3-D printing has been a hot topic in the health care industry. Organovo (ONVO -8.89%), which is working to print the world's first liver tissue, has captured the public's imagination with the theory that entire organs and body parts can eventually be bioprinted.

While the idea of printing replacement body parts sounds like the realm of science fiction, scientists and engineers have already made incredible advancements over the past few years in using 3D printers to create prosthetics, implants, and even pacemakers.

Open source 3-D printed prosthetics
Merging 3D printing with open source templates -- which are freely available to anyone to manufacture, distribute, and modify -- could spark a revolution in cheaper prosthetics for amputees around the world.

Hanger (NYSE: HGR), a leading provider of cutting-edge prosthetics, works with impressive products, such as the C-Leg prosthetic leg and the i-LIMB prosthetic hand. The C-Leg uses multiple motion sensors to allow amputees to jog and ride bicycles. The i-LIMB captures electrical signals for an amputee's remaining limb to allow finger-by-finger movement to type or dial the phone.

This makes them impractical products for rural, developing, and emerging markets. In response, several projects to create cheaper open source prosthetic hands have emerged. Joel Gibbard's Open Hand project, which started on crowdfunding site Indiegogo, aims to make its open source 3-D printed Dextrus Hand available across the world for under $1,000. A similar project, Richard Van As and Ivan Owen's Robohand, aims for an even lower price point of $150. The Robohand can be assembled from 16 3-D printed pieces and 28 off-the-shelf components.

Both the Dextrus and Robohand only have a single grip pattern, compared to the i-LIMB's individual finger movements, but they are still remarkable examples of the ability of open source prosthetics to aid amputees in lower income areas.

3-D printed medical implants
According to a recent study, 2% of Americans, or 7 million people, are living with new hips and knees. While that sounds like good news for leading orthopedic implant manufacturers Johnson & Johnson (JNJ -1.97%), Stryker, and Zimmer Holdings, investors should remember that all three companies were recently hit with troubling recalls.

Johnson & Johnson was arguably the worst offender, reaching an agreement to resolve over 7,500 lawsuits against its DePuy Orthopedics unit last November for over $2.5 billion. DePuy's all-metal ASR hip systems were intended to be more durable than traditional hip implants, which usually consist of plastic sockets with ceramic or metal balls.

Unfortunately, those all-metal devices can leave behind dangerous fragments of metal over time, causing bone fractures, dislocations, infections, as well as nerve and muscle damage. The British Orthopedic Association and the British Hip Society reported that DePuy's ASR devices had a failure rate between 21% after four years to 49% after six years. That failure rate leads to an interesting question -- can 3-D printing be used to create better fitting, safer implants?

Three years ago, Dr. Craig Gerrand, a doctor at Newcastle-upon-Tyne Hospitals NHS Trust, created a 3-D printed implant for a patient with a type of bone cancer which required the removal of half of the pelvis. Dr. Gerrand and a team of doctors scanned the patient to determine how much bone needed to be replaced, then designed a 3-D model based on the missing area. A new half-pelvis implant was then printed out of titanium and coated in a mineral to aid the natural regeneration of the connected bone.

Although Dr. Gerrand's procedure was an experimental one, it was only one of several recent successful implants using 3-D printers, indicating that these custom printed implants could eventually replace the current generation of troublesome metal-on-metal implants.

3-D printed pacemakers
Last but not least, the pacemaker industry dominated by Medtronic (MDT -1.07%) and St. Jude Medical (STJ) could eventually be altered by membrane-like 3-D printed pacemakers that look nothing like today's devices.

Earlier this month, researchers at the University of Illinois and Washington University created a thin sheath of flexible silicon with a 3-D printer that could be fit around a 3-D replica of a rabbit's heart. The silicon pouch was then filled with a web of sensors that could monitor the heart and keep its rhythm steady. The 3-D printed pacemaker functions like an artificial pericardium -- the outer sac that covers the heart.

Medtronic controls over half of the pacemaker market today, but it has trailed behind its chief rival St. Jude Medical in terms of overall innovation. In 2009, St. Jude created the first Wi-Fi pacemaker that can be continuously connected to the cloud, then acquired miniature pacemaker manufacturer Nanostim last October. Nanostim's tiny pacemaker, which is the size of a triple A battery, doesn't require any wires (known as leads) that connect to the heart -- a component that has been implicated in failures of traditional pacemakers. It can also be implanted and removed without surgery. Medtronic fired back with the Micra TPS, an even smaller pacemaker roughly the size of a multivitamin.

However, the 3-D printed "heart wrap" indicates that Medtronic and St. Jude's progressively smaller devices could also become less mechanical and much softer, thinner, and more organic in the near future -- which would render their traditional devices obsolete.

The Foolish takeaway
Prosthetics, implants, and pacemakers only represent three of the many possible applications of 3-D printing in the medical device industry.

When we put together the pieces of this puzzle, it's easy to picture a future where bioprinting technology converges with traditional 3-D printing, where prosthetics and implants can be printed from organic material instead of silicon or metal. When that day arrives, humans could realistically print their own replacement parts, dramatically altering the future for companies like Organovo, Hanger, Johnson & Johnson, Medtronic, and St. Jude.

Editor's Note: In a previous version of this article, Hanger, Inc. was incorrectly referred to as a prosthetic limb manufacturer. Also, the prices listed for some of the prosthetics the company provides were inaccurate and this section has been removed from the article. The Fool regrets the errors.