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. 2026 Sep 15;11(38):57362–57369. doi: 10.1021/acsomega.6c05682

Achieving Highly Efficient Repeatable Hair Perming by Bunte Salt-Thiol Exchange Reaction

Zhenyu Qin †, Kuan Chang †,*, Jing Wang †,*, Timson Chen ‡, Zhizhen Li ‡, Ya Chen ‡, Ling Ma ‡
PMCID: PMC13629021  PMID: 42825074

Abstract

Conventional chemical perming typically suffers from a gradual decrease in styling efficiency due to the cumulative degradation of original disulfide bonds. To address this issue, this study proposes a novel perming technology based on the Bunte salt–thiol exchange reaction. A bifunctional Bunte salt, sodium hexamethylene-1,6-bisthiosulfate (SHB), was successfully synthesized, with its high purity and structure validated by X-ray diffraction (XRD) and Raman spectroscopy. The perming performance of this Bunte salt method was evaluated and compared against the conventional oxidative perming method on Chinese virgin hair. X-ray photoelectron spectroscopy (XPS) was utilized to calculate the relative content of disulfide bonds and to elucidate their reconstruction mechanism. Furthermore, the hair morphology, surface properties, and mechanical performance were systematically evaluated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), tensile testing, and other auxiliary experiments. The results demonstrated that after six repetitive perming cycles, the SHB-permed hair maintained a high perming efficiency of 920%, whereas that of the H2O2-permed hair declined to 730%. Notably, the disulfide bond content in SHB-permed hair increased by 15.5%, whereas it decreased by 10.7% in the H2O2 group. Additionally, the Bunte salt perming successfully preserved both the surface and internal structural integrity of the hair fibers. After six cycles, the mechanical and surface properties of SHB-permed hair showed only a marginal reduction (13–16%) compared to virgin hair, in stark contrast to the approximately 45% degradation observed in the oxidative perming group. In conclusion, while repeated conventional oxidative perming causes extensive cleavage of disulfide bonds and severe fiber damage, the Bunte salt–thiol exchange technology effectively mitigates hair damage and maintains disulfide bond density. This methodology offers a robust alternative to conventional protocols and demonstrates significant potential for application in repetitive hair styling products.

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Introduction

Since the first inception in the late 19th century, chemical perming has evolved into a styling process of both practical and aesthetic significance. Conventional chemical perming protocols involve two primary stages: the initial application of a reducing agent to cleave disulfide bonds, which softens the hair fiber for reshaping, followed by the use of an oxidizing agent to reconstruct these covalent cross-links once the desired style is achieved. − Although chemical perming is characterized as a repeatable procedure, in practice, the efficiency of the treatment diminishes progressively with an increasing number of cycles.

Keratin, the primary structural component of hair, is characterized by the outstanding high concentration of cysteine. Cysteine exists predominantly as cystine, forming an extensive network of disulfide bonds, which is fundamental to maintaining the mechanical properties of hair. , However, disulfide bonds are susceptible to over-oxidation into sulfonates, thereby destroying the cross-linked network of hair, , especially during the chemical perming process. This chemical degradation manifests as cuticle desquamation and the depletion of cortical proteins, ultimately leading to frizz and fiber breakage. , Additionally, as the fundamental basis for chemical hair perming, reduction in disulfide bond content leads to an irreversible decline in perming efficiency. Repeated multiple cycles of oxidative perming ultimately result in poor mechanical properties and an extremely low perming efficiency. Therefore, oxidative perming cannot be defined as a repeatable perming technology.

Although researchers have attempted various mild oxidizing agents such as sodium bromate and sodium perborate in hair perming, oxidative damage cannot be completely avoided. Accordingly, a series of novel permings have been developed in recent years. Tinoco et al. investigated the use of bovine serum albumin, silk fibroin, keratin, and two recombinant fusion proteins as novel perming agents, achieving a high perming efficiency and improved moisture retention. Additionally, Chang et al. developed 1,4-butanediamine bismaleimide to reconnect thiol groups via click chemistry, thereby avoiding the oxidative steps. Despite these advancements in reducing damage, the fundamental problem of a declining disulfide bond content remains unresolved.

In recent decades, bunte salts have received increasing research attention. As a class of compounds with the general structure R-S-SO3 –, Bunte salts can undergo an exchange reaction with thiols, as shown in Scheme . Diz et al. utilized this reaction to graft a cationic surfactant onto damaged wool keratin via disulfide linkages, imparting softness, and antimicrobial properties. Georg et al. proposed the idea of using amino acid-containing Bunte salts for hair setting treatment but did not report any experimental data. However, the permeation performance and advantages of Bunte salts have not been systematically investigated.

1. Bunte Salt-Thiol Exchange Reaction.

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In this study, a Bunte salt, sodium hexamethylene-1,6-bisthiosulfate (SHB), was successfully synthesized. The purity and chemical structure of SHB were characterized and verified by using X-ray diffraction (XRD) and Raman spectroscopy. Thereafter, SHB was applied to the hair perming. The reaction mechanism between the Bunte salt and reduced hair fibers was elucidated by X-ray photoelectron spectroscopy (XPS). Finally, a comprehensive evaluation was performed to compare the Bunte salt perming method with the conventional oxidative perming method by assessing the surface morphology, surface characteristics, internal structure, and mechanical properties of the treated hair samples.

Materials and Methods

Reagents and Materials

Sodium thiosulfate (RG, ≥97.0%), 1,6-dichlorohexane (RG, 98%+), ethanol (AR), mercaptoacetic acid (AR), 30 wt % hydrogen peroxide (AR), and sodium hydroxide (GR, ≥98.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Methanol (AR, ≥99.5%), hydrochloric acid (AR) were purchased from Shanghai Titan Technology Co., Ltd. Chinese virgin black hair piece (18 cm × 3 cm × 6 g) were purchased from Shanghai Canyu Commercial Co., Ltd.

Before use, all hair samples were washed 3 times with 8 wt % sodium dodecyl sulfate solution, then rinsed with deionized water, air-dried, and stored under constant temperature and humidity conditions (temperature = 25 ± 2 °C, relative humidity = 50 ± 5%).

Synthesis and Characterization of SHB

SHB was synthesized in accordance with Scheme . Typically, sodium thiosulfate (6.6 g, 0.042 mol), deionized water (40 mL), and anhydrous ethanol (40 mL) were added to a 200 mL three-necked flask, and then mixed in a 70 °C water bath. Then, 1,6-dichlorohexane (3.10 g, 0.02 mol) was added, with the pH adjusted to 9 using ammonia water. A stirrer, a reflux condenser, and a thermometer were installed on the three-necked flask. The mixture was stirred at 80 °C for 5 h. The solvent was removed under reduced pressure, and the product was then dried under vacuum at 50 °C for 3 h. The obtained solid was recrystallized with 200 mL of methanol to afford a high-purity product.

2. Synthesis Method of SHB.

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The purified product was qualitatively analyzed by Raman spectrometer (LabRAM HR Evolution, HORIBA, Tokyo, Japan) and X-ray diffractometer (XRD, D8, Bruker AXS GmbH, Karlsruhe, Germany). The excitation wavelength of the Raman spectrum was 785 nm, and the scanning range was 500–4000 cm–1. The diffraction patterns were obtained by continuous scanning of the 2θ angle from 10 to 70° with 10 mA, 30 kV, 0.154 nm wavelength Cu (Kα) radiation.

Treatment of Hair Samples

Reduced hair samples were obtained by immersing Chinese virgin black hair in 8 wt % thioglycolic acid aqueous solution at a hair-to-solution mass ratio of 1:40, with the solution temperature adjusted to 40 °C and pH adjusted to 8. After soaking for 20 min, the hair was taken out and rinsed 3 times with deionized water.

Permed hair samples were obtained by the conventional oxidative perming method or the Bunte salt perming method. Chinese virgin black hair was wrapped around a curling iron, and the reduction process was carried out in accordance with the aforementioned method. After rinsing, the hair was immersed in 3 wt % H2O2 aqueous solution at pH 5 for oxidative perming or 1 wt % SHB aqueous solution at pH 9 for Bunte salt perming at a hair-to-solution mass ratio of 1:40, followed by treatment at 40 °C for 30 min, respectively. This process was repeated 6 times to obtain the repeatedly permed samples. The permed hair samples were rinsed 3 times with deionized water.

The hair samples in the control group were treated with deionized water at 40 °C and pH 7 for 30 min. This treatment exerted negligible effects on hair, so the label “virgin” instead of “control” was adopted in the figures for clearer visualization. The control group can also be regarded as hair treated with deionized water as the perming reagent.

For permed hair, shorter hair lengths and more curls correspond to higher absolute perming efficiency. Nevertheless, comparison with the length and curl count of hair before perming offers more intuitive results in practical applications; hence, (relative) perming efficiency is commonly adopted for the evaluation. During shampooing after perming, the conformations of hydrogen bonds and covalent bonds continuously change, reducing the hair curl retention. Accordingly, the concept of perming resistance is proposed to assess the curl retention capacity of the permed hair. The calculation formulas for perming efficiency and perming resistance are shown as follows

perming efficiency(%)=(N1/L1)/(N0/L0)×100% 1
perming resistance(%)=permingefficiencyaftertreatingpermingefficiencybeforetreating×100% 2

where N 1 is the number of loops after perming, L 1 is the fiber length after perming, N 0 is the number of loops before perming, and L 0 is the fiber length before perming.

Evaluation of Hair Properties

X-ray Photoelectron Spectroscopy (XPS)

An X-ray photoelectron spectrometer (SCIENTIFIC K-α, Thermo Fisher Scientific, Massachusetts, USA) was used to detect the existing forms of sulfur elements in hair samples and analyze the changes in their valence states. A hair fiber was randomly selected and fixed to the sample holder. XPS measurements were performed by using a focused X-ray source and a monochromatized Al target, and the photoelectron passing energy was 100 eV. For plotting, the binding energy value of C 1s was corrected at 284.8 eV.

Scanning Electron Microscopy (SEM)

The morphology of hair cuticles was observed by using a field emission scanning electron microscope (S-4800, Hitachi, Ltd., Tokyo, Japan). Hair samples were mounted onto the specimen stage using conductive carbon adhesive tape and subsequently sputter-coated with a 2 nm gold layer to enhance the electrical conductivity. Micrographs were acquired at an accelerating voltage of 5 kV.

Transmission Electron Microscopy (TEM)

To investigate the internal structure, cross sections of hair fibers were examined via transmission electron microscopy (HT7800, Hitachi, Ltd., Tokyo, Japan) operating at 80 kV. The specimens were prepared through a sequence of fixation, dehydration, embedding, and staining, followed by cryosectioning to obtain sufficiently thin sections for imaging.

Tribological Testing

The surface frictional properties of individual hair fibers were evaluated by using a single-fiber friction tester (XCF-1A, Shanghai Xinxian Instrument Co., Ltd., Shanghai, China). Each fiber was secured perpendicularly to the roller under a constant load of 2 cN. For each sample group, 30 fibers were tested to determine the mean frictional values, following established protocols.

Combing Testing

A multifunctional hair piece tester (TSH-50, Shanghai Yanzhuang Biotechnology Co., Ltd., Shanghai, China) was employed to assess the apparent smoothness through dry combing work assays. For the combing work test, a standardized comb-shaped mold was inserted into the piece. The operational parameters were defined by a displacement of 200 mm and a constant speed of 200 mm/min. Data were collected from the 20–80 mm segment. Each measurement was repeated 5 times to ensure reproducibility.

Static Contact Angle Test

Surface hydrophobicity was characterized using an optical contact angle goniometer (OCA-40, Beijing Oriental Defei Instrument Co., Ltd., Beijing, China). A deionized water droplet (2 μL) was deposited onto a suspended hair fiber. The resulting contact angle was recorded and analyzed. Ten fibers were measured per group to calculate the average contact angle.

Mechanical Property Test

Tensile properties were measured using a single-fiber strength tester (XS(80)­XT-3, Shanghai Xusai Instrument Co., Ltd., Shanghai, China). Hair fibers with average diameters ranging from 70 to 90 μm were pre-selected using a laser rangefinder (LSM 501S, Mitutoyo Co., Ltd., Kawasaki, Japan). Individual fibers were secured with a gauge length (clamp spacing) of 40 mm and stretched at a crosshead speed of 20 mm/min. A total of 30 fibers were tested per group. The tensile strength (σ) and Young’s modulus (E) were calculated according to eqs and

σ=F/S 3
E=σ/ε=(F/S)/(ΔL/L0) 4

Where F is the breaking strength, S is the cross-sectional area of the fiber, ε is the strain, ΔL is the displacement, and L 0 is the initial length.

Additionally, the bending resistance was assessed using a flexural tester (QM-25-10, Shanghai Yanzhuang Biotechnology Co., Ltd., Shanghai, China). Fibers were fixed onto the apparatus, where a steel wire subjected the same point of the fiber to continuous reciprocal friction until failure occurred. The average values were derived from 30 independent measurements.

Statistical Analysis

Statistical significance was assessed using t-test, with p-values calculated to determine the level of significance. Data are presented as mean ± standard deviation (SD), with error bars displayed in the figures. Different symbols in the figures denote statistically significant differences between the respective groups.

Results and Discussion

Perming with SHB

SHB was synthesized by 1,6-dichlorohexane and sodium thiosulfate as a white odorless powder. The XRD pattern presented in Figure a showed a characteristic peak at 29.8° and the absence of diffraction peak at 29.1° for SHB, indicating that the product is free of residual sodium thiosulfate. Additionally, the negligible intensity of the peak at 32.1° suggests a minimal concentration of sodium chloride byproduct. These findings demonstrate a new substance with high purity.

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XRD patterns (a) and Raman spectra (b) of SHB, sodium thiosulfate, and sodium chloride.

The structure of SHB was further characterized by Raman spectroscopy, as shown in Figure b. A prominent absorption peak at approximately 2893 cm–1 is observed, corresponding to the C–H stretching vibration of the methylene groups. The intense peak at 1445 cm–1 is attributed to the in-plane bending vibration of methylene (−CH2−). Furthermore, the characteristic peaks within the 500–1250 cm–1 range confirm the retention of the thiosulfate moieties. These spectral features suggest that the chlorine atoms in 1,6-dichlorohexane were successfully substituted by thiosulfate groups, yielding a Bunte salt structure.

The perming performance of SHB was evaluated and compared to that of the conventional oxidative perming method. As illustrated in Figure b, the perming efficiency of SHB (960%) was higher than that of H2O2 (950%), demonstrating an excellent perming efficacy. The perming resistance was evaluated after 10 washing cycles, referring to the capacity of the hair fibers to maintain their curled configuration under external stress. As presented in Figure c,d, after 10 washes, the perming resistance of the SHB-permed hair remained at 78%, which was 5% higher than that of the H2O2-permed hair. This result indicates that the SHB provides superior curl stability. This is because oxidative perming inevitably generates sulfonate groups, resulting in fewer cross-linked covalent bonds compared with SHB-permed hair. Thus, less force is available to bend keratin chains, which macroscopically manifests as a lower hair curl degree.

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(a) Schematic diagram of reaction principles for oxidative perming and Bunte salt perming; (b) perming efficiency of three hair samples; (c) photos of virgin hair (left), H2O2-permed hair (middle), and SHB-permed hair (right) after washing; (d) perming resistance of three hair samples (t-test, ***means p < 0.001).

High Efficiency and Repeatability Achieved by Bunte Salt Perming

In conventional oxidative perming, the incomplete reconnection of disulfide bonds between keratin chains inevitably leads to a decline in perming efficiency and progressive structural damage during repetitive treatments. In contrast, Bunte salts inherently possess disulfide bond structures, which provide the possibility for efficient and repeatable hair perming.

Perming repeatability was evaluated and compared in Figure a by six consecutive perming treatments with H2O2 and SHB, respectively. As presented in Figure b, perming efficiency of H2O2-permed hair decreased substantially with each cycle, falling from 970 to 730% after six treatments. In contrast, Bunte salt perming demonstrated exceptional stability, with the efficiency decreasing only slightly from 970 to 920%. This divergence is likely attributed to the continuous depletion of disulfide bonds during the oxidation process, which weakens the styling effect, while Bunte salt perming avoids this structural deficiency.

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(a) Photos of virgin hair (left), H2O2-permed hair (middle) and SHB-permed hair (right) after repeated perming; (b) perming efficiency of three hair samples (t-test, *means 0.01 < p < 0.05, **means 0.001 < p < 0.01).

To further investigate the changes of disulfide bonds after different perming methods, XPS was employed to monitor the oxidation state of sulfur element. Figure a–c present the XPS spectra and corresponding peak fitting results for repeatedly permed samples, with characteristic peaks at 164.0 and 168.1 eV assigned to disulfide bonds and sulfonates, respectively. The relative content of these groups is compared in Figure d. In H2O2-permed hair, sulfonate content increased by 10.7% compared to virgin hair, indicating that significant cysteine thiols underwent irreversible overoxidation. Conversely, SHB-permed hair exhibited a 15.5% increase in the proportion of disulfide bonds, confirming that the Bunte salt-thiol exchange reaction successfully introduced new covalent cross-links. These analyses verify that incomplete restoration of disulfide bonds accounts for the declining efficiency of oxidative perming. Although Bunte salt perming reinforces the cross-linked architecture by replenishing disulfide bonds, a marginal decrease in perming efficiency was still observed, likely because of limitations in reaction conversion rates and disulfide bond conformations.

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XPS spectra of virgin hair (a), H2O2-permed hair (b) and SHB-permed hair (c); (d) proportions of disulfide bonds and sulfonates in three hair samples.

Effect of Bunte Salt Perming on Hair Properties

Numerous studies have indicated that the damage induced by conventional chemical perming is closely associated with the use of oxidizing agents. As a nonoxidative perming method, Bunte salt perming can effectively avoid oxidative damage. Nevertheless, SHB-permed hair exhibits high reactivity, and whether it causes other forms of hair damage has not been thoroughly investigated. In this section, the effects of Bunte salt permeation on hair properties are systematically evaluated in terms of morphology, porosity, surface characteristics, and mechanical properties.

The surface of hair is protected by a hydrophobic barrier primarily composed of 18-methyleicosanoic acid (18-MEA). This layer determines critical surface properties, such as hydrophobicity and friction, but is highly susceptible to oxidative damage. Generally, a more intact surface cuticle correlates with superior hydrophobicity and tribological properties. SEM was utilized to evaluate the surface morphologies of the hair samples. As shown in Figure a, both virgin hair and SHB-permed hair exhibited clear, tightly arranged cuticle scales with no evident structural impairment. In contrast, H2O2-permed hair displayed severe damage, characterized by a roughened surface with cracked or even detached scales.

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SEM images of three hair samples.

Moreover, the surface properties after different perming methods were compared by evaluating the dynamic friction factor, dry combing work, and hydrophobicity. As shown in Figure a, the dynamic friction factor (μ) of H2O2-permed hair increased by 36.6% to 0.138, compared to 0.101 for virgin hair, indicating significant structural impairment of the cuticle. In contrast, the dynamic friction factor of SHB-permed hair exhibited a slight decrease (μ = 0.095). This suggests that the treatment not only preserved the cuticle structure but also reinforced the surface barrier through the introduction of alkyl chains. This divergence was further confirmed by contact angle measurements (Figure b). While the value for the H2O2-permed hair dropped to 73.6° compared to that of 115.7° for virgin hair, the SHB-permed hair (113.3°) showed only a marginal decrease of 2.4°, confirming the integrity of the hydrophobic barrier. This result is consistent with dry combing tests (Figure c). Compared to virgin hair (W = 9.96 J), the combing work for H2O2-permed hair increased by 157%, whereas the SHB-permed hair showed an increase of only 33%.

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Dynamic friction factor (a), contact angle (b) and dry combing work (c) of three hair samples (t-test, *means 0.01 < p < 0.05, **means 0.001 < p < 0.01).

The loss of disulfide bonds between keratin molecules is macroscopically manifested as the formation of internal pores. Therefore, TEM was employed on ultrathin cross sections of the hair fibers. It could be observed in Figure that H2O2-permed hair presented distinct cavities within the intercellular regions of the cuticle (indicated by white spots within red circles). In contrast, only a few cavities were observed in SHB-permed hair, which is because the relatively large molecular size of SHB prevents it from completely reacting with thiol groups located deepest inside hair fibers. This also implies that SHB-permed hair cannot be fully restored to the level of virgin hair in subsequent mechanical property analysis. The above analysis indicates that supplementing disulfide bonds by Bunte salt can strengthen the cross-linking network, prevent protein loss, and thus maximize the integrity of the internal structure.

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TEM images of three hair samples.

After pores form, the internal structure becomes loose, resulting in a decrease in mechanical properties. The tensile properties of different hair samples are compared in Figure . Compared to the tensile strength of 198.41 MPa for virgin hair, oxidative perming caused a substantial reduction of 42.2%, falling to 114.68 MPa. In contrast, SHB-permed hair retained a strength of 170.52 MPa. This may be because the new disulfide bonds introduced by Bunte salt enhance the inter-fiber forces, thereby increasing the breaking strength. Similarly, the Young’s modulus of H2O2-permed hair plummeted by 45.2% to 2.06 GPa, whereas SHB-permed hair maintained a modulus of 3.28 GPa, effectively preserving cortical toughness. Flexural fatigue testing quantified bending resistance through the number of cycles to failure. As shown in Figure c, oxidative perming reduced fiber durability to 367 cycles. Conversely, the SHB-permed hair maintained superior bending resistance at 557 cycles due to minimal disruption of the cross-linked structure. Overall, although Bunte salt perming still slightly weakens the mechanical strength of hair, the improvement is significant compared with oxidative perming. This conclusion is consistent with the above TEM image analysis results.

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Tensile strength (a), Young’s modulus (b), and number of abrasion cycles to fracture (c) of three hair samples (t-test, *means 0.01 < p < 0.05, **means 0.001 < p < 0.01, ***means p < 0.001).

Conclusion

Conventional oxidative perming method leads to a 10.7% depletion of disulfide bonds, which causes hair damage and limits the viability of repeatable perming. To address this limitation, a novel perming methodology based on the Bunte salt-thiol exchange reaction was developed using sodium hexamethylene-1,6-bisthiosulfate (SHB). Compared to oxidative perming, this method achieves efficient and repeatable perming by supplementing disulfide bonds and avoiding the use of oxidizing agents, with a efficiency of 920% even after six perming cycles, compared to that of 730% for conventional oxidative perming. The Bunte salt perming method also effectively preserves cuticle morphology and surface hydrophobicity, with the contact angle decreasing by only 2.4°. Additionally, due to reduced protein loss and the avoidance of pore formation, the mechanical properties of the hair fibers were improved by 49–59% compared with oxidative perming. This study demonstrates that the Bunte salt-thiol exchange technology maintains disulfide bond density and significantly mitigates hair damage, highlighting its substantial potential for application in repeatable hair styling products.

Acknowledgments

The authors acknowledge Adolph Innovation Laboratory for the support throughout the study and the team at the Key Laboratory of Synthetic and Biological Colloids for their assistance with the study management.

The work was fully supported by the Fundamental Research Funds for the Central Universities (No. JUSRP202404009).

The authors declare no competing financial interest.

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