The key to creating viable, living tissue is first understanding how it works.
As much as scientists know about the human body, the way tissue is formed at the cellular and sub-cellular level is still in large part a mystery. There are about 40 different cell types that make up a human liver, including Kupffer cells for removing debris from the blood, stellate cells for regenerating tissue that has died or been injured, and sinusoidal endothelial cells, which make up the interior surface of blood vessels and lymphatic vessels.
3D printed breast tissue could someday help to reconstruct breasts in cancer survivors. (Image: TeVido BioDevices)
"It's a complicated challenge," Miller said. "We don't know all the structures in the body. We're still learning. So we don't know to what extent we need to reconstitute all those features. We have some evidence that we may not need to re-create all those functions."
Miller and others believe that if they reconstitute a portion of tissue, even if it's not complete, there's a good chance it will continue to grow into a fully functioning organ once implanted in the body.
"We've had some success in thin tissues, skin, corneas and bladder," Miller said. "It gets more complicated when you're talking about biochemical functions in the liver or kidney. Those are fragile cells that don't do well in labs. Some of the most interesting cells we want to print are hardest to keep alive."
Instead of printing cells 10 layers deep, as might be needed for a skin graft, researchers are attempting to print cells 5,000 or 10,000 layers deep, Miller said.
Just add sugar and water, and voila — blood vessels
In order to print thick tissues, scientists must also be able to create the vascular system needed for sustainability.
One approach with 3D printing has been to print out a temporary "scaffolding" made of sugar glass (a sugar-and-water combination) that can act as a mold to support cells that eventually form blood vessels. It's similar to the way a bronze statue is created: First the mold is formed, then filled with metal. In this case, living cells are used instead of metal.
Miller and others have had some success re-creating those vascular structures through the use of sugar glass — the same substance that's used to make easily breakable bottles and windows for stunts in movies.
"You start with a template, cast it and then melt [the sugar glass] out, leaving the vascular structure behind," Miller said. "Sugar is great because it's very rigid."
Using sugar glass as the scaffolding, Miller and his team of researchers have had some success in re-creating liver tissue. To date, the researchers have been able to create a piece of tissue the size of a thumbnail and keep it alive for two weeks.
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