The Anti-Flyaway/Frizz Effect by Inducing the α-Helical Structure Transition of Hair

Abstract

In order to reduce chronic hair flyaways/frizz, both reducing and oxidizing agents have to be used, leaving aside the hair damage issues. This study presents changes in hair morphology caused by treatment with a shampoo containing only reducing agents, excluding oxidizing agents that affect critical hair damages. As a result of flyaway/frizz improvement rates calculated through monitoring of the area of light transmittance in the hair tresses, reducing agents, such as ammonium thioglycolate (ATG), L-cysteine, and sodium sulfite were found to be effective in decreasing hair flyaway/frizz. Additionally, the methods to maintain homeostasis and control damage caused by oxidation during washing were also used to see flyaway/frizz improvement rates. Measurements using electrostatic force microscopy (EFM) showed that the surface charge of hair tresses treated with shampoo containing reducing agents was lowered. Using Raman spectroscopic analysis, it has been suggested that these treatments with reducing agents induced a 3D structural transition of the hair from an α-helix to a random coil. In addition, this structural release was confirmed, identifying the reduction in the enthalpy of the α-helix using differential scanning calorimetry (DSC). Furthermore, we verified that this change causes no hair damage through a tensile strength test. Therefore, the formulation of shampoo with reducing agents can be used as an effective strategy to care for hair flyaway/frizz without hair damage issues.

1. Introduction

Hair flyaways are more common in individuals with straight or slightly curly hair than in those with curly hair [1]. This issue occurs due to static electricity in the hair or due to the specific shape of the hair [2]. This phenomenon can be controlled with styling products, brushing, or static electricity [1]. Frizz is a similar concept to flyaways. However, it is used to indicate a curled appearance. Even though the individual’s hair is straight, some noticeable curls can still exist. This kind of hair does not easily straighten even when subjected to straightening treatment. Frizzy hair can occur for a variety of reasons, including harsh towel drying, excessive brushing, heat damage, and moisture [1].

Flyaways/frizz are rare in individuals with curly hair, which is understood to be the result of its high lipid content [1]. The hair lipid content of individuals of African ethnicity, who often have curly hair, is higher than that of the hair of individuals of other ethnicities [3,4]. The application of lipids is an excellent method used to reduce flyaways/frizz in different hair types. However, since lipids move through the cell membrane complex, it is difficult to control the content of these compound inside the hair [5].

Such flyaway/frizzy hairs are characterized by hair that is misaligned, brushing the misaligned hair will cause it to break and become caught in the hairbrush being used [6]. There has always been a demand for products that aid in straightening and smoothing hair to solve the problem of frizzy/flyaway hair.

In this study, various methods, including those that change the internal structure of the hair, control moisture, or treat the hair with antioxidants, were used to remove flyaway/frizzy hair. In addition, we conducted research through tensile strength to establish safe methods that do not damage hair.

Perm treatment breaks the disulfide bonds in the hair, thus changing the physical properties and shape of the hair [7]. However, the reducing agents used in perm treatments damage hair [8]. Ammonium thioglycolate (ATG) is a widely known compound that gives hair a straight or wavy appearance. ATG reduces the cysteine of keratin and breaks the disulfide bond [9]. Studies in rats have shown that abundant ATG is toxic, can act as a skin irritant, and can cause sensitization both in animal and test tube experiments [10]. In this study, to reduce hair irritation, ATG was mixed with shampoo used to wash and treat hair, and changes in hair shape were observed. Additionally, L-cysteine, a reducing agent that changes hair structure by causing heterogeneous reactions between cysteine and keratin fibers [11], and sodium sulfite, used as a hair relaxant [1], were also tested.

In addition to reducing agents, substances that can treat flyaway/frizzy hair were also used and compared. Since the moisture inside the hair and the humidity in the external environment affect hair shape through the hair’s static electricity [12], in this study, we studied changes in hair shape by controlling moisture homeostasis.

Copper ions in water that come into contact with the hair and ultraviolet rays generate hydroxyl radicals, which cause hair damage [13,14,15]. The damage caused by these radicals can be prevented through the use of antioxidant ingredients [16]. Damaged hair can result in flyaways/frizz [1]; therefore, the anti-frizz effects of antioxidants have also been studied.

In this study, an instrumental evaluation method was established to objectively quantify the degree of flyaways/frizz, and to the best of our knowledge, this is the first study to establish such a method. In this study, an amide I band examined via the Raman spectrum and an enthalpy of α-helix were analyzed to identify a mechanism that reduces hair flyaways/frizz. To determine whether the examined hair was damaged, the X-ray crystal structure and tensile strength of the hair were measured. As a part of this study, research was conducted to determine a simple, effective, and safe method to reduce flyaways/frizz.

2. Materials and Methods

2.1. Materials

For use in this study, sodium laureth sulfate (SLES) was obtained from LG Household and Healthcare (Seoul, Republic of Korea). Ammonium thioglycolate (70%) was purchased from thermos scientific (Waltham, MA, USA). L-cysteine (>98%), sodium sulfite (>98%), and sodium metabisulfite (>99%) were obtained from Sigma Alderich (St. Louis, MO, USA). Erythritol was obtained from Cargill Incorporated (Wayzata, MN, USA) and xylitol was obtained from Futaste Pharmaceutical Co., Ltd. (Dezhou, China). Finally, H₂O₂ (30%) and NaOH (40%) were obtained from Daejoung Chemical Co. (Siheung, Republic of Korea).

2.2. Treamtent of Hair

Hair samples from the same Chinese people were purchased from Bulex (Happy Call, Seoul, Republic of Korea). A 2 g hair swatch was prewashed with 10% SLES solution. Wella Blondor Bleach (Wella, Petit-Lancy, Switzerland) and Wella Koleston Cream Developer 6% (Wella, Petit-Lancy, Switzerland) were mixed in a 1:2 weight ratio, applied at twice the weight of the hair, and left on for 15 min. The hair was thoroughly washed with 15% SLES solution and dried. After the hair sample was washed, it was gently dried with a 22 cm × 11 cm paper towel (Yuhan, Seoul, Republic of Korea) for 10 s and then combed with a brush. The hair was then dried using a hair dryer (JMW, Seoul, Republic of Korea) at a temperature of 70 °C with a comb for 2 min. The prepared bleached hair was partitioned into 26 cm long 2 g tresses and stored vertically for longer than half a day under constant temperature and humidity conditions (25 °C; 45% humidity) before measurement.

A base shampoo that contains SLES and does not contain any other oils or functional ingredients other than polyquaternium-10 was used. The formulation is described in detail elsewhere [17]. The target substance to be treated on the hair was mixed with the base shampoo at an appropriate concentration and then mixed using a stirrer (IKA Eurostar 40, Staufen, Germany) at a speed of 50 rpm for 1 h.

The prepared tresses were thoroughly moistened with water. The prepared shampoo [17] was applied in an amount equivalent to 1/10 of the hair weight, massaged for 60 s to create a lather, and left for 3 min. Afterward, it was rinsed under running water for 60 s, and this washing process was repeated three times with the same shampoo. In all experimental results, hair that was not treated with a reducing agent was indicated as the virgin group.

2.3. Flyaway/Frizz Measurement

The degree of flyaways/frizz was analyzed using a Bolero Lite system (Bossa nova vision, Los Angeles, CA, USA). Measurements were taken from 5 cm below the top of the hair to 20 cm below. The direction of the hair tress was changed, and photos were taken at different angles of 90 degrees (0°, 90°, 180°, and 270°). The FAF (flyaway/frizz) value of each hair (n = 3) produced in the same environment was calculated as a total n = 12 and then averaged to determine the degree of flyaways/frizz of the tresses. The ratio of the flyaway/frizz part of the hair tress was calculated by considering the part with low permeability, rather than the bulky part of the hair tress, as the flyaway part. The criterion for low transmittance was set at 0.5 or lower. The ratio of the area occupied by this transferred portion to the total area of the hair tress was calculated as the FAF value, taking into account the degree of flyaways/frizz. The change in FAF after washing compared to the FAF₀ of hair washed before treatment with the active substance (FAF − FAF₀)/FAF₀ was expressed as ΔFAF.

2.4. Atomic Force Microscope

To measure electrical properties, electrostatic force was evaluated using electrostatic force microscopy (EFM) by atomic force microscopy (AFM, XE-100, Park Systems, Suwon, Republic of Korea) using an NSC36/Cr-Au 10M cantilever (Parksystems, Republic of Korea). The cantilever had a typical spring constant of 0.6 N·m⁻¹ and a resonant frequency of 65 kHz. The hair surface (20 μm by 12 μm) of a total of three hairs was scanned in EFM mode.

2.5. Raman Spectroscopy

A semiconductor diode near-infrared laser operating at λ = 1064 nm was used as the excitation source and delivered a laser power of 2.5 mW measured at the hair fiber. The Raman signal was focused through a 500 μm wide slit and was dispersed by a diffraction grating of 1800 grooves per millimeter onto a deep depletion InGaAs detector (Horiba, Irvine, CA, USA) of a size of 30 × 30 pixels with a 1 µm × 1 µm area. The exposure time per pixel was 40 s, and the scan was repeated 10 times. The distribution of Raman spectra was analyzed using a software program (Labspec 6.5.2.5).

2.6. DSC

Dry-differential scanning calorimetry (DSC) experiments were performed with a DSC-400 (Perkin Elmer, Waltham, MA, USA). Each sample was subjected to heating and cooling treatments at a scanning rate of 10 °C/min under a nitrogen atmosphere in order to prevent oxidation. Chopped hair weighing 5 mg was placed in an aluminum open pan with two small pin holes and tested over a temperature range of 30–300 °C. A total of 6 hair tresses were used in this experiment.

2.7. Tensile Strength

A tensile tester (MTT175, Dia-Stron, East Anton, UK) was employed to fracture individual hair fibers and measure the breaking strength in gram force (gmf). Hair samples were prepared by crimping both ends of each hair fiber to a length of 30 mm using metal crimps (Dia-Stron, UK). A laser scan micrometer (LSM-501S, LSM-6200, Mitutoyo, Kawasaki, Japan) was employed to measure the cross-sectional area of the hair fiber. One end of the fiber, positioned in the fiber holder, was extended at a rate of 20 mm/min using converted energy from the rotation of the motor inside. The pulling force was measured and monitored on graphs using a load cell situated on the opposite side of the fiber holder. The break load per cross-sectional area of each fiber was calculated as the break stress, and the repair ratio (%) was subsequently evaluated to compare the impact of peptides on the hair.

3. Results

3.1. Degree of Hair Flyaways/Frizz

Unlike straight hair, flyaway and curly hair possesses hollow areas outside the bulky part. Light can pass through this part, and as shown in Figure 1a, the bulky part can be evaluated as green, and the light that passes through and reaches the other side can be evaluated as red. By calculating the ratio of the area of light reaching the other side and the total hair area, the degree of flyaways/frizz was calculated as FAF values. The degree of flyaways/frizz of the hair tresses was quantified as shown in Figure 1b. All experimental values of ΔFAF were expressed as the rate of change divided by the difference between the pre- and post-experimental values by the pre-experimental value.

In this study, three methods were used to control hair flyaway/frizz. The methods were used to control the internal bonds of the hair, maintain homeostasis, and control damage caused by oxidation during washing. Representative materials were selected to aid these processes.

In Figure 1a, after washing the hair, many flyaway and frizz areas were observed on the outside of the hair tress. When the hair tress was washed with water, the averaged ΔFAF value increased by 28%. The closer the value is to 28%, the less effective FAF is, and a negative value for ΔFAF means a decrease in the degree of frizz after washing.

When hair is treated successively with reducing and oxidizing agents, the disulfide bonds in the hair can be broken and recombined. In this case, a straight shape can be maintained by wearing a device that physically maintains the shape of the hair at high temperatures for a long period.

Treating the hair with ATG, L-cysteine, and sodium sulfite, used as reducing agents, was found to reduce flyaways and frizz. No oxidizing agents or other hair products were used, and the hair was straightened via simply brushing the hair while it was dry. The ΔFAF change value during the ATG treatment was −20.39, and it was confirmed with the naked eye that the hair showed a neat appearance. In the case of the hair treated with shampoo containing L-cysteine, the change value of ΔFAF was −6.24. In the case of the sodium sulfite treatment, the value was 7.13, with the hair showing more frizz after washing. However, the ΔFAF value was lower than when the hair was washed with water.

Flyaway/frizzy hair is affected by static electricity, which is greatly affected by the humidity in the surrounding environment [1]. Current frizz control shampoos contain a polyalcohol to control the moisture inside the hair [18]. In this study, erythritol and xylitol were utilized to examine to what extent they reduced FAF. The ΔFAF values of erythritol and xylitol were 17.24 and 19.44, respectively, making them less effective than the reducing agents.

Damaged, split, or broken hair undergoes changes in its internal density, causing it to become misaligned or subject to flyaways or frizz [1,19]. Radicals generated in hair are the main cause of physical damage to hair [16]. Radical oxidative damage can be prevented through treatment with the antioxidants of sodium metabisulfite and erythorbic acid, which are isomers of vitamin C [20,21]. Sodium metabisulfite and erythorbic acid are widely used in viscous liquid cosmetic formulations to exert antioxidant and physiological/biological activities. In this study, to prevent oxidation by radicals, sodium metabisulfite and erythorbic acid were used. As shown in Figure 1, treatment with antioxidants did not reduce hair flyaways and frizz but instead increased their degree after washing.

3.2. Measurements of Physical Property

In Figure 2a, the intensity of the electrostatic force acting on the hair surface is expressed in color. On the same hair surface, the cuticle edge has a relatively lower voltage. This result is consistent with other results showing that there are more polar values at the edges [22,23]. When comparing the six measured hairs, the virgin hair has an overall higher voltage. The hair treated with reducing agents appeared greener overall. Figure 2b graphs the voltage intensity at pixels in the scanned area. As shown in Figure 2c, the hair treated with reducing agents has an overall lower voltage distribution. In the case of the hair treated with erythritol, the surface charge value was evaluated to be relatively low.

As seen in Figure 1, an attempt was made to reduce hair frizz under the assumption that ATG, L-cysteine, and sodium sulfite act as reducing agents and change the structure of the hair. Based on this hypothesis, the internal structure of the hair was observed to determine the cause of the reduction in flyaways/frizz using Raman spectroscopy. As shown in Figure 3, Raman spectra were taken to determine the type of changes that occurred in the disulfide bonds inside the hair shown in Figure 1.

As shown in Figure 3a, the Raman spectra reveal peaks corresponding to disulfide, SO₃, and amide I at 513 cm⁻¹, 1042 cm⁻¹, and 1658 cm⁻¹, respectively [24,25]. ATG, L-cysteine, and sodium sulfite acted as reducing agents. However, no changes in the disulfide peak and SO₃ peak areas were observed.

The alterations in the internal structure of the hair resulting from oxidative damage can be deduced by observing variations in the α-helix, β-sheet, and random coil peaks within the amide I band [26,27]. In order to compare how different the ratios of components are for each hair type, fitting work was performed with a total of 6 components. As shown in Figure 3b, the amide I band was separated into an α-helical portion, a β-sheet, and a random coil portion at 1650 cm⁻¹, 1671 cm⁻¹, and 1685 cm⁻¹, respectively. Fitting analysis included three additional components by referring to previous research results (cyan, pink, and yellow).

Since the amount of change varies from hair to hair, the experiment was conducted on three different people’s hair. Compared to the analysis results from the untreated hair, the hair treated with ATG, L-cysteine, and sodium sulfite shows a lower α-helix structure compared to the β-sheet. A relative increase in the random coil ratio was also observed in the case of ATG, L-cysteine, and sodium sulfite. No such changes were observed in the hair treated with erythritol or sodium metabisulfite compared to the untreated hair, as shown in Figure 3c.

To determine the structural bond strength of the α-helix in the hair protein structure, the endothermic reaction that occurred was measured using DSC. Figure 4 shows a graph displaying the 215~260 °C range in which the endothermic reaction of the α-helix occurs. To calculate the enthalpy energy, the horizontal axis was expressed as the time when energy was applied at a rate of 10 °C per minute.

The area of the peak reaching its highest point on the dotted line in Figure 4 is the degree of α-helix alignment of the hair [28]. The averaged α-helix enthalpies of the untreated hair, the ATG-treated hair, and the erythritol-treated hair were calculated as 28.378 mJ/g, 24.18 mJ/g, and 27.92 mJ/g, respectively. Unlike the other peaks, in the case of ATG, the rate at which the α-helix began to melt was fast.

ΔFAF serves as a comparison of hair flyaway/frizz values before and after washing the hair with shampoo containing ATG, and if its value is less than 0, this means that the degree of frizz/flyaways has decreased. Even when the ΔFAF is a positive value, if the value is lower than that of the control group, in which the hair was washed with water, this indicates an effective reduction in frizz.

As shown in Figure 5, after washing the hair with shampoo containing various concentrations of ATG, changes in the hair ΔFAF value according to the ATG concentration were observed. When the ATG concentration was below 0.2%, the ΔFAF value increased in comparison to the pre-washing condition, leading to a positive ΔFAF value. When the ATG concentration in the shampoo exceeded 0.3%, the FAF value decreased compared to the pre-washing state, resulting in a negative ΔFAF value, as depicted in Figure 5.

According to the tensile strength measurement shown in Figure 6, it can be seen that the tensile strength of the hair was maintained without a significant difference even after washing the hair a dozen times with ATG-containing shampoo. This result implies that despite frequent contact with ATG during washing, there were no alterations observed in the physical characteristics of the hair.

4. Discussion

When washing the hair with water, the ΔFAF value actually increased compared to the value before washing. There are various lipids inside hair, and these lipids are extracted through the disturbance of water molecules during washing through a cell membrane complex [17,19]. Before applying shampoo, lipids exist on the surface of the hair to bind the hair strands together. Once the hair is washed, these lipids are removed, causing the hair to exhibit flyaways and frizz.

When the hair was treated with ATG, L-cysteine, and sodium sulfite, its ΔFAF value decreased. In the case of ATG, the reduction in FAF was the highest compared to the other materials, meaning that the reduction effect was the greatest. The fact that the hair was straightened without the use of a hair device or heat reaction seems to be the result of drying and combing the hair while the internal bonding force was weakened following ATG treatment.

Measurements of the polar charge on the hair surface revealed changes of electrostatic charge. Hair treated with a reducing agent had lower surface polarity values. This reduction in charge appears to have resulted in a decrease in ΔFAF. In the case of hair treated with erythritol, the amount of static electricity seems to have been evaluated low because it retains moisture. The lower electrical charge between the hairs would have had a significant effect on reducing flyaways/frizz.

To determine what factors reduce the charge, the internal structure of hair was evaluated using Raman spectra. By normalizing the Raman peak at 513 cm⁻¹, which reflects the disulfides in the hair, the size of amide bands or the intensity of peak for cysteic acid between hairs can be compared [17]. Contrary to expectations, no changes were observed in the disulfide and SO₃ peaks between the different hair treatments. This appears to be the result of the disulfide recombining even if decomposed by a reducing agent [26]. The fact that there was no difference in the intensity of the disulfide peak between the untreated hair and the ATG-treated hair may be due to the influence of the sulfur in ATG and the -SH contained in the hair, despite the breakdown of disulfide in the hair. Another possibility is that the SH group remaining in the hair was oxidized by the air and the disulfide bond was thus regenerated.

Studies using FT-IR spectra allow for the inference of a transition from an α-helix structure to a β-sheet structure through a change in the amide II band [29]. An α-helix to β-sheet transition in amide I has also been observed in other Raman studies performed on hair [30]. Using the same analysis method, changes in the hair protein structure can also be studied using Raman spectroscopy [26]. As a result of analyzing the amide I band in the Raman spectroscopy test, hair treated with ATG, L-cysteine, and sodium sulfite exhibited a higher prevalence of β-sheet/random coils compared to α-helical structures. This may represent the transition from α-helix to a random coil. Therefore, the anti-flyaway/frizz effect of hair treated with ATG, L-cysteine, and sodium sulfite may be due to the release of α-helices.

The peak positions of the α-helix and β-sheet can be affected by a downward shift of the amide I band with the helix number, and an increase in hydrogen bond strength [31]. In order for the transition from an α-helix to a random coil to be generalized, the same spot of hair must be measured and compared before and after a reducing agent treatment. There was AFM data where the same spot of hair was evaluated before and after treatment [32]. However, this method is difficult to measure the same area because each hair must be washed with water.

The charge was high when protein was aligned in an α-helix, and the charge was low when it was released as a random coil. The electrostatic contribution to protein folding energy is closely related [33]. When a random coil is folded, large hydrophobic groups are exposed to the outside, and charged groups are attached to the inside [34]. As this change in internal structure occurs, the static electricity exposed to the surface appears to change.

The enthalpy of hair as keratin is denatured and decayed by internal bonding breakdown, changes the amount of α-helical intermediate filament material in the hair, and causes structural defects. The partial transformation of keratin α-helices into a β-sheet structure or a random coil of keratin is closely related to the breakage of thiol groups in hair disulfide bonds. When the thiol groups are reconnected through disulfide bonds, the β-sheet can be restored to an α-helix [35]. The DSC results in this study showed that the endothermic reaction enthalpy of the α-helix decreased after ATG treatment, indicating that the α-helix of the ATG-treated hair was released. This is consistent with the decrease in the α-helix band of ATG-treated hair in the Raman spectrum. This change in internal bonding appears to have reduced the degree of hair flyaways/frizz when the hair was treated with reducing agents.

As the concentration of ATG in the shampoos increased, the treated hair exhibited a decreased level of changes in FAF. There are many factors that determine the appearance of an individual’s hair, including humidity and genetics [1]. The higher the concentration of ATG, the higher the alignment of hair after washing, meaning that the majority of the anti-flyaway/frizz effect is derived from ATG. Considering that the ΔFAF value becomes negative when the concentration of ATG exceeds 0.3%, it can be concluded that a concentration of around 0.3% should be used when washing actual hair.

The strength of the examined hair was evaluated, whereby the FAF reduction effect was observed when the hair was treated with various materials during the washing process. Intermediate filaments with many α-helices are known to make a major contribution to the tensile strength of hair [36]. It was demonstrated that 0.5% concentration of ATG did not change the tensile strength of the hair. This result supports the hypothesis that the use of ATG is a safe method even though it causes structural changes in the hair to produce a straightening effect. Washing number 56 in Figure 6 can be estimated to a usage of 3 months depending on the person, so it appears to be safe for hair. However, since skin irritation tests on the scalp were not conducted, additional research is needed. The 0.5% concentration of ATG used in Figure 6 cannot be considered a small concentration considering the raw material content of a typical shampoo formulation. However, before shampoos containing ATG can be used in practice in the future, further experiments should be conducted to determine the threshold of ATG concentration associated with hair and scalp irritation.

Each person has different hair types, and the degree of frizz varies depending on the type. The appearance of hair varies greatly depending on race. Ellipticity (major axis/minor axis) is 1.78 for Africans, 1.22 for Asians, and 1.33 for Caucasians [37,38]. Since this shape affects frizz/flyaway hair, this study should apply to hair of other races, not just Asians.

5. Conclusions

To reduce impact of hair flyaways/frizz, reducing agents were used. However, this in turn causes problems associated with hair damage. The results of this study show that reducing agents can react with hair during washing to reduce flyaways/frizz. Our Raman studies confirmed that when untreated hair was treated with a reducing agent, the intensity of the α-helix was reduced in the amide I band. As a result of selecting the ATG that reduced the degree of flyaways/frizz to the greatest degree and using DSC, we found that there was a reduction in the enthalpy of the α-helix.

Surface charge measurements using EFM showed a reduction in charge on hair where flyaways/frizz was reduced using ATG. This decrease in surface charge appears to be closely related to the increase in random coils identified through Raman analysis. Tensile strength did not decrease even after a dozen rounds of treatment, proving that adding a reducing agent to hair cleansers can safely reduce flyaways/frizz. The reason ATG minimizes hair damage when used as a shampoo is because its content is small. In the future, research on the concentration threshold at which damage occurs will likely be needed.