Evaluation of Densified Collagen Tubes as an Interposition Graft in Abdominal Aorta

The human body is a network of pipes, carrying life-sustaining blood to every corner. When a major pipe like the aorta becomes damaged or blocked, surgeons often need to replace a section with a new tube. For decades, the best options have been synthetic materials, such as expanded polytetrafluoroethylene or woven polyester. While these artificial grafts can hold blood under pressure, they are rigid and do not behave like living tissue. They often fail over time because the body cannot remodel them, and they frequently trigger clotting that blocks the flow. Scientists have long sought a better alternative: a graft made from natural biological materials that the body can accept, repair, and eventually integrate as its own. The challenge lies in making these biological tubes strong enough to handle the force of a heartbeat without collapsing, yet flexible enough to move with the artery, all while preventing blood from sticking to the inside and forming dangerous clots.

Researchers at the University of Cambridge have taken a significant step toward solving this problem by creating vascular grafts from collagen, the main structural protein found in human skin and connective tissue. In a study published as a preprint, the team demonstrated that they could turn a soft, jelly-like collagen gel into a tough, hollow tube capable of replacing a section of the abdominal aorta in rats. They tested two different versions of these tubes. The first was a thick-walled tube that had been chemically treated to make it stronger. While this version allowed new tissue to grow on the inside surface, it eventually failed because it was too stiff compared to the natural artery, leading to blood clots that blocked the vessel after several months. The second version was a breakthrough: a much thinner, non-treated tube that matched the flexibility of the natural artery. By adding a common substance called hyaluronic acid to the mixture, the team made the collagen fibers pack together more tightly, creating a tube that was both strong and flexible. These thin-walled tubes remained open and functional for up to 17 weeks, allowing the body to grow a new inner lining and integrate the graft into the surrounding tissue.

The journey to create these tubes began with a simple observation: collagen gel is naturally too weak to serve as a blood vessel. To fix this, the team used a process called densification. Imagine a sponge that is soaked with water; if you squeeze the water out, the sponge becomes much denser and stronger. The researchers applied this principle to a collagen gel, but instead of squeezing it with their hands, they used gravity and a specialized funnel to pull water out of the gel in a controlled way. As the water left, the collagen fibers were pulled together and aligned, transforming the soft gel into a dense, durable material. For the first set of experiments, they created thick-walled tubes using a funnel method that could easily be scaled up to human size. They then chemically crosslinked these tubes with a natural dye called genipin to stabilize them and prevent them from swelling when they touched blood. They also coated the surface with heparin, a common blood-thinning drug, to try to stop clots from forming.

When these thick-walled, chemically treated tubes were implanted into the abdominal aorta of rats, the results were mixed. For the first ten weeks, the grafts worked well. Blood flowed through them, and the body began to build a new inner layer, known as a neointima, composed of smooth muscle cells and a lining of endothelial cells. This new lining is crucial because it mimics the natural surface of a blood vessel. However, as time passed, the limitations of the thick wall became apparent. The graft was significantly stiffer than the natural rat aorta. This difference in stiffness created a mismatch at the point where the graft met the natural artery, disrupting the smooth flow of blood. By 26 weeks, this disruption had caused extensive clotting, blocking the vessel. The clots appeared to start in the natural artery and extend into the graft, suggesting that the mechanical mismatch was the primary cause of failure. The chemical treatment that made the tube strong also prevented the body's own cells from growing into the wall of the graft, leaving it as a foreign object that the body could not fully integrate.

Recognizing that the thickness and chemical treatment were the problems, the researchers developed a second, more refined approach. They wanted a tube that was as thin as the natural artery wall, roughly 100 micrometers thick, and did not require chemical crosslinking. To achieve this, they modified the funnel densification process. Instead of letting the gel flow through a narrow gap, they anchored the top and bottom of the gel with a metal mesh and porous membranes. As water was removed from the bottom, the gel was forced to densify inward, creating a very thin, high-density tube without the need for air drying or chemical stabilizers. To make the tube even thinner and stronger, they added hyaluronic acid, a substance naturally found in the body, to the collagen mixture before densification. This addition caused the collagen fibers to bundle together more effectively, allowing the gel to compress into a smaller volume. The result was a thin-walled tube with a collagen density nearly 50 percent higher than the unmodified version, yet it remained flexible and did not require chemical crosslinking.

The team then tested these new, thin-walled tubes in rats. Because the tubes were so thin and flexible, they matched the natural artery's ability to expand and contract with every heartbeat. This close match in mechanical properties meant there was no abrupt transition at the connection points, allowing blood to flow smoothly. The researchers also treated the surface of these tubes with a different anti-clotting agent called dipyridamole, which is less likely to wash away than heparin. The results were striking. Over a period of 17 weeks, the grafts remained open and functional. Unlike the thick-walled version, the body was able to grow cells into the wall of the thin graft. A new inner lining formed, complete with a layer of endothelial cells and underlying smooth muscle cells, effectively turning the artificial tube into living tissue. The outer surface also developed a new layer of connective tissue, known as a neo-adventitia, which helped anchor the graft in place.

One of the most surprising findings was that the addition of hyaluronic acid did more than just make the tube thinner; it also made the surface less likely to trigger blood clots. Even without the chemical coating, the hyaluronic acid-modified collagen resisted clotting significantly better than plain collagen. This suggests that the way the collagen fibers are arranged at a microscopic level plays a major role in how blood interacts with the surface. The researchers found that the bundled fiber structure created by the hyaluronic acid reduced the ability of platelets to stick and activate, providing a built-in protection against clotting that complemented the surface coating. This dual approach—improving the internal structure of the material while treating the surface—proved more effective than relying on surface treatments alone.

The study also highlighted the importance of matching the mechanical properties of a graft to the natural vessel. The failure of the thick-walled graft was not due to a lack of strength or an inability to heal the surface, but rather because it was too rigid. The stiffness created a disturbance in the blood flow that the body could not overcome, leading to clot formation. In contrast, the thin-walled graft, which moved in harmony with the natural artery, allowed the body to heal and integrate the material successfully. This finding suggests that for small-diameter blood vessels, flexibility and compliance are just as important as strength. The researchers noted that while the thin-walled method is currently difficult to scale up to human-sized vessels, the thick-walled method used for the first set of experiments can be scaled. This means that in the future, it may be possible to use the thick-walled technique for larger vessels where the stiffness mismatch is less critical, while the thin-walled technique could be refined for smaller, more delicate arteries.

The path forward involves testing these grafts in larger animals to see if the results hold true in a system that more closely resembles human physiology. The researchers are confident that the principles they have uncovered—specifically the need for mechanical compatibility and the benefits of modulating collagen structure—will guide the development of future vascular grafts. The study does not claim to have solved the problem of small-diameter vascular grafts entirely, as one of the thin-walled grafts did eventually fail due to a surgical issue rather than a material flaw. However, the fact that four out of five thin-walled grafts remained patent for 17 weeks, with full integration and new tissue growth, provides a strong foundation for further research. The work demonstrates that it is possible to create a vascular graft from a simple, natural material that the body can accept and remodel, offering a promising alternative to the synthetic options currently available. By focusing on how the material interacts with the body on both a mechanical and biological level, the team has moved closer to a future where damaged blood vessels can be replaced with living, breathing tissue.