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We have developed a perfusion bioreactor system that allows the formation of steady state oxygen gradients in cell culture. In this study, gradients were formed in cultures of rat hepatocytes to study the role of oxygen in modulating cellular functions. A model of oxygen transport in our flat-plate reactor was developed to estimate oxygen distribution at the cell surface. Experimental measurements of outlet oxygen concentration from various flow conditions were used to validate model predictions. We showed that cell viability was maintained over a 24-h period when operating with a physiologic oxygen gradient at the cell surface from 76 to 5 mmHg O2 at the outlet. Oxygen gradients have been implicated in the maintenance of regional compartmentalized metabolic and detoxification functions in the liver, termed zonation. In this system, physiologic oxygen gradients in reactor cultures contributed to a heterogeneous distribution of phosphoenolpyruvate carboxykinase (predominantly localized upstream) and cytochrome P450 2B (predominantly localized downstream) that correlates with the distribution of these enzymes in vivo. The oxygen gradient chamber provides a means of probing the oxygen effects in vitro over a continuous range of O2 tensions. In addition, this system serves as an in vitro model of zonation that could be further extended to study the role of gradients in ischemia–reperfusion injury, toxicity, and bioartificial liver design. © 2003 Wiley Periodicals, Inc. Biotechnol Bioeng 82: 253–262, 2003. [ABSTRACT FROM AUTHOR]

The temperature dependence of the electrical resistivity (r) and thermopower (S) of misfit cobaltite Ca3Co4O9 has been studied. The compound is a promising thermoelectric material, exhibiting very low r([?]1.3 × 10[?]4 O cm) and reasonably high S ([?]56 µV K[?]1) at room temperature, which is desirable for thermoelectric applications. At T < 80 K, S exhibits T3/2 dependence, suggesting that electron-magnon scattering has a pronounced influence on S at low temperatures. Ca3Co4O9 exhibits a temperature independent power factor in the temperature range 300-80 K. A comparison of the temperature dependence of the power factor of Ca3Co4O9 with various materials projected or used as a thermoelectric cooler (e.g. single crystal of short c-axis Ca3Co4O9, polycrystalline La0.95Sr0.05CoO3, Bi0.85 Sb0.15, Bi2Te3, Bi-SiO2 composite) indicates that title compound Ca3Co4O9 is a much superior candidate for thermoelectric applications. [ABSTRACT FROM AUTHOR]