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J. Cosmet. Sci., 75.5, 423–446 (September/October 2024)
*Address all correspondence to Trefor Evans, tevans@triprinceton.org.
Sustainable Hair
TREFOR EVANS
TRI Princeton, Princeton, New Jersey, USA
Accepted for publication September 06, 2024
Synopsis
The amazingly complex structure of hair is crafted deep within a hair follicle to produce what, in material
science terms, might be described as a bio-composite fiber. While this process continues throughout the
lifetime of the fiber, the portion that egresses out onto the scalp is biologically inert. Despite being very
tough and durable, this structure will still gradually degrade as the result of external stressors. In cosmetic
manipulations, some of these can be quite aggressive and sizably exacerbate this process. A considerable
historical scientific literature describes the myriad of ways by which different parts of the hair structure
may breakdown and change as the result of a plethora of different insults. Accordingly, physical and tactile
properties of fibers also change. At some threshold value, such differences become perceivable to the wearer—
who may then use a variety of expressions and descriptors to communicate their perception of these changes:
and, on some occasions, suggest their own presumptions to a solution. Often, these views are not the actual
cause of the symptom (nor the solution), but a desire to sell products to the consumer prompts the use of this
“consumer language.” Further, the words and expressions used by consumers frequently do not equate to what
scientists understand by these words (i.e., “scientific language”). This article attempts to describe how this
situation causes major complications in the hair care industry.
INTRODUCTION
The amazingly complex bio-composite structure of hair is one in a long line of evolutionary
marvels. Over time, material scientists have learned that making something strong, tough,
and durable is best attained by using multiple phases, rather than from just one. Yet, Mother
Nature realized this eons ago and has used it as a template for many natural fibers, including
hair. It is often hypothesized that our ancient ancestors evolved hair as a means of protection
against the outside world. Hair would protect bodies against the cold, it would help shield
skin from the harmful effects of the sun’s UV rays, and it could even help protect against
biting insects. These traits necessitate a tough, durable material, and hair certainly fits the
bill. In modern times, the term biomimicry has arisen as science looks to the natural world
for solutions to various needs. This has driven some research into the structural properties of
hair but, for the most part, this work has been fueled by the cosmetic industry.
At the time of writing, it is estimated that the global hair care market is worth
approximately $100 billion per annum,1 which has spurred the historic need to study
and better understand our substrate. (It is also worth noting that hair and wool have

424 JOURNAL OF COSMETIC SCIENCE
practically identical structure and chemistry, and so much can be gleamed from the sizable
literature on this important textile relative). At the same time, the cosmetic market is
focused on satisfying consumer needs, and a great deal of time and effort is spent by
marketers and consumer scientists to understand issues and wishes. In conducting such
interviews, a familiar list of terms begins to appear: softness, smoothness, strength, shine,
moisturization, frizz, and many more. We term these “consumer attributes,” and together
they constitute the lexicon of our industry as marketers attempt to communicate with
potential customers in their own language. As scientists, when we hear such expressions,
there is the natural tendency to take them literally. The concept of “strength” makes us
think about performing tensile experiments for quantification. “Moisturization” prompts
conceptualizing means of technical water evaluation. Yet, this creates a sizable industry
complication in that, as will be described herein, often “consumer language” does not
equate to “scientific language.”
The multifaceted structure of hair is under constant bombardment from everyday habits
and practices.2–4 Hair fibers are subjected to abrasion and fatiguing during grooming, are
bathed in reactive chemicals to alter color and shape, can be exposed to intense heat in an
attempt to quaff new styles, and constantly encounter the effect of the sun’s UV radiation.
All these (and others) compromise hair’s complex fundamental structure in a myriad of
ways with a concomitant change in various physical properties. The net effect of these
negative changes is expressed by one all-encompassing, nebulous, and wholly ineffectual
descriptor: “damage.” There is frequently the desire to measure how damaged (or how
healthy) a hair sample is however, it can hopefully be seen why this is a taxing proposition
as the various components of the hair’s structure can break down in numerous ways, at
differing rates, and as the result of countless insults.
The purpose of this article is to highlight how hair’s fundamental structure relates to the
various consumer attributes mentioned earlier. It will similarly cover how the effects of certain
insults can negatively impact such properties and contribute to consumer unrest. Insights
are attained through measuring changes in specific technical properties and attention will
be paid to the proper measurement of such properties. The nature of the cosmetic industry
is to promise consumer benefits, and hair-care products can unquestionably attain this
end. Yet, there is constantly the need/desire to push the marketing envelope where claims
frequently involve “protection” and/or “repair” of hair after using certain products. The
validity and subtleties of this consumer language will also be explored.
HAIR STRUCTURE
There are many excellent texts that describe the complex structure of hair, and the reader
is directed to these for higher level depth and detail.5–7 Herein, a brief overview is provided
as it specifically relates to the topic at hand. At the most fundamental level we can simply
think about the hair structure in terms of the “inside” and the “outside.” The outer
structure of hair is collectively termed the cuticle, where a conceptual analogy might involve
the bark at the outer edge of a tree. This structure is made up of many overlapping scales
or tile-like structures (i.e., the individual cuticle scales) that result in a tough exterior that
protects the more vulnerable interior. Figure 1 shows a high-magnification image of the
hair surface where this fine structure can be seen. Moreover, the greater cuticle structure
itself consists of generally five to six layers of these individual tiles. Figure 2 shows a similar