Sitting at a computer connected to a large microscope, Salman Khetani calls up a kaleidoscopic image: green islands of human liver cells in a hexagonal pattern, surrounded by a red sea of support cells. Sangeeta Bhatia, Khetani's advisor, says that the cells have been carefully patterned to hit the liver "sweet spot."Arranged just so -- in 37 colonies about 1,200 micrometers from each other -- the cells behave as though they were in the human body.

When grown in the lab using existing methods, liver cells can survive for a day or two, but over the course of a week, they lose the ability to perform their liver-specific functions and then die. Bhatia and Khetani's cells, on the other hand, function for about a month. They secrete the blood protein albumin, synthesize urea, and make the enzymes necessary to break down drugs and toxins. Bhatia believes that the cells act enough like human tissue that they could be used to screen new drugs for liver toxicity or to study metabolism and, possibly, hepatitis C, a virus that grows only in human tissue. Indeed, the researchers have already developed a drug toxicity test that uses liver cells arranged in their signature hexagonal pattern.