ATR-FTIR spectroscopy: a chemometric approach for studying the lipid organisation of the stratum corneum

Lipid Biomimetic Models as Simple Yet Complex Tools to Predict Skin Permeation and Drug-Membrane Biophysical Interactions

The barrier function of the skin is primarily determined by its outermost layer, the Stratum Corneum (SC). The SC consists of corneocytes embedded in a lipid matrix composed mainly of ceramides, cholesterol, and free fatty acids in equimolar proportions and is organised in a complex lamellar structure with different periodicities and lateral packings. This matrix provides a diffusion pathway across the SC for bioactive compounds that are administered to the skin. In this regard, and as the skin administration route has grown in popularity, there has been an increase in the use of lipid mixtures that closely resemble the SC lipid matrix, either for a deeper biophysical understanding or for pharmaceutical and cosmetic purposes. This review focuses on a systematic analysis of the main outcomes of using lipid mixtures as SC lipid matrix models for pharmaceutical and cosmetic purposes. Thus, a methodical evaluation of the main outcomes based on the SC structure is performed, as well as the main recent developments in finding suitable new in vitro tools for permeation testing based on lipid models.

Development of a novel human stratum corneum mimetic phospholipid -vesicle-based permeation assay models for in vitro permeation studies

OBJECTIVES

To develop and evaluate a novel human stratum corneum (SC) mimetic phospholipid vesicle-based permeation assay (PVPASC) model for in vitro permeation studies.

SIGNIFICANCE

Due to the increasing restrictions on the use of human and animal skins, artificial skin models have attracted substantial interest in pharmaceuticals and cosmetic industries. In this study, a modified PVPASC model containing both SC lipids and proteins was developed.

METHODS

The PVPASC model was optimized by altering the lipid composition and adding keratin in the formulation of large liposomes. The barrier properties were monitored by measuring the electrical resistance (ER) and permeability of Rhodamine B (RB). The modified PVPASC model was characterized in terms of the surface topography, solvent influence and storage stability. The permeation studies of the active components in Compound Nanxing Zhitong Plaster (CNZP) were performed to examine the capability of PVPASC in the application of skin penetration.

RESULTS

The ER and Papp values of RB obtained from the optimized PVPASC model indicated a similar barrier property to porcine ear skin. Scanning electron microscope analysis demonstrated a mimic 'brick-and-mortar' structure. The PVPASC model can be stored for three weeks at -20 °C, and withstand the presence of different receptor medium for 24 h. The permeation studies of the active components demonstrated a good correlation (r2 = 0.9136) of Papp values between the drugs' permeation through the PVPASC model and porcine ear skin.

CONCLUSION

Keratin contained composite phospholipid vesicle-based permeation assay models have been proven to be potential skin tools in topical/transdermal permeation studies.

Biomimetic Stratum Corneum Liposome Models: Lamellar Organization and Permeability Studies

The stratum corneum (SC), the outer layer of the skin, plays a crucial role as a barrier protecting the underlying cells from external stress. The SC comprises three key components: ceramide (CER), free fatty acid (FFA), and cholesterol, along with small fractions of cholesterol sulfate and cholesterol ester. In order to gain a deeper understanding about the interdependence of the two major components, CER and FFA, on the organizational, structural, and functional properties of the SC layer, a library of SC lipid liposome (SCLL) models was developed by mixing CER (phytosphingosine or sphingosine), FFA (oleic acid, palmitic acid, or stearic acid), cholesterol, and cholesterol sulfate. Self-assembly of the SC lipids into lamellar phases was first confirmed by small-angle X-ray scattering. Short periodicity and long periodicity phases were identified for SCLLs containing phytosphingosines and sphingosine CERs, respectively. Furthermore, unsaturation in the CER acyl and FFA chains reduced the lipid conformational ordering and packing density of the liposomal bilayer, which were measured by differential scanning calorimetry and Fourier transform infrared spectroscopy. The introduction of unsaturation in the CER and/or FFA chains also impacted the lamellar integrity and permeability. This extensive library of SCLL models exhibiting physiologically relevant lamellar phases with defined structural and functional properties may potentially be used as a model system for screening pharmaceuticals or cosmetic agents.

Probing Dynamic Behavior of Chemical Enhancers Passing In and Out of the Stratum Corneum and Modulation by Biodegradable Enhancer

Chemical enhancers (CEs) decreased the barrier of the stratum corneum (SC) to enhance drug permeation. This was a "dynamic" behavior, which involved three processes including passing in, acting on, and passing out of the SC. However, compared with mature "static" researches about acting on the SC, the other two processes were poorly understood. This work aimed to probe the dynamic behavior of CEs and modulate it for satisfactory effectiveness. The investigating method of CEs' dynamic behavior was established to obtain the rate of CEs passing in and out of the SC. An analysis attribution was conducted to obtain the possible reasons for the quite different dynamic behavior of CEs based on log P, solubility parameter, and minimum binging energy. It demonstrated the rate of CEs passing in and out of the SC was dependent on CE affinity with the SC and the interaction between CEs and the SC, respectively. The relevance between CEs' dynamic behavior and the extent of decreasing SC barrier was confirmed by transepidermal water loss (TEWL). The higher rate of CE passing in the SC and a lower rate of passing out of the SC may contribute to an increased concentration of CEs in the SC, leading to a stronger ability to decrease the SC barrier. More importantly, two biodegradable CEs (Leu-Dod and Ser-Dod) of dodecanol were synthesized and achieved a modulation of its dynamic behavior to obtain more satisfactory effectiveness of enhancing drug permeation. This work was meaningful for the guidance of rationally promoting CEs' effectiveness from a dynamic perspective, which was an unprecedented attempt in this field.

Nanoscale investigation of human skin and study of skin penetration of Janus nanoparticles

Janus nanoparticles (JNP) are innovative nanocarriers with an interesting pharmaceutical and cosmetic potential. They are characterized by the presence of a lipid compartment associated with an aqueous compartment delimited by a phospholipid bilayer containing phospholipids and non-ionic surfactants. The hydrodynamic diameter of JNP varies between 150 and 300 nm. The purpose of this study was to answer the following questions: after cutaneous application, are JNP penetrating? If so, how deep? And in which state, intact or degraded? It was essential to understand these phenomena in order to control the rate and kinetics of diffusion of active ingredients, which can be encapsulated in this vehicle for pharmaceutical or cosmetic purposes. An innovative technique called AFM-IR, was used to elucidate the behavior of JNP after cutaneous application. This instrument, coupling atomic force microscopy and IR spectroscopy, allowing to perform chemical analysis at the nanometer scale thanks to local absorption measurements. The identification of organic molecules at the nanoscale is possible without any labelling. Before cutaneous application of JNP, the nano-structure of untreated human skin was investigated with AFM-IR. Then, in vitro human skin penetration of JNP was studied using Franz cells, and AFM-IR allowed us to perform ultra-local information investigations.

Ultrasound-mediated topical delivery of econazole nitrate with potential for treating Raynaud's phenomenon

The study aims to assess the ultrasound-assisted econazole nitrate (EN) permeation from topically applied formulations with potential for treating Raynaud's phenomenon. Optimization of ultrasound parameters such as the distance of the horn, application time and amplitude were performed. In vitro percutaneous absorption studies were performed using econazole formulations (F2_HPMC dispersion, F4_Lipoderm® Activemax™ Cream) across the ultrasound-treated porcine skin and were compared with the control group (skin samples without ultrasound). Histology and ATR-FTIR studies were performed on treated skin samples. A constant frequency (20 kHz) ultrasound application with 40% amplitude, 0.5 cm distance between ultrasound horn and the skin surface for 2 min was optimized. The permeation of EN was found to be higher from ultrasound-treated skin samples than the control group. Drug permeation from F2_HPMC dispersion was found to be higher as compared to the other formulations and the marketed EN cream. Histological evaluation confirmed that F2_HPMC dispersion showed no signs of toxicity. ATR-FTIR studies revealed a slight increase in the CH₂ stretching vibrations (~2920 cm^-1 and 2850 cm^-1) in ultrasound-treated skin samples as compared with the control. In conclusion, the ultrasound-assisted transdermal delivery of F2_HPMC dispersion could be further studied as a new therapy for Raynaud's phenomenon.

Skin Electrical Resistance Measurement of Oxygen-Containing Terpenes as Penetration Enhancers: Role of Stratum Corneum Lipids

The measurement of skin electrical resistance (SER) has drawn a great deal of attention for the rapid screening of transdermal penetration enhancers (PEs). However, the mechanisms underlying the SER measurement are still unclear. This study was to investigate the effects and mechanisms of seven oxygen-containing terpenes on the SER kinetics. Stratum corneum (SC) lipids were proved to play a key role in SER measurement. Then, the factors affecting the SER measurement were optimized. By the determination of SER kinetics, cyclic terpenes (1,8-cineole, terpinen-4-ol, menthol and α-terpineol) were demonstrated to possess higher enhancement ratio (ER) values compared with linear terpenes (linalool, geraniol and citral). For the first time, the linear correlation was found between ER of terpenes and the interaction energy of terpene⁻ceramide complexes revealed by molecular simulation. The attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) analysis revealed that the effect of cyclic terpenes on SC lipid arrangement was obviously stronger than that of linear terpenes. In addition, by evaluating HaCaT skin cell viability, little difference was found between the toxicities of cyclic and linear terpenes. In conclusion, measurement of SER could be a feasible approach for the efficient evaluation of the PEs that mainly act on SC lipids.

Stable Liposome in Cosmetic Platforms for Transdermal Folic acid delivery for fortification and treatment of micronutrient deficiencies

Oral folate fortification has been successful in many developed nations, however, developing countries still face low compliance and high incidence of folate deficiency associated with low birth weight infants and preterm deliveries. We report safe and efficient approach for transdermal systemic folate delivery using fluidising liposomes (120 ± 4 nm) stabilised within 3D matrix of naturally occurring cosmetic bases: Fuller's earth and henna with room temperature stability. The proof of stratum corneum fluidisation was established ex-vivo by Langmuir-Blodgett film, FTIR and confocal imaging in rat skin. In-vivo topical application in rats showed 11-fold increase in plasma folate within 2 hr, confirming systemic delivery through skin. Efficacy study in folate deficient rats over 4 weeks showed significantly higher plasma levels compared to oral delivery with significant skin depot. Sub-acute toxicity studies in rats at 750-fold higher doses showed safety after 4 weeks daily application. Primary irritation patch test on 25 healthy human volunteers proved non-irritant nature of the nutricosmetics. The technology is first demonstration of transdermal folate fortification with nanosized liposome incorporated in cosmetics, without synthetic surfactants/ethanol or need of external energy. The platform technology opens the possibility of delivering multiple nutrients systemically through skin and can be scaled for affordable community fortification.

Development of essential oils as skin permeation enhancers: penetration enhancement effect and mechanism of action

CONTEXT

Essential oils (EOs) have shown the potential to reversibly overcome the stratum corneum (SC) barrier to enhance the skin permeation of drugs.

OBJECTIVE

The effectiveness of turpentine, Angelica, chuanxiong, Cyperus, cinnamon, and clove oils were investigated for the capacity and mechanism to promote skin penetration of ibuprofen.

MATERIALS AND METHODS

Skin permeation studies of ibuprofen across rat abdominal skin with the presence of 3% w/v EOs were carried out; samples were withdrawn from the receptor compartment at 8, 10, 22, 24, 26, 28, 32, 36, and 48 h and analyzed for ibuprofen content by the HPLC method. The mechanisms of penetration enhancement of EOs were further evaluated by attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) analysis and determination of the properties of EOs. Moreover, the toxicities of EOs on skin cells were also measured.

RESULTS

The enhancement ratio (ER) values of turpentine, Angelica, chuanxiong, Cyperus, cinnamon, clove oils and azone were determined to be 2.23, 1.83, 2.60, 2.49, 2.63 and 1.97, respectively. Revealed by ATR-FTIR analysis, a linear relationship (r = 0.9045) was found between the ER values and the total of the shift of peak position of SC lipids. Furthermore, the results of HaCaT skin cell toxicity evaluation revealed that the natural EOs possessed relatively lower skin irritation potential.

CONCLUSION

Compared with azone, the investigated EOs possess significantly higher penetration enhancement effect and lower skin toxicity. EOs can promote the skin permeation of ibuprofen mainly by disturbing rather than extracting the SC lipids.

Modified NLC-loaded coumarin for pharmaceutical applications: the improvement of physical stability and controlled release profile

BACKGROUND

Coumarin-6 is a lipophilic dye and is often used as a model in delivery system.

OBJECTIVE

The aim of this study was to improve the nonstructured lipid carrier (NLC) system loading with lipophilic molecule, coumarin-6, and to investigate its characteristics in terms of physical stability and controlled release profile.

MATERIALS AND METHODS

Initially, the selection of the coating polymer was observed. Then, the preparation of the conventional NLC-loaded coumarin-6 was compared to the modified NLC-loaded coumarin-6 via the probe sonication. The physical properties and stability were determined by Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) techniques. The release profile was established using fluorescent spectroscopic method.

RESULTS AND DISCUSSION

The size and zeta potential measurement showed significant decrease in the size range of the modified NLC-loaded coumarin and the lower intensity of the surface charge compared to the NLC-loaded coumarin. The change of crystallinity observed from DSC and XRD techniques indicated the molecular dispersion of coumarin-6 in the lipid matrix of NLC. The FT-IR spectra were also proven that coumarin-6 was entrapped in the NLC molecule. The result showed comparable controlled release profile to the conventional preparation with no difference on the cytotoxicity level.

CONCLUSIONS

The modified NLC delivery system, therefore, exhibited the acceptable potential as a nanocarrier.

Development of nanovesicular systems for dermal imiquimod delivery: physicochemical characterization and in vitro/in vivo evaluation

The aim of the current investigation was to develop and statistically evaluate nanovesicular systems for dermal imiquimod delivery. To this purpose, transethosomes were prepared with phospholipid, ethanol and different permeation enhancers. Conventional ethosomes, with soy phospholipid and ethanol, were used as control. The prepared vesicles were characterized for size, zeta potential, stability and entrapment efficiency. The optimal transethosomal formulation with mean particle size of 82.3 ± 9.5 nm showed the higher entrapment efficiency (68.69 ± 1.7%). In vitro studies, permeation results of accumulated drug and local accumulation efficiency were significantly higher for transethosomes (24.64 µg/cm(2) and 6.70, respectively) than control (14.45 µg/cm(2) and 3.93, respectively). Confocal laser scanning microscopy of rhodamine 6G-loaded transethosomes revealed an enhanced retention into the deeper skin layers as compared to conventional ethosomes. Besides, Fourier-transform infra-red spectroscopy studies were also performed to understand the mechanism of interaction between skin and carriers. What's more, results of in vivo studies indicated the transethosomes of imiquimod providing the most effectiveness for dermal delivery among all of the formulations. These results suggested that transethosomes would be a promising dermal carrier for imiquimod in actinic keratose treatment.

Biopolymer-based transdermal films of donepezil as an alternative delivery approach in Alzheimer's disease treatment

Matrix type transdermal films of donepezil (DNP) as an alternative delivery approach was designed to improve patient compliance to Alzheimer disease treatment. Sodium alginate, a natural polysaccharide, was used as matrix-forming agent in the optimization of transdermal films. Propylene glycol and dl-limonene was added into films as a plasticizer and permeation enhancer, respectively. As well as mechanical strength and bioadhesiveness of optimized transdermal films of DNP, the impact of dl-limonene concentration in films on DNP in vitro permeation across pig skin was assessed. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) measurements were carried out to examine the effects of enhancer on in vitro conformational order of the stratum corneum intercellular lipids following permeation study. Results showed that transdermal formulations of DNP were suitable due to both mechanical and bioadhesive features of the films. In vitro skin permeation study indicated that dl-limonene at a concentration of 3% was optimum with high drug flux. ATR-FTIR results confirmed a more fluidized stratum corneum lipid state in the presence of dl-limonene, indicating its permeation enhancement effect. Regarding to achieve therapeutic levels of DNP, it seems to be feasible deliver DNP with transdermal films for the management of Alzheimer disease.

Windowless ultrasound photoacoustic cell for in vivo mid-IR spectroscopy of human epidermis: low interference by changes of air pressure, temperature, and humidity caused by skin contact opens the possibility for a non-invasive monitoring of glucose in the interstitial fluid

The application of a novel open, windowless cell for the photoacoustic infrared spectroscopy of human skin is described. This windowless cavity is tuned for optimum performance in the ultrasound range between 50 and 60 kHz. In combination with an external cavity tunable quantum cascade laser emitting in the range from ~1000 cm(-1) to 1245 cm(-1), this approach leads to high signal-to-noise-ratio (SNR) for mid-infrared spectra of human skin. This opens the possibility to measure in situ the absorption spectrum of human epidermis in the mid-infrared region at high SNR in a few (~5) seconds. Rapid measurement of skin spectra greatly reduces artifacts arising from movements. As compared to closed resonance cells, the windowless cell exhibits the advantage that the influence of air pressure variations, temperature changes, and air humidity buildup that are caused by the contact of the cell to the skin surface can be minimized. We demonstrate here that this approach can be used for continuous and non-invasive monitoring of the glucose level in human epidermis, and thus may form the basis for a non-invasive monitoring of the glucose level for diabetes patients.

In vivo noninvasive monitoring of glucose concentration in human epidermis by mid-infrared pulsed photoacoustic spectroscopy

The noninvasive determination of glucose in the interstitial layer of the human skin by mid-infrared spectroscopy is reported. The sensitivity for this measurement was obtained by combining the high pulse energy from an external cavity quantum cascade laser (EC-QCL) tunable in the infrared glucose fingerprint region (1000-1220 cm(-1)) focused on the skin, with a detection of the absorbance process by photoacoustic spectroscopy in the ultrasound region performed by a gas cell coupled to the skin. This combination facilitates a quantitative measurement for concentrations of skin glucose in the range from <50 mg/dL to >300 mg/dL, which is the relevant range for the glucose monitoring in diabetes patients. Since the interstitial fluid glucose level is representative of the blood glucose level and follows it without significant delay (<10 min), this method could be applied to establish a noninvasive, painless glucose measurement procedure that is urgently awaited by diabetes patients. We report here the design of the photoacoustic experiments, the spectroscopy of glucose in vivo, and the calibration method for the quantitative determination of glucose in skin. Finally, a preliminary test with healthy volunteers and volunteers suffering from diabetes mellitus demonstrates the viability of a noninvasive glucose monitoring for patients based on the combination of infrared QCL and photoacoustic detection.

Penetration enhancer-containing vesicles (PEVs) as carriers for cutaneous delivery of minoxidil: in vitro evaluation of drug permeation by infrared spectroscopy

Recently, we carried out a research on new liposomal systems prepared by using in their composition a few penetration enhancers which differ for chemical structure and physicochemical properties. The penetration enhancer-containing vesicles (PEVs) were prepared by using soy lecithin and different amounts of three penetration enhancers, 2-(2-ethoxyethoxy) ethanol (Transcutol(®)), capryl-caproyl macrogol 8-glyceride (Labrasol(®)), and cineole.To study the influence of the PEVs on (trans)dermal delivery of minoxidil, in vitro diffusion experiments were performed through new born pig skin and the results were compared with that obtained applying the vesicular system without enhancer (control) after pretreatment of the skin with the various enhancers. In this study, Fourier transform infrared spectroscopy (FTIR), attenuated total reflectance FTIR (ATR-FTIR) and FTIR imaging were used to evaluate the effective penetration of minoxidil in the skin layers and to discover the influence of the enhancer on the drug topical delivery. These analytical studies allowed us to characterize the drug formulations and to evaluate the vesicle distribution into the skin. Recorded spectra confirmed that the vesicle formulations with penetration enhancers promoted drug deposition into the skin.

Basic nanostructure of stratum corneum lipid matrices based on ceramides [EOS] and [AP]: a neutron diffraction study

The goal of this study was to investigate the nanostructure of SC lipid model membranes comprising the most relevant SC lipids such as the unique-structured omega-acylceramide [EOS] in a near natural ratio with neutron diffraction. In models proposed recently the presence of ceramide [EOS] and FFA are necessary for the formation of one of the two existent crystalline lamellar phases of the SC lipids, the long-periodicity phase as well as for the normal barrier function of the SC. The focus of this study was placed on the influence of the FFA BA on the membrane structure and its localization within the membrane based on the ceramides [EOS] and [AP]. The internal nanostructure of such membranes was obtained by Fourier synthesis from the experimental diffraction patterns. The resulting neutron scattering length density profiles showed that the exceptionally long ceramide [EOS] is arranged in a short-periodicity phase created by ceramide [AP] by spanning through the whole bilayer and extending even further into the adjacent bilayer. Specifically deuterated BA allowed us to determine the exact position of this FFA inside this SC lipid model membrane. Furthermore, hydration experiments showed that the presented SC mimic system shows an extremely small intermembrane hydration of approximately 1 A, consequently the headgroups of the neighboring leaflets are positioned close to each other.

Chemical enhancer solubility in human stratum corneum lipids and enhancer mechanism of action on stratum corneum lipid domain

Previously, chemical enhancer-induced permeation enhancement on human stratum corneum (SC) lipoidal pathway at enhancer thermodynamic activities approaching unity in the absence of cosolvents (defined as Emax) was determined and hypothesized to be related to the enhancer solubilities in the SC lipid domain. The objectives of the present study were to (a) quantify enhancer uptake into SC lipid domain at saturation, (b) elucidate enhancer mechanism(s) of action, and (c) study the SC lipid phase behavior at Emax. It was concluded that direct quantification of enhancer uptake into SC lipid domain using intact SC was complicated. Therefore a liposomal model of extracted human SC lipids was used. In the liposome study, enhancer uptake into extracted human SC lipid liposomes (EHSCLL) was shown to correlate with Emax. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and differential scanning calorimetry (DSC) were used to evaluate lipid phase alterations in enhancer-treated intact SC. IR spectra demonstrated an increase in the lipid domain fluidity and DSC thermograms indicated a decrease in the phase transition temperature with increasing Emax. These results suggest that the enhancer mechanism of action is through enhancer intercalation into SC intercellular lipids and subsequent lipid lamellae fluidization related to enhancer lipid concentration.

Infrared spectroscopy studies of mixtures prepared with synthetic ceramides varying in head group architecture: coexistence of liquid and crystalline phases

The barrier function of the skin is provided by the stratum corneum (SC), the outermost layer of the skin.Ceramides (CERs), cholesterol (CHOL) and free fatty acids (FFAs) are present in SC and form highly ordered crystalline lipid lamellae. These lamellae are crucial for a proper skin barrier function. In the present study,Fourier transform infrared spectroscopy was used to examine the lipid organization of mixtures prepared from synthetic CERs with CHOL and FFAs. The conformational ordering and lateral packing of these mixtures showed great similarities to the lipid organization in SC and lipid mixtures prepared with native CERs.Therefore, mixtures with synthetic CERs serve as an excellent tool for studying the effect of molecular architecture of CER subclasses on the lipid phase behavior. In SC the number of OH-groups in the head groups of CER subclasses varies. Furthermore, acylCERs with a linoleic acid chemically bound to a long acyl chain are also identified. The present study revealed that CER head group architecture affects the lateral packing and conformational ordering of the CER:CHOL:FFA mixtures. Furthermore, while the majority of the lipids form a crystalline packing, the linoleate moiety of the acylCERs participates in a "pseudo fluid" phase.

Properties of ceramides and their impact on the stratum corneum structure. Part 2: stratum corneum lipid model systems

The stratum corneum (SC) represents the outermost layer of the mammalian skin, exhibits the main skin barrier and plays an important role in the water penetration pathway through the SC. Knowing the structure and properties of the SC at the molecular level is essential for studying drug penetration through the SC and for the development of new dermal drug delivery systems. Therefore, research interest is focused on the SC lipid matrix and on water diffusion through it. Thus, the ultimate aim is to design a lipid mixture that mimics the barrier properties of the human SC to a high extent and that can substitute the SC in drug delivery systems. This review summarizes various studies performed on either isolated animal or human ceramide based SC model systems, coming to the result that using synthetic lipids with a well-defined architecture allows good extrapolation to the in vivo situation. This review is the continuation of part 1 that is focused on a detailed description of the thermotropic and/or lyotropic phase behaviour of single ceramide types obtained by various experimental techniques. The objective of part 2 is to reflect the numerous studies on SC lipid model systems, namely binary, ternary and multicomponent systems, during the last decade. In this context, neutron diffraction as a prospective tool for analyzing the internal membrane structure is addressed in particular. Based on these new insights, current SC models are presented, whose validations are still under discussion. A profound knowledge about SC lipid organization at the molecular level is still missing.

Analysis of Quil A–phospholipid mixtures using drift spectroscopy

The aim of this study was to investigate molecular interactions between Quil A and phosphatidylcholine in the solid state using diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS). Analysis of the interactions was characterized on the different regions of phosphatidylcholine: hydrophobic chain, interfacial and headgroup regions. The spectra of the hydrocarbon region of phosphatidylcholine alone compared to that for the binary mixture of Quil A and phosphatidylcholine were similar. These findings suggest that Quil A did not cause conformational disorder of the fatty acyl chains of the phospholipid. In contrast, a shift in the wavenumber of the choline group and a broad band in this moiety indicate a modification of the phospholipid in the headgroup region due to interaction between Quil A and phosphatidylcholine. These results suggest possibly ionic interactions between the negatively charged glucuronic acid moiety of the Quil A molecule with the positively charged choline group. The findings could also be the result of conformational changes in the choline group because of the intercalation of sugar moieties in Quil A between the choline and phosphate groups due to hydrogen bonding. Shift of wavenumbers to lower values on the carbonyl group was observed suggesting hydrogen bonding between Quil A and phosphatidylcholine. The difference in degrees of wavenumber shift (choline>phosphate>carbonyl group) and observed broad bands indicated that Quil A preferentially interacted with phosphatidylcholine on the hydrophilic headgroup. Cholesterol influenced such interactions at relatively high concentration (60%, w/w).

Application of optical non-invasive methods in skin physiology: a comparison of laser scanning microscopy and optical coherent tomography with histological analysis

BACKGROUND/PURPOSE

Optical, non-invasive methods, such as fluorescence laser scanning microscopy (LSM) and optical coherent tomography (OCT), have become efficient tools for the characterization of the skin structure in vivo, as well as real-time investigation of distribution and penetration of topically applied substances.

METHODS

In the present paper, the results obtained with both non-invasive methods - OCT and LSM - were compared to conventional light microscopy of histological sections. Skin structure and the distribution of topically applied particulate and non-particulate substances on the skin surface and in the epidermis were analyzed.

RESULTS

None of the methods used are suitable for the realization of all diagnostic tasks, however, each method has advantages for particular applications. Fluorescence LSM is well suited for the investigation of the upper 150 microm of the skin as well as for the investigation of the kinetics of substances applied onto or into the epidermis. OCT can be applied for the investigation of vertical cross-sections of the skin up to a depth of 2 mm, albeit at lower resolution than achieved by LSM or conventional light microscopy. Conventional light microscopy of histological sections of biopsy specimens produces familiar high-resolution images of deeper tissue layers. However, the analysis of the kinetic processes is limited in this case.

CONCLUSIONS

LSM- and OCT-measurements are efficient non-invasive tools for the characterization of morphological structures of the skin. On the one hand, the optical methods have a clear advantage in the case of kinetic measurements. On the other hand, histological investigations are characterized by a high information density and a high resolution, also in deep tissue layers. The selection of the best method for the analysis of the skin morphology depends on the target and the task of the investigation.

Chemical imaging of latent fingerprint residues

In situ attenuated total reflection Fourier transform infrared (ATR-FT-IR) spectroscopic imaging has been used to obtain chemical images of fingerprints under controlled humidity and temperature. The distribution of lipid and amino acid components in the fingerprints from different donors left on the surface of the ZnSe crystal has been studied using an in situ FT-IR spectroscopic imaging approach under a controlled environment and studied as a function of time. Univariate and multivariate analyses were employed to analyze the spectroscopic dataset. Changes in the spectra of lipids with temperature and time have been detected. This information is needed to understand aging of the fingerprints. The ATR-FT-IR spectroscopic imaging offers a new and complementary means for studying the chemistry of fingerprints that are left pristine for further analysis. This study demonstrates the potential for visualizing the chemical changes of fingerprints for forensic applications by spectroscopic imaging.

Role of ceramide structure and its microenvironment on the conformational order of model stratum corneum lipids mixtures: an approach by FTIR spectroscopy

The aim of this study is to investigate the influence of ceramide head group architecture and free fatty acid (another main class of stratum corneum lipids) or protein (keratin), on the lamellar organization of the ceramide auto-associated in model films mimicking lipid organization within the stratum corneum. FTIR spectroscopy is a powerful technique for investigating the structure of such systems. This technique has already been used to characterize phase transitions of the SC and of related model systems. As temperature is known to modify the conformational order of lipids, we used it as a variable parameter to monitor the differences in the conformational stability of ceramides. Our study included four ceramides: ceramide 2, 3, 5 and 6 which differ by their head group architecture. Two kinds of lipid-lipid interactions were studied: non-polar and polar. We noted some structural factors which participated to the organizational behavior: insaturation of alkyl chain, alpha-hydroxyl on fatty acid moiety and sphingosine or phytosphingosine head group. There is a direct interaction of palmitic acid on alkyl chains organization and a weak interaction with polar head group in presence of keratin, both provoking a destabilization of the ceramidic orthorhombic organization.

Infrared spectroscopic study of stratum corneum model membranes prepared from human ceramides, cholesterol, and fatty acids

The outermost layer of the skin, the stratum corneum, consists of corneocytes surrounded by lipid domains. The main lipid classes in stratum corneum are cholesterol, ceramides (CER), and free fatty acids forming two crystalline lamellar phases. However, only limited information is available on whether the various lipid classes participate in the same crystalline lattices or if separate domains are formed within the lipid lamellae. In this article infrared spectroscopic studies are reported of hydrated mixtures prepared from cholesterol, human CER, and free fatty acids. Evaluation of the methylene stretching vibrations revealed a conformational disordering starting at approximately 60 degrees C for all mixtures. Examination of the rotational ordering (scissoring and rocking vibrations) of mixtures prepared from equimolar cholesterol and CER with a variation in the level of free fatty acids showed that at lower free fatty acid content orthorhombic and hexagonal domains coexist in the lipid lamellae. Increasing the fatty acid level to an equimolar cholesterol/CER/fatty acid mixture reveals the dominant presence of an orthorhombic lattice, confirming x-ray diffraction studies. Replacing the protonated free fatty acid chains by their perdeuterated counterparts demonstrates that free fatty acids and CER participate in the same orthorhombic lattice up to a level of slightly less than 1:1:0.75 cholesterol/CER/free fatty acids molar ratio but that free fatty acids also form separate domains within the lipid lamellae at equimolar ratios at room temperature. However, no evidence for this has been observed at 32 degrees C. Extrapolating these findings to the situation in stratum corneum led us conclude that in stratum corneum, fatty acids and CER participate in the orthorhombic lattice at 32 degrees C, the skin temperature.