J. Cosmet. Sci., 69, 335–346 (September/October 2018) 335 Morphological Changes of Human Hair Related to “Graying” LORENA BECHTHOLD, ERIK SCHULZE ZUR WIESCHE, and FRANZ J. WORTMANN, Henkel AG & Co. KGaA, Hamburg 22763, Germany (L.B., E.S.W.), School of Materials, The University of Manchester, Manchester M13 9PL, United Kingdom (L.B., F.J.W.) Synopsis The appearance of hair is a crucial factor of human well-being. Besides hair color and shine, the dynamic movement characteristics have a great impact on a youthful look, which is desirable at all ages. However, the hair follicle is subject to biochemical changes which tend to become obvious in the mid-30s by the appearance of the fi rst nonpigmented “gray” hairs. Especially, these fi bers seem to be unruly, hereby infl uencing the hair collective. In this investigation, the complex dynamic movement of swinging hair is modeled by an in vitro method. Using pigmented and nonpigmented hair strands, the results are related to the morphological and mechanical changes associated with the process of ageing. Furthermore, the in vitro method is extended toward a real life setting by monitoring the movement of women’s ponytails with different fractions of gray hair, while walking on a treadmill. The dynamic movement of hair is a complex phenomenon, which can be affected by several factors: the internal structure, thickness and waviness of single hair fi bers, the fi ber–fi ber interactions, and the shape and volume of hair collectives. As these properties change with age, they are expected to lead to differences in the dynamic hair movement. Using the in vitro method, the dynamic hair movement of pigmented and nonpigmented hair strands is quantifi ed. A harmonic bending oscillation of a hair collective is induced by rotational excitation at the upper strand end, which allows the analysis of the driven as well as the free oscillation mode. The maximum swing height of the hair collective, characterized by the parameter “relative amplitude,” is measured during the driven oscillation and correlates with the defl ection of the hair collective. Compared with pigmented hair, the relative amplitude is signifi cantly lower for nonpigmented hair strands. This indicates a stronger damping, i.e., energy loss, for the nonpigmented hair strands, which relates to higher waviness and larger hair collective volume. In addition, the larger diameter of the nonpigmented hair fi bers leads to a higher contribution of these fi bers to the collective’s bending stiffness. Furthermore, the natural frequency during the free oscillation stage of the measurement is signifi cantly lower for partly nonpigmented hair strands. The damping of hair collectives expressed by the logarithmic decrement is, in turn, signifi cantly higher for nonpigmented hair strands. This is attributed to increased fi ber–fi ber interactions and higher frictional forces within the strand and to increased air resistance. With the laboratory test (in vitro method), the oscillation of different hair qualities using hair strands with defi ned weights and lengths can be analyzed, providing the practical and theoretical concepts to determine the hair movement in a realistic setting. This enables the measurement of the ponytail movement for women walking on a treadmill. Like the in vitro method, the in vivo method allows the analysis of the driven and the free oscillation mode. It is shown that the results of both methods demonstrate a high degree of correspondence. Ponytails with ≥5% nonpigmented hair fi bers have a signifi cantly lower relative amplitude and a signifi cantly higher damping performance in comparison with ponytails with no or less than 5% nonpigmented hair Address all correspondence to Lorena Bechthold at lorena.bechthold@henkel.com.

JOURNAL OF COSMETIC SCIENCE 336 fi bers. This highlights the importance of even small fractions of “gray” hair for the dynamic movement and, as such, the appearance and perception of hair collectives. INTRODUCTION Ageing of the hair follicle leads to various morphological changes of the fi ber. Robbins et al. (1) have shown that the fi ber diameter increases up to an age of approx. 45 years and decreases afterward. The hair density (number of hairs/area of skin surface) increases after age 30 and hair thinning becomes increasingly more noticeable in the mid-40s to the late 50s (1). In addition, hair curliness and stiffness increase with age, whereas hair luster decreases (2). Furthermore, there is wide-spread anecdotal evidence that nonpigmented hairs strongly infl uence the physical behavior of a hair collective, especially the hair movement. Only few studies have been published dealing with the quantifi cation of hair collective movement. Hindley et al. (3) measured the movement of a hair tress that is attached to a sliding bar and moves horizontally backward and forward at a set speed. Focht et al. (4,5) described the stimulation of a bending oscillation of a roundly bundled hair strand by a rotation axis and developed the concepts to analyze the movement char- acteristics of pigmented and bleached hair. Other recent investigations (6) have shown that natural gray hair moves signifi cantly different in comparison with pigmented hair and that already small admixtures of “gray” hair can lead to signifi cant performance changes of a hair collective. THEORETICAL CONCEPTS OF THE HAIR MOVEMENT For the analysis of hair movement, the model of a harmonic oscillator can be used (7,8). An oscillator is characterized by a periodic motion where the motion is repeated at regu- lar intervals of time. A simple harmonic motion is described by a sine or cosine oscillation curve. Figure 1 shows a harmonic motion with the function: x(t)=Asin(ωt), (1) where A is the magnitude or amplitude with the units of x. The circular frequency ω is expressed in radians per second. T is the time period in seconds of one complete cycle (e.g., the time interval that a hair collective needs for one complete swing) of the signal x(t). The reciprocal value of the time period T in seconds is the frequency f (Hz) of the hair movement [equation (2)]: 1 = . f T (2) Fo r the analysis of the hair movement, two types of oscillations are used: Forced (or driven) and free oscillation. In a driven oscillation, a continuing periodic excitation is applied to the system. In this setting, the amplitude A or “swing height” of the collec- tives can be analyzed.