Researchers develop new method to create complex vascular networks in heart tissue, paving the way for advancements in organ transplantation.
The development of functional human organs outside the body has long been a goal in the field of organ transplantation, but despite decades of research, the ability to wholly engineer and manufacture viable organs for transplant remains beyond reach. Now a new study from Harvard’s Wyss Institute for Biologically Inspired Engineering and the John A Paulson School of Engineering and Applied Science (SEAS) marks a significant step forward in this space. The researchers have developed a method to 3D-print intricate vascular networks within human cardiac tissue, mimicking the natural structure of blood vessels with remarkable closeness.
This breakthrough, published in Advanced Materials, introduces a technique known as coaxial sacrificial writing in functional tissue (co-SWIFT). The method enables the creation of interconnected blood vessels, which feature a shell of smooth muscle cells and endothelial cells surrounding a hollow core through which fluid can flow. These printed vessels not only replicate the physical architecture of natural blood vessels but also demonstrate the potential to support living tissue, a crucial advancement towards the eventual goal of manufacturing implantable human organs [1].
Longevity.Technology: Discussions about transplant organs, whether the source is human donors, xenotransplantation or artificial creations, always comes down to vascularization. No man is an island, and no organ is either; organs need a healthy blood supply, and the importance of developing vascularized tissues in organ engineering cannot be overstated. The ability to generate complex, perfusable vasculature is a critical hurdle in tissue engineering, as it is essential for supplying oxygen and nutrients to maintain viable tissues.
Every step of progress in organ transplantation must be met with equal progress in vascularization; these ongoing advances could addressing the substantial unmet need for donor organs and significantly impact both lifespan and healthspan. With thousands of patients languishing on transplant waiting lists, often with dire outcomes, innovations that bring us closer to creating functional, lab-grown organs that are appropriately vascularized could potentially save countless lives.
The original SWIFT method (left) printed hollow channels through living OBBs (green), but had no structure to contain fluid as it flowed through. Co-SWIFT (right) creates a cell-laden vessel (red) surrounding the channel, which isolates blood flow from the tissue and improves their viability.
“In prior work, we developed a new 3D bioprinting method, known as ‘sacrificial writing in functional tissue’ (SWIFT), for patterning hollow channels within a living cellular matrix,” explains Paul Stankey, the study’s first author and a graduate student at SEAS. “Here, building on this method, we introduce coaxial SWIFT (co-SWIFT) that recapitulates the multilayer architecture found in native blood vessels, making it easier to form an interconnected endothelium and more robust to withstand the internal pressure of blood flow [2].”
The innovation at the heart of this research is the use of a core-shell nozzle, which features two independently controllable fluid channels for the ‘inks’ that form the printed vessels. The shell ink, composed of a collagen-based material, surrounds a gelatin-based core ink. The nozzle is designed to puncture previously printed vessels, creating branching networks capable of supporting the oxygenation required by human tissues and organs, and by adjusting printing speed and ink flow rates, the team can vary the size of the vessels, tailoring them to specific needs.
To validate the efficacy of the co-SWIFT method, the researchers first printed their multilayer vessels within a transparent granular hydrogel matrix. They then progressed to a more biologically relevant matrix known as uPOROS, a porous collagen-based material that closely resembles the dense, fibrous structure of living muscle tissue – in both instances, the team successfully created branching vascular networks. The process involved heating the matrix, which caused the collagen in both the matrix and the shell ink to crosslink, while the gelatin core ink melted and was removed, leaving behind an open, perfusable vasculature [1].
co-SWIFT prints 3D vessels that consist of an outer “shell” and an inner “core” that can be easily connected to each other to create a branching network of vasculature that can support living human tissues.
Further experimentation involved using a shell ink infused with smooth muscle cells (SMCs), which are integral to the outer layer of human blood vessels. After the removal of the gelatin core ink, endothelial cells (ECs), which form the inner layer of blood vessels, were perfused into the vasculature. Over a seven-day period, both SMCs and ECs remained viable and functioned as vessel walls, with a marked reduction in vessel permeability, indicative of a successful replication of natural blood vessel functionality [1].
The researchers then tested the method in living human tissue by constructing cardiac organ building blocks (OBBs) composed of tiny spheres of beating human heart cells compressed into a dense cellular matrix. Using co-SWIFT, they printed a biomimetic vessel network into this cardiac tissue. After removing the sacrificial core ink and seeding the SMC-laden vessels with ECs, the tissue was evaluated for performance. The results were promising: the printed vessels displayed the characteristic double-layer structure of human blood vessels, and in addition, after five days of perfusion with a blood-mimicking fluid, the cardiac OBBs began to beat synchronously – an indication of healthy, functional heart tissue [1]. The tissues also responded appropriately to common cardiac drugs, further demonstrating their potential use in medical applications.
“We were able to successfully 3D-print a model of the vasculature of the left coronary artery based on data from a real patient, which demonstrates the potential utility of co-SWIFT for creating patient-specific, vascularized human organs,” says Jennifer Lewis, co-senior author of the study and Hansjörg Wyss Professor of Biologically Inspired Engineering at SEAS [2].
Looking ahead, the team plans to expand upon their work by generating self-assembled networks of capillaries and integrating them with their 3D-printed blood vessel networks. This would more fully replicate the micro-scale structure of human blood vessels, potentially enhancing the functionality of lab-grown tissues. As noted by Wyss Founding Director Donald Ingber: “To say that engineering functional living human tissues in the lab is difficult is an understatement. I’m proud of the determination and creativity this team showed in proving that they could indeed build better blood vessels within living, beating human cardiac tissues. I look forward to their continued success on their quest to one day implant lab-grown tissue into patients [2].”
Photographs credit: Wyss Institute at Harvard University. Cover photo shows that co-SWIFT vessels are embedded with living smooth muscle cells and endothelial cells to replicate the structure of human blood vessels in vitro.
[1] https://onlinelibrary.wiley.com/doi/10.1002/adma.202401528
[2] https://wyss.harvard.edu/news/3d-printed-blood-vessels-bring-artificial-organs-closer-to-reality/
