JP6454498B2 - Method for measuring skin stratum corneum moisture content using terahertz waves - Google Patents

Description

  The present invention relates to a method for measuring skin stratum corneum moisture content.

  The stratum corneum, which is the outermost layer of the skin, is the main protective barrier against moisture evaporation, microorganisms, poisons and the like. In the stratum corneum, water is known to control physical properties such as substance permeability and plasticity of the stratum corneum and the activity of enzymes that have an important effect on the desquamation of stratum corneum cells on the skin surface (non-patented) Reference 1). Therefore, knowing in detail the amount of moisture in the stratum corneum and its changes is important for evaluating the stratum corneum function.

  Various methods have been devised so far for measuring the amount of moisture in the stratum corneum and its change. For example, paying attention to the fact that the electrical properties such as the electrical conductivity, capacitance, and dielectric constant of the stratum corneum change depending on the amount of moisture in the stratum corneum, the relative stratum corneum from the measured values of the electrical properties Devices that try to know the amount of moisture have been proposed (Patent Document 1, Non-Patent Document 2, Non-Patent Document 3).

  These devices were useful in that the water content of the stratum corneum can be known quickly and easily without damaging the skin. However, in the skin where cosmetics and skin preparations such as its raw materials are applied, the applied product may affect the measured value by inhibiting or promoting the flow of electricity. There was a problem that a measurement value reflecting only the amount of water was not obtained.

  In addition, paying attention to the fact that the near-infrared absorbance changes depending on the amount of water in the stratum corneum, the angle is measured by diffuse reflection spectroscopy using near infrared rays or attenuated total reflection spectroscopy (ATR method). A method for measuring the moisture content of the layer has been proposed (Non-Patent Document 4).

  These methods are also beneficial in that the amount of stratum corneum moisture can be known without damaging the skin. However, it is difficult to separate the near-infrared absorption spectrum of water from the substances contained in cosmetics and skin preparations, or the absorption spectrum of substances in the skin, multiple scattering in skin tissue, absorption by melanin pigments, etc. There is a problem that it is easily affected by substances other than the stratum corneum moisture, and it cannot be said to be a sufficient technique for evaluating the stratum corneum function simply and with high accuracy.

  Furthermore, a method for observing the distribution of moisture content in the skin from a signal obtained by nuclear magnetic resonance of hydrogen atoms constituting water has been proposed (Non-patent Document 5). In the method using nuclear magnetic resonance, the moisture content distribution can be acquired as an image without damaging the skin. Therefore, it is possible to acquire data according to the request of which part of the skin moisture content is to be obtained. There are advantages you can do. However, there is a possibility that accidents such as burns may occur depending on the necessity of very expensive equipment and facilities, the need to place the sample in a very strong magnetic field, and the composition of the external preparation for skin application. There was a problem.

In addition, focusing on the fact that the intensity of Raman scattering changes depending on the amount of moisture in the skin, a method for measuring the distribution of moisture in the skin by using a confocal laser scanning Raman spectroscopic device has been proposed (non- Patent Document 6). In this method, the stratum corneum is assumed to be composed of water and protein, and Raman scattering intensity based on OH stretching vibration derived from OH groups constituting water and CH ₃ stretching vibration derived from CH ₃ groups constituting protein are considered. By correcting the change in the detected intensity due to the depth from the sample surface from the ratio of the Raman scattering intensity based on it, it is possible to express the water content at each depth point in weight percent. However, in the skin where cosmetics and skin external preparations are applied, since the substance containing CH ₃ stretching vibration is mostly contained in the applied product, the Raman scattering intensity due to CH ₃ stretching vibration was used. There was a problem that the correction might not give the correct moisture weight percent.

  An electromagnetic wave of 0.1 to 10 terahertz called a terahertz wave is located in an intermediate region between the light wave and the radio wave. Terahertz waves are characterized by having good substance permeability, being able to be safely used for the human body, being less scattered in living tissue, and being very much absorbed by water. In recent years, attention has been paid to these features, and attempts have been made to image water distribution and perform spectroscopic measurement using terahertz waves. For example, using terahertz wave time-domain spectroscopy, echoes of terahertz waves generated on discontinuities in the group index resulting from differences in the group index of the stratum corneum, epidermis, dermis, and subcutaneous tissue stratum that make up skin tissue A method for measuring the amount of water from pulse intensity information has been proposed (Non-Patent Document 7).

  This method estimates the stratum corneum water content using the intensity of echo pulses obtained from the air-stratum stratum interface, but echoes from the stratum corneum-skin interface where no clear interface is formed are obtained. Due to the difficulty and the thinness of the stratum corneum with respect to the pulse width of the terahertz wave, it has been difficult to obtain information on moisture in the stratum corneum with high accuracy.

  In addition, a terahertz wave is incident on the sample under total reflection conditions, and the attenuation of the terahertz wave due to the interaction between the surface wave (evanescent wave) of the terahertz band generated at that time and the sample is measured. Attempts have been made to obtain a complex dielectric constant (Patent Document 2, Non-Patent Document 8). In Patent Document 2, a complex refractive index of a sample is obtained by measuring two spectra with different incident angle conditions without requiring an extremely thin liquid layer unlike the transmission method. In Non-Patent Document 8, in order to eliminate the influence of water stacked on an extremely thin thin film sample such as a single cell layer, the so-called two-interface model considering reflection at the interface between the thin film sample and water is used. The complex permittivity is obtained.

  However, both documents only show that the complex refractive index or the complex dielectric constant was measured. In addition, the stratum corneum of the skin is not a single cell layer, but is a complex stack of dead keratinocytes, and it was not known whether it could be used in a method for measuring the water content of the stratum corneum.

Japanese Examined Patent Publication No. 63-19016 JP 2008-304444 A

Rawlings A et al., The effect of glycerol and humidity on desmosome degradation in stratum corneum., Arch Dermatol Res, 287, 457-64, 1995. Tagami H et al., Evaluation of the skin surface hydration in vivo by electrical measurement., J Invest Dermatol, 75, 500-7, 1980. Hashimoto Kumiko et al., Comparison of high-frequency conductivity measuring device and Corneometer for stratum corneum moisture measurement, Journal of Cosmetic Society, 11, 252-8, 1987. Potts RO et al., A noninvasive in vivo technique to quantitatively measure water concentration of the stratum corneum using attenuated total-reflectance infrared spectroscopy., Arch Dermatol Res, 277, 489-95, 1985. Mirrashed F et al., In vivo quantitative analysis of the effect of hydration (immersion and Vaseline treatment) in skin layers using high-resolution MRI and magnetization transfer contrast.Skin Research and Technology, 10, 14-22, 2004. Caspers PJ et al., In vivo confocal Raman microspectroscopy of the skin: Noninvasive determination of molecular concentration profiles., J Invest Dermatol, 116, 434-42, 2001. Takeshi Yasui et al., Development of non-contact and local skin moisture measurement using terahertz electromagnetic pulse, Biomedical engineering, 42, 390-4, 2004. K. Shiraga et al., Determination of the complex dielectric constant of an epithelial cell monolayer in the terahertz region., Applied Physics Letters, 102, 053702, 2013.

  By the way, when measuring using the ATR method, the terahertz wave needs to be totally reflected on the surface of the skin. The total reflection condition is uniquely determined by the refractive index of the medium on the incident side, the refractive index of the medium on the outgoing side, and the incident angle. In other words, the ATR measurement condition depends on the incident angle of the terahertz wave, the refractive index of the prism (emitter-side medium) in contact with the skin, and the refractive index of the stratum corneum (incident-side medium) in contact with the prism.

  However, because the skin has a laminated structure of the stratum corneum, epidermis layer, dermis layer, and subcutaneous tissue layer, there is a risk of being affected by moisture in the epidermis layer under the stratum corneum, so far. However, there is no known method for accurately measuring the refractive index of the stratum corneum, and the actual situation is that the refractive index of the stratum corneum is unknown. Therefore, it is unclear whether the incident terahertz wave is totally reflected under the arbitrarily set conditions, and the condition for total reflection in the stratum corneum cannot be accurately grasped.

  This invention is made | formed in view of such a background art, Comprising: This invention makes it a subject to provide the method of measuring the moisture content of a stratum corneum using a terahertz wave.

  The measurement method of the present invention is a method for measuring the amount of moisture in the skin stratum corneum by total reflection attenuation spectroscopy using terahertz waves, and irradiates terahertz waves by bringing the skin surface into contact with the prism surface that is the terahertz wave emission surface. And an absorption coefficient is obtained.

  According to the present invention, the water content of the stratum corneum can be easily measured using terahertz waves.

FIG. 1 is a schematic diagram showing the measurement concept of the present invention. FIG. 2 is a schematic diagram showing the measurement concept of the two-interface model. FIG. 3 is a diagram showing an absorption spectrum of water. FIG. 4 is a diagram showing an absorption spectrum of an aqueous sucrose solution. FIG. 5 is a graph showing the relationship between the mass water content of the aqueous sucrose solution and the absorption coefficient, where (a) shows the measurement result at 0.5 THz, and (b) shows the measurement result at 1.0 THz. ● indicates experimental values and ○ indicates literature values. FIG. 6 is a diagram showing an absorption spectrum of the stratum corneum model. FIG. 7 is a diagram showing the relationship between the mass moisture content and the absorption coefficient using the stratum corneum model, where (a) shows the measurement result at 0.25 THz, (b) shows the measurement result at 0.5 THz, (C) is a figure which shows the measurement result in 1.0 THz. FIG. 8 shows the absorption spectrum of pig skin (dried for 48 hours). FIG. 9 is a diagram showing the relationship between the mass moisture content of pig skin and the number of stripping, where (a) shows the measurement result at 0.25 THz, (b) shows the measurement result at 0.5 THz, (c ) Is a diagram showing a measurement result at 1.0 THz. FIG. 10 is a graph showing an absorption spectrum of pig skin (dried for 48 hours) after application of the two-interface model. FIG. 11 is a diagram showing the relationship between the absorption coefficient of pig skin and the number of stripping after applying the two-interface model, where (a) shows the measurement results at 0.25 THz, (b) shows the results at 0.5 THz. (C) is a figure which shows a measurement result in 1.0 THz. FIG. 12 is an example of a diagram showing the complex refractive index of the porcine stratum corneum determined using the two-interface model, where (a) shows its real part and (b) shows its imaginary part. FIG. 13 is a diagram showing changes in the absorption coefficient before and after the application of the two-interface model, and shows the change rate with respect to before application. (A) shows the measurement result at 0.25 THz, (b) shows the measurement result at 0.5 THz, and (c) shows the measurement result at 1.0 THz. FIG. 14 is a diagram showing an absorption spectrum of an aqueous albumin solution. FIG. 15 is a graph showing the relationship between the mass moisture content of the albumin aqueous solution and the absorption coefficient. (A) shows the measurement result at 0.25 THz, (b) shows the measurement result at 0.5 THz, and (c) shows the measurement result at 1.0 THz. FIG. 16 is a diagram showing the relationship (0.5 THz) between the mass moisture content of pig skin and the number of stripping when an aqueous albumin solution is used as a stratum corneum model.

  The measurement method of the present invention is a method for measuring the amount of moisture in the skin stratum corneum by total reflection attenuation spectroscopy using terahertz waves, and irradiates terahertz waves by bringing the skin surface into contact with the prism surface that is the terahertz wave emission surface. And obtaining a skin absorption coefficient.

FIG. 1 is a schematic diagram showing the measurement concept of the present invention. In total reflection attenuation spectroscopy, a terahertz wave is incident on the prism under the condition that it is totally reflected at the interface between the surface of the ATR prism (hereinafter referred to as “prism”) and the skin surface, and is reflected at the interface between the prism surface and the skin surface. The terahertz wave is observed by a detection device (not shown). In this case, the electric field strength E ~ _out of the reflected terahertz wave is represented by the product of the Fresnel reflection coefficient r ~ ₁₂ at the interface of the electric field strength E _in the prism and the skin surface of the incident terahertz wave as shown in Equation 1, analysis relationship holds and to the _{E ~ out = r ~ 12 ×} E in is performed. The Fresnel reflection coefficient r ~ ₁₂ is obtained by the following formula 2. Here, in Equation 2, n ₁ is the refractive index of the prism, n ~ ₂ is the complex refractive index of the skin, theta is the angle of incidence of the terahertz wave.

(Equation 1)
E ~ _out = r ~ ₁₂ × E _in

Here, in the case of a substance having absorption of terahertz waves, if the thickness of the sample on the prism surface is t and the absorption coefficient of the sample is α, the absorption intensity A (t) of the ATR spectrum observed from the reflected terahertz wave is As shown by the following equation 3, when the thickness t of the sample is sufficiently larger than the evanescent wave oozing depth d _p (t → ∞), the absorption intensity A (∞) of the ATR spectrum is expressed by the following equation 4. The absorption intensity A (∞) is proportional to the evanescent wave bleeding depth d _p . The evanescent wave oozing depth d _p is a depth at which the electric field intensity of the incident wave is attenuated to 1 / e as shown in Equation 5, the wavelength λ of the incident terahertz wave, the refractive index n _{1 of the} prism, It is uniquely determined by the refractive index n _{2 of the} sample and the incident angle θ with respect to the prism surface.

Further, since the absorption intensity A (t) of the ATR spectrum depends on the evanescent wave bleeding depth d _p , that is, the wavelength λ of the incident terahertz wave, the ATR correction depends on the wavelength λ by performing the process of dividing the ATR spectrum by the wavelength λ. The peak intensity is made uniform.

  On the other hand, the propagation of electromagnetic waves in a substance having absorption can be expressed using a complex refractive index n˜. The complex refractive index n˜ is expressed by Equation 6 using the extinction coefficient κ of the substance. It is known that the extinction coefficient κ and the absorption coefficient α of the substance have a proportional relationship shown in Expression 7. In Equation 6, n is the refractive index of the substance, and i is an imaginary unit.

That is, when the thickness t of the sample is sufficiently larger than the evanescent wave bleeding depth d _p , the ATR spectrum A (∞) is proportional to the absorption coefficient α (Equation 4), and the absorption coefficient α is extinguished. The relationship is proportional to the coefficient κ (Formula 7). Therefore, the absorption coefficient α can be obtained by directly measuring the absorption intensity A (t) of the ATR spectrum or by obtaining the extinction coefficient κ of the substance.

The ATR spectrum after reflection can be observed by terahertz time domain spectroscopy. In this method, a reflection spectrum and a phase difference spectrum are obtained by performing Fourier transform on the acquired time waveform of the terahertz wave. When the thickness t of the sample is sufficiently larger than the evanescent wave bleeding depth d _p , the reflection spectrum R (ω) and the phase difference spectrum Φ (ω) are expressed by the following equations 8 and 9, respectively. . Further, the Fresnel reflection coefficient r ~ _{12 at} the interface between the prism and the sample is expressed by Equation 2. Therefore, the complex refractive index of the sample is obtained by obtaining the solutions of the continuous cubic numbers of Equations 2, 8 and 9 from the reflection spectrum R (ω) and the phase difference spectrum Φ (ω) obtained by the terahertz time domain spectroscopy. n ₂ ~ is required. Note that r _ref in Equations 8 and 9 is the Fresnel reflection coefficient when the sample is not placed. The absorption coefficient α of the substance is obtained from the imaginary part (extinction coefficient κ) of the obtained complex refractive index n ₂ ~ and Equation 7. That is, the measurement method of the present invention includes a step of measuring a reflection spectrum and a phase difference spectrum obtained by irradiating a terahertz wave while bringing the skin into contact with a prism surface which is a terahertz wave emission surface, and the obtained reflection spectrum and phase difference. Calculating an absorption coefficient of the stratum corneum from the spectrum.

  In the measurement method of the present invention, since the skin is the measurement object, the absorption coefficient α obtained above represents the absorption coefficient of the skin. However, the terahertz wave is very much absorbed by water, and the skin tissue and hair. Therefore, it can be said that the absorption coefficient α obtained above represents absorption by moisture of the skin.

  In the present invention, the terahertz wave is emitted under the condition that the terahertz wave is totally reflected at the interface between the prism and the skin surface. Here, since the incident angle of the terahertz wave is an angle larger than 0 ° and smaller than 90 °, the total reflection condition is that the refractive index of the prism is at least equal to or more than the complex refractive index of the stratum corneum. It will be necessary. As described below, according to the actual measurement by the present inventor, it was found that the real part of the complex refractive index of the stratum corneum of pig skin is about 2.5 or less when irradiated with terahertz waves of 0.1 THz or more. This was confirmed by calculation using a so-called two-interface model described below in consideration of the reflection coefficient generated at the interface between the epidermis layer and the stratum corneum under the stratum corneum. That is, the complex refractive index of the stratum corneum can be calculated by applying the two-interface model using the complex refractive index of the skin layer and the thickness of the stratum corneum. In addition, the complex refractive index of the skin layer used for calculation is obtained by measuring the skin layer obtained by removing the stratum corneum by stripping, for example, by the ATR method. In addition, the thickness of the stratum corneum can be measured values, but the number of stripping may be used as the thickness, or an estimated value may be used.

  Pig skin is similar in composition and structure to human skin, and it can be said that the complex refractive index of the stratum corneum of human skin is almost the same as that of porcine skin. Accordingly, the refractive index of the prism varies depending on the frequency of the terahertz wave used for the measurement, but generally has a refractive index of at least 2.0 or more, preferably 2.5 or more, desirably 3.0 or more at a frequency lower than 1 THz. In addition, a prism having a refractive index of 2.0 or higher is generally used in a high frequency band higher than 1 THz. That is, a prism having a refractive index of 3.0 or higher can be measured using a terahertz wave of 0.1 THz or higher. If it is a prism which has such a refractive index, the material will not be ask | required but it selects suitably. For example, the material is silicon (n = 3.4) and may be MgO (n = 3.1). Also, the incident angle varies depending on the refractive index of the prism and the frequency of the terahertz wave, and is set to an angle that satisfies the total reflection condition.

The terahertz wave that can be irradiated in the present invention may be an electromagnetic wave in the terahertz wave region. The lower limit of the frequency band is, for example, 0.1 THz, 0.25 THz, 0.5 THz, and 1.0 THz. obtain. The upper limit is, for example, 10.0 THz, 5.0 THz, 3.0 THz, and 2.0 THz. As described below, the higher the frequency of the terahertz wave, the higher the water absorption coefficient α, so the sensitivity is improved. However, when the frequency is increased, the bleeding depth d _p becomes too small and the stratum corneum moisture is sufficient. Tend not to be reflected. On the other hand, the bleeding depth d _p of the evanescent wave is proportional to the wavelength λ of the terahertz wave and inversely proportional to the frequency as shown in Equation 5, so that the bleeding depth d _p is large on the low frequency side. Become. Therefore, on the low frequency side, absorption in the epidermis layer below the stratum corneum is the main component, and it cannot be said that the absorption coefficient α of the stratum corneum is reflected. From such a viewpoint, a terahertz wave of 0.1 THz or more and 3.0 THz is preferable.

  The obtained absorption coefficient α is a value reflecting the moisture content of the stratum corneum, and the relative moisture content of the stratum corneum can be known from the calculated absorption coefficient α. Since the absorption coefficient α is proportional to the molar concentration of the substance, the molar concentration of the substance can be obtained from the absorption coefficient α. That is, the mass water content (moisture content) of the stratum corneum is obtained from the obtained absorption coefficient α using the relationship between the known mass water content and the absorption coefficient α. When calculating | requiring the mass water content of a stratum corneum, it can obtain | require by contrast with the absorption coefficient (alpha) measured, for example using the stratum corneum model whose mass water content is known. An arbitrary model is used as the stratum corneum model. Since the stratum corneum is a layer of dead keratinocytes, for example, a sheet model obtained by compressing protein component powder collected from the skin and containing moisture, the stratum corneum is composed of protein and water. The model which consists of an albumin protein aqueous solution assumed to be comprised is illustrated.

On the low frequency side, the evanescent wave bleed depth d _p is larger than the stratum corneum thickness d, and is considered to be affected by moisture in the epidermis layer. Thus, by applying a two-surface model described in Non-Patent Document 8, seeking reflection coefficient r ~ ₁₂₃ corrected, it can also be determined absorption coefficient alpha. As shown in FIG. 2, the two-interface model changes the reflection coefficient r to ₁₂ at the interface between the prism and the thin layer sample and measures the thin layer sample and the laminated sample. This means a model that uses the entire reflection coefficient (corrected reflection coefficient) r to ₁₂₃ in consideration of the reflection coefficient r to ₂₃ at the interface with. In other words, the interface between the thin film sample and the laminated sample is treated as having an attenuation proportional to the Fresnel reflection coefficient r- ₂₃ at the interface, and the attenuation in the thin film sample is extracted and analyzed by removing this attenuation. Since the skin has a structure in which an epidermis layer is laminated on an extremely thin stratum corneum, it is considered that an interface exists between the stratum corneum and the epidermis layer. Therefore, considering the reflection coefficient r ~ ₂₃ at this interface, it is possible to reduce the error when the evanescent wave is oozing from the thickness of the stratum corneum.

The reflection coefficient r ~ _{23 at} the interface between the stratum corneum and the skin layer is obtained by Expression 10, and the corrected reflection coefficient r ~ ₁₂₃ is obtained by Expression 11 below. Further, the absorption coefficient α is substituted with the corrected reflection coefficient r to ₁₂₃ in place of the reflection coefficient r to _{12 in} the above Expressions 8 and 9, and the simultaneous expressions of Expression 2, Expression 10, Expression 11, Expression 8, and Expression 9 are substituted. It can be obtained by finding a number of solutions. The corrected complex index of refraction n ~ ₃ of thickness d and the skin layers of the stratum corneum which is required is a measured value, an estimated value, and the like literature values.

Moreover, two case of applying the interfacial model, since an error due to the correction occurs when bleeding out depth d _p of the evanescent wave is smaller than the thickness d of the stratum corneum, the depth d _p seeped evanescent wave angular A frequency band that is larger than the layer thickness d is used. For example, it may be a terahertz wave that is preferably 3 THz or lower, 2 THz or lower, more preferably 1.5 THz or lower, and desirably 1 THz or lower.

  As described above, by using the measurement method of the present invention, it is possible to easily measure the moisture content of an extremely thin stratum corneum such as several tens of μm without damaging the skin.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on the following Example, this invention is not limited by the following Example.

[Measurement of water absorption coefficient]
In measuring the moisture in the stratum corneum, first, the water absorption coefficient α was measured. The terahertz total reflection type attenuation spectrophotometer used for the measurement includes an Si (n = 3.42) prism as an ATR module, and an incident angle is 57 ° (the same applies hereinafter). Measurement was performed by dropping water on the prism surface. The reference was air. The results are shown in FIG.

Next, sucrose aqueous solutions having different moisture contents were measured, and the relationship between the absorption coefficient α and the mass moisture content (sucrose concentration) of the sucrose aqueous solution was examined. Measurement was performed by dropping 1 mL of a sucrose aqueous solution onto the prism surface. The results are shown in FIGS. From this result, it was confirmed that the mass water content of the aqueous sucrose solution and the absorption coefficient α were proportional (FIG. 5). The coefficient of determination ^{R 2} is for example 0.5THz at this time, and at each 0.99 or more 1.0THz band absorption coefficient in 0.5THz literature value (Keiichiro Shiraga et al., Food Chemistry, 140,315-320 , 2013).

[Water absorption coefficient in the stratum corneum model]
The water absorption coefficient contained in the stratum corneum model was measured using the stratum corneum model. A sheet obtained by pressing interstitial powder (hide powder non-chromated, manufactured by SIGMA) collected from the skin dermis was used as a stratum corneum model (sheet model). Applying a pressure of 294 Pa or 589 Pa to 0.12 g of powder for 2 seconds was repeated 3 times to produce a sheet. 200 to 500 μL of distilled water was dropped onto the sheet, and left in a container with a humidity of 100% for 48 hours, so that moisture was uniformly distributed in the sheet. The water content was calculated from the wet mass immediately after measurement and the dry mass after drying at 70 ° C. for 24 hours.

  The obtained sheet was placed on a prism, and a weight was placed on the sheet for measurement. The results are shown in FIGS. As shown in FIG. 7, the mass moisture content and the absorption coefficient are proportional, and it was shown that the mass moisture content can be obtained from the absorption coefficient.

[Measurement of water content of pig skin]
Using pig skin, the water content of the stratum corneum of the pig was measured, and then the change in water content due to stripping was measured. The frozen porcine skin (2 cm × 2 cm) was thawed at room temperature (about 20 ° C.) and then left to dry at room temperature (about 20 ° C.) for 48 hours with the side of the skin covered with an adhesive tape. Next, a paper waste (trade name “Kimwipe”, manufactured by Nippon Paper Claire Co., Ltd.) containing 2 mL of distilled water was placed on the dried skin for 150 seconds and left standing. Thereafter, the surface of the skin was lightly wiped with a paper waste cloth to obtain water-containing skin. Further, after the measurement, the measurement was carried out while removing the stratum corneum by stripping using a cellophane tape, and the measurement was repeated until the stripping was performed 25 times. In this stripping, it is considered that one stratum corneum (thickness of about 1 μm) is peeled off by one stripping, and the stratum corneum is almost removed by stripping 25 times.

The pig skin was placed on a prism, and the measurement was performed with a weight. The moisture content was calculated from the measured absorption coefficient using a graph showing the relationship between the absorption coefficient obtained in Example 2 and the mass moisture content. The results are shown in FIGS. As in the case of water or an aqueous sucrose solution, the skin absorption coefficient tended to increase on the high frequency side. As the stripping was repeated, the water content of the pig skin decreased temporarily, but then tended to increase. As a result of water absorption in the dried skin in a very short time, water absorption occurred in the outermost layer, but in the subsequent intermediate layer, water was not sufficiently absorbed and the moisture content was low, and the deep part of the skin In this case, it was considered that the moisture content remained high as a result of insufficient drying. Under this measurement condition, the evanescent wave oozing depth d _p is calculated to be about 70 to 10 μm, and it is considered that the moisture content of the stratum corneum of pig skin is measured. These measurement results not only reflect the water content of the pig skin, but also report that there is a water gradient in the stratum corneum (Wu J et al., Confocal Raman microspectroscopy of stratum corneum: apre-clinical validation study, International Journal of Cosmetic Science, 30, 1, 47-56, 2008 .: Egawa M et al., In vivo Estimation of Stratum Corneum Thickness from Water Concentration Profiles Obtained with Raman Spectroscopy, Acta Dermato Venereologica, 87, 4-8 , 2007), it can be said that the method according to the present invention is a valid method. As described above, in the present invention, it is possible to accurately grasp the amount of water or a change in the amount of water in an extremely thin thin film sample such as a stratum corneum.

[Measurement of water content of pig skin (horny layer) using two-interface model]
From the measurement data obtained in Example 3, the absorption coefficient and mass water content of the stratum corneum of pig skin were calculated using a two-interface model. The results are shown in FIGS. An example of a diagram showing the relationship between the refractive index n (real part) and extinction coefficient κ (imaginary part) of the complex refractive index n ~ ₂ of the stratum corneum derived to calculate the absorption coefficient of the stratum corneum and the frequency. Is shown in FIG. In the two-interface model, the stratum corneum thickness d of pig skin necessary for calculating the reflection coefficient r to ₁₂₃ was assumed to be 30 μm. As the complex refractive index n to ₃ of the skin layer, the complex refractive index obtained by stripping 25 times in Example 3 was used.

As shown in FIG. 12, the real part of the complex refractive index n- ₂ of the stratum corneum was measured to be 2.5 or less. The relationship between the absorption coefficient of the skin and the number of stripping and the absorption coefficient showed the same tendency as in Example 3. On the other hand, when the presence or absence of the application of the two-interface model is examined, as shown in FIG. 13, when the two-interface model is applied, the skin absorption coefficient tends to be smaller as a whole, compared to the high frequency side on the low frequency side Became smaller. It can be said that the evanescent wave oozes deeper on the low frequency side, and the effect of the epidermis is eliminated. Further, the change in the absorption coefficient depending on the presence or absence of application was smaller on the high frequency side than on the low frequency side.

[Chorn layer model using water-soluble albumin solution]
There was a concern that the stratum corneum model used in Example 2 could not measure a moisture content with a mass moisture content of 30% or less due to the presence of voids. Therefore, the same measurement was performed using an aqueous solution of water-soluble albumin having a known mass concentration (w / w%) as a model without voids. FIG. 14 shows the relationship between the absorption coefficient and the frequency. FIG. 15 shows the relationship between the absorption coefficient and the mass moisture content. Further, FIG. 16 shows the relationship between the mass moisture content obtained from the measurement data obtained in Example 3 using a two-interface model and the number of stripping. The relationship between the mass moisture content and the absorption coefficient in FIG. In Equation 12, M _W is the mass of water, M _D is the mass of albumin, V is the volume of the aqueous solution, and W _M is the mass concentration of water. In the terahertz band, the absorption coefficient is considered to have a linear relationship with the molar concentration of water molecules, but in an albumin aqueous solution, not only the mass of water molecules but also the mass of albumin should be considered in calculating the mass water content. The mass moisture content is not proportional to the molar concentration. On the other hand, the decrease in the absorption coefficient of water when albumin concentration increases (mass moisture content decreases) is considered to be proportional to the increase in albumin (absorption is negligibly small), so albumin concentration and water absorption It is considered that the coefficient also has a linear relationship. If it does so, the density | concentration of water and albumin density | concentration can be represented as Numerical formula 13 and Numerical formula 14, respectively, using A, B, C, and D as constants. Accordingly, A, B, C, and D are obtained by approximating the measured absorption coefficient and the mass concentration of water, and the measured absorption coefficient and the albumin mass concentration by linear functions, respectively, and the relationship shown in FIG. )

  By using the albumin aqueous solution as a stratum corneum model, the effect of voids was reduced, and the water content of the stratum corneum could be measured even when the mass moisture content was 30% or less. Moreover, when the relationship between the mass water content and the number of stripping was determined from this calibration curve, the same tendency as in Example 4 was observed.

  The present invention provides a new measurement method for measuring the moisture content of the stratum corneum.

Claims (5)

A method of measuring the skin stratum corneum water content by attenuated total reflection spectroscopy using a terahertz wave, the step of the prism surface is a terahertz wave emitting surface in contact with the skin surface obtains the absorption coefficient is irradiated with the terahertz wave Have
The refractive index of the prism is larger than the complex refractive index of the stratum corneum calculated by total reflection attenuation spectroscopy using terahertz waves, and the complex refractive index of the stratum corneum is reflected at the interface between the dermis layer and the stratum corneum. A method for measuring skin stratum corneum moisture content, which is calculated by applying a two-interface model considering a coefficient .   The measurement method according to claim 1, wherein a refractive index of the prism is 2.0 or more.   The measurement method according to claim 1, wherein a refractive index of the prism is 3.0 or more. The method for measuring the skin stratum corneum moisture content according to any one of claims 1 to 3 , wherein a terahertz wave having a frequency of 0.1 THz or more and 3.0 THz or less is irradiated. 5. The method according to claim 1 , further comprising a step of obtaining a water content of the stratum corneum from a comparison between the obtained absorption coefficient and an absorption coefficient measured using a skin model having a known water content. The measurement method described.