When cancer researcher Rosalie Sears, PhD, clicks the print button, ink does not spray onto a page. Instead, actual human cells issue from different heads of her 3-D printer.

In a short while, she has before her a very small tumor -- an exact replica of a patient's cancerous growth. At that point, she and her colleagues can attack the printed copy with any number of cancer treatments.

"The hope is that it will allow us to test, in real time, how a patient's tumor will respond," says Sears, a professor of molecular and medical genetics at Oregon Health and Science University in Portland.

Sears's work is just one exciting aspect of 3-D printing's potential impact on medicine -- from prosthetics, to the bioprinting of cells, to lifelike models of organs, to the possibility of printable, implantable tissue.

Creating tailor-made robotic arms and hands is one of the more publicized uses of 3-D printing in health care. Volunteers working with free software available online have designed some of these devices. Such prosthetics are more functional than traditional ones, often at a tiny fraction of the cost. Think $50 vs. $30,000.

"It's more accessible than ever before," says Terry Yoo, PhD, a computer scientist and 3-D printing specialist at the National Institutes of Health. "Today, people are falling over their feet trying to come to the lab to do 3-D printing."

And this is just the start, says Cornell University associate professor of engineering Hod Lipson, PhD, author of Fabricated: The New World of 3D Printing. "The range of materials is expanding, the cost of machines is dropping, and we just keep seeing more and more applications. We haven't seen the least of it yet."

Pediatric cardiologist Matthew Bramlet, MD, has already witnessed the benefits of 3-D printing at Children's Hospital of Illinois in Peoria, where he practices. There, surgeons prepare for and plan surgeries for children with complex heart defects with the help of 3-D models of their patients' hearts.

The result? More effective operations. In one case, the model helped surgeons come up with a different way to repair a 3-year-old's heart. The boy -- who was at first expected to live 20 to 30 years -- now might lead a normal life.

"The model allows us to pull [a heart] out of 2-D screen and actually hold it in our hands and evaluate it in a dimension we never had before," says Bramlet. "They are real game-changers."

At Children's National Medical Center in Washington, D.C., engineer Axel Krieger, PhD, also uses his printer to assemble lifelike models of patients' imperfect hearts, using them as both surgical guides and teaching tools. His team has made about 40 model hearts, but a big question remains: Do they improve surgical outcomes? It's too early to tell, Krieger says.

"These surgeries are really complex, and it's difficult to tease out exactly what effect the model has, because it's just one little step in the work flow."

Krieger predicts that they'll have a better idea by next year, after many more heart surgeries.

Cancer researcher Sears also welcomes the ability to go beyond the shortcomings of two dimensions.

"We can grow tumor cells on a plate in the lab, but that's not how a tumor cell exists in the body, and responses in 2-D don't mimic what we see in the clinic," she says. "That's why we have thousands of targeted therapies that look promising in the lab but don't pan out in clinical trials in patients."

Using mice -- a common tool in cancer research -- also has serious drawbacks. One of them is time. Sears says it takes 6 months to implant and grow a tumor in a mouse. For fast-growing cancers like pancreatic cancer, patients can't wait that long. Enter 3-D printing or, more accurately, bioprinting, because this involves human cells.

"Within two weeks, we can learn whether these printed tumors respond or don't respond to a given therapy," Sears says.

Another advantage: Sears can print numerous identical tumors at the same time, allowing her to test multiple drugs at once.

"It's very exciting, both in terms of potentially getting patients the right drugs sooner and in understanding how cancer cells communicate with other cells," she says. "It's more promising to me than anything else that's out there right now."

But she says much research still needs to be done. The big question: Will patients' tumors respond to drugs in the same way as the 3-D printed models?

To Lipson, research such as Sears's is just the first step in 3-D printing's most exciting direction.

"That's the ultimate: bioprinting, or printing with live cells," Lipson says. He predicts we'll start to go beyond model making within the next several years. The next stop: implantable 3-D-printed tissue.

"I think that that is where the future is," he says. "We'll climb the ladder from simple tissue such as cartilage and bone to complex, heterogeneous tissue all the way to functioning organs, which is the Holy Grail indeed."

Lipson thinks FDA approval for the first such procedures remains at least 5 years away, though lab and animal testing is ongoing.

Interesting ethical questions come with the advancing technology, he says. For example, if scientists can bioprint a new knee to replace one ravaged by arthritis, do you go with a copy of your old knee, or do you allow a computer to design you a better one? Can we rebuild or improve our bodies?

In the short term, Lipson says, we'll see more and more 3-D-printed synthetic implants, such as hip and other joint replacements, custom-shaped to improve how well they work. Unlike prosthetics, though, implants can be costly.

"They're already on the market, but they're fairly new and expensive," he says.

Lipson says new developments in 3-D printing will be determined by economics: "It's a question of funding and market priorities rather than purely a technical challenge."

But we should expect big things, he says. "This is just the beginning. There's a lot more to come. This is not just a hype cycle."