Development of a method for the determination of human skin moisture using a portable near-infrared system

Measurement of human skin moisture via high-frequency spectroscopy

High-frequency spectroscopy (HFS) is an analytical method that is sensitive to slight changes in the dielectric properties of materials. Since water has high permittivity, HFS can be used to detect changes in water content in materials. In this study, we employed HFS to measure human skin moisture during a water sorption-desorption test. Skin without any treatment showed a resonance peak at approximately 1150 MHz. Furthermore, the peak shifted to lower frequency immediately after the application of water to the skin and gradually returned to its original frequency as time progressed. The resonance frequency obtained via least-squares fitting showed that the applied water remained in the skin after 240 s from the beginning of the measurement. These results illustrated that HFS measurements can monitor the progression of decreasing moisture content in human skin during a water sorption-desorption test.

Design and Analysis of a Continuous and Non-Invasive Multi-Wavelength Optical Sensor for Measurement of Dermal Water Content

Dermal water content is an important biophysical parameter in preserving skin integrity and preventing skin damage. Traditional electrical-based and open-chamber evaporimeters have several well-known limitations. In particular, such devices are costly, sizeable, and only provide arbitrary outputs. They also do not permit continuous and non-invasive monitoring of dermal water content, which can be beneficial for various consumer, clinical, and cosmetic purposes. We report here on the design and development of a digital multi-wavelength optical sensor that performs continuous and non-invasive measurement of dermal water content. In silico investigation on porcine skin was carried out using the Monte Carlo modeling strategy to evaluate the feasibility and characterize the sensor. Subsequently, an in vitro experiment was carried out to evaluate the performance of the sensor and benchmark its accuracy against a high-end, broad band spectrophotometer. Reference measurements were made against gravimetric analysis. The results demonstrate that the developed sensor can deliver accurate, continuous, and non-invasive measurement of skin hydration through measurement of dermal water content. Remarkably, the novel design of the sensor exceeded the performance of the high-end spectrophotometer due to the important denoising effects of temporal averaging. The authors believe, in addition to wellbeing and skin health monitoring, the designed sensor can particularly facilitate disease management in patients presenting diabetes mellitus, hypothyroidism, malnutrition, and atopic dermatitis.

Measurement of dermal water content using a multi-wavelength optical sensor

Skin hydration is crucial for overall skin health. Maintaining skin hydration levels preserves skin integrity and prevents tissue damage which can lead to several debilitating conditions. Moreover, continuous monitoring of skin hydration can contribute to the diagnosis or management of serious diseases. For instance, sugar imbalance in diabetes mellitus and kidney disease can lead to the loss of bodily fluids and cause dry skin. Therefore, continuous, accurate and non-intrusive monitoring of skin hydration would present a remarkable opportunity for maintaining overall health and wellbeing. There are various techniques to assess skin hydration. Electrical based Corneometers are currently the gold standard in clinical and non-clinical practice. However, these techniques have a number of limitations. In particular, they are costly, sizeable, intrusive, and operator dependent. Recent research has demonstrated that near infrared spectroscopy could be used as a non-intrusive alternative for the measurement of skin water content. The present paper reports the development and in-vitro validation of a noninvasive, portable, skin hydration sensor. The results indicate that the developed sensor can deliver reliable measurements of skin water content.

Collagen Hydrolysates for Skin Protection: Oral Administration and Topical Formulation

Antioxidants are molecules that delay or inhibit the oxidation of other molecules. Its use significantly increased in recent years in the diet of people. Natural antioxidants are replacing the use of synthetic antioxidant ingredients due to their safety, nutritional, and therapeutic values. Hydrolyzed collagen (HC) is a popular ingredient considered to be an antioxidant. This low molecular weight protein has been widely utilized due to its excellent biocompatibility, easy biodegradability, and weak antigenicity. It is a safe cosmetic biomaterial with good moisturizing properties on the skin. The antioxidant properties of HC are conditioned to the size of the molecule: the lower the molecular weight of peptides, the greater the ability to donate an electron or hydrogen to stabilize radicals. The antioxidant capacity of HC is mostly due to the presence of hydrophobic amino acids in the peptide. The exact mechanism of peptides acting as antioxidants is not clearly known but some aromatic amino acids and histidine are reported to play an important role in the antioxidant activity. Oral ingestion of HC increases the levels of collagen-derived peptides in the blood torrent and improves the skin properties such as elasticity, skin moisture, and transepidermal water loss. Additionally, daily intakes of HC protect the skin against UV melasma, enhances the fibroblast production and extracellular matrix of the skin. HC has been identified as a safe cosmetic ingredient for topical formulations with good moisturizing properties at the stratum corneum layer of the skin. It reduces the effects of skin aging (dryness, laxity, and wrinkles). The use of HC as a principal ingredient in safe formulations for skin protection was reviewed and compared when it is used by topical and/or oral administration.

Review of Modern Techniques for the Assessment of Skin Hydration

Skin hydration is a complex process that influences the physical and mechanical properties of skin. Various technologies have emerged over the years to assess this parameter, with the current standard being electrical probe-based instruments. Nevertheless, their inability to provide detailed information has prompted the use of sophisticated spectroscopic and imaging methodologies, which are capable of in-depth skin analysis that includes structural and composition details. Modern imaging and spectroscopic techniques have transformed skin research in the dermatological and cosmetics disciplines, and are now commonly employed in conjunction with traditional methods for comprehensive assessment of both healthy and pathological skin. This article reviews current techniques employed in measuring skin hydration, and gives an account on their principle of operation and applications in skin-related research.

Engineering Approaches to Assessing Hydration Status

Dehydration is a common condition characterized by a decrease in total body water. Acute dehydration can cause physical and cognitive impairment, heat stroke and exhaustion, and, if severe and uncorrected, even death. The health effects of chronic mild dehydration are less well studied with urolithiasis (kidney stones) the only condition consistently associated with it. Aside from infants and those with particular medical conditions, athletes, military personnel, manual workers, and older adults are at particular risk of dehydration due to their physical activity, environmental exposure, and/or challenges in maintaining fluid homeostasis. This review describes the different approaches that have been explored for hydration assessment in adults. These include clinical indicators perceived by the patient or detected by a practitioner and routine laboratory analyses of blood and urine. These techniques have variable accuracy and practicality outside of controlled environments, creating a need for simple, portable, and rapid hydration monitoring devices. We review the wide array of devices proposed for hydration assessment based on optical, electromagnetic, chemical, and acoustical properties of tissue and bodily fluids. However, none of these approaches has yet emerged as a reliable indicator in diverse populations across various settings, motivating efforts to develop new methods of hydration assessment.

Ex vivo optical measurements of glucose diffusion kinetics in native and diabetic mouse skin

The aim of this study was to estimate the glucose diffusion coefficients ex vivo in skin of mice with diabetes induced in vivo by alloxan in comparison to non-diabetic mice. The temporal dependences of collimated transmittance of tissue samples immersed in glucose solutions were measured in the VIS-NIR spectral range to quantify the glucose diffusion/permeability coefficients and optical clearing efficiency of mouse skin. The average thickness of intact healthy and diabetic skin was 0.023 ± 0.006 cm and 0.019 ± 0.005 cm, respectively. Considerable differences in optical and kinetic properties of diabetic and non-diabetic skin were found: clearing efficiency was 1.5-fold better and glucose diffusivity was 2-fold slower for diabetic skin. Experimental Setup for measuring collimated transmittance spectra of mouse skin samples.

Comparing the rates of absorption and weight loss during a desorption test using near infrared spectroscopy

The importance of determining skin hydration has over the years prompt the development of many instruments and methods, specifically designed to assess this parameter or water contents especially in the stratum corneum, and have greatly matured to suit different anatomical sites and measure multiple attributes. Of those, Near Infrared Spectroscopy (NIRS) has gained wide interest as a precise, safe, fast and noninvasive technique for determining skin hydration due to its high sensitivity to hydrogen bonding and ability to measure the amount of water in skin directly using the intensities of overtone and combination bands of OH and HOH water bonds occurring in the NIR region, that are good indicators of the state of skin hydration. This paper reports near infrared spectrophotometric measurements using a highly sophisticated spectrophotometer in the region of 1000-2500 nm to study the water uptake and dehydration properties of skin in vitro using samples of porcine skin. Initial results of pure liquid water and skin samples have clearly displayed the prominent bands associated with water content, and desorption tests have been able to verify changes in these bands associated with water content, although a clear correlation between the rates of weight loss and absorbance loss at various hydration periods has not yet been established. These preliminary results are expected to further explain the relationship between water and skin, and its role within, in hope to aid the future development of a portable instrument based on near infrared spectroscopy that would be capable of directly measuring skin hydration and/or water content in a fast and noninvasive manner.

In vivo optical investigation of short term skin water contact and moisturizer application using NIR spectroscopy

Nowadays, a number of noninvasive methods and instruments are available to inspect the biophysical properties and effects of various applicants on human skin, providing quantitative measurements and more details regarding the interactions between skin and various products. Such methods include Near Infrared Spectroscopy (NIRS), a technique which over the years, has gained quite a reputation in being able to accurately determine moisture levels and water contents due to its sensitivity to hydrogen bonding. This paper reports preliminary results of an in vivo study carried out on the skin of a small number of human participants, investigating the optical response of human skin after direct short-term contact with water followed by application of a moisturizer, using a highly advanced spectrophotometer in the region of 900-2100 nm, and equipped with a reflectance fibre optic probe. Results obtained here certainly raise some questions regarding the optical characteristics of different skin types and the influence of frequent moisturizer use, as well as the varying response between different water bands in the NIR region. Future work will focus on gaining more knowledge about these, in order to further improve optical skin measurements, and hopefully support the design and development of a portable and/or miniaturized optical device that could provide reliable, accurate and fast skin hydration readings in real time.

In vitro spectrophotometric near infrared measurements of skin absorption and dehydration

The application of Near Infrared Spectroscopy(NIRS) for measuring skin water content has long been established, and has gained w ide interest as a precise, safe, fast and noninvasive technique for determining skin hydration.This paper reports near infrared spectrophotometric measurements using a highly sophisticated spectrophotometer in the region of 1000-2500 nm to study the water uptake and dehydration properties of skin in vitro using samples of porcine skin. Initial results of pure liquid water and skin samples have clearly displayed the prominent bands associated with water content, and desorption tests have been able to verify changes in these bands associated with water content, although a clear correlation between the rates of weight loss and absorbance loss at various hydration periods has not yet been established. These preliminary results are expected to further explain the relationship between water and skin, and its role within, in hope to aid the future development of a portable instrument based on near infrared spectroscopy that would be capable of directly measuring skin hydration and/or water content in a fast and noninvasive manner.

Quantitative analysis of dehydration in porcine skin for assessing mechanism of optical clearing

Dehydration induced by optical clearing agents (OCAs) can improve tissue optical transmittance; however, current studies merely gave some qualitative descriptions. We develop a model to quantitatively evaluate water content with partial least-squares method based on the measurements of near-infrared reflectance spectroscopy and weight of porcine skin. Furthermore, a commercial spectrometer with an integrating sphere is used to measure the transmittance and reflectance of skin after treatment with different OCAs, and then the water content and optical properties of sample are calculated, respectively. The results show that both the reduced scattering coefficient and dehydration of skin decrease with prolongation of action of OCAs, but the relative change in former is larger than that in latter after a 60-min treatment. The absorption coefficient at 1450 nm decreases completely coincident with dehydration of skin. Further analysis illustrates that the correlation coefficient between the relative changes in the reduced scattering coefficient and dehydration is ∼1 during the 60-min treatment of agents, but there is an extremely significant difference between the two parameters for some OCAs with more hydroxyl groups, especially, glycerol or D-sorbitol, which means that the dehydration is a main mechanism of skin optical clearing, but not the only mechanism.

Nondestructive assessment of engineered cartilage constructs using near-infrared spectroscopy

Noninvasive assessment of engineered cartilage properties would enable better control of the developing tissue towards the desired structural and compositional endpoints through optimization of the biochemical environment in real time. The objective of this study is to assess the matrix constituents of cartilage using near-infrared spectroscopy (NIRS), a technique that permits full-depth assessment of developing engineered tissue constructs. Mid-infrared (mid-IR) and NIR data were acquired from full-thickness cartilage constructs that were grown up to 4 weeks with and without mechanical stimulation. Correlations were assessed between established mid-IR peak areas that reflect the relative amount of collagen (amide I, amide II, and 1338 cm(-1)) and proteoglycan (PG), (850 cm(-1)), and the integrated area of the NIR water absorbance at 5190 cm(-1). This analysis was performed to evaluate whether simple assessment of the NIR water absorbance could yield information about matrix development. It was found that an increase in the mid-IR PG absorbance at 850 cm(-1) correlated with the area of the NIR water peak (Spearman's rho = 0.95, p < 0.0001). In the second analysis, a partial least squares method (PLS1) was used to assess whether an extended NIR spectral range (5400-3800 cm(-1)) could be utilized to predict collagen and proteoglycan content of the constructs based on mid-IR absorbances. A subset of spectra was randomly selected as an independent prediction set in this analysis. Average of the normalized root mean square errors of prediction of first-derivative NIR spectral models were 7% for 850 cm(-1) (PG), 11% for 1338 cm(-1) (collagen), 8% for amide II (collagen), and 8% for amide I (collagen). These results demonstrate the ability of NIRS to monitor macromolecular content of cartilage constructs and is the first step towards employing NIR to assess engineered cartilage in situ.

Visualization technique for water content distribution of skin tissue by dualband polarization imaging

The advanced imaging technique for visualizing two-dimensional water content distribution in the stratum corneum of human skin is proposed. The method involves two elemental principles of spectral filtering and imaging with polarization control. It is found in the in vivo experiment in the visible and the near-infrared range that the reflectance spectra of skin tissue in the wavelength range shorter than 600 nm are affected largely by water content. The polarization imaging technique is, on the other hand, introduced for emphasizing the subsurface reflection from skin tissue. The superficial reflection can be separated from the deeply penetrated light by controlling the orientations of the polarizer and the analyzing polarizer put in front of the light source and the imaging device, respectively. The combination of spectral bandpass filtering and polarization imaging enables us to acquire the water content distribution of the stratum corneum.

The use of near-infrared spectroscopy in skin care applications

BACKGROUND

Near-infrared (NIR) spectroscopy was used to document the skin water content as a function of product usage and changes in the % relative humidity (%RH) in vitro and in vivo. The objective of the investigation was to determine if the NIR could provide comparable skin water content information as that obtained using gravimetric, conductivity, or visual assessment methods without having to invoke complex chemometric calculations.

METHODS

NIR data were obtained using an NIR5000 spectrophotometer with a fiber optic probe (Smartprobe) attachment to complete the clinical studies and a Direct Contact Analyzer module to carry out the in vitro experiments. Conductivity measurements were completed using the Skicon 200, which measures conductance at a fixed frequency of 3.5 MHz. Three moisturization-based clinical studies were carried out assessing the NIR's ability to detect skin hydration changes. In Clinical Study A, NIR and Skicon data were collected for panelists who had only washed their outer calf with water over a 4-week period. During this time, the daily average %RH was recorded. In Clinical Study B, 10% solutions of glycerin, choline chloride, and the sodium salt of pyrrolidone carboxylic acid were applied to the panelist's outer calf and biophysical measurements were completed to assess the hydration and desorption properties of these humectants. In Clinical Study C, a 10% solution of choline chloride was applied to panelist's outer calf and the cumulative effect of using this product was evaluated over a 3-week period. For all in vitro studies, porcine skin was used as the substrate.

RESULTS

Comparable NIR, Skicon, and visual dryness results were obtained for most of the product usage-based clinical experiments completed. However, the NIR was particularly more effective at detecting skin water content differences as a function of %RH changes. In the absence of abrupt (>50%) relative humidity variations, there was a direct correlation between the NIR readings and the %RH (R(2)=0.83) unlike what was observed for the Skicon measurements (R(2)=0.22).

CONCLUSIONS

NIR spectroscopy demonstrated the changes in the skin water content as a function of product usage; the results were consistent with those obtained using the Skicon conductivity meter and visual dryness assessment scores. More importantly, the differences detected were obtained without having to use chemometric manipulations in the data analysis as is the common practice. Of all the methods used, the NIR gave the best linear regression for %RH-induced skin water content changes.

In vivo measurement of water content of the fingernail and its seasonal change

BACKGROUND/AIMS

Recently, there has been increasing awareness regardless keeping nails in good condition. Maintenance of an optimum water level in the nail plate is very important since water content affects the nail condition. However, a simple method for in vivo measurement of water in the nail plate quantitatively has not been reported. In this study, we applied Near infrared spectroscopy (NIR)-based prediction of water content of the cut nail plate to in vivo measurement to obtain information about the seasonal change of water in the fingernail.

METHODS

Diffused reflectance spectra of the nail plate of the fingernail were collected in the NIR region with an NIR spectrometer in vivo both in summer and winter from the same 15 Japanese female subjects. The water content of the nail plate in vivo was calculated from the NIR spectrum using partial least square regression analysis.

RESULTS

It was possible to apply NIR-based prediction of water content of the cut nail plate to in vivo measurement. We also found that the water content of the nail plate varied from person to person but not with the finger of the individual. The average water content of the nail plate was significantly lower in winter than in summer (P<0.05). In summer, the water content in the double-layered nail tended to be higher than a nail in good condition (P<0.1) and significantly lower in the split nail (P<0.05). There was no significant difference between the groups in winter. On the contrary, the amount of water absorption tended to vary depending on the nail condition. A large amount of water was absorbed in the double-layered nail and a small amount in the split nail both in summer and winter.

CONCLUSION

The water content of the nail plate of fingernail was predicted by in vivo measurement using NIR spectrometry. Physiological information of the nail in vivo, such as aging and other disorders could be obtained by this method.

Regional difference of water content in human skin studied by diffuse-reflectance near-infrared spectroscopy: consideration of measurement depth

Diffuse reflectance (DF) spectra in the 1250-2500 nm region were measured in vivo for the skin of the forehead, cheek, jaw, elbow, volar forearm, palm, knee, and heel of seven healthy volunteers, using a Fourier transform near-infrared (FT-NIR) spectrophotometer with a fiber-optic probe. Apparent regional differences of water content in the skin, as estimated from the diffuse reflectance NIR spectra, are discussed in relation to the influence of measurement depth. The NIR spectra were collected with or without a 300 microm gap between the fiber-optic probe and the skin surface. For comparison, in vitro NIR spectra of stratum corneum sheets equilibrated at 41, 50, 63, and 81% relative humidity, at 25 degrees C, were also obtained. There was a difference in the ratio of the two water bands centered near 1450 nm and 1900 nm between the contact and non-contact measurements. In addition, regional differences of water content calculated from the peak height of the 1900 nm water band, which was normalized to the peak height of the 2175 nm amide band, were compared. The results of Monte Carlo simulation indicated that the apparent regional differences arise at least in part from differences in the measurement depth due to differences in specular reflection at the skin surface and in the thickness of the stratum corneum.

Depth profile of diffuse reflectance near-infrared spectroscopy for measurement of water content in skin

BACKGROUND/PURPOSE

The penetration depth of light in diffuse reflectance near-infrared spectroscopy for measuring water content in skin is assessed both from theoretical and experimental points of view.

METHODS

The Monte Carlo simulation was implemented to investigate the dependencies of the light penetration depth on a source-detector distance. To compare with the simulation results, an in vivo experiment for water contents of skin was performed introducing two different optical fiber probes.

RESULTS

It is found that the minimum separation between a source and detector fibers influences largely the measurement depth. The larger separation leads to a deeper measurement depth at a particular wavelength. The measurement depth is also influenced fairly by the absorption coefficient of the tissue. The larger absorption coefficient results in a shallower measurement depth.

CONCLUSION

The correlations between the water contents measured by the optical and capacitance techniques were discussed. The dependencies of the light penetration depth on the source-detector geometry and wavelength are presented.

Non-contact skin moisture measurement based on near-infrared spectroscopy

Non-contact skin moisture measurement based on near-infrared (NIR) spectroscopy is proposed in the spectral range from 1300 to 2000 nm. A gap is introduced between the optical fiber probe and the skin surface in order to avoid occluding surface vapor. In vitro and in vivo experiments for measuring the water content of skin are implemented. The measured absorbance spectra are processed by multivariate analyses. Processed results are compared with the water content values obtained by a capacitance method. The correlations between the optical method and the capacitance method obtained by partial-least squares regression are higher than those obtained by multiple linear regression. In addition, a Monte Carlo simulation is implemented to evaluate measurement depths of the optical methods. It is presented that the measurement depth of the optical method depends largely on the water absorption. The simulation result also shows that the measurement depth of the optical method is much deeper than the depth of the capacitance method, especially in the spectral range where water absorption is relatively weak.

Mid-infrared in vivo depth-profiling of topical chemicals on skin

BACKGROUND/PURPOSE

Thermal emission decay-Fourier transform infrared (TED-FTIR) spectroscopy is a non-contact and non-destructive analytical technique and was used in this study to detect the presence of external chemicals on human skin in vivo. The detection was possible due to the ability of the TED-FTIR technique to acquire the mid-infrared spectrum of the outmost layers (less than 10 microm) of Stratum Corneum (SC) and the ability to identify the absorption bands of the chemical.

METHODS

As an illustration of such measurements, propylene glycol (PG) was applied on human stratum corneum and depth-resolved TED-FTIR spectra of the SC were measured to quantify the concentration of PG in deeper layers of SC.

RESULTS

The mid-infrared spectrum of the surface 0.7 microm layer of skin had 50% contribution from SC and 50% from PG. At 3 h after application, the contribution of PG at the surface decreased to 7% as PG molecules diffused deeper into the skin and were lost at the surface. At a depth of 6 microm, the maximum concentration was 20% after 25 min after PG application.

CONCLUSIONS

This work shows the feasibility of the TED-FTIR technique to detect the presence of chemicals on human SC in vivo and without contact, and for a wide range of other applications, such as detection of toxic chemicals used as warfare (vesicant agents like sulphur mustard and organophosphate nerve agents), pesticides, and other toxins on fruit and vegetable skins, water, or even other contaminated surfaces.

Determining water content in human nails with a portable near-infrared spectrometer

The water content of human nail plates was determined using a portable near-infrared (NIR) spectrometer with an InGaAs photodiode array detector. NIR diffuse reflectance (DR) spectra were collected from 108 cut nail plates with different relative humidity and in vivo from fingernails. Partial least-squares (PLS) regression was applied to the NIR spectra in the 1115-1645 nm region to develop calibration models that determine the water content in the cut nail plates and fingernails. A good correlation was obtained between the NIR spectra and the water content measured by nuclear magnetic resonance (NMR) for the NIR measurement of both cut nail plates and fingernails. The results indicate that the water content in the nails can be determined very rapidly (1 s) by means of the portable NIR spectrometer and PLS regression.