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.

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