Nanomagnets Could Aid Study Of Hearing

As people age, the tiny hairlike cells lining the inner ear can become damaged, leading to hearing loss. A new method that uses magnetic nanoparticles to stimulate these hair cells could help researchers better understand how the cells function and fail (ACS Nano 2014, DOI: 10.1021/nn5020616).

Some 16,000 hair cells line the cochlea in the inner ear, detecting motion produced by sound waves and transmitting electrical signals to nerves in a process that results in hearing. To understand how to protect or repair these cells, researchers must first understand how they work under normal circumstances. And so far, that process has been cumbersome and complicated.

Traditionally, researchers use a glass probe, attached directly to a hair-cell bundle, to physically push the bundle and stimulate the hair cells. Because the probe is so heavy, it adds mass to the delicate hairs in an uncontrolled way that can interfere with the experiment. “That loading could change what you’re measuring,” says Dolores Bozovic of the University of California, Los Angeles.

Bozovic and Jinwoo Cheon of Yonsei University, in Seoul, thought that magnetic nanoparticles could manipulate the hair bundles without adding extra load. They and their colleagues synthesized 50-nm-wide cubic nanoparticles from zinc and iron. They coated the particles with silica to increase their solubility in water and with polyethylene glycol to prevent them from clumping together. Then the researchers attached a protein called concanavalin A to the particles. This protein binds to glycoproteins on the surface of hair cells, which allowed the scientists to attach nanoparticles to hair cells taken from the ear of the North American bullfrog (Rana catesbeiana). By applying an oscillating magnetic field to a plate containing the ear cells, the researchers found that they could push and pull the nanoparticle-laden hairs at frequencies ranging up to 10,000 Hz. They recorded the movements with a high-speed camera and used software to determine the frequencies.

Peter G. Barr-Gillespie, a researcher at Oregon Health & Science University, who was not involved with the study, calls the new method a useful alternative to existing ways to stimulate hair cells. He points out that the magnetic force pulls hair cells in one direction rapidly, but the hairs move back passively to their original state when the magnetic field is turned off. Although it’s not a huge problem, an ideal method for stimulating a hair bundle would move it over and back as fast as possible, Barr-Gillespie says.

Cheon says his team’s method is similar to optogenetics, which allows researchers to use light to stimulate processes in the brain. Cheon would like to see so-called magnetogenetics close gaps in understanding the sensory system that governs hearing. He says this new method offers a way to explore the mechanics of the inner ear in a more natural way.