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.”