The remarkable, complex structure of hair is progressively compromised by a variety of insults relating to everyday habits and maintenance practices. High magnification Scanning Electron Microscopy (SEM) is frequently performed to visualize this breakdown of the surface cuticle; but there is no easy way to see internal changes within the cortex.
Hair’s mechanical properties are a consequence of this internal structure – and therefore provide a proxy for investigation. That is, any changes in mechanical properties must be the result of internal structural modification. Such alterations can be temporary or permanent. Chemical treatments (perms, relaxers, permanent color, etc.), high heat and the sun’s UV radiation are all well-recognized to irreversibly alter hair protein chemistry. On the other hand, plasticizers (most notably water) solvate secondary electrostatic bonding within the hair and lower mechanical properties – although these effects are reversed upon their removal. To this end, it becomes evident that wet state and dry state tensile experiments probe different regions of the hair structure.
Furthermore, hair’s complex hierarchical structure produces anisotropy, whereby different mechanical stimuli can yield different results. For example, the extensional properties of hair are widely recognized to be reflective of the inner cortex structure and possess no contribution from the cuticle. However, the cuticle has a considerable influence on the hair’s bending and twisting properties. This presentation will how a combination of various mechanical testing techniques, which themselves can performed under a variety of environmental conditions, provides a comprehensive means for probing unseen regions of the hair structure.