Growing fully functional human organs outside the body has long been considered the ultimate goal in organ transplantation medicine, yet it remains a significant challenge. However, new research from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Wyss Institute for Biologically Inspired Engineering has brought this aspiration closer to reality.
Scientists have developed an innovative method for 3D printing vascular networks. These networks feature interconnected blood vessels with a distinct “shell” of smooth muscle cells and endothelial cells surrounding a hollow “core,” enabling fluid flow, all embedded within human cardiac tissue. This intricate vascular structure closely resembles natural blood vessels, marking substantial progress towards manufacturing implantable human organs. The groundbreaking findings are detailed in the journal Advanced Materials.
“In our previous work, we introduced a novel 3D bioprinting technique called ‘sacrificial writing in functional tissue’ (SWIFT), which allowed us to create hollow channels within a living cellular matrix. Building upon this, we now present coaxial SWIFT (co-SWIFT). This enhanced method replicates the multi-layered architecture of native blood vessels, facilitating the formation of interconnected endothelium and increasing robustness against internal blood flow pressure,” explained Paul Stankey, the 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 Wyss Core Faculty member.
Co-SWIFT vessels are embedded with living smooth muscle cells and endothelial cells to replicate the structure of human blood vessels in vitro.
The core innovation lies in a specialized core-shell nozzle featuring two independently controlled fluid channels for the printing “inks.” These inks consist of a collagen-based shell ink and a gelatin-based core ink. The nozzle’s interior core chamber extends slightly beyond the shell chamber, allowing it to puncture previously printed vessels and create interconnected branching networks. This is crucial for effective oxygenation of human tissues and organs through perfusion. The vessel size can be adjusted during printing by modifying the printing speed or ink flow rates.
To validate the co-SWIFT method, the team initially printed multi-layered vessels into a transparent granular hydrogel matrix. Subsequently, they printed vessels into uPOROS, a newly developed matrix composed of a porous collagen-based material that mimics the dense, fibrous structure of living muscle tissue. They successfully printed branching vascular networks in both cell-free matrices. Following printing, the matrix was heated, causing collagen in the matrix and shell ink to crosslink and the sacrificial gelatin core ink to melt away. This process resulted in an open, perfusable vasculature.
Advancing to more biologically relevant materials, the team repeated the process using a shell ink infused with smooth muscle cells (SMCs), the outer layer of human blood vessels. After removing the gelatin core ink, they perfused endothelial cells (ECs), which form the inner layer, into the vasculature. After seven days of perfusion, both SMCs and ECs were viable and functioning as vessel walls. Notably, vessels with ECs showed a three-fold reduction in permeability compared to those without.
The original SWIFT method (left) printed hollow channels through living OBBs (green), while Co-SWIFT (right) creates a cell-laden vessel (red) surrounding the channel, enhancing blood flow isolation and tissue viability.
Finally, the method was tested within living human tissue. Researchers constructed hundreds of thousands of cardiac organ building blocks (OBBs) – 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 via perfusion, they assessed the performance.
The printed biomimetic vessels exhibited the characteristic double-layer structure of human blood vessels. Moreover, after five days of perfusion with a blood-mimicking fluid, the cardiac OBBs began to beat synchronously, indicating healthy and functional heart tissue. The tissues also responded appropriately to common cardiac drugs: isoproterenol increased beating rate, and blebbistatin halted beating. The team even 3D-printed a model of a patient’s left coronary artery vasculature into OBBs, demonstrating the potential for personalized medicine using 3d Printed Organs.
“We successfully 3D-printed a vascular model of the left coronary artery based on patient data, highlighting co-SWIFT’s potential for creating patient-specific, vascularized human organs,” stated Lewis.
Looking ahead, Lewis’s team aims to develop self-assembled capillary networks and integrate them with their 3D-printed blood vessel networks. This advancement would further replicate the structure of human blood vessels at the microscale and improve the functionality of lab-grown tissues, bringing us closer to the reality of 3d printed organs for transplantation.
Additional contributors to the research paper include Katharina Kroll, Alexander Ainscough, Daniel Reynolds, Alexander Elamine, Ben Fichtenkort, and Sebastien Uzel. Funding was provided by the Vannevar Bush Faculty Fellowship Program, the Basic Research Office of the Assistant Secretary of Defense for Research and Engineering, the Office of Naval Research, and the National Science Foundation.
