Co-SWIFT 3D printed blood vessels, incorporating living smooth muscle cells and endothelial cells, successfully mimic the intricate structure of human blood vessels in a laboratory setting. (Image courtesy of the Wyss Institute at Harvard University)
The dream of creating fully functional human organs outside the human body has long been a major objective in the field of organ transplantation. This ambitious goal, often referred to as the “holy grail” of regenerative medicine, has remained largely out of reach—until now. Groundbreaking research emerging from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Wyss Institute for Biologically Inspired Engineering is bringing this vision significantly closer to reality.
Scientists have unveiled a revolutionary technique for 3D printing intricate vascular networks. These networks are composed of interconnected blood vessels, meticulously engineered with a distinct “shell” of smooth muscle cells and endothelial cells surrounding a hollow “core”. This core allows for the seamless flow of fluids, mirroring the functionality of natural blood vessels and is embedded within human cardiac tissue. This sophisticated vascular architecture closely replicates the complexity of naturally occurring blood vessels, marking a substantial leap forward in the ability to manufacture implantable human organs through 3D printing. The details of this remarkable achievement have been published in the prestigious journal Advanced Materials.
“Building upon our previous work where we introduced ‘sacrificial writing in functional tissue’ (SWIFT), a novel 3D bioprinting method for creating hollow channels within living cellular matrices, we have now developed coaxial SWIFT (co-SWIFT). This enhanced method allows us to replicate the multi-layered structure inherent in native blood vessels,” explains Paul Stankey, the study’s lead author and a bioengineering Ph.D. student at SEAS, working in the lab of Jennifer Lewis, Hansjorg Wyss Professor of Biologically Inspired Engineering at SEAS and a Wyss Core Faculty member. Stankey further emphasizes that co-SWIFT facilitates the formation of interconnected endothelium more efficiently and enhances the robustness of the printed vessels to withstand the internal pressure of blood flow, crucial for their functionality when 3d Printing An Organ.
The cornerstone of this innovative co-SWIFT method is a uniquely designed core-shell nozzle. This nozzle features two independently controlled fluid channels that dispense specialized “inks” to construct the vessels. These inks include a collagen-based shell ink and a gelatin-based core ink. The nozzle’s interior core chamber extends slightly beyond the shell chamber, enabling it to precisely puncture a previously printed vessel layer. This precise control is essential for creating interconnected branching networks, which are vital for ensuring sufficient oxygenation and nutrient delivery throughout the engineered human tissues and organs via perfusion – a critical step in 3D printing an organ that is functional. The dimensions of the vessels can be finely tuned during the printing process by adjusting either the printing speed or the flow rates of the inks, providing flexibility in creating complex vascular structures.
To validate the efficacy of the new co-SWIFT method, the research team initially printed these multi-layered vessels within a transparent granular hydrogel matrix. Subsequently, they extended their printing to uPOROS, a recently developed matrix. UPOROS is composed of a porous collagen-based material that closely emulates the dense, fibrous structure of living muscle tissue. Impressively, they successfully printed branching vascular networks within both of these cell-free matrices, demonstrating the versatility of their approach to 3D printing an organ. Following the bioprinting of these biomimetic vessels, the matrix was carefully heated. This heating process induced crosslinking of the collagen in both the matrix and the shell ink, while simultaneously melting the sacrificial gelatin core ink. The melted gelatin was then easily removed, leaving behind an open and perfusable vasculature, ready for the next stage of biointegration.
Advancing towards more biologically relevant applications in 3D printing an organ, the team repeated the printing process using a shell ink that was enriched with smooth muscle cells (SMCs). SMCs are crucial components of the outer layer of human blood vessels. After removing the gelatin core, they introduced endothelial cells (ECs), which form the inner lining of human blood vessels, into their newly printed vasculature through perfusion. After a seven-day perfusion period, both SMCs and ECs demonstrated viability and functionality as vessel walls. Notably, vessels integrated with ECs showed a threefold reduction in permeability compared to those without ECs, highlighting the importance of this dual-layered cellular structure for vessel integrity and function.
A visual comparison highlighting the advancements of Co-SWIFT over the original SWIFT method in 3D bioprinting. SWIFT (left) created hollow channels but lacked structural integrity for fluid containment. Co-SWIFT (right) introduces a cell-laden vessel wall, isolating blood flow and enhancing tissue viability, crucial for 3D printing an organ. (Image courtesy of the Wyss Institute at Harvard University)
Finally, to truly test the method’s potential for 3D printing an organ, the researchers applied co-SWIFT within living human tissue. They engineered hundreds of thousands of cardiac organ building blocks (OBBs)—tiny spheres composed of beating human heart cells—which were then compressed into a dense cellular matrix. Utilizing co-SWIFT, they printed a biomimetic vessel network directly into this cardiac tissue. Subsequently, the sacrificial core ink was removed, and the inner surface of the SMC-laden vessels was seeded with ECs via perfusion. The performance of these bioprinted vessels within living cardiac tissue was then rigorously evaluated.
The results were compelling. These 3D printed biomimetic vessels not only exhibited the characteristic double-layer structure of natural human blood vessels but also demonstrated functionality within the living tissue. After five days of perfusion with a blood-mimicking fluid, the cardiac OBBs began to beat synchronously, a clear indication of healthy and functional heart tissue. Furthermore, the tissues exhibited expected responses to common cardiac drugs: isoproterenol accelerated their beating rate, while blebbistatin halted their beating entirely. In a powerful demonstration of personalized medicine potential through 3D printing an organ, the team even 3D-printed a model of the branching vasculature of a real patient’s left coronary artery directly into cardiac OBBs.
“The ability to successfully 3D-print a vascular model of a patient’s left coronary artery from real patient data underscores the potential of co-SWIFT for creating patient-specific, vascularized human organs,” concludes Lewis, highlighting the clinical translation possibilities of their work in 3D printing an organ.
Looking ahead, Lewis and her team are focused on further refining their approach to 3D printing an organ. Their future endeavors include generating self-assembled capillary networks and integrating these with their 3D-printed blood vessel networks. This integration aims to more comprehensively replicate the intricate structure of human blood vessels at the microscale and to further enhance the functionality of lab-grown tissues, pushing the boundaries of what is possible in 3D printing an organ for transplantation and regenerative medicine.
This research was made possible with contributions from Katharina Kroll, Alexander Ainscough, Daniel Reynolds, Alexander Elamine, Ben Fichtenkort, and Sebastien Uzel. Funding was provided by the Vannevar Bush Faculty Fellowship Program sponsored by the Basic Research Office of the Assistant Secretary of Defense for Research and Engineering through the Office of Naval Research Grant N00014-21-1-2958 and the National Science Foundation through CELL-MET ERC (#EEC-1647837).
