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For adherent processes that need to be scaled up beyond HYPERStack/CellSTACK scale, we have used an iCELLis® bioreactor. This system utilizes single use fixed bed bioreactors with up to 500 m2 of growth area, the equivalent of nearly 800 CellSTACKs. The system has been designed to enable optimal gas exchange, but it would take some work to ensure that you have a process that can match or exceed your current productivity/volume. They do offer an iCELLis Nano system for smaller scale feasibility work (up to 4 m2 of growth area). There are also other vendors who have or are developing fixed bed bioreactors for use when adherent processes need to be scaled up beyond the HYPERStack/CellSTACK scale when moving to suspension is not feasible.