Skin hydration testing has evolved significantly over the decades, and today’s options range from low-cost probes to advanced imaging technologies. Here’s a look at the methods available and what each one is and isn’t suited for.
Probe-Based Assays
Probe devices remain one of the most widely used and cost-effective ways to measure skin hydration. These instruments measure the electrical properties of the stratum corneum, such as capacitance and conductance, which change based on the amount of trapped water. Common examples include the Corneometer®, MoistureMeter, Skicon, and Novameter.
Probes generally work well, though they have limitations with products that act as electrical insulators, such as certain makeup and lip products.
Transepidermal Water Loss (TEWL)
TEWL measurement is a valuable tool for products that rely on occlusion. Rather than directly measuring hydration, TEWL confirms how well a product reinforces the epidermal permeability barrier, making it especially useful for occlusive-based formulations and products that aren’t compatible with electrical probes. Devices such as the VapoMeter and Tewameter are commonly used, and TEWL is also considered an early indicator of dry skin, making it useful for tracking progression over time.
High-Tech Imaging Methods
For research applications, advanced technologies like Raman spectroscopy, optical coherence tomography, and nuclear magnetic resonance can directly measure water content, and sometimes other skin components, rather than relying on electrical properties. These methods can be powerful research tools, but their cost typically makes them impractical for most claims support applications.
The Leg Regression Method
Before low-cost probes became widely available, the leg regression method was the standard approach, and it remains valuable today, particularly for multi-day hydration claims. The method takes advantage of the fact that lower legs tend to be relatively dry and flaky year-round, and relies on expert grading of features like flaking and roughness across three phases:
Washout Phase
Both lower legs are washed with a mild cleanser for 5–7 days, with no leave-on products, followed by baseline (Day 0) readings.
Treatment Phase
The test product is applied to one leg for 2–4 weeks to reach a peak hydration state, with the duration adjustable based on study goals.
Regression Phase
Following the final application, dryness is assessed over 1–4 days to measure how quickly signs of dry skin reappear.
Choosing the Right Method for Your Product
The best method depends on your formulation’s chemistry and the type of claim you want to support:
- Creams, lotions, or serums with humectants (with or without occlusives) are typically best suited to probe methods. This offers a fast, cost-effective option for percentage-increase and all-day hydration claims.
- Products that rely primarily on occlusion, or that are incompatible with probes, are often better suited to TEWL testing, keeping in mind that occlusion-based hydration takes time, so “immediate” claims aren’t appropriate.
- Multi-day hydration claims are best supported by the leg regression method.