Porogens can be any of the following except:

Porogens are materials used in tissue engineering to create pores within a scaffold, which play a critical role in the scaffold's ability to support cell growth and tissue formation. They enhance the permeability and surface area of the scaffold while allowing for nutrient and waste exchange.

An inorganic salt is suitable as a porogen because it can be easily removed from the scaffold after the desired porosity is achieved. Similarly, paraffin spheres can be utilized to create controlled porosity since they can be dissolved or removed after the scaffold's formation. Crystals of saccharose also serve effectively as porogens because they can be leached out from the scaffold, leaving behind a porous structure.

On the other hand, ammonium bicarbonate, while it may seem plausible, does not function as a porogen in the same way. It can release gas during the scaffold formation process, leading to some porosity, but it does not provide a clear mechanism for creating consistent pores like the other materials mentioned. Therefore, ammonium bicarbonate is not typically classified within the conventional porogen categories used in tissue engineering.