New spinal cord interface restores bladder control and sensation

Approximately 308,000 people in the United States live with spinal cord injury. Nearly all lose bladder control. And yet the vast majority of research and engineering attention in neurotech has poured into motor restoration-making paralyzed limbs move again.

"When you can't control your bladder, that's all you think about," says Charles Liu, Dr. Charles Liu, professor of neurological surgery and neurology at the Keck School of Medicine of USC who directs the USC Neurorestoration Center and is co-senior author on a new paper, "Intraspinal Microstimulation of Dorsolateral Funiculus for Coordinated Bladder Control, published in IEEE Transactions on Neural Systems and Rehabilitation Engineering along with Vasileios Christopoulos, an assistant professor in the Alfred E. Mann Department of Biomedical Engineering at USC Viterbi School of Engineering. Shan Zhong, a postdoctoral researcher at USC Viterbi, is a first author on the paper.

Liu explains that odor is a concern for his patients. "It's socially a huge problem. And medically, all of my brain-computer interface patients have a severe episode of urosepsis every year. I've known patients who died from this."

A spinal cord machine interface

Brain-computer interfaces have made headlines for years. The team is building is a spinal cord machine interface-and the distinction matters.

The spinal cord is a surprisingly elegant engineering target: specific fiber bundles carry specific signal types in consistent anatomical locations across individuals. And yet, Christopoulos notes, it has been largely dismissed by the neuroscience community. "When I was talking to people in neuroscience, most of the response was that it's a cable." Almost no functional neuroimaging research exists on the spinal cord-a striking gap for a structure that governs so much of human experience.

The spinal cord is not just a cable. Bladder control is sparsely distributed in the brain. But here, we can directly target one region and trigger the sense of bladder filling."

Shan Zhong, postdoctoral researcher at USC Viterbi

The practical payoff matters too. Most existing technologies that help patients void rely on fixed schedules: an alarm goes off, and they catheterize. "The best thing about this," Zhong says, "is that it can actually make people feel that there is a need for voiding, instead of depending on alarm clocks."

That region is the dorsolateral funiculus, or DLF, a thin bundle of ascending sensory fibers near the spinal cord's surface. Normally, as the bladder fills, signals travel up through the DLF to the brain, which registers fullness and sends a coordinated command back down: contract the bladder muscle and relax the sphincter simultaneously. After spinal cord injury, that loop is severed. The patient loses not just voluntary control but the ability to feel the need to go.

The team's question: could you find the exact address of that signal, and replay it artificially?
Using custom microelectrode arrays developed by Ecate LLC, a USC-affiliated startup, the team mapped neural activity in rats during controlled bladder filling. The arrays-with electrodes smaller than a human hair-were inserted into candidate spinal cord regions. Most were silent. But in the DLF, one or two adjacent channels lit up with rhythmic bursting that tracked filling precisely, climbing from 30 Hz with the first drops of saline to nearly 100 Hz just before voiding. Electrodes just 65 micrometers away stayed completely silent. The responsive zone, roughly 100 by 100 micrometers, was consistent enough across animals to serve as a reliable anatomical address.

In a separate animal model group, they delivered patterned electrical pulses at those same coordinates, timed to mimic the biological signal of a full bladder. Coordinated voiding followed in 91.7% of trials, rising to 100% when the bladder was pre-filled to the volume where natural DLF activity begins. Leg muscle electrodes stayed silent throughout: the response was bladder-specific, not a generalized motor reflex.

The envisioned full system named BLISS, for Bladder-Linked Stimulation System, would pair this sensory interface with a bladder volume sensor and a motor stimulator, creating a closed-loop neuroprosthesis that restores both the sensation and the act of voiding.

The pathway to patients

The team is already working with larger animal models, whose anatomy is closer to human scale. Liu estimates that with adequate funding, initial human recordings could begin within 18 months; not in spinal cord injury patients first,but piggybacked onto spinal cord tumor surgeries that are already far more invasive. A brief recording during an existing surgery adds minimal risk, while those patients often face bladder complications themselves and have a direct stake in what's being built.