The P3D chambers have been designed to create flow perfusion conditions on porous scaffold. Thus, the surfaces in the direction which is perpendicular to the flow has been minimised and the dimensions of the chamber have been reduced to closely fit with the scaffold dimensions. This allows getting the most of the flow rate going through the pores of the scaffold and not throught the gaps left between the scaffold and the chamber, which must be taken into account when working with non-porous or low porosity scaffolds.
If the porosity of the scaffold is very reduced, as it happens with hydrogels, it will exert a strong resistance to the flow, which will results in a very high flow velocity at the pores and in an increase of the pressure at the scaffold. If the mechanical consistency of the scaffold is low, it will probably be damaged under the combination of high pressure and high velocity at the pores.
Thus, you are recommended to carefully use low flow rates when dealing with hydrogels or low porosity scaffolds.