Three-dimensional tubes of living tissue have been printed using modified desktop printers filled with suspensions of cells instead of ink. The work is a first step towards printing complex tissues or even entire organs.
"This could have the same kind of impact that Gutenberg's press did," claims tissue engineer Vladimir Mironov of the Medical University of South Carolina.
Many labs can now print arrays of DNA, proteins or even cells. But for tissue engineers, the big challenge is creating three-dimensional structures. Mironov became interested when Thomas Boland of Clemson University, also in South Carolina, told Mironov how he could print biomaterials using modified ink-jet printers.
The printers are adapted by washing out the ink cartridges and refilling them with suspensions of, say, cells. The software that controls the viscosity, electrical resistances and temperature of the printing fluids is reprogrammed and the feed systems altered.
To create 3D structures, Boland and Mironov used a "thermo-reversible" gel recently developed by Anna Gutowska at the Pacific Northwest National Laboratory in Richland, Washington. The non-toxic, biodegradable gel is liquid below 20 °C and solidifies above 32 °C.
The team has done several experiments using easily available tissues such as hamster ovary cells. By printing alternate layers of the gel and clumps of cells onto glass slides, they have shown 3D structures such as tubes can be built up.
Biologists have long known that bits of tissue placed next to each other can fuse. The researchers found that as long as the layers were thin enough for the clumps to come into contact with each other, the bits of tissue fused. Once a structure is complete, the gel is easily removed. Details of the team's initial work will soon be published.
Like printing with different colours, placing different types of cells in the ink cartridges should make it possible to recreate complex structures consisting of multiple cell types. "I think this is extremely exciting technology that has the potential to overcome some of the major obstacles [to tissue engineering] we have seen in the past," says leading tissue engineer Anthony Atala of Harvard Medical School in Boston.
Other groups have developed ways of building up tissues layer by layer (New Scientist print edition, 4 January), but none is as simple and quick as printing. Most tissue engineers first create a degradable scaffold and then seed it with cells. This technique can be used to create complex shapes, such as the infamous "ear on a mouse", but placing different cell types precisely is very difficult.
Printing should make it easier to position cells, but many other problems will have to be overcome before entire organs can be created. A huge challenge in tissue-engineering solid organs, for example, is supplying enough oxygen and nutrients to sustain cells deep within the structure.
"It's been the holy grail of tissue engineering, to be able to create adequate circulatory networks for complex organs," Atala says.
Mironov and Boland hope it will be possible to print the entire network of arteries, capillaries and veins that nourish organs. But to keep cells alive, the organs would have to be completed within a couple of hours and a growth medium circulated through the fragile new vessels.
Large structures might not be strong enough to hold together if the gel is removed after such a short period. However, the team is already experimenting with adding substances such as the skin protein collagen to speed fusion and reinforce structures.
Printing is not the only promising new technique for creating entire organs. It might one day be possible to grow them in situ. In December, scientists in Israel reported that they had managed to grow miniature but fully functional kidneys by implanting fetal pig or human cells into immunodeficient mice.
But growing organs from scratch will take much longer than printing them, Mironov says. "Patients don't always have the luxury to wait."
Exclusive from New Scientist Print Edition