j. Cosmet. sci., 51, 103-125 (March/April 2000) Photodegradation of human hair: An SEM study SIGRID B. RUETSCH, Y. KAMATH, and H.-D. WEIGMANN, TRI/Princeton, P.O. Box 625, Princeton, NJ 08542. Accepted for publication January 3 I, 2000. Synopsis This study uses field emission scanning electron microscopy (FESEM) to monitor the effects of UV irra- diation on the physical nature of hair fibers. Long-term UV irradiation/humidification cycling causes thinning and fusion of the surface cuticle cell, as well as fusion of the cuticular sheath into a solid, rigid, and brittle unit. While intercellular cohesion within the cuticular sheath is high, possibly due to crosslink- ing of the proteins in the intra- and intercellular domains, the cells themselves are brittle. A newly observed fracture pattern of long-term UV-exposed fibers suggests fusion of the regions attacked most severely by UV light into one rigid and brittle mass, incapable of extension due to loss of all original elastic properties. Unlike chemical oxidation, which results in partial dissolution (1 h H202) and then complete solubilization (4 h H202) of the melanin granules, photochemical oxidation produces entirely different results. Even after long-term UV irradiation/humidification (95% RH) cycling, the melanin granules appear physically intact. Loss of color does not occur as long as the melanin granules are intact. The severity of photodegradation during UV irradiation/humidification cycling becomes apparent upon brief (seconds) contact of these fibers with alkaline hydrogen peroxide. Such contact results in instantaneous disintegration of the components within the cuticle cells. Formation of sac-like structures (AllwiSrden sacs) occurs due to osmotic pressure within seconds of exposure to alkaline hydrogen peroxide caused by pho- tochemically degraded proteins within the surface cuticle cells. The cells swell until they burst and their contents drain, leaving behind cuticular membranes, which may detach or fuse to the fiber surface. UV irradiation has also severely photodegraded the melanin granules and preconditioned them for accelerated solubilization upon contact of the fibers with alkaline hydrogen peroxide. The effects of both relative humidity and spectral energy distribution on the photochemical oxidation of the hair fiber are studied. Results obtained at various relative humidities in two different fading units, namely, the QUV Accelerated Weathering Tester and the Atlas Weather-Ometer © ("AW") are compared. Scale thinning and fusion observed during UV/humidification cycling are greatly reduced with exposure at low humidities without humidification cycles. Upon post-treatment with water, fibers irradiated at a constant 10% RH in the QUV show scale thinning and fusion similar to that of fibers exposed to UV/humidification cycling. This indicates that photodegradation occurs at low humidity as well. Fibers exposed at constant 20%, 50%, and 70% humidity in the "AW" show only moderate scale thinning, even after post-treatment with water. The total solar spectrum used in the "AW" apparently causes less severe photodegradation of the proteins than the UV light of the QUV. INTRODUCTION When exposed to sunlight, hair is known to undergo changes in morphological, chemi- cal, and mechanical characteristics (1-4). The lower wavelength range of the UV com- 103

104 JOURNAL OF COSMETIC SCIENCE ponent of sunlight is known to be responsible for these changes. In recent years, UV radiation at lower wavelengths has significantly increased due to the deterioration of the ozone layer. While the earth's atmosphere filters out most radiation below 295 nm, depletion of the ozone layer, and therefore the reduction in the screening effect of the atmosphere, permits lower wavelength components to reach the earth's surface. These lower wavelength regions of the UV radiation received by the earth are the most energetic and therefore can cause severe photodegradation. The UV range of the sunlight can be divided into three wavelength regions, namely UV-A (320-400 nm), UV-B (280-320 nm), and UV-C (280 nm). UV-C radiation is totally filtered out by the atmosphere and is only experienced in space. We have carried out extensive studies, investigating by various microscopic techniques the effects of photochemical oxidation on specific aspects of hair damage such as the appearance of the surface cuticle cell, intercellular cohesion, scale lifting during longi- tudinal extension, the integrity of the cuticular sheath, the physical nature of the melanin granules, characteristic fracture patterns, and loss in hair color. In other words, we have examined radiation-induced changes in the physical rather than chemical nature of the hair fiber. Hair fibers used in these studies were exposed to UV radiation/ humidification cycling in a QUV Accelerated Weathering Tester. Various follow-up treatments of these long-term UV-irradiated fibers illustrate the extent of photodegra- dation inflicted upon the hair proteins. More recently, we have also examined the effects of both the relative humidity and the spectral energy distribution in the radiation on the photochemical oxidation of the hair fiber. Comparisons were made between the results obtained at various relative humidi- ties in two different fading units, namely, the QUV Accelerated Weathering Tester (290-400 nm UV-A kn• = 340 nm) and the Atlas Weather-Ometer ©, "AW" (Xenon solar simulator, 250-800 nm). Using FESEM, we characterized by different approaches the extent of photodamage inflicted upon the physical nature of the hair fiber in general and the cuticula in particular. EXPERIMENTAL MATERIALS Hair type. Root sections of 14-in-long, brown European hair fibers from DeMeo Brothers were used. The major axis of these hair fibers ranged from 70 lam to 120 lam. Because of this variation in size, single hair fibers also varied in the depth of shade. Fibers with the larger major axis were more elliptical than fibers with a smaller major axis. UV EXPOSURE CONDITIONS QUV Accelerated Weathering Tester. The QUV simulates the sunlight in the range of 290-400 nm, with an irradiance maximum of the fluorescent bulb at 340 nm. The irradiance intensity factor has been chosen to be 1.35 compared to 1.0 for regular sunlight. The energy density at the 340-nm wavelength is kept constant at 0.97 W/m 2. The total energy density of the UV light in the wavelength range of 300-400 nm is 5.06 mW/cm 2. Hair fibers were exposed to two sets of conditions in the QUV: (a) 0, 100, 300, 500, and