A new project led by Rice University Bioengineers combines unique bioprinting experiences to clear a major hurdle on the path to 3D printing replacement organs, using a photopolymerization-based bioprinting technique. As featured in the May 3 issue of Science, the technique allows scientists to create more complex, entangled vascular networks that mimic the body’s natural passageways for blood, air, lymph, and other vital fluids.
The research comes with a visually stunning proof-of-principle — a bioprinted hydrogel model of a lung-mimicking air sac in which airways deliver oxygen to surrounding blood vessels (see the video below). The work was led by bioengineers Jordan Miller of Rice University and Kelly Stevens of the University of Washington (UW), along with 15 collaborators from Rice, UW, Duke University, Rowan University and the well known generative 3D printing studio, Nervous System, an MIT-originating design firm based in Somerville. After establishing a clear leadership in generative design and 3D printing innovation, the studio co-founded by Jessica Rosenkrantz has been exploring materials such as ceramics and, recently, hydrogels and cells.
“One of the biggest roadblocks to generating functional tissue replacements has been our inability to print the complex vasculature that can supply nutrients to densely populated tissues,” said Miller, assistant professor of bioengineering at Rice’s Brown School of Engineering. “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 multi-vascularization in a direct and comprehensive way.”
Stevens, assistant professor of bioengineering in the UW College of Engineering, assistant professor of pathology in the UW School of Medicine, and an investigator at the UW Medicine Institute for Stem Cell and Regenerative Medicine said multi-vascularization is important because form and function often go hand in hand.
“Tissue engineering has struggled with this for a generation,” Stevens said. “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 liver is especially interesting because it performs a mind-boggling 500 functions, likely second only to the brain,” Stevens continued. “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 address this challenge, the team created a new open-source bioprinting technology dubbed the “stereolithography apparatus for tissue engineering,” or SLATE. The system uses a photopolymerization additive manufacturing process to make soft hydrogels one layer at a time.
Layers are printed from a liquid pre-hydrogel solution that becomes a solid when exposed to blue light. A digital light processing projector shines light from below, displaying sequential 2D slices of the structure at high resolution, with pixel sizes ranging from 10-50 microns. With each layer solidified in turn, an overhead arm raises the growing 3D gel just enough to expose liquid to the next image from the projector. The key insight by Miller and Bagrat Grigoryan, a Rice graduate student and lead co-author of the study, was the addition of food dyes that absorb blue light. These photoabsorbers confine the solidification to a very fine layer. In this way, the system can produce soft, water-based, biocompatible gels with intricate internal architecture in a matter of minutes.
Tests of the lung-mimicking structure showed that the tissues were sturdy enough to avoid bursting during blood flow and pulsatile “breathing,” a rhythmic intake and outflow of air that simulated the pressures and frequencies of human breathing. Tests found that red blood cells could take up oxygen as they flowed through a network of blood vessels surrounding the “breathing” air sac. This movement of oxygen is similar to the gas exchange that occurs in the lung’s alveolar air sacs.
To design the study’s most complicated lung-mimicking structure, which is featured on the cover of Science, Miller collaborated with study co-authors Jessica Rosenkrantz and Jesse Louis-Rosenberg, co-founders of Nervous System. “When we founded Nervous System it was with the goal of adapting algorithms from nature into new ways to design products,” Rosenkrantz said. “We never imagined we’d have the opportunity to bring that back and design living tissues.”
Miller said the new bioprinting system can also produce intravascular features, like bicuspid valves that allow fluid to flow in only one direction. In humans, intravascular valves are found in the heart, leg veins and complementary networks like the lymphatic system that have no pump to drive flow. “With the addition of multi-vascular and intravascular structure, we’re introducing an extensive set of design freedoms for engineering living tissue,” Miller said. “We now have the freedom to build many of the intricate structures found in the body.”
Miller and Grigoryan are commercializing key aspects of the research through a Houston-based startup company called Volumetric, whose products are now commercialized through a partnership with emerging bioprinting segment leader CELLINK. The company, which Grigoryan has joined full time, is designing and manufacturing bioprinters and bioinks.
“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,” Miller said. Miller said his lab is already using the new design and bioprinting techniques to explore even more complex structures. “We are only at the beginning of our exploration of the architectures found in the human body,” he said. “We still have so much more to learn.”
Additional study co-authors include Rice’s Samantha Paulsen, Daniel Sazer, Alexander Zaita, Paul Greenfield, Nicholas Calafat and Anderson Ta; UW’s Daniel Corbett, Chelsea Fortin and Fredrik Johansson; Duke’s John Gounley and Amanda Randles; and Rowan’s Peter Galie.
The work was supported by the Robert J. Kleberg, Jr. and Helen C. Kleberg Foundation, the John H. Tietze Foundation, the National Science Foundation, the National Institutes of Health and the Gulf Coast Consortia.