Since you've said that they are studying the kidneys, they already have some idea of what these organs do, so maybe next you could ask, "What do you think would happen if you couldn't pee any more?" Let their imaginations run wild! Then you can raise the idea of artificial kidneys.
This prompted me to contact the middle school that my son is going into next year, since it seems very relevant as they do a sponsored event for juvenile diabetes. Very positive response towards my offer so far.