Morphological Changes of Human Hair Related to 'Graying'

The appearance of hair is a crucial factor of human wellbeing. Besides hair colour 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 first non-pigmented ‘grey’ hairs. Especially these fibres seem to be unruly, hereby influencing the hair collective.
In this investigation, the complex dynamic movement of swinging hair is modelled by an in vitro method. Using pigmented and non-pigmented 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 towards a real life setting by monitoring the movement of women’s ponytails with different fractions of grey hair, whilst 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 fibres, the fibre-fibre-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.
For the quantification of hair movement with the in vitro method, the dynamic bending behaviour of pigmented and non-pigmented hair, as well as ‘grey’ hair strands are analysed using novel experimental as well as theoretical concepts. 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, characterised by the parameter ‘relative amplitude’, is measured during the driven oscillation and correlates with the deflection of the hair collective.
Compared to pigmented hair, the relative amplitude is significantly lower for non-pigmented hair strands. This indicates a stronger damping, that is energy loss, for the non-pigmented hair strands, which relates to higher waviness and larger hair collective volume. Additionally, the larger diameter of the non-pigmented hair fibres leads to a higher contribution of these fibres to the collective’s bending stiffness.
Furthermore, the natural frequency during the free oscillation stage of the measurement is significantly lower for partly non-pigmented hair strands. The damping of hair collectives expressed by the logarithmic decrement is, in turn, significantly higher for non-pigmented hair strands. This is attributed to increased fibre-fibre interactions and higher frictional forces within the strand, as well as to increased air-resistance.
With the laboratory test (in vitro method), the oscillation of different hair qualities using hair strands with defined weights and lengths can be analysed, 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 as well as the free oscillation mode.
It is shown that the results of both methods demonstrate a high degree of correspondence. Ponytails with ≥ 5% non-pigmented hair fibres have a significantly lower relative amplitude and a significantly higher damping performance in comparison to ponytails with no or less than 5% non-pigmented hair fibres. This highlights the importance of even small fractions of ‘grey’ hair for the dynamic movement and, as such, the appearance and perception of hair collectives.