Also, what contributes to elasticity of hair, and how can we maintain or improve the quality in our own locks? These are important questions, and as always, much insight can be gleaned by an examination of the fundamental principles as well as the molecular structures that make up the hair.
What Does Elasticity Mean?
Elasticity is a term used to describe how a material responds to the application and removal of a specific type of mechanical load (pulling and/or bending). When a stress (force per unit area) is applied to a material, it stretches a certain amount beyond is original length. This deformation is dependent upon the stiffness or rigidity of the material. The ratio of applied stress to the amount of deformation/elongation that occurs is called the elastic (or Young’s) modulus.
Rigid materials, such as iron, stretch very little with an applied force, while other materials, such as synthetic rubber, can stretch many times their original length without breaking. Dry hair can stretch to approximately 1.2 – 1.3 times its original length and still return to its dimensions, while wet hair is less rigid than dry hair and can stretch up to 1.5 times its length. Curly hair can stretch even than straight hair, as it is highly coiled in its relaxed state.
A material is said to exhibit elastic behavior if it returns to its previous shape and size once an applied force is removed. This is called reversible deformation. Simple materials such as elemental metals typically display purely elastic behavior. These tend to stretch to a certain point and then experience sudden fracture if the stress is not removed. Materials such as these are described as being brittle.
More complicated materials such as polymers, proteins, biomaterials and some inorganic amorphous solids exhibit elastic behavior until a certain stress is exceeded (yield strength). Beyond this point, less force is required to induce further deformation, and the material is unable to recover its size and shape once the load is removed. This phenomenon is referred to as irreversible deformation, plastic deformation, or permanent set. The applied force causes something to change inside the substance at a molecular level that causes it to become fundamentally different in its physical structure. The change can be a rearrangement of crystalline lattice structure from one type to another, shifting or slippage of molecular alignment in an amorphous or semi-crystalline material, change of protein tertiary structure, or breaking of bonds in polymeric compounds. Materials with this property are referred to as being ductile or having greater toughness than brittle substances.
Plastic deformation is particularly relevant to the health hair and its appearance. If excessive force is used to style or comb hair, the yield strength can easily be exceeded, and the hair can no longer bounce back when it is pulled out of shape. This can adversely affect its ability to hold a style or retain curl and can result in shapeless, frizzy hair.
Additionally, special caution should be taken with wet hair. Hair saturated with water is fragile and can stretch much more easily than when it is dry. It is very easy to exceed the yield strength when hair is wet and permanently diminish its elasticity, or even cause breakage. For this reason, it is crucial to use extreme care when handling and combing wet hair. The use of a good conditioner helps protect wet hair from plastic deformation by decreasing combing forces (less force is required to get the comb through tangles).
What Affects Hair Elasticity?
The interior of the hair shaft, the cortex, is the portion of the hair structure that carries the bulk of an applied load and contributes most significantly to elasticity. Although it is very important, the cuticle is only significant in this regard for its role in guarding the integrity of the inner shaft of the hair.
The cortex is an elaborate structure of clusters of fibrils of keratin protein embedded within a matrix with high water content. The individual molecules of keratin are in the alpha-helical conformation. There are many different inter- and intramolecular interactions and bonds that occur both between amino acids on the same protein strand, amino acids on adjacent protein chains, and between proteins and water molecules within the matrix.
Hydrogen bonds are weak physical bonds that occur between aqueous hydrogen and amino acid nitrogen and oxygen atoms. These interactions are easily formed and broken and are responsible for a large portion of the elastic behavior of hair. For this reason, it is very important to maintain a proper amount of moisture inside the hair shaft. Without adequate hydration, hydrogen bonding will be decreased, which adversely affects elasticity of hair strands.
Salt bonds are weak physical interactions that occur between amino acids and require hair to be maintained at an optimum pH. Cystine bonds, also known as disulfide bonds, are chemical bonds which impart a high degree of elasticity to hair by providing crosslinks between different amino acids on a single protein fiber and also between protein strands. All of these various types of bonds act to hold strands of protein together and allow them to stretch just so far and to snap back into their original shape.
Another factor that influences the elasticity of hair is its diameter. Hair of smaller diameter cannot withstand the same forces as hair of thicker diameter. Remember, stress = force per unit area, so thinner hair experiences greater stresses at the same forces. This means that those with finer hair may have more trouble with their hair losing curl, not holding styles, and developing frizz and breakage. African hair typically has the smallest diameter, with Caucasian hair having medium diameter, and Asian hair having the thickest diameter. There is no known way to overcome this, so one must take care to treat fine hair with the same care one would afford your most precious cashmere sweater.
How to Improve Hair Elasticity
We have learned that hair elasticity is heavily dependent upon two key factors: 1.) hydrogen bonding between water molecules and keratin strands and 2). disulfide bonds between adjacent cystine amino acid groups, both of which are dependent upon preservation of the protein structure and hydration of the cortex. The best approach to ensure excellent elasticity is to maintain an intact protein structure inside the cortex and an adequate level of hydration.
In an ideal world, prevention of damage to the cortex protein structure is achieved by maintaining a pristine cuticle layer, avoiding high temperature treatments and processes, avoiding chemical processes such as color, permanent waves and relaxers, minimizing UV exposure, limiting hygral fatigue (excessive water exposure), and using only the most gentle mechanical forces for combing and styling. Of course, we don’t live in an ideal world, so most people will experience varying levels of degradation of the internal protein structure of their hair, accompanied by a gradual deterioration of the desirable elastic properties. Minimizing exposure to destructive processes and frequent trims helps defray damage, as does use of a good deep conditioner and gentle treatment of hair at all times.
The use of protein treatments and protein-containing conditioners is often recommended to help improve or restore elasticity. This approach can be useful for those who do have damaged proteins in the cuticle structure or within the hair shaft. Hydrolyzed proteins in these products are in amino-acid form and lower molecular weight poly-peptide form, and can penetrate the cortex. They are retained there in subsequent washings and can contribute to hair strength and integrity to some extent, preserving the tendency for elastic, reversible deformations at low stresses. However, it is most likely that these materials act only as a patch over a hole rather than actually assimilating themselves into the protein strand and fibrillar structure. One word of caution about these types of treatments is that they can potentially contribute to brittle behavior (breakage) if used in excess or if the hair already has sufficient protein content.
For a substance that seems mostly decorative, hair never ceases to amaze me in its complexity. The intricacies of this biopolymeric composite are simply amazing. The elastic properties of healthy hair can serve us well and allow for much versatility in our coiffure, if proper care is taken to keep hair in the best shape possible.