Preparation and in vitro evaluation of protective effects of Silibinin‐loaded polymeric micelles on human hair against UV‐B radiation

The purpose of this study was to investigate the protective effect of Silibinin‐loaded polymeric micelles from human hair against UV‐B radiation.


| INTRODUC TI ON
The destructive effects of sunlight on hair have been proven.The extent of these destructive effects varies depending on the wavelength, intensity, and time of the radiation, the color and thickness of hair, and the composition of hair components.This radiation can change the color of hair and cause structural changes in it, including the destruction of proteins and melanin. 1 Sunlight consists of 3 wavelength ranges UV (100-400 nm), VIS (400-740), and IR (>740) that UV includes 3 ranges UV-A (315-400 nm), UV-B (280-315 nm), and UV-C (100-280 nm) 2,3 Structural changes in the hair are mainly due to UV wavelengths, especially UV-B (280-315), 1 which is why our experiments focus on the UV-B range.UV-B can affect all layers of hair, but in undamaged hair, due to its penetration depth of 5 μm, its effects are limited to the cuticle (the outermost layer of hair). 3,4-B radiation on hair breaks the R-S and S-S bonds and causes carbonyl and amine groups and finally destroys the structural proteins of hair, which can be calculated by measuring the amount of protein loss in hair after radiation. 3,5In addition, UV-B causes the formation of reactive oxygen species (ROS) in hair, which also causes chemical damage to hair structure.The presence of natural pigments in hair structure helps prevent the destructive effects of this process on hair structure, and this is a natural protective mechanism in hair structure; however, during this process, these pigments are destroyed. 3,6Chemical filters, silicones, and antioxidants can be used to improve UV-B protection.Antioxidants are compounds that stop the ROS formation process in hair and protect structural hair proteins from ROS oxidation.For these compounds to be able to exert these protective effects, they must penetrate hair structure and eventually interact with melanin. 3,7 this experiment, the protective properties of Silibinin were used to prevent hair damage by UV-B.Silibinin is a flavonoid isolated from the seeds and fruits of the milk thistle plant (Silybum marianum l). 8,9Previous research has shown that the protective effects of Silibinin against UVB-induced chemical damage are due to its antioxidant and DNA protection effects.The only problem with Silibinin is its low aqueous solubility, which limits its penetration into hair due to the lipid structure of the cuticle.Also, because it is a polyphenolic compound with non-polar properties, it simultaneously has the problem of solubilization in water and oil. 10,11If Silibinin is loaded into the matrix of nanocapsules, it can show controlled release, solubility, and better effects.3][14] Silibinin nanoencapsulation provided better spreadability and rheology and higher bioadhesive potential than the formulation containing free Silibinin. 15lymeric micelles are composed of an aggregate of amphiphilic copolymer blocks, which have surfactant properties, a hydrophilic part, and a hydrophobic part in their structure. 16celles are composed of two parts, the shell and the nucleus.In an aqueous solution, the micelle nucleus is formed by placing the hydrophobic parts of the amphiphilic copolymer block away from the solvent molecules in the center of the micelle, and also the shell is formed by placing the hydrophilic parts in contact with the solvent molecules.The nucleus is the storage site for hydrophobic drugs.[19][20] Considering the importance of hair health in beauty and selfconfidence and its effect on maintaining the health of the scalp, in this experiment, while examining the destructive effects of UV-B on human hair, the protective effect of Silibinin-loaded polymeric micelles with pseudoplastic behaviors, in protecting hair from destructive UV-B is evaluated.

| Experimental design and optimization
Several parameters could affect the final polymeric micelle properties and its permeability through hair.As it has already been determined that polyethylene glycol and oleic acid significantly increase the penetration and absorption of a water-insoluble drug into skin. 21Therefore, in this study, a factorial design with three independent variables at two levels were used to design the experiment.Independent variables in the determination of micelle's properties were Silibinin, Pluronic F127, and Labrafil M1944 + labrasol (1:1) concentrations.Eight different formulations with low and high levels of Silibinin percentage (0.01%and 0.02% w/v), Pluronic F127 percentage (0.5% and 0.75% w/v), Labrafil M1944 + Labrasol (1:1) percentage (0.012% and 0.12% w/v), 0.1% w/v Polyethylene glycol 300 and 0.014% w/v Oleic Acid were prepared and their properties such as particle size and drug loading were evaluated.
The Selection done based on a checkpoint analysis and performed with Minitab 16 software to find the level of independent variables that would obtain a minimum value of particle size and the maximum amount of drug loading.The value of particle size and drug loading efficiency are critical parameters that determine Silibinin concentration and penetration into hair as the main site for protection against UV-B.

| Preparation of polymeric micelles
In a ballon, 0.1% w/v Polyethylene glycol 300 and 0.014% w/v Oleic acid are introduced and Silibinin (0.01% and 0.02% w/v) is dissolved in it with the help of a stirrer and then sonicated for 2 min and it is then dissolved in an appropriate amount of chloroform and methanol (in a ratio of 3-1) and placed in a rotary apparatus at 60°C to dry.Then it is kept in a vacuum overnight until all solvent is removed from the environment.The aqueous solution containing Labrafil M1944 + Labrasol in a 1:1 ratio as surfactant-cosurfactant (0.012% and 0.12% w/v) in which Pluronic F127 is dispersed as a polymer (0.5% and 0.75% w/v) (it is considered to open the polymer chains in water with sufficient time) is prepared and added to the fatty phase and stirred well and then placed it in a sonicator bath with a power of 500 w at a temperature of 25°C for 3 min.

Particle size measurement
The particle size of polymeric micelles is determined by a Malvern® particle size analyzer.Three samples are taken from each formulation and each sample is measured three times.

Drug loading measurement
The amount of drug loading in the formulations calculated by the indirect method.In this way, the prepared formulation is placed in a centrifuge for 15 min at 25000 rpm and the aqueous phase is separated and passed through the membrane with a cut-off 3 to 4 KDa and the concentration of the drug in it is measured.In this method, the amount of unloaded drug is calculated and from its difference with the initial amount of the drug, the amount of loaded drug is obtained.

In vitro drug release of selected formulation measurement
A 5 mL of the selected formulation is poured into a dialysis bag with cut-off 4-7 KDa and placed in 10 mL of acceptor phase (phosphate buffer pH = 7 + 1% tween 80) and at 37°C temperature and 200 rpm, sampling is done at 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 24, and 48 h and the amount of Silibinin released is determined with UV spectrophotometry.Each time, 0.5 mL of the sample is taken from the acceptor phase and 0.5 mL of the new solution is replaced.The amount of released drug is measured by the above method and the release kinetics is studied.The blank formulation was used as a control.

Zeta potential of the selected formulation measurement
The zeta sizer device used to measure the zeta potential.For this purpose, the selected formulation's sample diluted 50 times with distilled water and then the sample transferred into the capillary tube with a syringe and the capillary tube placed in its special place in the device.This measurement is performed at pH = 7.4, temperature 25°C, and power 149 w. 22

| UV-B protective experiments
Samples Dark brown hair collected from one young male volunteer with no history of chemical treatments.The hair sample washed for 1 min with 1 mL of 0.2% w/w aqueous solution of sodium lauryl sulfate and then washed with water for 30 s.This process is repeated twice.
The hair then dried at room temperature and stored in a plastic bag until use.

Radiation source
The UV-B radiation was produced by F6T5.BLACK LIGHT 6 w lamp (HITACHI®, Japan) equipped with a UV-B meter (UV-340A, Lutron Electronic®, Taiwan) to adjust the required dose of UV-B radiation (1.6 J cm −2 ).UV-B meter showed that a distance of 15 cm between the UV-B lamp and hairs provided the required dose of UV-B.

Samples treating and classification
For this step, the selected polymeric micelle formulation containing Silibinin with a concentration of 0.015%, the selected formulation without Silibinin, solution of Silibinin with a concentration of 0.015% in solvent (phosphate buffer PH = 7 + 1% tween 80), solvent without Silibinin (Phosphate buffer pH = 7 + 1% tween 80) and Eucerin® sunscreen with SPF = 50 is used.
A hair sample will only be exposed to light without receiving any of the above.Therefore, hair is classified into six groups and according to the mentioned methods, the characteristics of hair before and after receiving UV-B light in each of the six groups will be examined.
The groups are: Group 1: hair without any treatment (the negative control group).
Group 4: treated hairs with the selected formulation without Silibinin.

UV-B radiation protocol
The UV-B lamp is placed at a distance of 15 cm from the location of the hair samples and the six groups mentioned above, are placed separately in the appropriate place.The radiation pattern is 10 h of radiation and 14 h of darkness.In this way, after 10 h of radiation, the hair is placed in a closed container.Radiation is done in periods of 20, 30, 40, 50, and 60 days.
For each of the above groups, 0.15 g of hair is washed and dried according to the mentioned method, and before contact with UV-B light, it is first placed in the desired solution or formulation for 5 min, then rinsed and dried for 2 min.Then it will be exposed to light at a distance of 15 cm from the light source.To better guide the light, the sample chamber under the UV-B lamp is covered with aluminum foil.

Evaluation of peak-to-valley roughness and RMS roughness of hair surface with AFM imaging technique
For this purpose, images of hair in the noncontact mode will be prepared with a scanning probe microscope.Images will be taken at room temperature before and after light exposure.Then, by image processing software, roughness factors such as peak-to-valley roughness and RMS roughness (according to the following equation) are obtained. 1ere z ave is the average of the z value within a specific area, z n is the z value for a given point within this area and N is the number of measured points.

Evaluation of the chemical changes of samples with FTIR
For this purpose, five hairs are examined separately with a resolution of 4 cm −1 before and after contact with UV-B light, and the relevant spectra are prepared.Based on the location and intensity of changes in the relevant spectra, chemical changes in hair caused by UV-B radiation are evaluated. 1

Amount of hair protein loss evaluation
To evaluate the effect of light on hair fiber proteins, pour 0.15 g of hair into a 10 mL Erlenmeyer flask, add 2.5 mL of distilled water then place in a sonicator bath at room temperature for 30 min. 4Then 0.5 mL of the aqueous phase is taken and the amount of protein is measured by UV-Vis spectrophotometer by Bradford protein assay method.Bovine serum albumin (BSA) is used to prepare the standard solution.

| Silibinin assay method
The determination of the amount of Silibinin was carried out by UV spectrophotometry.This determination is performed by a spectrophotometer (CE250, CECIL®, England) at 293 nm.The limit of quantification (LOQ), accuracy, repeatability, and linearity were evaluated as method validation.

| Data analysis and statistics
The experiments were repeated three times and data were expressed as the mean values.Statistical data were analyzed using the one-way analysis of variance (ANOVA) and p < 0.05 was considered to be significant with a 95% confidence interval.To figure out the relationship between the dependent and independent variables, the simultaneous multi-regression test was used.

| Validity of the drug measurement method
Regression analysis showed that in the concentration-absorption curve, according to the correlation coefficient (R 2 = 0.991) and p = 0.001, there is a linear relationship between concentration and UV absorption for concentrations of 0.025-0.2mg/mL.the lack of fit was 0.183 mg/mL which showed that this linear relationship is true for all relevant data and it can be used to determine the unknown concentration based on UV absorption.The LOQ was 0.001 mg/mL, so all concentrations reported in this study are higher than this value.
The relative standard deviation in within and between days studies was less than 3%, indicating that this measurement is repeatable.Also, the difference between the values obtained and the actual values in the within and between days studies was 3.7% and 3.1%, respectively, which shows that the error of this method is low.

| The particle size of polymeric micelles
The particle size results of polymeric micelle formulations are presented in Table 1 The prepared formulations have a particle size range between 25 and 120 nm, and the lowest and highest particle sizes belonging to formulations 5 and 1, respectively.Therefore, polymeric micelle formulations usually have a particle size below 120, which is very suitable for penetration into hair tissue and increases the chance  of drug penetration into hair fiber by creating a large surface area.
Data regression analysis showed that there is a significant relationship between polymer concentration (p = 0.020) and not significant correlation with surfactant concentration (p = 0.093) with particle size.
Particle size increased with increasing in polymer concentration.
Investigation of the effect of interference between variables and particle size showed that there is no significant relationship between the two.Also, considering that this regression analysis (p = 0.06), showed that another factor other than the variables considered in this study does not affect the particle size.Therefore, by adjusting the concentration of surfactant and polymer, the appropriate particle size can be achieved.

| The loading efficiency of polymeric micelles
The drug loading efficiency of the polymeric micelle formulations is presented in Table 1.According to the results, the highest and lowest loading efficiency are related to formulations 3 and 7. Regression analysis showed that there was a significant relationship between polymer concentration (p = 0.048), and Silibinin concentration (p = 0.038) and non-significant surfactant concentration (p = 0.095) with loading efficiency.The loading increases with increasing polymer and Silibinin concentrations.Also, regression analysis showed that there is no significant relationship between the interaction between variables and loading efficiency; and another factor other than the variables considered in this study does not affect the amount of loading (p = 0.36).So by adjusting the amount of these three variables, the desired loading efficiency can be achieved.

| Choose the selected formulation and review its features
The selected formulation includes low concentration of surfactant (0.12 mg/mL) and medium concentrations of polymer (0.625 mg/mL) and high concentration of Silibinin.The effects of polymer concentration on particle size and loading efficiency are opposite so the medium concentration (between high and low level of polymer concentration) was chosen for selected formulation.

| Particle size and loading efficiency of the selected formulation
The loading efficiency and particle size of the selected formulation were 45.34 ± 1.31 percent and 43.19 ± 4.29 nm.Due to the increase in the concentration of surfactant, in addition to increasing the particle size, it also reduces the loading efficiency, so a low level of its concentration was chosen for the selected formulation.Also, considering that increasing the polymer concentration increases both the particle size and the loading efficiency, then the average level was chosen for the selected formulation.

| In vitro release of Silibinin from selected formulation
The Silibinin release profile of the selected polymeric micelles formulation is presented in Figure 1.Based on the results of the Silibinin release profile from the selected formulation, it has two parts: rapid release and sustained release.Accordingly, about 40% of the drug is released in the first 10 h and about 70% during the first 48 h.This sustained release is a positive factor in creating a proper drug depot on hair.The correlation coefficient of the results was determined for all kinetic models and based on this, the highest correlation coefficient was for the PEPAS model (r = 0.996).In this model, the released drug component compared to the total drug is related to the nth power of time, and the n estimation showed that the release mechanism follows the Fick's laws of diffusion.

F I G U R E 1
Silibinin release profile of selected polymeric micelles formulation.

| Zeta potential of the selected formulation
This feature of the formulation is very effective in its interaction with hair.The zeta potential of the selected formulation was −5.9 ± 2.22 mv (Figure 2), indicating a small electrical charge on the particles diffusion layer.This low electrical charge is a useful feature because it does not cause much electrostatic interference between the formulation particles and hair.shows that UV-B light has not been able to cause damage in these two groups.The difference in the amount of this factor in Groups 5 and 6 with the negative control group, especially from 300 h onwards, shows that these two groups had a good protective effect.
Comparison of the parameter of the peak-to-valley roughness in the two Groups 5 and 6 from 300 h onwards shows a significant difference (p = 0.022) between these two groups, which shows that the selected formulation had a better protective effect than the positive control formulation.It can be concluded that since the placebo formulation had no protective effect, the protective effect of the selected formulation is due to the presence of Silibinin.

F I G U R E 2
The zeta potential distribution of the selected formulation.

F I G U R E 3
Peak-to-valley roughness changes in six groups according to radiation time.
F I G U R E 4 AFM results of (A) untreated and unirradiated hair; (B) negative control group; (C) Group receiving the selected formulation; and (D) positive control group (the duration of radiation is specified in the images).

| RMS roughness
The results of changes in RMS roughness, depending on the UV light radiation time, in the six groups of hairs in Figure 5 are presented.According to the results, in the negative control group, RMS roughness increased with increasing radiation time, which indicates the destructive effect of UV on hair.RMS roughness after 600 h of UV radiation in Groups 2, 3, and 4 was not significantly different from the negative control group, so the treatment performed in these groups was not effective to prevent an increase in RMS roughness.However, in Groups 5 and 6, the amount of this factor after 600 h of radiation was significantly lower than the negative control group and the amount of this factor did not differ significantly between 0 and 600, so it can be concluded that the selected formulation contains Silibinin and the positive control group was able to prevent the increase of RMS roughness and hair destruction.
Since Silibinin alone and placebo formulation have not been able to provide adequate protection, it can be concluded that the selected formulation has been able to provide adequate protection by creating the appropriate concentration of Silibinin in hair layers.
Since RMS roughness after 600 h in the two Groups 5 and 6 did not differ significantly (p = 0.37), these two groups had an equal protective effect on the RMS roughness.The FTIR spectrum prepared from the negative control group on Day 0 (untreated and unirradiated hair) had a similar result to the spectrum mentioned in the study by Richena et al. (Figure 6).

| Samples FTIR
According to this study, the rate of chemical changes in hair is judged by the intensity of the two peaks, 1041 cm −1 (indicating the rate of cysteine oxidation) and 2926 cm −1 (indicating the rate of lipid degradation).In our study, the peak of cystine oxidation in the range of 1078-1081 cm −1 has been shown.
The intensity of this peak was significantly increased in the negative control groups and the group receiving pure solvent after 20 and 60 days of radiation.While the intensity of this peak in the groups receiving the drug in the solvent, the selected formulation, and the positive control increased less after 20 and 60 days, and in the group receiving the blank formulation, this increase was less than the negative control group.In the comparison between the group receiving the selected formulation and the positive control group, the results show the superiority of the selected formulation over the Eucerin® formulation because in this group the peak has increased less.
But the intensity of the peak at 2926 cm −1 in all six groups on Days 20 and 60, there is no obvious difference with the negative control group on Day 0, which indicates that UV radiation did not change the structure of hair lipids.In general, it can be said that the selected formulation has a good protective effect on the S=O group in the disulfide bond and prevents its oxidation.intensities.This increase was not significantly different in Groups 2, 3, and 4 compared to the negative control group, while in Groups 5 and 6 this difference was significant.In other words, the two positive control groups and the selected formulation containing Silibinin were able to effectively reduce hair protein loss.

| DISCUSS ION
For better drug delivery to hair, a polymeric micelle carrier was used to dissolve Silibinin, which is a hydrophobic molecule, in an aqueous base.Also, by using polymer, the viscosity of the product can be adjusted; it has better spreading on the surface of hair and with the bioadhesive properties of the polymer, the durability of the product on the surface of hair can be increased.
To determine the selected formulation, particle size, and drug loading of eight formulations that were prepared with different concentrations of Silibinin, surfactant, and polymer at two levels; were measured.The results showed that particle size is directly affected by surfactant and polymer concentration.Also, the percentage of loading increases with the increase of Silibinin and polymer concentration and the decrease of surfactant concentration.Considering that smaller particle size and higher loading efficiency are more suitable for the selected formulation, the selected formulation was To determine the effectiveness of the selected formulation, the results of four factors including peak-to-valley roughness, RMS factor, hair chemical changes (measured by FTIR), and the amount of hair protein loss in six sample groups were compared.
Previously, it has been proven in a study that with UV radiation with an intensity of 1.6 J cm −2 on human hair, after 500 h, a significant increase in peak-to-valley roughness and RMS was created. 1 The results of the negative control group showed that hair damage increased over time and the peak-to-valley roughness increased significantly, especially after 300 h of radiation.While both the selected formulation and the positive control formulation have prevented the increase of peak-to-valley roughness and its graph has not changed significantly even after 600 h of UV radiation compared to zero time.
The protective effect of the selected formulation was significantly higher than the positive control group from 300 h of radiation onwards.
It was also found that the placebo formulation had no protective effect.Therefore, the observed protective effect is related to Silibinin; and the polymeric micelle as a carrier has increased the durability of Silibinin on hair.These results for the RMS factor are exactly the same as the peak-to-valley roughness results; with the difference that the protective effect of the selected formulation was not significantly different in comparison with the positive control group.
The results of the FTIR spectrum of hair showed that UV-B radiation destroyed hair structure, especially structures such as S=O group, amide III, C-H bond of protein and lipid parts, and amide I.All the structural changes were created especially from Day 20 to Day 60 of UV-B radiation.UV-B has been able to induce the oxidation of the amino acid cystine, which is effective in changing hair color.In this case, both the selected formulation and the positive control formulation have been able to prevent these structural changes from occurring, and the superiority of the selected formulation over the positive control formulation was significant.Also, the placebo formulation has protective effects and can prevent the oxidation of cystine to some extent.Therefore, the carrier and Silibinin have strengthened each other's effect in protecting hair.
Before this, the results of a study on the FTIR spectrum of hair showed that with 500 h of UV radiation on hair, the oxidation of cystine has increased, which leads to chemical changes in hair protein structure.But the lipid structures of hair did not change significantly 1 ; which was consistent with the results obtained in our study.
The results of several studies state that UV radiation on hair with a destructive effect on proteins increases their solubility with increasing radiation time. 5,6Also, the results of another study proved that the effect of the UV-B spectrum is more significant than other sunlight spectrums in increasing the solubility of proteins. 4 In our study, in the protein loss test, it was found that in all groups, with the passage of radiation time, the amount of protein loss increased significantly, even in the selected formulation and positive control group; but with a lower slope.So these two formulations have shown an acceptable protective effect to reduce the amount of hair protein loss caused by UV-B radiation.

| CON CLUS ION
Based on the obtained results, Silibinin can be introduced with the dose proposed in this study as a molecule that protects hair against UV-B radiation, as well as the polymeric micelle formulation, both by increasing the durability of Silibinin on hair and by Having protective effects can be introduced as a suitable carrier for Silibinin in the design of hair care formulations.

ACK N OWLED G M ENTS
This study was extracted from the Pharm.D thesis (Akbari H) and was sponsored by Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran.

DATA AVA I L A B I L I T Y S TAT E M E N T
Data sharing not applicable to this article as no datasets were generated or analysed during the current study.

E TH I C S S TATEM ENT
Authors declare ethics approval was not required for this study.

1
The composition and properties of prepared polymeric micelles (mean ± SD, n = 3).

3. 5 |
Results of the effect of UV-B light on the hair3.5.1 | Peak-to-valley roughnessThe results of peak-to-valley roughness changes and RMS roughness of hair surface with AFM imaging technique in the six groups of hairs are presented in Figures3 and 4, respectively.Peak-tovalley roughness is one of the factors that show the rate of hair damage.According to the figure, over time, the amount of this factor has increased in all groups except Groups 5 and 6, which

FTIR technique was used
to investigate the chemical changes caused by UV-B irritation on hair.According to a study by Richena et al. on the FTIR spectrum of healthy human hair, the peak at 1041 cm −1 due to oxidant products (S=O) related to di-sulfides bond in keratin structure, the peak at 1076 cm −1 due to C-C structure vibration, and the peak at 1237 cm −1 related to amide III.Also, the peaks at 1395 cm −1 and 1451 cm −1 are due to C-H band vibration in CH2 and CH3 groups in lipid and protein structures.The Peaks at 2853 cm −1 and 2926 cm −1 correspond to the C_H symmetric and asymmetric vibration of CH2 groups and the peaks at 2877 and 2958 cm −1 due to the same vibrations of CH3 groups.The two obvious peaks of 1518 and 1633 cm −1belong to the N-H and amide I groups and the peak at 3287 cm −1 belongs to the O-H structure.1

3. 5 . 4 | 7 .
The amount of hair protein loss The BSA standard curve was used to calculate the amount of hair protein loss.The results of the rate of protein loss, depending on the time of UV light radiation, in the six groups of hair are presented in Figure The results showed that in the negative control group, the amount of protein loss increased significantly with increasing radiation time.This trend was observed in all groups with different F I G U R E 5 RMS roughness changes in six groups according to radiation time.

6
The FTIR spectrum of (A) untreated and unirradiated hair on Day 0; (B): untreated and irradiated hair (the negative control group) after 60 days of radiation; (C) Group receiving the selected formulation after 60 days of radiation; and (D) positive control group after 60 days of radiation.preparedwith a low level of surfactant concentration and a medium level of polymer and Silibinin concentration.The loading efficiency of the selected formulation was 45.34 ± 1.31% and also d90 = 43.19± 4.29 nm; which indicates the appropriate loading and particle size of the selected formulation.The measurement of the drug release from the selected formulation showed that the release of Silibinin has two parts: rapid release and continuous release, which creates a sufficient concentration of the drug in hair in a short period of time and the persistence of this concentration over time.As compared to formulations containing free Silibinin, Silibinin nanoencapsulation causes better penetration of Silibinin into layers of skin and maintains its concentration in the skin over time15 ; this confirms that the release pattern of Silibinin from the selected formulation has a sustained release phase.The zeta potential of the formulation shows minimal electrostatic interference with hair and the formulation and good stability of the micelles.It means that the formulation can cover the hair surface well and the micelle structure is protected from destruction caused by electrostatic interactions.