Organ printing is getting closer and closer to reality.

In a breakthrough study, researchers at Wake Forest Institute for Regenerative Medicine announce that they’ve used an advanced 3-D printer to create sections of bone, muscle and cartilage that all functioned like the real thing when implanted in animals.

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The advance could make it possible to custom print replacement body parts and organs for anyone.

The idea of printing tissue is not new. For years now, scientists have been using bioprinters to precisely lay down cells in specific patterns with the goal of creating a piece of bone or organ.

But the technology has not made its way into mainstream medicine because of the limitations that still exit.

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In the human body, cells grow and thrive thanks to blood vessels that supply them with oxygen and nutrients.

But when living cells are printed, they must be suspended in some kind of biologically compatible matrix until they can grow and develop into the final body part.

Keeping them alive has been a challenge since these structures lack blood vessels, which are too small and delicate to be printed.

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And even if scientists have figured out a way to keep cells alive, the resulting structures have proved unstable and too fragile to be implanted into a living being.

At Wake Forest, a research team led by Anthony Atala, developed a matrix embedded with microchannels — a sort of bio-sponge — that allows nutrients and oxygen to flow freely to the cells anywhere in the structure.

To make a specific kind of tissue, the scientists shaped the bio-sponge, which is made from a biodegradable material, into the custom form. Next, they infused it with a water-based gel that contained the living cells.

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The cells were allowed to grow and then the structure was implanted in an animal. Over time, the matrix biodegraded and the cells took hold on their own in the desired shape.

In the study, the scientists found that after two months the ears, implanted in mice, had kept their shape.

Muscles cells prompted nerve formation in rats and bone implants actually triggered the formation of blood vessels after five months.

Eventually, such tissues will need to be studied in humans. Because this research was funded in part by the US Army, it’s only a matter of time before this kind of technology will be used to help people, perhaps starting with soldiers injured on the battlefield.