SUBJECTIVE IRRITATION BY SOAPS 161 hydration and emollient application and correlation with perceived skin feel, J. Soc. Cosmet. Chem., 32, 55-65 (1981). (6) E. L. Cussler. "Predicting Skin Feel," in Cosmetic Science, Vol. I, M. M. Breuer, Ed. (Academic Press, New York, 1978), pp. 117-152. (7) P. J. Frosch. "Irritancy of Soap and Detergent Bars," in Principles of Cosmetics for the Dermatologist, P. Frost and S. N. Horwitz, Eds. (C. V. Mosby, St. Louis, 1982), pp. 5-12. (8) M. C. Gacula, Jr. and R. W. Washam II, Scaling word anchors for measuring off flavors, J. Food Quality, 9, 57-65 (1986). (9) E. L. Lehmann, Non-Parametric Statistical Methods Based on Ranks (Holden-Day, Inc., San Francisco, 1975), p. 130.

j. Soc. Cosmet. Chem., 82, 163-169 (May/June 1987) Crosslinking in coilaDen (rat tail tendon) D. G. KAPLAN, A. B. FURTEK, and J. J. AKLONIS, Department of Chemistry, University of Southern California, Los Angeles, CA 90089 Received October 9, 1986. Synopsis Mechanical creep experiments have been carried out on swollen denatured tail tendons from rats of various ages. Even though covalent crosslinks may be present in these tendons, our results show that there are not enough effective crosslinks to result in a network structure capable of exhibiting equilibrium elastic be- havior in mechanical experiments. Tendons with additional synthetically incorporated crosslinks clearly show such equilibrium behavior. This being the situation, it is doubtful that the natural crosslinks in collagenous materials are an important influence on mechanical strength of tendon as has been claimed. INTRODUCTION Crosslinking can cause substantial changes in the physical and chemical properties of polymeric materials. The aging of collagen, a high molecular weight polymer, and subsequent changes in its properties, have frequently been ascribed to variations in the extent of crosslinking (1-8). The high tensile strength of collagenous tissues has also been associated with the presence of these covalent crosslinks (9-11). There have been numerous chemical methods used to study crosslinking in collagen (12-19), and such work has resulted in the identification of several chemical species which bind collagen chains together. However, not all high-strength polymeric materials contain covalent crosslinks. For example, semi-crystalline polymers are known which have mechanical strengths supe- rior to corresponding amorphous polymers even when uncrosslinked. Examples of such materials are polyethylene, polypropylene, and Mylar. In such materials, even moderate crosslinking results in negligible changes in physical properties below the crystal melting temperature. Kevlar, one of the strongest synthetic polymers known, is a crys- talline polymer in which no covalent crosslinks are present. Partial crystallinity is found in naturally occurring polymers also. Since the crystalline phase generally has a higher density than the amorphous phase, a partially crystalline structure will contain density and refractive index fluctuations. The white opalescent character of some collagenous tissues results from the scattering of visible light by the partially crystalline structure of native collagen. Thus, the semicrystalline nature of collagen alone could be the major contributor to its high mechanical strength. Despite the precision of various chemical methods which might be used to quantify the level of crosslinking in collagen, a chemical assessment of the importance of such cross- 163