In 1987, American inventor Chuck Hull built the first successful 3D printer, a robot that could extrude material in layers, slowly turning a digital blueprint file into a physical object. Over the next 30 years, Hull and other engineers __(6)__ the concept, and today 3D printing has become one of the core technologies of modern design and manufacturing. It’s ideal for creating one or a few copies of a complex shape, such as a product prototype.

Custom-built, complex shapes are also common in biology, leading many biomedical researchers to ask whether it’s possible to print working tissues and organs. Beginning with __(7)__ inkjet printers in the early 2000s, bioprinting pioneers have since moved on to 3D printing, __(8)__ increasingly complex living structures. At the same time, cell biologists have refined techniques for creating induced pluripotent stem cells (iPSCs).

By combining these two technologies, future physicians might bioprint replacement tissues and organs from a patient’s own cells—eliminating the need to find donors and the risk of __(9)__ rejection. Researchers still have to solve some tricky problems to get bioprinting into the clinic, especially for complex organ replacement, but a series of recent developments and the availability of relatively cheap, user-friendly bioprinters has made the technique more accessible than ever.

Hearts are a natural choice for bioprinting proof-of-concept experiments. They have fewer cell types than some other complex organs, and provide clear visual evidence of whether the cells are functioning properly, as __(10)__ spontaneously start beating when they’ve formed working tissue.

Using their __(11)__ printing approach, the investigators built complex sections of __(12)__ heart tissue, seeded with iPSCs that were stimulated to form cardiac cells. It worked. “They go from basically a(n) __(13)__ beating, where each individual organoid is beating at a different rate; and as they fuse together, they start to beat together, they __(14)__, and then the whole tissue is contracting at the same time,” says Jennifer Lewis, professor of engineering and applied sciences at Harvard University in Cambridge, Massachusetts.

While the result is impressive, Lewis cautions that it’s still several steps removed from building a functional heart. One problem is that the __(15)__ remain immature, with contractions that are much weaker than those of an adult heart. Making a working heart will require more postprocessing to increase the tissue’s strength, and more advances in stem cell biology to improve the cells’ maturation. It will also entail printing a full organ, something Lewis is reluctant to do at this point. “We purposely didn’t even go anywhere near to a construct that looked like a heart,” says Lewis, adding that she didn’t want to create false hope with images of a complete but nonfunctional organ.

 (Excerpted from: https://www.science.org/content/article/prints-pieces)