j. Cosmet. sci., 52, 377-389 (November/December 2001) Hair photoprotection by dyes CHANDRA M. PANDE, LINDA ALBRECHT, and BRIAN YANG, Clairol Inc., 2 Blachley Road, Stamford, CT 06922. Accepted for publication August 15, 2001. Synopsis We have found tinat hair dyes protect hair against photodamage. The efficacy of photoprotection of dye products has been measured by monitoring the tensile strength of hair fibers and the integrity of the disulfide bonds in the fibers. Although the hair dyed with permanent dye products are initially weakened due to oxidative chemical damage, they also show a slower rate of degradation upon photoirradiation, compared to the undyed hair. Thus, a less appreciated benefit of using hair colors is their ability to provide protection against sun damage. INTRODUCTION Human hair has a complex structure, with at least three distinct phases. The cuticle cells form a hard outer protective barrier for the cortical cells within. The latter contain fibrillar proteins, which provide mechanical strength to the fiber, and melanin pigments responsible for color, embedded in an amorphous protein matrix. Occasionally, the fibers also contain a medulla, located in the center of the fiber, whose function is not well understood. The dry mass of hair fibers is primarily made up of keratin proteins, with small contribution from lipids and melanin pigments. Under ambient conditions of 25øC and 50% RH, hair binds ca. 10% water by weight (1-3). The natural hair color originates from a combination of absorption and scattering of the incident light by the pigment granules that are distributed within the cortical cells. Exposure of hair fibers to sunlight, or simulated solar light, leads to bleaching of the color, as well as damage to the fiber itself. At the molecular level the fiber damage involves the amino acids, constituting the proteins, and the lipids (3-9). One of the manifestations of this structural damage is a loss in the mechanical strength of the affected fibers (8). It has been shown that the mechanical strength of the unpigmented fibers is compromised at a rate that is significantly faster than that seen for the pig- mented fibers (9). It has also been shown that the rate of scission of the cystine disulfide bonds due to weathering is also faster for the unpigmented fibers, compared to the pigmented fibers (10). This correlation between the loss in the mechanical strength of the fibers and the scission of the disulfide bonds is not surprising since the latter contribute significantly to the wet mechanical strength of hair fibers (1, and references cited therein). 377

378 JOURNAL OF COSMETIC SCIENCE The above discussion clearly shows that the natural hair-coloring pigments, which are located in the cortex, protect hair against sun damage as judged by the above criteria. It is, therefore, tempting to expect that hair-coloring products would perform even better, since they also color the cuticles. However, dye molecules may not be passive optical filters and, instead, accentuate light-induced damage by initiating additional photochemical reactions. Here, we have investigated the effect of hair dyes on the photodegradation of hair. We have found that hair colors, in general, provide protection against sun damage. This protective effect is immediately realized with direct or semipermanent colors. Hair dyed with products utilizing oxidation dyes, although initially weakened due to chemical damage, also shows net protection upon prolonged photo-exposure. These data also reveal, not surprisingly, that the darker shades that deposit more color on hair also provide better photoprotection. EXPERIMENTAL Different methods were employed to measure the efficacy of products using oxidation dyes and those using direct or semipermanent dyes. The efficacy of the former was evaluated using tensile measurements. This method was not suitable for evaluating the semipermanent products because a significant amount of color would bleed during measurements. It has been shown that the mechanical strength of hair fibers in the wet state correlates very well with the amount of disulfide bonds in hair (11). A Raman spectroscopic method, which measures the integrity of the disulfide bonds in hair, was, therefore, used for these products. This method, however, is not suitable for testing hair colored with oxidation dye products because of thermal heating associated with absorp- tion of the near-infrared probe beam. PERMANENT AND DEMIPERMANENT PRODUCTS Piedmont or medium brown hair supplied by DeMeo Brothers (New York) was used. The hair was cleaned prior to testing using a sodium lauryl sulfate (SLS) solution (10%, w/w). Tensile measurements. Tensile measurements were performed on single fibers in the wet state. To reduce the data scatter due to differences between fibers, the effects of dyeing and photoirradiation were measured individually for each fiber. Hair fibers, 30-ram in length, were used in a Dia-Stron MTT 600 tensile tester (Dia- Stroh Ltd) to obtain the F•5 (force needed to extend the fiber by 15%) in deionized water. The fibers were extended at a rate of 20 mm per minute up to 20% extension and then released. Upon completion of the tensile measurements, the fibers were immersed in deionized water for two hours at room temperature (ca. 20øC) and then air dried. Although an "overnight" soak is typically used in this type of experiment (1,12), we have found that a two-hour soak, under our experimental conditions, was sufficient to restore the original tensile properties of the fibers. Sets of 25 fibers, which had undergone initial tensile testing, were dyed using various shades of a representative permanent and a demipermanent or tone-on-tone color for 25