Probe Delivering Optoelectronic Stimulation May Strengthen Understanding of Spinal Cord

A rubber-like fiber that can flex and stretch with the human spine while delivering both optical impulses and electrical connections for stimulation and monitoring of the spine could be used in the study of spinal cord neurons and potentially to help restore spinal cord function.

A team of researchers from the Massachusetts Institute of Technology (MIT), the University of Washington and Oxford University has created a hybrid probe that maintains low optical transmission losses in the visible range and that can stand up under strains exceeding those occurring in mammalian spinal cords.

The team combined a transparent elastomer that could act as a waveguide for optical signals and a coating formed of a mesh of silver nanowires to produce a conductive layer for electrical signals. To process the elastomer, the material was embedded in a polymer cladding, then drawn into a highly stretchable, flexible fiber. The cladding was dissolved once the drawing process was completed, leaving a transparent fiber with electrically conductive, stretchy nanowire coatings. The fiber can stretch by 20 to 30 percent without any effect on its properties.

Researchers have developed a rubber-like fiber, shown here, that can flex and stretch while simultaneously delivering both optical impulses, for optoelectronic stimulation, and electrical connections, for stimulation and monitoring. Courtesy of Chi (Alice) Lu and Seongjun Park.

The fiber probes were used for optical and electrophysiological interrogation of spinal cord circuits in a mouse model. The probes maintained optical and electrical properties under bending and stretching deformations, exceeding those experienced by the mouse spinal cord during normal motion. The mechanical, optical and electrical characteristics of the probes enabled acute recordings of spontaneous neural activity, sensory-evoked potentials, and the simultaneous recording of optically evoked spinal potentials and optical control of hind limb muscles.

“I wanted to create a multimodal interface with mechanical properties compatible with tissues, for neural stimulation and recording,” said researcher Chi (Alice) Lu.

It was essential for the device to be stretchable, because “the spinal cord is not only bending but also stretching during movement,” Lu said.

According to professor Polina Anikeeva, the spinal cord “undergoes stretches of about 12 percent during normal movement.”

The findings suggest that the fiber platform could, in the future, permit monitoring and controlling of neural activity to promote recovery following spinal cord injury. Isolating single-neuron action potentials from the mouse spinal cord during free behavior remains a goal of the team.

“Eventually, we’d like to be able to use something like this to combat spinal cord injury. But first, we have to have biocompatibility and to be able to withstand the stresses in the spinal cord without causing any damage,” said Anikeeva.

“We're the first to develop something that enables simultaneous electrical recording and optical stimulation in the spinal cords of freely moving mice, so we hope our work opens up new avenues for neuroscience research,” said Lu. “There are many different types of cells in the spinal cord, and we don't know how the different types respond to recovery, or lack of recovery, after an injury.”

These new fibers, the researchers hope, could help to fill in some of those blanks.

The research was published in Science Advances (doi: 10.1126/sciadv.1600955).