A light-and-heat show for exciting neurons

In what could be an innovative use of nanomaterials in biomedical application, a team of India-born scientists in Washington University (WU) in St. Louis, has used nanoparticles as heaters to manipulate the electrical activity of neurons in the brain as well as heart muscles cells.

Srikanth Singamaneni, a materials scientist, and Barani Raman, a biomedical engineer, and their teams at McKelvey School of Engineering at WU collaborated to develop a non-invasive technology that inhibits the electrical activity of neurons using polydopamine (PDA) nanoparticles and near-infrared light. The negatively charged PDA nanoparticles, which selectively bind to neurons, absorb near-infrared light that creates heat, which is then transferred to the neurons, inhibiting their electrical activity.

"We showed we can inhibit the activity of these neurons and stop their firing, not just on and off, but in a graded manner," said Singamaneni, a professor in the Department of Mechanical Engineering & Materials Science. "By controlling the light intensity, we can control the electrical activity of the neurons. Once we stopped the light, we can completely bring them back again without any damage," he said, of their work published in Advanced Materials journal in July.

In addition to their ability to efficiently convert light into heat, the PDA nanoparticles are highly biocompatible and biodegradable. The nanoparticles eventually degrade, making them a convenient tool for use in in vitro and in vivo experiments in the future.

"When you pour cream into hot coffee, it dissolves and becomes creamed coffee through the process of diffusion," Singamaneni explained. "It is similar to the process that controls which ions flow in and out of the neurons." Diffusion, he notes, depends on temperature, "so if you have a good handle on the heat, you control the rate of diffusion close to the neurons. This would in turn impact the electrical activity of the cell. This study demonstrates the concept that the photothermal effect, converting light into heat, near the vicinity of nanoparticles-tagged neurons can be used as a way to control specific neurons remotely."

The team also designed a photothermal foam that is similar to a sugar cube, forming a dense population of nanoparticles in tight packaging that acts more quickly than individual sugar crystals that disperse, Raman said.

"With so many of them packed in a small volume, the foam is quicker in transducing light to heat and give more efficient control to only the neurons we want," he said. "You don’t have to use high-intensity power to generate the same effect."

In addition, the team, which included Jon Silva, associate professor of biomedical engineering, applied the PDA nanoparticles to cardiomyocytes, or heart muscle cells. Interestingly, the photothermal process excited the cardiomyocytes, showing that the process can increase or decrease the excitability in cells depending on their type.

"The excitability of a cell or tissue, whether it be cardiomyocytes or muscle cells, depends to a certain extent on diffusion," Raman said. "While cardiomyocytes have a different set of rules, the principle that controls the sensitivity to temperature can be expected to be similar."

Now, the team is looking at how different types of neurons respond to the stimulation process. They will be targeting particular neurons by selectively binding the nanoparticles to provide more selective control.