basal medium compartment 30 the weight is transferred to membrane 20 membrane support 110 keeps membrane 20 from sagging and displacing culture medium 50 into culture medium head space 180 membrane support 110 makes minimal contact with membrane 50 so the surface area for dialysis is not substantially diminished membrane support 110 is designed such that it will allow gas bubbles to move freely to culture medium head space 180 membrane support 110 can be made of any biocompatible material in the preferred embodiment it is clear plastic such as polystyrene or polycarbonate if membrane 20 is a material cast onto a stiff mesh backing or precise control of the volume of culture medium 50 residing above gas permeable film 120 is not required membrane support 110 is optional the embodiment of fig3 shows a configuration where portions of gas permeable film 120 project into cell culture compartment 40 to provide support of membrane 20 without the need for membrane support 110 silicone is a good choice for material as it can be readily molded to form an appropriate shape wall thickness can be minimized to allow additional gas transfer into cell culture compartment 40 in the case of silicone average wall thickness should be kept below 0 015 inches preferably about 0 004 to 0 012 inches the embodiment shown in fig4 keeps membrane 20 from sagging and insures.