​This is where we define the tissue contact.  The three categories for biocompatibility are surface contact, externally communicating, or implant.  Surface contacting and implants are pretty typically self explanatory.  But, externally communicating can sometimes confuse some people.  Externally communicating, is something that will serve as route or method to move gases, liquids or solids into or out of the body.  In terms of biocompatibility evaluations, in the body means whatever won't get wet when you jump in a pool.  The concept that the GI tract or lungs are the outside of the body, will trip up many.  Think of it this way, if you inhaled or ingested something, it can have an effect on you.