Tiny livers have been printed in the lab by Organovo in San Diego, California. These mini-livers are just four millimeters across and half a millimeter deep, but they perform most of the functions of real livers.
(Organovo 3D prints mini-livers)
To create them, a printer builds up about 20 layers of hepatocytes and stellate cells Ė two major types of liver cell. Crucially, it also adds cells from the lining of blood vessels. These form a delicate mesh of channels that supply the liver cells with nutrients and oxygen, allowing the tissue to live for five days or longer. The cells come from spare tissue removed in operations and biopsies.
Existing liver assays, based on single or double layers of cells, only last two days and don't have the same range of functions as the micro-discs.
The realistic structure and functioning of the mini-livers make them good predictors of the toxicity of drugs and other substances. They produce albumin, the liver protein that bulks up blood and ferries hormones, salts and drugs throughout the body. They also make cholesterol, which carries fat in the bloodstream, and produce major detoxification enzymes, called cytochrome P450s, that metabolise drugs in the liver.
Artificially grown livers for transplantation was one of the issues discussed by science fiction writer Larry Niven in his 1968 novel A Gift from Earth:
...For a while your colonists will have to come up to the Hospital to get treatment with the ramrobot symbiots, but eventually we can build culture tanks in Gamma and Delta and Eta."
(Read more about Larry Niven's artificially grown organs)