In a fourth-floor lab at Harvard University, Adam Feinberg is peering through a low-magnification microscope and using a scalpel to cut out triangles and rectangles from a thin polymer. What's impossible to see with the naked eye is a one-cell-thick layer of heart tissue coating each shape. When Feinberg connects the petri dish holding the triangles and rectangles to a pacemaker, the tissue begins to rhythmically contract, and the shapes come alive--twisting, pinching, and even swimming through a solution.

The pieces of "muscular thin films" are just a few millimeters long and only 30 micro­meters thick; at first glance they resemble small worms you might find wiggling in a mud puddle. Kevin Kit Parker, the professor of biomedical engineering who heads the Harvard lab, jokes that he's planning to retire to the South, where he was raised, and sell them as customizable lures in a bait shop.