International Journal of Trichology

: 2022  |  Volume : 14  |  Issue : 6  |  Page : 197--203

Impact of acid (”progressive brush”) and alkaline straightening on the hair fiber: Differential effects on the cuticle and cortex properties

Maria Valéria Robles Velasco1, Tânia Cristina de Sá-Dias1, Michelli Ferrera Dario1, Valcinir Bedin2, Marjory Bernardes Fileto1, Andressa Costa de Oliveira1, Claudinéia Aparecida Sales de Oliveira Pinto1, André Rolim Baby1,  
1 Department of Pharmacy, Faculty of Pharmaceutical Sciences of University of São Paulo, São Paulo, Brazil
2 Department of Dermatology, Faculty BWS, São Paulo, Brazil

Correspondence Address:
Marjory Bernardes Fileto
Department of Pharmacy, Faculty of Pharmaceutical Sciences of University of São Paulo, 580 Prof. Lineu Prestes Avenue, Bl-13/15, 05508-900 São Paulo


Background: Glyoxylic acid has emerged as a safe alternative to formol (formaldehyde) use as a hair straightener/relaxer. However, the possible damage to the hair fiber after its application is low known and/or published in the literature. Aims: This work aims to characterize hair locks treated with glyoxylic acid compared to traditional alkaline straighteners such as sodium and guanidine hydroxide and ammonium thioglycolate. Materials and Methods: The morphology of the hair cuticles was observed by scanning electron microscopy. Protein loss was assessed by the Lowry method modified by Peterson and as mechanical properties that were expressed in terms of tensile strength. Results: All products (sodium and guanidine hydroxides and ammonium thioglycolate) caused protein loss of about 2.5 μg/g, except glyoxylic acid that caused the worst damage (3.5 μg/g), in relation to the untreated (virgin) hair (1.12 μg/g), indicating that the chemical treatments can cause hair damage in both cuticles and cortex. The force to break the fibers treated with traditional straighteners based on sodium hydroxide, guanidine hydroxide, and ammonium thioglycolate was statistically the same. Conclusion: The treatment with glyoxylic acid showed rupture tensile statistically equivalent to the alkaline straighteners. The mechanism of action of glyoxylic acid does not appear to be based on breaking and rearrangement of disulfide bridges, but altered them, that influenced the hair strength. However, it is also essential to consider other factors relevant: technical application technique, reaction time, and interval of reapplication of the product, as this can change the pattern of the results obtained.

How to cite this article:
Velasco MV, de Sá-Dias TC, Dario MF, Bedin V, Fileto MB, de Oliveira AC, Pinto CA, Baby AR. Impact of acid (”progressive brush”) and alkaline straightening on the hair fiber: Differential effects on the cuticle and cortex properties.Int J Trichol 2022;14:197-203

How to cite this URL:
Velasco MV, de Sá-Dias TC, Dario MF, Bedin V, Fileto MB, de Oliveira AC, Pinto CA, Baby AR. Impact of acid (”progressive brush”) and alkaline straightening on the hair fiber: Differential effects on the cuticle and cortex properties. Int J Trichol [serial online] 2022 [cited 2023 Mar 24 ];14:197-203
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The hair consists of three concentric areas: cuticle, which is the outer covering of the hair; cortex, which is the largest portion of the hair and imparts strength to the hair shaft; and medulla, which is the central area of the hair and is not always present, and the cell membrane complex represents about 2% of the hair's mass and has the function to unite the cuticle layers of hair.

The shaft of hair is composed mainly of keratin protein (90%) and a small amount of lipid (1%–9%). The major function of keratin cuticles is to protect the cortex of the hair from damage caused by several factors including heat, ultraviolet (UV) radiation, pollution, and cosmetic products. The intermolecular bonds present in the threads are responsible for the shape of the hair and structural stability of the keratin, consequently, for the mechanical resistance and shape of the hair shaft. These intermolecular interactions can be classified into disulfide (-S-S-), hydrophobic bonds that exist in the presence of water, van der Waals, and ionic (saline bonds). Basic straighteners, for example, have as their main mechanism of action the interaction with keratin, breaking the structural bonds (S-S) of the hair that can then be rearranged, in order to leave the hair straight.[1],[2],[3]

The straighteners such as based on sodium, potassium, and lithium hydroxides, guanidine carbonate, bisulfites, and thiols have several action mechanisms. The hydroxide-based straighteners have as their main mechanism of action, the “lantionization” process, where the hydroxyl ion (OH-), when penetrating the cuticles, reacts with the disulfide bonds, forming the sulfenic acid, product unstable, with subsequent reactions leading to the formation of lanthionine which stabilizes the new form. The main difference between cysteine and lanthionine is that the latter has a sulfur atom. The high pH of the hydroxides (pH 12–13) breaks the disulfide bonds. The shaft then becomes malleable and, when mechanically stretched, the disulfide bonds are rearranged. A substance is then applied that acidifies the pH, interrupting the process and re-closing the disulfide bridges in the new desired wire shape. Acidic shampoos (pH 4.0–6.0) are generally used to restore bonds.[4]

The thiol-based straighteners have ammonium or ethanolamine thioglycolate as representatives. The main mechanism of action of thioglycolate is the reduction of disulfide bonds, leaving the ions negatively charged and facilitating the desired shape. This process depends on intermediary pH that depends on the hydroxide (sodium or ammonium).[1],[5]

Regardless of the straightening active chosen, in order to finish the reaction, a neutralizing agent is used. In the case of products based on thioglycolate, the hair is rinsed with a product with an oxidizing agent (hydrogen peroxide) so that neutralization occurs. The application of an alkaline relaxer is followed by a washing procedure with a product containing acid and an acid-base indicator, usually phenolphthalein.[6] On the other hand, glyoxylic acid and derivative application is not followed by neutralization reaction. In this case, a flat iron is applied several times to reach the desired smooth effect due to the polymer film formation on the surface of the cuticle. Then, it is necessary to wait 20–30 min before rinsing.[7]

Straightening products based on glyoxylic acid and derivatives have been used for 5 years as an alternative to formaldehyde (forbidden by Brazilian law) or to traditional alkaline and reducing products, and they have pH below 2.0. The action mechanism of glyoxylic as well as related ingredients is not entirely understood, but apparently, they cause significant changes in amino acid structure. Additionally, heating to 200°C by a flat iron promotes biopolymerization reactions with hydrophobic film formation in the fiber surface which is related to the smoothing effect. Other reactions with components present in melanin pigments and artificial hair dyes affect hair color, often fading by the formation of free radicals.[7]

In the study carried out by Boga (2014), it was demonstrated that in straightening with glyoxylic acid, rearrangements in the secondary structure of keratin occurred, probably due to the formation of new compounds, unusual in the hair, such as hemi(thio)acetals and imines. It has been proven that this rearrangement occurs mainly in the cortex of the hair shaft, and there are also conformational changes in the keratin from α-helix to β-pleated leaf. The straightening products with formol are forbidden, but the consumers seek cosmetics that give benefits, such as smoother hair with less volume. These products appear on the market under different names, such as “capillary or restructuring or remodeling” and “smart brush.” There is no end, mostly, they are products based on glyoxylic acid or its derivatives and mixtures, and the acid straightener released in Brazil until June 2020 is glyoxyloyl carbocysteine (and) glyoxyloyl keratin amino acids (and) water, although the regulation of new substances is under study, such as glyoxyloyl hydrolyzed wheat protein/sericin and glyoxylic acid. The mentioned substances release formaldehyde in a reduced concentration and sufficient to form the biopolymer film around the hair strand when it is submitted to the heating process by the curling iron.[1],[4],[5]

The study of Colenci (2017) explains the mechanism of action of formation of the biopolymer film from the formaldehyde (formaldehyde), with the formation of a polymer, polyacetal, in which it is formed when the formaldehyde comes into contact with the hair, and there is heating of the strands (~200°C) using flat iron. Because, it is necessary to remove the water present in the hair, and provide energy for the formation of the film. Polymerized capillary fiber can acquire the characteristics of polyacetal such as greater rigidity, shine, and resistance to moisture (hydrophobic), which prevents interaction with the external environment, leaving the hair dry inside.[8]

However, in Brazil, many beauty salons still insist on using formaldehyde in their straightening formula, but the use of formaldehyde as a hair straightener has been banned by ANVISA since 2009. The agency does not register products that contain formaldehyde in their formulations, because this component can cause several health problems, such as irritation, itching, burning, swelling, scaling and redness of the scalp, and hair loss. This research aimed to analyze and compare protein loss, mechanical properties, and cuticle morphology of hair tresses treated with traditional alkaline straightening and glyoxylic acid (acid brush) products, in order to identify and compare the real damage caused in the cuticles and cortex hair.[9]

 Materials and Methods

Straightening products

The qualitative composition of straightening formulations commercially available is described in [Table 1].{Table 1}

Hair tress preparation

Prewashing procedure

Curly brown hair tresses, 30 cm in length, classified as type IV according to the De la Mettrie et al. system,[10] were purchased from Andrea Lopes® (Brazil). They were moistened in warm water (40.0°C ± 3.0°C) for 30 s and approximately 0.4 g of sodium lauryl sulfate (16% w/v in water) per gram of hair was added, and it was cleaned for 30 s with the fingers. Then, the tresses were rinsed in the warm water for 30 s, the excess of water was removed, and they were dried at room temperature (22.0°C) at least 12 h.[11]

Application of straightening/relaxing formulations

Hair straightening products were applied to tresses according to manufacturer's instructions as described next.

Glyoxylic acid

The straightening emulsion with glyoxylic acid was applied to cleaned and moistened hair tresses (1:1), followed by a massage for 1 min. The product remained in contact with hair for 20 min. Then, a thin, nonmetallic comb was passed to remove the excess of the product (without washing procedure). The tresses were dried with a hairdryer and a round brush, and then, a flat iron at 200°C was used for at least five times in order to form the biofilm on the cuticles.

Ammonium thioglycolate

About 1.0 g of sodium thioglycolate emulsion per gram of hair was applied and allowed to act for 5 min without combing. Then, the hair was straightened with a nonmetallic comb for 1 min followed by 15 min of rest. This procedure was repeated, and the neutralization washing procedure was performed. About 1.0 g/g of neutralizing lotion containing hydrogen peroxide was applied on hair strands and the product acted for 15 min. After the neutralization, the washing procedure “prewashing procedure” was performed again.

Sodium or guanidine hydroxide

Guanidine hydroxide was prepared just before the application, by mixture of guanidine carbonate and calcium hydroxide [Appendix A]. The product was applied in the proportion 1:1 (1.0 g of straightening cosmetic/1 g of dry hair tresses), followed by 1 min of massage. The tresses were smoothed using a nonmetallic comb for 1 min and the product acted for 15 min. After the straightening process, the washing procedure was performed as described earlier, using an indicator shampoo containing phenolphthalein.

Evaluation of hair tresses

Protein loss quantification by Lowry method modified by Peterson

This assay was based on the reduction of the Folin–Ciocalteu reagent by protein of hair previously treated with copper in alkaline medium.[12],[13] The analytical curve was prepared using standard bovine albumin (Fraction V), secondary reference standard Sigma-Aldrich® (purity content = 96.1%). An amount of 100.0 mg of each tress (about 2.0 cm in length) was cut and transferred to 250 ml Erlenmeyer containing 15 ml of distilled water. The mixture was submitted to 40 min of an ultrasonic bath and filtered through a quantitative filter paper. Exactly, 2.0 ml was reacted with 2.0 ml of Reagent A for 10 min and then for 30 min with 1.0 ml of Reagent B (triplicate) in the dark. The absorbance was evaluated at 750 nm (UV-visible spectrophotometer, Shimadzu® UV-1203) and distilled water used as blank. The values were presented as equivalent in albumin (standard protein with purity of 100.0%).[14]

The Reagent A was prepared as follows: 25 ml of a solution containing copper sulfate (0.1% w/v), potassium tartrate (0.2% w/v), and sodium carbonate (10% w/v) was mixed with 25 ml of a sodium hydroxide solution (0.8 N) and 25 ml of sodium dodecyl sulfate (10% w/v) in a 100-mL volumetric flask diluted to volume with distilled water. The Reagent B was prepared with 8.4 ml of Folin–Ciocalteu in a 50-ml volumetric flask diluted to volume with water.[14]

Mechanical properties

Analysis was performed on Diastron MTT175® load cell for testing, operating at speed traction from the clutches of 50 mm/min, distance of 30 mm. Twenty fibers from each treatment were used. Their diameters were measured with micrometer Mitutoyo®, in three positions (root, the middle portion, and tip), and the mean value was used to calculate the total area of the hair fiber. The tests were conducted at 25°C and 65% relative humidity.[15]


Possible significant differences in the results were analyzed by one-way ANOVA and the differences between treatments were identified by Tukey test (α =0.05).

Morphology analysis

Five fibers of each tress were sputter coated with platinum (Baltec® Model 020) before observation of the morphology using scanning electron microscopy (Leica® Model 440). All micrographs were collected at an accelerating voltage of 10 kV.

 Results and Discussion

Protein loss

Hair fibers are constituted mainly by protein. When exposed to physical or chemical treatments, it may suffer damage to its structure and thus may be altered in the protein composition. This can be evaluated easily by a quantitative assay of the extracted hair amino acids.

Various methods of protein loss are mentioned in the scientific literature, such as Lowry, Bradford, and Kjeldahl. Lowry's method was chosen for the tests in this study with straighteners with our research group which employs the Folin–Ciocalteu reagent. It has been used to quantify the damage to human hair; it is based on the reduction of the Folin reagent by protein previously treated with copper in alkaline medium. A copper atom bonds to four residuals of amino acid. This complex reduces the Folin reagent, with the solution becoming blue with maximum absorbance at about 715 nm. The higher the damage to the cuticle causes, bigger the protein loss compared to virgin hair.

Lucarini and Kilikian (1999) carried out work in which they comparatively evaluated the Lowry and Bradford methods regarding the level of interference of some substances used for the precipitation of glucoamylase by ethyl alcohol. The Bradford method does not present any interference, while the Lowry method resulted in 20% higher values of the protein concentration in the presence of ethyl alcohol and tris buffer. Despite these interferences, Lowry method can more accurately assess the increase in purity during fractionation, due to its greater sensitivity to proteins and peptides of low molar mass (<6 kDa).

The protein values are relative, since the analytical curve is constructed with the absorbance values for bovine serum albumin (SBA). This method is more sensitive for cystine, cysteine, histidine, tryptophan, and tyrosine than for other amino acids. Since the SBA is composed of these amino acids, compared to 20%–25% (w/w) in human hair, the quantification of protein loss is similar to the absolute values and can be used to compare the damage caused by the procedures.[4],[16],[17]

Considering the results presented in [Figure 1], all chemical straighteners caused protein loss and damage to hair, but glyoxylic acid caused the most protein loss (albumin equivalent). A protein loss of 1.12 μg/g was seen for untreated hair, about 2.5 μg/g for strands treated with traditional alkaline straightening products, and 3.56 μg/g for those treated with glyoxylic acid. Thus, sodium and guanidine hydroxide as well as ammonium thioglycolate caused equal protein loss, this result may be linked or to the reduction of the amount of cysteine caused by the smoothing process. When sodium or potassium hydroxide or sodium carbonate in combination with guanidine in the concentration of 1.5%–3% for 15 min under the strands, they irreversibly reduce the cystine content of the hair, resulting from the union of two cysteines, to two-thirds of the original amount.{Figure 1}

All results were higher than the observed loss for virgin hair but lower (statistically significant) than glyoxylic acid. In the study by Sá-dias, 2015, it was noticed that the scope evaluated the damage to the hair fiber from untreated samples and in those that received application of traditional and alternative straighteners/relaxers. The glyoxylic acid and formaldehyde straighteners significantly reduced the breakdown stress, making them more fragile and the greater protein loss. In another study by Colenci (2017), he analyzed the effects of using straighteners containing carbocysteine, glyoxylic acid, formaldehyde, and ammonium thioglycolate with one application and successive applications; the results obtained showed that all the straightening systems studied affected irreparable shape to the hair matrix and the breaking energy was also affected with the use of all straighteners, indicating a loss of mechanical strength and the consequent loss of fiber elasticity.[18],[19]

The intense damage caused by treatment with glyoxylic acid can also be associated with cortex and cuticles, or both, because of the acid pH of the product (<2.0) and also due to the excessive heat treatment necessary for the smoothing effect (about 200°C) on hair shaft. However, the cleavage of cystine disulfide bonds by glyoxylic acid is unlikely. Its action is probably based on its reaction with some amino acids, besides rearrangements in the secondary structure distribution as well as some conformational changes at the level of disulfide bridges within the hair cortex rather than in the cuticle.[7]

Mechanical and surface properties

Human hair has about 65%–95% of its weight in proteins, more 32% of water, lipid pigments, and other components. Chemically, about 80% of human hair is formed by a protein known as keratin. Hair fiber is strong since a Caucasian one can support 50–100 g before breaking, which equals to 12 kg/mm2. The hair mechanical strength depends on the keratin helical structure which is parallel to the longitudinal axis of the hair. When a load is applied, the hair tresses and elongation are proportional to the force used (plastic region). If the force continues to be applied, the hair extends quickly (around 25%–30% of the original size), featuring a plastic behavior, and if the same force continues to be used, the elongation is proportional to the elastic-plastic region up to breakage.[5],[20]

Keratin, a protein that has cysteine as its main amino acid, responsible for the disulfide bridges (S-S), which form three-dimensional network of cystine, with a high density of cross-links. Keratin IF and keratin-associated proteins located in the matrix are the main proteins present in the hair fiber. In keratin, most of the cystine residues (>95%) are involved in disulfide bonds providing mechanical, chemical resistance and elastic properties. The straighteners (ammonium thioglycolate and sodium or guanidine hydroxides) have a high pH value (>9.0) and smooth by breaking and reorganizing the disulfide bridges (SS) present in the keratin, structural protein of the hair fiber. Acid straighteners based on formaldehyde, glutaraldehyde, and glyoxylic acid involve a reaction between the aldehyde groups of these molecules and some functional groups present in the capillary fiber structure, causing rearrangements in the distribution of the secondary structure of keratin, as well as conformational changes in the level of disulfide bonds. Additionally, plasticization of the cuticle occurs, forming a hydrophobic film.[1],[11] As shown in [Figure 2], traditional relaxers (sodium and guanidine hydroxides and ammonium thioglycolate) reduced the mechanical resistance of virgin hair (statistically similar values but different from natural hair) but caused small damage to the cuticles, at least as a single application, as observed in [Figure 3]b, [Figure 3]c, [Figure 3]d, in comparison with virgin hair [Figure 3]a.{Figure 2}{Figure 3}

The technique using alkaline products can change the mechanical properties in an irreversible way and with plastic characteristics of extension and elongation. This technique is more effective with greater durability of the smooth effect due to the modification of the hair fiber, but the transformation of the amino acid cystine into lanthionine after the straightening causes the weakening of the hair fiber, related to the reduction in its mechanical resistance and propensity to the break.[19]

Furthermore, as expected, glyoxylic acid did not cause a significant injury in the shaft hair, because although the small molecule penetrates quickly reaching cortex, its relaxing mechanism seems not to be based on the break of disulfide bonds rearrangement, the main responsible for hair strength, but it changes the structure of α-keratin.[7] Besides, it forms a biopolymerized film on the cuticles, as observed in [Figure 3]e, with hydrophobic character that attracted the natural sebum of the scalp, which protect and maybe keep a certain level of resistance to avoid hair damage. This result indicated that the integrity of the keratin protein in the cortex was less affected, and the protein loss found was related to the damage in both layers: cuticles and cortex. However, the cuticles are protected with a noncontinuous film which explains the possible protein loss.

The film formation is felt in practice use by hairdressers and users, especially when it is also performed other procedures such as dye application, which requires partial removal of the film by a washing process with shampoo. Furthermore, additional applications that occur in approximate 3 to 6 months may ensure better coverage of the product on hair shaft. These events may explain brightness noticed by consumers that straighten the hair with acid products. Thus, additional studies will be 24 necessary in order to elucidate the action mechanism to 25 elucidate the interaction with keratin structure, potential 26 damages, and how to minimize these.

The tests carried out direct us to confirm the behavior of the hair fiber in the face of chemical procedures as causing damage to the layers of the cuticle and cortex. Future studies will investigate other methodologies and damage factors and how to prevent them.


All products (sodium and guanidine hydroxides and ammonium thioglycolate) caused protein loss of about 2.5 μg/g, except glyoxylic acid that caused the worst damage (3.5 μg/g), in relation to the untreated (virgin) hair (1.12 μg/g), indicating that the chemical treatments caused hair damage in both cuticles and cortex.

The tensile strength of natural hair and those treated with glyoxylic acid are statistically equivalent, probably due to the formation effect of the polymeric film of the straightening. The force required to break the fibers treated with traditional straighteners based on sodium hydroxide, guanidine hydroxide, and ammonium thioglycolate was statistically the same, but less than that required for virgin hair, suggesting that alkaline straighteners damage the hair. Furthermore, the treatment with glyoxylic acid showed rupture tensile results statistically equivalent to the alkaline straighteners. These findings were obtained for only one application, and the results may change with repeated applications.

In image analysis, changes in cuticular layers were observed in the straightening treatment with sodium hydroxide and guanidine and ammonium thioglycolate, with fragments adhered to the surface. When straightening with glyoxylic acid, a possible film formation is observed.

It is fundamental also to consider other relevant factors such as product application technique, reaction time, and reapplication local, which may alter the pattern of results obtained because it is possible to observe overlapping effects or deposition of product in splice areas between the old and new applications of straightening product in hair. Thus, this research area requires additional studies with other complementary techniques.


We thank Daniela D' Almeida Peres for support in statistical analysis, to Dow Corning®, especially to Alvaro Gomes, Caroline Freitas, and Daniel Almeida.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.


1Goshiyama AM. Evaluation of hair fibers properties treated with acid straightener at different pH values. In: Masters Dissertation. University of São Paulo: Faculty of Pharmaceuticals Sciences; 2019.
2Lai-cheong JE, Mcgrath JA. Structure and function of skin, hair and nails. Med 2013;41:317-20.
3Villa AL, Aragão MR, Santos EP, Mazotto AM, Zingali RB, Souza EP, et al. Feather keratin hydrolysates obtained from micro-bial keratinases: Effect on hair fiber. BMC Biotechnol 2013;13:15.
4Ferreira LA, Braga DC. Active substances of capillary straightening and their action mechanisms. Ele J Pharm 2016;8:56-6.
5Sá-dias TC. In vitro Evaluation of the Effect of Different Chemical/Thermal Straightening Processes on Hair Fiber. In: Doctoral Thesis. São Paulo: Faculty of Pharmaceutical Sciences of University of São Paulo; 2015.
6Draelos ZD. Cosmetic Dermatology: Products and Procedures. Oxford: Wiley-Blackwell; 2010.
7Boga C, Taddei P, Micheletti G, Ascari F, Ballarin B, Morigi M, et al. Formaldehyde replacement with glyoxylic acid in semipermanent hair straightening: A new and multidisciplinary investigation. Int J Cosmet Sci 2014;36:459-70.
8Colenci AV. Degradation of human hair caused by the use of straighteners contemporary and other chemical processes effect of a formulation containing the chitosan biopolymer on the caucasian hair fiber. 102p. In: Master Dissertation. São Paulo: School of Engineering of São Carlos, University of São Paulo; 2017.
9Brazilian Society of Dermatology. Straightning Brush; 2017. Available from: [Last accessed on 2020 Jun 20].
10De la Mettrie R, Saint-Léger D, Loussouarn G, Garcel A, Porter C, Langaney A. Shape variability and classification of human hair: A worldwide approach. Hum Biol 2007;79:265-81.
11Dario MF. Development and evaluation of the effectiveness of bioactive cationic nanoemulsion in protecting hair photo-oxidative damage. In: Doctoral Thesis. São Paulo: Faculty of Pharmaceutical Sciences, University of São Paulo; 2016.
12Lowre OH, Rosebrough NJ, Farr AL, Randall RJ. Protein measurement with the folin phenol reagent. J Biol Chem 1951;193:265-75.
13Peterson GL. A simplification of the protein assay method of Lowry et al. which is more generally applicable. Anal Biochem 1977;83:346-56.
14Faria PM, Camargo LN, Carvalho RS, Paludetti LA, Velasco MV, Gama RM. Hair protective effect of argan oil (Arganiaspinosa kernel oil) and cupuassu butter (Theobroma grandiflorum seed butter) post treatment with hair dye. J Cosmet Dermatol Sci Appl 2013;3:40-4.
15Dario MF, Pahl R, de Castro JR, de Lima FS, Kaneko TM, Pinto CA, et al. Efficacy of Punica granatum L. hydroalcoholic extract on properties of dyed hair exposed to UVA radiation. J Photochem Photobiol B 2013;120:142-7.
16Gama RM, Baby AR, Velasco MV. In vitro methodologies to evaluate the effects of hair care products on hair fiber. Cosmetic 2017;4:1-10.
17Gama RM. Evaluation of the damage to the hair shaft caused by oxidative dye with without additives of conditioning. In: Master Dissertation. São Paulo,: Faculty of Pharmaceuticals Sciences, University of São Paulo; 2010.
18Mauricio LP. Characterization of the structural integrity of the hair fiber treated with different chemicals. In: Master Dissertation. Natal: Federal University of Rio Grande do Norte; 2019. p. 108f.
19França-Stefoni SA, Dario MF, Sá-Dias TC, Bedin V, de Almeida AJ, Baby AR, et al. Protein loss in human hair from combination straightening and coloring treatments. J Cosmet Dermatol 2015;14:204-8.
20Velasco MV, Sá-dias TC, Freitas AZ, Júnior NDV, Pinto CA, Kaneko TM, et al. Hair fiber characteristics and methods to evaluate hair physical and mechanical properties. Braz J Pharm Sci 2009;45:153-62.