The cardiovascular system is a fundamental biological trait that appeared over 600 million years ago and is still present in modern vertebrates and invertebrates alike.

Its tree-like structure helps distribute gases and nutrients to tissues, allows cell waste removal, and plays a central role in the immune system. Thus, engineering hierarchical vascularized tissues has been one of the main foci since the field’s beginnings, and despite many recent advances, it remains an unsolved challenge to this day.

Current techniques for in vitro engineering of implantable vascularized tissues center on creating self-assembled microvascular networks or fabricating vessel replacements of intermediate size. but recreating the system’s hierarchical structure was beyond the scope of these achievements. 

In the human body, the heart pumps blood into the aorta – the main artery that carries blood away from your heart to the rest of your body – that then branches out into progressively smaller blood vessels, transporting oxygen and nutrients to all the tissues and organs. 

When a cardiac patient needs a transplant, the donor heart needs similar support of blood vessels, and so do tissues that are engineered for transplantation. Engineering hierarchical vasculatures is critical for creating implantable functional thick tissues. Until now, experiments with engineered tissue containing hierarchical vessel networks have involved an intermediary step of transplanting first into a healthy limb, allowing the tissue to be permeated by the host’s blood vessels, and then transplanting the structure into the affected area.

Now, a research team at the Technion-Israel Institute of Techology who were led by Technion Prof. Shulamit Levenberg – who has for many years specialized in tissue engineering – have succeeded in creating a hierarchical blood vessel network that is necessary for supplying blood to implanted tissue. Their work is considered an important breakthrough. 

In the study, published in Advanced Materials under the title “3D Bioprinting of Engineered Tissue Flaps with Hierarchical Vessel Networks (VesselNet) for Direct Host-To-Implant Perfusion,” Dr. Ariel Alejandro Szklanny used 3D printing to create large and small blood vessels to form for the first time a system that contained a functional combination of both. With Szklanny’s new achievement, the intermediary step of transplanting first into a healthy limb could become unnecessary.

The breakthrough took place in Levenberg’s stem cell and tissue engineering laboratory in the Technion’s Faculty of Biomedical Engineering. 

To create in the lab a tissue flap with all the vessels necessary for blood supply, Szklanny combined and expanded on two separate techniques. First, using 3D printing technologies, he created a fenestrated polymeric scaffold (that has openings or “windows”) that mimics the large blood vessel.

The fenestration served to create not just a hollow tube, but a tube with side openings that allowed the connection of smaller vessels to the engineered larger vessel. Using a collagen bio-ink, tissue was then printed and assembled around that scaffold, and a network of tiny blood vessels formed inside. Finally, the large vessel scaffold was covered with endothelial cells, which are the type of cells that constitute the inner layer of all blood vessels in the body. After a week of incubation, the artificial endothelium created a functional connection with the smaller 3D bio-printed vessels, mimicking the hierarchical structure of the human blood vessel tree.

The resulting structure was then implanted in a rat, attached to its femoral artery. Blood flowing through it did what the scientists wanted blood to do – it spread through the vessel network, reaching to the ends of the structure, and supplied blood to the tissue without leaking from the blood vessels.

They noted that while previous studies used collagen from animals to form the scaffolds, here, tobacco plants were engineered by the Israeli company CollPlant to produce human collagen, which was successfully used for 3D bioprinting the vascularized tissue constructs.

This study constitutes an important step towards personalized medicine, they said. Large blood vessels of the exact shape necessary can be printed and implanted together with the tissue that needs to be implanted. This tissue can be formed using the patient’s own cells, thus eliminating rejection risk.