For Patients Who Require Bone Grafts, a 3-D Printer Could Provide Relief

Scientists have successfully 3-D-printed splints for babies’ airways, faux brains to study cortical folding — and now they’ve generated synthetic bone. A team of researchers at Northwestern University has created an extremely pliable, artificial bone that helps expedite recovery and that can be easily manipulated by surgeons in the operating room.

“This could also be adapted someday to accommodate Nasal Implants and Facial Implants in general.” — Dr. Slupchynskyj

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

“It really fulfills a major need in the clinical world,” remarked Senior Author Ramille Shah, a Biomaterials Engineer at Northwestern University.

In order to repair damaged bone, surgeons typically use ceramic fillers or scaffolds made of hydroxyapatite, a mineral full of calcium and phosphate. Because a modified form of hydroxyapatite comprised the majority of human bones, these man-made ceramics should meld well with the natural tissue, enabling cells to grow.

There are numerous problems with these materials. They’re usually really stiff, which makes it extremely difficult for a surgeon to modify the implant without breaking it. And if doctors choose a more malleable putty instead, full of tiny granules, these tend to 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 wanted to 3-D-print a composite material that would have several key qualities: biocompatible, to avoid inflammation or rejection; porous, to permit 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 comprised of hydroxyapatite alone, they’d be extremely brittle. But because they’re composite materials, they also have some collagen, which is much softer. The combination of the two enables bones to be stiff while also being flexible enough to withstand all kinds of forces without snapping (most of the time, anyway).

Shah’s composite material operates in a similar way. In weight, its contents are 90% hydroxyapatite, which should make it very brittle, and yet it can be squeezed and stretched and still recover. There are probably two reasons for this. First of all, the remaining 10% is made of a soft polymer that coats the hydroxyapatite, providing just enough flexibility. In addition, the material is very porous, which gives tiny filaments of material enough space to bend when they’re bearing a load, and then recover without actually 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% of its original height without suffering damage.

“It can get pretty squished and then still bounce back,” Shah noted.

Oddly enough, if the scientists put more soft polymer into the mix (hence 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 discovered that the hyperelastic bone actually helped expedite 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 normally probably have taken two to three times as long.

Shah says this technology could be useful for infants born with craniofacial birth defects, not just for more typical bone breaks. She’s interested in learning whether different types of bones or kinds of defects will require that they 3-D-print different kinds of shapes.

Those designs can probably be very sophisticated, as the researchers can already print artificial bone that might be very stiff in one direction but very pliant in another. They can also incorporate antibiotics or other drugs into the scaffolding, for the sake of reducing infection or encouraging growth.

Shah’s team can print fast, as much as 275 cubic centimeters per hour. The scientists are optimistic that they’ll be able to scale up production of their artificial bone, theoretically providing such materials as an affordable option for more patients.