The skin barrier is today one of the most intensively claimed territories in cosmetics. Hydration, repair, protection, tolerance: behind each of these promises lies a scientific proof requirement that brands can no longer avoid. This guide reviews the methods available to evaluate barrier function, from in vitro tests to in vivo studies, covering biometrological parameters, molecular biomarkers, and reference measurement devices.
A thin layer for a vital protective role
The barrier function of the epidermis plays a vital multifunctional role in protecting the body from the environment and in modulating transcutaneous penetration. The stratum corneum, the outermost layer of the skin, provides essential and dynamic features: mechanical resistance, photoprotection, antimicrobial and antioxidant defenses, body temperature regulation, hydration regulation, and waterproofing.
This barrier maintains a crucial balance for skin health, and its disruption is directly implicated in the itching, burning, and stinging symptoms associated with sensitive skin. Numerous factors can impair it: genetic factors, microorganisms, physical trauma, chemical agents. The consequences are well-documented: excessive water loss, microbial infection, skin inflammation, pruritus, and diseases such as psoriasis and atopic dermatitis.
Structurally, the stratum corneum is organized in stacked layers following a two-compartment “brick and mortar” model: a proteic structure (corneocytes) embedded in an intercellular lipid matrix. Corneocytes are ultra-differentiated, metabolically inactive, tightly associated keratinocytes, continuously renewed from undifferentiated keratinocytes in the basal layer that proliferate and differentiate as they migrate toward the surface.
In vitro evaluation approaches: molecular precision and reconstructed models
In vitro tests allow evaluation of the effect of an ingredient or formula on specific barrier markers under controlled, reproducible conditions. Several models are available: skin explants, 3D reconstructed epidermis (EPI-200, SkinEthic RHE), full-thickness models, or 2D keratinocyte cultures in monoculture or coculture with immune or neuronal cells.
Evaluating barrier efficiency
Barrier integrity can be measured by transepidermal water loss (TEWL), transepithelial/transendothelial electrical resistance (TEER), or the passage of molecules across the epidermis via Franz Cell (OECD 428) or other percutaneous penetration techniques. These approaches directly quantify barrier permeability under standardized conditions.
Barrier formation and renewal
Barrier integrity requires appropriate formation and renewal correlated to keratinocyte proliferation, differentiation, and desquamation. Multiple biomarkers allow assessment of these processes:
| Function | Key biomarkers |
|---|---|
| Undifferentiated keratinocytes | K5, K14 |
| Stemness | K15, K19 |
| Proliferation | Ki67 |
| Differentiation | K1, K10, Loricrin, Involucrin, Filaggrin |
| Cell-cell and cell-matrix cohesion | K6, K16 |
| Cornified envelope crosslinking | Transglutaminases 1, 3 and 5 |
| Filaggrin synthesis | Sirtuin-1 |
| Filaggrin degradation | Caspase-14 |
| Desquamation | Kallikreins |
Filaggrin degradation produces the Natural Moisturizer Factor (NMF), a key factor in skin hydration. Appropriate skin pH and hydration condition the proper functioning of stratum corneum enzymes involved in cellular cohesion.
Tight junctions and intercellular cohesion
Tight junctions ensure cohesion between corneocytes and prevent the transfer of molecules through the stratum corneum. Their integrity is evaluated using markers such as Corneodesmosin, Zonula Occludens 1 (ZO1), Occludin, E-Cadherin, Desmoglein-1, and Claudin 1. Stratum corneum cohesion also involves proteins such as envoplakin and periplakin, which connect intracellular keratins to membranes and cellular junctions.
Antimicrobial peptides
The first line of defense against pathogens is provided by antimicrobial peptides secreted on the skin surface: human cathelicidin LL-37, types 1-4 beta-defensins, psoriasin (S100A7), calprotectin (S100A8/9), koebnerisin (S100A15), and RNase 7. Their expression constitutes a relevant indicator of the functional state of the cutaneous immune barrier.
Stratum corneum lipid barrier
Lipid composition and organization are critical for barrier function. Epidermal thickness, stratum corneum thickness, and lipid organization are assessed using Raman microspectroscopy, while lipid composition is obtained by liquid chromatography coupled to high-resolution mass spectrometry. These approaches allow evaluation of ceramide synthesis, subclasses, and organization in particular.
In vivo methods: quantifying the barrier on subjects
Barrier function can be studied on human subjects using various protocols: clinical evaluation by dermatologists, scoring by experts or AI algorithms, sensory and neurosensory analysis, or instrumental evaluation. Biometrological studies allow visualization of skin structure and surface, and quantification of different physiological parameters.
Biometrological parameters and reference equipment
| Parameter measured | Reference equipment |
|---|---|
| TEWL (reference method) | Tewameter 300 and Nano (Courage+Khazaka), Aquaflux (Biox), Dermalab (Cortex), Evaporimeter, Vapometer |
| Stratum corneum hydration | Corneometer® (C+K), Dermalab, Epsilon (Biox), MoistureMeter SC, Skicon-200, DPM 9003 |
| Epidermal hydration | MoistureMeterEpiD |
| Dermal hydration | MoistureMeterD |
| Microtopography | MoistureMap® (C+K), Epsilon (Biox) |
| Surface visualization | Visioscan (C+K), SkinCam, SpectraCam, NomadCam (Newtone), Dermalab Video (Cortex), AEVA-HE2-M and Evasurf (Eotech), C-Cube (Pixience), Antera3D (Miravex), VideometerLab |
| Structure visualization | Ultrasound, MPT Flex Optical Multiphoton Tomography, Vivascope and Vivosight, Laser Confocal Microscopy, LC-OCT 3D, Infrared spectrometers |
| Molecular water content | Confocal microscopy LBRAM 800, Raman spectroscopy gen2-SCA (RiverD) |
| Microbiome balance | 16S-qPCR, Ms/MS PCR, metabolomic analysis |
TEWL: the gold standard for barrier permeability
Transepidermal water loss (TEWL) remains the absolute reference method for evaluating stratum corneum permeability. It measures the flux of water evaporated through the skin under non-occlusive conditions. An increase in TEWL signals barrier impairment. For measurements to be comparable and statistically valid, standardization is mandatory: subject acclimatization of at least 20 to 30 minutes in a controlled room, temperature of 20°C (±1°C), and relative humidity of 50% (±10%).
Choosing the right protocol for each claim
The choice of measurement method does not precede the claim: it derives from it. This is one of the most common errors in study design.
| Target claim | Recommended protocol | Population |
|---|---|---|
| Strengthens the barrier | TEWL and Corneometer before/after on healthy skin | Normal to dry skin |
| Repairs the barrier | TEWL on challenge model (SLS or tape stripping) + recovery kinetics | Compromised skin |
| Suitable for sensitive skin | TEWL + irritation scores + neurosensory evaluation | Self-declared sensitive skin |
| Protection against external aggressors | Vexation model (cold/wind) + TEWL + impedance | Normal skin |
Skin challenge models: creating a controlled impaired barrier
To substantiate a repairing claim, the study must work on a previously compromised barrier. Three models are commonly used in CROs:
SLS model (Sodium Lauryl Sulfate): application of an SLS solution under an occlusive chamber (24 to 48 hours) on the forearm. Standardized, reproducible method, widely accepted by labs and scientific authorities. TEWL is measured before, during, and after application of the test product to trace the recovery kinetics.
Repeated tape stripping model: mechanical disruption by successive removal of superficial stratum corneum layers using adhesive strips. Allows evaluation of barrier recovery kinetics, and can be combined with analysis of extracted proteins (differentiation markers) or Raman measurement.
Cold/wind model (skin vexation): exposure to aggressive environmental conditions (cold, wind, low humidity). Used for claims on winter-sensitive skin or weather protection formulas.
What the Skinobs platform reveals
Skin barrier studies today mobilize an interdisciplinary approach combining neuroscience, epigenetics, and cosmetic science. On the Skinobs platform, the substantiation of skin barrier strengthening draws on 40 referenced methods across 138 laboratories in 38 countries. This density reflects both the complexity of the field and the diversity of approaches available to address each type of claim.
Conclusion
Evaluating the barrier function is not limited to measuring TEWL. It means building a coherent body of evidence, structured around a precise claim, a defined target population, an appropriate study model, and validated measurement equipment. In vitro for molecular mechanisms, in vivo for clinical demonstration: both approaches are complementary and often both necessary for a solid substantiation file.
Looking for the right laboratories to evaluate your formula’s barrier function? Skinobs lists specialized CROs and helps you identify the protocol adapted to your claims. [Access the platform]
