Currently, if you need a replacement organ you'll have to join a long waiting list or have a donor ready and willing. An emerging alternative is to 3D print new organs out of a patient's own cells, but that technology is still going through some teething issues. Now researchers have found a surprisingly simple answer to the complex problem of printing detailed vascular networks, and shown it off with a dramatic model of a breathing lung that passes oxygen into surrounding blood vessels.

According to the team, growing living cells in the lab is relatively straight-forward with today's technology – the tricky part is keeping them alive and functioning. In the body, vast vascular networks pump nutrients to organs and tissues to keep them running, and trying to accurately replicate these has been a huge hurdle so far.

But the new study, conducted by researchers from the universities of Washington, Rice, Duke and Rowan, as well as a design firm named Nervous System, solves this complex issue with a simple fix – food dyes.

First the team created a new open-source technology for bioprinting called the stereolithography apparatus for tissue engineering (SLATE). Like regular 3D printing, the system works by depositing layer upon layer of a material, slowly building up the structure of the object being printed. In this case, that material is made up of living cells and hydrogel scaffolds.

After each liquid layer is added, it's cured with a blue light shone from below. Then the solid gel structure is lifted up by a robotic arm, and a new layer is added underneath. The team improved on the process by adding food dyes that absorb blue light, which focuses the solidification onto very thin layers. That, in turn, allows the printer to create objects in very fine detail, including the necessary vasculature.

"One of the biggest road blocks to generating functional tissue replacements has been our inability to print the complex vasculature that can supply nutrients to densely populated tissues," says Jordan Miller, lead researcher on the study. "Further, our organs actually contain independent vascular networks – like the airways and blood vessels of the lung or the bile ducts and blood vessels in the liver. These interpenetrating networks are physically and biochemically entangled, and the architecture itself is intimately related to tissue function. Ours is the first bioprinting technology that addresses the challenge of multivascularization in a direct and comprehensive way."

To demonstrate the new system, the team bioprinted a kind of model lung. On the inside is an air sac made of hydrogel, into which air is pumped in a fashion that mimicked breathing. Surrounding the air sac is a network of tubes through which blood flows. Like the real thing, the sac is designed to allow oxygen to pass through the membrane into the blood vessel, oxygenating the blood.

In another test, the team bioprinted new "livers" for mice with liver disease. They created small devices made of hydrogel loaded with liver cells and fed by vascular networks, then implanted them into mice. After two weeks, the team found that the liver cells were still surviving, indicating that the vascular network was keeping them alive and functional.

The research could be instrumental in helping to speed up the development of bioprintable organs, which could reduce the need for human organ transplants.

The research was published in the journal Science. The lung model can be seen in action in the video below.

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