Recent market trends have increased the need for hair protection as consumers experiment with a diverse range of hair colors and styles, and seek to change their look regularly. This has led the industry to develop bond multipliers that protect and repair hair during processing. Bleaching processes, in particular, can be very damaging as they oxidize amino acids, cleave bonds and increase fiber porosity. This ultimately leads to dry, brittle-feeling hair and fiber breakage.
In hair care, the use of keratin ingredients, including peptides and other derivatives, is well-established for effects such as moisture retention and hair softness. Methods to prepare keratins suitable for personal care have most commonly involved hydrolysis in order to convert the high molecular weight insoluble keratin protein into lower molecular weight, soluble peptides. These can be applied in formulations and are capable of penetrating the hair and skin for biological performance.1 The cleavage of disulfide bonds via sulfitolysis has been used to isolate high molecular weight, soluble keratins that protect the hair and maintain hair surface health.2
On the other hand, the use of oxidized keratins, where the disulfide bond is cleaved through oxidation to create a soluble protein, has been limited to medical applications to date. These include wound-healing products for the acceleration of skin healing3 and stimulation of cellular production of specific collagens.4 Such approaches are well-established as chronic wound treatments5 and for the management of the dermatological disorder Epidermolysis bullosa.6 This oxidized form of keratin exploits the established biological function of keratin in cell growth, development and healing.7 Extending the application of oxidized keratins to hair care, however, is a new approach.
The interaction of oxidized keratin with the skin and its potential to positively affect biological functions is due to the keratin’s ability to interact with cells in a manner similar to the body's own keratin. This is dependent on the high degree of homology between the oxidized keratin and human keratin, which is ensured by similarities in the amino acid sequences of the two proteins.
Sheep wool intermediate filament keratin protein has a 91% homology to human intermediate filament keratin protein, based on a side-by-side comparison of the amino acid sequences. As such, experiments were undertaken, as described here, to investigate the effect of oxidized sheep wool keratin on human hair when used in bleach and as a post-bleach recovery treatment.
The results were compared with that of a leading industry synthetic bond multiplier (BM) based on bis-aminopropyl diglycol dimaleate. Effects were quantified using single fiber tensile testing, combing breakage and sensorial tests. Oxidized keratin also was applied to severely damaged hair in order to investigate its repair capabilities. This was followed by multiple washing cycles to examine the longevity of effects.
Materials and Methods
Hair tresses: Dark brown (grade 2) hair tresses (~2 g, 20 cm) were pre-bleached without oxidized keratin or with the synthetic BM. Bleaching paste (1 part bleach powder: 1.5 parts of 20 volume (6%) peroxide solution) was applied evenly to the hair tresses with a brush. Tresses were left in ambient conditions for 1 hr, rinsed under warm water for 1 min and gently toweled dry. Tresses were hung to air dry and a comb was passed through them whilst still damp.
Bleached control: Control bleached tresses were prepared by application of the bleaching paste described previously.
Bleach with oxidized keratin: An OK bleach paste was prepared by adding one part oxidized keratin (as supplied) to four parts bleach paste.
Bleach with BM: The BM bleach paste was prepared by addition of 1.0 mL of BM to 10 g of bleach paste (per manufacturer specification).
Hair treatment protocol: All tresses were wrapped in plastic film and placed in an oven at 43 ± 3°C. After 20 min, tresses treated with bleach only were removed from the oven and rinsed under warm water for 1 min. They were then gently toweled dry and partially dried off under a stream of hot air whilst combing for 10 strokes. The OK and BM bleached tresses were left under heat until they achieved the same lightness as the bleached control tresses. After approximately 40 min, all tresses were removed, rinsed and dried as previously described.
Post treatment with oxidized keratin: OK was rubbed into the tresses and they were heated at 43 ± 3°C for 20 min, rinsed under warm water, conditioned with Paul Mitchell Original Conditioner, and rinsed and dried as previously described.
Post treatment with BM: Some tresses bleached with BM were further treated with bond protector cream following the manufacturer’s guidelines. The cream was applied and combed through. This was left for 20 min, rinsed off and the tresses were shampooed using Herbal Essences Naked Shampoo, rinsed, and conditioned with Paul Mitchell Original Conditioner. Following a final rinse, the tresses were dried as previously described.
Tensile testing: Hair tresses were supplied for single fiber tensile testing using a standard method based on ISO 5079:1995(E), Textile Fibers—Determination of Breaking Force and Elongation at Break of Individual Fibers. All strands were conditioned for 24 hr at 50% RH and 20°C prior to testing using a tensile testerd. The student’s t-test was used to determine the statistical significance of the data sets.
Bond multipliers relying on cross-linking are able to restore strength to damaged fibers, but this increased strength is associated with increased brittleness.
Combing breakage: Each hair tress tested was first secured onto a clamp stand. A plastic comb was run through from root to tip taking approximately 2 sec to achieve a combing stroke. Each tress was combed 1,600 times and the broken fibers were collected and counted.
Sensorial assessment: A blind, paired comparison test was conducted on hair tresses. Non-expert volunteers were provided with sets of paired tresses and asked to assess each pair based on several parameters related to appearance and feel.
Durability of OK treatment: Hair tresses were bleached twice using 6% peroxide solution and permed three times using the Perfect Touch Perm by Fancy-full brand following the manufacturer’s guidelines before being subjected to a series of washes. Each hair wash cycle included hand washing with Paul Mitchell’s Baby Don’t Cry Shampoo followed by Paul Mitchell Conditioner and rinsing. Hair was blown dry between washes. Hair tresses were submitted for single fiber tensile testing using a method based on ISO 5079:1995(E) and tensile testere apparatus. Hair was soaked in distilled water for 30 min prior to testing.
Further bleaching: Bleached tresses were subjected to a further bleaching process either in the presence or absence of OK and synthetic BM. Some of the tresses were exposed to a second treatment with OK or bond protector cream, according to the manufacturer’s instructions.
Results: Hair Strength
Tensile analysis was then performed on the samples; results are summarized in Table 1 and Figure 1 and Figure 2. The first bleaching process (pre-bleached) appeared to adversely affect tensile properties and this effect was significant (p < 0.05) for all parameters except highest force. A second bleaching (bleached control) resulted in all tensile parameters significantly decreasing compared with undamaged hair.
When OK was used in the bleaching treatment (bleach/OK), the % extension at peak was increased (p < 0.05) relative to the bleached control and became closer to that of undamaged virgin hair. The total energy at peak appeared to have increased compared with bleached hair, although not significantly (p = 0.091).
Application of BM to the bleach (bleach/BM) did not seem to retard fiber damage, compared with the bleached control. There was also a significant decrease in the % extension of the bleached/BM hair (p < 0.001), relative to the bleached/OK, indicating increased brittleness and decreased flexibility (see Figure 1). The total energy at peak of bleached/BM hair was significantly decreased as well (p < 0.01), relative to the bleached/OK hair (see Figure 2).
These results give insight into the mechanism of protection afforded by OK, which appears to work within the fiber to protect against brittleness during the bleaching process. This contrasts with the BM, which reacts covalently and irreversibly with available sulfur species to create cross-links that ultimately compromise the flexibility and extensibility of the fiber.
Post-bleach application of OK to bleached/OK hair, and BM to bleached/BM hair resulted in the highest force and total energy at peak parameters. These were equivalent to one another (p > 0.1) and improved over the bleached control; i.e., p = 0.087 and p < 0.05, respectively. Note that these samples were subjected to post treatment with conditioner following the protocol outlined by the manufacturer. The BM post treatment itself is, however, a relatively complex formulation containing conditioning agents, therefore information about the fiber brittleness may be masked by the plasticizing effects of conditioning molecules.
The work at a 25% extension parameter was not significantly different for the bleached/OK and post-OK, and the bleached/BM and post BM-treated hair. Both appeared to be higher than the bleached control (p = 0.058 and p = 0.051, respectively).
Results: Combing Breakage
The dry combing of chemically bleached hair has been documented to increase the amount of short-segment (< 2.5) hair breakage with increasing strokes.8 Here, the results for the combing breakage tests of bleached control hair, bleached/OK and OK post-treatment hair, and bleached/BM and BM post-treatment hair are shown in Figure 3.
Repeated dry combing of the control bleached hair resulted in 201 broken fibers, of which nine were long segment breaks and 192 were short segment breaks. Hair treated with OK yielded 43 short segment breaks after 1,600 combing strokes, and hair treated with BM gave 46 short segment breaks and one long segment break. Clearly, both the OK and BM treatments improved the dry combing property of bleached hair fibers to a similar degree.
Results: Sensory Properties
A panel of 20 non-expert volunteers evaluated pairs of hair tresses by indicating which tress they associated most with specific attributes. Results for bleached/OK and post-treated OK as well as bleached/BM and post-treated BM are summarized in Figure 4 and Figure 5. All differences in the OK and BM treated hair were significant to at least a p value < 0.005.
A strong preference was observed for OK-treated bleached hair for the attributes of condition, shine, nourishment, softness, silkiness and color. The OK treated bleached hair also was perceived to be less damaged, less dry and to have the least amount of frizz, compared with BM treated bleached hair—and note that these preferences were amongst tresses treated with conditioners. Therefore, the BM post treatment cream or hair conditioners alone did not compensate for the deterioration in hair fibers. The greater extensibility of the OK-treated hair appears to be contributing to attributes that includee softness, nourishment and silkiness.
Oxidized keratin-treated bleached hair was perceived to be less damaged, less dry and to have the least amount of frizz.
Results: Durability of OK Treatment
As noted, an experiment was undertaken to investigate the durability of a post-OK treatment on hair exposed to a harsh bleaching and perming protocol. Tensile measurements were used to quantify damage and repair, and to determine the persistence of the observed changes in fiber characteristics following multiple washing and drying cycles. Results are summarized in Table 2 and shown in Figure 6.
A combination of bleach and perming had an extremely detrimental effect on the tensile properties of the hair, as evidenced by a decrease in all parameters. Application of OK to the damaged hair, however, significantly (p < 0.001) increased all tensile parameters, returning them closer to those of undamaged virgin hair. This improvement, relative to damaged hair, persisted at a significance level of p < 0.001 for 20 washes.
After 30 washes, highest force (p < 0.005), extension at peak (p < 0.001) and total energy (p < 0.001) parameters were still increased, whereas energy at 20% extension was no longer significantly different. Thus, the application of OK ameliorated the damage caused by the severe bleaching and perming protocol and this improvement persisted after 30 washes, indicating the beneficial effects on fiber properties was long-lasting.
The virgin hair structure is highly ordered and well-understood.9 In undamaged hair, the cuticle prevents the ingress of higher molecular weight materials; thus polymers and proteins are typically understood to be restricted to interactions on the fiber surface. The intact cortical cell structure—which at the most fundamental level consists of fibrous intermediate filament proteins coiled together in tightly wound helical structures—is grouped into higher order structures to give hair its characteristic strength and flexibility. The binding of fibrous proteins into coiled structures therefore relies on a combination of interactions between amino acid sequences that align and bond through ionic, hydrophobic and hydrophilic interactions, leading to a zipper-like binding between neighboring keratins. These binding sequences for intermediate filaments are well-understood.10
In damaged hair, the protection afforded by the cuticle is disrupted and cortical proteins are damaged. Higher molecular weight materials are then able to penetrate, and in the case of oxidized keratin, the intact segments of the keratin-specific binding regions of the protein can bind to damaged cortical proteins, repairing the cortical structure and restoring strength and flexibility.
Bond multipliers relying on cross-linking mechanisms to bind damaged protein components together within a fiber, such as the dimaleate system used in this study, are clearly able to restore strength to damaged fibers. However, as is well-known in protein and polymer systems, cross-linking reduces flexibility, so increased strength is associated with increased brittleness. This was observed in the reduced extension at break of BM-treated hair.
The OK system of interest in the present study was capable of binding intermediate filament keratin proteins into the damaged hair fiber. As a result, hair strength was improved along with improved flexibility—all while maintaining the natural protein structure. Even in the case of severely damaged fibers, the effect appears durable. Brittleness to the fiber may have accounted for the negative sensorial effects observed for BM-treated fibers; where the combination of factors leading to perception of softness, silkiness and nourishment were undermined by cross-linking, giving fibers a rigidity that appeared absent with the OK treatment.
The oxidized sheep’s wool-derived keratin described here appears to be a viable, natural alternative to synthetic bond multiplier systems for repairing damage caused during the bleaching process. The novel treatment interacts with the internal structure of hair via a different mechanism, and repairs strength and flexibility in a way not possible with synthetic cross-linker-based bond multiplier systems.
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