Scientists have been looking for ways to stimulate the brain for decades. Deep brain stimulation, for example, is an invasive technique that can be used to manage symptoms of neurological conditions including Parkinson’s disease and epilepsy. A non-invasive approach could benefit more people and possibly be deployed earlier in the course of a disease.
“For over a decade, our group has been working on magnetic approaches to control neuronal activity. However, typically these methods relied on specialized receptors – those sensing heat or tension or particular chemicals. But there’s one signal that all neurons can understand: voltage,” says corresponding author Polina Anikeeva, chair of MIT’s Department of Materials Science and Engineering and director of the K. Lisa Yang Brain-Body Center. “So, it was somewhat of a ‘holy grail’ for us to create a particle that would efficiently convert magnetic field into electrical potential.”
Ye Ji Kim, a PhD candidate and lead author on the paper, decided to tackle this problem. The result is a magnetic nanoparticle, called a magnetoelectric nanodisc (MEND), that could be injected into a specific location in the brain and stimulated with an electromagnet located outside of the body. “MENDs harness the signalling mechanisms naturally present in all neurons. This capability marks a significant advancement,” Kim explains.
MENDs, which are approximately 250 nm across, have two layers. One is a magnetostrictive core that changes shape when magnetized and induces a strain in the second layer, a piezoelectric shell. In response to this strain, the shell is electrically polarized, facilitating the delivery of electrical pulses to neurons in response to the external magnetic field.
Characterizing and testing the MENDs also required design work.
“In our simulations, we had to account for the evolution of the non-uniform magnetization and thus non-uniform strain,” says Noah Kent, a postdoctoral fellow at MIT involved in the research. “The comprehensive pipeline composed of Ye Ji’s innovative electrochemical measurements coupled with nanomagnetic simulation will be extremely valuable not only for biological applications of these materials, but more generally for the design of magnetoelectrics.”
Another scientist at MIT, Emmanuel Vargas Paniagua, facilitated tests involving mice. The scientists injected MENDs in solution into specific brain regions of mice and turned on a weak electromagnet in the vicinity to stimulate neurons. They found that MENDs could stimulate the ventral tegmental area – a deep brain region involved with feelings of reward – and the subthalamic nucleus – a brain region associated with motor control that’s typically stimulated in patients receiving deep brain stimulation for management of Parkinson’s disease. Additional results of their in vivo experiments are detailed in Nature Nanotechnology.
Characterization experiments demonstrated that the magnetostrictive effect was amplified by approximately 1000 relative to that achieved with conventional spherical particles. Meanwhile, conversion of the magnetic effect into an electrical output was only four times greater, which the scientists say suggests areas for improvement. Their next steps include applying MENDs to basic research using animal models, and they have suggested possible designs for future human models.
“These particles are very interesting from a translational standpoint, as they do not require genetic modification,” Anikeeva says. “Additionally, the magnetic fields are weak, and the frequencies are low – making electronics safe, simple and potentially portable for human patients.”
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