Controlling Neurons with Light

Opto­genetics is a bio­logical technique that uses light to turn specific neuron groups in the brain on or off. For example, researchers might 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,” University of Arizona biomedical engi­neering professor Philipp Gutruf said. “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.”

Combined image analysis with MRI and CT results superimposed on a 3D rendering of the animal implanted with the programmable bilateral multi µ-ILED device. (Source: P. Gutruf)

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 fibers, 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.

But these devices still came with their own limi­tations. They were bulky and often attached visibly outside the skull, they didn’t allow for precise control of the light’s frequency or inten­sity, and they could only stimulate one area of the brain at a time. “With this research, we went two to three steps further,” Gutruf said. “We were able to implement digital control over inten­sity and frequency of the light being emitted, and the devices are very minia­turized, 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.”

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.”

Devices are implanted with a simple surgical procedure similar to surgeries in which humans are fitted with neuro­stimulators, or brain pacemakers. They cause no adverse effects to subjects, and their func­tionality doesn’t degrade in the body over time. This could have impli­cations for medical devices like pace­makers, which currently need to be replaced every five to 15 years. (Source: U. Arizona)

Reference: P. Gutruf et al.: Fully implantable optoelectronic systems for battery-free, multimodal operation in neuroscience research, Nat. Elec. 1, 652 (2018); DOI: 10.1038/s41928-018-0175-0

Link: Center for Bio-Integrated Electronics, Northwestern University, Evanston, USA

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