One of the best ways to appreciate the beautiful intricacy of something is to try to replicate it yourself. This is especially so in the quest to bioprint human organs for the hundreds of thousands of people on the waitlist for a lifesaving transplant. Now, bioengineers have taken a valuable step forward with a “breakthrough technique” for printing vascular tissues. 

Vascular networks are vital passages for blood, air, lymph, and other nutrient transportation. The architecture of such a system is vast and complex, with independent networks “like the airways and blood vessels of the lung or the bile ducts and blood vessels in the liver" physically and biochemically entangled. 

"The liver is especially interesting because it performs a mind-boggling 500 functions, likely second only to the brain," said co-lead author Kelly Stevens, from the University of Washington School of Medicine and the UW College of Engineering, in a statement. "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.”

To work towards such a goal, the team forged a new bioprinting technology called "stereolithography apparatus for tissue engineering" (SLATE) that makes soft hydrogels layer by layer. Once the gel layers are printed, they solidify under blue light. A food dye widely available in the food industry is used to absorb the blue light and refine where the fine, delicate layers form. This process allows the team to produce an intricate, biocompatible, water-based gel in a matter of minutes. 

The team worked with Nervous System to design and test the technology by fashioning a lung-mimicking structure that was stress-tested with blood and air pushed through its tissues – the bio-printed design stood up to the test. They found that red blood cells could even take up the oxygen as the air sac “breathed”.

"When we founded Nervous System it was with the goal of adapting algorithms from nature into new ways to design products," said study co-author Jessica Rosenkrantz. "We never imagined we'd have the opportunity to bring that back and design living tissues.”

The 3D-printing system can even make bicuspid valves – those that only allow blood to flow in one direction. Such one-way valves are found in the human heart, leg veins, and lymphatic system. 

"With the addition of multivascular and intravascular structure, we're introducing an extensive set of design freedoms for engineering living tissue," said Jordan Miller, co-lead author, of Rice University. "We now have the freedom to build many of the intricate structures found in the body.”

To replicate such an evolutionary feat would benefit transplant patients by providing them with replacement organs from their own cells. Currently, transplant recipients must take a lifetime of immune-suppressant drugs to prevent a rejection of the foreign donor organ.

Bioprinted organs for transplant patients are not on the table at this point and will likely take much more time to perfect. To help speed progress in this much-needed arena, the team said their creation is open-source and freely available. The study is published in the journal Science. 

"Making the hydrogel design files available will allow others to explore our efforts here, even if they utilize some future 3D printing technology that doesn't exist today," added Miller.

"We are only at the beginning of our exploration of the architectures found in the human body. We still have so much more to learn.”