462 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS depilatory preparations. Advantages of this method are that it is reproducible and rapid to perform, and could probably be run on most commercially available tensile testing devices. Solutions, lotions, or creams may be tested with equal facility. We have chosen T95 % as a convenient parameter to evaluate the relative performance of reducing systems in a depilatory context. We have found most systems likely to be of interest, as depilatories will produce 95% tensile weakening within a reasonable time. The exact percent chosen is not critical, as examination of our raw data indicates that any percentage between 85% and 97% would have led to all of the systems evaluated in this report being ranked in exactly the same order as they were by T95 •. MATERIALS AND EQUIPMENT Our studies were conducted chiefly with human scalp hair for the sake of availability plus the ability to run more than one experiment from a single strand. For most experi- ments, the hair was from a single donor who had never subjected her hair to chemical treatment. The hair was given a double lathering with Prell © shampoo and rinsed thorougly prior to use. Tensile measurements were made on an Instron © tensile tester. Measurements of hair diameter were made on an optical diameter gauging system made by the Diffracto Corporation, Windsor, Ontario. Wetting force measurements were made on a Perkin Elmer AD-2Z electrobalance. THE SFTK METHOD The single fiber tensile kinetics method is based on stress relaxation caused by the breakage of disulfide bonds. The hair is first extended to 2% strain in a buffer solution and rapidly stress relaxed to a final extension of 1.5% strain by the following procedure: After 20 seconds of stress relaxation at 2%, the strain is reduced to -1.0% for 20 seconds, increased to --1.8% for 20 seconds, reduced to --1.2% for 20 seconds, in- creased to --1.7% for 20 seconds, and then reduced to the final value of 1.5%. A constant level of stress is reached within less than two minutes at 1.5% strain. The buffer is then replaced by the reducing agent of interest and reaction is followed by loss of tensile stress. Data may be analyzed by either of two kinetic models (3) or by com- paring the time required to reach a given level of force reduction (4). Data can be displayed graphically as the normalized stress versus time. Normalized stress is defined as the tensile stress at time t, divided by the equilibrium stress reached before addition of reducing agent. RESULTS AND DISCUSSION REDUCTION AND DIAMETRICAL SWELLING Reduction of disulfide bonds in hair leads to tensile weakening and increased swelling in water (1,5). We have followed swelling in the wet state by a modification of the wettability measurement described by Kamath eta/. (6). The hair is placed in surfactant solution and the receding wetting force, F, is measured. By assuming that the receding hair liquid-contact angle, {), is zero, the wet perimeter, P, can be calculated from: F = y P cos {)

SINGLE FIBER DEPILATORY STUDIES 463 where 'y is the liquid-air surface tension. Work in our laboratory indicates that the assumption of a zero receding hair-liquid contact angle is reasonable, especially since cos 0 is relatively insensitive when 0 is small. Figure 1 shows the percent of diameterical swelling and loss in tensile stress as different sections of the same hair were reduced in sodium dihydrolipoate (6,8-dithiooctanoate) containing 0.1% of an ethoxylated linear fatty alcohol nonionic surfactant. When the tensile stress was reduced by 50%, the hair had swollen to --112% of its original wet diameter. By the time the tensile force was reduced by 95%, the hair was swollen to about 145%. At this point, the hair, which had been dark brown, was translucent and was easily broken by rubbing between the fingers. CORRELATION TO IN VIVO DEPILATION Figure 2 shows SFTK curves obtained for two commercial depilatory products con- taining thioglycolate, using a subject's leg hair. Product B is seen to reduce the tensile force much faster than product A. The time to reduce the tensile stress by 95%, T95 %, was 5 minutes for product B. Product A reduced the tensile force to a constant level after about 12 minutes. The hair was totally weakened by this point, and we believe that the small difference from baseline is due to the high viscosity of the product, which did not allow the Instron © clamp to settle to exactly the same position as in the buffer solution. The time to reach 95% of this final level was approximately 8 minutes. Table I shows the approximate percent of hair removed from the legs of this same individual lO[ 80- 60- 40- 20- I i I I I • i 0 10 20 30 40 50 60 70 Time (minutes) Figure 1. Swelling of hair measured by wetting force, and reduction measured by SFTK, 0.14 M lipoate, pH 9.0, 22øC.