Here's how they did it: after collecting 73 donor hearts that had been deemed unfit for transplantation, the scientists used a detergent solution to wash away almost all of the hearts' cells. What they were left with was a milky-white, heart-shaped scaffold of protein called an "extracellular matrix"—an eerie object that looks right out of the HBO show Westworld. Next, they used messenger RNA to reprogram human skin cells to become pluripotent stem cells—a kind of master cell that can turn into any cell in the body—which they then turned into heart cells. They added those cells to the left ventricular wall of each heart matrix, then mounted the hearts in a specially developed bioreactor that mimicked the conditions of the human body while feeding the cells a steady diet of nutrients. After two weeks, they checked on their project. The new tissue looked just like that of immature human hearts. Even better, when the scientists gave the hearts a jolt of electricity, they started beating.