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As the main building blocks of hair, the loss of amino acids not only directly affects the structure of the hair shaft, but also its overall appearance and behavior. Significant quantities of certain amino acids are depleted from the hair during the weathering process, as a result of mechanical, photochemical, and chemical damage. Standard amino acid extraction/analysis techniques revealed that 6 key amino acids are preferentially lost during the weathering process, as measured by root (undamaged) versus tip (damaged) amino acid content. Subsequent experiments demonstrated that it is possible to reintroduce some of these amino acids via cosmetic preparations. Deuterium labeling coupled with scanning focused ion beam analysis was used to prove the penetration of the amino acids into the cortical region of the hair shaft. Additional deuterium labeling, followed by deuterium trace analysis, revealed that the quantity of the reintroduced amino acids may be increased as a function of both concentration and exposure frequency. Leave-in treatments are the most effective vehicle for amino acid deposition and penetration. Further, the reintroduction of these select amino acids had a direct effect on the strength of the hair, resulting in significant advantages for tensile properties. Thus it has been demonstrated that key amino acids depleted within the hair fiber may be reintroduced using cosmetic treatments and that the reintroduction increases with repeated usage or with products of higher amino acid concentration. Finally, the reintroduction has a demonstrable benefit on the strength of the hair fiber, which is closely linked to long-term improvements in aesthetic properties, such as the look and the feel of healthy hair, as well as reduced hair damage and breakage.
Beth Huggins, PhD, Chris Gummer, PhD, Cheri McMaster, MS, Emily Pack, BS, The Procter & Gamble Company, Cincinnati, OH, United States