Sensitive skin is a widespread concern, affecting nearly 50% of the global population, with a higher prevalence in women [1]. Factors such as environmental conditions, lifestyle choices, and physiological aspects can exacerbate symptoms. The characteristics of sensitive skin include overt sensory perceptions or decreased skin tolerance thresholds. Some individuals may experience sensations such as skin tightness, stinging, burning, tingling, pain, or itching. Objective perceptions observed by trained professionals include various cutaneous reactions such as erythema.

Defining sensitive skin is essential for the cosmetics industry to develop and market products that cater to this condition, but this remains a challenge, with no clear consensus on a definition of “sensitive skin” between consumers, dermatologists, scientists, and the cosmetics industry (see Figure 1, for the agreed sensitive skin characteristics). Sensitive skin remains by and large a self diagnosed condition, however various research groups have defined different types of skin sensitivity (see Table 1) [2] .

Table 1 Classification of sensitive skin

Figure 1 Sensitive skin characteristics which are agreed upon by the majority of experts

Supporting sensitive skin claims

To make sensitive skin claims, cosmetic products must meet global legislative requirements. Assessments and measurements on the appropriate test panel of subjects with sensitive are the key to a reliable and scientifically sound claim support.

Volunteer selection and characterisation

Volunteers in a clinical study should reflect the target population of the product under investigation. When a sensitive skin panel in required the main question is how the sensitivity of the subjects is determined. Sometimes it may be enough to rely of the volunteer’s self-perceived skin sensitivity (assessed via sensitive skin questionnaires where volunteers grade and score various sensitive parameters), other times it may be necessary to perform a chemical trigger test in order to further characterize their type of sensitivity (see Table 2) and there are also various biophysical methods for identification of sensitive skin (see section below on biophysical measurements).

Table 2 Characterization of Sensitive Skin According to Chemical Responses.

Accelerated sensitive skin volunteer panel recruitment

Incorporating documentation of volunteer self-perceived skin sensitivity upon registration in the SGS Proderm databased ensures immediate access to more than 1,000 subjects with self-perceived skin sensitivity and more than 150 subjects with skin sensitivity determined in a Facial Stinging Test (stingers), vastly accelerating the recruitment process (see Figure 2, for the sensitive skin panels included in the SGS proderm database).

Figure 2 Sensitive skin panels at SGS Proderm. (N = number of volunteers in each panel)

Methods for testing sensitive skin products

Sensitive skin product testing involves objective approaches, including tolerability assessments and efficacy assessments using biophysical and imaging parameters.

Tolerability studies aim to exclude irritation potential or skin discomfort during product usage. These studies often involve patch testing, in-use studies with expert grading and tolerability assessment, and facial stinging tests using lactic acid with subjective stinging severity grading. These studies are carried out on a defined “sensitive” panel of volunteers, who are either “Stingers” as determined by Lactic Acid Facial Stinging Tests or have sensitive skin as determined by Questionnaires or self-estimation.

Figure 3 Facial stinging test – Simultaneous application of two test products to the naso-labial folds of face of lactic acid stingers.

Efficacy studies should comprise both subjective and objective assessments as well as measurements where appropriate. They may include anti-facial stinging testing on “stingers” with subjective evaluation of stinging potential or the capsaicin / methyl nicotinate test on volunteers with sensitive skin and evaluation of erythema/stinging via a combination of subjective and objective assessments/measurements if applicable.

Clinical photography (with USR-CliP MacIS or Visia-CR) can be used in objective assessment of certain skin sensitivity parameters such as: skin redness or erythema; skin texture and surface roughness; pore size and distribution.

Biophysical measurements can be used to evaluate the physiological aspects of sensitive skin. These include transepidermal water loss (TEWL), skin hydration capacitance (corneometry), skin surface roughness (profilometry), skin redness (via chromametry), skin temperature (IR-Thermography), stratum corneum thickness (confocal microscopy) and skin elasticity (cutometer). Imaging measurements provide in-depth evaluations of vascular parameters, such as blood flow, using techniques like Full-Field Laser Perfusion Imaging (FLPI). Additionally, itch sensations can be assessed experimentally using topical application or intra-dermal injections of various substances like histamine.

Figure 4 Full Field Laser Perfusion Imaging (FLPI) of facial blood flow.

Quantitative Sensory Analysis (QSA) can evaluate sensations like heat and cold pain using thermal cutaneous sensation testing. Methods like the thermo-sensory analyzer (TSA) measure the threshold for warm and cold sensation, as well as hot and cold pain. Touch-evoked sensitivity can also be assessed using Von Frey filaments.

Figure 5 Heated Thermode on the skin.

Measurement of Heat Pain Threshold (HPT) with a Thermal Sensory Analyser (TSA).

Biomarker analysis of inflammation markers and skin surface lipids offers insight into skin reactivity, irritation, and barrier function [3, 4]. Non-invasive methods can be used to collect these biomarkers for analysis with immune-assays or spectroscopic and chromatographic technologies. Dermal transcriptome analysis is possible with transdermal patches and next-generation sequencing [5].

In conclusion, to effectively support sensitive skin claims, it is essential to understand target consumers as well as regulatory requirements, while utilizing an appropriate sensitive skin volunteer panel in clinical studies. Combining both subjective and objective assessments as well as measurements ensures rigorous testing, ultimately helping build the necessary evidence for successful product development and marketing.

References

Please contact SGS proderm for the cited literature.

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CONTACT

Sascha Faust

SFaust@proderm.de

www.proderm.de

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