3D printed ice template of blood vessels shown on the left. The right shows imaging of cells forming a blood vessel-like structure on the template one week later. (Image: Feimo Yang)

More than 100,000 individuals in the United States are currently in need of organ transplants. The demand for organs far exceeds the available supply, and people sometimes wait years to receive a donated organ. Approximately 6,000 Americans die while waiting each year.

Tissue engineering to create lab-grown organs and tissues aims to close the gap between the availability of organs and the demand for transplants. But one big challenge in tissue engineering is creating blood vessel networks in artificial organs that work like natural ones, from tiny capillaries to larger arteries. Traditional artificial blood vessel designs often don't mimic the natural design needed to function properly in the body.

However, new research shows the possibility of using 3D ice printing to help create structures that resemble blood vessels in the body. 3D ice printing generally involves adding a stream of water to a very cold surface.

“What makes our method different from other kinds of 3D printing is that instead of letting the water completely freeze while we’re printing, we let it maintain a liquid phase on top,” said Feimo Yang, a graduate student in the labs of Philip LeDuc and Burak Ozdoganlar at Carnegie Mellon University. “This continuous process, which is what we call freeform, helps us to get a very smooth structure. We don’t have the layering effect typical with many 3D printing methods,” said Yang.

They also used heavy water, a form of water where the hydrogen atoms are replaced by deuterium, which gives the water a higher freezing point, and helps create the smooth structure.

These 3D-printed ice templates are then embedded in a gelatin material, GelMA. When exposed to UV light, the gelatin hardens, and the ice melts away, leaving behind realistic blood vessel channels.

The researchers successfully demonstrated that they could introduce endothelial cells, like those in blood vessels, into the fabricated blood vessels. The cells survived on the gelatin for up to two weeks. In the future, they intend to culture those cells for a longer duration.

In addition to potential use for organ transplants, Yang pointed out that 3D-printed blood vessels could be used for testing the effects of drugs on blood vessels. They could also be coated with a patient’s own cells to see how the cells respond to a drug treatment before giving it to the patient.

The approach could be a significant step forward in creating complex, lifelike blood vessel networks for use in tissue engineering.

Here is an exclusive Tech Briefs interview — edited for length and clarity — with Yang and LeDuc.

Tech Briefs: Can you give us a little background on the project?

LeDuc: We've been working on this general idea for probably a decade now. And this has been a problem, I think, since the early ‘90s. A group had this cartilage thing that was a piece of plastic, and they put cells on it. It looked like an ear. The cells grew, and everyone was excited.

And then it was, ‘OK, the problem we have now is vasculature.’ Because without the vasculature you can't deliver blood to whatever it is, and it dies. I think it's 200 microns away from the vasculature that the cells just die because they can't get the nutrients that they need. So, there's been a problem for about 30 years. We're sitting in 2024 — guess what's still a problem.

People have tried all kinds of different things, and we've been working on this for probably 10 years. So, we came up with these ideas to create our artificial vasculature. And the reason we could do it is we were looking for thermally changing materials, and wax, you can melt it and control the temperature of wax when it melts.

So, we made wax, and we made it in vascular-looking systems. Then we could put collagen around it, which is actual real biological material. Then one day one of my students said, ‘Well, what if we 3D printed ice?’

I call it the Frozen of science: Elsa's making her ice castles with her fingers. We're making ice structures, but at half the width of a human hair.

Tech Briefs: What was the biggest technical challenge you faced while designing this 3D ice printing method?

Yang: In terms of the biggest challenge? Probably printing the ice itself because, unlike other materials, ice requires a very low temperature. So, let's say we print something that's a few millimeters tall; we're printing on this very cold substrate, but there is a temperature gradient from that substrate — maybe 30-40 °C difference. And then it becomes ‘How do we control this printing without the ice melting while we're doing it?’

TB: Can you explain in very simple terms how the 3D ice printing method works?

Yang: All the printers have a printing nozzle, and then you have this print plate on the bottom. Instead of having a print plate, we have a very cold plate: -35 °C. Then, instead of dropping, say, plastic, or using a continuous method, we drop droplets that are roughly 50 micrometers in diameter. We drop them at a frequency, of about 200 hertz — every second you drop maybe 200 drops. When the water hits the cold platform, it freezes, and then it continues on and forms a pillar-like thing.

TB: What's the difference between 3D ice printing and 3D printing?

LeDuc: I guess we also call this 3D freeform ice printing. One thing is you use ice instead of any other kind of filaments or materials. A lot of times when you 3D print, you have different layers. So, you print first layers and you put the second layer on top of that, then you bind them together. But in freeform printing, you have a continuous process; on the bottom it’s freezing. On the top, there is a liquid cap, which in our case is the water that you keep depositing.

So, you have a continuous freezing process going on. And what it does is, instead of having a very rough surface, like how you print layer by layer, this will give you a very smooth surface.

TB: Do you have any advice for engineers or researchers aiming to bring their ideas to fruition?

LoDuca: I think that the most important things are persistence and patience. There’s a balance between being stubborn and being patient. I understand that, but really tough ideas, sometimes they take time. I would say, if it were really easy, then other people would've done it. That's a very consistent thing that I say to my lab. And so that's why you just have to be persistent with it.