Skin is one of the most significant parts of the human body. It connects us with the environment and has a vast number of functions, among which defensive function is of a high importance. Skin structure and its layers may vary with a number of factors such as sight, age, sex, race and the overall health state of the individuals. The latter affects skin water to lipids ratio and their depth profile in the skin. Smaller changes in the water to lipids ratio may result in skin type variations. In both cases, skin appearance will change along with variations of skin conditions. Given the great importance of the state of the skin, a number of methods and devices for measuring water and lipids content were developed over the years. The research presented in this thesis proposes methods to achieve simultaneous measurements of water and lipids content of the skin and their ratio. We also analysed the impact of these measurements on determining the skin condition. Skin appearance is also addressed through measurement of the skin gloss, using several methods such as the ratio of specular to diffuse component of the image, the slope of the gradient intensity of the image from specular to the diffuse component, and an approached based on number of weighted pixels. The method proposed for simultaneous water and lipids content measurement is described in the Chapter 2, and is based on light measurements, comprising 3 wavelengths that are sensitive to primarily lipids, primarily water and equally sensitive to both, these wavelengths are: 1720 nm, 1770 nm, and 1750 nm, respectively. We benchmarked our measurement with those obtained with a corneomenter and sebumeter – benchmark devices, on induced skin conditions corresponding to combinations of high, low and neutral levels of water and lipids content in the skin. The study showed good agreement. The state of the protective function of stratum corneum (SC) and distribution as function of depth of skin lipids and water are addressed by means of short wave infrared spectroscopy. The method does not give information as a function of depth. This obstacle was overcome by tape stripping of one SC layer at a time. Comparative measurement was performed with Raman confocal microscopy and is described in the Chapter 3. Our proposed method showed similar pattern of the depth profile for water as obtained with the corneometer and with Raman confocal microscopy, while trans epidermal water loss measurement indicated the point of the barrier breaking point. Lipids measurements obtained with our method also showed similar trends as Raman confocal microscopy. As expected, water concentration increased and lipids concentration decreased with increasing depth into the stratum corneum. Additionally, a low-cost method for quantifying skin appearance by measuring skin gloss is proposed in Chapter 4. The method has proven to be reliable for skin gloss measurements via comparison with benchmark devices, and it also shows a great potential for other gloss measurements in a wide range, i.e., from an almost absolutely matte surface to a mirror like one. The proposed method comprises surface imaging by hand-held low-cost camera with ring-illumination along with image post processing based on weighting specular and diffuse components of the image. A gloss value is assigned as the result of the processing. Looking ahead, we discuss in Chapter 5 how the methods developed in this thesis could potentially be combined in one hand-held device. There will be several challenges such as the presence of other chromophores in the skin along with the low absorption coefficient of water and lipids in the spectral region suitable for the camera. The abovementioned obstacles can be solved by measuring absorption and scattering coefficients separately by means of illumination with spatial frequency modulation. The presence of several chromophores will as well require separating their impact on the absorption coefficient, potentially using more extensive data processing algorithms than those used in this research. @en

We describe non-invasive, shortwave-infrared spectroscopy for quantitative profiling of stratum corneum barrier function and wide field reflectance imaging for measurement of skin gloss.

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We demonstrate the feasibility of short wave infrared (SWIR) spectroscopy combined with tape stripping for depth profiling of lipids and water in the stratum corneum of human skin. The proposed spectroscopic technique relies on differential detection at three wavelengths of 1720, 1750, and 1770 nm, with varying ratio of the lipid-to-water absorption coefficient and an ‘isosbestic point’. Comparison of the data acquired using SWIR spectroscopy with that obtained by a gold standard for non-invasive quantitative molecularspecific skin measurements, namely confocal Raman spectroscopy (CRS), revealed specificity of the proposed modality for water and lipid quantification. At the same time, we provide evidence showing aberrant sensitivity of Corneometer hydration read-outs to the presence of skin surface lipids, and a lack of sensitivity of the Sebumeter when attempting to measure the lipids of the cornified lipid envelope and intracellular lipid layers. We conclude that a spectroscopic SWIR-based spectroscopic method combined with tape stripping has the potential for depth profiling of the stratum corneum water and lipids, due to superior measurement sensitivity and specificity compared to the Corneometer and Sebumeter.

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We report a low-cost optical method with high sensitivity for the quantitative assessment of the gloss of human skin in the low gloss regime relevant for physiological skin gloss conditions. Using Monte Carlo simulations, experiments on gloss calibration standards and in-vivo skin gloss experiments using an optical prototype, we demonstrate the improved sensitivity of the proposed method in the low gloss regime compared to professional industrial and skin gloss measurement devices.

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We show the feasibility of short wave infrared spectroscopy combined with tape stripping as a simple and noninvasive method for the analysis of lipids and the degree of hydration as a function of depth in the stratum corneum. The spectroscopic method utilizes differential detection with three wavelengths 1720, 1750, and 1770 nm, corresponding to the lipid vibrational bands that lay "in between" the prominent water absorption bands. The results are compared with other biophysical devices such as Corneometer and Sebumeter.@en

We demonstrate a low-cost optical method for measuring the gloss properties with improved sensitivity in the low gloss regime, relevant for skin gloss properties. The gloss estimation method is based on, on the one hand, the slope of the intensity gradient in the transition regime between specular and diffuse reflection and on the other on the sum over the intensities of pixels above threshold, derived from a camera image obtained using unpolarized white light illumination. We demonstrate the improved sensitivity of the two proposed methods using Monte Carlo simulations and experiments performed on ISO gloss calibration standards with an optical prototype. The performance and linearity of the method was compared with different professional gloss measurement devices based on the ratio of specular to diffuse intensity. We demonstrate the feasibility for in-vivo skin gloss measurements by quantifying the temporal evolution of skin gloss after application of standard paraffin cream bases on skin. The presented method opens new possibilities in the fields of cosmetology and dermatopharmacology for measuring the skin gloss and resorption kinetics and the pharmacodynamics of various external agents.

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Skin barrier function relies on well balanced water and lipid system of stratum corneum. Optimal hydration and oiliness levels are indicators of skin health and integrity. We demonstrate an accurate and sensitive depth profiling of stratum corneum sebum and hydration levels using short wave infrared spectroscopy in the spectral range around 1720 nm. We demonstrate that short wave infrared spectroscopic technique combined with tape stripping can provide morequantitative and more reliable skin barrier function information in the low hydration regime, compared to conventional biophysical methods.

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Skin health is characterized by heterogeneous system of water and lipids in upper layers providing protection from external environment and preventing loss of vital components of the body. Skin hydration (moisture) and sebum (skin surface lipids) are considered to be important factors in skin health; a right balance between these components is an indication of healthy skin and plays a central role in protecting and preserving skin integrity. In this manuscript we present inter-and intra-individual variation in skin hydration and surface lipids measured with a home-built experimental prototype based on infrared spectroscopy. Results show good agreement with measurements performed by commercially available instruments Corneometer and Sebumeter used for skin hydration and sebum measurements respectively.

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Skin health characterized by a system of water and lipids in Stratum Corneum provide protection from harmful external elements and prevent trans-epidermal water loss. Skin hydration (moisture) and sebum (skin surface lipids) are considered to be important factors in skin health; a right balance between these components is an indication of skin health and plays a central role in protecting and preserving skin integrity. In this manuscript we present an infrared spectroscopic method for simultaneous and quantitative measurement of skin hydration and sebum levels utilizing differential detection with three wavelengths 1720, 1750, and 1770 nm, corresponding to the lipid vibrational bands that lie "in between" the prominent water absorption bands. The skin sebum and hydration values on the forehead under natural conditions and its variations to external stimuli were measured using our experimental set-up. The experimental results obtained with the optical set-up show good correlation with the results obtained with the commercially available instruments Corneometer and Sebumeter.

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We report a method on quantitative and simultaneous noncontact in-vivo hydration and sebum measurements of the skin using an infrared optical spectroscopic set-up. The method utilizes differential detection with three wavelengths 1720, 1750, and 1770 nm, corresponding to the lipid vibrational bands that lay "in between" the prominent water absorption bands. We have used an emulsifier containing hydro- and lipophilic components to mix water and sebum in various volume fractions which was applied to the skin to mimic different oily-dry skin conditions. We also measured the skin sebum and hydration values on the forehead under natural conditions and its variations to external stimuli. Good agreement was found between our experimental results and reference values measured using conventional biophysical methods such as Corneometer and Sebumeter.

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