Hydration effects on the barrier function of stratum corneum lipids: Raman analysis of ceramides 2, III and 5

Progress in Topical and Transdermal Drug Delivery Research-Focus on Nanoformulations

Skin is the largest organ and a multifunctional interface between the body and its environment. It acts as a barrier against cold, heat, injuries, infections, chemicals, radiations or other exogeneous factors, and it is also known as the mirror of the soul. The skin is involved in body temperature regulation by the storage of fat and water. It is an interesting tissue in regard to the local and transdermal application of active ingredients for prevention or treatment of pathological conditions. Topical and transdermal delivery is an emerging route of drug and cosmetic administration. It is beneficial for avoiding side effects and rapid metabolism. Many pharmaceutical, technological and cosmetic innovations have been described and patented recently in the field. In this review, the main features of skin morphology and physiology are presented and are being followed by the description of classical and novel nanoparticulate dermal and transdermal drug formulations. The biophysical aspects of the penetration of drugs and cosmetics into or across the dermal barrier and their investigation in diffusion chambers, skin-on-a-chip devices, high-throughput measuring systems or with advanced analytical techniques are also shown. The current knowledge about mathematical modeling of skin penetration and the future perspectives are briefly discussed in the end, all also involving nanoparticulated systems.

Effect of a low-mineralized thermal spring water on skin barrier mechanical properties using atomic force microscopy

The mineral content of thermal spring water (TSW) applied to the skin surface can directly influence the skin barrier. Indeed, our previous study showed that Avène TSW (ATSW), a low mineral content thermal spring water, protects the stratum corneum from dehydration compared to a mineral-rich TSW (MR-TSW) and maintains skin surface ultrastructure. While many TSWs have been recognized to have beneficial effects on skin, little is known about their localized and specific effects on skin barrier biomechanics at the nanometric scale. The aim of this study was to compare the effects of ATSW with a reference, MR-TSW, on the biomechanical barrier properties of the skin under homeostasis conditions using atomic force microscopy (AFM). AFM was used to obtain a precise nanomechanical mapping of the skin surface after three applications of both TSW. This provides specific information on the skin topographical profile and elasticity. The topographic profile of skin samples showed a specific compaction of the skin layers after application of MR-TSW, characterized by an increase of the total number of external skin layers, compared to non-treated samples. By contrast, ATSW did not modify the skin topographic profile. High-resolution force/volume acquisitions to capture the elastic modulus showed that it was directly correlated with skin rigidity. The elastic modulus strongly and significantly increased after MR-TSW application compared to non-treated skin. By contrast, applications of ATSW did not increase elastic modulus. These data demonstrate that applications of MR-TSW significantly modified skin barrier properties by increasing skin surface layer compaction and skin rigidity. By contrast, ATSW did not modify the topographical profile of skin explants nor induce mechanical stress at the level of the stratum corneum, indicating it does not disrupt the biophysical properties linked to skin surface integrity.

Non-invasive biochemical analysis and comparison of atopic dermatitis and psoriasis skin using handheld confocal Raman spectroscopy

A handheld non-invasive confocal Raman system (CRS) was used to evaluate the differences in skin biochemicals between atopic dermatitis (AD) and psoriasis, which are inflammatory skin conditions. Raman spectral measurements in the fingerprint and high wavenumber region were acquired using a portable in-house CRS system with excitation lasers operating at 671 and 785 nm. It was deduced that relative amount of water decreases in the following sequence of skin: healthy, psoriasis and AD. Moreover, differential trends were observed for the subclasses of ceramides such that ceramide 3 is lower in the lesional AD and psoriasis skin as compared to healthy, while ceramide 2 showed a contrasting trend of decrease in lesional AD and increase in lesional psoriasis as opposed to healthy skin. Amount of cholesterol was significantly higher in lesional psoriasis as compared to lesional AD and healthy skin. These differences can aid in an objective classification of the skin conditions and in the formulation of new disease-specific topical treatments.

Strat-M® positioning for skin permeation studies: A comparative study including EpiSkin® RHE, and human skin

In the development and optimization of dermatological products, In Vitro Permeation Testing (IVPT) is pivotal for controlled study of skin penetration. To enhance standardization and replicate human skin properties reconstructed human skin and synthetic membranes are explored as alternatives. Strat-M® is a membrane designed to mimic the multi-layered structure of human skin for IVPT. For instance, in Strat-M®, the steady-state fluxes (JSS) of resorcinol in formulations free of permeation enhancers were found to be 41 ± 5 µg/cm2·h for the aqueous solution, 42 ± 6 µg/cm2·h for the hydrogel, and 40 ± 6 µg/cm2·h for the oil-in-water emulsion. These results were closer to excised human skin (5 ± 3, 9 ± 2, 13 ± 6 µg/cm2·h) and surpassed the performance of EpiSkin® RHE (138 ± 5, 142 ± 6, and 162 ± 11 µg/cm2·h). While mass spectrometry and Raman microscopy demonstrated the qualitative molecular similarity of EpiSkin® RHE to human skin, it was the porous and hydrophobic polymer nature of Strat-M® that more faithfully reproduced the skin's diffusion-limiting barrier. Further validation through similarity factor analysis (∼80-85%) underscored Strat-M®'s significance as a reliable substitute for human skin, offering a promising approach to enhance realism and reproducibility in dermatological product development.

Effects of solar radiations on stratum corneum hydration: Part I, protective role of skin surface lipids

This study used Raman spectroscopy to develop a new approach to evaluate the effects of solar radiation on the stratum corneum (SC). The method measures the SC's hydration and dehydration kinetics by calculating the vOH/vCH ratio to monitor the relative water content during the drying process. The study also investigated the role of skin surface lipids (SSLs) in protecting the SC from solar radiation. The SSLs film is a complex mixture of free fatty acids, triglycerides, wax esters, squalene, free and esterified cholesterols, that play a crucial role in the skin's barrier function. The results showed that solar radiation alters the water content and balance within the SC, and SSLs provide protection by acting as an optical filter by absorbing some of the energy of the solar light. This is confirmed by high temperature gas chromatography coupled to mass spectrometry analyses by revealing a decrease in specific lipids after irradiating the SSLs .

Machine Learning Assisted Handheld Confocal Raman Micro-Spectroscopy for Identification of Clinically Relevant Atopic Eczema Biomarkers

Atopic dermatitis (AD) is a common chronic inflammatory skin dermatosis condition due to skin barrier dysfunction that causes itchy, red, swollen, and cracked skin. Currently, AD severity clinical scores are subjected to intra- and inter-observer differences. There is a need for an objective scoring method that is sensitive to skin barrier differences. The aim of this study was to evaluate the relevant skin chemical biomarkers in AD patients. We used confocal Raman micro-spectroscopy and advanced machine learning methods as means to classify eczema patients and healthy controls with sufficient sensitivity and specificity. Raman spectra at different skin depths were acquired from subjects’ lower volar forearm location using an in-house developed handheld confocal Raman micro-spectroscopy system. The Raman spectra corresponding to the skin surface from all the subjects were further analyzed through partial least squares discriminant analysis, a binary classification model allowing the classification between eczema and healthy subjects with a sensitivity and specificity of 0.94 and 0.85, respectively, using stratified K-fold (K = 10) cross-validation. The variable importance in the projection score from the partial least squares discriminant analysis classification model further elucidated the role of important stratum corneum proteins and lipids in distinguishing two subject groups.

Impact of Alkanediols on Stratum Corneum Lipids and Triamcinolone Acetonide Skin Penetration

Alkanediols are widely used as multifunctional ingredients in dermal formulations. In addition to their preservative effect, considering their possible impact on drug penetration is also essential for their use. In the present study, the influence of 2-methyl-2,4-pentanediol, 1,2-pentanediol, 1,2-hexanediol and 1,2-octanediol on the skin penetration of triamcinolone acetonide from four different semisolid formulations was investigated. Furthermore, confocal Raman spectroscopy measurements were performed to examine the influence of the alkanediols on stratum corneum lipid content and order. Alkanediols were found to increase the penetration of triamcinolone acetonide. However, the extent depends strongly on the formulation used. In certain formulations, 1,2-pentanediol showed the highest effect, while in others the penetration-enhancing effect increased with the alkyl chain length of the alkanediol used. None of the tested alkanediols extracted lipids from the stratum corneum nor reduced its thickness. Notwithstanding the above, the longer-chained alkanediols cause the lipids to be converted to a more disordered state, which favors drug penetration. This behavior could not be detected for the shorter-chained alkanediols. Therefore, their penetration-enhancing effect is supposed to be related to an interaction with the hydrophilic regions of the stratum corneum.

Retaining Skin Barrier Function Properties of the Stratum Corneum with Components of the Natural Moisturizing Factor-A Randomized, Placebo-Controlled Double-Blind In Vivo Study

The influence of a topically applied formulation containing components of natural moisturizing factor (NMF) on barrier-related parameters of the stratum corneum (SC) was investigated in vivo using confocal Raman microspectroscopy in a randomized, placebo-controlled double-blind study on 12 volunteers for 14 days. This method allowed for the elucidation of subtle differences between the verum and the placebo even though the components of the verum naturally occur in the SC. This differentiation is not possible non-invasively by conventional methods. In this study, we found that the applied verum and placebo formulations disrupted the equilibrium of water, NMF and lipids in the SC. The adverse effects of the formulation could be mitigated by incorporating it into a simplified supplementation of NMF molecules. As a long-term effect, the amount of strongly bound water increases at 30–40% SC depth (p < 0.05) and the amount of weakly bound water decreases at 30–40% SC depth (p < 0.05) for the verum. This supplement was also unexpectedly able to prevent intercellular lipids (ICL) disorganization in selected depths. In the long term, the verum treatment limited the lateral disorganization of the ICL to the upper 20% SC depth. Further research is required to elucidate the interplay of these factors in the SC, to better understand their contribution to the equilibrium and barrier function of the skin. This understanding of the interaction of these naturally occurring components could help in the future to develop and optimize topical treatments for diseases like psoriasis, atopic dermatitis, ichthyosis where the skin barrier is disrupted.

Raman spectroscopic analysis of skin penetration and moisturizing effects of Bionics vernix caseosa cream compared with Vaseline

BACKGROUND:

The stratum corneum (SC) is the outermost layer of human skin and deemed as barrier against chemical exposure and water loss. Moisturizers have beneficial effects in treating dry skin, especially the SC. Confocal Raman spectroscopy (CRS) was used to evaluate the efficacy of moisturizers on skin hydration and penetration, with such agents posing inherent characteristics of being noninvasive, nondestructive, timesaving, and cost effective. Bionics vernix caseosa (BVC) cream mimics the composition of vernix caseosa (VC), which could protect the newborn skin.

METHODS:

This research applied CRS to evaluate the penetration depth and water content variation during the intervention with two moisturizers, BVC cream and Vaseline. Volunteers received the 2 h application of BVC cream and Vaseline on the forearms. The evaluations on 0 h, 2 h, 4 h and 6 h were performed clinical assessment. Experimental data was processed by least square method and analysis of variance (ANOVA).

RESULTS:

The penetration depth of Vaseline was deeper than that of Bionics vernix caseosa cream. Specifically, BVC cream penetrated 18 μm into human skin, while Vaseline penetrated at least 20 μm. Compared with Vaseline, only BVC cream increased skin hydration, with a moisturizing effect lasting for 4 h. At 6 h, the Vaseline moisturizing effect decreased significantly.

The importance of 12R-lipoxygenase and transglutaminase activities in the hydration-dependent ex vivo maturation of corneocyte envelopes

Background

Terminally differentiated keratinocytes acquire corneocyte protein envelopes (CPE) complexed with corneocyte lipid envelopes (CLE). These two structural components of the corneocyte envelopes (CEs) undergo maturation by gaining in hydrophobicity, rigidity and surface area. Linoleoyl acylceramides are processed by 12R‐lipoxygenase (12R‐LOX) and other enzymes before transglutaminase (TG) attaches ω‐hydroxyceramides to involucrin in the CPE. Concurrently, structural proteins are cross‐linked by TG that has been activated by cathepsin D (CathD).

Objectives

The primary aim of this work was to demonstrate the impact of relative humidity (RH) during ex vivo CE maturation. Low, optimal and high RH were selected to investigate the effect of protease inhibitors (PIs) on CE maturation and TG activity; in addition, 12R‐LOX and CathD activity were measured at optimal RH. Finally, the effect of glycerol on ex vivo CE maturation was tested at low, optimal and high RH.

Methods

The first and ninth tape strip of photo‐exposed (PE) cheek and photo‐protected (PP) post‐auricular sites of healthy volunteers were selected. Ex vivo CE maturation was assessed via the relative CE maturity (RCEM) approach based on CE rigidity and hydrophobicity. The second and eighth tapes were exposed to RH in the presence of inhibitors.

Results

Irrespective of tape stripping depth, CEs from PE samples attained CE rigidity to the same extent as mature CEs from the PP site, but such improvement was lacking for CE hydrophobicity. 70% RH was optimal for ex vivo CE maturation. The inhibition of 12R‐LOX activity resulted in enhanced CE rigidity which was reduced by the TG inhibitor. CE hydrophobicity remained unchanged during ex vivo maturation in the presence of TG or 12R‐LOX inhibition. CE hydrophobicity was enhanced in the presence of glycerol at 44% RH and 100% RH but not at 70% RH. Furthermore, TG activity was significantly diminished at 100% RH compared to the commercial inhibitor LDN‐27219. However, a protease inhibitor mix reversed the negative effect of overhydration.

Conclusion

The study adds to the understanding of the roles of 12R‐LOX and TG activity in CE maturation and gives further insight into the effect of glycerol on the SC.

Holistic dermatology: An evidence-based review of modifiable lifestyle factor associations with dermatologic disorders

BACKGROUND

Holistic dermatology focuses on treating the human body as a whole and implementing lifestyle changes to enhance the treatment and prognosis of skin disease. Understanding the interplay between modifiable lifestyle factors and patients' dermatologic health will help physicians better inform patients on self-care methods to mitigate the burden of their skin disease(s).

OBJECTIVE

To review the current scientific literature on the relationship between modifiable lifestyle factors and the dermatologic outcome of skin disorders.

METHODS

A systematic literature search on PubMed, Cochrane, and Web of Science was conducted to identify research articles examining the relationship between dermatology and 6 major categories of modifiable lifestyle factors: diet, sleep, exercise, stress, alcohol, and smoking.

RESULTS

A substantial amount of evidence supports the relationship between modifiable lifestyle factors and dermatologic outcomes. There were the most studies on diet, stress, alcohol, and smoking, but all lifestyle factors were supported by some degree of scientific evidence.

CONCLUSION

All modifiable lifestyle factors explored in this review play a critical role in modulating the onset and progression of skin disease. We anticipate more research studies in the future and an increasing integration of holistic dermatology into patient care.

The non-homogenous distribution and aggregation of carotenoids in the stratum corneum correlates with the organization of intercellular lipids in vivo

The human stratum corneum (SC) contains an abundant amount of carotenoid antioxidants, quenching free radicals and thereby protecting the skin. For the precise measurements of the depth-dependent carotenoid concentration, confocal Raman microscopy is a suitable method. The quantitative concentration can be determined by the carotenoid-related peak intensity of a Gaussian function approached at ≈1524 cm^-1 using non-linear regression. Results show that the carotenoid concentration is higher at the superficial layers of the SC then decreases to a minimum at 20% SC depth and increases again towards the bottom of the SC. In the present work, two carotenoid penetration pathways into the SC are postulated. The first pathway is from the stratum granulosum to the bottom of the SC, while in the second pathway, the carotenoids are delivered to the skin surface by sweat and/or sebum secretion and penetrate from outside. The carotenoids are aggregated at the superficial layers, which are shown by high correlation between the aggregation states of carotenoids and the lateral organization of lipids. At the 30%-40% SC depths, the ordered and dense lipid molecules intensify the lipid-carotenoid interactions and weaken the carotenoid-carotenoid interaction and thus exhibit the disaggregation of carotenoids. At 90%-100% SC depths, the carotenoid-lipid interaction is weakened and the carotenoids have a tendency to be aggregated. Thus, the molecular structural correlation of carotenoid and SC lipid might be reserved in the intercellular space of the SC and also serves as the skeleton of the intercellular lipids.

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.

Physical and compositional analysis of differently cultured 3D human skin equivalents by confocal Raman spectroscopy

Three-dimensional skin equivalents are increasingly gaining acceptance as non-animal based experimental models of human skin. They are particularly suited to studying differences in physical and compositional properties of normal and diseased skin and their impact on the skin's barrier function. Typically, a culture protocol yielding a model of normal skin is modified to create a model simulating a pathology. Skin layer thicknesses and lipid/protein contents are compared using methods that are invasive, precluding further experiments on the same replicates, and which may be prone to artefacts. We show here that confocal Raman spectroscopy (CRS) is a valuable method for non-invasive discrimination of skin equivalents grown under different culture conditions. Using 3D full-thickness skin equivalents developed in-house, we measure significant differences in stratum corneum and viable epidermis apparent thicknesses resulting from a 7-day difference in the cultures' air-lift phase and from supplementation of the culture medium with interleukin 4. Furthermore, stratum corneum thicknesses obtained by CRS are up to 2.6-fold higher than values measured from histological photomicrographs. Regarding composition, CRS reveals the differential effects of the culture protocol modifications on ceramide, cholesterol and protein composition as a function of depth in the stratum corneum.

Human skin in vivo has a higher skin barrier function than porcine skin ex vivo-comprehensive Raman microscopic study of the stratum corneum

Porcine skin is widely used as a human skin model in dermatology. For both, porcine stratum corneum (SC) ex vivo and human SC in vivo, the hydrogen bonding states of water, the secondary and tertiary structures of keratin, the natural moisturizing factor (NMF) concentrations and the intercellular lipids' (ICL) lateral organization are investigated depth-dependently using confocal Raman microscopy. The SC depth profiles show that porcine SC ex vivo is characterized by lower hydrogen bonding states of water (10%-30% SC depth), lower NMF concentration in the whole SC, more β-sheet form of keratin (10%-90% SC depth), more folded tertiary keratin structures (30%-70% SC depth) and higher hexagonal lateral packing order of ICL (10%-50% SC depth) compared to human SC in vivo. The results clearly show a higher value of skin barrier function of human SC in vivo than of porcine SC ex vivo. Thus, the human SC in vivo is less permeable for lipophilic and hydrophilic substances than porcine SC ex vivo. Considering the porcine SC as an ex vivo model of human SC in vivo, these findings should be taken into consideration.

In vivo confocal Raman microscopic determination of depth profiles of the stratum corneum lipid organization influenced by application of various oils

BACKGROUND

The intercellular lipids (ICL) of stratum corneum (SC) play an important role in maintaining the skin barrier function. The lateral and lamellar packing order of ICL in SC is not homogenous, but rather depth-dependent.

OBJECTIVE

This study aimed to analyze the influence of the topically applied mineral-derived (paraffin and petrolatum) and plant-derived (almond oil and jojoba oil) oils on the depth-dependent ICL profile ordering of the SC in vivo.

METHOD

Confocal Raman microscopy (CRM), a unique tool to analyze the depth profile of the ICL structure non-invasively, is employed to investigate the interaction between oils and human SC in vivo.

RESULTS

The results show that the response of SC to oils' permeation varies in the depths. All oils remain in the upper layers of the SC (0-20% of SC thickness) and show predominated differences of ICL ordering from intact skin. In these depths, skin treated with plant-derived oils shows more disordered lateral and lamellar packing order of ICL than intact skin (p<0.05). In the intermediate layers of SC (30-50% of SC thickness), the oils do not influence the lateral packing order of SC ICL (p>0.1), except plant-derived oils at the depth 30% of SC thickness. In the deeper layers of the SC (60-100% of SC thickness), no difference between ICL lateral packing order of the oil-treated and intact skin can be observed, except that at the depths of 70-90% of the SC thickness, where slight changes with more disorder states are measured for plant-derived oil treated skin (p<0.1), which could be explained by the penetration of free fatty acid fractions in the deep-located SC areas.

CONCLUSION

Both oil types remain in the superficial layers of the SC (0-20% of the SC thickness). Skin treated with mineral- and plant-derived oils shows significantly higher disordered lateral and lamellar packing order of ICL in these layers of the SC compared to intact skin. Plant-derived oils significantly changed the ICL ordering in the depths of 30% and 70-90% of the SC thickness, which is likely due to the penetration of free fatty acids in the deeper layers of the SC.

Comparative atomic-scale hydration of the ceramide and phosphocholine headgroup in solution and bilayer environments

Previous studies have used neutron diffraction to elucidate the hydration of the ceramide and the phosphatidylcholine headgroup in solution. These solution studies provide bond-length resolution information on the system, but are limited to liquid samples. The work presented here investigates how the hydration of ceramide and phosphatidylcholine headgroups in a solution compares with that found in a lipid bilayer. This work shows that the hydration patterns seen in the solution samples provide valuable insight into the preferential location of hydrating water molecules in the bilayer. There are certain subtle differences in the distribution, which result from a combination of the lipid conformation and the lipid-lipid interactions within the bilayer environment. The lipid-lipid interactions in the bilayer will be dependent on the composition of the bilayer, whereas the restricted exploration of conformational space is likely to be applicable in all membrane environments. The generalized description of hydration gathered from the neutron diffraction studies thus provides good initial estimation for the hydration pattern, but this can be further refined for specific systems.

A depth-dependent profile of the lipid conformation and lateral packing order of the stratum corneum in vivo measured using Raman microscopy

The intercellular lipid structure of the stratum corneum (SC) plays a key role in skin barrier function. A depth profile of the intercellular lipid conformation and the lipid lateral packing order were measured in vivo in the human SC using confocal Raman microscopy. The depth profiles of the 2880 cm(-1)/2850 cm(-1) peak ratio intensity, which represent the C-H stretching and lateral packing order of lipids, and the 1080 cm(-1)/(1130 cm(-1) + 1060 cm(-1)) peak ratio, which represents the C-C skeleton vibration and trans-gauche conformation order of lipids, were investigated. The influence of keratin on the lipid peaks at 2850 cm(-1) and 2880 cm(-1) was excluded by the developed mathematical algorithm. The results show that the trans-conformation and lateral packing order of the intercellular lipids reach their maximum value in the SC at 20-40% of its depth and then decrease towards the stratum granulosum. These results show that at a depth of 20-40% (normally corresponding to a depth of 4-8 μm) the SC exhibits the most ordered lipids and therefore the highest skin barrier function. The lateral packing of lipids is more disordered on the surface and in the deeper parts of the SC, which may be associated with a reduced skin barrier function.

Human as the Ultimate Wound Healing Model: Strategies for Studies Investigating the Dermal Lipidome

PURPOSE OF REVIEW

Educate the reader of the multiple roles undertaken by the human epidermal lipidome and the experimental techniques of measuring them.

RECENT FINDINGS

Damage to skin elicits a wound healing process that is capped by the recreation of the lipid barrier. In addition to barrier function, lipids also undertake an active signaling role during wound healing. Achievement of these multiple functions necessitates a significant complexity and diversity in the lipidome resulting in a composition that is unique to the human skin. As such, any attempts to delineate the function of the lipidome during the wound healing process in humans need to be addressed via studies undertaken in humans.

SUMMARY

The human cutaneous lipidome is unique and play a functionally significant role in maintaining barrier and regulating wound healing. Modern mass spectrometry and Raman spectroscopy based methods enable the investigation epidermal lipidome with respect to those functions.

Non-invasive Quantitative Analysis of Specific Fat Accumulation in Subcutaneous Adipose Tissues using Raman Spectroscopy

Subcutaneous adipose tissue (SAT), visceral adipose tissue (VAT), and fat beneath the dermis layer were investigated using a ball lens top hollow optical fiber Raman probe (BHRP). Hamsters were fed with trilinolein (TL) and tricaprin (TC) for six weeks and measurements were carried out every two weeks. The BHRP with an 800 μm diameter fused-silica ball lens was able to obtain information on the subcutaneous fat in a totally non-invasive manner. Changes in the concentration of TL and TC during the treatment were analyzed, and the relationship between fat accumulation and dietary fat was studied. It was found that SAT had, in general, a higher degree of unsaturation than VAT. The accumulation rate of TC found in SAT and VAT was 0.52 ± 0.38 and 0.58 ± 0.4%, respectively, while the TL accumulation rate was 4.45 ± 1.6 and 4.37 ± 2.4%, respectively. The results suggest different metabolic pathways for TC, a typical medium-chain fatty acid, and TL, a long-chain unsaturated fatty acid. Raman subsurface spectra were successfully obtained and used to analyze the subcutaneous fat layer. The accumulation rates of TL and TC found in skin fat were 5.01 ± 3.53% and 0.45 ± 0.36%, respectively. The results demonstrate the high feasibility of Raman spectroscopy for non-invasive analysis of adipose tissue.

Depth profiles of hydrogen bound water molecule types and their relation to lipid and protein interaction in the human stratum corneum in vivo

Confocal Raman microscopy has been used to measure depth-dependent profiles of human SC in vivo in the high wavenumber (HWN) region. In order to keep the linearity of HWN region boundaries and to not remove an informative signal from Raman spectra, a new baseline subtraction procedure has been introduced. After baseline subtraction, the HWN spectrum was deconvoluted using 10 Gaussian functions with individual chemical meanings. The results show that the hydrogen bound water molecule types contributed differently to the water diffusion process in the SC. The most concentrated double donor-double acceptor (DDAA) and single donor-single acceptor (DA) water molecule types in the SC represent more than 90% of the SC's water and mostly contribute to the water flux in the skin. Single donor-double acceptor (DAA) and weakly-bound water molecule types represent less than 10% of the SC's water content. The most tightly hydrogen bound water molecule type, DAA, reaches its maximum concentration near the skin surface and does not take part in the water diffusion process via the SC. The results show that the hydrogen bonding state of water (DA/DDAA water molecule type ratio) reaches its maximum at the depth of approx. 30% of the SC thickness, which correlates well with the maximum lateral packing order of intercellular lipids (ICL) and the natural moisturizing factor (NMF), and does not coincide with the folding/unfolding state of keratin. The NMF's contribution to the bonding of water in the SC is supposed to dominate over that of ICL.

Heavy Cigarette Smokers in a Chinese Population Display a Compromised Permeability Barrier

Cigarette smoking is associated with various cutaneous disorders with defective permeability. Yet, whether cigarette smoking influences epidermal permeability barrier function is largely unknown. Here, we measured skin biophysical properties, including permeability barrier homeostasis, stratum corneum (SC) integrity, SC hydration, skin surface pH, and skin melanin/erythema index, in cigarette smokers. A total of 99 male volunteers were enrolled in this study. Smokers were categorized as light-to-moderate (<20 cigarettes/day) or heavy smokers (≥20 cigarettes/day). An MPA5 was used to measure SC hydration and skin melanin/erythema index on the dorsal hand, forehead, and cheek. Basal transepidermal water loss (TEWL) and barrier recovery rates were assessed on the forearm. A Skin-pH-Meter pH900 was used to measure skin surface pH. Our results showed that heavy cigarette smokers exhibited delayed barrier recovery after acute abrogation (1.02% ± 13.06 versus 16.48% ± 6.07), and barrier recovery rates correlated negatively with the number of daily cigarettes consumption (p = 0.0087). Changes in biophysical parameters in cigarette smokers varied with body sites. In conclusion, heavy cigarette smokers display compromised permeability barrier homeostasis, which could contribute, in part, to the increased prevalence of certain cutaneous disorders characterized by defective permeability. Thus, improving epidermal permeability barrier should be considered for heavy cigarette smokers.

Probing the Role of Ceramide Headgroup Polarity in Short-Chain Model Skin Barrier Lipid Mixtures by ²H Solid-State NMR Spectroscopy

The thermoptropic phase behaviors of two stratum corneum model lipid mixtures composed of equimolar contributions of either Cer[NS18] or Cer[NP18] with stearic acid and cholesterol were compared. Each component of the mixture was specifically deuterated such that the temperature-dependent (2)H NMR spectra allowed disentanglement of the complicated phase polymorphism of these lipid mixtures. While Cer[NS] is based on the sphingosine backbone, Cer[NP] features a phytosphingosine, which introduces an additional hydroxyl group into the headgroup of the ceramide and abolishes the double bond. From the NMR spectra, the individual contributions of all lipids to the respective phases could be determined. The comparison of the two lipid mixtures reveals that Cer[NP] containing mixtures have a tendency to form more fluid phases. It is concluded that the additional hydroxyl group of the phytosphingosine-containing ceramide Cer[NP18] in mixture with chain-matched stearic acid and cholesterol creates a packing defect that destabilizes the orthorhombic phase state of canonical SC mixtures. This steric clash favors the gel phase and promotes formation of fluid phases of Cer[NP] containing lipid mixtures at lower temperature compared to those containing Cer[NS18].

Analysis of the effects of dietary fat on body and skin lipids of hamsters by Raman spectroscopy

Raman spectroscopy has previously been applied for studying lipid metabolism. In this study, a ball lens-installed hollow optical fiber Raman probe (BHRP) was used for the noninvasive measurement of skin lipids in hamsters. Our analysis suggested that multi-unsaturated lipids, once converted into a structure containing conjugated double bonds, were oxidized to form peroxides. These results were applied for analyzing lipid metabolism in adipose and skin tissues in hamsters fed tricaprin, saturated medium-chain triglyceride and trilinolein, unsaturated long-chain triglyceride fat diets. Unsaturated lipids formed conjugated structures in skin tissue but not in adipose tissue. Principal component analysis (PCA) revealed that the dietary fat intake correlated strongly with lipid composition in body and skin tissues. Hence, the present results successfully demonstrate that Raman spectroscopy with a BHRP can be a powerful tool for analyzing lipid metabolism.

The relationship between water loss, mechanical stress, and molecular structure of human stratum corneum ex vivo

Proper hydration of the stratum corneum (SC) is important for maintaining skin's vital functions. Water loss causes development of drying stresses, which can be perceived as 'tightness', and plays an important role in dry skin damage processes. However, molecular structure modifications arising from water loss and the subsequent development of stress has not been established. We investigated the drying stress mechanism by studying, ex vivo, the behaviors of the SC components during water desorption from initially fully hydrated samples using Raman spectroscopy. Simultaneously, we measure the SC mechanical stress with a substrate curvature instrument. Very good correlations of water loss to the mechanical stress of the stratum corneum were obtained, and the latter was found to depend mainly on the unbound water fraction. In addition to that, the water loss is accompanied with an increase of lipids matrix compactness characterized by lower chain freedom, while protein structure showed an increase in amount of α-helices, a decline in α-sheets, and an increase in folding in the tertiary structure of keratin. The drying process of SC involves a complex interplay of water binding, molecular modifications, and mechanical stress. This article provides a better understanding of the molecular mechanism associated to SC mechanics.