The team coated the conductive structures with a thin gold layer via a high vacuum. (Image Credit: Andreas Heddergott / TUM)
Humans often receive pain treatment through nerve stimulation. However, fine nerves become more difficult to treat because electrodes cannot attach as easily. So researchers from the Technical University of Munich (TUM) and NTT Research took advantage of 4D printing technology to create flexible electrodes. These electrodes fold and wrap around thin nerves when they come in contact with moisture.
Stimulating nerves that are tens to hundreds of micrometers in diameter is very difficult and requires electrodes designed with extreme fineness and precision. Plus, it's particularly challenging to insert and attach electrodes to micrometer-sized nerves.
That's where the team's 4D-printed electrodes play a crucial role — they wrap around extremely thin nerve fibers after insertion into moist tissue. All of this is achieved through its design. The electrode's outer sheath consists of a biocompatible hydrogel, which inflates due to moisture contact. Meanwhile, the flexible, internal material retains its shape.
Schematic diagram of the team’s foldable electrodes. (Image Credit: TUM)
Each electrode's internal structure has a titanium-gold coating responsible for transmitting electrical signals between the electrodes and nerve fibers."The close contact between the folded cuffs and the nerves allows us to both stimulate the nerves and measure nerve signals with the electrodes," says Bernhard Wolfrum, Professor of Neuroelectronics at the Munich Institute of Biomedical Engineering (MIBE) at TUM and head of the study.
The team’s electrodes could have some practical biomedical applications, like treating sleep apnea. People dealing with that condition usually experience airway blockage because their tongue drops back toward the throat. Stimulating muscles responsible for moving the tongue forward may address that issue. "Currently, however, selectively stimulating only those muscles that move the tongue forward is difficult. This is where the flexible electrodes might be applied to facilitate stimulating nerves more selectively in the future," says Professor Clemens Heiser, senior physician at the Department of Otolaryngology at the TUM University Hospital Klinikum rechts der Isar.
So far, the researchers have used these robust and easy-to-manage electrodes in locusts. In that case, 100-micrometer nerve fibers were sheathed without leading to nerve damage, allowing muscle stimulation in a specific target. “We interfaced a nerve of the locust Locusta migratoria to validate the in vivo performance of the 4D cuff electrodes. The nerve 5 (N5) emerges from the locust's metathoracic ganglion and innervates the animal's hind leg,” the team wrote in the paper. “To validate the stimulation capabilities of the 4D cuff electrode, we stimulated the nerve with biphasic current pulses and recorded the elicited leg response on camera. The pulses were delivered between two electrodes every 2 s. The stimuli triggered the leg's fast extensor tibiae neuron (FETi), provoking it to extend its tibiofemoral joint.”
Although it’s still in early development, the team believes the electrodes can deploy peripheral nerve stimulation for clinical applications in the future.
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