Hair damage from environmental and cosmetic treatments continues to be a concern. Consequently, continuing efforts are under way to understand the mechanisms of hair degradation and to devise means to protect or repair hair from damage. This short review summarizes recent advances in the quest to protect and repair human hair.
It is well-known that hair is comprised of three structural elements: the cortex, which resides in the interior of the fiber; the cuticle, which is wrapped around the exterior; and the medulla, which is a tubular region of low density that resides within the cortex region. It is also well-known that hair can be damaged by exposure to mechanical stress and abrasion from combing and brushing, oxidative stress due to UV irradiation in an oxygen-rich environment, and chemical treatments designed to perm, color, straighten, relax or bleach the hair.
For example, perming causes hair within a molecular weight range of 48 kDa and 60 kDa to lose about 58% of its protein.1 After such damaging treatments, it is standard protocol to use conditioners to screen the damage by improving wet and dry combing attributes, imparting a soft feel and alleviating static flyaway. However, conventional conditioners do little to restore hair strength.
Synthetic copolymers of styrene and acrylates can be utilized to cover damaged hair and improve shine. One advantage to using polymers as hair fixatives is that they minimize the need for more commonly employed silicones, which can compromise fixative properties. In one case, styrene, butyl acrylate, methacrylic acid and other co-monomers were synthesized via emulsion polymerization and formulated as aerosol hair spray and gel. Shine measurement, film gloss, stiffness analysis, high humidity curl retention and dynamic mechanical analysis were then used to characterize the treated hair.2 This method does not repair damaged hair, but instead relies on the high refractive index of latex polymers as the means of attaining gloss/luster of the polymer-treated hair.
Aminofunctional silicones have been shown to offer protection from the damage incurred from bleaching and oxidative dyeing; they also confer effective conditioning, and visual shine and smoothness to hair.3 However, early aminofunctional silicones, with few cationic groups, did not adsorb sufficiently to resist removal by normal washing and rinsing processes. This issue has been addressed by the synthesis and use of multi-sequenced cationic polydimethylsiloxane-polyether copolymers, which are claimed to improve the combability, softness, volume, shapeability, manageability, tangling and shine of hair, both damaged and undamaged.4
MQ resins, i.e, those having the functional siloxane units Me3SiO (M) and SiO4 (Q), are highly branched silicone resins that have been used extensively to confer transfer-resistance to color cosmetics. Emulsions incorporating MQ resins have now been demonstrated as being useful for the repair of split ends.5 Achieving stable emulsions of MQ resins is not straightforward. In this case, the inventors addressed the additional complexity of matching the surface tension of the emulsion to the surface energy of hair in order to facilitate spreading of the emulsion on the hair surface and uniformly distribute the deposited MQ resin on the hair.
The surface energy of healthy hair and damaged hair are reported to be about 24-28 dynes/cm and 20-50 dynes/cm, respectively. The intent, in this case, is to ensure that the external phase of the emulsion has such an affinity for the hair that it spontaneously spreads along the hair shaft, thereby causing the emulsion to break and coat the hair with a film of coalesced, plasticized MQ resin that is sufficiently strong enough to resist removal through many wash, blow drying and styling cycles. The appropriate emulsion is achieved by emulsifying the plasticized MQ by sequential addition of emulsifiers (see Figure 1).
As described by the patent,5 a dime-sized drop of the emulsion was applied to the split ends of a hair tress, which was then washed, rinsed, blown dry and subjected to two passes with a hot iron at a temperature of approximately 425°C. This cycle was repeated multiple times without reapplication of the MQ resin emulsion. After the first cycle, all of the split ends were mended. After the sixth cycle, 75% of the split ends remained intact.
Tyrosine and Tryptophan
Hair is damaged by both UVA and UVB radiation through the photochemical degradation of hair proteins, which produces free radicals. UVB radiation, 280–320 nm, is mostly absorbed by the cuticle, and causes protein loss by breaking disulfide bonds on the surface of the cuticle. UVA radiation, 320–400 nm, is able to penetrate the cuticle layers—where it produces reactive oxygen species and causes oxidation of amino acids, sterols and fatty acids, decomposition of lipids, and a decrease of melanin within the cortex.6, 7 Tyrosine and tryptophan residues in hair proteins absorb strongly in the UV region, and act as photosensitizers for photochemical damage.8
Melanin and Copper
The melanin pigment in hair also protects against photodamage by absorbing across the visible and UV spectrum and acting as a free radical scavenger, thus preventing radicals from entering the keratin matrix.9, 10 However, even melanin is photo-degraded, which is manifested as the fading of hair color upon UVA exposure.11 For this reason, kojic acid (5-hydroxy-2-hydroxymethyl-4-pyranone) has been used for hair lightening because it is a tyrosinase inhibitor, operating by chelating the copper atoms in the enzyme. In this context, it is interesting that exogenous copper increases UV damage in hair.12 It has been proposed that copper acts as catalyst with photo-produced hydrogen peroxide to form reactive alkoxy radicals, which degrade tryptophan and similar amino acid components of the hair protein. Menthyl carbamate compounds have been developed to lighten hair and skin without tyrosinase inhibition.13
Keratins are a group of more than 30 cytoskeletal proteins that are intertwined and linked through covalent disulfide bonds, Coulombic salt bonds, hydrogen bonds and dispersion forces to form microfibrils in the hair cortex. They are divided into two subsets: type I keratins that are acidic and have molecular weights in the range 40 kDa to 48 kDa, and type II keratins that are basic to neutral and have molecular weights from 58 kDa to 65 kDa.14 UV radiation causes structural changes within keratin by oxidation of amino acids, resulting in the rupture of sulfur linkages.15 This type of damage results in decreased shine, color fade, weakened structural integrity and breakage.
Over the years, many approaches have been implemented to prevent, repair or mask the damage caused to UV-exposed hair. These methods often employ natural extracts as well as improved polymeric materials. Researchers have shown that treating hair with a hydroalcoholic extract from Punica granatum L. (pomegranate) prior to UV exposure can prevent color fading by up to 60.8%. This is because pomegranate contains various polyphenols that act as antioxidants and scavenge free radicals produced by UV irradiation, thus preventing damage.16 Another approach to implementing natural antioxidants employs extracts from rice and artichoke as pre-treatments to UV exposure. Hair treated in this manner showed improved hair fiber integrity, increased mechanical properties, and preserved color and shine.17
Protein hydrolysates have been incorporated into hair products for decades, and soluble keratin is one material offered as a hair strengthener, protectant and conditioner.18 Peptides with strong affinity to hair and nails have been bio-engineered,19 and in studies, sulfonated keratin from the alkaline oxidation of wool has been shown to condition and strengthen bleached hair.20 Specifically, the sulfonated keratin was applied to the hair via a conditioner formulation with stearyl amidopropyl dimethyl diamine. A suggested mechanism for its binding to the surfaces of damaged hair is that the anionic sulfonated keratin forms a complex with the cationic surfactant, and this complex has an affinity for the negatively charged damaged hair.
The discovery of amphipathic self-associating peptides by researchers at Leeds University21 is an interesting development that could have an impact on hair strengthening systems. These peptides form gels that can be triggered by pH to self-assemble into beta sheet tapes, ribbons or fibrils that can be spun into high tensile strength fibers. These self-assembled structures can be seeded with cells, and in this capacity, could serve as scaffolds for tissue engineering. Such structure-building peptides may offer the potential of strengthening damaged hair by adsorption thereto.
Of particular interest, the peptides can be constructed to have selectively negative or positive charges at physiological pH values. Thus, they can be designed to adhere to negatively charged hair keratin. Examples of these positively charged peptides are shown in Figure 2, and the self-assembled structures attainable from them are shown in Figure 3.
Polyglycerol partial esters with an average degree of polymerization from 2 to 8 have been disclosed as additives that enhance the grip and elasticity of hair. They also protect the hair against heat damage,22 and have been shown to be hair repair and strengthening agents. The mechanism by which these compounds achieve their results is not clear from the patent, and in this author’s view, it is unlikely that such large compounds could penetrate into the cortex of undamaged hair. However, it is possible they could reach the cortex of damaged hair from which the cuticle was stripped.
In conclusion, the industry’s understanding of the processes by which hair is damaged is advancing. However, protection of hair from damage by environmental and chemical stresses continues to be a challenge, but by using cleverly crafted materials such as MQ resin emulsions, keratin sulfonates and amphipathic self-assembling polypeptides, it appears that the damage can be repaired—at least temporarily.
- M-O Han, Effects of permanent waving on changes of protein and physicomorphological properties in human head hair, J Cos Sci 59 (3) 203-215 (2008)
- F Zeng et al, ACS symposium series, in Polymers for Personal Care and Cosmetics, A Patil et al, American Chemical Society, Washington, DC, USA (2013)
- MD Berthiaume, Effects of silicone pretreatment on oxidative hair damage, J Soc Cosmetic Chem 46 (5) (1995)
- US Pat Application 20120308494, Novel linear polydimethylsiloxane-polyether copolymers having amino and/or quaternary ammonium groups and use thereof, F Schubert et al (Dec 6, 2012)
- US Pat Application 20130295028, Emulsified MQ resins: Composition and methods, WA Lee and G Hawkins (Nov 7, 2013)
- E Fernández et al, Efficacy of antioxidants in human hair, J Photochemistry and Photobiology B: Biology 117 146–156 (2012)
- MF Dario et al, Efficacy of Punica granatum L. hydroalcoholic extract on properties of dyed hair exposed to UVA radiation, J Photochemistry and Photobiology B: Biology 120, 142–147 (2013)
- MJ Davies, Reactive species formed on proteins exposed to singlet oxygen, Photochem Photobiol Sci 3 533-537 (2004)
- K Sebetic et al, Review: UV damage of the hair, Coll Antropol 32, suppl 2 163-165 (2008)
- W Korytowski et al, Photo-induced generation of hydrogen peroxide and hydroxyl radicals in melanins, Photochem Photobiol Sci 45 185-190 (1987)
- N Santos et al, Hair color changes and protein damage caused by ultraviolet radiation, J Photochem Photobiol Sci B74 109-117 (2004)
- JM Marsh et al, Role of copper in photochemical damage of hair, Int J Cos Sci 36 32-38 (2014)
- US Pat Application 20130129646, Menthyl carbamate compounds as skin and/or hair lightening actives, G Vielhaber, H Oertling, K Schaper, C Gomann and R Brodhage, assigned to Symrise AG (May 23, 2013)
- AL Miranda-Vilela, AJ Botelho and LA Muehlmann, An overview of chemical straightening of human hair: Technical aspects, potential risks to hair fiber and health and legal issues, Intl J Cos Sci 36 2-11 (2014)
- T Herrling, K Jung and J Fuchs, UV-generated free radicals in skin and hair–Their formation, action, elimination and prevention, A general view, SÖFW 133 2-11 (2007)
- Ibid Ref 7
- Ibid Ref 6
- Croda direct database, https://pc.crodadirect.com/home.aspx?d=content&s=158&r=280 (Accessed Mar 27, 2014)
- US Patent 7,544,353, Peptide-based conditioners and colorants for hair, skin and nails, X Huang, H Wang and Y Wu, assigned to du Pont de Nemours Company (Jun 9, 2009)
- US Pat Application 20140044664, Compositions and methods for treating a keratin-based substrate, JM Cardamone, P Erazo-Majewicz and N Naouli (Feb 13, 2014)
- US Pat Application 20140044649, Beta sheet tapes, ribbons in tissue engineering, N Boden, A Aggeli, E Ingham and J Kirkham, assigned to the University of Leeds (Feb 13, 2014)
- US Pat Application 20120308503, Cosmetic composition containing polyglycerol partial ester, HH Wenk and P Lersch, assigned to Evonik Goldschmidt GmbH (Dec 6, 2012)