regions of human hair, J. Cosmet. Sci. 56, 1–16 (2005). (7) J. M. Marsh, M. G. Davis, M. J. Flagler, Y. Sun, T. Chaudhary, M. Mamak, D. W. McComb, R. Williams, K. D. Greiss, L. Rubio, and L. Coderch, Advanced hair damage model from ultra-violet radiation in the presence of copper, Int. J. Cosmet. Sci, 37, 532–541 (2015). (8) R. McMullen, D. Laura, S. Chen, D. Koelmel, G. Zhang, and T. Gillece, Determination of physio- chemical properties of delipidized hair, J. Cosmet Sci, 64, 355–370 (2013). (9) G.A Camacho-Bragado, G. Balooch, F. Dixon Parks, C. Porter, and H. Bryant, Understanding breakage in curly hair, Br. J. Dermatol, 173(Suppl. 2), 10–16 (2015). (10) J. M. Marsh, M. A. Brown, T. J. Felts, H. D. Hutton, M. L. Vatter, S. Whitaker, F. C. Wireko, P. B. Styczynski, C. Li, and I. D. Henry, Gel network shampoo formulation and hair health benefi ts, Int. J. Cosmet. Sci., 39, 543–549 (2017). Table III Fatigue breakage data for hair treated with gel network shampoo Treatment Alpha (α) value Beta (β) value Control chemically treated hair (no treatment) 6,955 0.693 16 cycles shampoo with no gel network 8,328 0.682 16 cycles shampoo with gel network (total FaOH in shampoo = 2.3%) 12,142 0.569