The creation of functional human organs outside the body has long been considered the ultimate goal in organ transplantation medicine, yet it remains a significant challenge. Groundbreaking research from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Wyss Institute for Biologically Inspired Engineering marks a substantial leap forward in this pursuit.
Scientists have pioneered a novel 3D printing technique to construct vascular networks. These networks are composed of interconnected blood vessels that replicate the complex structure of natural vessels, featuring a distinct “shell” of smooth muscle cells and endothelial cells surrounding a fluid-carrying “core.” Crucially, these printed vessels are embedded within human cardiac tissue. This advancement in mimicking natural blood vessel architecture is a major step towards manufacturing transplantable human organs. The details of this achievement are published in Advanced Materials.
“Building upon our previous work with ‘sacrificial writing in functional tissue’ (SWIFT), a 3D bioprinting method for creating hollow channels in living cellular matrices, we developed coaxial SWIFT (co-SWIFT). This new method replicates the multi-layered structure of native blood vessels, facilitating the formation of interconnected endothelial linings and enhancing the vessels’ ability to withstand blood flow pressure,” explained Paul Stankey, the lead author and bioengineering Ph.D. student at Harvard SEAS, working in the lab of Jennifer Lewis, Hansjorg Wyss Professor of Biologically Inspired Engineering at SEAS and Wyss Core Faculty member.
The core innovation lies in a specialized core-shell nozzle featuring two independently controlled fluid channels for the vessel “inks.” These inks consist of a collagen-based shell and a gelatin-based core. The nozzle’s inner core chamber extends slightly beyond the shell, enabling it to penetrate previously printed vessels. This creates interconnected branching networks essential for effective oxygenation of human tissues and organs through perfusion. The vessel size can be adjusted during printing by altering the printing speed or ink flow rates.
To validate the co-SWIFT method, the team initially printed multi-layered vessels within a transparent granular hydrogel matrix. Subsequently, they printed vessels in uPOROS, a recently developed porous collagen-based matrix that imitates the dense, fibrous structure of living muscle tissue. Branching vascular networks were successfully printed in both cell-free matrices. Following printing, the matrices were heated to crosslink the collagen in the matrix and shell ink, while the gelatin core ink melted away. This process allowed for the easy removal of the core, resulting in an open, perfusable vascular system.
Expanding to more biologically relevant materials, the team used a shell ink infused with smooth muscle cells (SMCs), the primary component of the outer layer of human blood vessels. After removing the gelatin core, they introduced endothelial cells (ECs), which form the inner lining of blood vessels, into the printed vasculature through perfusion. After seven days, both SMCs and ECs remained viable and functioned as vessel walls. Notably, vessels with ECs exhibited a threefold reduction in permeability compared to those lacking ECs.
Finally, the researchers tested their method within living human tissue. They created hundreds of thousands of cardiac organ building blocks (OBBs) – tiny spheres of beating human heart cells compressed into a dense cellular matrix. Utilizing co-SWIFT, they printed a biomimetic vessel network within this cardiac tissue. After removing the sacrificial core and seeding the SMC-laden vessels with ECs via perfusion, they assessed the performance of the printed vessels.
The printed biomimetic vessels not only exhibited the characteristic double-layer structure of human blood vessels, but also demonstrated functionality within living tissue. After five days of perfusion with a blood-mimicking fluid, the cardiac OBBs began to beat synchronously, indicating healthy and functional heart tissue. Furthermore, the tissues responded appropriately to common cardiac drugs: isoproterenol increased their beating rate, while blebbistatin halted their beating. Demonstrating the potential for personalized medicine, the team even 3D-printed a model of a patient’s left coronary artery vasculature into OBBs, based on real patient data.
“Our ability to successfully 3D-print a vascular model of a left coronary artery from patient-specific data underscores co-SWIFT’s potential for creating personalized, vascularized human organs,” Lewis emphasized.
Looking ahead, Lewis’s team aims to develop self-assembling capillary networks and integrate them with their 3D-printed blood vessel networks. This integration will more comprehensively replicate the intricate structure of human blood vessels at the microscale, further enhancing the functionality of lab-grown tissues and bringing the promise of 3D printed organs closer to clinical reality.
