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Neuroprosthetics Powered by the Brain

Powering a neuroprosthetic is a tricky issue. As most of these devices are implanted either on the surface of the brain, or deep inside its folds, it is not a trivial matter to pop it out and change the battery. Every operation does a little more damage to the brain, and even worse, if you move the neuroprosthetic even fractionally during the replacement procedure there is no guarantee it will hook up with active dendrites from the correct brain region in its new position.

One major solution is to incorporate a power receiver under the skull during the first implantation. Power recharges can then be sent via a transmitter affixed to a shaved region of the scalp. However, this is still far from an ideal solution. Ideally a neuroprosthetic would be powered by the body itself; functioning as long as the person is alive.

But, how to do that? Normally such efforts to power prosthetics from the body, rely on piezoelectric power sources. Piezoelectricity is the accumulation of a charge in a solid in response to mechanical stress. In other words, as the body moves, the flesh deforms, and the piezoelectric material deforms along with it. This creates an electrical charge which is used to either power or recharge the prosthetic device.

Unfortunately, the brain is completely encased in the skull. If there is any twisting or deformation, it is not a good thing, and certainly not normal. Piezoelectricity will not work for devices attached to or inside the central nervous system itself.

A group of researchers based at MIT have developed a different approach. If we cannot depend on movement to power the prosthetic, why not use the same power source the cells of the brain use? Why not derive power from the sugars in the blood?

Their prototype glucose fuel cells are tiny; small enough to implant into a silicon wafer. The largest being 64mm by 64mm, the smallest functional cell just 2mm by 2mm.


A single 128mm silicon wafer holding all prototype cells

The fuel cell, described in the June 12 edition of the journal PLoS ONE, strips electrons from glucose molecules to create a small yet continuous electric current proportional to its surface area.

The idea of a glucose fuel cell is not new: In the 1970s, scientists showed they could power a pacemaker with a glucose fuel cell, but the idea was abandoned in favor of lithium-ion batteries, which could provide significantly more power per unit area than glucose fuel cells. These glucose fuel cells also utilized enzymes that proved to be impractical for long-term implantation in the body, since they eventually ceased to function efficiently.
The new twist to the MIT fuel cell described in PLoS ONE is that it is fabricated from silicon, using the same technology used to make semiconductor electronic chips. The fuel cell has no biological components: It consists of a platinum catalyst that strips electrons from glucose, mimicking the activity of cellular enzymes that break down glucose to generate ATP, the cell’s energy currency.

So far, the fuel cell can generate up to hundreds of microwatts — enough to power an ultra-low-power and clinically useful neural implant.
“It will be a few more years into the future before you see people with spinal-cord injuries receive such implantable systems in the context of standard medical care, but those are the sorts of devices you could envision powering from a glucose-based fuel cell,” says Benjamin Rapoport, a former graduate student in the Sarpeshkar lab and the first author on the new MIT study.
Rapoport calculated that in theory, the glucose fuel cell could get all the sugar it needs from the cerebrospinal fluid (CSF) that bathes the brain and protects it from banging into the skull. There are very few cells in the CSF, so it’s highly unlikely that an implant located there would provoke an immune response. There is also significant glucose in the CSF, which does not generally get used by the body. Since only a small fraction of the available power is utilized by the glucose fuel cell, the impact on the brain’s function would likely be minimal. The brain's own glucose reserves would be unaffected.

Karim Oweiss, an associate professor of electrical engineering, computer science and neuroscience at Michigan State University, says the work is a good step toward developing implantable medical devices that don’t require external power sources.
“It’s a proof of concept that they can generate enough power to meet the requirements,” says Oweiss, adding that the next step will be to demonstrate that it can work in a living animal.

References

New energy source for future medical implants: sugar

A Glucose Fuel Cell for Implantable Brain–Machine Interfaces (paper)

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