For years now, medical researchers have been reproducing human cells in laboratories by hand to create blood vessels, urine tubes, skin tissue and other living body parts. But engineering full organs, with their complicated cell structures, is much more difficult.

Enter 3-D printers, which because of their precise process can reproduce the vascular systems required to make organs viable. Scientists are already using the machines to print tiny strips of organ tissue. And while printing whole human organs for surgical transplants is still years away, the technology is rapidly developing.

“The mechanical process isn’t all that complicated. The tricky part is the materials, which are biological in nature,” said Mike Titsch, editor-in-chief of 3D Printer
World, which covers the industry. “It isn’t like 3-D printing plastic or metal. Plastic doesn’t die if you leave it sitting on an open-air shelf at room temperature for too long.”

The idea of printing a human kidney or liver in a lab may seem incomprehensible, even creepy. But to many scientists in the field, bioprinting holds great promise. Authentic printed organs could be used for drug or vaccine testing, freeing researchers from less accurate methods such as tests on animals or on synthetic models.

Then there’s the hope that 3-D printers could someday produce much-needed organs for transplants. Americans are living longer, and as we get deeper into old age our organs are failing more. Some 18 people die in the United States each day waiting in vain for transplants because of a shortage of donated organs — a problem that Anthony Atala, director of the Wake Forest Institute for Regenerative Medicine and a pioneer in bioprinting, calls “a major health crisis.”

Bioprinting works like this: Scientists harvest human cells from biopsies or stem cells, then allow them to multiply in a petri dish. The resulting mixture, a sort of biological ink, is fed into a 3-D printer, which is programmed to arrange different cell types, along with other materials, into a precise three-dimensional shape. Doctors hope that when placed in the body, these 3-D-printed cells will integrate with existing tissues.

The process already is seeing some success. Last year a 2-year-old girl in Illinois, born without a trachea, received a windpipe built with her own stem cells. The U.S. government has funded a university-led “body on a chip” project that prints tissue samples that mimic the functions of the heart, liver, lungs and other organs. The samples are placed on a microchip and connected with a blood substitute to keep the cells alive, allowing doctors to test specific treatments and monitor their effectiveness.