a standard perfusion culture experiment shows constant partial pressures of at least 160 mmHg O2 [7]. Modulation of oxygen content It has been shown that growing cells and tissues have very individual oxygen requirements. Due to this reason it is important that the content of oxygen can be adapted in individual perfusion culture experiments. The technical solution is a gas exchanger module containing a gas inlet and outlet (Fig. 6a). Further a spiral with a long thin-walled silicon tube for medium transport is mounted inside the module. Since the tube is highly gas-permeable, it guarantees optimal diffusion of gases between culture medium and internal atmosphere of the gas exchange module. In consequence, the desired gas atmosphere can be adjusted by a constant flow of a specific gas mixture through the module. This way the content of oxygen or any other gases can be modulated in the medium by diffusion. Applying this simple protocol it became possible to decrease the oxygen partial pressure within the transported medium during long term culture experiments under absolutely sterile conditions [7]. Elimination of harmful gas bubbles Performing perfusion culture experiments it always has to be considered that gas bubbles are forming during slow transport of culture medium. They arise during suction of medium in the storage bottle, during transport within the tube, during distribution within the culture container and during elimination on the way to the waste bottle. Due to unknown reasons gas bubbles accumulate especially at material transitions between tubes, connectors and perfusion containers. First these gas bubbles are so small that they cannot be observed with the human eye, but during ongoing transport of culture medium they increase in size and are able to form an embolus that massively impedes medium flow. Within a culture container gas bubbles are leading to a regional shortage of medium supply and are causing breaks in the fluid continuum so that massive fluid pressure changes result. In a gradient perfusion culture container, where two media are transported at exactly the same speed, embolic effects can lead to pressure differences destroying in turn the contained epithelial barrier [5,9]. To avoid the concentration of gas bubbles within a perfusion culture experiment, a gas expander module was developed (Fig. 6b). This module removes gas bubbles from the medium during transport. When medium is entering the gas expander module, it rises within a small reservoir and expands before it drops down