In addition to the brain, and a peripheral nervous system (think spinal cord), the LLNL scientists have created a rudimentary heart and a tangle of 3D-printed blood vessels, which they plan to use to link organs together. The end product won't look like tiny human innards, but the network of cells, vessels, and neurons will carry out the same functions. Right now, the biggest challenge is "getting the cells to be happy on the device," says Elizabeth Wheeler, the project's principal investigator. Cells are picky, and can die in response to subtle changes in acidity or humidity. That's why it was notable when, in July, the team announced that they kept a section of a human nervous system functioning for a record 23 full days.