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Biomedicine

3-D Printing’s Next Act: Nerve Regeneration

Researchers demonstrate customized implants that help injured nerves regenerate in rats.

Additive manufacturing opens new opportunities for making anatomically precise medical implants.

Bridging the gap between the ends of a torn nerve is the latest biomedical trick performed with the help of a 3-D printer.

Additive manufacturing, or 3-D printing, makes it possible to build more customized biomedical implants, and has become a popular way to make dental implants and even windpipes. A new 3-D printed structure meant to “guide” the regrowth and reconnection of the loose ends of an injured nerve suggests that the technique could appeal to neurosurgeons as well.

Peripheral nerve injuries, caused by a variety of things including disease and trauma, are fairly common—doctors perform more than 200,000 nerve repair procedures each year in the United States alone. The most common surgery entails using nerve tissue taken from another spot in the body to fill the gap. But this requires an additional surgery to harvest that tissue, and can lead to chronic pain, sensory loss, or other problems at the site from which it was cut. An alternative approach involves using an artificial scaffold, generally tube-shaped, that sits between the two ends of the broken nerve and serves as a conduit for regeneration, often with the help of biochemical cues known to prompt nerve growth.

But nerves and nerve injuries are often not so straightforward, and 3-D printing technology makes it possible to design and make guides that are conducive to more complicated shapes, says Michael McAlpine, a professor of mechanical engineering at the University of Minnesota. To demonstrate the new technique, McAlpine and his collaborators, including neurosurgeons and biomedical engineers, showed in rats that they could regenerate the original Y-shaped structure after a 10-millimeter piece of the sciatic nerve—including the point where it branches—had been cut out.

The researchers used a 3-D scanner to record information about the geometry of the missing piece, and fed that data into their custom printer. In an intact and functioning sciatic nerve, the base of the Y shape contains a mix of motor and sensory nerve fibers. It splits into branches that contain either mostly sensory nerve fibers, which send information to the brain, or mostly motor neurons, which send information to the muscles. On the inside of the silicone guide, the same printer deposits precise amounts of biochemical “cues” chosen to promote nerve growth. Each branch of the Y shape gets a different cue—one is meant to encourage sensory nerve growth and the other is meant to encourage the growth of motor nerves.

If the technology is to eventually advance to the clinic, it will not be essential that the missing piece of an injured nerve be available for 3-D scanning, says McAlpine, which is important because many injuries likely won’t afford that luxury. In that case, a corresponding nerve on the opposite side of the body may be able to stand in for scanning, he says, or perhaps there could even be existing “libraries” of geometrical data based on nerves from cadaver models.

The new technique should be thought of as a starting point that “opens the door” for the development of new regeneration schemes that take advantage of 3-D printing to make implants with complex shapes, says Xiaofeng Jia, a collaborator on the project and a professor of neurosurgery at the University of Maryland School of Medicine. It should be possible to use this approach to create more complicated branching designs suited for other nerves, he says, and it’s also possible that alternative materials and biochemical schemes will be more effective at promoting the healing of injured nerves.

McAlpine says the group will continue to experiment with different materials, and in particular he’d like to try to use a biodegradable material that would dissolve in the body once it has served its function. He estimates that the technology could be ready for testing in humans in five to 10 years.

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