This silicon wafer consists of glucose fuel cells of varying sizes; the largest is 64 by 64 mm. Image: Sarpeshkar Lab |
Massachusetts
Institute of Technology (MIT) engineers have developed a fuel cell that runs on
the same sugar that powers human cells: glucose. This glucose fuel cell could
be used to drive highly efficient brain implants of the future, which could
help paralyzed patients move their arms and legs again.
The
fuel cell, described in PLoS ONE, strips electrons from glucose
molecules to create a small electric current. The researchers, led by Rahul
Sarpeshkar, an associate professor of electrical engineering and computer
science at MIT, fabricated the fuel cell on a silicon chip, allowing it to be
integrated with other circuits that would be needed for a brain implant.
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 used
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. (Platinum has a proven record of long-term biocompatibility within
the body.) So far, the fuel cell can generate up to hundreds of microwatts—enough
to power an ultralow-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 laboratory 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
small.
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.
A
team of researchers at Brown University, Massachusetts General
Hospital, and other
institutions recently demonstrated that paralyzed patients could use a
brain-machine interface to move a robotic arm; those implants have to be
plugged into a wall outlet.
Mimicking biology with microelectronics
Sarpeshkar’s group is a leader in the field of ultralow-power electronics,
having pioneered such designs for cochlear implants and brain implants. “The
glucose fuel cell, when combined with such ultralow-power electronics, can
enable brain implants or other implants to be completely self-powered,” says
Sarpeshkar, author of the book “Ultra Low Power Bioelectronics.” This book
discusses how the combination of ultra-low-power and energy-harvesting design
can enable self-powered devices for medical, bioinspired, and portable
applications.
Sarpeshkar’s
group has worked on all aspects of implantable brain-machine interfaces and
neural prosthetics, including recording from nerves, stimulating nerves,
decoding nerve signals and communicating wirelessly with implants. One such
neural prosthetic is designed to record electrical activity from hundreds of
neurons in the brain’s motor cortex, which is responsible for controlling
movement. That data is amplified and converted into a digital signal so that
computers—or in the Sarpeshkar team’s work, brain-implanted microchips—can
analyze it and determine which patterns of brain activity produce movement.
The
fabrication of the glucose fuel cell was done in collaboration with Jakub
Kedzierski at MIT’s Lincoln Laboratory. “This collaboration with Lincoln Laboratory
helped make a long-term goal of mine—to create glucose-powered bioelectronics—a
reality,” Sarpeshkar says. Although he has just begun working on bringing ultralow-power
and medical technology to market, he cautions that glucose-powered implantable
medical devices are still many years away.
