The back diffusion of glucose across human skin in vitro

In-vitro model for assessing glucose diffusion through skin

Pig ear skin membrane-covered glucose biosensor based on oxygen electrode has been assessed as a tool to evaluate glucose penetration through skin in-vitro. For this, glucose oxidase (GOx) was immobilised on oxygen electrode and covered with the skin membrane. Exposing this electrode to the solution of glucose resulted in glucose penetration though skin membrane, its oxidation catalysed by GOx, consumption of O₂ and decrease of the current of the oxygen electrode. By processing the biosensor responses to glucose, we found that glucose penetration through 250 µm thick skin membrane is slow; 90% of steady-state current response was reached in 32( ± 22) min. Apparent diffusion coefficient for glucose in skin was found to be equal to 0.15( ± 0.07)* 10^-6 cm² s^-1. This value is 45 times lower than glucose diffusion coefficient in water. Tape-stripping of stratum corneum (SC) allows considerably faster glucose penetration. The electrodes covered with tape-stripped skin reached 90% of steady-state current response in 5.0(± 2.7) min. The theoretical estimate of glucose flux through SC was considered exploiting four-pathway theory of transdermal penetration. Theoretical flux values were more that three orders lower than measured experimentally. This high discrepancy might indicate that glucose penetration through healthy human skin could be even slower, allowing much lower flux, than it was found in our study for skin membranes from pig ears.

Water Activity and Its Significance in Topical Dosage Forms

Unique properties of thermodynamic activity of solvents in topical semisolids and its effects on in vitro product performance have not been fully understood. Mechanistic investigation was undertaken to demonstrate the significance of thermodynamic potential of solvents [water activity (a_w) or solvent activity (a_s)] on in vitro performance of model topical formulations. Drug transport across synthetic membranes was found to decrease with decreasing water activity of formulations. Similarly, in vitro permeation of model permeant (caffeine) across porcine epidermis was found to decrease with decreasing water activity of formulations. Notably, relatively low water activity formulations (a_w, 0.78) induced dehydration in porcine skin associated with significant structural changes like detachment of individual stratum corneum layers. Inclusion of hydrating agents (propylene glycol) in low water activity (a_w, 0.78) formulations restored hydration levels and structural integrity of porcine skin. Most importantly, incremental inclusion of propylene glycol in low water activity formulations (a_w, 0.78) enhanced in vitro permeation of model permeant (fluorescein sodium). Further investigation revealed that variability in processing conditions (high shear mixing during emulsification step) could modulate water activity in semisolid formulations despite their compositional sameness. In retrospect, water activity was found to be a critical quality attribute of topical semisolid products which impacts overall product performance and drug delivery.

Passive Diffusion of Transdermal Glucose: Noninvasive Glucose Sensing Using a Fluorescent Glucose Binding Protein

The motivation for this study was to determine if a statistically significant correlation exists between blood glucose (BG) and transdermal glucose (TG) collected by passive diffusion. A positive outcome will indicate that noninvasive passive TG diffusion is a painless alternative to collecting blood through a break on the skin. Sampling involves placing a small volume of buffer solution on the surface of membrane or skin for 5 minutes. The sample is then assayed with fluorescent GBP. In vitro testing was done on regenerated cellulose and a porcine skin model to determine diffusion of standard glucose solutions. In vivo testing was done on a healthy subject and a subject with type 2 diabetes. Glucose diffused readily through the regenerated cellulose membrane with good correlation between surface and internal glucose concentrations (R ² = .997). But the porcine skin model required a surface prewash to achieve the same good correlation R ² = .943). Based on this, an optimum prewash step was determined for the in vivo studies. The resulting correlation coefficients between TG and BG after a 15-minute prewash in a healthy subject and type 2 subject were .87 and .93, respectively. Removal of the extraneous glucose in the skin by prewashing was an important step in achieving good correlation between TG and BG. The results suggest that passive collection of TG is a noninvasive alternative to current practice of breaking the skin. Further studies are under way to determine the lag time between TG and BG and for the sampling protocol to be more amenable to point-of-care application.

Effects of Membrane Type and Liquid/Liquid Phase Boundary onIn VitroRelease of Ketoprofen from Gel Formulations

The aim of this study was to test the hypothesis that the most appropriate model for studying the diffusional release of an active from a topical formulation is one in which the membrane offers minimal resistance to release and involves a receptor phase that presents the least possible interfacial discontinuity. Using ketoprofen as the active, a series of simple gels were prepared consisting of PEG400 thickened with Cabosil M5. Using Franz-type diffusion cells, three different types of membrane (two porous and one non-porous) were compared, as were receptor phases of PEG400 (component of formulation) and PBS. Of the membranes tested only 0.2 microm nylon provided consistent first order kinetics for a range of gel consistencies, indicating negligible influence of the membrane. The non-porous silicone membrane did not show first order kinetic profile confirming the diffusional nature of such a membrane. From the non-thickened formulations, diffusional release into a receptor phase of PEG400 was some 3x that into PBS, whereas from the formulation thickened with 5% Cabosil M5, diffusional release into a receptor phase of PEG400 was 6x lower than that into PBS. Diffusional release into PBS did not follow first order kinetics while diffusion into PEG400 did, suggesting that the existence of a discontinuity affected the release process. Although the importance of zero-resistance membranes is of perhaps obvious importance, it is often not stated in the literature. The existence of phase/hydrodynamic boundaries in release studies can be a source of significant inaccuracy.

Glucose partition coefficient and diffusivity in the lower skin layers

PURPOSE

This work aims to estimate the diffusivity and partitioning of glucose in the dermis and the viable epidermis of human skin.

METHODS

The partition coefficient of glucose between phosphate-buffered saline and dermis, tape-stripped epidermis (TSE), stratum corneum (SC), and split-thickness skin, was measured in vitro using human cadaver skin. Glucose permeability across dermis and tape-stripped split-thickness skin (TSS) was measured using side-by-side diffusion cells. Glucose desorption from TSE and human epidermal membrane (HEM) was measured. All measurements were conducted at 32 degrees C.

RESULTS

The partition coefficient for glucose [mean +/- SD (no. of samples)] was 0.65 +/- 0.09 (n = 25) for dermis, 0.81 +/- 0.06 (n = 10) for TSE, and 0.53 +/- 0.12 (n = 9) for SC. Glucose diffusivity in dermis was calculated to be 2.64 +/- 0.42 x 10(-6) cm2/s (n = 14). Glucose diffusivities in the viable epidermis estimated from TSS permeation, TSE desorption, and HEM desorption were 0.075 +/- 0.050 x 10(-6) cm2/s (n = 5), 0.037 +/- 0.018 x 10(-6) cm2/s (n = 4), and 1.0 +/- 0.6 x 10(-6) cm2/s (n = 4), respectively.

CONCLUSION

The tissue/buffer partition coefficient of glucose in all skin layers was found to be less than unity, suggestive of excluded volumes in each layer. Glucose diffusivity in human dermis was found to be one third of its value in water, indicative of hindered diffusion related to the structural components of the tissue. A substantially lower value for glucose diffusivity in viable epidermis is suggested.

Noninvasive and minimally invasive methods for transdermal glucose monitoring

Noninvasive and minimally invasive techniques for monitoring glucose via the skin are reviewed. These approaches rely either on the interaction of electromagnetic radiation with the tissue or on the extraction of fluid across the barrier. The structure and physiology of the skin make the technical realization of transdermal glucose monitoring a difficult challenge. The techniques involving transdermal fluid extraction circumvent and/or compromise the barrier function of skin's outermost and least permeable layer, the stratum corneum, by the application of physical energy. While sonophoresis and microporation methods, for example, are in relatively early-stage development, a device using reverse iontophoresis [the GlucoWatch Biographer (Cygnus, Inc., Redwood City, CA)] is already commercially available. Optical techniques to monitor glucose are truly noninvasive. The tissue is irradiated, the absorbed or scattered radiation is analyzed, and the information is processed, to provide a measure proportional to the concentration of glucose in the dermal tissue. These techniques include near-infrared and Raman spectroscopy, polarimetry, light scattering, and photoacoustic spectroscopy. By contrast, impedance spectroscopy measures changes in the dielectric properties of the tissue induced by blood glucose variation. Large-scale studies in support of efficacy of these methodologies are as yet unavailable. At present, therefore, transdermal fluid extraction technologies are offering greater promise in terms of practical and realizable devices for patient use. The truly noninvasive allure of the optical approach assures continued and intense research activity--for the moment, however, an affordable, efficient, and portable system is not on the immediate horizon.

Noninvasive glucose monitoring by back diffusion via skin: chemical and physical enhancements

Blood glucose levels are routinely obtained by invasive and painful methods using glucose meters and test strips. The development of less invasive or non invasive techniques would be beneficial for diabetes patients. In this study, a noninvasive method was evaluated using the back diffusion of glucose across skin with or without permeation enhancement methods. An in vitro model was utilized. The stratum corneum (SC) was the predominant barrier for both back and forward diffusion of glucose across skin. Surfactants with various charges and essential oils (cyclic monoterpenes) were used as chemical enhancers to promote the back diffusion of glucose. A cationic surfactant (benzalkonium chloride) showed the highest enhancement, followed by anionic and nonionic surfactants. d-Limonene and 1,8-cineole dispersed in appropriate proportions of ethanol could enhance the glucose diffusion after pretreatment of the skin surface. Electroporation, defined as a physical method, significantly increased the amount of glucose that diffused back. The percentages of diffused glucose by 300 V (volts) and 500 V high voltage pulses on skin for 10 min were found to be 45 and 75 times greater than the control group, respectively.

Reverse iontophoretic monitoring in premature neonates: feasibility and potential

Premature neonates represent a fragile patient population, often subjected to intensive clinical care and multiple drug therapy, which must be monitored carefully and continuously. The difficult and painful nature of repetitive blood sampling, particularly in this population, has provided considerable impetus for the development of noninvasive methods for monitoring blood analytes. Reverse iontophoresis, a relatively new technology already used for the transdermal monitoring of blood glucose levels in adults, may be particularly well-suited to exploit the unique properties of preterm neonatal skin. The underdevelopment of the premature infant's epidermis, and more specifically the stratum corneum (SC), results in an increased permeability to molecular transport. In this study, we have investigated the feasibility of reverse iontophoretic monitoring of two model drugs, caffeine and theophylline, which are often administered to premature neonates. To this purpose, tape-stripped porcine skin in vitro, which has been previously demonstrated to be an excellent model for premature neonatal skin, was employed. Reverse iontophoresis across intact membranes enabled a quantifiable extraction of both drugs predominantly at the cathode compartment. The mechanism of extraction of these essentially neutral drugs (caffeine and theophylline being uncharged at pH 7.4) was electroosmosis. However, when the SC was removed by progressive tape-stripping, the amounts of drugs extracted by reverse iontophoresis were equivalent to those obtained by passive diffusion. In these circumstances, therefore, the benefit and usefulness of the applied electric field had been lost. In summary, the absence of an at least partially functional skin barrier obviates, in the case of neutral molecules, the control (and directional transport) offered by iontophoresis; in contrast, for ionized species, where the principal iontophoretic transport mechanism is electromigration, the approach should be valid.