In vitro transdermal moisture loss detection of cosmetics

In vitro transdermal moisture loss detection of cosmetics

In vitro transdermal moisture loss detection of cosmetics:The integrity of the skin barrier function is directly related to the moisturizing effect of cosmetics, and transcutaneous water loss (TEWL) testing is the gold standard for evaluating this function. This test quantifies the rate of moisture evaporation on the skin surface, which not only scientifically verifies the actual efficacy of moisturizing cosmetics, but also provides accurate data support for product formula optimization. Professional testing institutions, with standardized testing processes and high-precision instruments, have made TEWL testing a key technical link in cosmetics research and development, filing, and market access.

Analysis of Detection Principles and Physiological Mechanisms

Transdermal water loss refers to the process in which the stratum corneum of the skin loses water through diffusion and evaporation, and its value directly reflects the barrier function status of the stratum corneum. The TEWL value of healthy skin is usually maintained in a dynamic equilibrium range of 2-10 g/(m ² · h). When the barrier is damaged (such as excessive exfoliation or sensitive skin), the TEWL value can soar to over 30 g/(m ² · h). The core principle of detection is based on Fick's diffusion law, which states that the diffusion rate of water through the stratum corneum is proportional to the difference in water vapor pressure on both sides of the skin and inversely proportional to the thickness of the stratum corneum.

Modern skin physiology research has shown that TEWL values are regulated by three major factors: the content of ceramides in the stratum corneum (accounting for 50% of intercellular lipids), the integrity of the tight junction structure of keratinocytes, and the hydration status of the skin surface. For example, moisturizers containing ceramides can reduce TEWL values by 25% -40%, which can be captured in real-time by high-precision instruments. During the detection process, small fluctuations in environmental temperature and humidity (such as temperature ± 1 ℃ changes) may lead to a measurement error of 5% -8%. Therefore, strict control of the detection environment parameters is a prerequisite for ensuring data reliability.

Core testing methods and instrument operation specifications

At present, the TEWL testing method for industry professionals is mainly based on the ISO 13099-2:2019 standard, which specifies in detail the instrument performance requirements, environmental condition control, and data processing procedures. The mainstream detection instruments adopt open capacitive sensor technology. Taking the Tewameter TM300 from Germany's Courage+Khazaka company as an example, its core detection unit consists of a pair of humidity sensors and temperature sensors. Quantitative analysis is achieved by measuring the water vapor pressure gradient of a 1cm ² area on the skin surface.

The standard testing process consists of four key steps: firstly, the environmental adaptation stage, which requires the subjects to sit quietly in a constant temperature and humidity chamber with a temperature of 23 ± 1 ℃ and a relative humidity of 50 ± 5% for 30 minutes, avoiding touching the testing area during this period; Next is the determination of the baseline value, which is measured three times continuously on the inner side of the forearm after cleaning, and the average value is taken as the blank control; Next is product intervention, applying the test sample at a dosage of 0.5mg/cm ² and waiting for the specified time (usually 2 or 24 hours); Finally, dynamic monitoring was conducted to continuously record the TEWL change curve within 15 minutes, and the area under the curve (AUC) was calculated to evaluate the moisturizing persistence.

The instrument calibration process is equally strict. Before daily testing, a saturated salt solution (such as potassium sulfate saturated solution corresponding to 98% RH) should be used for humidity calibration to ensure that the sensor error is ≤ ± 3%. The testing data of a certain international brand of moisturizing cream showed that its 2-hour TEWL value reduction rate reached 32.6%, and the relative standard deviation (RSD) of 3 repeated measurements was only 2.8%, far below the industry allowed threshold of 5%, fully verifying the precision of the testing method.

Standard system and data validity control

The standardization system for TEWL testing has formed a multi-level specification. In addition to the international standard ISO 13099, China's "Technical Specification for Safety of Cosmetics" (2022 edition) also lists it as a mandatory item for evaluating moisturizing efficacy. For data validity, the industry generally follows three quality control requirements: first, instrument performance, sensor response time should be ≤ 10 seconds, and resolution should reach 0.1 g/(m ² · h); The second is sample volume control, applying a precise dose of 0.5 μ L/mm ² and using an automatic pipetting device to ensure an error of ≤± 2%; The third requirement is statistical, with a parallel experiment sample size of no less than 15 people. Paired t-test is used for inter group comparison, and the significance level is set at p<0.05

It is worth noting that there are significant differences in the baseline TEWL values of different parts, such as the forehead (about 8 g/(m ² · h)) being higher than the inner forearm (about 5 g/(m ² · h)). Therefore, the standard method specifies that the detection site is the 3cm × 3cm area on the inner forearm, where the skin thickness is uniform and less affected by external stimuli. A comparative study conducted in a certain laboratory showed that the intra day coefficient of variation of TEWL for the same subject under the same conditions was 3.2%, and the inter day coefficient of variation was 6.7%. This provides an important reference baseline for data interpretation.

Technical advantages of professional testing institutions

Quanwei Testing Agency has established a full range of technical barriers in the field of TEWL testing. Taking CNAS accredited laboratories as an example, their core competitiveness is reflected in three aspects: environmental control adopts a dual stage air conditioning system, achieving temperature fluctuations of ≤± 0.5 ℃ and humidity fluctuations of ≤± 2% RH, which is superior to ISO standard requirements; Personnel operations shall follow standardized SOP procedures, and testing personnel shall undergo proficiency testing twice a year (such as TEWL measurement comparison organized by the National Institute of Statistics); Establish a full process quality control record from instrument calibration to report issuance for data traceability, ensuring that each set of data can be traced back to international standard substances.

In addition, the Xianjin laboratory is equipped with a skin mirror system, which can synchronously observe the microstructural changes of the stratum corneum and correlate TEWL data with skin morphological features for analysis. For example, cosmetics containing hyaluronic acid not only reduce TEWL values by 28%, but also improve texture clarity due to increased moisture content in the stratum corneum observed through a mirror system. This multi-dimensional detection capability makes the data reports issued by professional institutions the core evidence chain for cosmetics companies to claim moisturizing effects.

With the upgrading of consumer demand for functional cosmetics, TEWL testing has transformed from a simple scientific research tool to a technological high ground in product market competition. By strictly adhering to international standards, using high-precision instruments, and implementing full process quality control, professional testing institutions are providing solid technical support for the high-quality development of the cosmetics industry. In the future, with the maturity of alternative methods such as 3D skin models, TEWL detection will further improve detection efficiency while maintaining scientificity, opening up new paths for green beauty research and development.