Wireless remote brain interfaces
Dr. Polina Anikeeva is an assistant professor of Materials Science and Engineering at MIT and a principle investigator of the Bioelectronics group. Her research lies in the field of neuroprosthetics and brain-machine interfaces. Together with her group she explores optoelectronic, fiber-based and magnetic approaches to minimally invasive neural interrogation. Her group was first to demonstrate multifunctional flexible fibers for simultaneous optical stimulation, electrical recording and drug delivery in the brain and spinal cord, as well as magnetic nanomaterials for wireless magnetic deep brain stimulation.
A study in mice points to a less invasive way to massage neuronal activity, by injecting metal nanoparticles into the brain and controlling them with magnetic fields. The technique could eventually provide a wireless, nonsurgical alternative to traditional deep brain stimulation surgery.
Optogenetics has revolutionized how neuroscientists study the brain by allowing them to directly manipulate specific neural circuits. But it isn’t practical for human deep brain stimulation. The technique requires that animals be genetically modified so that their neurons respond to light. Light also scatters in brain tissue. So rodents in optogenetics experiments must remain tethered to a surgically implanted, fiber optic cable that delivers laser beams directly to the brain region of interest.
Unlike light, low-frequency magnetic fields pass straight through brain tissue as if it were “transparent,” Anikeeva says. That makes those types of magnetic fields an ideal vehicle for delivering energy into the brain without damaging it. Clinicians have long tried to do just that by placing magnetic field coils near a patient’s head. This so-called transcranial magnetic stimulation (TMS) triggers the flow of small electrical currents in neural circuits beneath the coils. But the magnetic fields used in TMS affect only brain tissue near the brain’s surface. Anikeeva, who is now at the Massachusetts Institute of Technology (MIT) in Cambridge, decided to see if she could use magnetic nanoparticles to go deeper.
Previous cancer studies had shown that by injecting tumors with magnetic nanoparticles made of iron oxide—“essentially rust, with well-tuned magnetic properties,” Anikeeva says—then exposing them to rapidly alternating magnetic fields, excited nanoparticles can be used to heat and destroy cancer tumors while leaving surrounding, healthy tissue intact. Anikeeva wondered if a similar method could be used to merely stimulate select groups of neurons deep within the brain.
To find out, she and her MIT colleagues targeted a class of proteins called TRPV1 channels, which are found in neurons that respond to heat and certain chemicals in food. Every time you touch a hot iron or eat a spicy pepper, TRPV1-containing neurons fire. Anikeeva and her colleagues injected custom-made, 20-nanometer iron oxide particles into a region of the rodents’ brains called the ventral tegmental area (VTA), a well-studied deep brain structure essential to the experience of reward, which plays a central role in disorders such as addiction and depression in people.
TRPV1-containing neurons are abundant in this region in humans, but sparse in mice. So the team also injected the rodents with a virus that increased cell expression of the channel just within that brain area. Such an approach would not be feasible in people, but made the experiment easier to evaluate, Anikeeva says.
he team put the mice underneath a custom-built, 6.35-centimeter-diameter coil that emits magnetic waves alternating between 10 hertz and 10 millihertz. Hours after the team applied the magnetic fields, they sacrificed the animals and examined their brain tissue under a microscope. The mice were a strain previously engineered to produce a bright green fluorescent marker in any active neurons. A large network of neurons connected to the VTA glowed green, suggesting that the magnetic fields had effectively stimulated the circuit, the team reports online today in Science. Anikeeva and colleagues found similar results when they waited a month before applying the magnetic stimulation, suggesting that the nanoparticles endured in place.
To make the approach feasible in humans, researchers need to design nanoparticles that are “very, very selective” in their ability to target specific brain structures and neurons, Sastre says. TRPV1 channels are widely distributed throughout the human brain, so another major challenge is figuring out how to deliver stimulation only to the cells researchers want to target, he adds.
In a “perfect, futuristic picture,” Anikeeva says, people suffering from depression or other neurologic or psychiatric disorders could come in for a simple intravenous injection of finely tuned, targeted nanoparticles that reach the region of the brain needing stimulation. In theory, such stimulation could take place every time patients go to sleep, if the magnetic coil were installed in their bed or a specialized pillow, she suggests. For now, however, the technique is most promising as a potential method of studying brain activity in animals that allows them to roam their enclosures without being tethered to wires, she says. “We’re not necessarily thinking of a clinical perspective yet,” Anikeeva emphasizes.