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Moisture retention of glycerin solutions with various concentrations: a comparative study

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Various methods of evaluating a humectant’s moisture retention have unique mechanisms. Hence, for designing advanced or efficient ingredients of cosmetic products, a clear understanding of differences among methods is required. The aim of this study was to analyze the moisture-retention capacity of glycerin, a common ingredient in cosmetic products. Specifically, this study applied gravimetric analysis, transepidermal water loss (TEWL) analysis, and differential scanning calorimetry (DSC) to examine the evaporation of glycerin solutions of different concentrations. The results revealed that the moisture-retention capacity of glycerin increased with the glycerin concentration from 0 to 60 wt%, and glycerin at concentration of 60–70 wt% did not exhibit weight change during the evaporation process. When the glycerin concentration exceeded 70 wt%, moisture sorption occurred in the glycerin solution. Furthermore, the results revealed a deviation between the evaporation rates measured using gravimetric analysis and those measured using TEWL analysis. However, normalizing the results of these analyses yielded the relative evaporation rates to water, which were consistent between these two analyses. DSC thermograms further confirmed the consistent results and identified two hydrated water microstructures (nonfreezable water and free water) in the glycerin solutions, which explained why the measured evaporation rate decreased with the glycerin concentration. These findings can be applied to prove the moisture-retention capacity of a humectant in cosmetic products by different measuring methods.
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Moisture retention of glycerin
solutions with various
concentrations: a comparative
study
H. J. Chen1,2,7, P. Y. Lee3,7, C. Y. Chen4, S. L. Huang4,5, B. W. Huang5, F. J. Dai1,2, C. F. Chau1,
C. S. Chen1 & Y. S. Lin4,5,6*
Various methods of evaluating a humectant’s moisture retention have unique mechanisms. Hence, for
designing advanced or ecient ingredients of cosmetic products, a clear understanding of dierences
among methods is required. The aim of this study was to analyze the moisture-retention capacity
of glycerin, a common ingredient in cosmetic products. Specically, this study applied gravimetric
analysis, transepidermal water loss (TEWL) analysis, and dierential scanning calorimetry (DSC)
to examine the evaporation of glycerin solutions of dierent concentrations. The results revealed
that the moisture-retention capacity of glycerin increased with the glycerin concentration from 0
to 60 wt%, and glycerin at concentration of 60–70 wt% did not exhibit weight change during the
evaporation process. When the glycerin concentration exceeded 70 wt%, moisture sorption occurred
in the glycerin solution. Furthermore, the results revealed a deviation between the evaporation
rates measured using gravimetric analysis and those measured using TEWL analysis. However,
normalizing the results of these analyses yielded the relative evaporation rates to water, which
were consistent between these two analyses. DSC thermograms further conrmed the consistent
results and identied two hydrated water microstructures (nonfreezable water and free water) in the
glycerin solutions, which explained why the measured evaporation rate decreased with the glycerin
concentration. These ndings can be applied to prove the moisture-retention capacity of a humectant
in cosmetic products by dierent measuring methods.
e moisture-retention capacity of ingredients is crucial in cosmetics1. An eective moisture-retaining agent
in cosmetic products can be benecial against skin aging2,3. A humectant is a hygroscopic substance that can
maintain skin moisture and hydration3,4. Loss of skin hydration engenders skin dryness, wrinkling, sagging, and
laxity. Accordingly, several studies have sought humectants that exhibit high ecacy in retaining moisture on
the human stratum corneum5.
A humectant’s moisture-retention capacity can be measured through various methods such as gravimetric
analysis, transepidermal water loss (TEWL) analysis, dierential scanning calorimetry (DSC), thermogravimetric
analysis, dilatometry, infrared spectroscopy, and nuclear magnetic resonance spectroscopy–based relaxation
time analysis69. Among these methods, gravimetric analysis can be easily applied to measure the weight change
of an analyte in a material through evaporation within a specic period; a low level of weight loss indicates high
moisture retention. However, because of the detection limit of balance machines used for gravimetric analy-
sis, considerable time is required for accumulating detectable weight changes in order to measure a solution’s
OPEN
1Department of Food Science and Biotechnology, National Chung Hsing University, No. 145, Xingda Rd.,
South Dist., Taichung City 402204, Taiwan, ROC. 2Healthmate Co., Ltd., No. 14, Pinghe 1st St., Changhua
City 500016, Taiwan, ROC. 3Department of Optoelectronics and Materials Technology, National Taiwan Ocean
University, No. 2, Beining Rd., Zhongzheng Dist., Keelung City 202301, Taiwan, ROC. 4Ph.D. Program in
Materials and Chemical Engineering, National United University, No. 2, Lienda Rd., Miaoli City 360302, Taiwan,
ROC. 5Department of Chemical Engineering, National United University, No. 2, Lienda Rd., Miaoli City 360302,
Taiwan, ROC. 6Institute of Food Safety and Health Risk Assessment, National Yang Ming Chiao Tung University,
No. 155, Sec. 2, Linong St. Beitou Dist., Taipei City 112304, Taiwan, ROC. 7
These authors contributed equally:
H. J. Chen and P. Y. Lee. *email: linys@nuu.edu.tw
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evaporation rate, which is an indicator of the solution’s moisture-retention10. erefore, in gravimetric analysis,
obtaining accurate evaporation rates is a time-consuming process11.
In general, TEWL refers to the amount of water vapor that permeates a certain area of membrane per unit of
time and can be measured using a probe. A TEWL probe is an open-chamber system that applies two pairs of
temperature and moisture sensors on a cylinder to determine water loss (in grams per hour per square meter)
through evaporation12. e measuring principle of a TEWL probe is based on Ficks law of diusion, which relates
to the mass transfer rate of water per unit area within a specic period. Compared with water loss measurement
methods that involve weighing an analyte, a TEWL probe can aord a more stable measurement of water loss
in a few minutes13.
DSC is a powerful tool for exploring the microstructure and thermal behavior of a liquid sample14; it can
also be applied for evaluating the moisture retention of a humectant15. According to the freezing temperature
criterion, the microstructure of water in a humectant can be categorized into three types: nonfreezable water,
intermediate water, and free water8,1619, as shown in Fig.1 for three hydrated water types. Nonfreezable water
and intermediate water can easily bind to a humectant through hydrogen bonding and are thus called bound
water. Intermediate water and free water can exhibit phase transitions and are thus called freezable water20. Non-
freezable water tightly binds to the hydrophilic sites of a humectant and has low mobility because of the strong
water–humectant interactions. Specically, nonfreezable water involves very weak free water–water interactions.
Intermediate water is oriented around nonfreezable water and the humectant as a hydration shell, forming cage-
like structures through which the maximum number of hydrogen bonds is achieved in the available space21.
e molecular interactions of intermediate water involve both water–humectant and water–water interactions.
Molecular interactions of free water mainly involve water–water interactions.
Various methods of evaluating a humectant’s moisture retention have unique mechanisms. Hence, for design-
ing advanced or ecient ingredients of cosmetic products, a clearer understanding of the dierences among
such methods is required. Accordingly, this study used glycerin—a common humectant—as a model to examine
moisture retention; specically, the study examined the moisture-retention capacity of glycerin solutions of dif-
ferent concentrations by using three convenient methods, namely; gravimetric analysis, TEWL assessment and
DSC, for comparison.
Materials and methods
Glycerin (First Cosmetics Manufacture Co., Ltd., Taiwan) and deionized water were used in this study. Glycerin
solutions of dierent concentrations (wt%) were prepared by diluting glycerin with various amounts of deion-
ized water; these solutions were then subjected to evaporation experiments. Each evaporation experiment was
conducted by placing 3mL glycerin solution in a vial with an internal diameter of 9mm. ese experiments
were conducted in a closed system at 30°C and 70% relative humidity.
e weight change of the glycerin solutions during evaporation was automatically monitored using a precise
ve-digit electronic balance machine (AS 60/220.R2, Radwag Wagi Elektroniczne, Poland) for 35h. Additionally,
a well-known TEWL probe (Courage + Khazaka Electronic, Köln, Germany) was used to detect the evaporation
rate of the glycerin solutions at the beginning of the evaporation process according to the international guidelines.
A single measurement was collected every 2s until the standard deviation was below 0.1g/hr/m2.
DSC experiments were performed using a dierential scanning calorimeter (Q10, TA Instruments, New
Castle, USA) with a ermo Model FC100AX0TA refrigerated cooling system and ermal Advantage Universal
Analysis soware. A 5-mg sample was weighed and sealed in the aluminum pan of the calorimeter. e sample
pan along with a reference pan was then placed in the DSC instrument, cooled from 40 to − 50°C, and heated up
again to 40°C at a rate of 1°C/min to avoid the response time lag caused by a faster heating rate. e temperature
and enthalpy peak associated with the phase transition during the heating process were analyzed. e enthalpy
in unit of J/g was calculated by integration of enthalpy peak and normalization of water weight in the glycerin
solution6. e experiments were repeated at least three times to ensure the reproducibility of the DSC results.
Results and discussion
Gravimetric analysis. Figure2a illustrates the uctuation of the instantaneous evaporation rate of a water
solution with time; the rate was measured using an electronic balance. e weight of the water solution was
measured automatically every minute during the evaporation process to calculate the instantaneous evaporation
rate. e instantaneous evaporation rate uctuated considerably because of a limited change in the weight of the
water solution during the evaporation process and detection limitation of the balance. Figure2b displays the
accumulative average evaporation rate dened as the overall evaporation rate from start to a certain time, pre-
sented in Fig.2a. As indicated in this gure, the accumulative average evaporation rate also uctuated consider-
Figure1. ree hydrated water types in a humectant.
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ably during the early phases of the evaporation process due to the small weight change; however, the uctuation
decreased gradually with evaporation time because of the relatively large accumulated weight change. A stable
accumulative average evaporation rate may be obtained aer more than 5h. erefore, the gravimetric analysis
was determined to consume considerable time before yielding a stable evaporation rate.
Evaporation rates measured using gravimetric and TEWL analyses. e evaporation rate of a
humectant can be an indicator of the moisture-retention capacity of the humectant. Figure3 presents evapora-
tion rates measured through gravimetric and TEWL analyses for glycerin solutions of dierent concentrations
(wt%). e evaporation rate of 10wt% glycerin measured through TEWL analysis was determined to be consist-
ent with that revealed by an invivo report on 20 healthy volunteers22. e results of the two analyses indicated
that the evaporation rate decreased with the glycerin concentration, demonstrating that a concentrated glycerin
solution has a high moisture-retention capacity. No obvious evaporation rate could be measured when the glyc-
erin concentration was at 60–70wt%. is phenomenon can be attributed to the equilibrium between glycerin
evaporation and moisture sorption. A glycerin molecule has three hydroxyl groups and is hygroscopic. When
the glycerin concentration exceeded 70wt%, a considerable amount of moisture sorption occurred, resulting in
an increase in the weight of the glycerin solution and a negative evaporation rate.
is study revealed a deviation between the evaporation rates measured using gravimetric analysis and those
measured using TEWL analysis. e rates measured using gravimetric analysis were higher than those meas-
ured using TEWL analysis. is deviation can be attributed to the dierent mechanisms of these two analyses.
In gravimetric analysis, the direct evaporation rate of a solution is measured in terms of weight loss (in grams
per hour per square meter) during the evaporation process. By contrast, in TEWL analysis, evaporation rate is
0
10
20
30
40
50
60
70
Instantaneous evaporation rate (g/hr/m2)
Time (hr)
0
10
20
30
40
50
60
70
80
0510 15 20 25 30 35 0510 15 20 25 30
35
Accumulated average evaporation rate (g/hr/m2
)
Time (hr)
(a() b)
Figure2. Gravimetric analysis of water evaporation rate with time: (a) instantaneous evaporation rate and (b)
accumulative average evaporation rate.
-40
-30
-20
-10
0
10
20
30
40
0102030405060708090
100
Gravimetric
TEWL
Glycerin concentration (wt %)
Evaporation rates (g/hr/m²)
Figure3. Evaporation rate of glycerin solutions of various concentrations measured by using gravimetric and
TEWL analyses.
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evaluated as the rate of water vapor diusion through a TEWL probe, as determined through the calculation of
vapor density gradient using Fick’s law of diusion.
To ensure a fair comparison between the analyses, this study normalized their results. e relative evapora-
tion rate to water (RERW) was dened as the ratio of the water evaporation rate of glycerin solution to the water
evaporation rate of pure water. Figure4 displays the RERW measured using gravimetric and TEWL analyses.
e rates derived from the two analyses were consistent, verifying the accuracy of this evaporation experiment.
According to the denition of RERW, moisture sorption started when the RERW was less than 0%, where no
water loss occurred. erefore, as revealed in Fig.4, when the RERW was 0%, the glycerin concentration was
approximately 60–70wt%. Glycerin concentrations that were lower than 60wt% were associated with positive
and less than 100% RERWs, indicating that glycerin at this concentration can achieve moisture retention and
reduced evaporation. However, when the glycerin concentration was higher than 70wt%, the RERW became
negative, demonstrating that glycerin at this concentration can gain water. is nding agrees with the reports
of Fluhr etal.23 and Kiran etal.24 that glycerin is an excellent humectant and hygroscopic agent. Humectancy
or hygroscopicity is the tendency of a substance to absorb moisture from the surrounding atmosphere. Pure
glycerin absorbs its own weight in water over 3 days23.
DSC analysis. DSC analysis was conducted to investigate the microstructure of water in the glycerin solu-
tions. Figure5 displays DSC thermograms of the glycerin solutions of dierent concentrations. e melting
curves varied considerably with the glycerin concentration, with an obvious peak appearing at a glycerin con-
centration of 0wt% and no signal appearing aer a glycerin concentration of 70wt% indicating existence of
nonfreezable water. ese peaks were ascribed to the melting of frozen water including bulk water and free
water18. Dierent types of frozen water have dierent transition temperatures and peak shapes. e transition
-150
-100
-50
0
50
100
150
0102030405060708090
100
Gravimetric
TEWL
Glycerin concentration (wt %)
Relative evaporation rate to water (%)
Figure4. RERWs of glycerin solutions of various concentrations measured by using gravimetric and TEWL
analyses.
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
-50-40 -30-20 -100 10 20
0 % glycerin
10 % glycerin
20 % glycerin
30 % glycerin
40 % glycerin
50 % glycerin
60 % glycerin
70 % glycerin
80 % glycerin
90 % glycerin
100 % glycerin
Temperature ()
Heat flow (mW)
Endo
Figure5. Heating curves of DSC thermograms at a 1°C/min scanning rate for glycerin solutions of various
concentrations.
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temperature of intermediate water is lower than that of free water19. Nevertheless, no melting peak was observed
for intermediate water in this study. is result corresponded to a previous study reporting that poly(2-meth-
oxyethylacrylate) analogous polymers had just two hydrated water types, nonfreezable water and free water18.
Table1 presents a summary of the peaks observed for the glycerin solutions of dierent concentrations. e
melting enthalpy observed for 0wt% glycerin was noted to be consistent with the value obtained for pure water
in a previous study25, indicating that the DSC method and conditions considered in the present study could be
applicable to other study settings. e results also revealed that the peak temperature decreased with the glycerin
concentration and that only 0wt% glycerin was associated with a positive peak temperature. e positive melting
peak indicates that the microstructure type of the water in 0wt% glycerin was bulk water18. However, the melt-
ing peaks associated with 10–60wt% glycerin were lower than 0°C, signifying that the microstructure type of
the water in the material also included free water except bulk water. When the glycerin concentration exceeded
70wt%, no melting peak was observed, revealing that the microstructure of the water was nonfreezable water.
Figure6 illustrates the microstructure type of the water in the glycerin solutions at dierent concentrations.
e melting enthalpy peak decreased with the glycerin concentration, and no melting enthalpy was observed
when the glycerin concentration exceeded 70wt% (Table1). is nding was consistent with the results of the
evaporation experiments conducted using gravimetric analysis and the TEWL probe. e melting enthalpy
resulted from frozen water (bulk water and free water), which can evaporate. e melting enthalpy increases with
the amount of frozen water evaporating. is thus explains why the evaporation rate of the glycerin solutions
decreased with the glycerin concentration. For concentrated glycerin solutions, the microstructure of the water
tended to be nonfreezable water without evaporation.
To more clearly demonstrate the microstructures of water, the DSC thermograms for glycerin solutions with
concentrations of < 10wt% are displayed in Fig.7 for comparison. e curves for 0.1, 1, and 5wt% glycerin were
between those for 0 and 10wt% glycerin. e melting peaks associated with 0.1, 1, and 5wt% glycerin shied
le from 0wt% glycerin toward lower temperature regions; additionally, the melting temperatures ranged from
Table 1. DSC thermogram analysis of 0 to 100wt% glycerin solutions.
Glycerin concentration (wt%) Peak temperature (°C) Enthalpy (J/g)
0 0.64 334.3
10 − 2.28 213.8
20 − 5.48 167.0
30 − 9.80 112.1
40 − 15.77 67.7
50 − 23.50 50.8
60 − 32.45 8.5
70
80
90
100
Figure6. A schematic diagram to illustrate the microstructure type of the water in the glycerin solutions with
various concentrations.
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both above and below 0°C. is phenomenon signies that free water was formed when glycerin molecules were
added to the bulk water solution18. When the glycerin concentration reached 10wt%, the melting peak was in
the negative temperature region because of the large amount of free water. Furthermore, as revealed in Table2,
in addition to the peak temperature, the melting enthalpy decreased with the glycerin concentration.
Conclusions
is study compared three methods used to evaluate the moisture-retention capacity of glycerin solutions of
dierent concentrations. e results indicate that the moisture-retention capacity of glycerin increases with the
glycerin concentration. Although a deviation was observed between the results of gravimetric analysis and TEWL
analysis, normalizing the results of these analyses revealed reasonably high consistency levels between them. In
addition to conrming the consistency between the gravimetric and TEWL analyses results, this study gener-
ated DSC thermograms to further identify two hydrated water forms in the glycerin solutions, which explained
the measured evaporation rates of the glycerin solutions. ese ndings can be applied to prove the moisture-
retention capacity of a humectant in cosmetic products by dierent measuring methods.
Data availability
All data generated or analysed during this study are included in this published article.
Received: 9 April 2022; Accepted: 13 May 2022
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Author contributions
C.Y. Chen and B.W. Huang did data curation. H.J. Chen, P.Y. Lee, S.L. Huang, F.J. Dai, C.F. Chau, C.S. Chen,
and Y.S. Lin performed formal analysis and discussion. Y.S. Lin wrote the main manuscript text. All authors
reviewed the manuscript.
Funding
is research was funded by the Ministry of Science and Technology, Grant Number 110-2622-E-239-003.
Competing interests
e authors declare no competing interests.
Additional information
Correspondence and requests for materials should be addressed to Y.S.L.
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Controlling the biotechnical properties of synthetic hydrogels allows their application in a wide range of biomedical fields. Cross-linker concentration and monomer mole ratio of poly(2-hydroxyethylmethacrylate-co-N-(2-hydroxypropyl) methacrylamide) [poly(HEMA-co-HPMA)]-based hydrogels were used to control the degree of hydration and water distribution within constructs. Cross-linker concentrations corresponding to 0.1, 0.5, 1.0 and 3.0 mol% tetraethylene glycol (TEGDA) with HEMA:HPMA mole ratios of 1:0 and 4:1, and poly(HEMA-co-HPMA) of cross-linker concentration corresponding to 1.0 mol% TEGDA with HEMA:HPMA ratio of 1:1 were investigated for their degree of hydration, water distribution and corresponding physiochemical and mechanical properties. Copolymerization of HEMA and HPMA was confirmed by FTIR. Both cross-linker concentration and chemical composition (HEMA:HPMA) systematically changed the water content and free:bound water distribution in the polymer which resulted in different biochemical and transport properties. The addition of 20% HPMA (poly(HEMA-co-HPMA) (4:1)) increased total hydration (25%) and glass transition temperature (9%) and decreased elastic modulus (31%) and non-freezable bound water (33%) of the hydrogel. Increasing cross-linker concentration resulted in a stiffer hydrogel with less total water but larger non-freezable water content. Evaluation of poly(HEMA-co-HPMA) (1:1) revealed that further increase of HPMA content increased the degree of hydration by 25% and decreased non-freezable water content and elastic modulus by 33% and 16%, respectively compared to poly(HEMA-co-HPMA) (4:1). The hydrogel correspondingly had a higher void fraction and rougher freeze-fractured surface. The diffusion-related processes depended more on water distribution within the hydrogel. The poly(HEMA) showed the fastest swelling kinetics with a concomitant burst release profile of FITC-dextran (a drug surrogate) while the compositions containing HPMA showed a sustained release pattern. The biotechnical properties are illustrative examples of key properties that are influenced by the water distribution rather than the absolute water content of hydrogels.
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Background Skin hydration is essential for maintaining stratum corneum (SC) flexibility and facilitating maturation events. Moisturizers contain multiple ingredients to maintain and improve skin hydration although a complete understanding of hydration mechanisms is lacking. The ability to differentiate the source of the hydration (water from the environment or deeper skin regions) upon application of product will aid in designing more efficacious formulations. Materials and Methods Novel confocal Raman microscopy (CRM) experiments allow us to investigate mechanisms and levels of hydration in the SC. Using deuterium oxide (D2O) as a probe permits the differentiation of endogenous water (H2O) from exogenous D2O. Following topical application of D2O, we first compare in vivo skin depth profiles with those obtained using ex vivo skin. Additional ex vivo experiments are conducted to quantify the kinetics of D2O diffusion in the epidermis by introducing D2O under the dermis. Results Relative D2O depth profiles from in vivo and ex vivo measurements compare well considering procedural and instrumental differences. Additional in vivo experiments where D2O was applied following topical glycerin application increased the longevity of D2O in the SC. Reproducible rates of D2O diffusion as a function of depth have been established for experiments where D2O is introduced under ex vivo skin. Conclusion Unique information regarding hydration mechanisms are obtained from CRM experiments using D2O as a probe. The source and relative rates of hydration can be delineated using ex vivo skin with D2O underneath. One can envision comparing these depth‐dependent rates in the presence and absence of topically applied hydrating agents to obtain mechanistic information.
Chapter
Gylcerol was discovered in 1779 by the Swedish chemist Scheele and is among the most effective humectant polyols such as sorbitol and mannitol. It is a versatile chemical, and moisturization is due to its high degree of hydroxyl groups, which bind and retain water. Glycerol is found in baby care products and in embalming fluids used by morticians, in glues and explosives; in throat lozenges and in suppositories. Glycerol is a colorless, viscous liquid, and stable under most conditions. Glycerin is nontoxic, easily digested, and is environmentally safe. It has a pleasant taste and odor, which makes it an ideal ingredient in food and cosmetic applications.1
Article
Background: Oxidative stress and photodamage resulting from ultraviolet radiation exposure play key roles in skin aging. Fermented Cyclopia intermedia, which is used to brew honeybush tea, exerts antioxidant and anti-wrinkle effects by inhibiting reactive oxygen species production and downregulating matrix metalloproteinase activity. Objectives: This randomized, double-blinded, placebo-controlled study aimed to evaluate the efficacy and safety of fermented honeybush (Cyclopia intermedia) extract (HU-018) for skin rejuvenation. Methods: 120 Korean subjects with crow's feet wrinkles were randomized to receive either low-dose extract (400 mg/day), high-dose extract (800 mg/day), or placebo (negative control, only dextran) for 12 weeks. Wrinkles were evaluated using JANUS® and PRIMO pico®. Skin elasticity, hydration and transepidermal water loss were measured. Results: Global skin wrinkle grade was significantly improved in both low-dose and high-dose groups compared to placebo group, as well as for skin hydration and elasticity. Both the low- and high-dose groups showed significantly decreased TEWL compared to the placebo group. There were no adverse effects during the entire study period. Conclusion: Our data indicate that HU-018 is effective for improving skin wrinkles, elasticity, and hydration. Therefore, daily supplementation with fermented honeybush could be helpful for protecting against skin aging.