3D printed implants can prompt new bone to grow in animals, scientists reported today in the journal Science Translational Medicine. Called hyperelastic “bone,” their new bioengineered material could also be cheap, versatile and easy to print and use for repairing or regenerating bones in people.
The grafts, which helped mend bone injuries in rats and monkeys, are made from hydroxyapatite, a mineral found in bones and teeth, and a biodegradable polymer. “Despite the fact that is majority ceramic, which is usually very brittle, it possesses very unique…properties that makes it highly elastic,” coauthor Ramille Shah, of Northwestern University in Evanston, Illinois, said in a press conference on Tuesday. “When we squeezed or deformed it, it bounced right back to its original shape.”
When Shah and her colleagues placed human stem cells taken from bone marrow on a sample of hyperelastic “bone,” its mere presence was enough to prompt them to mature into bone cells (this type of stem cell can also make fat or cartilage). The “bone” scaffold served as a source for the cells to create their own natural materials, fellow team member Adam Jakus, also of Northwestern University, said in the press conference.
To test whether the grafts are safe to implant, the team placed hyperelastic “bone” under the skin of mice. The biomaterial is porous, which allowed the rodents’ blood vessels to quickly infiltrate the graft and incorporate it into the body without prompting a response from the immune system.
And when implanted into the spines of rats, the 3D printed grafts helped generate bone to help fuse and heal the animals’ vertebrae. It performed as well as treated tissue from a fellow rat, which is commonly used for bone grafting. It was also absorbent, meaning it could be laced with antibiotics or proteins that encourage bone to grow.
Finally, the team used hyperelastic “bone” to replace a weak, unhealthy piece of skull in a rhesus macaque. The surgeons weren’t sure how extensive the damage was, so the researchers printed a large graft for them to trim down to size in the operating room. This means that an implant could easily be tweaked at the last minute in people, too. After four weeks, the monkey’s skull had mended and filled the graft with blood vessels.
Other materials currently used in bone repair tend to be brittle and difficult for surgeons to manipulate. “Hyperelastic ‘bone,’ on the other hand, can be easily cut, rolled, folded, and sutured to tissue,” Shah said. “And since it is elastic, it can be pressed, fit into a defect, and expand to mechanically fix itself into a space without glue or sutures.” The biomaterial is also sturdy; when the team printed a segment of human femur, it could support loads up to 150 pounds before it buckled.
It’s not clear why these materials are so effective when printed this way. One possibility is that the grafts emulate natural bone, but not perfectly. “Cells might actually see it as maybe incomplete bone,” Jakus said. “So it spurs them even further to remodel it and make it into natural bone.”
He and his colleagues envision their new inks being useful for reconstructive and plastic surgery. The material could also prevent children from having to have later surgeries to replace ill-fitting grafts. “It is designed to degrade and remodel into natural bone, and therefore, it can grow with the patient,” Shah said.
The inks can be stored and then used to quickly print scaffolds at room temperature. “I think ideally it would be great if we could have these printers in a hospital setting where we can provide them the hyperelastic ‘bone’ ink and then they can then make patient specific implants that day—within 24 hours,” Shah said. She and her team hope to start clinical trials within five years.
