A new development involves a network of plastic tubes being printed within an organ. After the whole organ is printed, these plastic tubes are dissolved, leaving a network of empty tunnels. Why does the organ need to have the empty tunnels within it?

These are needed to mimic the blood vessels which these organs will use for the supply or drainage of fluids e.g blood.

In addition the empty tunnels serve as the routes for nutrients and oxygen supply  for the organ.

In summary the tunnels provided  a vascular structure to support the organ

Explanation:

answer: the blood-brain barrier is a layer of specialized membranes and cell processes designed to keep undesirable substances from passing from the bloodstream into the precious tissues of the brain. while the blood-brain barrier does keep most bad things out of the central nervous system, it is not impermeable.

explanation: