In a 3D printing first, scientists have figured out how to print artificial versions of the body's complex vascular networks, which mimic our natural passageways for blood, air, lymph, and other vital fluids.

“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, assistant professor of bioengineering at Rice’s Brown School of Engineering, who helped lead the team, in a press statement.

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Miller says our organs contain their own vascular networks, like the lung's blood vessels and airways, and 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," he says.

But Miller and his team are the first to develop bioprinting technology that "addresses the challenge of multivascularization in a direct and comprehensive way."

Creating functional tissue replacements is a high scientific priority because of its potential impact on organ donations. The crisis of organ shortages is a long-lasting one; around 114,000 people are on transplant waiting lists in the United States alone. Even after a successful transplant, patients have to take immune-suppressing drugs to prevent organ rejection for the foreseeable future. Bioprinting organs could play an important role in reducing both problems.

“Tissue engineering has struggled with this for a generation,” says Kelly Stevens of the University of Washington, who led the team of bioengineers with Miller, in the press statement. “With this work, we can now better ask, ‘If we can print tissues that look and now even breathe more like the healthy tissues in our bodies, will they also then functionally behave more like those tissues?’ This is an important question, because how well a bioprinted tissue functions will affect how successful it will be as a therapy.”

The heart and brain are often thought to be the most complex human organs, but other, equally nuanced body parts have proven to be just as difficult to recreate in the lab.

“The liver is especially interesting because it performs a mind-boggling 500 functions, likely second only to the brain,” Stevens says. “The liver’s complexity means there is currently no machine or therapy that can replace all its functions when it fails. Bioprinted human organs might someday supply that therapy.”

The team created a new open-source bioprinting technology that they called “stereolithography apparatus for tissue engineering,” or SLATE. During the SLATE process, layers are printed one at a time from a liquid pre-hydrogel solution. When that solution is exposed to blue light, it becomes solid.

Images from a DLP projector display sequential 2D slices of the structure at extremely high resolution, down pixel sizes ranging from 10 to 50 microns. When each layer solidifies, an overhead arm raises the growing 3D gel slight, enough to expose liquid to the next image from the projector.

The scientists made a lung-mimicking structure as a test. SLATE held up, showing itself to be sturdy enough to create a rhythmic intake and outflow of "breathing." Red blood cells had enough room to carry oxygen through the body.

For size comparison, Rice placed a penny next to their scale-model of a lung-mimicking air sac with airways and blood vessels . Brandon Martin/Rice University

“We are only at the beginning of our exploration of the architectures found in the human body,” Miller says in the statement. “We still have so much more to learn.”

Reprinting human organs has a number of potential uses. In addition to transplants, scientists are using 3D printed organs to better understand how they are affected by cancer.

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