Each Xpansion plate has a surface area of 612 cm2, which is similar to the size of a plate in a multitray stack, but the difference is in the layout. Typically, the gap between the plates in a stack would be around 15 mm. By shifting the gas exchange in the central column, this gap is greatly reduced in Xpansion.
As a result, the largest Xpansion unit is 80 cm high yet contains 200 plates with a gap of just 1.6 mm between each pair of plates to keep the same ratio of medium per surface as in multitray stack. The capacity of 122,400 cm2 means that one Xpansion bioreactor can replace up to 20 traditional stacked trays.
The Xpansion bioreactor is attached to a digital controller unit on a separate skid that includes all the necessary control and measurement hardware. It also incorporates a gassing system that is connected to gas supply lines that, in turn, are connected to the gas-in filter on the bioreactor. Air, oxygen, carbon dioxide, and nitrogen options can be used to create the correct composition of gas for injection into the bioreactor.
Gas aeration occurs depending on pH and dissolved oxygen (DO) measurements in the bioreactor. The gas coming from the controller enters via the gas-in filter of the bioreactor and passes through very thin silicone tubing placed in the central column. The gases are then diffused through this tubing.
Medium circulation at a minimum flow of 1–2 mm/s is coupled with the aeration system when required to minimize the shear stress. Each plate contains 16 radial channels supporting media circulation through the plates. A magnetic impeller at the bottom acts as a centrifugal pump to support smooth medium circulation.
The liquid is sucked from the central column, pushed to the bottom and then flows sequentially over the plates from bottom to top. Once it reaches the top, it is returned to the bottom of the stack, via the column central, where gas exchange occurs.
Any changes in the surface the cells adhere to, or minute differences in the conditions within the bioreactor, can have a dramatic effect on the growth process and even the morphology of the cells. As such, it is imperative to keep the environment as stable as possible throughout the cell growth process.