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.

427 SUSTAINABLE HAIR
Figure 5 is a frequently used schematic for illustrating the multifaceted structure of hair.8
It shows how α-helical keratin protein chains wrap together to form a coiled rope-like
structure, which further entwines to produce intermediate filaments (frequently described
as microfibrils). These pack within an amorphous keratin matrix (sometimes termed
keratin-associated protein) to form a microfibril (often termed a cortical cell). These all
pack together within the lipid structure of the cell membrane complex (CMC), and the
whole assembly is encased within the outer cuticle. In short, the result is a bio-composite
fiber made primarily out of keratin proteins.
A protein falls into the keratin family due to the presence of the amino acid cystine
somewhere within its structure. The disulfide bond at the heart of this molecule is
thought to be the main crosslinking structure within the protein chains and is believed
to provide most of hair’s permanent structure. Yet this is a relatively reactive moiety. In
permanent waving, this bond is attacked by reducing agents to deconstruct a portion of
the protein structure.9 Attempts are later made to reform this bonding, with the hair in a
new conformation, using oxidizing treatments. Inevitably not all the broken bonds can be
re-formed, and a portion are lost to cysteic acid formation. These bonds can also be directly
oxidized into cysteic acid via oxidizing treatments.10 For example, bleaching treatments
utilize these chemicals to degrade the melanin pigment granules that provide hair’s natural
color. Similarly, these bonds can be photochemically oxidized under the action of the sun’s
UV rays.11 The obvious implication of losing strength-supporting bonds is that the hair is
less well-structured, and its strength is compromised (examples will be presented later in
this article). From a practical stance, this might be anticipated to make hair more prone to
breakage.
Another consequence of this diminished structuring is that fibers will swell more when
wetted by water.12 It has been suggested that water should be considered part of hair’s
structure, due to the sizable effect its presence has on many of the hair’s properties.13 Simply
Figure 5. Schematic of hair structure. Reproduced from reference 8. Copyright 1991, Elsevier.