Small, magnetically-powered neural stimulator

A battery-free neuro stimulator implant could soon be treating epilepsy, Parkinson’s disease, chronic pain, and other conditions. Powered by magnetic energy, the miniature device produces the same high-frequency signals as clinically approved, battery powered implants.

Developed by Rice University neuroengineers, the implant is about the size of a grain of rice.

A thin film of magnetoelectric material coating, the implant converts magnetic energy into electrical voltage. This method avoids the drawbacks of radio waves, ultrasound, light, and magnetic coils, all of which have been shown to suffer from interference with living tissue or harmful amounts of heat.

While battery-powered implants are frequently used to treat epilepsy and Parkinson’s disease, research has shown that neural stimulation could be useful for treating depression, obsessive-compulsive disorder (OCD), and chronic, intractable pain that can lead to anxiety, depression, and opioid addiction.

Creating easily implantble battery-free, wireless devices could make neural stimulation therapy more widely available. The researchers’ device is small enough to implant almost anywhere in the body with a minimally invasive procedure, similar to the one used to place stents in blocked arteries.

To demonstrate viability of the magnetoelectric technology, researchers implanted the device in rodents. During the test, rodents were free to roam about their enclosures. The study found rodents preferred to be in portions of their enclosures where a magnetic field activated the stimulator, providing a small voltage to the reward center of their brains.

“That proof-of-principle demonstration is important, because it’s a huge technological leap to go from a benchtop demonstration to something that could be used to treat people,” says Jacob Robinson, a member of the Rice Neuroengineering Initiative.

Amanda Singer, an applied physics student in Robinson’s lab, solved the problem of powering the device wirelessly by joining layers of two different materials in a single film. The first layer, a magnetostrictive foil of iron, boron, silicon, and carbon, vibrates at a molecular level when placed in a magnetic field. The second, a piezoelectric crystal, converts mechanical stress directly into electric voltage.

“The magnetic field generates stress in the magnetostrictive material,” Singer says. “The material generates acoustic waves, and some of those are at a resonant frequency that creates an acoustic resonant mode.”

The acoustic reverberations activate the piezoelectric half of the film. The magnetoelectric films harvest plenty of power but operate at a frequency that’s too high to affect brain cells.

“A major piece of engineering that Amanda solved was creating the circuitry to modulate that activity at a lower frequency that the cells would respond to,” Robinson says. “It’s similar to AM radio – you have very high-frequency waves, but they’re modulated at a low frequency that you can hear.”

Creating a modulated, bi-phasic signal that could stimulate neurons without harming them was a challenge, as was miniaturization.

“When you have to develop something that can be implanted subcutaneously in small animals, your design constraints change significantly,” says Caleb Kemere, a member of the Neuroengineering Initiative. “Getting this to work on a rodent in a constraint-free environment forced us to push size and volume to the minimum.”

Robinson and Kemere are associate professors of electrical and computer engineering and bioengineering. The research was supported by the National Science Foundation and the National Institutes of Health.