Guo Ruoxi,Wang Peng,Zhang Hongyan,Zhang Lei

Tianjin Shangmei Cosmetics Co.,Ltd.,China

Abstract This paper briefly introduces the human body evaluation test method of cosmetics moisturizing efficacy test,according to stratum corneum moisture content test,skin moisturizing capacity test and other test methods and the instruments used are briefly introduced,so as to provide ideas for scientific and rigorous experimental scheme design and scientific basis for the claim of moisturizing effect.

Key words moisturizing;efficacy evaluation;human body test

The apparent age of the skin is closely related to its moisture content,adequate moisture is one of the important factors for maintaining the health and good visual effect of skin.Therefore,the moisturizing effect is the most basic requirement of using cosmetics.The human body evaluation test of cosmetics moisturizing efficacy can be divided into two types:subjective evaluation and objective instrument evaluation.Among them,subjective evaluation types include expert evaluation and subject self-evaluation,namely,2 categories,analytical sensory evaluation and preference sensory evaluation in cosmetic sensory evaluation.Expert evaluation is a qualitative or graded assessment of the skin status indicators of volunteers;The forms of volunteers’ self-assessment include interviews,questionnaires,consumer diaries,etc.,so that the subjects can rated their own skin moisturizing degree and other aspects.However,due to the strong interference of subjective factors in subjective evaluation,it is impossible to evaluate and analyze the moisturizing effect of cosmetics comprehensively,accurately and objectively.This paper mainly introduces the objective instrument evaluation method in the human body evaluation test method of cosmetics moisturizing efficacy.

1 Stratum corneum moisture content test

It is very important that the stratum corneum contains enough moisture,which can keep the surface of skin smooth and flexible,certain humidity and maintain the complete protection function of epidermal barrier,an appropriate moisture content of the stratum corneum is the primary condition for maintaining the basic structure and functional activities of the skin.There are two methods for measuring the moisture content of the stratum corneum:direct method of measurement and indirect method of measurement.

1.1 Direct method of measurement

The main function of moisturizing cosmetics is to supplement or help with maintain the moisture content of the application site.The moisturizing effect of cosmetics can be verified by direct detection of the moisture content in stratum corneum.For example:Skin Composition Analyzer gen2-SCA(RIVERD INTERNATIONAL B.V.,Netherlands)is an instrument for testing the composition of substances in the skin based on the principle of Roman Spectroscopy,it is a non-invasive,fast and high spatial resolution detection method for human skin.It can test the moisture content distribution in the depth direction of skin,the content distribution of natural moisturizing factors and the absorption distribution of drugs or cosmetics smeared on the surface of skin along the depth direction of skin.

In addition,nuclear magnetic resonance spectroscopy(NMR),attenuated total reflectionfourier transform infrared spectrometry(ATR-FTIR),near-infrared spectroscopy,NIR,and other methods can detect water molecules in the skin directly.These methods are highly accurate and reliable,but these kinds of instruments has not been widely used because of its high overall requirements,complex operation and high price.[1]Besides,some instruments can also measure skin moisture content directly,such as:skin acoustic wave propagation time test probe Reviscometer RVM600,the test principle is based on the measurement of resonant acoustic wave propagation time.Due to the difference of skin surface moisture and elasticity,the speed and time of sound shock wave propagation are also different.Therefore,skin moisture content can be estimated by the propagation time of sound wave.The propagation time will be shorter in the skin with better elasticity,sufficient moisture and less wrinkles;conversely,the propagation time will be longer.

1.2 Indirect method of measurement

Compared with the above direct method of measurement,it is more widely used in the industry by using these three electrical parameters:capacitance,resistance and conductance to measure the moisture content of the skin stratum corneum indirectly.

The capacitance method can quantify the moisture content in the stratum corneum of skin,which has many advantages such as sensitivity,accuracy,good reproducibility,simple operation and low cost.The capacitance method is more sensitive especially for measuring the low hydration degree,and is one of the commonly used methods for the evaluation of moisturizing efficacy.

Common instruments for the capacitance method are German CK skin moisture content test probes Corneometer CM 820 and 825;Finland Delfin skin stratum corneumhydration meter MoistureMeterSC,tissue water content and edema meter MoistureMeterD.

MoistureMeterD can measure the moisture content of the dermis and subcutaneous tissue,the effective measuring depth of probe one is 500 μm,and the effective measuring depth of probe two can reach 1,500 μm.Among them,the Corneometer is common in the literature,[2]and it is also a common method for testing.The test principle is that the water molecule is more polar and has a higher dielectric constant(?≈81),which is significantly different from the dielectric constant of other substances(ε<7 ),the measurement principle can be expressed by the following formula:

In this formula,F is the current frequency;R is the resistance;C is the capacitance;Z is the total resistance of the skin.One of the main parts of the instrument is a gold-plated probe(electrode),the surface of the probe is covered with a layer of insulating vitrified substance,and the whole probe works under a constant pressure of 0.16 N/m2.The stratum corneum of dry skin is an insulating medium.when the stratum corneum is hydrated,the capacitance value of the instrument probe will change after contact with the skin.The higher the capacitance is,the more moisture content of the stratum corneum in skin will be.

Relatively speaking,the test results of the capacitance method are more reliable,because there is no unnatural contact between the skin and the probe during the test,and there is almost little or no electric current passing through,which is less affected by the polarization effect and ionic conductivity.During the test,there is no inertial effect in the process of establishing moisture balance,and it can be measured quickly and accurately,which also eliminates the influence of living skin on the accuracy of measurement results.

The measurement of skin moisture content by the resistance method is based on the hydration degree,tissue composition and state of stratum corneum the principle is based on the capacitance value of impedance,impedance value is obtained by applying the cumulative measurement of alternating current at different frequencies.The representative instruments applying this principle include SCIM(surface characterizing impedance monitor )[3]and Nova DPM 9003.[4]

The principle of the conductance method for measuring skin moisture content is that highfrequency conductance is related to the moisture content of skin stratum corneum,and the size of conductance is positively correlated to the moisture content of skin stratum corneum.The conductivity of dry stratum corneum is weak,when the moisture content of the stratum corneum increases,the dielectric constant will increase and then the skin electrical conductivity will increase.The correlation coefficient between the moisture content of stratum corneum and its electrical conductivity reaches 0.99,[5]and the conductivity of skin has a certain correlation with the moisture content in the superficial and deep stratum corneum.

Skicon-200 is a common instrument for testing the moisture content of skin stratum corneum based on the change of electrical characteristics when hydration occurs in the skin stratum corneum,it can also measure the high frequency conductance of skin accurately and reliably and then quantify the moisture content of skin.Compared with the Corneometer,Skicon has a slightly poor reproducibility,but it is more suitable for measuring the increase of skin moisture content.[5]

2 Skin moisture retention test

2.1 Trans Epidermal Water Loss test

Another function of moisturizing cosmetics is to help reduce the moisture loss of the application site.Although Trans Epidermal Water Loss(TEWL)(unit:g/h·m2)cannot reflect the moisture content of the skin stratum corneum directly,it is an important parameter to characterize the barrier function of the skin stratum corneum and can represent the strength of the skin moisture retention ability,so it is also one of the important parameters for the efficacy evaluation of moisturizing cosmetics.The instrument commonly used in the literature is Tewameter,[3]and its TEWL test principle is comes from Fick's diffusion law:

In this formula,m is the diffusion of water,g;t is the diffusion time,h;D is the diffusion constant of water vapor in air,0.087 7g/(m·h·mmHg);A is the diffusion area,m2;p is the steam pressure,mmHg;x is the distance between the measurement points,m.

The instrument uses a specially designed cylindrical cavity measuring probe with open ends to form a relatively stable test environment on the skin surface.The water vapor pressure gradient on the skin surface is measured by two groups of temperature and humidity sensors,and then the water loss through the epidermis is obtained.The lower the value of trans epidermal water loss is,the less the transcutaneous water loss is,and the better the barrier function of stratum corneum is.

The Vapometer(Delfin,Finland)is also used to measure trans epidermal water loss.The value of TEWL can also be applied to skin patches,contact dermatitis,occupational skin diseases,physical therapy,systematic observation of newborns,monitoring of burn and new tissue.The stratum corneum moisture content test combined with the trans epidermal water loss test can comprehensively evaluate the moisturizing effect of cosmetics.

2.2 Water load test of stratum corneum

The water load test of stratum corneum is also called the Sorption-desorption test,[6-8]which can evaluate the ability of skin to maintain and absorb water,and is a simple and quick method to detect the hydration kinetics of the stratum corneum in vivo.

In normal environment,the moisture content of human skin is MMV0,one drop of distilled water or normal saline was dropped on the test skin surface,after a certain period of time(10 s),moisture on the surface of the skin was wiped off to form an environment with a relative humidity of 100%.At this time,the moisture content of the skin is MMVmax.The difference between MMVmaxand MMV0is called water absorbing capacity,the larger the difference is,the stronger the ability of the skin to absorb water will be in this area.Continue to measure the skin moisture content at different times,and get the change law that MMV gradually decreases with the extension of time,that is,the moisture release curve was MMVt=MMVmax·e-λt.

In the above formula,λ is the water release constant,which reflects the capacity to maintain moisture .The smaller the λ value is,the slower the water release rate is,and the better the water holding capacity of the skin.The water load test of stratum corneum is a scientific and effective method to study the skin dynamic hydration process and the water holding capacity in stratum corneum.

3 Other test methods

To a certain extent,the moisture content in the stratum corneum of skin can be measured by sampling and analyzing the stratum corneum of the skin,detecting the adhesion of the stratum corneum,serine protease activity,NMF(Natural Moisturizing Factor)content,etc.The drying degree of the skin can also be evaluated by the shedding index.The cells and scales that are relaxed on the surface of the stratum corneum are adhered by transparent tape,a certain area is selected as the measurement boundary to calculate the proportion of the thickness and area of the shedding cells.The shedding index is calculated according to the following formula :

In the above formula,D.I.is the shedding index;A is the proportion of keratinocyte area;Tn is the proportion of keratinocyte thickness;n is the thickness.[9]The operation of this method is relatively complex,but the evaluation effect is good,and it is an effective method to evaluate the moisturizing of cosmetics.

Because the skin itself cannot produce enough moisturizing ingredients,if cosmetics cannot increase the moisture content of the skin effectively,it will lead to rough and dry skin and changes in smoothness,softness,texture,elasticity and other indicators.Therefore,through the test of these indicators,the moisture content of the skin can be characterized from the side,and then the moisturizing effect of cosmetics can be verified.

For example:the skin moisture distribution tester MoistureMap MM100,the image sensor chip composed of capacitors is pressed on the skin surface with a certain pressure,and the skin surface moisture distribution map and the microstructure map of the tissue can be obtained.The gray value in the gray distribution histogram of all the lattices in the tested image can represent the moisture content and distribution.

The skin friction test probe Frictiometer FR700 can measure the dynamic friction coefficient of the skin.[10]There is a micro-motor and a gear transmission inside the test probe,and a movable Teflon material cylindrical friction head is connected to the tip of the probe.When the friction head rotates,the rotation speed of the cylinder will change due to the friction force on the skin surface.The smoother the skin of the tested part is,the smaller the torque required when the motor rotates,resulting in a smaller value of skin friction force.For dry and wrinkled skin,the friction value is relatively high.The profilometer(Hommel-Werke,Villingen-Schwenningen,Germany)can measure that the roughness of the skin is positively proportional to the degree of dryness.[11]

4 Conclusion

The moisturizing efficacy of cosmetics is an important prerequisite for the realization of other effects,and its status as a mainstream claim,it also puts forward higher requirements for its efficacy evaluation methods.[12]

Human moisturizing efficacy test of cosmetics is the process of selecting a certain number of qualified volunteers,designing the test plan scientifically and rigorously,selecting appropriate testing equipment,in a test site that meets the requirements,[13]makeing an objective evaluation conclusion on the efficacy of the product through the subjective evaluation,objective measurement and statistical analysis of human test results.