The use of FT-IR for the determination of stratum corneum hydration in vitro and in vivo

Measurement of the water content of stratum corneum plays an important role in physiological and therapeutic inquiries in dermatology. There are many techniques available for non-invasive determination of skin hydration such as measurement of electrical, mechanical, thermal and spectroscopic properties of the skin. Most techniques, however, suffer from the fact that they do not employ a direct measurement of water content rather a property caused by skin hydration. Recently, Potts et al., (Arch. Derm. Res. 277, 489–495, 1985) developed an FT-IR method for the determination of water content of the skin both in vitro and in vivo. The method employed attenuated total reflectance infrared (ATR-IR) to measure a weak OH stretch formed by the presence of water at 2100 cm⁻¹. This absorbance is distant from interferences due to skin and most topically applied substances and therefore may be used in the quantitation of skin water content (hydration). This report describes the use of this technique in an investigation into the effect of occlusion on the water content of the skin. Method development and validation employing an in vitro system is also discussed.     

Alteration of skin hydration and its barrier function by vehicle and permeation enhancers: a study using TGA, FTIR, TEWL and drug permeation as markers

Vehicles and permeation enhancers (PEs) used in transdermal drug delivery (TDD) of a drug can affect skin hydration, integrity and permeation of the solute administered. This investigation was designed to study the effect of the most commonly used vehicles and PEs on rat skin hydration, barrier function and permeation of an amphiphilic drug, imipramine hydrochloride (IMH). An array of well-established techniques were used to confirm the findings of the study. Thermogravimetric analysis (TGA) and Fourier transform infrared (FTIR) spectroscopy were used to determine changes in skin hydration. Alteration of the stratum corneum (SC) structure was investigated using FTIR studies. To monitor the barrier function alteration, transepidermal water loss (TEWL) measurement and permeation studies were performed. Our findings indicate that with hydration, there was an increase in the bound water content of the skin, and pseudoequilibrium of hydration (a drastic decrease in hydration rate) was achieved at around 12 h. Hydration increased the ratio between amide-I and amide-II peaks in FTIR and reduced the C-H stretching peak area. Both propylene glycol (PG) and ethanol (EtOH) dehydrated skin, with the latter showing a predominant effect. Furthermore, it was confirmed that PG and EtOH decreased the bound water content due to alteration in the protein domains and extraction of SC lipids, respectively. The effect of hydration on the SC was found to be similar to that reported for temperature. Permeation studies revealed that the dehydration caused by vehicles decreased IMH flux, whereas the flux was enhanced by PEs. The role of partition was predominant for the permeation of IMH through dehydrated skin. A synergistic effect was observed for PG and menthol in the enhancement of IMH. Further findings provided strong evidence that PG affects protein domains and EtOH extracts lipids from the bilayer. Both PG and EtOH, with or without PEs, increased TEWL. Initial TEWL was well correlated with the flux of IMH through the same skin. It was found that both PG and EtOH affect the permeation of solute and TEWL by dehydration. The experiments also proved that the initial TEWL value has a strong potential as a predictive tool for the permeation of the solute.     

Determination of water content in skin by using a ft near infrared spectrometer

The water content of skin was determined using a FT near infrared (NIR) spectrometer. NIR diffuse reflectance spectra were collected from hairless mouse, in vitro, and from human inner arm, in vivo. It was found that the variation of NIR absorbance band 1450 nm from OH vibration of water and 1940 nm from the combination involving OH stretching and OH deformation, depending on the absolute water content of separated hairless mouse skin, in vitro, using the FT NIR spectrometer. Partial least squares regression (PLSR) was applied to develop a calibration model. The PLS model showed good correlation. For practical use of the evaluation of human skin moisture, the PLS model for human skin moisture was developed in vivo on the basis of the relative water content of stratum corneum from the conventional capacitance method. The PLS model predicted human skin moisture with a standard errors of prediction (SEP) of 3.98 at 1130-1830 nm range. These studies showed the possibility of a rapid and nondestructive skin moisture measurement using FT NIR spectrometer.     

Influence of hydration on epidermal tissue

The influence of hydration on the dynamic properties of rat epidermal skin has been determined with the use of solid-state NMR. Samples were prepared with varying degrees of hydration using deuterium oxide and then subjected to CP-MAS (cross-polarization-magic-angle spinning) with a variable contact time. By fitting the carbon magnetization as a function of contact time, estimates of the rates of polarization transfer and decay of magnetization were obtained. Increasing water content up to 50% (w/w) caused a progressive decrease in the apparent rate of proton-carbon transfer of polarization as well as an accelerated decay of the carbonyl magnetization. This may indicate that there is greater mobility of the skin components in the presence of water. The results are suggestive for providing quantitative information for the change in the effective diffusion coefficient of water in skin with increasing hydration.     

药用库拉索芦荟活性多糖的护肤特性研究

目的研究库拉索芦荟多糖的保湿特性、经皮吸收特性以及皮肤安全性。方法采用超声辅助提取芦荟活性多糖,并通过人体皮肤水分含量和水分散失量测试法对所制备的芦荟活性多糖的保湿特性（包括锁水、保水两个方面）进行评价;Franz扩散池体外透皮吸收法评价其透皮吸收特性;人体斑贴试验评价其皮肤安全性。结果与空白对照组比较,库拉索芦荟多糖可显著提高皮肤水合状态（P〈0.05）,同时降低经皮失水（P〈0.05）;多糖质量浓度为5%时,皮肤渗透速率达到0.2510mg/（h·cm2）,24h累计渗透量达（4.1519±0.0462）mg/cm2;人体皮肤斑贴试验中全部评定为0级反应。结论库拉索芦荟活性多糖对皮肤具有显著的保湿功效以及较强的皮肤渗透力,并且高度安全,可作为优良的保湿添加剂在化妆品等肤用产品中广泛应用。     

Iron concentrations in human dermis assessed by microdialysis associated with atomic absorption spectrometry

Until recently, the determination of metallic elements concentrations in normal skin, in vivo, was rare due to the lack of non-invasive techniques. Microdialysis has the advantage of being slightly invasive when applied to the collection in vivo of endogenous or exogenous substances from the skin. Iron is an active element in different cutaneous disorders. The aim of this work was to assess iron by atomic absorption spectrometry (AAS) after the collection of samples by microdialysis from human dermis. A first essential step, before determining the in vivo iron concentration in human dermis, was to establish an experimental protocol applicable to ex vivo as well as in vivo conditions. For this reason, this work deals only with the assessment of iron in ex vivo human dermis. A skin microdialysis technique and a calibration method, the No Net Flux, were used to quantify basal iron concentrations in human dermis and the same method was also used to determine in vitro and ex vivo iron recoveries. No differences were detected between in vitro and ex vivo recoveries. Ex vivo basal iron dermis concentrations ranged from 3.6 to 7.7 microg/l. This study shows that non-invasive microdialysis is an efficient method for sampling iron from human skin. A sensitive and accurate AAS technique was able to assess low iron concentrations in human dermis. The strategy adopted for this work was efficient and appropriate for the determination of iron in human skin and experiments will be carried out in vivo.     

An improved in vitro method for measuring skin permeability that controls excess hydration of skin using modified Franz diffusion cells

When liquid donors/receivers are used for in vitro skin permeation studies, excess hydration can change skin properties compared to in vivo conditions. A novel in vitro method of determining the permeability of drugs through skin was developed that avoids exposing the membrane to dilute donor/receiver solutions. The drug is dissolved in an unstirred donor gel, and diffuses through a membrane into an unstirred gel receiver that can potentially be adjusted to mimic physiological conditions. Pulsatile microdialysis (PMD) was used to sample local concentrations in the receiver medium, and a model was developed to allow the determination of permeability. For Doxepin HCl, permeabilities through artificial membranes and human cadaver skin were determined using the new and previously reported methods. For artificial membranes that minimally hydrate, the new method gave consistent but slightly lower permeability values. For human cadaver skin, the permeability determined using the new method was 1/6 that of the fully hydrated skin. Limitations of the model, their relations to experimental design and data analysis were evaluated. It was concluded that this method can be applied to characterize membrane permeabilities using experiments that may avoid membrane breakdown and more closely mimic physiological conditions.     

Phospholipid polymer, 2-methacryloyloxyethyl phosphorylcholine and its skin barrier function

The effect of poly[2-methacryloyloxyethyl phosphorylcholine] (pMPC) on the skin permeation property was investigated by performing in vitro skin permeation study of a model drug, nicotinic acid (NA). Effect of pMPC polymer in donor solution on skin permeation rates was evaluated using side-by-side diffusion cells. Also, the structural alterations in the stratum corneum (SC), inter-lamellar bilayer (ILB) and dermis layers in pMPC-treated and -untreated skin sections were investigated with transmission electron microscopy (TEM). The permeation profile of NA without pMPC in donor solution showed biphasic mode: initial 1st phase and 2nd hydration phase. The sudden, more than 10-fold increase in flux from the initial steady state (43.5 microg/cm2/hr) to the 2nd hydration phase (457.3 microg/cm2/h) suggests the disruption of skin barrier function due to extensive hydration. The permeation profile of NA with 3% pMPC in the donor solution showed monophasic pattern: the steady state flux (10.9 microg/cm2/h) without abrupt increase of the flux. The degree of NA permeation rate decreased in a concentration-dependent manner of pMPC. TEM of skin equilibrated with water or 2% pMPC for 12 h showed that corneocytes are still cohesive and epidermis is tightly bound to dermis in 2% pMPC-treated skin, while wider separation between corneocytes and focal dilations in inter-cellular spaces were observed in water-treated skin. This result suggests that pMPC could protect the barrier property of the stratum corneum by preventing the disruption of ILB structure caused by extensive skin hydration during skin permeation study.     

Effect of Hydration on Opacity in the Bovine Corneal Opacity and Permeability (BCOP) Assay

Abstract

The bovine corneal opacity and permeability (BCOP) assay has been proposed an an in vitro method for predicting the irritancy or toxicity of chemical substances. A loss of corneal transparency, which reduces visual acuity, can be the result of an increase in corneal hydration or direct damage to corneal tissues. The BCOP assay as currently practiced does not differentiate between these mechanisms. The purpose of this study was to determine the effect of increased hydration on transparency of the bovine cornea and to measure the hydration levels of corneas treated with compounds and substances known, from previous BCOP studies, to cause corneal opacity. Corneas were treated according to the BCOP protocol and corneal opacity was determined by measurement of light absorbance at 570 nm (A₅₇₀). Corneal hydration was determined by measurement of wet and dry weights. Hydration of the intact cornea was 3.86 mg H₂O/mg dry weight and A₅₇₀ = 0.048. After removal of the epithelium and incubation in MEM, corneal hydration increases to 6.27 mg H₂O/mg and A₅₇₀ = 0.11. Hydration of the maximally hydrated cornea, following removal of the epithelium and endothelium and incubation in deionized water, is 16.33 mg H₂O/mg and A₅₇₀ = 0.67. The hydration of corneas treated with compounds that are ranked as moderate to severe irritants based on the BCOP assay is not correlated with hydration levels of these corneas, since the A₅₇₀ of these corneas was usually higher than predicted from hydration levels. For example, after exposure to isopropanol hydration = 4.7, A₅₇₀ = 0.59; acetone: hydration = 5.69, A₅₇₀ = 1.38; 30% trichloroacetic acid: hydration = 4.28, A₅₇₀ = 1.43; 1% NaOH: hydration = 8.22, A₅₇₀ = 1.7. In contrast, after exposure to 30% sodium lauryl sulfate (SLS), which only removes epithelium, hydration = 5.38, A₅₇₀ = 0.095. This suggests that opacity in excess of that caused by increased hydration is caused by damage to corneal tissue, including epithelium and stroma. This was confirmed by light and transmission electron microscopy. Determining the cause of an increase in corneal opacity in the BCOP assay is important since tissue damage may be irreversible, while the cornea can recover from moderate increases in hydration. On the other hand, since increases in hydration alone cause relatively low A₅₇₀ readings, a potential irritant that causes a visually significant increase in cornea hydration might be ranked as a mild irritant. It is therefore proposed that measurement of corneal hydration be added to the BCOP protocol.     

Multi-species assessment of electrical resistance as a skin integrity marker for in vitro percutaneous absorption studies.

Assessment of percutaneous absorption in vitro provides key information when predicting dermal absorption in vivo. Confirmation of skin membrane integrity is an essential component of the in vitro method, as described in test guideline OECD 428. Historically, assessment of the membrane's permeability to tritiated water (T2O) and the generation of a permeability coefficient (Kp) were used to confirm that the skin membrane was intact prior to application of the test penetrant. Measuring electrical resistance (ER) across the membrane is a simpler, quicker, safer and more cost effective method. To investigate the robustness of the ER integrity measure, the Kp values for T2O for a range of human and animal skin membranes were compared with corresponding ER data. Overall, for human, rat, pig, mouse, rabbit and guinea pig skin, the ER data gave a good inverse association with the corresponding Kp values; the higher the Kp the lower the ER values. In addition, the distribution across a large dataset for individual skin samples was similar for Kp and ER, allowing a cut-off value for ER to be established for each skin type. Based on CTL's (Syngenta Central Toxicology Laboratory) standard static diffusion cells and databridge, we propose that intact skin should have an ER equal to or above (in kOmega): human (10), mouse (5) guinea pig (5), pig (4) rat (3), and rabbit (0.8). We conclude that measurement of ER across in vitro skin membranes provides a robust measurement of skin barrier integrity and is an appropriate alternative to Kp for T2O in order to identify intact membranes that have acceptable permeability characteristics for in vitro percutaneous absorption studies.     

Effect of bicellar systems on skin properties

Bicelles are discoidal aggregates formed by a flat dimyristoyl-glycero-phosphocholine (DMPC) bilayer, stabilized by a rim of dihexanoyl-glycero-phosphocholine (DHPC) in water. Given the structure, composition and the dimensions of these aggregates around 10-50 nm diameter, their use for topical applications is a promising strategy. This work evaluates the effect of DMPC/DHPC bicelles with molar ratio (2/1) on intact skin. Biophysical properties of the skin, such as transepidermal water loss (TEWL), elasticity, skin capacitance and irritation were measured in healthy skin in vivo. To study the effect of the bicellar systems on the microstructure of the stratum corneum (SC) in vitro, pieces of native tissue were treated with the aforementioned bicellar system and evaluated by freeze substitution applied to transmission electron microscopy (FSTEM). Our results show that bicelles increase the TEWL, the skin elastic parameters and, decrease skin hydration without promoting local signs of irritation and without affecting the SC lipid microstructure. Thus, a permeabilizing effect of bicelles on the skin takes place possibly due to the changes in the phase behaviour of the SC lipids by effect of phospholipids from bicelles.     

Effects of Cream Containing Ficus carica L. Fruit Extract on Skin Parameters: In vivo Evaluation

This study was aimed to investigate the effects of cream containing Ficus carica L. fruit (Fig) extract on various skin parameters such as skin melanin, erythema, moisture content, trans-epidermal water loss and sebum. For this purpose, formulation with 4% concentrated extract of F. carica fruit and base without extract were developed. Base served as a control. Both base and formulation were applied to the cheeks of human volunteers for 8 weeks to investigate the effects on different skin parameters using non-invasive bioengineering instruments. Formulation decreased the skin melanin, trans-epidermal water loss and skin sebum significantly. Formulation increased the skin hydration significantly and insignificant effects on skin erythema. We concluded that a stable topical cream (w/o emulsion) containing F. carica fruit extract have effects on skin melanin, trans-epidermal loss, hydration values and sebum content and possibly could be used against for hyper pigmentation, acne, freckles and wrinkle.     

An alkylpolyglucoside surfactant as a prospective pharmaceutical excipient for topical formulations: The influence of oil polarity on the colloidal structure and hydrocortisone in vitro/in vivo permeation

There is a growing need for research into new skin- and environment-friendly surfactants. This paper focuses on a natural surfactant of an alkylpolyglucoside type, which can form both thermotropic and lyotropic liquid-crystalline phases. The aim of this study was to relate some physicochemical properties (characterised by polarisation and transmission electron microscopy, thermal analysis and rheology) of the three formulations based on cetearyl glucoside and cetearyl alcohol, to the results of in vitro and in vivo bioavailability of hydrocortisone (HC). The three formulations contained oils of different polarity (medium chain triglycerides: MG, isopropyl myristate: IPM and light liquid paraffin: LP), respectively. In vitro permeation was followed through the artificial skin constructs (ASC), while the parameters measured in vivo were erythema index: EI (using instrumental human skin blanching assay), transepidermal water loss (TEWL) and stratum corneum hydration (SCH). The vehicles based on cetearyl glucoside and cetearyl alcohol showed a complex colloidal structure of lamellar liquid-crystalline and lamellar gel-crystalline type, depending on oil polarity. Rheological profile of the vehicle was directly related to the in vitro profile of the HC permeation. In vivo results suggested that the vehicle with MG retarded the HC permeation, whereas less polar IPM and non-polar LP enhanced it. It is suggested that the enhancement is achieved either by a direct interaction with lipid lamellae of the SC or indirectly by improving skin hydration.

There were no adverse effects during in vivo study, which indicates a good safety profile of this alkylpolyglucoside surfactant.     

Effect of lipid-containing, positively charged nanoemulsions on skin hydration, elasticity and erythema--an in vivo study

Dry skin and other skin disorders such as atopic dermatitis are characterized by impaired stratum corneum (SC) barrier function and by an increase in transepidermal water loss (TEWL) leading to a decrease in skin hydration. The possibility that dermatological and cosmetic products containing SC lipids could play a part in the restoration of disturbed skin barrier function is of great interest in the field of dermatology and cosmetics. The aim of the present study was to evaluate the effect of positively charged oil/water nanoemulsions (PN) containing ceramide 3B and naturally found SC lipids (PNSC) such as ceramide 3, cholesterol, and palmitic acid on skin hydration, elasticity, and erythema. Creams of PNSC were compared to PN creams, to creams with negatively charged o/w nanoemulsion and SC lipids (NNSC) and to Physiogel cream, a SC lipid containing formulation, which is already on the market. The formulations (PN, PNSC, and NNSC) were prepared by high-pressure homogenization. After adding Carbopol 940 as thickener, particle size and stability of the creams were not significantly changed compared to the nanoemulsions. The studies were carried out on three groups, each with 14 healthy female test subjects between 25 and 50 years of age, using Corneometer 825, Cutometer SEM 575 and Mexameter 18 for measurements of skin hydration, elasticity, and erythema of the skin, respectively. The creams were applied regularly and well tolerated throughout the study. All formulations increased skin hydration and elasticity. There was no significant difference between PNSC and Physiogel. However, PNSC was significantly more effective in increasing skin hydration and elasticity than PN and NNSC indicating that phytosphingosine inducing the positive charge, SC lipids and ceramide 3B are crucial for the enhanced effect on skin hydration and viscoelasticity.     

Influence of DMPS on the water retention capacity of electroporated stratum corneum: ATR-FTIR study

Anionic lipids like phosphatidylserine are known to significantly enhance electroporation mediated transepidermal transport of polar solutes of molecular weights up to 10 kDa. The underlying mechanism of the effect of anionic lipids on transdermal transport is not fully understood. The main barrier to transdermal transport lies within the intercellular lipid matrix (ILM) of the stratum corneum (SC) and our previous studies indicate that dimyristoyl phosphatidylserine (DMPS) can perturb the packing of this lipid matrix. Here we report on our investigation on water retention in the SC following electroporation in the presence and the absence of DMPS. The water content in the outer most layers of the SC of full thickness porcine skin was determined using ATR-FTIR-spectroscopy. The results show that in the presence of DMPS, the SC remains in a state of enhanced hydration for longer periods after electroporation. This increase in water retention in the SC by DMPS is likely to play an important role in trans-epidermal transport, since improved hydration of the skin barrier can be expected to increase the partitioning of polar solutes and possibly the permeability.     

Biophysical study of porcine ear skin in vitro and its comparison to human skin in vivo

The goal of this work was to establish, using biophysical characterization, that porcine ear skin in vitro is a valid model for its human counterpart. Specifically, stratum corneum (SC) barrier function was evaluated during its progressive removal by adhesive tape-stripping using the techniques of transepidermal water loss (TEWL) and impedance spectroscopy. TEWL increased slowly at first and then more rapidly with the degree of SC impairment. In contrast, low-frequency skin impedance declined exponentially as a function of progressive SC removal. The methods provide complementary and correlated information about SC barrier function. Biophysical parameters, including the diffusivity and permeability coefficient of water across the SC, and the thickness of the barrier were determined from the TEWL data using Fick's first law of diffusion. Furthermore, an ionic partition coefficient-mobility product was estimated from the skin impedance measurements. Comparison of the results with those previously reported for human skin in vivo strongly supports the validity of the porcine membrane as an in vitro model.     

Determination of the free/included piroxicam ratio in cyclodextrin complexes: comparison between UV spectrophotometry and differential scanning calorimetry.

Few analytical techniques allow to evaluate the inclusion yield of cyclodextrin-drug complexes, because most manufacturing processes give amorphous products. In this study, we have developed an alternative method to differential scanning calorimetry, to accurately determine the free/complexed piroxicam ratio by UV spectroscopy. This method is based on the differential solubility of the piroxicam-beta-cyclodextrin 1:2.5 mol/mol complex in water-acetonitrile (1:1, v/v) (Solvent A) or in anhydrous acetonitrile (Solvent B), both containing 0.05 M HCl. In anhydrous acetonitrile, beta-cyclodextrin is insoluble and the included drug remains entrapped, allowing the free piroxicam determination, while with 50% of water, the complex is totally dissolved, allowing the determination of the total guest content. This method was validated for linearity, precision and accuracy. The presence of cyclodextrin does not influence the assays, but more than 0.5% of water in Solvent B significantly affects the determination of the free piroxicam content. In comparison with differential scanning calorimetry, both detectability and precision were improved. It is now possible to analyse complexes with an inclusion purity greater than 99%.     

The influence of water mixtures on the dermal absorption of glycol ethers

Glycol ethers are solvents widely used alone and as mixtures in industrial and household products. Some glycol ethers have been shown to have a range of toxic effects in humans following absorption and metabolism to their aldehyde and acid metabolites. This study assessed the influence of water mixtures on the dermal absorption of butoxyethanol and ethoxyethanol in vitro through human skin. Butoxyethanol penetrated human skin up to sixfold more rapidly from aqueous solution (50%, 450 mg/ml) than from the neat solvent. Similarly penetration of ethoxyethanol was increased threefold in the presence of water (50%, 697 mg/ml). There was a corresponding increase in apparent permeability coefficient as the glycol ether concentration in water decreased. The maximum penetration rate of water also increased in the presence of both glycol ethers. Absorption through a synthetic membrane obeyed Fick's Law and absorption through rat skin showed a similar profile to human skin but with a lesser effect. The mechanisms for this phenomenon involves disruption of the stratum corneum lipid bilayer by desiccation by neat glycol ether micelles, hydration with water mixtures and the physicochemical properties of the glycol ether-water mixtures. Full elucidation of the profile of absorption of glycol ethers from mixtures is required for risk assessment of dermal exposure. This work supports the view that risk assessments for dermal contact scenarios should ideally be based on absorption data obtained for the relevant formulation or mixture and exposure scenario and that absorption derived from permeability coefficients may be inappropriate for water-miscible solvents.     

The determination of ofloxacin in human skin tissue by high-performance liquid chromatography and correlation between skin tissue concentration and serum level of ofloxacin

Abstract— A simple high-performance liquid chromatographic (HPLC) assay was developed for the measurement of ofloxacin in skin tissue, and the correlation between serum levels and skin tissue levels of ofloxacin was determined. The homogenized skin samples were directly introduced into an HPLC system after filtering through a Molcut II membrane filter to remove proteins. The filtrate was concentrated on a pre-column using a phenyl-stationary phase and was then introduced to an analytical column using an ODS-stationary phase by column-switching techniques. Ofloxacin and lomefloxacin as an internal standard were detected by UV absorbance at 300 nm. Determination was possible for ofloxacin over the concentration range 250–3000 ng (g skin tissue)^?1; the limit of detection was 100 ng g^?1. The absolute recovery of ofloxacin added to skin tissue homogenate was over 72% with a coefficient of variation of less than 4·1%. This method is applied to determination of skin tissue level of ofloxacin in patients after treatment with ofloxacin. The correlation between serum levels and skin tissue levels of ofloxacin was determined for 30 patients after oral administration of ofloxacin. Good correlation was obtained and the coefficient of correlation was 0·84.     

Interaction of hydration, aging, and carbon content of soil on the evaporation and skin bioavailability of munition contaminants

Water plays a key role in enhancing the permeability of human skin to many substances. To further understand its ability to potentially increase the bioavailability of soil contaminants, artificial sweat was applied to excised pig skin prior to dosing with munition-contaminated soils. Skin was mounted in chambers to allow simultaneous measurement of evaporation and penetration and to control air flow, which changed the dwell time of skin surface water within a l-h period post application of test materials. Additional variables included type of compound, aging of spiked soil samples, and carbon content of soil. To this end, the evaporation and skin penetration of C-14 labeled hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), 2,6-dinitrotoluene (26DNT), and 2,4,6-trinitrotoluene (TNT) were determined from two soil types, Yolo, having 1.2% carbon, and Tinker, having 9.5% carbon. RDX soil samples aged 27 mo and 62 mo were compared to freshly spiked soils samples. Similarly, 26DNT samples aged 35-36 mo and TNT samples aged 18 mo were compared to freshly spiked samples. Approximately 10 microg/cm(2) of radiolabeled compound was applied in 10 mg/cm(2) of soil. Radiolabel recovered from the dermis and tissue culture media (receptor fluid) was summed to determine percent absorption from the soils. Radiolabel recovered from vapor traps determined evaporation. Mean skin absorption of all compounds was higher for low-carbon soil, regardless of soil age and skin surface water as affected by air flow conditions. For 26DNT, a simultaneous increase in evaporation and penetration with conditions that favored enhanced soil hydration of freshly prepared samples was consistent with a mechanism that involved water displacement of 26DNT from its binding sites. A mean penetration of 17.5 +/- 3.6% was observed for 26DNT in low-carbon soil, which approached the value previously reported for acetone vehicle (24 +/- 6%). 26DNT penetration was reduced to 0.35% under dryer conditions and to 0.08% when no sweat was applied. When soil hydration conditions were not varied from cell to cell, air flow that favored a longer water dwell time increased penetration, but not evaporation, consistent with a mechanism of enhanced skin permeability from a higher hydration state of the stratum corneum. Profiles of 26DNT penetration versus air flow conditions were exponential for freshly prepared soil samples, suggesting strong and weak binding sites; corresponding profiles of 26DNT penetration from aged samples were linear, suggesting a conversion of weak to strong binding sites. Absorption and evaporation was less than 5% for TNT and less than 1% for RDX, regardless of soil type and age. Fresh preparations of RDX in Tinker soil and aged samples of TNT in Yolo soil showed a significant decrease in skin absorption with loss of surface moisture. The penetration rate of radiolabel into the receptor fluid was highest during the 1-2 h interval after dosing with 26DNT or TNT. High-performance liquid chromatography (HPLC) analysis of 26DNT in receptor fluid at maximum flux indicated no metabolism or breakdown. For TNT, however, extensive conversion to monoamino derivatives and other metabolites was observed. Relatively little radioactivity was found in the dermis after 26DNT and TNT applications, and dermal extracts were therefore not analyzed by HPLC. RDX was not sufficiently absorbed from soils to allow HPLC analysis. This study has practical significance, as the use of water for dust control at remediation sites may have the unintended effect of increasing volatilization and subsequent absorption of soil contaminants. Soil in contact with sweaty skin may give the same result. Skin absorption of 26DNT from soil was over 50-fold higher than the value for dryer skin and over 200-fold higher than the value obtained when there was no sweat application. While the hydration effect was less dramatic for RDX and TNT, soil contaminants more closely matching the physical properties of 26DNT may be similarly affected by hydration.