Scientists have created a sophisticated, wireless system that uses light to control neurons in the brain, a therapy that could be used to turn off pain receptors or reducing the effects of severe neurological disorders.
Researchers may use optogenetic stimulation to restore movement in case of paralysis or, in the future, to turn off the areas of the brain or spine that cause pain, eliminating the need for - and the increasing dependence on - opioids and other painkillers.
"We're making these tools to understand how different parts of the brain work," said Philipp Gutruf, a professor at University of Arizona in the US.
"The advantage with optogenetics is that you have cell specificity: You can target specific groups of neurons and investigate their function and relation in the context of the whole brain," said Gutruf, first author of the research published in Nature Electronics.
In optogenetics, researchers load specific neurons with proteins called opsins, which convert light to electrical potentials that make up the function of a neuron. When a researcher shines light on an area of the brain, it activates only the opsin-loaded neurons.
The first iterations of optogenetics involved sending light to the brain through optical fibres, which meant that test subjects were physically tethered to a control station. Researchers went on to develop a battery-free technique using wireless electronics, which meant subjects could move freely.
However, these devices still came with their own limitations -- they were bulky and often attached visibly outside the skull, they did not allow for precise control of the light's frequency or intensity, and they could only stimulate one area of the brain at a time.
"We were able to implement digital control over intensity and frequency of the light being emitted, and the devices are very miniaturised, so they can be implanted under the scalp. We can also independently stimulate multiple places in the brain of the same subject, which also wasn't possible before," Gutruf said.
The ability to control the light's intensity is critical because it allows researchers to control exactly how much of the brain the light is affecting -- the brighter the light, the farther it will reach.
In addition, controlling the light's intensity means controlling the heat generated by the light sources, and avoiding the accidental activation of neurons that are activated by heat.
The wireless, battery-free implants are powered by external oscillating magnetic fields, and, despite their advanced capabilities, are not significantly larger or heavier than past versions.
In addition, a new antenna design has eliminated a problem faced by past versions of optogenetic devices, in which the strength of the signal being transmitted to the device varied depending on the angle of the brain: A subject would turn its head and the signal would weaken.
"This system has two antennas in one enclosure, which we switch the signal back and forth very rapidly so we can power the implant at any orientation," Gutruf said.
"In the future, this technique could provide battery-free implants that provide uninterrupted stimulation without the need to remove or replace the device, resulting in less invasive procedures than current pacemaker or stimulation techniques," he said.
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