Preparation and characterization of a stratum corneum substitute for in vitro percutaneous penetration studies

The intercellular stratum corneum (SC) lipids form the main barrier for diffusion of substances through the skin. A porous substrate covered with synthetic SC lipids would be an attractive model to study percutaneous penetration, hereby replacing native human SC. Prerequisite is that this stratum corneum substitute (SCS) is prepared with a uniform lipid composition and layer thickness. Furthermore, the lipid organization and orientation should resemble that in SC. The objective of this study was to investigate the utility of an airbrush spraying device to prepare a SCS composed of cholesterol, ceramides and free fatty acids on a polycarbonate filter. The results demonstrate that a proper choice of solvent mixture and lipid concentration is crucial to achieve a uniform distribution of the applied lipids over the filter surface. A smooth and tightly packed lipid layer is only obtained when the equilibration conditions are appropriately chosen. The SCS possesses two crystalline lamellar phases with periodicities similar to those present in native SC. The orientation of these lamellae is mainly parallel to the surface of the polycarbonate filter, which resembles the orientation of the intercellular SC lipids. In conclusion, the airbrush technique enables generation of a homogeneous SCS, which ultimately may function as a predictive in vitro percutaneous penetration model.     

Preparation and characterization of a stratum corneum substitute for in vitro percutaneous penetration studies

The intercellular stratum corneum (SC) lipids form the main barrier for diffusion of substances through the skin. A porous substrate covered with synthetic SC lipids would be an attractive model to study percutaneous penetration, hereby replacing native human SC. Prerequisite is that this stratum corneum substitute (SCS) is prepared with a uniform lipid composition and layer thickness. Furthermore, the lipid organization and orientation should resemble that in SC. The objective of this study was to investigate the utility of an airbrush spraying device to prepare a SCS composed of cholesterol, ceramides and free fatty acids on a polycarbonate filter. The results demonstrate that a proper choice of solvent mixture and lipid concentration is crucial to achieve a uniform distribution of the applied lipids over the filter surface. A smooth and tightly packed lipid layer is only obtained when the equilibration conditions are appropriately chosen. The SCS possesses two crystalline lamellar phases with periodicities similar to those present in native SC. The orientation of these lamellae is mainly parallel to the surface of the polycarbonate filter, which resembles the orientation of the intercellular SC lipids. In conclusion, the airbrush technique enables generation of a homogeneous SCS, which ultimately may function as a predictive in vitro percutaneous penetration model.     

Is an orthorhombic lateral packing and a proper lamellar organization important for the skin barrier function?

The lipid organization in the stratum corneum (SC), plays an important role in the barrier function of the skin. SC lipids form two lamellar phases with a predominantly orthorhombic packing. In previous publications a lipid model was presented, referred to as the stratum corneum substitute (SCS), that closely mimics the SC lipid organization and barrier function. Therefore, the SCS serves as a unique tool to relate lipid organization with barrier function. In the present study we examined the effect of the orthorhombic to hexagonal phase transition on the barrier function of human SC and SCS. In addition, the SCS was modified by changing the free fatty acid composition, resulting in a hexagonal packing and perturbed lamellar organization. By measuring the permeability to benzoic acid as function of temperature, Arrhenius plots were constructed from which activation energies were calculated. The results suggest that the change from orthorhombic to hexagonal packing in human SC and SCS, does not have an effect on the permeability. However, the modified SCS revealed an increased permeability to benzoic acid, which we related to its perturbed lamellar organization. Thus, a proper lamellar organization is more crucial for a competent barrier function than the presence of an orthorhombic lateral packing.     

Lipid organization in human and porcine stratum corneum differs widely, while lipid mixtures with porcine ceramides model human stratum corneum lipid organization very closely

The conformational disordering and lateral packing of lipids in porcine and human isolated stratum corneum (SC) was compared using Fourier transform infrared spectroscopy (FTIR). It was shown that SC of both species differ markedly, porcine SC lipids being arranged predominantly in a hexagonal lattice while lipids in human SC are predominantly packed in the denser orthorhombic lattice. However, the lipid organization of equimolar ceramide:cholesterol:free fatty acid (CER:CHOL:FFA) mixtures prepared with isolated porcine CER or human CER is very similar, only the transition temperatures differed being slightly lower in mixtures with porcine CER. Therefore, the difference in lateral packing between human and porcine stratum corneum is not due to the difference in CER composition. Furthermore, it is possible to use more readily available porcine CER in model lipid mixtures to mimic lipid organization in human SC. As the equimolar porcine CER:CHOL:FFA mixtures closely mimic the lipid organization in human SC, both human SC and this mixture were selected to examine the effect of glycerol on the lipid phase behaviour. It was found that high concentrations of glycerol change the lamellar organization slightly, while domains with an orthorhombic lateral packing are still observed.     

Lipid organization in human and porcine stratum corneum differs widely, while lipid mixtures with porcine ceramides model human stratum corneum lipid organization very closely

The conformational disordering and lateral packing of lipids in porcine and human isolated stratum corneum (SC) was compared using Fourier transform infrared spectroscopy (FTIR). It was shown that SC of both species differ markedly, porcine SC lipids being arranged predominantly in a hexagonal lattice while lipids in human SC are predominantly packed in the denser orthorhombic lattice. However, the lipid organization of equimolar ceramide:cholesterol:free fatty acid (CER:CHOL:FFA) mixtures prepared with isolated porcine CER or human CER is very similar, only the transition temperatures differed being slightly lower in mixtures with porcine CER. Therefore, the difference in lateral packing between human and porcine stratum corneum is not due to the difference in CER composition. Furthermore, it is possible to use more readily available porcine CER in model lipid mixtures to mimic lipid organization in human SC. As the equimolar porcine CER:CHOL:FFA mixtures closely mimic the lipid organization in human SC, both human SC and this mixture were selected to examine the effect of glycerol on the lipid phase behaviour. It was found that high concentrations of glycerol change the lamellar organization slightly, while domains with an orthorhombic lateral packing are still observed.     

Combined LC/MS-platform for analysis of all major stratum corneum lipids,and the profiling of skin substitutes

Ceramides (CERs), cholesterol, and free fatty acids (FFAs) are the main lipid classes in human stratum corneum (SC, outermost skin layer), but no studies report on the detailed analysis of these classes in a single platform. The primary aims of this study were to 1) develop an LC/MS method for (semi-)quantitative analysis of all main lipid classes present in human SC; and 2) use this method to study in detail the lipid profiles of human skin substitutes and compare them to human SC lipids. By applying two injections of 10 μl, the developed method detects all major SC lipids using RPLC and negative ion mode APCI-MS for detection of FFAs, and NPLC using positive ion mode APCI-MS to analyze CERs and cholesterol. Validation showed this lipid platform to be robust, reproducible, sensitive, and fast. The method was successfully applied on ex vivo human SC, human SC obtained from tape strips and human skin substitutes (porcine SC and human skin equivalents). In conjunction with FFA profiles, clear differences in CER profiles were observed between these different SC sources. Human skin equivalents more closely mimic the lipid composition of human stratum corneum than porcine skin does, although noticeable differences are still present. These differences gave biologically relevant information on some of the enzymes that are probably involved in SC lipid processing. For future research, this provides an excellent method for (semi-)quantitative, ‘high-throughput’ profiling of SC lipids and can be used to advance the understanding of skin lipids and the biological processes involved.     

Long periodicity phase in extracted lipids of vernix caseosa obtained with equilibration at physiological temperature

The outermost layer of the skin, the stratum corneum (SC), comprises the main barrier function between body and environment. The SC features a highly structured lipid organization: a short periodicity phase and a long periodicity phase (LPP) with a repeat distance of 6 and 13 nm, respectively. Like SC, vernix caseosa (VC), the creamy white skin-surface biofilm of the newborn, also contains barrier lipids, i.e. ceramides, cholesterol and free fatty acids. Aim of this study was to investigate whether isolated VC lipids also form the characteristic LPP. Several preparation methods were examined and only when the solution of the lipid mixture, isolated either from VC or SC, was dried under nitrogen at 37 °C and subsequently spread onto a support, the LPP was formed. When VC barrier lipids were first exposed to elevated temperatures and subsequently cooled down, the LPP was formed at around 34 °C, which is at a much lower temperature than observed with the lipids in SC. In conclusion, we showed for the first time that depending on the preparation method, (i) VC lipids also form the LPP and (ii) the LPP in VC lipids and SC lipids was obtained at a low equilibration temperature, mimicking the physiological condition.     

Novel lipid mixtures based on synthetic ceramides reproduce the unique stratum corneum lipid organization

Lipid lamellae present in the outermost layer of the skin protect the body from uncontrolled water loss. In human stratum corneum (SC), two crystalline lamellar phases are present, which contain mostly cholesterol, free fatty acids, and nine types of free ceramides. Previous studies have demonstrated that the SC lipid organization can be mimicked with model mixtures based on isolated SC lipids. However, those studies are hampered by low availability and high interindividual variability of the native tissue. To elucidate the role of each lipid class in the formation of a competent skin barrier, the use of synthetic lipids would offer an alternative. The small- and wide-angle X-ray diffraction results of the present study show for the first time that synthetic lipid mixtures, containing only three synthetic ceramides, reflect to a high extent the SC lipid organization. Both an appropriately chosen preparation method and lipid composition promote the formation of two characteristic lamellar phases with repeat distances similar to those found in native SC. From all synthetic lipid mixtures examined, equimolar mixtures of cholesterol, ceramides, and free fatty acids equilibrated at 80 degrees C resemble to the highest extent the lamellar and lateral SC lipid organization, both at room and increased temperatures.     

Structure of the skin barrier and its modulation by vesicular formulations

The natural function of the skin is to protect the body from unwanted influences from the environment. The main barrier of the skin is located in the outermost layer of the skin, the stratum corneum. Since the lipids regions in the stratum corneum form the only continuous structure, substances applied onto the skin always have to pass these regions. For this reason the organization in the lipid domains is considered to be very important for the skin barrier function. Due to the exceptional stratum corneum lipid composition, with long chain ceramides, free fatty acids and cholesterol as main lipid classes, the lipid phase behavior is different from that of other biological membranes. In stratum corneum crystalline phases are predominantly present, but most probably a subpopulation of lipids forms a liquid phase. Both the crystalline nature and the presence of a 13 nm lamellar phase are considered to be crucial for the skin barrier function. Since it is impossible to selectively extract individual lipid classes from the stratum corneum, the lipid organization has been studied in vitro using isolated lipid mixtures. These studies revealed that mixtures prepared with isolated stratum corneum lipids mimic to a high extent stratum corneum lipid phase behavior. This indicates that proteins do not play an important role in the stratum corneum lipid phase behavior. Furthermore, it was noticed that mixtures prepared only with ceramides and cholesterol already form the 13 nm lamellar phase. In the presence of free fatty acids the lattice density of the structure increases. In stratum corneum the ceramide fraction consists of various ceramide subclasses and the formation of the 13 nm lamellar phase is also affected by the ceramide composition. Particularly the presence of ceramide 1 is crucial. Based on these findings a molecular model has recently been proposed for the organization of the 13 nm lamellar phase, referred to as "the sandwich model", in which crystalline and liquid domains coexist. The major problem for topical drug delivery is the low diffusion rate of drugs across the stratum corneum. Therefore, several methods have been assessed to increase the permeation rate of drugs temporarily and locally. One of the approaches is the application of drugs in formulations containing vesicles. In order to unravel the mechanisms involved in increasing the drug transport across the skin, information on the effect of vesicles on drug permeation rate, the permeation pathway and perturbations of the skin ultrastructure is of importance. In the second part of this paper the possible interactions between vesicles and skin are described, focusing on differences between the effects of gel-state vesicles, liquid-state vesicles and elastic vesicles.     

The role of ceramide chain length distribution on the barrier properties of the skin lipid membranes

The skin barrier function is provided by the stratum corneum (SC). The lipids in the SC are composed of three lipid classes: ceramides (CERs), cholesterol (CHOL) and free fatty acids (FFAs) which form two crystalline lamellar structures. In the present study, we investigate the effect of CER chain length distribution on the barrier properties of model lipid membranes mimicking the lipid composition and organization of SC. The membranes were prepared with either isolated pig CERs (PCERs) or synthetic CERs. While PCERs have a wide chain length distribution, the synthetic CERs are quite uniform in chain length. The barrier properties were examined by means of permeation studies using hydrocortisone as a model drug. Our studies revealed a reduced barrier in lipid membranes prepared with PCERs compared to synthetic CERs. Additional studies revealed that a wider chain length distribution of PCERs results in an enhanced hexagonal packing and increased conformational disordering of the lipid tails compared to synthetic CERs, while the lamellar phases did not change. This demonstrates that the chain length distribution affects the lipid barrier by reducing the lipid ordering and density within the lipid lamellae. In subsequent studies, the effect of increased levels of FFAs or CERs with a long acyl chain in the PCERs membranes was also studied. These changes in lipid composition enhanced the level of orthorhombic packing, reduced the conformational disordering and increased the barrier of the lipid membranes. In conclusion, the CER chain length distribution is an important key factor for maintaining a proper barrier.     

Comparison of rat epidermal keratinocyte organotypic culture (ROC) with intact human skin: lipid composition and thermal phase behavior of the stratum corneum.

The present report is a part of our continuing efforts to explore the utility of the rat epidermal keratinocyte organotypic culture (ROC) as an alternative model to human skin in transdermal drug delivery and skin irritation studies of new chemical entities and formulations. The aim of the present study was to compare the stratum corneum lipid content of ROC with the corresponding material from human skin. The lipid composition was determined by thin-layer chromatography (TLC) and mass-spectrometry, and the thermal phase transitions of stratum corneum were studied by differential scanning calorimetry (DSC). All major lipid classes of the stratum corneum were present in ROC in a similar ratio as found in human stratum corneum. Compared to human skin, the level of non-hydroxyacid-sphingosine ceramide (NS) was increased in ROC, while alpha-hydroxyacid-phytosphingosine ceramide (AP) and non-hydroxyacid-phytosphingosine ceramides (NP) were absent. Also some alterations in fatty acid profiles of ROC ceramides were noted, e.g., esterified omega-hydroxyacid-sphingosine contained increased levels of oleic acid instead of linoleic acid. The fraction of lipids covalently bound to corneocyte proteins was distinctly lower in ROC compared to human skin, in agreement with the results from DSC. ROC underwent a lipid lamellar order to disorder transition (T2) at a slightly lower temperature (68 degrees C) than human skin (74 degrees C). These differences in stratum corneum lipid composition and the thermal phase transitions may explain the minor differences previously observed in drug permeation between ROC and human skin.     

Comparison of rat epidermal keratinocyte organotypic culture (ROC) with intact human skin: Lipid composition and thermal phase behavior of the stratum corneum

The present report is a part of our continuing efforts to explore the utility of the rat epidermal keratinocyte organotypic culture (ROC) as an alternative model to human skin in transdermal drug delivery and skin irritation studies of new chemical entities and formulations. The aim of the present study was to compare the stratum corneum lipid content of ROC with the corresponding material from human skin. The lipid composition was determined by thin-layer chromatography (TLC) and mass-spectrometry, and the thermal phase transitions of stratum corneum were studied by differential scanning calorimetry (DSC). All major lipid classes of the stratum corneum were present in ROC in a similar ratio as found in human stratum corneum. Compared to human skin, the level of non-hydroxyacid-sphingosine ceramide (NS) was increased in ROC, while α-hydroxyacid-phytosphingosine ceramide (AP) and non-hydroxyacid-phytosphingosine ceramides (NP) were absent. Also some alterations in fatty acid profiles of ROC ceramides were noted, e.g., esterified ω-hydroxyacid-sphingosine contained increased levels of oleic acid instead of linoleic acid. The fraction of lipids covalently bound to corneocyte proteins was distinctly lower in ROC compared to human skin, in agreement with the results from DSC. ROC underwent a lipid lamellar order to disorder transition (T₂) at a slightly lower temperature (68 °C) than human skin (74 °C). These differences in stratum corneum lipid composition and the thermal phase transitions may explain the minor differences previously observed in drug permeation between ROC and human skin.     

The influence of two azones and sebaceous lipids on the lateral organization of lipids isolated from human stratum corneum

The main problem with topical application of compounds to administer drugs to and regulate drug levels in a human body, is the barrier formed by the intercellular lipid matrix of the stratum corneum (SC). In a search for possibilities to overcome this barrier function, a good understanding of the organization and phase behavior of these lipids is required. SC lipid model studies especially provide a wealth of information with respect to the lipid organization and the importance of certain subclasses of lipids for the structure. Previously, we have shown that electron diffraction (ED) provides detailed information on the lateral lipid packing in both intact SC (G.S.K. Pilgram et al., J. Invest. Dermatol. 113 (1999) 403) and SC lipid models (G.S.K. Pilgram et al., J. Lipid Res. 39 (1998) 1669). In the present study, we used ED to examine the influence of two azones and sebaceous lipids on the lateral phase behavior of lipids isolated from human SC. We established that human SC lipids are arranged in an orthorhombic packing pattern. Upon mixing with the two enhancers the orthorhombic packing pattern was still observed; however, an additional fluid phase became more apparent. In mixtures with sebaceous lipids, the presence of the hexagonal lattice increased. These findings provide a basis for the mechanism by which these enhancers and sebaceous lipids interact with human SC lipids.     

The nanoscopic molecular pathway through human skin

BackgroundKnowledge regarding the barrier properties of human skin is important for understanding skin pathology, developing of transdermal drug delivery systems and computational skin absorption models; however, the molecular pathways through human skin remains to be fully investigated on a nanoscopic level. In particular the nanoscopic pathway of molecules passing the intercellular lipid bilayers separating the corneocytes in the stratum corneum (SC) is not fully elucidated.MethodsUsing stimulated emission depletion microscopy (STED) and Förster resonance energy transfer (FRET) the molecular pathways through the SC, the main barrier of the skin, are determined for lipophilic and water-soluble molecules at a nanoscopic resolution.ResultsUsing STED and confocal microscopy, water-soluble dyes, were observed to be present in both the corneocytes and in the intercellular lipid matrix, whereas the lipophilic dyes were predominately in the intercellular lipid bilayers. FRET was observed in the SC between the lipophilic and water-soluble dyes, the existence of a minimum possible distance between acceptor and donor molecules of 4.0 ± 0.1 nm was found.ConclusionsThe results indicate that lipophilic molecules penetrate the stratum corneum via the intercellular lipids bilayers separating the corneocytes in the SC, while the more water-soluble molecules penetrate the stratum corneum via the transcellular route through the corneocytes and intercellular lipid bilayers via the polar head groups of lipid molecules in the bilayers.General significanceKnowledge of the nanoscopic molecular pathways through human skin will help understand the skin barrier function and will be of use for computational skin absorption models and transdermal drug delivery strategies.     

The skin barrier in healthy and diseased state

The primary function of the skin is to protect the body for unwanted influences from the environment. The main barrier of the skin is located in the outermost layer of the skin, the stratum corneum. The stratum corneum consists of corneocytes surrounded by lipid regions. As most drugs applied onto the skin permeate along the lipid domains, the lipid organization is considered to be very important for the skin barrier function. It is for this reason that the lipid organization has been investigated quite extensively. Due to the exceptional stratum corneum lipid composition, with long chain ceramides, free fatty acids and cholesterol as main lipid classes, the lipid organization is different from that of other biological membranes. In stratum corneum, two lamellar phases are present with repeat distances of approximately 6 and 13 nm. Moreover the lipids in the lamellar phases form predominantly crystalline lateral phases, but most probably a subpopulation of lipids forms a liquid phase. Diseased skin is often characterized by a reduced barrier function and an altered lipid composition and organization. In order to understand the aberrant lipid organization in diseased skin, information on the relation between lipid composition and organization is crucial. However, due to its complexity and inter-individual variability, the use of native stratum corneum does not allow detailed systematic studies. To circumvent this problem, mixtures prepared with stratum corneum lipids can be used. In this paper first the lipid organization in stratum corneum of normal and diseased skin is described. Then the role the various lipid classes play in stratum corneum lipid organization and barrier function has been discussed. Finally, the information on the role various lipid classes play in lipid phase behavior has been used to interpret the changes in lipid organization and barrier properties of diseased skin.     

The skin barrier in healthy and diseased state

The primary function of the skin is to protect the body for unwanted influences from the environment. The main barrier of the skin is located in the outermost layer of the skin, the stratum corneum. The stratum corneum consists of corneocytes surrounded by lipid regions. As most drugs applied onto the skin permeate along the lipid domains, the lipid organization is considered to be very important for the skin barrier function. It is for this reason that the lipid organization has been investigated quite extensively. Due to the exceptional stratum corneum lipid composition, with long chain ceramides, free fatty acids and cholesterol as main lipid classes, the lipid organization is different from that of other biological membranes. In stratum corneum, two lamellar phases are present with repeat distances of approximately 6 and 13 nm. Moreover the lipids in the lamellar phases form predominantly crystalline lateral phases, but most probably a subpopulation of lipids forms a liquid phase. Diseased skin is often characterized by a reduced barrier function and an altered lipid composition and organization. In order to understand the aberrant lipid organization in diseased skin, information on the relation between lipid composition and organization is crucial. However, due to its complexity and inter-individual variability, the use of native stratum corneum does not allow detailed systematic studies. To circumvent this problem, mixtures prepared with stratum corneum lipids can be used. In this paper first the lipid organization in stratum corneum of normal and diseased skin is described. Then the role the various lipid classes play in stratum corneum lipid organization and barrier function has been discussed. Finally, the information on the role various lipid classes play in lipid phase behavior has been used to interpret the changes in lipid organization and barrier properties of diseased skin.     

Ethanol effects on the stratum corneum lipid phase behavior.

The stratum corneum is considered to be the diffusional barrier of mammalian skin for water and most solutes. The intercellular lipid multilayer domains of the stratum corneum are believed to be the diffusional pathway for most lipophilic solutes. Fluidization of the lipid multilayers in the presence of ethanol is frequently conceived to result in enhanced permeation. Current investigations address the effect of ethanol on the phase behavior in terms of stratum corneum lipid alkyl chain packing, mobility and conformational order as measured by Fourier transform infrared (FTIR) spectroscopy. Phospholipid multilamellar vesicles were also studied as model systems. There appeared to be no effect of ethanol on either the solid-solid phase transition or the gel phase interchain coupling of the stratum corneum lipids. However, there was a reduction in the mobility of the alkyl chains in the presence of ethanol. Possible mechanistic relationships between the current FTIR spectroscopic results with available literature data of ethanol induced lipophilic solute penetration enhancement through the skin are discussed.     

Influence of different ceramides on the structure of in vitro model lipid systems of the stratum corneum lipid matrix

Human stratum corneum (SC) consists of several layers of keratinized corneocytes embedded in a lipid matrix of ordered lamellar structure which is considered to constitute the major barrier to percutaneous penetration. Artificial mixtures of SC lipids are often used as model systems to mimic the skin barrier or to investigate the effects of substances on the phase behaviour of the models. In the present study a SC lipid model composed of cholesterol, fatty acids and ceramides was used to investigate the effect of three different commercially available ceramide types on the microstructure and the physicochemical behaviour of the lipids. Polarized light microscopy, transmission electron microscopy, small-angle X-ray diffraction, wide-angle X-ray diffraction and differential scanning calorimetry (DSC) were used for physicochemical characterization. The results revealed a lamellar structure for all models but showed differences with regard to the thermal and optical behaviour depending obviously on the composition of the ceramide mixtures. A model containing a mixture of Cer[AS] was comparable to human SC lipids.     

Enhancement of Galantamine HBr Skin Permeation Using Sonophoresis and Limonene-Containing PEGylated Liposomes

This study aimed to investigate the effect of low-frequency sonophoresis (SN) and limonene-containing PEGylated liposomes (PL) on the transdermal delivery of galantamine HBr (GLT). To evaluate the skin penetration mechanism, confocal laser scanning microscopy (CLSM), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC) were employed. The application of SN led to more GLT penetration into and through the skin than GLT solution alone. The liposomes also improved GLT permeation, and 2% limonene-containing PL (PL-LI2%) exhibited the highest GLT permeation, followed by PL-LI1%, PL-LI0.1%, and PL. The CLSM images of PL-LI2% resulted in the highest fluorescence intensity of fluorescent hydrophilic molecules in the deep skin layer, and the rhodamine PE-labeled liposome membrane was distributed in the intercellular region of the stratum corneum (SC). PL-LI2% induced significant changes in intercellular lipids in the SC, whereas SN had no effect on intercellular lipids of the SC. DSC thermograms showed that the greatest decrease in the lipid transition temperature occurred in PL-LI2%-treated SC. SN might improve drug permeation through an intracellular pathway, while limonene-containing liposomes play an important role in delivering GLT through an intercellular pathway by increasing the fluidity of intercellular lipids in the SC. Moreover, a small vesicle size and high membrane fluidity might enhance the transportation of intact vesicles through the skin.     

Interactions of dipalmitoylphosphatidylcholine with ceramide-based mixtures

The outermost layer of the skin, the stratum corneum (SC), acts as the natural physical barrier. The SC consists of corneocytes embedded in a crystalline lipid matrix consisting of ceramides, free fatty acids and cholesterol.Although phospholipids are frequently present in topical formulations, no detailed information is reported on the interactions between phospholipids and SC lipids. The aim of this study was to examine the interactions between a model phospholipid, dipalmitoylphosphatidylcholine (DPPC) and synthetic ceramide-based mixtures (referred to as SC lipids).(Perdeuterated) DPPC was mixed with SC lipids and the lipid organization and mixing properties were examined. The studies revealed that DPPC participates in the same lattice as SC lipids thereby enhancing a hexagonal packing. Even at a high DPPC level, no phase separated pure DPPC was observed.When a DPPC containing formulation is applied to the skin surface it must partition into the SC lipid matrix prior to any mixing with the SC lipids. To mimic this, DPPC was applied on top of a SC lipid membrane. DPPC applied in a liquid crystalline state was able to mix with the SC lipids and participated in the same lattice as the SC lipids. However, when DPPC was applied in a rippled gel-state very limited partitioning of DPPC into the SC lipid matrix occurred. Thus, when applied to the skin, liquid crystalline DPPC will have very different interactions with SC lipids than DPPC in a (rippled-)gel phase.