A 3-D printer may soon be making bone grafts

Scientists have 3-D-printed splints for babies’ airways, faux brains to study cortical folding — and now they’ve done it with bone. A team of researchers at Northwestern University has created a highly flexible artificial bone that helps speed up recovery and that can be easily manipulated by surgeons in the operating room.

The “hyperelastic bone,” described in the journal Science Translational Medicine, could be cut, folded and sutured to tissues, and could pave the way toward cheaper, customized and more effective bone grafts.

“It really fulfills a major need in the clinical world,” said senior author Ramille Shah, a biomaterials engineer at Northwestern University.

To repair damaged bone, surgeons often use ceramic fillers or scaffolds of hydroxyapatite, a mineral full of calcium and phosphate. Because a modified form of hydroxyapatite makes up much of human bones, these man-made ceramics should blend well with the natural tissue and allow cells to grow.

But there are several problems with these materials. They’re often really stiff, which makes it difficult for a surgeon to modify the implant without breaking it. And if doctors instead choose a more malleable putty, full of tiny granules, these often get washed away by blood flow during the operation. Many materials are also too dense, lacking the pores that would allow blood cells to colonize the area and for the body to integrate the graft.

Shah and her team sought to 3-D-print a composite material that would have several key qualities: biocompatible, to avoid inflammation or rejection; porous, to allow blood vessels to grow; and pliable, easily handled by surgeons. They also wanted to make a material that didn’t need to be cured by heat.

If human bones were made of hydroxyapatite alone, they’d be exceedingly brittle. But because they’re composite materials, they also have some collagen, which is much softer. The combination of the two allows bones to be stiff while also being flexible enough to withstand all kinds of forces without snapping (most of the time).

Shah’s composite material operates in a similar way. In weight, its contents are 90 percent hydroxyapatite, which should make it very brittle, and yet it can be squeezed and stretched and still recover. The remaining 10 percent is made of a soft polymer that coats the hydroxyapatite, giving it just enough flexibility. The material is also extremely porous, which gives tiny filaments of material enough space to bend when they’re bearing a load, then recover without breaking.

“That porosity also leads to the very unique mechanical properties that we see with hyperelastic bone that has never been seen with this type of composite before,” Shah said. The result is a material that can be compressed to less than 50 percent of its original height without suffering damage.

Strangely enough, if the scientists put more soft polymer into the mix (thus filling many of those pores), they found that the material actually became more brittle. Those empty spaces were key to the flexibility.

“That’s why it was very surprising,” she said.

“You wouldn’t imagine, with such little elastomer in there, [that] you can make a huge difference in mechanical properties.”

The scientists found that the hyperelastic bone actually helped speed up spinal fusion in rats; in a macaque with a skull defect, they saw the start of bone regrowth in just four weeks — a process that would otherwise probably have taken two to three times as long.