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.