A team of bioengineers has taken a step closer to the day when it will be possible to regenerate new organs from patient’s own cells. The researchers have “printed” 3D patterns of blood vessel networks out of sugar that allow tissue to grow around them and then dissolve, leaving behind a hollowed-out “vascular architecture”.

Once the sugar dissolves, the hollowed-out blood vessel pattern can rapidly be perfused with nutrient-rich fluid and oxygen to stop the tissue cells from dying.

(A common problem when trying to engineer thicker tissue like that of the liver, is that without a decent vascular system to deliver nutrients and oxygen and remove waste products, the cells deep inside perish.)

Although tissue engineering has made great strides in recent years, it is still impossible to recreate the complex 3D blood vessel networks that are present in naturally grown organs.

Writing in the 1 July online issue of Nature Materials, researchers from the University of Pennsylvania (Penn) and Massachusetts Institute of Technology (MIT) in the US, say their 3D sugar printing method is a significant step in the right direction and is also free of some of the problems that arise when trying to make 3D tissue and its internal vasculature by other means.

For instance, one common approach that bioengineers use is to grow the tissue and its vascular network layer by layer, but this has a significant problem in that the nutrient fluid can push open the seams between the layers.

Christopher S Chen, the Skirkanich Professor of Innovation in the Department of Bioengineering at Penn, is one of the lead researchers on this work. He told the press:

“This new platform technology, from the cell’s perspective, makes tissue formation a gentle and quick journey, because cells are only exposed to a few minutes of manual pipetting and a single step of being poured into the molds before getting nourished by our vascular network.”

The rapid casting technique that Chen and colleagues have developed relies on making a material that is rigid enough to hold up as a 3D network of filaments, but that can also easily dissolve in water without poisoning the cells.

The other requirement is that the material has to be compatible with a 3D printer, so it can make more complex vascular networks much faster than the layer by layer approach, and on a larger scale.

After lots of trial and error, they found the perfect material was sugar. Sugar is mechanically strong and abundant in nature. For instance, in the form of cellulose, it is the most common material in the Earth’s biomass. Another advantage is the building blocks of sugar are typically added and dissolved in nutrient media that nourish cells.

Postdoctoral fellow Jordan S Miller is another co-leader of the research team and member of Chen’s Tissue Microfabrication Laboratory in the Department of Bioengineering at Penn. He said they tested lots of different formulations of sugar until they got the best possible match to these requirements.

“Since there’s no single type of gel that’s going to be optimal for every kind of engineered tissue, we also wanted to develop a sugar formula that would be broadly compatible with any cell type or water-based gel,” he explained.

They eventually settled on a formula that combined sucrose and glucose with dextran for structural strength. They printed it with a RepRap, an open-source 3D printer with a custom-designed extruder and controlling software.

An important part of the method is that the sugar needs to be stable after printing, so it is coated with a thin layer of a degradable polymer made from corn that allows the sugar structure to dissolve and flow out of the gel medium through the channels they create without stopping the gel from setting and without damaging the growing cells nearby.

Once the sugar is out of the way, the researchers feed liquid through the vascular framework, to nourish the cells with nutrients and oxygen, similar to how it happens naturally with blood in the body.

They say the whole process is quick and inexpensive, and they can swap easily between multiple computer simulations and physical models of vascular frameworks.

When the researchers injected human blood vessel cells into the hollowed out vascular network, they spontaneously started making new capillary sprouts, thus increasing the penetration of the network. This is how blood vessels grow naturally in the body.

To test this effect further, the researchers then made gels containing primary liver cells. When they then pumped nutrient-rich fluid through the hollowed-out vascular architecture, they found the liver cells increased the amount of albumin and urea they made, which is a sign of healthy behaviour in liver cells.

There was also evidence that more cells were surviving around the vascular channels carrying the nutrient fluid.

Another challenge for bioengineering organs is to create sufficient numbers of healthy and functioning cells: and current technology is very far from achieving the cell densities of a fully functioning liver.

But with their printed vascular system, Chen and colleagues achieved cell densities that approached clinical relevance, suggesting the new technique could spur further research into lab-grown organs and organ-like structures.

The therapeutic threshold for human-liver therapy is thought to be around 10 billion functioning liver cells. Chen and colleagues have managed to get closer to that number, but they are still a long way off: per gel they are reaching about tens of millions of liver cells, they said.

And there is still lots of work to do in other areas, such as how to connect these vascular networks to real blood vessels, and testing how the artificial vasculature interacts with liver cells.

Funds from the National Institutes of Health, the Penn Center for Engineering Cells and Regeneration and the American Heart Association-Jon Holden DeHaan Foundation helped pay for the research.

Written by Catharine Paddock PhD