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j. Cosmet. Sci., 54, 579-588 (November/December 2003) Effects of conditioners on surface hardness of hair fibers: An investigation using atomic force microscopy S. B. RUETSCH, Y. K. KAMATH, L. KINTRUP, and H.-J. SCHWARK, TRI/Princeton, Princeton, NJ 08542 (S.B.R., Y. K.K.), and Henkel KGaA, Diisseldorf, Germany (L.K., H.-J.S.). Accepted •br publication July 29, 2003. Synopsis Conditioners are known to have a prophylactic effect on hair damage caused by cosmetic chemical treatments or mechanical grooming procedures (1). They are known to impart softness and smoothness to hair by moisturizing the fiber (2). Since the amount of conditioners deposited on the fiber is very small in quantity, it is conceivable that mainly the surface is moisturized. This is especially true of polymeric conditioners, which deposit preferentially on the surface of the fiber, rather than penetrate into the cortex. Therefore, this study strictly investigates whether cationic polymeric conditioners impart softness to the surface cuticle cell as a result of their hydrophilicity, with no regard to its applicability to cosmetic effects. Such softening can be detected by indentation of the surface and can be quantified by measuring the depth of the indent in real time. Atomic force microscopy (AFM), equipped with nano-indentation capability, is ideally suited for this purpose. In this work it was used to determine changes in the microhardness (micromechanical properties) of the hair fiber surface as a result of fiber/conditioner/moisture interactions. In a preliminary study, we observed that the scale faces of hair treated with Polyquaternium 10 (PQ-10) conditioner gave deeper indents, while scale edges yielded shallower ones in comparison to cuticle cells of untreated hair. This suggests that the conditioner softens the scale face and hardens the scale edges. However, because of significant amounts of conditioner residues left on the scale face, this conclusion was rather ambiguous. Therefore, the study was repeated in which multiple indentations were made on the surface cuticle cells of a larger number of the same hair fibers before and after multiple applications of the conditioner. This reduces errors due to fiber-to-fiber variation in pre-existing microhardness differences in surface cuticle cells. Also, the larger number of fibers investigated in the current work allowed for a statistical outcome. This latter study has led to a rather definite conclusion that the scale face is indeed softened by polymeric conditioners such as Polyquaternium-10 (PQ-10). These studies will ultimately help in the development of conditioners with suitable moisturizing and softening effect on hair. INTRODUCTION Conditioners are applied to hair fibers to protect them from abrasion/ablation during everyday grooming (1), to improve general manageability, combability, and luster, and to reduce static charging (flyaway). In this study we strictly want to show whether a cationic conditioning compound present on the fiber surface imparts softness to the surface cuticle cell as a result of its hydrophilicity, with no regard to its applicability to 579