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Citation: Junior, C.M.; Vieira, M.H.;
Cacoci, É.S.P.; Abelan, U.S.; Sarruf,
F.D.; Lima, C.C.; Chin, C.M.
Comparative Assessments of New
Hair-Straightening Cosmetic
Formulations on Wavy Type 2 Hair.
Cosmetics 2024,11, 222. https://
doi.org/10.3390/cosmetics11060222
Academic Editor: Desmond J. Tobin
Received: 14 November 2024
Revised: 3 December 2024
Accepted: 10 December 2024
Published: 16 December 2024
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Article
Comparative Assessments of New Hair-Straightening Cosmetic
Formulations on Wavy Type 2 Hair
Celso Martins Junior 1, *, Matheus Henrique Vieira 1,Érica Savassa Pinto Cacoci 1, Ursulandrea Sanches Abelan 1,
Fernanda Daud Sarruf 2, Cibele Castro Lima 3and Chung Man Chin 1,4
1Laboratory for Drug Design (LAPDESF), Drugs and Medicines Department, School of Pharmaceutical
Sciences, University of São Paulo State, UNESP, Araraquara 14800-903, SP, Brazil; chung.man-chin@unesp.br
or chung@unilago.edu.br (C.M.C.)
2Department of Pharmacy, Faculty of Pharmaceutical Sciences, University of São Paulo,
São Paulo 05508-000, SP, Brazil
3Institute of Physics, University of São Paulo, São Paulo 05508-000, SP, Brazil; cibelecl@if.usp.br
4Advanced Research Center in Medicine (CEPAM), School of Medicine, Union of the Colleges of the Great
Lakes (UNILAGO), São Josédo Rio Preto 15030-070, SP, Brazil
*Correspondence: celso.junior@tricologia-abt.com.br
Abstract: Hair straighteners are among the most technically complex products to be safely and
effectively developed, and this challenge has increased even more with the higher incidence of
resistant hair among consumers. This underscores the importance of studying new active ingredients,
combinations and carrier formulations to improve performance without compromising safety. In
this research, we compared eight hair-straightening formulations with different active ingredients
and/or concentrations to develop new, safer and more effective texture modifiers. Eight formulations
were developed and compared with each other and to controls (virgin and bleached hair) regarding
mechanical and thermal resistance, cuticle morphology, hair shine and fiber diameter. Results showed
that all formulations were safe and effective at straightening hair. Specifically, 13.3% and 9.4%
ammonium thioglycolate (G03 and G04) were more suitable for wavy and curly hair, 12.5% and 7.9%
amino methyl propanol thioglycolate (G05 and G06) for finer or chemically processed hair, 5% and
4% sodium cysteamine (G07 and G08) for curly and tight curly hair to control volume, and 2% and
1% of a combination of ammonium thioglycolate with sodium thioglycolate (G09 and G10) for more
resistant wavy and curly hair.
Keywords: hair straightening; hair discoloration; hair fiber; oxireductor straightening; hair
efficacy assessment
1. Introduction
Hair has been long-considered important for consumers’ self-esteem, and the cosmetics
industry has continuously worked to develop products to improve both hair scalp and
fiber quality while ensuring they are safe and effective [
1
]. One of the most critical hair
product categories are hair straighteners, a globally used product category that presents
technical challenges related to tolerance levels and chemical compatibility when developed
and applied to different hair fiber types and conditions. This highlights the importance of
studying formulation approaches, including active ingredients and their free concentrations,
carrier emulsions, pH ranges, viscosity control polymers and their effects on different hair
types and conditions, mainly concerning safety and efficacy [2].
Hair straightener active components are divided into three main groups: lanthion-
izers, oxiredutors and organic acids. Lanthionizers correspond to hydroxides (sodium,
guanidine, lithium and potassium), which convert cystine amino acids into lanthionines
on the outermost portion of the hair. Oxireductors are salts or esters of thioglycolic acid.
Organic acids (like formaldehyde, glutaraldehyde, glyoxylic acid and their compounds) are
Cosmetics 2024,11, 222. https://doi.org/10.3390/cosmetics11060222 https://www.mdpi.com/journal/cosmetics
Cosmetics 2024,11, 222 2 of 17
prohibited in some countries such as Brazil, as their improper use poses risks to consumers’
and professionals’ health; however, they are still available in some market products [3–5].
Relevant research involving combinations of texture-modifying active ingredients and
polymeric associations enabled important improvements in active ingredients regarding
straightening potential, better safety and hair resistance, and lower fiber wear, highlighting
the importance of additional formulation components [6–9].
In the past decade, there has been a consumer movement to return to natural hair,
which has increased the number of individuals with more resistant hair seeking texture
modifications. This shift has presented technical challenges for salon professionals who
use products with higher concentrations of ammonium thioglycolate. To address these
challenges, combinations of ammonium thioglycolate with lower percentages of sodium
thioglycolate have been suggested to enhance product penetration into resistant fibers,
providing better release kinetics and more effective initial straightening. Although these
combinations have not been used before, they have been mentioned in the literature as a
technical possibility for improving straightening efficacy [10,11].
Based on the aforementioned considerations, this study evaluates eight hair straight-
ening formulations, each featuring distinct active components, combinations and concen-
trations, in comparison to control treatments (virgin and bleached hair). The objective was
to develop a new generation of hair texture modifiers that are both safer and more effective
for use. Additionally, the study identified the most suitable formulations for hair exhibiting
varying levels of resistance.
2. Materials and Methods
2.1. Formulations Development
Eight hair-straightening emulsions were developed according to Table 1. Four of
these emulsions contained higher concentrations while the remaining four had lower
concentrations of the same straighteners for comparison. Formulations’ stability was
evaluated over 120 days in accordance with the Brazilian Stability Guideline prior to further
testing [
12
]. Formulations’ specifications including pH ranges (measured by pHmetry) and
viscosities (measured using a viscosimeter) are also presented. These parameters are crucial
for determining product efficacy and permeation into hair fibers [2].
Table 1. Hair-straightening formulations and treatment groups.
Formulations and Groups Composition 1
Virgin hair (G01)—control Not applicable
Market bleaching product (G02)—control
Association of ammonium, sodium and
potassium persulfates + oxidizing cream 40 V
(12% H2O2)
Straightener with ammonium
thioglycolate—high concentration (G03)
Ammonium thioglycolate 59%: 22.50%
Ammonium Hydroxide 29%: 2.50%
Viscosity: 80,000–120,000 cPs
pH: 9.0–9.5
Straightener with ammonium
thioglycolate—low concentration (G04)
Ammonium thioglycolate 59%: 16.00%
Ammonium Hydroxide 29%: 1.50%
Viscosity: 80,000–120,000 cPs
pH: 9.0–9.5
Straightener with amino methyl propanol
thioglycolate—high concentration (G05)
Amino methyl propanol: 12.50%
Thioglycolic acid: 11.00%
Viscosity: 100,000–200,000 cPs
pH: 7.5–8.5
Cosmetics 2024,11, 222 3 of 17
Table 1. Cont.
Formulations and Groups Composition 1
Straightener with amino methyl propanol
thioglycolate—low concentration (G06)
Amino methyl propanol: 7.90%
Thioglycolic acid: 7.50%
Viscosity: 100,000–200,000 cPs
pH: 7.5–8.5
Straightener with sodium cysteamine—high
concentration (G07)
Cysteine: 5.00%
Sodium Hydroxide: 4.05%
Sodium Metabisulfite: 0.50%
Viscosity: 50,000–100,000 cPs
pH: 11.0–13.0
Straightener with sodium cysteamine—low
concentration (G08)
Cysteine: 4.00%
Sodium Hydroxide: 3.00%
Sodium Metabisulfite: 0.50%
Viscosity: 200,000–300,000 cPs
pH: 11.0–13.0
Straightener with ammonium thioglycolate
and sodium thioglycolate—high
concentration (G09)
Ammonium thioglycolate 59%: 18.00%
Sodium Hydroxide: 2.75% 2
Thioglycolic acid: 2.00%
Viscosity: 100,000–200,000 cPs
pH: 8.5–9.5
Straightener with ammonium thioglycolate
and sodium thioglycolate—low
concentration (G10)
Ammonium thioglycolate 59%: 20.00%
Sodium Hydroxide: 2.20 %2
Thioglycolic acid: 1.00
Viscosity: 45,000–80,000 cPs
pH: 8.5–9.5
1
A polymeric standard base emulsion was used for all formulations in amount sufficient to 100% complete
treatment. The composition of the base emulsions was not revealed due to intellectual property.
2
In these
formulations, sodium hydroxide is not used as a straightening agent itself. It reacts with thioglycolic acid to form
the straightening agent sodium thioglycolate. Therefore, straightening active ingredients in G09 and G10 are
ammonium thioglycolate and sodium thioglycolate.
2.2. Preparation of Hair Locks Samples
Virgin, dark brown, naturally wavy, type 2 hair locks (considered as resistant) with
roots’ extremities glued were obtained from De Meo Brothers Inc.
®
(New York, NY, USA),
weighing 3 g and measuring 15
±
2 cm long (per lock). They were prepared per assay,
identified and treated as described in Table 1(G01 to G10).
All locks samples (G01 to G10) were pre-washed with a 10% sodium lauryl ether
sulfate solution at 0.5 mL/g hair, massaged for 1 min, totally rinsed off and completely
dried with hot hair drier with 15 cm distance.
For G02 (bleached hair), the locks were pre-washed, completely dried with hot hair
drier with 15 cm distance, bleached with commercial bleach and 40 V oxidizing cream
according to manufacturer’s instructions with 60 min contact, and again dried with hot
hair drier with 15 cm distance.
For G03 to G10 (straightened hair), the locks were pre-washed, dried with hot hair drier
with 15 cm distance, then straightener was applied and left in contact for 60 min. After that,
hair was completely rinsed with warm water, neutralizing shampoo was applied (0.5 mL/g)
and left for 5 min, hair was rinsed with flowing water for 1 min, neutralizing shampoo
was reapplied (0.5 mL/g) and left for 5 min, hair was rinsed with flowing water for 1 min,
commercial product Complex Balm Neutralizer was applied (0.5 mL/g), massaged on hair
for 1 min and left in contact for 15 min. Finally, hair was totally rinsed with flowing water,
and locks were dried with hot hair drier with 15 cm distance.
2.3. Hair Diameter Measurement
One hundred fibers per group among triplicate locks were selected for diameter
measurement using Mitutoyo
®
(Kanagawa, Japan) electronic micrometer IP54. Average
Cosmetics 2024,11, 222 4 of 17
diameter was compared between groups using variance statistical analysis (ANOVA) with
Tukey post-test and a confidence interval of 95% using Minitab®19.0.
2.4. Scanning Electronic Microscopy
We selected three fibers from one lock per group, which were cut to obtain a 5 cm
portion from the central part. These samples were transferred to a sample-holder for
sputtering (gold coating) to improve electron conductivity and were subjected to Scanning
Electronic Microscopy (SEM) using JSM-6460LV (Jeol
®
—Tokyo, Japan) microscope to obtain
3 hair surface 1000X zoom images per sample for morphology integrity assessment [13].
Image Analysis of SEM Images
One SEM image per treatment was visually selected for mathematical image analysis
of cuticle damage with Image Pro Premier software (Media Cybernetics
®
—Rockville, MD,
USA). The selection was based on program’s requirements to obtain a reliable and coherent
analysis, allowing the quantification of the percentage area of the cuticular shadow.
Cuticular shadow area can be related to hair porosity, as large shadow areas indicate
greater damage points in hair morphology and more porous hair. The exception to this
correlation is when cuticle damage is so extensive that it is removed from hair, causing total
cortex exposure. In this situation, the observed %area will be lower than for intact hair [
2
].
Images were processed with the software to enhance highlighting of hair characteristic;
cuticle shadow area was calculated (Pixels
2
); and %area was determined. The %area
corresponds to the area affected by damage (cuticle cell edges’ openings promoted by
morphology modification) in relation to the region of interest selected on the microscopy
image [2].
We selected 3 fibers per group after treatment and took 3 images per fiber with
1000x zoom. We selected 1 of the 9 images per group for image analysis using visual
selection criteria to choose the best image that would allow the most robust assessment by
the software.
2.5. Hair Thermal Analysis
Hair fragments from one lock sample per treatment were cut from the central portion
of the selected fibers to be weighted in crucibles using a Shimadzu AUW220D analytical
balance (Kyoto, Japan). Thermogravimetry/Derived Thermogravimetry (TG/DTG) and
Differential Scanning Calorimetry (DSC) analyses were performed as follows.
DSC: Around 2 mg of each hair sample was weighed and placed in pin hole alu-
minum crucibles (non-hermetic pan for dry analysis) and analyzed with Exstar DSC 7020
Differential Scanning Calorimeter (Hitachi, Tokyo, Japan), with a 25–300
◦
C heating ramp,
10
◦
C/min heating rate and inert dynamic atmosphere of nitrogen with 50 mL/min flow.
Data were analyzed with TRIOS software version 5.5.1.5 (TA Instruments, New Castle,
DE, USA) for hair dehydration (water loss) and denaturation of alpha-helix chains. In this
methodology, pyrolysis occurs simultaneously with hair fiber structures’ pyrolysis [14].
TG/DTG: Around 2–5 mg of each hair sample was weighed and placed in to platinum
hermetic crucibles and analyzed with TG/DTA Discovery TGA 5500 (TA Instruments,
New Castle, DE, USA), with a 25–500
◦
C heating ramp, 10
◦
C/min heating rate and inert
dynamic atmosphere of nitrogen with 100 mL/min flow. Data were analyzed with TRIOS
software version 5.5.1.5 (TA Instruments) for mass loss determination [14].
2.6. Assessment of Hair Loss by Breakage
Treated hair locks were inserted in triplicates into the thermal cycle machine to be
combed with rotating brushes while being subjected to a hot hair drier. Brushes passed
through each lock 1500 times divided into 3 cycles of 500, with a standardized speed of
25 rpm and fixed drier distance. After each cycle, all fallen hair was manually counted and
summed per treatment for comparison.
Cosmetics 2024,11, 222 5 of 17
2.7. Mechanical Resistance
One treated hair lock per group was selected to cut 30 fibers for mechanical resistance
assessment using Instron 4505 with 1.0 kgf load cell coupled with tensile grips probe and
Tracomp Windows TRC v61288 software for data collection. The claws were positioned
5 cm apart, and the traction speed was set at 50 mm/min. We obtained stress–strain curves
and calculated maximum force for each fiber (n = 30). Average force values were obtained,
and groups’ data were statistically compared using Minitab 19.0 by ANOVA with Tukey
post-test (alpha = 5%) [2].
2.8. Fluorescence Confocal Microscopy
For fluorescence confocal microscopy assessment, rhodamine fluorescent marker was
added to the investigational products (hair straighteners) prior to locks’ treatments, aiming
to verify/compare products’ penetration into hair fibers. Treatment groups G03 to G10
were divided into 2 subgroups:
(a)
“Virgin”: virgin locks were treated as described in Section 2.2 with straightener
products (G03 to G10) impregnated with rhodamine.
(b) “Bleached”: locks were previously bleached (as described for group G02 in Section 2.2)
and then treated as described in Section 2.2 with straightener products (G03 to G10)
impregnated with rhodamine.
All locks samples were kept under standardized environmental conditions during the
whole assay, at 50 ±5% relative humidity and 21 ±2◦C temperature.
Thirty fibers of each subgroup were selected and cut to 10 cm length and then sub-
jected to cross-sectional cuts with a razor. Cuts were transferred to slides and introduced
in the fluorescence confocal microscope LSM 700 (Zeiss
®
—Oberkochen, Germany) for
image capturing.
2.9. Hair Gloss Measurement
Hair gloss (luster values) was assessed in triplicate and at 10 reading points, using
Samba Hair equipment (Bossa Nova Technologies
®
—Culver City, CA, USA), at room
temperature (22
±
2
◦
C and 55
±
5% relative humidity). Luster values were calculated
according to Equation (1) [
15
]. Data were statistically compared between groups using
Minitab 19.0 by ANOVA with Tukey post-test (alpha = 5%).
LBN T =100 ×Sin
(D+Sout )×1
Wvisual
(1)
where:
LBNT = Bossa Nova Technologies luster;
Sin = specular profile value in central light distribution;
Sout = specular profile value for extreme angle;
D = integral value of the diffuse profile;
Wvisual = average width of brightness band.
3. Results
3.1. Formulations’ Development and Straightening Efficacy
All eight straightening emulsion formulations were successfully developed and were
stable during the whole stability assay (120 days) in all experimental environmental con-
ditions (room temperature at 25
◦
C
±
2
◦
C, refrigerator at 5
◦
C
±
2
◦
C and stove at
45
◦
C
±
2
◦
C). Therefore, all of them were selected for further assessments. Table 2presents
the formulations’ obtained pH and viscosity values. All parameters varied within specifica-
tions during stability assays.
Cosmetics 2024,11, 222 6 of 17
Table 2. Formulations’ pH and viscosity values on initial measurements.
Group pH Value Viscosity Spindle
and Rotation (rpm) Viscosity Value (cPs)
G03 9.32 S63–0.6 95,180
G04 9.23 S63–1.0 112,000
G05 8.44 S63–0.6 118,000
G06 7.84 S63–0.6 174,000
G07 12.71 S63–1.5 60,707
G08 11.27 S63–0.3 250,000
G09 8.80 S63–6.0 12,157
G10 9.00 S63–1.5 51,269
Figure 1shows images of hair tresses after product application for each group.
Cosmetics 2024, 11, x FOR PEER REVIEW 6 of 18
3. Results
3.1. Formulations’ Development and Straightening Efficacy
All eight straightening emulsion formulations were successfully developed and were
stable during the whole stability assay (120 days) in all experimental environmental
conditions (room temperature at 25 °C ± 2 °C, refrigerator at 5 °C ± 2 °C and stove at 45 °C
± 2 °C). Therefore, all of them were selected for further assessments. Table 2 presents the
formulations’ obtained pH and viscosity values. All parameters varied within
specifications during stability assays.
Table 2. Formulations’ pH and viscosity values on initial measurements
Group pH Value
Viscosity Spindle and
Rotation (rpm) Viscosity Value (cPs)
G03 9.32 S63–0.6 95,180
G04 9.23 S63–1.0 112,000
G05 8.44 S63–0.6 118,000
G06 7.84 S63–0.6 174,000
G07 12.71 S63–1.5 60,707
G08 11.27 S63–0.3 250,000
G09 8.80 S63–6.0 12,157
G10 9.00 S63–1.5 51,269
Figure 1 shows images of hair tresses after product application for each group.
Figure 1. Hair tresses after product application per group (in left to right sequence: from G01 to
G10).
When visually comparing treatments, we observed that straightening performance
was directly proportional to active concentration. This was particularly noticeable in
treatments G07 and G08, where the difference is more evident. The highest straightening
potential was observed in treatments G03 and G09, while the lowest was found in G06
and G08.
3.2. Hair Diameter Measurement Results
The average hair fiber diameter results per group are presented in Figure 2.
Figure 1. Hair tresses after product application per group (in left to right sequence: from G01 to G10).
When visually comparing treatments, we observed that straightening performance
was directly proportional to active concentration. This was particularly noticeable in
treatments G07 and G08, where the difference is more evident. The highest straightening
potential was observed in treatments G03 and G09, while the lowest was found in G06
and G08.
3.2. Hair Diameter Measurement Results
The average hair fiber diameter results per group are presented in Figure 2.
Cosmetics 2024, 11, x FOR PEER REVIEW 7 of 18
Figure 2. Hair fiber average diameter per treatment group and time. G01 = virgin hair; G02 =
bleached hair; G03 = ammonium thioglycolate 13.3%; G04 = ammonium thioglycolate 9.4%; G05 =
AMP thioglycolate 12.5%; G06 = AMP thioglycolate 7.9%; G07 = sodium cysteamine 5%; G08 =
sodium cysteamine 4%; G09 = combination of ammonium thioglycolate with sodium thioglycolate
2%; G10 = combination of ammonium thioglycolate with sodium thioglycolate 1%. Groups that do
not share a leer are significantly different.
Difference in diameter between times (delta before and after treatment) was
calculated per group and in the results are as follows: G01 = 1.36; G02 = −10.18; G03 = 4.08;
G04 = 5.07; G05 = 7.61; G06 = 4.06; G07 = 7.10; G08 = 8.47; G09 = 5.50; G10 = 7.39. These
results show that only bleaching caused diameter reduction, and all straightening
formulations increased diameter (positive delta values).
After statistical ANOVA analysis followed by Tukey post-test, treatments with no
statistical difference were gathered under the same grouping leer as follows: variable
“a”: G01, G05, G07, G08, G09, G10; variable “b”: G01, G04, G05, G07, G08, G09; variable
“c”: G01, G03, G04, G05, G06, G07, G09; and variable “d”: G02. Groups that do not share
a leer are significantly different.
3.3. Scanning Electronic Microscopy (SEM) with Image Analysis Results
The SEM technique enables detailed 3D-amplified imaging of hair fibers, allowing us
to assess morphological integrity and cuticle damage [2,13]. Table 3 lists the selected fibers
and images for each group and the image analysis results (area percentage). Figure 3
shows the analyzed SEM images.
Table 3. Hair fibers and images selected and SEM image analysis results 1.
Group Selected Fiber Selected Image Percentage Area of Cuticle Shadow
G01 3 3 9.06%
G02 1 3 8.09%
G03 2 1 5.38%
G04 1 1 7.35%
G05 3 1 6.10%
G06 1 3 7.08%
G07 2 3 7.82%
G08 2 2 5.95%
G09 2 3 7.04%
G10 2 1 5.51%
1 Image analysis corresponds to the percentage area of cuticle shadow of each fiber, which
indicates fiber damage and porosity.
Figure 2. Hair fiber average diameter per treatment group and time. G01 = virgin hair;
G02 = bleached hair; G03 = ammonium thioglycolate 13.3%; G04 = ammonium thioglycolate 9.4%;
Cosmetics 2024,11, 222 7 of 17
G05 = AMP thioglycolate 12.5%; G06 = AMP thioglycolate 7.9%; G07 = sodium cysteamine 5%;
G08 = sodium cysteamine 4%; G09 = combination of ammonium thioglycolate with sodium thiogly-
colate 2%; G10 = combination of ammonium thioglycolate with sodium thioglycolate 1%. Groups
that do not share a letter are significantly different.
Difference in diameter between times (delta before and after treatment) was calcu-
lated per group and in the results are as follows: G01 = 1.36; G02 =
−
10.18; G03 = 4.08;
G04 = 5.07; G05 = 7.61; G06 = 4.06; G07 = 7.10; G08 = 8.47; G09 = 5.50; G10 = 7.39. These
results show that only bleaching caused diameter reduction, and all straightening formula-
tions increased diameter (positive delta values).
After statistical ANOVA analysis followed by Tukey post-test, treatments with no
statistical difference were gathered under the same grouping letter as follows: variable “a”:
G01, G05, G07, G08, G09, G10; variable “b”: G01, G04, G05, G07, G08, G09; variable “c”:
G01, G03, G04, G05, G06, G07, G09; and variable “d”: G02. Groups that do not share a letter
are significantly different.
3.3. Scanning Electronic Microscopy (SEM) with Image Analysis Results
The SEM technique enables detailed 3D-amplified imaging of hair fibers, allowing
us to assess morphological integrity and cuticle damage [
2
,
13
]. Table 3lists the selected
fibers and images for each group and the image analysis results (area percentage). Figure 3
shows the analyzed SEM images.
Table 3. Hair fibers and images selected and SEM image analysis results 1.
Group Selected Fiber Selected Image
Percentage Area of Cuticle Shadow
G01 3 3 9.06%
G02 1 3 8.09%
G03 2 1 5.38%
G04 1 1 7.35%
G05 3 1 6.10%
G06 1 3 7.08%
G07 2 3 7.82%
G08 2 2 5.95%
G09 2 3 7.04%
G10 2 1 5.51%
1
Image analysis corresponds to the percentage area of cuticle shadow of each fiber, which indicates fiber damage
and porosity.
3.4. Hair Thermal Analysis Results
Thermal analysis was used to study samples’ behaviors after being subjected to
a controlled temperature program to provide information about the physical–chemical
characteristics of each material [
14
,
16
,
17
]. Using the DSC technique, we measure the
energy difference between the sample and a reference material as a function of temperature,
allowing us to identify specific thermal events.
Ascending peaks correspond to exothermic events and descending peaks to endother-
mic events. The TG/DTG analysis monitors mass variations as a function of temperature
and/or time and provides mass data on mass loss events [
14
,
16
,
17
]. The resulting thermal
curves are shown in Figure 4.
3.5. Hair Breakage Results
This analysis allows us to verify fragility points on hair fiber by counting the number
of broken fibers after standardized cycles of hair combing and drying in a thermal cycling
machine [
18
]. The higher the number of broken fibers, the more damage has been caused
to the hair (i.e., more fragile hair). The sum of broken fibers per treatment are as follows:
Cosmetics 2024,11, 222 8 of 17
G01 = 99; G02 = 115; G03 = 85; G04 = 70; G05 = 93; G06 = 72; G07 = 67; G08 = 60; G09 = 103;
G10 = 96.
Cosmetics 2024, 11, x FOR PEER REVIEW 8 of 18
Figure 3. Analyzed SEM images per group.
3.4. Hair Thermal Analysis Results
Thermal analysis was used to study samples’ behaviors after being subjected to a
controlled temperature program to provide information about the physical–chemical
characteristics of each material [14,16,17]. Using the DSC technique, we measure the
energy difference between the sample and a reference material as a function of
temperature, allowing us to identify specific thermal events.
Ascending peaks correspond to exothermic events and descending peaks to
endothermic events. The TG/DTG analysis monitors mass variations as a function of
temperature and/or time and provides mass data on mass loss events [14,16,17]. The
resulting thermal curves are shown in Figure 4.
Figure 3. Analyzed SEM images per group.
Cosmetics 2024, 11, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/cosmetics
Figure 4. Thermal analysis profiles of the hair samples per treatment group: (a) DSC; (b) TG/DTG.
Figure 4. Thermal analysis profiles of the hair samples per treatment group: (a) DSC; (b) TG/DTG.
Cosmetics 2024,11, 222 9 of 17
When comparing these results, we noticed that bleached hair promoted the high-
est number of broken fibers, as expected (115 total). Also, some treatments promoted
lower breakage than on virgin hair, with G04, G06, G07 and G08 having the lowest (best
performance in this assay). Only G09 treatment led to more breakage than for virgin hair.
3.6. Mechanical Resistance Results
A dynamometer was used to measure the force (stress) required to deform hair fiber
(strain) until rupture, obtaining a stress–strain curve. A typical hair fiber curve is composed
of an elastic/Hookean region (0–2% deformation), a plastic region (2–30%), a post-plastic
region (>30% deformation), and a breaking point. In the elastic region, deformation of the
fiber increases proportionally with the applied force. In the plastic region, fiber elongation
increases significantly without requiring a substantial increase in force. This is caused by
the conversion of alpha-keratin to beta-keratin, offering less resistance to applied force. In
the post-plastic region, the elongation becomes proportional to tension, with the resistance
to deformation caused by the beta-keratin structure. Stretching continues until the fiber
reaches the breaking point (fiber rupture) [18].
The average mechanical resistance results (maximum force) per treatment were as
follows: G01 = 0.0664; G02 = 0.0520; G03 = 0.0600; G04 = 0.0765; G05 = 0.0799; G06 = 0.0903;
G07 = 0.0711; G08 = 0.0615; G09 = 0.0672; G10 = 0.0843. Figure 5represents the results’
interval graph.
Cosmetics 2024, 11, x FOR PEER REVIEW 10 of 18
Figure 5. Mechanical resistance results per treatment—average maximum force values 1. 1 Leers a,
b, c and d represent grouping variables of ANOVA analysis with Tukey post-test. Averages that do
not share a leer are statistically different.
Leers represent grouping variables of ANOVA analysis with Tukey post-test.
Averages that do not share a leer are statistically different. Variable “a”: G01, G04, G05,
G06, G07, G08, G10; variable “b”: G01, G05, G06, G07, G08, G09, G10; variable “c”: G03,
G05, G06, G07, G09, G10; and variable “d”: G02.
After ANOVA analysis (α = 0.05), G02 (bleached hair) was significantly different than
all other groups, demonstrating force reduction, as expected. Hair bleaching knowingly
fragilizes the hair fiber cuticle and cortex, thus negatively influencing its mechanical
properties [2]. When comparing straighteners to virgin hair (G01), G03 was significantly
different, with a lower force, unlike all other treatments. Still, G03 was beer than
bleached hair.
3.7. Fluorescence Confocal Microscopy Results
Fluorescence analysis was conducted to assess the extent of product penetration into
hair fibers. Rhodamine, a fluorescent marker, was added prior to application of the
treatment products. The fluorescence observed in the images indicates areas where the
product reached after contact with hair. This allowed us to visualize whether each
treatment successfully penetrated to the cortex [19]. Products (straighteners) were applied
to both virgin and bleached hair to examine the influence of bleaching treatment on hair
permeation to straighteners. As all treatments were in contact with hair for the same
duration, this analysis enabled us to assess the intensity of product diffusion. Fluorescence
microscopy images are listed in Figure 6.
Figure 5. Mechanical resistance results per treatment—average maximum force values
1
.
1
Letters a,
b, c and d represent grouping variables of ANOVA analysis with Tukey post-test. Averages that do
not share a letter are statistically different.
Letters represent grouping variables of ANOVA analysis with Tukey post-test. Aver-
ages that do not share a letter are statistically different. Variable “a”: G01, G04, G05, G06,
G07, G08, G10; variable “b”: G01, G05, G06, G07, G08, G09, G10; variable “c”: G03, G05,
G06, G07, G09, G10; and variable “d”: G02.
After ANOVA analysis (
α
= 0.05), G02 (bleached hair) was significantly different
than all other groups, demonstrating force reduction, as expected. Hair bleaching know-
ingly fragilizes the hair fiber cuticle and cortex, thus negatively influencing its mechan-
ical properties [
2
]. When comparing straighteners to virgin hair (G01), G03 was signifi-
cantly different, with a lower force, unlike all other treatments. Still, G03 was better than
bleached hair.
Cosmetics 2024,11, 222 10 of 17
3.7. Fluorescence Confocal Microscopy Results
Fluorescence analysis was conducted to assess the extent of product penetration
into hair fibers. Rhodamine, a fluorescent marker, was added prior to application of the
treatment products. The fluorescence observed in the images indicates areas where the
product reached after contact with hair. This allowed us to visualize whether each treatment
successfully penetrated to the cortex [
19
]. Products (straighteners) were applied to both
virgin and bleached hair to examine the influence of bleaching treatment on hair permeation
to straighteners. As all treatments were in contact with hair for the same duration, this
analysis enabled us to assess the intensity of product diffusion. Fluorescence microscopy
images are listed in Figure 6.
Cosmetics 2024, 11, x FOR PEER REVIEW 11 of 18
Figure 6. Fluorescence microscopy images of straightened bleached and virgin hair fibers.
Among the bleached straightened hair locks, no difference in diffusion was observed
between treated groups. Differences in product diffusion were observed between groups
on virgin hair. Specifically, G05 and G06 showed the lowest penetration, and G09 and G10
exhibited the highest fiber penetration.
3.8. Hair Gloss Results
The assessment of hair gloss (luster) serves as an indicator of cuticle integrity, which
affects light reflection. Luster results obtained by Samba Hair equipment per group are
described in Figure 7. The average values (G01 to G10) obtained were as follows: 24.52;
2.66; 27.93; 29.32; 29.21; 27.71; 23.91; 29.03; 29.48; 28.02.
Figure 6. Fluorescence microscopy images of straightened bleached and virgin hair fibers.
Among the bleached straightened hair locks, no difference in diffusion was observed
between treated groups. Differences in product diffusion were observed between groups
on virgin hair. Specifically, G05 and G06 showed the lowest penetration, and G09 and G10
exhibited the highest fiber penetration.
3.8. Hair Gloss Results
The assessment of hair gloss (luster) serves as an indicator of cuticle integrity, which
affects light reflection. Luster results obtained by Samba Hair equipment per group are
described in Figure 7. The average values (G01 to G10) obtained were as follows: 24.52;
2.66; 27.93; 29.32; 29.21; 27.71; 23.91; 29.03; 29.48; 28.02.
Cosmetics 2024,11, 222 11 of 17
Cosmetics 2024, 11, x FOR PEER REVIEW 12 of 18
Figure 7. Hair gloss values (average and interval—BNT luster) per treatment group.
In the statistical analysis, G02 luster values were significantly lower than all other
groups, as expected due to damage from the treatment process. Compared to virgin hair
(G01), G07 showed no statistically significant difference in luster indicating neither
improvement nor substantial worsening in hair shine. However, G07 did differ
significantly in luster compared to all other groups. Overall, all straightened hair groups,
except G07, showed significantly improved shine compared to virgin hair.
4. Discussion
In this study, we developed hair straighteners with different active ingredients for
evaluation. Each selected active ingredient was incorporated into a cosmetic emulsion
with a standardized polymeric association to enhance performance.
Ammonium thioglycolate has been widely used as hair straightener since 1932, and
formulations with this active ingredient have been progressively improving over time.
While olfactory discomfort during application was once considered an issue, it has
improved considerably over the last decade. New research on polymeric associations
combined with active fiber modifiers has also allowed significant improvement in
ammonium thioglycolate’s performance regarding straightening potential, reducing
mechanical resistance loss and improving compatibility with colored hair by lowering
oxidation levels [3,20].
Amino methyl propanol (AMP) thioglycolate, patented as BR 10 2013 017,342 8–INPI,
is an option for reshaping and texturizing hair that has already undergone chemical
processing, such as in cases of hair transitioning. AMP thioglycolate operates within lower
pH ranges, resulting in a reduced cuticle dilation and minimal permeation, making it
gentler and more suitable for fine and more sensitive hair. On the other hand, it is less
effective in resistant hair [2,20].
Sodium cysteamine, patented as BR 102022004790-1, is a straightening agent
designed to modify the texture of curly and tight curly hair. It is formed by the
combination of L-cystine (weak acid) with sodium hydroxide (strong base) with a pH
range of 11.00–12.50 due to the base strength predominance in its reaction [3,20].
The combination of ammonium thioglycolate with lower percentages of sodium
thioglycolate was designed to enhance permeation into more resistant hair, which exhibits
higher release kinetics and more intense effects during the beginning of the straightening
Figure 7. Hair gloss values (average and interval—BNT luster) per treatment group.
In the statistical analysis, G02 luster values were significantly lower than all other
groups, as expected due to damage from the treatment process. Compared to virgin
hair (G01), G07 showed no statistically significant difference in luster indicating neither
improvement nor substantial worsening in hair shine. However, G07 did differ significantly
in luster compared to all other groups. Overall, all straightened hair groups, except G07,
showed significantly improved shine compared to virgin hair.
4. Discussion
In this study, we developed hair straighteners with different active ingredients for
evaluation. Each selected active ingredient was incorporated into a cosmetic emulsion with
a standardized polymeric association to enhance performance.
Ammonium thioglycolate has been widely used as hair straightener since 1932, and for-
mulations with this active ingredient have been progressively improving over time. While
olfactory discomfort during application was once considered an issue, it has improved
considerably over the last decade. New research on polymeric associations combined with
active fiber modifiers has also allowed significant improvement in ammonium thioglyco-
late’s performance regarding straightening potential, reducing mechanical resistance loss
and improving compatibility with colored hair by lowering oxidation levels [3,20].
Amino methyl propanol (AMP) thioglycolate, patented as BR 10 2013 017,342 8–INPI,
is an option for reshaping and texturizing hair that has already undergone chemical
processing, such as in cases of hair transitioning. AMP thioglycolate operates within lower
pH ranges, resulting in a reduced cuticle dilation and minimal permeation, making it
gentler and more suitable for fine and more sensitive hair. On the other hand, it is less
effective in resistant hair [2,20].
Sodium cysteamine, patented as BR 102022004790-1, is a straightening agent designed
to modify the texture of curly and tight curly hair. It is formed by the combination of
L-cystine (weak acid) with sodium hydroxide (strong base) with a pH range of 11.00–12.50
due to the base strength predominance in its reaction [3,20].
The combination of ammonium thioglycolate with lower percentages of sodium
thioglycolate was designed to enhance permeation into more resistant hair, which exhibits
higher release kinetics and more intense effects during the beginning of the straightening
Cosmetics 2024,11, 222 12 of 17
process. While this specific association is novel in practice, it has been mentioned in
literature [10].
All formulations have been assessed and proved to be safe and effective for hair
straightening, with different behaviors concerning some assessed parameters, and differ-
ences in straightening performance. These differences are the basis of decisions about
applications directed to different hair types/conditions.
Average diameter may reflect product deposition and/or hair swelling, and a correla-
tion between the diameter, the level of chemical interference of the proposed formulas and
their concentrations was determined. This parameter tends to directly correlate with hair
mechanical resistance: the larger the diameter, the greater the hair tensile strength [
2
,
21
].
Among all groups, bleached hair presented the smallest diameter with a statistically sig-
nificant difference compared to all other groups (p> 0.05). Also, bleached hair caused a
significant decrease in hair strength after stress–strain assessment, and fluorescence mi-
croscopy showed that it drastically increased hair permeation. This was expected and
corroborates findings in the literature, as bleaching fragilizes hair and wears the cuticle [
3
].
Discoloration can modify around 20% of hair fiber structure, compromising up to 45%
of the mechanical resistance, associated with the denaturation of the sulfur bridges of
the external structure (cuticle layers), increasing the permeation of cosmetics [
3
]. When
comparing straightened to virgin hair (G01), there was no statistically significant difference
for any of the treatments, with some showing a slight increase in average diameter (mainly
G08 and G10).
Hair diameter results can be correlated with hair force results, as a higher diame-
ter value tends to increase force. Concerning mechanical properties’ assessment among
straighteners, G03 promoted the highest damage to hair fiber (force reduction). As in
diameter assessment, hair force (tensile strength) can also be correlated to the polymeric
association added to the formulations, which can be used to control active liberation
kinetics [22–24].
Results concerning hair loss by breakage also corroborate hair diameter and force
results. In this assay, we can highlight the good performances of G04, G06 and G08, all of
which had lower concentrations of active ingredients, thus evidencing a smoother active
force and the exceeding polymeric association in formulations, which protected the hair.
Hair morphology, cuticle damage and porosity were assessed using SEM combined
with image analysis. The cuticle shadow area in image analysis can be related to hair
porosity: the larger the cuticle shadow area, the larger the damage points observed in
hair tress morphology and, consequently, the more porous the hair. The exception to this
condition occurs when the damage to the cuticle is so extensive that it is removed from the
hair, causing total cortex exposure. In this case, the percentage area observed will be lower
than that of intact hair [
2
]. This assay allows us to analyze the hair surface damage level
by evaluating the sum of the area occupied by the openings at the edges of the cuticular
cells, caused by the morphological change due to chemical procedures, also simulating the
potential for contraction and reorganization of the hair structure, indirectly determining the
level of damage for each group [
2
]. The SEM images were selected for image analysis based
on software’s requirements to achieve a more robust and coherent analysis concerning
visual morphology.
After analyzing SEM images, we observed good surface uniformity with more pre-
served cuticles for virgin hair. However, some porosity was observed in G01 probably due
to pre-washing with sodium laurate sulfate. Bleached hair (G02), as expected, presented in-
tense wear and lixiviation of cuticle layer, corroborating diameter and mechanical resistance
findings. In addition, no significant alterations were observed for hair straightener groups,
with porosity values occurring in accordance with literature [
3
]. This demonstrated that all
formulations reached an adequate balance level for hair shape and texture modification
without excessive damage.
The lowest porosity results were found in groups with higher active ingredient con-
centrations such as G03 and G05 (ammonium thioglycolate and amino methyl propanol
Cosmetics 2024,11, 222 13 of 17
thioglycolate). This may be explained by a better polymeric coverage in these formulations,
which might contribute to controlled release kinetics of these active ingredients. On the
other hand, other groups with high active ingredient concentrations presented the highest
porosity values, such as G07 (sodium cysteamine) and G09 (combination of ammonium
thioglycolate and sodium thioglycolate). For G07 and G09, slightly higher porosity was
expected, as these active ingredients are known to promote greater hair permeation and hair
structure modification induction. A more pronounced difference in cuticle wear is expected
for more alkaline straighteners (sodium cysteamine and hydroxides) and oxidizing agents
(thioglycolic acid salts and esters) [3].
Another parameter that reflects cuticle integrity is hair gloss/luster, which was mea-
sured with Samba Hair [
25
]. In our findings, we observed that straightened hair samples,
except for G07, showed significantly improved luster compared to virgin hair. This result
supports the findings of Bloch and coworkers in 2021, in which hair straightened with
ammonium thioglycolate demonstrated better hair shine results after sensory analysis
by trained panelists compared to virgin hair. The authors concluded that this improve-
ment could be attributed to the greater alignment of fibers after straightening, enhancing
light reflection [
26
]. Goshiyama (2019) also evaluated hair luster by Samba Hair of locks
straightened at different acid pH values (pH 1.00 and pH 2.00), comparing them to each
other and to virgin hair, and concluded that “the higher the shine, the more aligned the
thread”. The author also mentioned the interference of different shades and curl level on
this parameter [27].
As for the thermal analysis results for DSC (dry methodology—pin hole crucible), all
samples exhibited similar thermal profiles composed of three endothermic events: water
loss between 30
◦
C and 160
◦
C, denaturation and pyrolysis of intermediate keratin fibers
between 229
◦
C and 236
◦
C. In the DSC curves, we also observed a third peak around
250
◦
C for all samples. Wortmann and Deutz (1998) attributed this peak to ortho-cortex
cells, which have lower melting temperature than the para-cortex. This difference may be
due to the varying cysteine contents and disulfate bonds between these cortex cell types:
ortho-cortex cells have a lower number of disulfate bonds than para-cortex cells [28]. Our
findings also support the work of Popescu and Gummer (2016), who reported that keratin
fibers present a typical endothermic peak around 230
◦
C, which was attributed to thermal
denaturation of the keratin alpha-helixes [29].
When analyzing our Dry-DSC results comparing treatments, we observed that thermal
denaturation peaks for G02 and G07 occurred at higher temperatures than G01—increasing
from 235
◦
C to 239 and 238
◦
C, respectively. Other straighteners behaved similarly to G01.
This suggests that G02 and G07 produced more significant alteration in the organization
of intermediate fibers. Wortmann and coworkers (2020) attribute an increase in hair
denaturation temperature to a reorganization of organic chains after denaturation and the
reduction in denaturation enthalpy to the disorganization of the keratin structure after
bleaching [
30
]. We could also attribute this result to the considerable difference in pH
range for sodium cysteamine (G07 and G08—pH around 4 points more alkaline), which
increases its protein-denaturing capacity. These endothermal events for sodium cysteamine
suggest a greater potential for hair structure modification, indicating the beginning of
the transformation of crystalline proteins into amorphous and more flexible forms, with
a reduction in peripheral disulfate bonds. From a practical marketing perspective, this
implies less need for thermal interferences (such as hair dryers and ironing) in procedures
that aim to modify hair texture with this active ingredient.
For the TG results, we observed that all samples presented thermogravimetric profiles
typical of hair samples, with three thermal events:
•First event (peak 1): water loss at 50–150 ◦C;
•
Second event (peaks 2 and 3): onset of matrix pyrolysis and disorganization of the
keratin structure at 250–350 ◦C;
•Third event (peak 4): degradation of keratin’s carbon structure until 500 ◦C.
Cosmetics 2024,11, 222 14 of 17
These findings support the findings in the literature. Monteiro and coworkers (2005)
tested virgin hair and observed a first mass loss event at 25–131
◦
C attributed to water
liberation, a second and a third mass loss event attributed to keratin denaturation with
degradation of macrofibrils and matrices at 280–350
◦
C, and an event attributed to total
degradation of hair keratin carbonic chains at 350–550
◦
C. The authors also observed
different behavior for bleached hair, with complete protein degradation at higher tempera-
tures [
31
]. Lima (2016) compared thermal events of Caucasian, Asian and Afro-ethnic hair
and observed three thermal events: a water loss event at lower temperatures (25–200
◦
C
for Caucasian hair; 25–195
◦
C for Asian; and 25–170
◦
C for Afro-ethnic hair); a thermal
keratin degradation/decomposition at intermediate temperatures (200–460
◦
C, 200–460
◦
C
and 170–432
◦
C, respectively, for Caucasian, Asian and Afro-ethnic hair); and a complete
degradation of keratin carbonic chains event (460–690
◦
C, 460–685
◦
C and 432–650
◦
C,
respectively, for Caucasian, Asian and Afro-ethnic hair) [14].
In our work, when comparing mass loss temperatures between treatments, there
was a significant difference between G01 (virgin hair) and G02 (bleached hair) concerning
degradation events (specifically peak 3), which corroborates findings from Monteiro and
coworkers (2005). According to the authors, damage to Caucasian hair causes a reduction
in the number of mass loss stages, which is compatible with the DTG peak softening
in degradation onset of G02 compared to G01. Also for G02, degradation occurred at a
lower temperature (317
◦
C) than G01 (329.3
◦
C), indicating hair damage [
31
]. Regarding
straighteners, most samples behaved similarly in terms of degradation events (peak 3),
except for G03 and G07. Only G03 presented a degradation peak (peak 3) lower than G02,
with a 47.1% mass loss at 294 ◦C for G03 versus a 54.5% mass loss at 317 ◦C for G02.
The lowest observed mass loss and consequently largest residue at the end of the assay
were groups G02 and G03, likely due to the complex hair cortical and cuticular structures.
Treatments may have caused a reduction in cuticle, matrix or other components, which
contributed to the lower mass loss in degradation events (peaks 2 and 3), leading to a
greater residue after 500 ◦C [2].
G03 and G05 presented the highest mass loss percentages due to evaporation under
lower temperatures (11.8% at 43
◦
C and 10.8% at 49
◦
C, respectively), which could be
considered proportional to their strength in dilating cuticle and facilitating permeability of
thioglycolic acid. They also reduced onset temperature of mass loss by dehydration, which
could be attributed to increased fiber porosity given by these dilators. This may result in a
greater potential for hair dryness after straightening, highlighting the need for additional
products for hair maintenance such as shampoos, conditioners and modelling products to
compensate for this increased water loss [2].
All performed assessments highlighted damages and structure alterations caused by
bleaching to fiber, and fluorescence microscopy reinforced this fact, as expected. Bleaching
makes hair more permeable to substances, as observed in the bleached groups for all
treatments. This pre-treatment significantly increased permeability of all straighteners com-
pared to virgin hair. When comparing straighteners on virgin hair regarding permeation in
cortex by fluorescence microscopy we observed that the following:
•
G03 permeated slightly more than G04, which was favored by the higher pH range
and higher ammonium hydroxide (dilator) concentration.
•
G05 and G06 penetrated the least among all treatments and had a very smooth action,
which was expected, as they act at the lowest pH values (7.5–8.5) and their dilator
(amino methyl propanol) is softer.
•
When comparing G07 and G08, G07 permeated more, which could be attributed to the
difference in active ingredients available in free form as well as a difference in active
ingredient’s release kinetics due to the polymeric associations.
•
G09 permeated slightly more than G10, which was probably favored by the increased
pH range and higher amount of the dilator ammonium hydroxide. In these groups,
sodium thioglycolate acted as a permeation accelerator and increased the liberation
kinetics of the reductor active—thioglycolic acid.
Cosmetics 2024,11, 222 15 of 17
Based on all obtained results, we can infer that all assessed straighteners proved to be
safe and effective on wavy hair. Each formulation showed optimal performance for specific
hair condition.
Ammonium thioglycolate (G03 and G04) presented good straightening potential and
permeability as shown in fluorescence microscopy. The formulations were well-balanced in
their proportion of active ingredient concentrations and are suitable for type 2 hair with
marked waves [2,5].
Amino methyl propanol thioglycolate (G05 and G06) showed a milder straightening
effect compared to other groups, remained effective and induced the least alterations to the
hair fiber. Therefore, these formulations are best suited for finer type 2, hair oxidized with
30 or 40 volumes (9% or 12% H2O2, respectively) or chemically processed hair [2].
Sodium cysteamine (G07 and G08) proved to be ideal for more rigid structures with
more need of modifications on external structures, such as in hair types 3 and 4 (curly
and tight curly hair). The observed anticipation of thermal events in DSC along with
increased diameter and resistance, luster improvement and improved structural flexibility
(resulting in reduced breakage) suggest that these formulations are best suited for these
hair types [11].
The combination of ammonium and sodium thioglycolates (G09 and G10) led to
stable and safe results with well-controlled straightening strength, mainly for G09, which
presented greater straightening efficacy. This combination is most appropriate for type 2
hair with average to thick textures, higher resistance and more pronounced waves [2,11].
5. Conclusions
In conclusion, developing safe and effective hair straighteners is a significant challenge
for the cosmetic industry, as these products must modify hair texture with minimal damage.
Given the diversity in hair types, each with unique resistance levels and responses to
straighteners, there is a need for products with varied kinetics, behavior, and intensity.
This study successfully developed texture-modifying formulations with different active
ingredient combinations, each tailored to specific hair types and conditions, and all of
which demonstrated safety, low damage potential, and efficacy.
The results indicate that ammonium thioglycolate (G03 and G04) is well-suited for
type 2 wavy hair, while amino methyl propanol thioglycolate (G05 and G06) is optimal for
finer type 2 wavy hair and chemically processed hair. Sodium cysteamine (G07 and G08) is
ideal for tight curly (Type 4) and curly (Type 3) hair, and the combination of ammonium
thioglycolate with sodium thioglycolate (G09 and G10) is best suited for type 2 wavy hair
with medium-to-coarse textures with higher resistance and pronounced waves.
Furthermore, the results underscore the importance of polymeric associations in the
preparation of emulsions, which played a decisive role in achieving the ideal viscosity
and controlled release kinetics of the active ingredients. These findings contribute to
the development of targeted, safe, and effective hair straighteners for diverse hair types,
meeting the industry’s demand for products with specialized performance.
Author Contributions: Conceptualization, C.M.J. and C.M.C.; methodology, C.M.J., F.D.S., C.C.L. and
U.S.A.; Validation, C.M.J. and C.C.L.; formal analysis, C.M.J. and F.D.S.; investigation, C.M.J., M.H.V.
and É.S.P.C.; resources, C.M.J.; data curation, C.M.J., F.D.S., C.C.L. and U.S.A.; writing—original
draft preparation, C.M.J. and F.D.S.; writing—review and editing, all authors; visualization, C.M.J.;
supervision, C.M.C. and C.C.L.; project administration and funding acquisition, C.M.J. All authors
have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The raw data supporting the conclusions of this article will be made
available by the authors on request.
Cosmetics 2024,11, 222 16 of 17
Acknowledgments: The authors acknowledge Grandha Professional Hair Care (Brazil) for donat-
ing materials and resources for this research work; the IPclin Group—Integrated Clinical Research
Institute Ltd. (Brazil)–and DEINFAR—Pharmacotechnical development and innovation laboratory—
University of São Paulo (Brazil)–for helping to conduct part of the assays; and the National Coun-
cil for Scientific and Technological Development (CNPq) for the productivity fellowship level 2,
306009/2022-6 (C.M.C.)
Conflicts of Interest: The authors declare no conflicts of interest.
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