To better understand neuromuscular diseases and how the body recovers from spinal cord injuries, scientists require tools that can record and modulate neural activity in the spine during normal movement. But neural probes to date, such as those used on the brain, are unable to bend and flex with the spine. A research team led by scientists from the Massachusetts Institute of Technology (MIT) has now created tiny, rubber-like neural implants for both electrophysiological recording and optical neuromodulation of rodent spinal cords. Described recently in Science Advances, the probes, made of thermally drawn elastomer fibers coated with a mesh of silver nanowires, could accurately record the neural activity in the spines of freely moving mice and elicit leg movements in anesthetized mice through optical impulses.

“We now have a stretchable optoelectronic fiber, which is able to perform chronic probing and interrogation in spinal cord circuits in a freely moving subject,” says study lead author Polina Anikeeva, an MIT materials scientist. “It’s useful not only in the spinal cord, but may also find applications in smart fabrics and other types of sensors.”

In recent years, genetic manipulation technologies, specifically optogenetics (genetically modifying neurons to make them sensitive to light), have made rodent models vital tools for neuroscience research. Being able to record neural activity in the spine during optical or electrical stimulation could allow scientists to discover neural pathways important for recovery from spinal cord injuries, which frequently cause loss of organ function or voluntary limb control. However, given that the spine can undergo up to 12% strain during normal activities, it is difficult to design devices able to bend and flex with the spine’s natural movement while retaining function. Furthermore, there is less functional redundancy of the neurons in the spinal cord than the brain—damage to spinal tissue from a stiff, brittle implant could lead to paralysis.

To create probes ready for rodent spinal cords, Anikeeva and her colleagues looked to polymer fibers, which have an ideal geometry for interfacing with the fibrous structure of the spinal cord. “We are very inspired by the physiology of the spinal cord,” she says. Polymer fibers are also suitable for neural probes because processing techniques are highly versatile and scalable, able to reduce the final device dimensions by up to 200 times. Naturally, the rubber-like properties of elastomers make them the perfect material to turn into fibers for the spinal cord, but elastomers are not suitable for the thermal fiber drawing technique, which involves heating and pulling the material through a furnace. Most elastomers just melt (instead of flowing like honey), and those that do not are still challenging to pull through the furnace due to their elasticity.

The researchers introduced a process to draw an elastomer into a fiber, which involves confining the fibers in a protective polymer cladding. The team confined a cyclic olefin copolymer elastomer core in a cladding of poly(methyl methacrylate), which they thermally drew into fibers. Following the drawing, they stripped off the cladding, leaving a stretchable thread. The elastomer core acts not only as a structural component, but also an optical component (for optoelectronic stimulation) due to its transparency in the visible range. To provide the fibers with electrical connections for electrical stimulation and monitoring, the team deposited uniform 1-mm-thick conductive mesh layers of silver nanowires using dip coating. “We figured mesh would perform best because solid films will crack while the mesh can stretch and deform,” Anikeeva says. Next, they encapsulated the fibers into polydimethylsiloxane (PDMS) to minimize direct contact between the nanowires and body tissue. The final device had a diameter of just 105–135 µm and it maintained low impedance at strains up to 100%.

The researchers implanted their device into mice’s spinal cords, which is only about 0.5 mm thick, to see if it could be used for neural recording and optoelectronic modulation. Study first author and MIT graduate student Alice Lu worked with collaborators at the University of Washington, Seattle to develop the surgical procedures to interface the device with the tiny mouse spinal cord, Anikeeva says. Once they implanted the floppy probes, the team was able to record not only global neural activity from the spinal cord, but also isolated action potentials from individual neurons. The team also implanted the device into transgenic mice that have spinal neurons that respond to light. They pulsed lasers through the devices, and this light activated neurons; that activity propagated down to the sciatic nerve, resulting in hind limb movement.

Seung Hwan Ko, a materials scientist at Seoul National University, said the work was “very interesting and somewhat exciting,” adding that the difficulty in making stretchable optoelectronic stimulators lies mainly in finding highly stretchable materials that are good electrical conductors. “The authors showed a very smart way to combine a highly stretchable optical stimulator and a highly stretchable metal electrode in a highly stretchable fiber form,” says Ko, who was not involved in the study. Because of the stretchability of the human body, “I think this study really extends the application field of stretchable electronics even to the translational science level.”

Given that silver has some biocompatibility issues, the team is looking into eventually using gold nanowires, which are more expensive. They are also hoping to expand the resolution of the device by increasing its numbers of electrodes, as well as looking at other elastomer materials that may allow for additional optical channels or drug delivery. “We’re working on expanding the pallet of functionalities,” Anikeeva says.

Read the article in Science Advances.