Neuron mechanical stimulation through Nano-magnetic particle

In addition to responding to electrical and chemical stimuli, many of the body's neural cells can also respond to mechanical effects, such as pressure or vibration. But these responses have been more difficult for researchers to study, because there has been no easily controllable method for inducing such mechanical stimulation of the cells. Now, researchers at MIT and elsewhere have found a new method for doing just that.

Unlike those systems, which require an external wire connection, the new system would be completely contact-free after an initial injection of particles, and could be reactivated at will through an externally applied magnetic field.

The new method opens a new pathway for the stimulation of nerve cells within the body, which has so far almost entirely relied on either chemical pathways, through the use of pharmaceuticals, or on electrical pathways, which require invasive wires to deliver voltage into the body. This mechanical stimulation, which activates entirely different signaling pathways within the neurons themselves, could provide a significant area of study, the researchers say.

"An interesting thing about the nervous system is that neurons can actually detect forces,"

"That's how your sense of touch works, and also your sense of hearing and balance." The team targeted a particular group of neurons within a structure known as the dorsal root ganglion, which forms an interface between the central and peripheral nervous systems, because these cells are particularly sensitive to mechanical forces.

The key to the new process was developing minuscule discs with an unusual magnetic property, which can cause them to start fluttering when subjected to a certain kind of varying magnetic field. Though the particles themselves are only 100 or so nanometers across, roughly a hundredth of the size of the neurons they are trying to stimulate, they can be made and injected in great quantities, so that collectively their effect is strong enough to activate the cell's pressure receptors. "We made nanoparticles that actually produce forces that cells can detect and respond to,"

Anikeeva says that conventional magnetic nanoparticles would have required impractically large magnetic fields to be activated, so finding materials that could provide sufficient force with just moderate magnetic activation was "a very hard problem." 

These discs, which are hundreds of nanometers in diameter, contain a vortex configuration of atomic spins when there are no external magnetic fields applied. This makes the particles behave as if they were not magnetic at all, making them exceptionally stable in solutions. When these discs are subjected to a very weak varying magnetic field of a few millitesla, with a low frequency of just several hertz, they switch to a state where the internal spins are all aligned in the disc plane. This allows these nanodiscs to act as levers—wiggling up and down with the direction of the field.

vortex state에 있을 때에는 얌전하다가 in plane state가 되면 자성이 띈다고 한다. 그래서 외부에서 약하게 magnetic field fluctuation을 가하면 particle이 반응을 하고, neuron에 자극을 한다고 한다. 

 

reference

Danijela Gregurec et al. Magnetic Vortex Nanodiscs Enable Remote Magnetomechanical Neural Stimulation, ACS Nano (2020). DOI: 10.1021/acsnano.0c00562