Controlling Prosthetic Limbs with Electrode Arrays

A new nerve-cell-support design could give amputees better control over prosthetic limbs.

By Nidhi Subbaraman

To design prosthetic limbs with motor control and a sense of touch, researchers have been looking at ways to connect electrodes to nerve endings on the arm or leg and then to translate signals from those nerves into electrical instructions for moving the mechanical limb. However, severed nerve cells on an amputated limb can only grow if a structure is present to support them—much the way a trellis supports a growing vine. And they are notoriously fussy about the shape and size of that structure.

"Cells are like people: they like furniture to sit in that's just the right size," says David Martin, a biomedical engineer at the University of Delaware. "They're looking for a channel that's got the 'Goldilocks'-length scale to it—how far apart the ridges are, how tall they are, how [wide] they are."

Ravi Bellamkonda's lab at Georgia Tech has designed a tubular support scaffold with tiny channels that fit snugly around bundles of nerve cells. The group recently tested the structure with dorsal root ganglion cells and presented the results at the Society for Biomaterials conference earlier this month.

The scaffold begins as a flat sheet with tiny grooves, similar to corrugated iron or cardboard. It is then rolled to form a porous cylinder with many tiny channels suited for healthy nerve-cell growth. The floors of the conduits double as electrodes, brushing up close to the nerve bundles and picking up nerve signals. "The thing that's different is that the patterns can be much more precisely controlled, and the orientation of the nerve bundles is essentially perfect here," says Martin. "It's a nice model system, and the ability to control nerve growth is what's really going to be valuable."

The ultimate goal is to enable two-way communication between the prosthetic limb and the wearer. Eventually, this design could separate the two kinds of nerve cells within a bundle, so neural cues directing hand movement would travel along one channel and information about touch and temperature from the prosthetic limb would travel to the brain along another channel. "The 'jellyroll' should in principle allow [them] to select through those channels—that to me is where the real excitement is," says Martin. "That's news for the future, but you've got to be able to walk before you can run."

In previous attempts to tap into neural signals, scientists have fitted severed nerve cells with "sieve electrodes"—flat metal disks with holes intended for nerves to grow through. "The problem with the sieve electrode is that the nerves wouldn't grow into it reliably," says Bellamkonda.

Current work on growing aligned nerve bundles includes foam supports with pores suited for nerve growth, and fabrics with aligned nanofibers along which nerves are intended to grow. But the jellyroll design has the potential to be a cut above the rest.

The multichannel scaffold could give added dexterity to prosthetic limbs. "You need to be able to stimulate as many axons as possible for movement, and you need to be able to pick up signals from as many axons as possible," says Akhil Srinivasan, primary researcher on the project. The most sophisticated of the electrodes currently used at nerve endings have about 16 channels to control movement. But the arm has 22 degrees of freedom. "You need at least 22 reliable channels," says  Mario Romero-Ortega, associate professor of bioengineering at the University of Texas, Arlington. "That's the limitation—we only have a few, but you need more."

"The novelty, from my perspective, is the materials they use [are ones they can] scale up," Romero-Ortega says. The electrode-roll design builds on previous work, but the new scaffold is made of materials that are safe for biological use. "They're the first to show in vitro growth," Romero-Ortega says.

To make the microarrays, a coat of the polymer polydimethyl siloxane is laid down on a glass slide to create a thin, uniform base, and a layer of a light-sensitive polymer, SU-8, is added. Ultraviolet light is shined on the SU-8 through a grating, and the parts of the surface exposed to the light bond together to form walls. The unbonded sections in between are then washed away, leaving behind row upon row of conduits. The grooved surface is capped with a second layer of base polymer, and the polymer sandwich is rolled into a cylinder.

So far, the rolled-up microarray still lacks electrodes, but Srinivasan says the next steps will be to insert gold electrodes into the base of the scaffold. The wired microarray will then be tested in a rat model.

"I think it's a clever design," says Dominique Durand, a professor of biomedical engineering at Case Western Reserve University. "They still haven't shown the electrodes, but that's a problem for another day."

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