Silicon transistors and the brain don’t mix.
At least not optimally. As scientists and companies are increasingly exploring ways to interface your brain with computers, fashioning new hardware that conforms to and compliments our biological wetware becomes increasingly important.
To be fair, silicon transistors, when made into electrode arrays, can
perform the basics: record neural signals, process and analyze them
with increasingly sophisticated programs that detect patterns, which in
turn can be used to stimulate the brain or control smart prosthetics.
The problem? They’re not biocompatible in the long term. Without
modification, implanted electrodes invariably activate the brain’s
immune system, resulting in scar tissue around the implantation site as
the cells eagerly attack the foreign invader.
The trick is to encase them in plastics that the body tolerates. But
if you’ve tried squeezing a sleeve-protective laptop into a small bag,
you’ll know that increasing bulk stretches out the bag (my struggle
everyday). In the case of brain-machine interfacing electronics, brain
tissue is the bag.
To Dr. Dion Khodagholy at Columbia University, the cure isn’t making
smaller transistors—we’ve almost hit the limit. Rather, it’s to
fabricate entirely new transistors that comfortably interface with human
tissue, brain or otherwise. This month, the team described a soft,
flexible, and biocompatible transistor that operates on ions, rather
than electrons in traditional transistors, in Science Advances.
Because neurons rely on ions for their communication, the new
transistors are far more efficient at processing body signals in real
time compared to current generation electronics. In a series of tests,
the team was able to string multiple transistors together to amplify
signals and form logic gates, similar to those used in silicon-based
computing.
The devices, made of flexible, biocompatible materials through
microfabrication, allowed the team to accurately measure EEG “brain
wave” signals without requiring additional adhesives, and lowered the
contact space between gadget and scalp by five times compared to the
usual setup. If that doesn’t seem particularly impressive, the team has
only just begun exploring the potential of their ion-drive transistors.
“Our transistor…makes communication with neural signals of the body
more efficient. We’ll now be able to build safer, smaller, and smarter
bioelectronic devices, such as brain-machine interfaces, wearable
electronics, and responsive therapeutic stimulation devices, that can be
implanted in humans over long periods of time,” said Khodagholy.
How Do Transistors Work Anyways?
By making novel transistors, Khodagholy’s team is digging into the very basics of computation—brain or otherwise.
In a nutshell, a transistor is a mini electrical component that does
two things very well: one, it works as an amplifier to boost input
current, which is what hearing aids or microphones rely on. Two, it
works as a switch, allowing a small current to trigger a larger one—this
is how computer chips work, with their billions of transistors that can
store 0s and 1s and each operating individually.
Silicon transistors, even fancy ones modified for biocompatibility,
require ion-to-electron conversion during operation. They act as
translators to turn the body’s operating language (ions, a type of
charged particle) to one that computers use. Most of them are
susceptible to water damage and need to be sequestered inside a
protective casing, which introduces bulk and decreases performance.
Scientists have been able to minimize some of those issues with
organic electrochemical transistors, which rely on biocompatible
molecules linked to each other to form a “channel” that allows signals
to flow through with the help of external electrolytes—liquids that
conduct electricity. These transistors, however, can’t be individually
controlled, making it impossible to build logic gates and circuits, and
they’re painfully slow compared to the brain’s operations.
To Khodagholy, an ideal transistor for the brain needs four things:
one, it’s built from biocompatible and stable materials; two, it’s soft
and flexible to avoid mechanical mismatches with the brain; three, it
needs high speed and efficient amplification mechanisms that can tease
out and boost useful brain chattering from background noise; and
finally, it has to have independent gating, in the sense that each
transistor can be controlled separately, which allows them to be linked
up into integrated circuits.
Meet the Internal Ion-Gated Organic Electrochemical Transistors (IGTs)
The team’s answer to bio-transistors is the IGT.
In a nutshell, IGTs are built from biocompatible material similar to
those previously used. However, they have mobile ions directly embedded
into the conducting material that makes up the transistor channel. In
this way, they no longer rely on external electrolytes, but are
themselves the full package for conducting information.
The secret ingredient? Sugar.
“Sugar molecules attract water molecules and not only help the
transistor channel to stay hydrated, but also help the ions travel more
easily and quickly within the channel,” Khodagholy explained.
Because the mobile ions are directly in the transistor channels, they
don’t have to travel far to modulate the transistor compared to
external electrolytes—the typical solution. This makes the IGT respond
orders of magnitude faster than electrolyte-gated transistors to changes
in external signaling, said study author Dr. George D. Spyropoulos.
Sticking sugar, which provides an ion reservoir, directly into the
transistor had another perk: it allowed each transistor to be made
independent. Rather than bathing in and sharing external electrolytes,
IGTs have the capacity to have their own gates—that is, a membrane that
controls whether they’re on or off. In one experiment, the team
microfabricated two separate logic gates and confirmed that each
operated accurately, performing their intended arithmetic.
This confirms “the scalability of IGT architecture for use as bioelectronics computational modules,” they said.
In another study, the team found that the devices could reliably
amplify tiny signals by as much as four-fold. Because neural and other
body signals often require multi-stage boosting before they’re
accurately picked up and deciphered, IGTs seem perfectly suited for the
job.
The No-Fuss EEG
As a proof of concept of IGT’s biocompatibility, the team turned its
focus on EEG. Widely used in clinics and labs, EEG picks up brain waves
using a cap of electrodes on the surface of the scalp.
It’s not a fun process: the scalp often has to be exfoliated (ouch!)
and an adhesive is used to better stick on the metal electrodes, which
causes irritation at best and rashes at worst. Hair also gets in the way
and muddies signals.
IGT, in contrast, is a dream. Its small size meant that the team
could slip it between hair follicles. Its flexibility and bendiness made
it possible to slap it straight onto the scalp—no pretreatments
required. In a test that measures brain signals when people are awake
with their eyes closed, IGTs reliably and consistently picked up the
brain’s activity.
A portable, lightweight EEG device that can be manipulated by hand could already change the future of neurology.
But that’s just the beginning. The tiny size of IGTs means it will be
possible to apply more devices to smaller areas to measure signals at a
finer scale, or stick them into areas normally too small or irregular to
accommodate electrodes. Because they’re intrinsically soft,
conformable, and biocompatible, they can be used on extremely delicate
tissue, such as a newborn’s scalp or inside the brain.
But most importantly, said study author Dr. Jennifer Gelinas, IGTs
can perform circuit computations. This means they could one day be part
of a closed-loop system capable of detecting the brain’s electrical
patterns and stimulate accordingly, with far less risk than current
electrode-based interfaces.
“With such speed and amplification, combined with their ease of
microfabrication, these transistors could be applied to many different
types of devices. There is great potential for the use of these devices
to benefit patient care in the future,” said Khodagholy.
