Medical Design Briefs - June 2022 - 23

Flexible Backing Improves Brain-Computer Interface
The flexible backing allows
arrays of micro-scale
needles to conform to the
contours of the brain.
UC San Diego
San Diego, CA
Engineering researchers have invented
an advanced brain-computer interface
with a flexible and moldable backing and
penetrating microneedles. Adding a
flexible backing to this kind of braincomputer
interface allows the device to
more evenly conform to the brain's complex
curved surface and to more uniformly
distribute the microneedles that pierce
the cortex. The microneedles, which are
10 times thinner than the human hair,
protrude from the flexible backing, penetrate
the surface of the brain tissue without
piercing surface venules, and record
signals from nearby nerve cells evenly
across a wide area of the cortex.
The novel brain-computer interface has
thus far been tested in rodents. It is on par
with and outperforms the Utah Array,
which is the existing gold standard for
brain-computer interfaces with penetrating
microneedles. The Utah Array has been
demonstrated to help stroke victims and
people with spinal cord injury. People with
implanted Utah Arrays can use their
thoughts to control robotic limbs and other
devices to restore some everyday activities
such as moving objects.
The backing of this brain-computer
interface is flexible, conformable, and reconfigurable,
while the Utah Array has a
hard and inflexible backing. The flexibility
and conformability of the backing of
the novel microneedle-array favors closer
contact between the brain and the electrodes,
which allows for better and more
uniform recording of the brain-activity signals.
Working with rodents as model species,
the researchers have demonstrated
stable broadband recordings producing
robust signals for the duration of the implant,
which lasted 196 days.
In addition, the way the soft-backed
brain-computer interfaces are manufactured
allows for larger sensing surfaces,
which means that a significantly larger
area of the brain surface can be monitored
simultaneously. The researchers
demonstrate that a penetrating microMedical
Design Briefs, June 2022
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MDB Tech Briefs 0622_1.indd 23
Artist rendition of the flexible, conformable, transparent backing of the new brain-computer interface
with penetrating microneedles developed by a team led by engineers at the University of California
San Diego in the laboratory of electrical engineering professor Shadi Dayeh. The inset image
on the left shows an illustration of hard-backed Utah Arrays, as a comparison. (Credit: Shadi
Dayeh/UC San Diego/SayoStudio)
needle array with 1,024 microneedles successfully
recorded signals triggered by
precise stimuli from the brains of rats.
This represents 10 times more microneedles
and 10 times the area of brain coverage,
compared to current technologies.
n Thinner and Transparent Backings
These soft-backed brain-computer interfaces
are thinner and lighter than the traditional,
glass backings of these kinds of
brain-computer interfaces. The researchers
note in their paper that light, flexible
backings may reduce irritation of the brain
tissue that contacts the arrays of sensors.
The flexible backings are also transparent.
In the new paper, the researchers
demonstrate that this transparency can
be leveraged to perform fundamental
neuroscience research involving animal
models that would not be possible otherwise.
The team, for example, demonstrated
simultaneous electrical recording
from arrays of penetrating micro-needles
as well as optogenetic photostimulation.
n Two-Sided Manufacturing
The flexibility, larger microneedle array
footprints, reconfigurability, and
transparency of the backings of the new
brain sensors are all thanks to the
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double-sided lithography approach the
researchers used.
Conceptually, starting from a rigid silicon
wafer, the team's manufacturing
process allows them to build microscopic
circuits and devices on both sides of
the rigid silicon wafer. On one side, a
flexible, transparent film is added on top
of the silicon wafer. Within this film, a
bilayer of titanium and gold traces is embedded
so that the traces line up with
where the needles will be manufactured
on the other side of the silicon wafer.
Working from the other side, after
the flexible film has been added, all the
silicon is etched away, except for
free-standing, thin, pointed columns of
silicon. These pointed columns of silicon
are, in fact, the microneedles, and
their bases align with the titanium-gold
traces within the flexible layer that remains
after the silicon has been etched
away.
These titanium-gold traces are patterned
via standard and scalable microfabrication
techniques, allowing scalable
production with minimal manual
labor. The manufacturing process offers
the possibility of flexible array design
and scalability to tens of thousands
of microneedles.
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Medical Design Briefs - June 2022

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