Modern methods for assessing hydration and biomechanical properties of the skin

Olga Panova

Elena Gubanova

Natalia Lapatina

Elena Hernandez

Alisa Sharova

Human skin is one of the main organs involved in maintaining homeostasis of the body. The study of its functional state is one of the promising areas in dermatology and dermatocosmetology. In modern dermatology, when assessing the effectiveness of medicinal and cosmetic products that improve skin condition, non-invasive biophysical methods for studying the skin in vivo are considered the most popular. The increasing interest of researchers in such methods is associated with their accessibility, simplicity and speed of execution, information content, and the possibility of statistical processing of results.

The first developments of methods for measuring physiological parameters of the skin appeared more than 30 years ago, but they were used exclusively to solve scientific problems related to skin research and evaluate the effectiveness/safety of dermatotropic drugs. Since the mid-80s of the last century, functional skin analysis methods have been used for the following purposes:

• when conducting observational, so-called epidemiological studies aimed at obtaining new, more detailed knowledge about skin changes that occur with age and under the influence of various environmental factors;
• when conducting clinical trials of new cosmetic and medicinal products and external use products;
• when conducting comparative post-marketing clinical studies of the effectiveness of the use of means and methods in the treatment of age-related changes;
• for diagnosing the functional state of the skin and correct prescription of therapy in the daily practice of dermatologists.

When assessing the functional state of the skin, one of the most important parameters are:

• moisture (level of hydration of the stratum corneum);
• biomechanical properties of the skin (viscoelastic properties, elasticity, tensile strength);
• microrelief;
• color characteristics (pigmentation, erythema);
• surface pH;
• transepidermal water loss;
• fat content

Methods for assessing hydration

Measuring the moisture content of the stratum corneum has become widespread in assessing the effectiveness of procedures using moisturizers. In addition, this parameter is of great importance in clinical dermatology and allergology.

The effect of moisture content in the stratum corneum on its mechanical properties has been known for a long time. In 1952, I. Blank described changes in the elasticity and firmness of the stratum corneum associated with its ability to retain exogenous water. The first biophysical methods were based on the study of electrical and thermal conductivity of the skin, since it is known that the more moisture the tissue contains, the better it conducts heat and electricity.

In the stratum corneum, water is found in two different thermodynamic forms (which is proven mainly by differential calorimetric scanning and thermogravimetry methods):

• free water in which numerous ionized and non-ionized substances (metal salts, amino acids, urea) are dissolved. This water is located between the lipid layers of the intercellular space of the stratum corneum, where it comes from the underlying layers of the epidermis. Water slowly moves towards the surface of the skin, upon reaching which it evaporates into the atmosphere (the process is called “transepidermal water loss”). This water can freeze at temperatures below 0°C;
• bound water makes up about 20–30% of the total water content in the stratum corneum. It is bound by electrostatic (non-covalent) bonds to keratin, natural moisturizing factor components and stratum corneum lipids and freezes at lower temperatures.

The amount of bound water can be assessed by differential calorimetry, thermogravimetry, as well as methods based on the study of the resonance effect (IR spectroscopy, nuclear magnetic resonance). Thanks to these methods, the interaction of the aqueous and lipid phases in the stratum corneum has been proven: with an increase in the moisture content of the stratum corneum, a change in the structure of its lipid layers is observed. Subsequently, it was shown that most skin characteristics, such as its relief and microrelief, lipid and water balance, are closely related to each other. It is the water component that plays an important role in changing characteristics such as relief and mechanical properties of the skin. Thus, the level of hydration can be judged not only using direct methods for assessing the water content in the stratum corneum, but also indirectly, relying on data from methods that determine other characteristics.

Corneometry

Corneometry is a widely used method for directly assessing the hydration of the corneal epidermis. The corneometer uses the principle of capacitor capacitance (changes in the dielectric properties of the skin depending on the amount of moisture contained in the stratum corneum). Skin is a dielectric medium, and any changes in the dielectric constant as a result of changes in the water content in the surface layers of the skin lead to a change in the capacitive characteristics of the measuring system.

The corneometer has a number of undeniable advantages [1]:

• the depth of penetration of electric waves is reliably small, so humidity is measured precisely on the surface of the skin within the stratum corneum;
• the short duration of measurements (about 1 s) prevents possible occlusion, which affects the accuracy of measurements;
• the influence of the capacity of deep-lying tissues is excluded;
• the measuring sensor is lightweight and easy to use;
• The small size of the sensor's measuring head (diameter 1 cm) makes it possible to take measurements on any part of the body.

Taking into account the fact that corneometry makes it possible to determine the total water content in the stratum corneum, this method can be used both for the primary diagnosis of skin pathology and for assessing the effectiveness of cosmetics or procedures aimed at increasing the degree of hydration of the skin surface.

Corneometry refers to semi-quantitative methods, since its result is expressed in conventional units (points or corneometric units). Each modification of the device has its own scale. For example, the corneometer “Monaderm Combined Unit CM825/SM 815/CT 580” (Courage Khazaka) has a scale from 0 to 120 units. Value below 30 units. characterizes very dry skin, from 30 to 45 units. – dry skin, above 45 units. – hydrated skin to varying degrees.

It has been established that both temperature and relative humidity significantly affect corneometry parameters. [2, 3]. Our study of seasonal influences on the biomechanical parameters of the lips and perioral area also confirmed the influence of temperature and air humidity on skin hydration: in the summer, the skin is more hydrated than in the winter.

"SkinChip"

Another device designed to assess skin hydration, based on the same physical phenomenon (change in dielectric resistance of a material depending on its water content) as corneometry, is the SkinChip, developed and patented by L'Oreal Research Laboratory.

"SkinChip" is an electronic contact sensor consisting of many microsensors that measure the dielectric conductivity of the skin. Each microsensor of the device transmits information to a computer, where it is transformed into shades of gray. Overall, they form an image that reflects the texture of the skin and its level of moisture. The darker the image, the higher the moisture content of the skin in this area. Skin hydration is assessed using statistical analysis of the brightness level (gray level) of the area of interest.

A study conducted at the Moscow laboratory “SkinLab” using the two above-mentioned devices led to the conclusion that the use of the “SkinChip” device, due to the presence of a large number of microsensors in it, in some cases allows one to obtain more reliable measurement results [4–6].

Alternative methods for measuring hydration include infrared spectroscopy, frequency resonance, and nuclear magnetic resonance. In addition, a number of studies have shown that the level of water content in the stratum corneum is a necessary condition for maintaining natural exfoliation processes [7]. Determining the degree of skin desquamation using adhesive tape allows you to indirectly judge its hydration. After applying and removing the adhesive tape from the surface of the skin, desquamated corneocytes remain on the sticky surface of the tape. The tape is then photographed in transmitted light, after which the resulting image is analyzed. Based on the data obtained, the peeling index is calculated, which is inversely proportional to the degree of skin hydration.

Vaporimetry (tevametry)

Indirect methods for assessing the level of hydration also include vaporimetry, since the state of skin hydration is directly related to the state of the hydrolipid mantle and the lipid barrier of the stratum corneum.

Vaporimetry (a method for assessing the transepidermal water loss index - TEWL) is based on measuring the pressure of water vapor above the surface of the skin. The signal enters a digital analyzer, which calculates how much water has evaporated per unit time. The TEWL index is measured in g/m2/h.

The TEWL index is often used in pharmacological studies, as it reflects the barrier properties of the stratum corneum. An increase in the TEWL index above normal indicates damage/weakening of barrier properties; a decrease in the TEWL index is observed in cases where there is an occlusive layer on the skin surface that prevents water evaporation.
The TEWL indirectly characterizes the state of the skin's hydrolipid barrier. Since this parameter correlates with the level of hydration, it is advisable to combine vaporimetry with corneometry and sebumetry. As a rule, with increasing hydration, a slight increase in TEWL is also observed [8]. There is also a correlation between different concentrations of the damaging chemical agent (acids) and the degree of change in TEWL and corneometry parameters [9].
In addition, it has been established that the TEWL index and, to a greater extent, corneometry indicators correlate with the severity of certain skin diseases, for example, atopic dermatitis [3].

When studying the influence of environmental conditions (temperature and relative air humidity) on changes in TEWL, a strong correlation between TEWL and temperature and a weak correlation between the TEWL index and air humidity were revealed. Our studies also revealed the dependence of moisture content and TEWL on the skin of the face and hands on changes in humidity and air temperature in the spring-summer period (Fig. 1).

Rice. 1. Seasonal influences of humidity and air temperature on corneometry and vapometry in the perioral zone

Study of skin lipid balance

An additional parameter that allows us to characterize the state of the hydrolipid mantle of the skin is the assessment of the function of the sebaceous glands. To do this, they use devices that record changes in the optical density of lipophilic films that absorb fat from the surface of the skin over a certain period of time (sebumetry), or conduct a visual analysis using a special camera or film that changes color when absorbing sebum.

Sebumetry

A sebumeter is the most famous measuring instrument for determining the amount of fat on the surface of smooth skin and scalp. The device detects even minor changes in the sebum content on the skin surface. Various in vivo and in vitro tests and experiments described in the scientific literature, comparing it with other measurement methods, have confirmed the high informativeness of the sebumeter in dermatological and cosmetological studies.

The sensor used in sebumetry is called a sebumetric cassette. Inside the cassette there is a roll of special synthetic tape that can absorb fat. The area of the measuring head is 64 mm2. One cassette is designed for 450 measurements.

During measurement, a small section of tape is applied to the surface of the skin. When sebum is absorbed, this film becomes transparent. To quantify secretion, a measuring sensor is inserted into the hole of the device, where the degree of transparency of the film is analyzed using the photometric method. Light scattering on the film correlates with the sebum content on the skin surface. The microprocessor calculates the result, which is displayed on the display in conventional units - from 0 to 350 (“Monaderm Combined Unit CM825/SM 815/CT 580”, Courage Khazaka).

An important aspect of correctly performed sebumetry is preliminary cleansing of the skin with an alcohol-containing solution 1–2 hours before taking measurements.

Advantages of the method [1]:

• short measurement time prevents the occlusion effect, which can change the final result;
• slight pressure of the sensor on the skin makes it possible to repeat measurements without affecting the physiological functions of the skin;
• selective measurement of skin oiliness;
• The small size of the measuring sensor allows measurements to be taken on all parts of the body, including the scalp.

The sebumetry indicator is widely used to objectify the results of anti-acne therapy, hormone replacement therapy, and the use of cleansers for different skin types (Fig. 2).

Rice. 2. Results of sebumetry of the forehead skin. 1 – women 20–35 years old; 2 – women 40–55 years old; 3 – women 50–60 years old taking hormone replacement therapy (HRT); 4 – women 50–60 years old who are not taking HRT. The significance of the differences is indicated in comparison with groups 1, 2, 4

When studying age-related changes in the fat content on the surface of the skin, we showed that with age there is a decrease in sebum production, and the use of hormone replacement therapy during the postmenopausal period leads to an increase in its secretion.

Methods for studying mechanical parameters of skin

Skin aging processes are characterized by changes at the level of the protein structures of the dermis - collagen and elastin, as well as changes in the intercellular matrix of the dermis. These processes are manifested by a decrease in elasticity and weakening of skin turgor, which is a typical sign of skin aging. Therefore, the study of the mechanical properties of the skin is considered as an obligatory element of the overall assessment of skin changes, as well as the effectiveness of the therapy [10]. In addition, indicators of skin elasticity and density can be used to form typological and morphological groups and identify age-related trends.

When studying the mechanical properties of skin, the concepts of “elasticity”, “elasticity” and “density” (stiffness) are most often used [11].

Elasticity is the property of a body or material to resist changes in its volume or shape under the influence of mechanical stress, due to an increase in its internal energy.

Elasticity is the ability of a body or material to form, with relatively little effort, elastic, reversible deformations without its destruction.

Density (rigidity) is the resistance of a body or material to indentation. The disadvantage of this characteristic is that hardness is not a physical constant of materials and is a complex property that depends on both elasticity and plasticity and the measurement method.

Unfortunately, at the moment there is no single technique that allows one to objectively and in detail characterize the biomechanical properties of the skin in vivo. Therefore, all modern descriptions of these properties largely depend on the measurement method used. The starting point for measuring the mechanical properties of skin is to create a deformation using a fixed force and then analyze the degree (depth) of deformation, which characterizes the density (hardness) and elasticity of the skin, and the recovery characteristics of the skin, which reflects elasticity.

Basic methods for assessing mechanical properties:

• Transverse deformation methods: ballistometry (indentation method), cutometry (suction method);
• Longitudinal deformation methods: strain gauging (tension method), torsiometry/tocmetry (torsion method).

The most common and accessible methods for measuring biomechanical parameters are methods based on the creation of transverse deformation. Although they are based on a similar principle, when comparing the suction method with the indentation method, it was shown that both methods describe related, but not identical aspects of the mechanical properties of the skin. The difference in measurement principle suggests that the suction method (cutometry) primarily measures skin elasticity, while the indentation method (ballistometry) primarily measures skin density. In addition, there is evidence that the results of cutometry are influenced by the relief and thickness of the skin [12].

When choosing instrumentation, the anatomical features of the area being studied should be taken into account. Taking into account the small size of the ballistometer measuring sensor, it is advisable to use this device on limited areas of the skin that have properties that significantly distinguish this area from nearby ones. In particular, this device can be successfully used to study the biomechanical properties of such a delicate area as the lips.

Ballistometry

The ballistometer (Dia-Stron Torsional Ballistometer BLS 780, combined with a PC) is a kind of pendulum that strikes the surface of the skin from a constant height. Shock wave propagation and skin response depend on the condition of the elastic fibers and water content. The impact transfers kinetic energy to the skin and causes the sensor to jump after the impact. Atonic skin absorbs a large amount of energy; accordingly, there is little energy left for the response, and the amplitude of vibrations when acting on such skin is lower than when acting on dense, elastic skin. The built-in sensor records the response vibrations of the skin and builds a graph of these vibrations (Fig. 3). Analysis of the parameters of the resulting curve allows us to assess the degree of skin deformation and its elasticity.

The main ballistometry indicators reflecting the viscoelastic properties of the skin include indentation depth, ALPHA (absorption profile) and AREA (area under the curve).

Rice. 3. Ballistometry indicators

Indentation depth (IND – Indentation) – the height of the first peak under the curve on the graph. This parameter shows how deeply the skin is pressed during the initial impact of the pendulum ball, and is measured in millimeters (mm). IND directly characterizes skin density - the higher the IND, the lower the skin density.

Absorption Profile (ALPHA) - This indicator reflects the degree to which the two effective rebounds of the pendulum ball from the skin are reduced. As it increases, it means that the rebound becomes less and less pronounced and the leather becomes less elastic and more viscous.

Area under the curve (AREA) is the area under the curve that corresponds to the number of bounces. Young, elastic skin is characterized by a lot of rebounds, so the AREA in this case will be large. This parameter directly correlates with the ALPHA indicator.

Our studies have shown that the ALPHA level increases with age, and the appointment of anti-aging procedures (injections of stabilized hyaluronic acid, Thermage) leads to its decrease.

Cutometry

Assessment of viscoelastic parameters of the skin using cutometry (Cutometer, Courage Khazaka) is based on the well-known principle of vertical deformation. The sensor is a hollow tube, inside of which negative pressure is created. At the point where the hole contacts the surface of the body, the skin is lifted (sucked into the tube). The height of the skin tubercle and the time it takes to return to its original state after the pressure inside the tube is restored are recorded using an optical sensor. The optical system consists of a light source and detector, as well as two opposing prisms that reflect light from the source to the detector. The light intensity varies depending on the height of the tubercle. The result can be presented in the form of a curve (Fig. 4).

Rice. 4. Cutometry indicators. Explanations in the text

Main parameters of the curve:

• Ue – immediate deformation under external influence;
• Uv – increasing deformation due to the viscoelastic properties of the skin;
• Uf – general skin deformation;
• Ur – immediate recovery upon cessation of external influence;
• Ud – progressive recovery due to skin viscosity;
• Ua – general skin restoration.

The main indicators of cutometry are F - general skin deformation (or skin resistance to negative pressure) and U, reflecting skin elasticity (ability to return to its original state), which is presented in Fig. 4 as R2=Ua/Uf. The U parameter is considered more informative. The closer it is to one (100%), the more elastic the skin, i.e. after creating external deformation, it completely returns to its original state.

Torsiometry is also widely used to assess the mechanical properties of skin and is based on the creation of torsional deformation. The results obtained are presented in the form of a curve with parameters similar to those of cutometry.

Discussion

When choosing an apparatus for studying the biomechanical properties of the skin, you should be guided not only by the capabilities of the method, but also by the presence of a sensor that allows you to measure the necessary indicators in the anatomical area under study.

When studying hydration indicators and biomechanical properties of the skin, identifying correlations between them is of great importance [3, 9]. In our studies, we did not find a direct correlation between some of the described parameters of ballistometry, corneometry and vapometry. However, the results of our latest studies examining the effect of anti-aging therapy on the biomechanical parameters of the skin showed that changes in skin elasticity parameters during treatment correlated with positive dynamics in the level of hydration and/or TEWL indicators [13].

Conclusion

Thus, the use of functional diagnostic methods makes it possible to conduct an objective study of the biophysical properties of the skin and its barrier functions, as well as evaluate the effectiveness of external therapies and technologies that improve skin condition.

Literature

Source: Bulletin of Dermatology and Venereology, No. 2/2009, pp. 80–87

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