Substantiating a moisturizing claim is one of the most common challenges in cosmetic product development. Yet with a growing number of biophysical measurement techniques available, selecting the right in vivo hydration test can be complex. Corneometer, TEWL, MoistureMeter, impedance-based methods — each offers specific advantages depending on your formulation, your target skin area, and the claim you need to support.
This guide reviews the main in vivo methodologies used in accredited CROs (contract research organizations) and explains how to align your testing protocol with regulatory expectations and marketing needs.
1. Understanding Skin Hydration: Two Distinct Parameters
Before choosing a test, it is essential to distinguish between two complementary aspects of skin hydration:
- Stratum corneum water content (SC hydration): the amount of water present in the outer layer of the skin
- Transepidermal water loss (TEWL): the passive flow of water evaporating through the skin, reflecting barrier function integrity
A product can increase SC hydration while leaving TEWL unchanged (humectant effect), or it can reduce TEWL by reinforcing the lipid barrier (occlusives, ceramides). Your claim must reflect which mechanism is targeted.
2. Main In Vivo Hydration Measurement Methods
| Method | Parameter measured | Key device | Typical application |
| Corneometry | SC hydration (capacitance) | Corneometer CM 825 | Face, arms — standard moisturizing claim |
| TEWL measurement | Transepidermal water loss | Tewameter TM 300 / VapoMeter | Barrier function, occlusives |
| Electrical impedance | SC water content (multi-freq) | MoistureMeter SC / Epsilon | Deeper layers, comparison studies |
| Near-infrared (NIR) | Water content in dermis | SkinLab, MoistureMap | Innovative long-lasting claims |
| Confocal Raman | Depth profile of water | River Diagnostics 3510 | Research, premium substantiation |
Corneometry: the gold standard for superficial hydration
Corneometry is the most widely used method to evaluate SC hydration. It measures the dielectric constant of the skin surface via capacitance. The Corneometer CM 825 (Courage + Khazaka) remains the industry reference. It delivers rapid, non-invasive, reproducible measurements and is well-accepted by regulatory bodies worldwide (ISO 22717).
Best suited for: humectant-based formulas (glycerin, hyaluronic acid, urea), immediate and 24h moisturization claims, before/after comparisons on face and arms.
TEWL: assessing barrier function
TEWL measures the rate of water vapor loss from the skin surface in g/m²/h. A decrease in TEWL after product application indicates improved barrier function. Closed-chamber devices (Aquaflux, VapoMeter) have gained traction over open-chamber methods as they reduce sensitivity to ambient conditions.
Best suited for: occlusives, emollients, ceramide-based formulations; sensitive or atopic skin claims; combined efficacy/tolerance testing.
Electrical impedance and MoistureMeter SC
Unlike corneometry (single frequency), impedance spectrometry uses multiple frequencies to probe different skin layers. The MoistureMeter SC (Delfin Technologies) provides a deeper assessment of SC water content. This is particularly relevant for long-lasting or deep hydration claims.
3. Study Design: Key Variables to Control
Choosing the right device is only part of the equation. Study design strongly influences result reliability and regulatory acceptability:
- Acclimatization time: minimum 20–30 minutes in a controlled environment (20–22°C, 45–55% RH)
- Wash-out period: typically 12–24h without any skin care product
- Panel size: minimum 20–30 subjects for a robust claim; 50+ for a premium substantiation dossier
- Time points: at T0, T+1h, T+4h, T+8h, T+24h depending on the claim (immediate vs. long-lasting)
- Body area: forearm (standard) vs. face (specific probes required) vs. hands/legs (adapted protocols)
4. Matching Your Claim to the Right Method
| Claim type | Recommended method(s) | Suggested time points |
| Instant moisturization | Corneometry | T0 → T+1h |
| 24h hydration | Corneometry + TEWL | T0, T+8h, T+24h |
| Long-lasting deep moisture | MoistureMeter SC or NIR | T0, T+24h, T+72h |
| Barrier repair | TEWL (closed-chamber) | T0, T+7 days, T+28 days |
| Sensitive skin tolerance + hydration | Corneometry + TEWL + clinical scoring | T0, T+14 days, T+28 days |
5. Regulatory Considerations
In the EU, cosmetic claims must comply with Regulation (EC) No 655/2013 and the associated technical document. Claims must be supported by adequate evidence, which may include in vivo instrumental data. ISO 22717 (corneometry) and ISO 22718 (TEWL) provide standardized methodological frameworks.
In the US, the FDA does not formally regulate cosmetic claims in the same way, but the FTC requires that claims be substantiated. Clinical studies using validated methods remain the best risk-mitigation strategy.
In South Korea — one of Skinobs’ key markets — MFDS guidelines also require evidence-based claim substantiation, with a strong preference for clinical data backed by recognized protocols.
Conclusion
Selecting the right in vivo hydration test is not a purely technical decision: it reflects your product positioning, the population you are targeting, and the regulatory environment of your launch markets. Corneometry remains the reference for most standard moisturizing claims, while TEWL and impedance methods enrich the clinical picture for premium or therapeutic-adjacent formulas.
To explore which CROs on Skinobs offer these specific protocols — including panel recruitment for sensitive or reactive skin — visit www.skinobs.com and use the platform’s 11 interconnected search filters to identify your ideal testing partner.
