272 JOURNAL OF COSMETIC SCIENCE hair is detectably photochemically damaged due to its low pigment content or pigmen- tation with pheomelanin. Hair damage causes a reduction in tensile and fatigue properties (3). According to Robbins and Bahl (4) and Speakman and McMahon (5), the reduction is largely attrib- uted to chemical damage to the protein chains, especially the S-S covalent crosslinks in cystine. They have found that photochemical damage, the loss of crosslinks, leads to an increase in solvent swelling, a finding not shared by Wolfram (6), who argues that crosslinking between amino acid residues is induced by irradiation and that these new crosslinks would limit the solvent swelling in the case of keratin fibers. According to the hair photolysis mechanism proposed by Tolgyesi (1), cystine, tyrosine, phenylalanine, and tryptophan residues absorb UV radiation, resulting in the formation of free radicals. Homolytic scission of disulfide bonds occurs. Melanin, the natural coloring matter in hair, provides partial protection to the hair fiber. It acts in a sacrificial manner, resulting in lightening of hair color. Tolgyesi explained that melanin, by its comprehensive system of double bonds and conjugated carbonyl groups, protects hair by scavenging free radicals generated by exposure to light. Wei (7) furthered the concept of the free-radical mechanism, showing that, all other factors being equal, the extent of photodamage is proportional to the square root of the amount of free-radical initiators present in the fiber. The mechanism assumes that the S-S group is the initiator, giving two -S free radicals by homolytic scission. A study using field emission scanning electron microscopy (FESEM) was carried out by Ruetsch et aL (8) to monitor the effects of UV irradiation 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. Swift and Smith (9) reported the advantages of the use of AFM, such as the greater details in the hair's cuticular surfaces that appear not to be as smooth as had been previously supposed when evaluated by SEM. They observed that the surface of the hair gradually changes as a function of its length, measured from the hair root. This tech- nique and the effects of subsequent damage are herein discussed. The study reported in this paper verified these initial findings on the damage of human hair exposed to sunlight, and also determined the photochemical damages associated with the physical morphological changes in hair structure. Scanning electronic micros- copy (SEM) analyzed the damage of UV radiation on hair fibers. This technique was employed to observe the damage induced on hair fibers by UV radiation. The use of atomic force microscopy (AFM) provides a new understanding of the mechanism of UV radiation damage to hair by physical quantification of the nature and damage of hair cuticle as roughness and thinning. MATERIALS AND METHODS MATERIALS Untreated black Caucasian hair was obtained from De Meo Bros., New York. Hair samples were pre-treated using 3% LAS (lauryl ammonium sulfate).

EVALUATION OF UV HAIR DAMAGE BY AFM 273 IRRADIATION OF HAIR Hair tresses weighing 2.0 g were exposed to simulated solar radiation in the Atlas Weather-Ometer, model 65 XW-WR1. It utilized a 6500W xenon lamp. The specific exposure condition of hair fibers to UV/visible radiation was 160 hours. The program- ming used was 102 minutes of insolation at relative humidity (RH) levels of 50 + 5% and 18 minutes of insolation with rain simulation with 100% RH. The mean tempera- ture in a cycle of two hours was 60øC. The energy density at the 340-nm wavelength was maintained at 0.35 W/m 2. In the wavelength range of 250-800 nm, the energy densities were 397 W/m 2, resulting in a total energy density of 41.27 mW/cm 2. TESTING Two different techniques were employed to assess hair photodamage: (1) the scanning electronic microscopy (SEM) technique was used to identify morphological changes on the hair surface and (2) the atomic force microscopy (AFM) technique was used to analyze hair morphology as the environmental conditions changed, allowing the direct study of roughness and cuticle thickness. SCANNING ELECTRON MICROSCOPY (SEM) SEM has been widely used over many years for claim substantiation of various toiletry treatments on the architecture of the hair's surface. It has been used to evaluate the effectiveness of hair shampoos in the removal of sebum and detritus, to investigate the effects of permanent waving systems, and to observe the deposition of polymeric mate- rials from hair products. Although SEM analyses provide valuable information about the hair surface, which helps to understand some of the existing hair products and aids in the development of new ones, it should not be seen as a key technique to understand all product systems. Its use is limited by many factors because pictures produced by SEM usually cause a high visual impact, but the technique is poor in terms of quantitative analysis. Longitudinal segments of untreated and UV-exposed fibers were mounted on an alu- minum substrate and coated with an approximately 90 • thickness of gold. The hair fiber topography was examined in a ZEISS 940-A spectrometer digital scanning electron microscope coupled to EDS and PGT, Digital Prism model. The objective was to analyze the morphological alterations in hair exposed to UV radiation. ATOMIC FORCE MICROSCOPY (AFM) The atomic force microscopy (AFM) technique has received a great deal of attention in recent years due to its enormous potential for the study of the surface of materials. AFM can be used for imaging non-conductive surfaces. The equipment consists of a probe, a laser beam, and a photodiode used to detect the movement of the probe. The probe, in the form of a sharp tip attached to a cantilever, scans the surface by using force in the order of 10 to 20 nN in the contact mode, and 0.1 nN in the tapping mode. The tapping mode is used to measure the surface characteristics of soft materials such as some