Article by: Kelly Dobos
In order to objectively evaluate the efficacy of the cosmetic products we create, numerous skin measurement devices have been developed over the years. The primary devices are reviewed below but first, here are a few guidelines to follow when taking instrumental measurements of the skin.
Skin Testing Guidelines
• The environment should be controlled to prevent sweating, typically 20 ± 1°C and 30-40% relative humidity
• Subjects should equilibrate for 30 minutes
• Subjects with obvious skin diseases, tattoos, or injury in the area of measurement should be excluded
Common Skin Measurements
Transepidermal Water Loss – Commonly referred to as TEWL, transepidermal water loss is the measurement of water transport through the skin by passive diffusion. TEWL can be used to measure the occlusive effects of moisturizers and demonstrate barrier repair or damage (TEWL increases with skin damage). For healthy skin, TEWL is in the range of 5-10 g/m2. The Vapometer from Delfin is an example of a device that can measure TEWL
Conductance – Water has a high dielectric constant and when skin is hydrated conductance increases and impedance decreases. Conductance can be used to estimate the water content of the skin. The Bioderm Skin Conductance meter can be used for this measurement.
Skin Surface pH – pH is the measure the acidity or basicity of an aqueous solution. The definition has been stretched to quantify the skin’s apparent pH due to diffusion on hydrogen ions. It is easy to measure by hard to interpret what skin pH really means. The typical range for skin is 4.0-5.7. A device like the Skin pH Meter from CK Electronic would work to get this value.
Mechanical Properties of the Skin – By applying a known stress and measuring the strain the elastic properties of the skin can be measured and used to support certain anti-aging claims. Some have used a durometer for this measurement. See this paper for a discussion about measuring the mechanical properties of skin.
Skin Color – Chromameters measure the color of the skin using the L*A*B* scale. L* ranges from white to black, A* ranges from red to green, and B* covers spans from blue to yellow. Erythema, natural skin pigment and tan induced by the sun or sunless tanners can be measure. This paper describes a comparison of different devices for measuring skin color.