Problems with the Gas Supply
With the high gas flow and strong agitation seen in bacterial cultures, the length of the pipes from the gas supply to the vessel is hardly relevant. Gases are distributed by inlet pressure and agitation and thus reach the cells rapidly. The efficient supply of required gases to plant and animal cells, which are less aerated and agitated, is not as simple.
Plant and animal cells tend to require a highly defined gas atmosphere and a stable pH. Long pathways for the gases and relatively high retention times in large filter housings could delay pH and pO2 control in many cell cultures and thus lead to losses in productivity.
To counteract these effects, three steps must be taken—a shortening of the pipes between the gas-mixing station and reactor, a reduction in pipe diameter, and a reduction in the dead volume of filters and filter housings.
The first two design changes are relatively straightforward and depend on plant geometry. But the alteration of filters and filter housings is more problematic.
The smallest dead leg and the shortest gas flow path are to the autoclavable single-use filters. These filters can not be sterilized with in situ sterilizable bioreactors and therefore can not be integrated into the plant. If the filters are autoclaved separately and mounted to the plant after sterilization of the reactor, a source of contamination is created.
The solution to the problem, as developed by Bioengineering, is based on the utilization of single-use filters with hydrophobic PTFE membranes and optimized dead legs for in situ sterilizable systems. Thus the retention time of the gases in the filters is minimized. Gas retention during the passage through filter housings is thusly avoided, since the new system simply dispenses with additional housings. Because of the small size of the filters the pipe diameters of the aeration lines can be kept small and the gas passage short.
How can the incompatibility of autoclavable single-use filters with in situ sterilizable bioreactors be circumvented and the full integration of filters in the plant be assured?
In Bioengineering’s new aeration system, single-use filters for each of the four gases in the gas-mixing station are located in an autoclave. This autoclave is integrated into the piping system of the plant and fitted with a viewing glass. A special set of valves, which are steam permeable, is mounted in the autoclave. The steam generated by the autoclave sterilizes the aeration line to a valve in the sterile cross. The pipe from the vessel to the sterile cross is sterilized in the course of the vessel sterilization, and the single-used filters are automatically sterilized in the autoclave.
Thus full integration of the autoclave and the filters into the plant is assured. The automatic sterilization of the entire plant, including single-use filters, is rendered possible. No manipulation of the filters is conducted after sterilization, minimizing the contamination risk and ensuring conformity with applicable regulations.
The advantages of the Bioengineering system are many. Cell culture operations in bioreactors can be supplied with the required sterile gases in the shortest possible time—this creates fast control loops for pH and pO2 control and therefore efficient response to all changes in cell metabolism during fermentation. Integration and automation of the aeration system significantly minimizes the work effort with single-use filters and provides the same hygienic safety as is standard with larger filters and filter housings.