How Frozen Inspired a New, Icy Blood Vessel Transplant

Immediately after its release in 2013, the fantasy children’s movie Frozen skyrocketed to popularity. It earned $1.28 billion in sales and is currently the second highest-grossing animated film of all time, only outperformed by its sequel, Frozen II. This movie series focuses on two princesses, one of which has the power to create ice structures with her hands. This franchise left its mark on popular culture by inspiring a multitude of ice-themed products, from dolls to theme-park attractions to…blood vessels? 

Blood vessels are especially important to humans because they connect our vital organs, transporting oxygen and other nutrients throughout the body. Blood vessels are tubes made of connective tissue and smooth muscle which facilitate the circulation of blood. These walls are lined by an extremely thin layer of endothelial cells, which are cells that regulate the transfer of nutrients between the blood and its surrounding tissues. These blood vessels form a larger network called the vascular system.

Recently, the Frozen movies inspired a team of researchers at Carnegie Mellon University to design artificial blood vessels using three-dimensionally (3D) printed ice. The researcher leading the artificial blood vessel project, Burak Ozdoganlar, revealed that “Frozen was what gave us the idea that we can make amazing shapes [using ice].” To execute this idea, the research team used 3D printing to freeze water into an icy blood vessel template. Water was chosen due to its ability to rapidly freeze into the vessel templates while still being compatible with biological structures.

To build the ice structures, the researchers used a droplet-on-demand system called inkjet printing. Inkjet printing traditionally consists of a nozzle moving across paper while distributing individual ink droplets. Inkjet 3D printing is often used in biological engineering because it enables the controller to place cells with extreme accuracy. The scientists used this method to place water instead of cells. First, the team of researchers mounted their nozzle on a computer-controlled, three-axis motion system which could ensure accuracy to the nearest micrometer, a millionth of a meter. Next, they installed a platform below the nozzle, which was kept at -35 degrees Celsius so the water would freeze at an appropriate frequency. To create a smooth and unified structure, the drop rate had to be fast enough so a droplet didn’t freeze before the next arrived. In addition, the entire printer was kept in a nitrogen-purged enclosure. This means that gaseous nitrogen was routed into the environment to displace the heavier, moist air, preventing the presence of extra water in the air, which could form unwanted frost on the ice structures.

When the ice blood vessel was finished printing, which could take 30 seconds, it was encased in gelatin. Gelatin is widely used as the primary material in tissue engineering because it originates from collagen, which is a protein that naturally strengthens skin, bones, and tendons. This provides low toxicity but high flexibility. Then, scientists shined an ultraviolet light on the artificial vessel to melt the ice while hardening the gelatin. When the ice was melted, it left behind a durable gelatin mold of a blood vessel. 

In the future, these man-made blood vessels could be used as transplants after vascular injury. Blood vessels can be harmed by blunt trauma, but also by internal diseases like Coronary Artery Disease (CAD). CAD is a type of heart disease that affects the large blood vessels on the surface of the heart called coronary arteries. Factors like increased saturated fat consumption can cause a buildup of cholesterol in the coronary arteries which forms plaque. Plaque buildup can block blood flow to the heart and lead to a heart attack. In cases where blocked vessels can’t be cleared internally, a blood vessel transplant could treat conditions like CAD. Additionally, the research team hopes that their artificial vessels will be used to test medicine without using a live subject. Pharmaceutical developers could observe the man-made blood vessel to see how the body might react to certain substances. While they might not realistically account for interactions between other body systems, the vessels could still provide a limited biological playground to harmlessly develop medical treatments. 

Though this new engineering process seems promising, the research team has a long way to go before their creation becomes implementable in live creatures. For instance, when scientists introduced endothelial cells to the gelatin vessels, they only survived for two weeks, despite having an average lifespan of over a year in humans. Despite the cells’ reduced longevity, this performance is a start to realistically integrating the artificial blood vessels into wider body systems, but there is still much improvement required. One major limitation to their work is that the ice structures are restricted in size, because larger creations are not able to stay cold due to increased rates of heat transfer. The researchers could create structures up to 400 micrometers in diameter, but the mean diameter of the larger leg veins are 10.5 millimeters, almost 27 times larger than the model’s maximum capability. The researchers must strike a balance between their temperature and droplet release rate to stably create larger veins. Even if the artificial blood vessels are successfully created, health hazards are present when using any transplant. One of these risks is transplant rejection, where the recipient's immune system perceives the transplant as a foreign object and attacks it. This rejection may lead to fatal infections, so universally biocompatible materials must be discovered to ensure the total safety of any transplant.

The Carnegie Mellon researchers have pioneered a method to 3D print ice on a microscopic scale, which holds promise in artificially engineering blood vessels. Although there is much more testing to be done, this research is a promising start to a completely new approach to creating biological structures—all thanks to the children’s princess movie which took the world by storm.