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A Swiss Army Knife for Neuroscience

Neural probes that combine optics, electronics, and drugs could help unlock the secrets of the brain.
Left: locks of polymers are the starting point for making a multifunctional neural probe. In a machine shop, patterns of conductive metal rods, transparent plastics, or hollow spaces will be added, creating a “preform.”

Right: The preform is loaded into this 12-foot-tall fiber-drawing tower.

Various powerful new tools for exploring and manipulating the brain have been developed over the last few years. Some use electronics, while others use light or chemicals.

At one MIT lab, materials scientist Polina Anikeeva has hit on a way to manufacture what amounts to a brain-science Swiss Army knife. The neural probes she builds carry light while collecting and transmitting electricity, and they also have tiny channels through which to pump drugs.

That’s an advance over metal wires or silicon electrodes conventionally used to study neurons. Anikeeva makes the probes by assembling polymers and metals into large-scale blocks, or preforms, and then stretching them into flexible, ultrathin fibers.

Polina Anikeeva

Multifunctional fibers offer new ways to study animal behavior, since they can record from neurons as well as stimulating them. New types of medical technology could also result. Imagine, as Anikeeva does, bionic wiring that bridges a spinal-cord injury, collecting electrical signals from the brain and transmitting them to the muscles of a paralyzed hand.

Anikeeva made her first multifunctional probe while studying at Stanford. It was crude: she simply wrapped metal wires around a glass filament. But this made it possible to combine standard electrode measurements with a new technology, optogenetics, in which light is fired at neurons to activate them or shut them down.

Now Anikeeva, a professor of materials science and engineering, makes probes using a fiber-drawing technology developed by another MIT researcher, Yoel Fink. It’s based on the way silica is heated and pulled to form telecommunications fiber. But it works at lower temperatures, at which many useful polymers become soft enough to stretch.

Polymer fibers have a couple of important advantages. One is that they are flexible and mimic the physical properties of tissue. That could allow them to work longer than the stiff metal electrodes neuroscientists have relied on, permitting long-term studies in animals. The second feature of the fibers is that they can combine many functions. Probes made so far have incorporated as many as 36 microwires, optical waveguides, and hollow channels for carrying medicine. There’s no reason not to incorporate sensors to measure temperature or pressure as well. Inside the body, the right materials and structures might even entice nerves to attach to the fibers, the way bone fuses to a hip implant.

The fiber-drawing process shrinks large patterns into microscopic ones, preserving the details. But there are challenges. The tiny wires and tubes have to be stripped, splayed, and soldered by hand to connect them to components such as a recording device a mouse wears on its head. That’s quite a nightmare, says Andres Canales, a graduate student, who hopes to resolve the problem.

Will polymer bio-wires be what ultimately cures paralysis—say, by ferrying nerve signals across an injured spinal cord? “I think it will be a version of this technology, a more sophisticated version,” says Anikeeva. “At least we are going to pursue this route.”

3. A collection of preform leftovers after drawing. Indium-tin rods are visible in what’s left of the preform at center.
4. Fiber is pulled from the furnace after being heated to 350 °C. A micrometer (red light) monitors the fiber’s size.

5. Each preform is drawn into as much as one kilometer of fiber. It is now about 1/100th as thick as it was originally.
6. A fiber soaks in THF, a solvent, to remove a protective cladding.
7. A cross section of a 0.35-millimeter- wide fiber containing four electrodes, a fluid channel, and a ring-shaped waveguide. At right, light shines through the waveguide.
8. This mouse has a fiber implanted in its brain. Visible on its head are a circuit board, a port to introduce light, and two more to inject drugs.
9. Optically stimulating the mouse’s brain produces the electrical activity recorded here.

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