I am trying to design carbonate apatite nanoparticles that might cross the endothelial layer of brain capillary (blood brain barrier-BBB). Thus, an in vitro system would be a better option (easy to understand and faster) to check the permeability of particles before moving to an in vivo model. I am looking for an in vitro BBB system that can be used for this purpose. I have seen the co-culturing systems discussed by many researchers. Are the co-culturing system with the filter available commercially?