In vitro detection of moisture content in the stratum corneum of cosmetics

In vitro detection of moisture content in the stratum corneum of cosmetics

Detection principle and technical mechanism

The detection of moisture content in the stratum corneum of cosmetics outside the body is the core means of evaluating the moisturizing effect of skincare products, and its technical principle is based on the correlation between skin electrical characteristics and moisture content. As the outermost barrier of the skin, the moisture content of the stratum corneum directly affects the softness, elasticity, and permeability of the skin. When the moisture content of the stratum corneum increases, its capacitance and conductivity will significantly increase, which becomes the basis for quantitative detection. At present, the mainstream detection methods are divided into two categories: capacitance method, which reflects the moisture content (unit: a.u.) by measuring the change in capacitance value of the skin to high-frequency AC current, represented by instruments such as Corneometer CM825; The conductivity law evaluates moisture content by measuring the skin's ability to conduct direct current, with typical equipment including Skicon-200EX. Both methods have the advantages of non invasiveness and fast response (single detection time<2 seconds), among which the capacitance method is widely used due to its small influence by skin temperature (error<5%).

Core testing technology and standardized process

Professional testing institutions use Corneometer CM825 as the benchmark testing equipment, with core technical parameters including: measurement frequency of 3.5MHz, detection depth of 50-100 μ m (accurate coverage of the stratum corneum), resolution of 0.1 a.u., and repeatability error<3%. The standardized testing process must strictly follow ASTM E1970-17 "Guidelines for Measuring Skin Moisture Content", and the key steps are as follows:

Sample pretreatment: Apply the cosmetic sample evenly onto the surface of a 3D recombinant skin model (such as MatTek EpiDarm), with a dosage controlled at 2 μ L/cm ², and incubate for 2 hours at 37 ℃ and 50% RH environment.

Environmental control: Before testing, the skin model needs to be transferred to a constant temperature and humidity chamber (25 ± 1 ℃, 40 ± 5% RH) and equilibrated for 30 minutes to eliminate the interference of temperature fluctuations on the measurement results (a temperature change of 1 ℃ can cause a reading deviation of ± 2%).

Multi point measurement: Select 5 uniformly distributed detection points on the surface of the model, conduct 3 consecutive measurements at each point, and take the average as the final result. To avoid electrode compression, the contact pressure between the probe and the skin should be controlled at 150 ± 10g.

Data validity assessment: When the coefficient of variation (CV) of 5 detection points is less than 8%, the data is considered valid. If CV>12%, resampling and testing are required.

Standard basis and methodological validation

The detection of moisture content in the stratum corneum needs to meet multiple standards. In China, it mainly follows GB/T 35828-2018 "Guidelines for the Evaluation of Moisturizing Efficacy of Cosmetics", while internationally it is based on ISO 15716:2003 "Cosmetics - Evaluation of Moisturizing Efficacy of Skin". Methodological validation requires the following key indicators:

Precision: intra day precision (n=10) RSD<8%, inter day precision (n=5) RSD<12%

Accuracy: deviation from standard hydrogel sample (80% water content) shall be<± 5%

Detection limit: The maximum detectable moisture content is 5% (corresponding to a capacitance value of 25 a.u.)

Linear range: Linear correlation coefficient r ²>0.99 within the range of 5% -95% moisture content

The typical test data of a moisturizing face cream showed that the water content of the blank control group was 32.6 ± 2.1 a.u., and that of the sample group reached 58.3 ± 3.5 a.u. 2 hours later, with the moisturizing rate increased by 78.8%. The paired t-test showed that the difference between the groups was statistically significant (p<0.01).

Technical advantages of professional testing services

As a CNAS accredited laboratory (certificate number CNAS L22006), Zhongke Testing has established a comprehensive technical support system in the field of stratum corneum moisture content detection

Environmental control: Equipped with a walk-in constant temperature and humidity chamber (temperature control accuracy ± 0.5 ℃, humidity ± 2% RH), it can simulate the moisturizing effect under different climatic conditions (such as the tropical environment mode of 30 ℃/60% RH).

Quality traceability: Equipment calibration is carried out using NIST traceable standard humidity blocks (25%, 50%, 75% RH) to ensure international mutual recognition of test data.

Model validation: Establish a 3D model library containing normal skin, dry skin (treated with SDS), and aged skin (UV irradiated) to evaluate the improvement effect of the product on different skin states.

Data integrity: The entire experiment was conducted using an electronic data recording system (compliant with FDA 21 CFR Part 11), with raw data stored for at least 5 years and supporting audit tracking.

Industry application and compliance value

The detection of moisture content in the stratum corneum has become the core supporting data for cosmetic efficacy claims. According to the "Evaluation Specification for Cosmetic Efficacy Claims", moisturizing products are required to submit in vitro or human efficacy evaluation reports. Professional testing services can help businesses:

Formula optimization: By comparing the dose-response curves of different moisturizers (such as hyaluronic acid and ceramide), determine the optimal addition ratio (such as 1% hyaluronic acid can increase water content by 40% -60%)

Competitor benchmarking: Establish a competitor database and quantitatively analyze the competitive advantages of products in key indicators such as moisturizing persistence (such as 8-hour decay rate)

Compliance filing: Provide testing reports that meet the requirements of the National Medical Products Administration and support claims of efficacy such as' improving skin moisture status'

A typical case shows that a certain brand has significantly enhanced its market competitiveness by optimizing its formula to increase the moisture retention rate of its product to 85% in an 8-hour moisturizing test (industry average of 62%). With the increasing attention of consumers to the authenticity of efficacy, standardized detection of stratum corneum moisture content will become a necessary step in cosmetics research and development.