Static diffusion cell

Clowes, H.M., R.C. Scott and J.R. Heylings (1994). Skin absorption flow-through or static diffusion cells, Toxicol. In Vitro, 8, 827-830. 

Percutaneous absorption studied in vitro is normally characterized either by a permeability constant or by the time course of the penetration process. Direct measurements of absorption require intermittent sampling of fluid contained in the receptor half of a diffusion cell. Permeability constants are frequently calculated by removing and assaying microaliquots of receptor fluid at various intervals during the early time course of absorption ( static" diffusion cell technique), until steady state (ss) flux is obtained the permeability coefficients are then derived according to Pick s First Law of Diffusion ... 

The skin permeability of a hydrophobic fragrance material (clnnamyl anthranilate) was examined using 1-chambered static diffusion cell technlaues ( ). In vitro percutaneous absorption measurements with rat skin using a normal saline receptor solution resulted in values that were much lower than the corresponding in vivo data for the compound. This difference is shown in the time course for the absorption of cinnamyl anthranilate (Figure 1). 

Chilcott et al. reported the in vitro measurements of S-labelled SM penetration through human skin (heat-separated epidermal membranes and full thickness skin) using Franz type static diffusion cells at 30-32 °C. The measurements were made after exposing the skin surface to pure SM liquid (finite, 10 pi, and infinite, 20 pi, doses) under occluded and unoccluded conditions and after exposure to saturated SM vapour. 

OECD, 2004b, Test Guideline 428, Skin Absorption In vitro Method (Either Elow-Through or Static Diffusion Cells, Excised Pig or Human Skin). 

Both static and flow-through diffusion cells are approved by the authorities, and data are available on their relevance for predicting the in vivo situation [2, 61-64], Basically, a donor and an acceptor compartment are separated by a membrane of either native skin or bioengineered materials. These materials can be of human, animal, or artificial origin. Sampling from the acceptor compartment is performed either continuously or at predetermined time intervals. 

F. Netzlaff, K.-H. Kostka, C.-M. Lehr, and U. F. Schaefer. TEWL measurements as a routine method for evaluating the integrity of epidermis sheets in static Franz type diffusion cells in vitro. Limitations shown by transport data testing. Em J. Pharm. Biopharm. 63 44—50 (2006). 

In terms of catalysis, important equilibrium processes include low-temperature gas adsorption (capillary condensation) and nonwetting fluid invasion, both of which are routinely used to characterize pore size distribution. Static diffusion in a Wicke-Kallenbach cell characterizes effective diffusivity. The simultaneous rate processes of diffusion and reaction determine catalyst effectiveness, which is the single most significant measure of practical catalytic reactor performance. 

Skin permeability is best measured on excised human skin. Skin can be heat-separated epidermis, dermatomed to a particular thickness (typically 200-500 pm) or whole skin with subcutaneous fat removed. The following protocol outlines a standard skin permeability experiment using human epidermis in static Franz-type diffusion cells ... 

Type of diffusion cell system employed (static, flow-through)... 

Figure 14.8 Basic diffusion cell designs. Static horizontal cells may be jacketed [as in the Franz-type) or unjacketed [and temperature controlled using water bath or heating block). Flow-through cells usually have a small receptor chamber to maximize mixing. Side-by-side cells are used mainly for solution vehicles.

The primary approach to assess dermal absorption is the in vitro diffusion cell. In thi,s model, skin sections (full thickness, deimatomed to a specific thickness) are placed in a two-chambered diffusion cell in which receptor fluid is placed in a reservoir (static cells) or perfused through a receiving chamber (flow-through cells) to simulate cutaneous blood flow. Chemical may either be dosed under ambient conditions neat or dissolved in a vehicle (Franz and Bronaugh cells) or in water (sLdc-by-side diffusion... 

Epidermis consists of the stratum comeum and adjacmt viable layer. The system coefficimts of the epidermis can be obtained by measuring the permeation coefficient of the probe compounds in the donor solution of the chranical mixtures using static Franz and flow-through diffusion cells. The log k value of a given probe compound is scaled to the solute descriptors of the compound via the LFER equation (Equation 5.1, where log SP = log k ). A LFER equation matrix is genraated from the permeation coefficients and the solute descriptors of all the probe compounds (Equation 5.2). The system coefficients of the epidermis and the ehemieal mixture can be obtained by multiple linear regression analysis of the LFER equation matrix Pog kp, R, Jt, a, p, V]. The system coefficients of the epidomis would be different from... 

Spatio and Spatio-Temporal Patterns. An exotic form of diffusional encounter should be mentioned, which arises from sets of reaction-diffusions equations (48). In 1952, the mathematician Alan Turing postulated the existence of two-dimensional and three-dimensional spatio and spatio-temporal patterns for certain classes of reactive systems (49). The physical realization of these mathematical solutions has been observed in a variety of systems (50). It suffices to say that since these patterns have been observed in both simple chemical systems and complex biological systems, their possibility in homogeneous catalysis should certainly not be ruled out. In this regard, static spectroscopic cells may be particularly prone to such spatial variation because of the lack of mixing. 

Static Spectroscopic Cells. ESR and NMR tubes can be sealed for gas-liquid reactions, and high pressure tubes have been constructed. However, ESR and NMR tubes represent extremely poor contacting patterns. Gas-liquid mass transfer will be controlled by diffusion across the more-or-less static interface and hence very low mass-transfer rates will occur. 

Small fuel cell stacks can operate on either diffused or forced reactant air. Diffused-air stacks are generally limited in their applicability because of air supply rate issues and their impact on geometry the requisite openness of the air compartments in such devices also tends to render them vulnerable to atmospheric conditions. Hence, diffused-air (static) fuel cells are usually practical only in certain particularly low-power applications, up to perhaps 25 Watts. ... 

In summary, skin penetration tests consist of placing the piece of skin in a diffusion chamber (e.g., Franz-cell-type static diffusion chamber or flow-through diffusion cell) with two compartments. The external side of the skin is turned toward the upper compartment in which the test solution is applied. After a defined contact time with the skin surface, the effect of the surfactant on skin permeability properties is determined, often by means of a penetration marker (e.g., tritiated water). Many variations in penetration studies have been described and the reader is referred to other articles for more details [34,35]. 

However, for molten semiconductors, particularly for molten silicon, the temperature dependence of diffusion constants has not been made clear. The measurement of the diffusion constant under microgravity was a driving force to improve the measurement techniques development of diffusion cells made of ceramics, appHcation of a shear cell technique, and so on [94]. Application of a strong static magnetic field could also be effective in suppressing convection on Earth [95]. The diffusion... 

These compounds have been the subject of several theoretical [7,11,13,20)] and experimental[21] studies. Ward and Elliott [20] measured the dynamic y hyperpolarizability of butadiene and hexatriene in the vapour phase by means of the dc-SHG technique. Waite and Papadopoulos[7,ll] computed static y values, using a Mac Weeny type Coupled Hartree-Fock Perturbation Theory (CHFPT) in the CNDO approximation, and an extended basis set. Kurtz [15] evaluated by means of a finite perturbation technique at the MNDO level [17] and using the AMI [22] and PM3[23] parametrizations, the mean y values of a series of polyenes containing from 2 to 11 unit cells. At the ab initio level, Hurst et al. [13] and Chopra et al. [20] studied basis sets effects on and y. It appeared that diffuse orbitals must be included in the basis set in order to describe correctly the external part of the molecules which is the most sensitive to the electrical perturbation and to ensure the obtention of accurate values of the calculated properties. 

Pollutants emitted by various sources entered an air parcel moving with the wind in the model proposed by Eschenroeder and Martinez. Finite-difference solutions to the species-mass-balance equations described the pollutant chemical kinetics and the upward spread through a series of vertical cells. The initial chemical mechanism consisted of 7 species participating in 13 reactions based on sm< -chamber observations. Atmospheric dispersion data from the literature were introduced to provide vertical-diffusion coefficients. Initial validity tests were conducted for a static air mass over central Los Angeles on October 23, 1968, and during an episode late in 1%8 while a special mobile laboratory was set up by Scott Research Laboratories. Curves were plotted to illustrate sensitivity to rate and emission values, and the feasibility of this prediction technique was demonstrated. Some problems of the future were ultimately identified by this work, and the method developed has been applied to several environmental impact studies (see, for example, Wayne et al. ). 

Other flexible framework calculations of methane diffusion in silicalite have been performed by Catlow et al. (64, 66). A more rigorous potential was used to simulate the motion of the zeolite lattice, developed by Vessal et al. (78), whose parameters were derived by fitting to reproduce the static structural and elastic properties of a-quartz. The guest molecule interactions were taken from the work of Kiselev et al. (79), with methane treated as a flexible polyatomic molecule. Concentrations of 1 and 2 methane molecules per 2 unit cells were considered. Simulations were done with a time step of 1 fs and ran for 120 ps. 

Figure 19.3—Polarographic cell and diffusion current. Dissolved oxygen, which leads to an interfering double wave, has to be removed from the sample solution by degassing. On the right features of the diffusion current are shown. These increase with time for every drop of mercury in a static (unstirred) solution. Direct polarography is a slow method of analysis. More than 100 droplets are needed to record the voltammogram.

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