3D-printing body parts

A new form of 3D-printing that can turn out replicas of a patient's organs has been developed by scientists in the US.

Traditional 3D-printing processes use "additive manufacturing" - or layer-by-layer - techniques to build up an object. But this approach is inherently constrained by the material that can be used by the printing process. A runny suspension of cells, for example, wouldn't work.

Now, University of Florida scientist Tommy Angelini and his team have found a way to print, they say, a huge diversity of soft materials in three dimensions.

They do this by printing the desired material, be it living cells, hydrogels or other substances using a hollow needle inserted into a gel matrix. The matrix supports the structure while it is taking shape and is simply washed away when the object is finished.

According to Angelini, who has published the technique this week in the journal Science Advances, the key to breakthrough is the composition and behaviour of the gel. "It's 99.8% water," he explains. "But the 0.2% comprises polymer grains of polyethylene glycol and polyacrylic acid."

These are massive molecules that form an extended meshwork structure, holding the gel together. The molecules are too big to move around on their own, so the gel behaves like a solid. But when a small force is applied by the needle, the gel separates like a liquid, allowing the needle to pass and deposit a stream of cells or other material before it returns to a stiffer, solid structure.

Owing to this plastic behaviour, the matrix permits multiple passes to be made by the probe, so different materials can be deposited sequentially. "We could lay down endothelial cells, and then muscle cells to make a blood vessel," Angelini explains.

The team aren't quite at the stage where they can print a working kidney yet, but they are working with cancer specialists to print small three-dimensional tumours that can be used, reproducibly, for drug testing.

They're also collaborating with neurosurgeons to help them to plan their operations on brain tumours.

"We can take the CT or MRI scans of a patient's brain and then print a replica of the brain in three dimensions using a soft, solidifying material that the surgeon or his trainees can then practise on," says Angelini.

"This will totally transform the standard of surgical practices."