Growing functional human organs outside the body is a long-sought “holy grail” of organ transplant medicine that remains elusive. New research from the Wyss Institute for Biologically Inspired Engineering and the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) brings that quest one step closer to realization.
A team of scientists has created a new method for 3D printing vascular networks consisting of interconnected blood vessels that possess a distinctive “shell” of smooth muscle cells and endothelial cells surrounding a hollow “core” through which fluid can flow, embedded within human heart tissue. This vascular architecture closely mimics that of natural blood vessels and represents a significant advance toward the possibility of manufacturing implantable human organs. The achievement is published in Advanced materials.
“In previous work, we developed a novel 3D bioprinting method, known as ‘sacrificial writing in functional tissue’ (SWIFT), to pattern hollow channels within a living cellular matrix. Here, building on this method, we introduce coaxial SWIFT (co-SWIFT) that recapitulates the multilayered architecture found in native blood vessels, facilitating the formation of an interconnected endothelium and making it more robust to withstand the internal pressure of blood flow,” said first author Paul Stankey, a SEAS graduate student in the lab of co-lead author and Wyss Core Faculty Member Jennifer Lewis, Sc.D.
The key innovation developed by the team was a unique core-shell nozzle with two independently controllable fluid channels for the “inks” that form the printed vessels: a collagen-based shell ink and a gelatin-based core ink. The nozzle’s inner center chamber extends slightly beyond the shell chamber so that the nozzle can completely pierce a previously printed vessel and create interconnected branching networks for sufficient oxygenation of human tissues and organs. through Perfusion. Vessel size can be varied during printing by changing the printing speed or ink flow rates.
To confirm that the new co-SWIFT method worked, the team first printed their multilayer vessels in a transparent granular hydrogel matrix. Next, they printed the vessels in a newly created matrix called uPOROS, composed of a collagen-based porous material that replicates the dense, fibrous structure of living muscle tissue. They were able to successfully print branching vascular networks in both cell-free matrices. After printing these biomimetic vessels, the matrix was heated, causing the collagen in the matrix and the shell ink to cross-link, and the sacrificial gelatin core ink to melt, allowing for easy removal and resulting in an open, perfusable vasculature.
Moving on to even more biologically relevant materials, the team repeated the printing process using a shell ink that was infused with smooth muscle cells (SMCs), which make up the outer layer of human blood vessels. After melting the ink from the gelatin core, they perfused endothelial cells (ECs), which form the inner layer of human blood vessels, into their vasculature. After seven days of perfusion, both SMCs and ECs were alive and functioning as vessel walls – there was a three-fold decrease in vessel permeability compared to those without ECs.
Finally, they were ready to test their method inside living human tissue. They built hundreds of thousands of cardiac organ building blocks (OBBs)—tiny spheres of beating human heart cells, which are compressed into a dense cellular matrix. Next, using co-SWIFT, they printed a network of biomimetic vessels into the heart tissue. Finally, they removed the ink from the sacrificial core and seeded the inner surface of their spinal cord stem cell-laden vessels with endothelial cells. through perfusion and evaluated its performance.
Not only did these printed biomimetic vessels display the characteristic double-layer structure of human blood vessels, but after five days of perfusion with a blood-mimicking fluid, the cardiac OBBs began beating in a synchronized manner, indicating healthy, functional heart tissue. The tissues also responded to common cardiac medications: isoproterenol made them beat faster, and blebbistatin stopped them. The team even 3D-printed a model of the branching vasculature of a real patient’s left coronary artery into OBBs, demonstrating their potential for personalized medicine.
“We were able to successfully 3D print a model of the left coronary artery vasculature based on data from a real patient, demonstrating the potential utility of co-SWIFT for creating patient-specific vascularized human organs,” said Lewis, who is also the Hansjörg Wyss Professor of Biologically Inspired Engineering in the seas.
In future work, Lewis’s team plans to generate self-assembling capillary networks and integrate them with their 3D-printed blood vessel networks to more fully replicate the structure of human blood vessels at the microscale and improve the function of lab-grown tissues.
“To say that engineering functional living human tissue in the lab is difficult is an understatement. I am proud of the determination and creativity this team showed in showing that they could indeed build better blood vessels inside living, beating human heart tissue. I hope they continue to succeed in their mission to one day implant lab-grown tissue into patients,” said Wyss Founding Director Donald Ingber, Ph.D. Ingber is also the Professor of Vascular Biology Judah Folkman at HMS and Boston Children’s Hospital and Hansjörg Wyss Professor of Biologically Inspired Engineering in the seas.
Additional authors on the paper include Katharina Kroll, Alexander Ainscough, Daniel Reynolds, Alexander Elamine, Ben Fichtenkort, and Sebastien Uzel. This work was supported by the Vannevar Bush Faculty Fellowship Program, sponsored by the Under Secretary of Defense for Research and Engineering’s Office of Basic Research through Office of Naval Research Grant N00014-21-1-2958 and the National Science Foundation through the CELL-MET ERC (#EEC-1647837).