Heart Stamps: Adam Feinberg (left), a postdoc at Harvard, and Kevin Kit Parker, a professor of biomedical engineering, make tiny machines out of rat heart tissue. Credit: Porter Gifford

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.

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