A Microfluidic Diffusion Cell for Fast and Easy Percutaneous Absorption Assays.

PURPOSE: Percutaneous absorption assays of molecules for pharmaceutical and cosmetology purposes are important to determine the bioavailability of new compounds, once topically applied. The current method of choice is to measure the rate of diffusion through excised human skin using a diffusion cell. This method however entails significant drawbacks such as scarce availability and poor reproducibility of the sample, low sampling rate, and tedious assay setup. METHODS: The objective of the present work is to propose an alternative method that overcomes these issues by integrating an experimental model of the skin (artificial stratum corneum) and online optical sensors into a microfluidic device. RESULTS: The measurement of the diffusion profile followed by the calculation of the permeability coefficients and time lag were performed on seven different molecules and obtained data positively fit with those available from literature on human skin penetration. The coating of the lipid mixture to generate the artificial stratum corneum also proved robust and reproducible. The results show that the proposed device is able to give fast, real-time, accurate, and reproducible data in a user-friendly manner, and can be produced at a large scale. CONCLUSION: These assets should help both the cosmetics and pharmaceutics fields where the skin is the target or a pathway of a formulated compound, by allowing more candidate molecules or formulations to be assessed during the various stages of their development.

Reaction-diffusion phenomena in a PDMS matrix can modify its topography.

Various reagents and solvents can be absorbed into polydimethylsiloxane (PDMS), which may be a concern for many applications. We hypothesize that these absorbed reagents can also react with each other within the elastomer matrix. Here we demonstrate this phenomenon and use it as a means to physically modify the surface topography of the PDMS by generating wrinkles or pores.

Low O2 metabolism of HepG2 cells cultured at high density in a 3D microstructured scaffold.

Among the features of in vivo liver cells that are rarely mimicked in vitro, especially in microchips, is the very high cell density. In this study, we have cultured HepG2 in a plate-type PDMS scaffold with a three-dimensional ordered microstructure optimally designed to allow cells to attach at a density of 10(8) cells/mL. After the first step of static open culture, the scaffold was sealed to simulate the in vivo oxygen supply, which is supplied only through the perfusion of medium. The oxygen consumption rate at various flow rates was measured. An average maximal cellular oxygen consumption rate of 3.4 x 10(-17) mol/s/cell was found, which is much lower than previously reported values for hepatocytes. Nevertheless, the oxygen concentration in the bulk stream was not the limiting factor. It has been further confirmed by the reported numerical model that the mass transport resistance on the surface of a cell that limits the oxygen supply to the cell. These results further emphasize that access to a sufficient quantity of oxygen, especially through the diffusion-limited layer on the surface of a cell, is very important for the metabolism of hepatocytes at such a high density.

A method for the design of 3D scaffolds for high-density cell attachment and determination of optimum perfusion culture conditions.

The application of in vitro cultured cells in tissue engineering or drug screening, aimed at complex soft tissues such as liver, requires in vivo physiological function of the cultured cells. For this purpose, the scaffold in which cells are cultured should provide a microenvironment similar to an in vivo one with a three-dimensional extracellular matrix, a high supply capacity of O(2) and nutrients, and high cell density. In this paper, we propose a method to design (1) the geometry of the scaffold, with a surface/volume ratio optimized to allow high-density (5 x 10(7)cells/mL) cell culture and (2) culture conditions that will supply optimal quantities of oxygen and nutrients. CFD modeling of mass transport was used to determine the shear stress as well as O(2) and glucose metabolism in the scaffold (20 mm width-35 mm length) for various flow rates. Validation of the model was done through comparison with flow resistance and micro-PIV experiments. CFD analysis showed the maximum metabolic rate densities for this scaffold are 6.04 x 10(-3)mol/s/m(3) for O(2) at 0.71 mL/min and 1.91 x 10(-2)mol/s/m(3) for glucose at 0.35 mL/min.