425 SUSTAINABLE HAIR
high-magnification end-on image of the outer edge of a broken hair fiber where these
various layers can be seen stacked upon each other.
Despite its toughness, this structure will progressively degrade and break down as a result
of the cumulative insults to which it is exposed. The rate of this process will be dependent
on the habits and practices of the individual where a greater incidence of the previously
mentioned insults will lead to faster manifestations. Figure 3 shows an example of a rather
extreme incidence, where the cuticle scales have become highly uplifted. In Figure 4, the
whole cuticle layer has been eroded away from the upper portion of the fiber.
There are many practical implications of this occurrence. Perhaps first and foremost, when
hair is touched, fingertips encounter this outer surface. Accordingly, it can be seen how
tactile properties are likely to become compromised as the above changes occur. As per
a previous point, consumers frequently ascribe all kind of complex reasons for negative
changes to the feel properties of their hair for example, “the hair has dried out,” or “the hair
has lost its natural oils.” In the vast majority of cases where feel properties are diminishing,
there is a very simple and straightforward explanation: the cuticle has degraded to an
unacceptable level.
Figure 1. Outer cuticle layer of hair.
Figure 2. Overlapping cuticle layers.

426 JOURNAL OF COSMETIC SCIENCE
A degrading surface will lead to increased friction between individual fibers, with
subsequent negative effects on hair manageability. Hair may become more difficult to
groom, with snags, and tangles becoming more frequent. Furthermore, these outcomes will
lead to still higher surface abrasion which, in turn, exacerbates further surface degradation.
A self-perpetuating, cascading cycle is set up: a rougher surface leads to more friction, and
more friction leads to a rougher surface. Indeed, it is often the case that already damaged
hair is considerably more susceptible to further damage.
Despite the toughness of this cuticle structure, it is widely believed that it has no meaningful
contribution to hair’s tensile strength. This, in part, relates to the nature of the cuticle
structure itself, but also its relatively minor presence from a cross-sectional perspective.
Caucasian hair is frequently listed as having an average diameter of ≈70 µm. The 5-6
cuticle layers represent an outer rim of ≈3 µm and a quick calculation suggests only a 15%
contribution to the overall fiber cross-sectional area. Instead, hair’s impressive extensional
mechanical properties are a consequence of the inner cortex structure. Although, as shown
later in this article, the cuticle will still have contribution to mechanical properties in the
bending mode.
Figure 3. Severely uplifted cuticle scales.
Figure 4. Cuticle abrasion.