Now that 3D printers are cheaper to produce, experts predict it won't be long before they are common in our homes. Even today, more doctors realize the potential for 3D-printed applications in their own practices. Since the technology moved from the theoretical to the real, soctors are expanding the boundaries of what's possible to print from very practical applications for medical procedures and medical devices.

Here are just a few of the parts of the body that researchers are currently 3D printing:

1. Bionic Eye

Researchers at the University of Minnesota, Minneapolis, MN, have 3D printed a prototype bionic eye. The device is an array of semiconductor photodetectors, made of polymers, printed on a glass hemisphere. Their research was recently published in Advanced Materials

Bionic eyes are usually thought of as science fiction, but now we are closer than ever using a multimaterial 3D printer," said study co-author Michael C. McAlpine, PhD, associate professor, Department of Mechanical Engineering, College of Science and Engineering, University of Minnesota.

The next step, he said, is to augment the prototype with more—and more efficient—photodetectors. He and his team are also searching for a way to print on a soft hemispherical material that can be implanted into a real eye.

Dr. McAlpine explained the impetus for his work: "My mother is blind in one eye, and whenever I talk about my work, she says, 'When are you going to print me a bionic eye?'"

In related eyeball research, investigators in the UK have used stem cells to 3D print human corneas.

2. Antibacterial tooth

Researchers in Groningen, the Netherlands, have developed not just a 3D-printed tooth but one that kills bacteria. The tooth's material is made from conventional artificial tooth resins combined with positively-charged quaternary ammonium ions. When this material encounters the negatively-charged membranes of bacteria, it kills the microbes.

"The trick…was to get the mixture right to enable 3D printing and minimize any leakage of the antimicrobials. You don't want them to enter the mouth and thus the intestines, where they could kill off gut microbes," explained material scientist Andreas Herrmann, PhD, professor, Zernike Institute for Advanced Materials, University of Groningen. He and his coauthors published their research in Advanced Functional Materials.

Dr. Hermann and colleagues tested both the tooth polymer and conventional artificial tooth resins in a saliva solution containing Streptococcus mutans, the bacteria in the mouth that contributes to tooth decay. The researchers found that 99% of the bacteria on the antibacterial polymer were killed while only 1% were killed on the conventional material.

3. Heart

Scientists in Switzerland have 3D printed a silicone heart that works and pumps like a real human heart. There's just one problem: It only lasts for about 3,000 beats.

"Our goal is to develop an artificial heart that is roughly the same size as the patient's own one and which imitates the human heart as closely as possible in form and function," said its developer Nicholas Cohrs, doctoral student, Functional Materials Engineering, ETH Zurich.

When the researchers evaluated the performance of the 3D-printed heart, they concluded that it works and moves much like a human heart—but lasts for only about 30 to 45 minutes before giving out under the strain. They published their results in Artificial Organs.

"This was simply a feasibility test," Cohrs said. "Our goal was not to present a heart ready for implantation, but to think about a new direction for the development of artificial hearts."

4. Skin

Researchers in Canada have developed a handheld 3D printer that creates narrow sheets of skin tissue for covering and healing deep wounds. It's the first device that produces tissue in situ and could revolutionize burn care, according to the researchers, who published their findings in Lab on a Chip.

Most current 3D bioprinters are bulky, low-speed, expensive, and incompatible with clinical application, said researchers at the University of Toronto, Toronto, ON, Canada.

"Our skin printer promises to tailor tissues to specific patients and wound characteristics," said PhD student Navid Hakimi, who led the research. "And it's very portable."

Weighing less than 1 kg (about 2.2 lbs), the handheld bioprinter produces strips of "bio ink" made of collagen and fibrin. The process takes 2 minutes or less.

The researchers plan to expand the size of the printed strips to cover larger wound areas, and anticipate doing more in vivo studies with the ultimate goal of performing human clinical trials.

5. Bionic ear

Scientists at Princeton University, Princeton, NJ, have 3D printed a "bionic ear" that combines a small coil antenna with cartilage. The ear can "hear" radio frequencies far beyond the range of normal human capability, the researchers reported in Nano Letters.

Using an off-the-shelf 3D printer, the scientists combined a matrix of hydrogel and bovine (calf) cells with silver nanoparticles that formed the antenna. The calf cells later developed into cartilage. Electrical signals produced by the ear could be connected to a patient's nerve endings, similar to a cochlear implant.

This is the first time that 3D printing was effectively used for interweaving tissue with electronics, the researchers said.

In principle, the bionic ear could be used to restore or enhance human hearing. However, the scientists cautioned that further work and testing still need to be done before the technology can be used on patients.

6. Elastic bone

What good is a squishy, elastic bone? If your patient is a growing child who needs a bone implant or repair, then an elastic bone could be very good indeed.

Bioengineers at Northwestern University, Chicago, IL, have 3D printed artificial hyperelastic bones that quickly induce bone regeneration and growth. The bone material is made mostly of hydroxyapatite, a human bone mineral, along with a small percentage of a biocompatible, biodegradable polymer. The high concentration of hydroxyapatite and the bone's scaffold-like structure encourages rapid bone regeneration in the body. While hydroxyapatite materials are brittle on their own, the 3D printed bone material is flexible thanks to the polymer.

So far, the hyperelastic bones have been successfully implanted in animal models, as the researchers reported in Science Translational Medicine. Furthermore, the 3D printing process can customize any shape of bone necessary, with a potential turnaround time of less than a day.

Check out this video by the National Science Foundation on hyperelastic bones.

7. Ovary

An all-female team of scientists at Northwestern University's Feinberg School of Medicine and its McCormick School of Engineering, Evanston, IL, have created functioning 3D-printed ovaries. The bioprosthetic ovaries were implanted in sterilized mice, which were able to ovulate and birth live pups (see photo). The researchers documented their findings in a study published in Nature Communications.

The 3D-printed ovaries were made of gelatin, a biological hydrogel derived from collagen, which is abundant in both human and mouse ovaries. Also, as the researchers found, the design of the scaffold within the ovary was vitally important—their study was the first to demonstrate that scaffold architecture made a difference in the survival of follicles (the structures that house the eggs).

The scientists' sole objective for developing the bioprosthetic ovaries was to help restore fertility and hormone production in those who've had cancer and may be experiencing diminished ovarian function.

Creating organ structures that function and restore the health of reproductive organs "is the holy grail of bioengineering for regenerative medicine," said one researcher.