Rice University neuroengineers have created a tiny surgical implant that can electrically stimulate the brain and nervous system without using a battery or wired power supply. The neural stimulator draws its power from magnetic energy and is about the size of a grain of rice.
It is the first magnetically powered neural stimulator that produces the same kind of high-frequency signals as clinically approved, battery-powered implants that are used to treat epilepsy, Parkinson's disease, chronic pain and other conditions. The research is available online today in the journal Neuron. The implant's key ingredient is a thin film of "magnetoelectric" material that converts magnetic energy directly into an electrical voltage.
Rice neuroengineers created the bi-layered film to power implantable neural stimulators that are approximately the size of a grain of rice. The film converts energy from a magnetic field directly into an electrical voltage, eliminating the need for a battery or wired power connection.
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The method avoids the drawbacks of radio waves, ultrasound, light and even magnetic coils, all of which have been proposed for powering tiny wireless implants and have been shown to suffer from interference with living tissue or produce harmful amounts of heat.
Tiny implants capable of modulating activity of the brain and nervous system could have wide-ranging implications. While battery-powered implants are frequently used to treat epilepsy and reduce tremors in patients with Parkinson's disease, research has shown that neural stimulation could be useful for treating depression, obsessive-compulsive disorders and more than a third of those who suffer from chronic, intractable pain that often leads to anxiety, depression and opioid addiction.
Robinson said the miniaturization by study lead author and graduate student Amanda Singer is important because the key to making neural stimulation therapy more widely available is creating battery-free, wireless devices that are small enough to be implanted without major surgery. Devices about the size of a grain of rice could be implanted almost anywhere in the body with a minimally invasive procedure similar to the one used to place stents in blocked arteries, he said.
Singer, an applied physics student in Robinson's lab, solved the wireless power problem by joining layers of two very different materials in a single film. The first layer, a magneto strictive foil of iron, boron, silicon and carbon, vibrates at a molecular level when it's placed in a magnetic field. The second, a piezoelectric crystal, converts mechanical stress directly into an electric voltage.
"The magnetic field generates stress in the magneto strictive material," Singer said. "It doesn't make the material get visibly bigger and smaller, but it generates acoustic waves and some of those are at a resonant frequency that creates a particular mode we use called an acoustic resonant mode."
Acoustic resonance in magneto strictive materials is what causes large electrical transformers to audibly hum. In Singer's implants, the acoustic reverberations activate the piezoelectric half of the film. Robinson said the magnetoelectric films harvest plenty of power but operate at a frequency that's too high to affect brain cells.
Singer said creating a modulated biphasic signal that could stimulate neurons without harming them was a challenge, as was miniaturization. "When we first submitted this paper, we didn't have the miniature implanted version," she said. "Up to that point, the biggest thing was figuring out how to get that biphasic signal that we stimulate with, what circuit elements we needed to do that.
"When we got the reviews back after that first submission, the comments were like, 'OK, you say you can make it small. So, make it small,'" Singer said. "So, we spent another a year or so making it small and showing that it really works. That was probably the biggest hurdle. Making small devices that worked was difficult, at first."