A Quantitative Study of the Effect of

J. Soc. Cosmetic Chemists, 19, 699-706 (Sept. 16, 1968) A Quantitative Study of the Effect of Depilatory Solutions Upon Hair HARVEY ALLEN YABLONSKY, Ph.D., and ROBERTA WILLIAMS, B.S.* Synopsis--A procedure is described for determining the efficacy of depilatories. It involves the measurement of two parameters, the mean initial cross-sectional hair diameter and the time of maximum hair swelling. Plots of both the length and the width of swelling hair vs. time are sigmoid. The t vo curves are sequential, the longitudinal sigmoid curve commencing on termination f the cross-sectional one. Neither the time of occurrence, nor any other parameter oœ the sigmoid curve appears correlated to hair-break time. Seemingly identical hairs, in identical soluti{ms, show breaking times which vary from less than one hour to more than ten days. It is shown that slope maxima of the sigmoid curves may be used to define an in vitro index oœ depilatory effectiveness. INTRODUCTION The practice of hair removal for cosmetic purposes is an ancient one. Records of recipes for hair epilation and depilation date back as far as 1500 B.C. (1). Methods of attaining this end run the gamut from physically tearing the hair by its root to the application in an acceptable cosmetic base of disulfide bond-cleavers. Brute force methods of hair removal are usually dependent upon the imbedding or attachment of the undesired hair to a matrix which is first applied to the skin. Both the matrix and incorporated hair are then "painlessly" removed by tearing from the skin (2-5). Due to the questionable painlessness o[ this process, epilatories have, in this country, been largely superseded by them- * Bristol-Myers Products, Physical Chemistry Dept., Research Laborat rics, 135{) Liberty Ave., Hillside, N.J. 07207. 699

700 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ical depilatories. The most commonly used have been metallic sulfide and aliphatic mercapto compounds (6-12). This paper will be concerned with chemical depilatories, rather than epilatories. Evaluation of the efficacy of depilatories has, for the most part, been qualitative or, at best, crudely quantitative. In vitro methods employed at these laboratories in the past were dependent upon the break time o[ mechanically stressed hair during treatment with the preparatiou under consideration. These methods suffer from two major disadvantages: 1. Tests which rely on hair breaking serve merely to indicate the effect of the preparation upon the weakest point along the hair shaft, rather than upon the hair as a whole. Such a point is usually the thinnest portion of the hair or some previously damaged region. Analysis of both these regions is unrealistic since depilatories are required to remove hair at the skin line, where the hair is usually strongest and thickest. 2. The application of sizable stress to the hair creates an artificial condition. No such stress will be employed when the preparation is actually applied. The choice of stress must therefore, of necessity, be arbitrary. Correlation of break point data for different stress situations will be difficult since no simple relationship exists. A simple and realistic procedure for quantitative determination of depilatory effectiveness is proposed. It will be independent of cuticle condition, break time, and in most cases provide a degree of precision that is at least an order of magnitude higher than that obtainable through use of break time data. PROCEDURE A ll)-cm hair, having a small weight at its bottom, is hung in a standard taper capped test tube containing the solution of interest (Fig. 1) By means of a traveling microscope, the rate of swelling, both longitudinal and cross-sectional, is evaluated. The time at which the rate is maximized is taken after normalization as being inversely proportional to the efficacy of the preparation. The weight is nominally about 1 g and serves merely to keep the hair taut, thus facilitating optical measurement. The stress imparted to the hair by this weight is minimal and will not affect longitudinal swelling in nondepilating solutions. The extension observed for hair in distilled water under these conditions is less than 1% in 24 hours. A good corn-

QUANTITATIVE STUDY OF DEPILATORIES UPON }fair 7111. ::.:..x.x.:...:... Figure 1. Apparatus for the determination of hair swelling mercial depilatory will exhibit approximately 50% extension in under ten minutes. If a differential plot of swelling per unit time rs. time is made, then the times of rate maximum Td and Tz are clearly defined (Fig. 2). They are seven minutes for Td, the time at which the rate of cross-sectional swelling is a maximum, and ten minutes for Tz, the time at which the longitudinal swelling is maximum. DISCUSSION Inherent in the above procedure are several points of potential difficulty. Consideration and compensation for these factors will appreciably facilitate the analysis. In the determination of Ta, the question arises: Which point along the length of the hair, and in what orientation, should the rate of swdling be determined? Figure 3 shows the rate of swelling of four equallyspaced positions along a length of hair. No attempt was made to orient

_ 7O2 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS CROSS SECTIONAL / /! i _.. -,--'/[ 2 4 I0 12 14 16 IS 20 22 24 26 28 TIME, MIN. Figure 2. Cross-sectional and longitudinal swelling in depilatory solution.014.013 z.ol2,,,.011 0.010.009 ------J. o o. 5 1.0 1.5 Z.O Z.5 3,0 3.5 4.0 4.5 5.0 TIME, HRS. Figs/re,2. Cross-sectional swelling at different positions along a hair length the hair. The orientation, however, remained consistent throughout the determinations. The swelling behavior is typical of randomly chosen hair segments, varying according to the conditions of the hair at the particular points being observed. From these curves it is evident that 2', to be meaningful must be an average value. The hair shaft resembles a tapered ellipsoidal column. Its diameter will consequently be a function of position, as will also be T and Tt. and 2' in order to be meaningful must each therefore be "normalized" to make them independent of hair diameter. The normalization process may be simplified if the following precautions are taken:

QUANTITATIVE STUDY OF DEPILATORIES UPON IIAIR 703 (a) Comparisons are made within a given hair type. Caucasian should be compared to caucasian, negroid to negroid, and orieutal to oriental. This is done since the ellipsoidal cross-sectional characteristics of hair vary with hair type (e.g., oriental being much more circular than caucasian). Diameters will have meaning only if observed in some consistent manner in terms of the reference type chosen. (b) Only sections of hair of fairly uniform width, in which the tapering effect is minimal, are utilized for the determination of T /and If these two precautions are taken, the normalization process then consists of dividing Td and T by the mean diameter of the hair, as averaged from two points at its extremes. For convenience of measurement, the diameter of the major axis is utilized. The resulting normalized quantities are denoted by To and T. In hair with a marked difference between major and nilnor axes, the degree of swelling of the minor axis is appreciably greater than that of the major axis; on completion of swelling, the cross-sectional area will invariably be more circular. The authors were concerned that the hair, while swelling in diameter, might at the same time also be uncoiling longitudinally. This would create two problems: (a) Longitudinal swelling would mean that the point on the hair shaft, whose diameter was determined at one position of the traveling microscope, would no longer be at that position when the next determination was made. This, however, did not prove to be a serious source o[ error. From Fig. 2 it may be seen that most of the cross-sectional swelling is complete before appreciable longitudinal swelling sets in. (b) The possible twisting of hair on longitudinal swelling could create apparent changes in diameter merely by reorienting the axis being measured. The following two experiments were devised to deternfine whether this twisting did in fact exist: A thin steel wire in the form of a pointer was cemented perpendicular to the long axis and at the lower portion of the hair. A reference point was marked on the outside of the test tube. The position of the pointer, after swelling, was noted and found unchanged. The second, more sensitive, experiment utilized a small mirror cemented to the hair in the manner of the pointer in the first determination. This created, in effect, a configuration similar to that found in ballistic galvanometers, where a light beam is utilized to create a long pointer

704 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 0.6 -- _ 443 - I 2 :5 4 5 6 TIME, MIN. Fig re 4. Differential longitudinal swelling of two hairs in the same depilatory preparation arm. Very small changes in are may be discerned by this procedure. The result of this experiment was in agreement with the first. No rota- tion was detected. In contrast to the determination of cross-sectional swelling, the measurement of maximum rate of longitudinal swelling is quite straightforward, independent of hair orientation, and practically independent of surface damage. The independent nature of the maximum rate of extension is readily apparent if one considers its time of occurrence to be a functiou of the summation of the individual rates of extension along the entire hair length. If a small section of hair is damaged such that the time at which its maximu rate of extension occurs differs appreciably from that which would be expected of the hair as a whole, its extension, being small by comparison to the total extension, will not affect a change iu T,. This may be seen in Fig. 4, where two hairs are placed in thioglycolate solutions of identical concentration and ph. Although the plots are quite dissimilar (T/(443) = 4.0 minutes, T/(444) : 4.5 minutes) when T is normalized to T, the values obtained are equal (Tt(44a) = 4.0 minutes, and T/(444) : 4.2 minutes). These values approach the maximum precision attainable when T is small since readings are normally taken every 15 seconds. Sections of several hairs were abraded with wooden spatulas, and their T values compared with the corresponding unabraded section. No appreciable change in T was discernible.

QUANTITATIVE STUDY OF DEPILATORIES UI'ON IiAiR 2.6 2.4 2.2-2.0-1.8-1.6-1.2 0 4 6 8 I0 12 14 16 18 20 TI ME, MIN. Figure 5. Longitudinal swelling of hair in a depilatory si)lution The measurement of T. offers one other distinct advantage over the measurement of TD. Although cross-sectional swelling occurs when hair is placed in solutions which are not capable of depilation (e.g., distilled water), longitudinal swelling does not. The plotting of differential curves is often cumberso ne and timeconsuming. An alternate method has therefore been adopted (viz., plotting hair lengths vs. time). This procedure produces a sigmoid curve (Fig. 5). If the segments of the curve comprising both sides of the first knee are considered linear, the intersection of these two lines is an excellent approximation of (T q- T )/2 (Figs. 2 and 5). This average ' is also inversely proportional to reactant activity. The approximation may be easily verified by comparison of Fig. 2 with Fig.,5, both having been derived from the same determination. (T, q- T )/2, when obtained from Fig. 2, is 8.5 minutes; by extrapolation in Fig. 5, it is 8.,5 minutes. This approximation should not be used interchangeably with either Ta or T ; to be meaningful it must be normalized in the same nmnher as those functions. The normalized average approximation J'./o has been utilized successfully in the evaluation of preparations produced in this laboratory. It is comparable to the time required to remove human axillary hair completely and is equal to approximately three times the time necessary for complete depilation of belly fur on test rabbit. When evaluating opaque depilatories, the procedure may be modified by having the 1-g weight in the form of a thin steel rod. The sufficiently long rod is attached to the bottom of the hair in a manlmr such

7O6 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS that it will hang parallel to it and also extend above the solution. The depth of the rod in the solution may then be monitored either optically or through use of a linear voltage differential transformer. CONCLUSION T,, Tf,, and T,/z may all be correlated with depilatory efficacy. However, Tj,/D was found to be the most convenient. Evaluation of nearly 500 depilatory preparations has shown this method to be reproducible to a high degree of precision and essentially independent of the physical condition of the hair under test. The simplicity of procedure and paucity of necessary equipment make it ideally suited for t he rapid evaluation of formulation modifications. ( 0) (1l) (12) REFERENCES (Received November 20, 1967) (1) The Papyrus Ebers, trans. by B. Ebbell, Vol. 63, Copenhagen and London, 1937, p. 76; Ibid., Vol. 67, p. 80. (2) Hiss, A. E., and Elbert, A. E., New Standard Formulary, G. P. Engelhard, Chicago, 1910, p. 1001. (3) Weinhold, P.M., Brit. Patent 478,176 (1938). (4) Lucas, II. V., U.S. Patent 2,067,909 (1937). (5) Gemsback, H., U.S. Patent 1,620,$39 (1927). (6) Perl, J., U. S. Patent 450,032 (1891). (7) Evans, R. L., and McDonough, E.G., U.S. Patent 2,3 2, 24 (1944). (8) Alexander, P., Review of progress in cosmetics, Specialties, 16 $ (1965). (9) Walker, G. T., The chemistry and formulation of depilatories, Am. Perfumer Cosmetics, 77, 86 (1962). Schimmel Britfs, Cosmetic formulations from the patent literature, H340 (July 1963)., Rousell Labs Ltd., Calcium thioglyeollate as dipilating agent, Soap, Perfumery Cosmetics, 31, 799 (1955). Bull, K. B., Chemical depilatories, Ibid., 31 63 (1965).