The process of creating the heart started with a biopsy of fatty tissue taken from patients. The cellular material from the tissues was used as the "ink" for the print job. That allowed researchers to create complex tissue models including cardiac patches and eventually an entire heart. It should be noted that the heart isn't very big -- it's only about the size of a rabbit's heart. But the technology that made it possible could eventually lead to the production of a human-sized organ. Currently, the hearts can only contract but researchers plan on culturing the 3D printed hearts and teaching them how to operate like the real deal. Once that process is complete, they will attempt to transplant them into animal models.
Researchers have been working on 3D-printed tissues for years, with the eventual goal of creating functioning organs for transplant. The scientists involved in the heart project at Tel Aviv University theorized that within 10 years, organ printers could be available at hospitals.