Popular Mechanics - USA (2022-01 & 2022-02)

January/February 2022 9

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nickel oxide as it reacted with oxygen in the air.

“You can think of the material as breathing oxy-

gen in and out,” explains Ramanathan. When

oxygen left the nickel oxide for the surrounding

air, that chemical reaction left electrons behind

in the semiconductor. And when the inf lux of oxy-

gen gas entered the material, it bonded with those

extra electrons. Those changes in the availability

of electrons, he says, changed how easy it was for an

electrical current to travel within the nickel oxide.

The team repeatedly added and withdrew the

hydrogen gas, reducing the time between exposures,

to see how this reaction changed over time. The first

time the nickel oxide was exposed to hydrogen gas,

it reacted strongly. After 15 minutes, around 99.

percent less electrical current was passing through.

But if scientists shortened the period between

hydrogen exposures to between 45 and 15 seconds,

they found there wasn’t as much oxygen available

to react with the second round of hydrogen. That

meant the electrical resistance didn’t change as

much during subsequent rounds—mirroring the

reaction-damping effect of habituation.

Researchers then sensitized the material using

ozone, a highly reactive gas made of three oxygen

molecules. When immersed in ozone, the nickel

oxide took in oxygen quickly, decreasing its electri-

cal resistance. Next time it was exposed to hydrogen

gas, there was more oxygen to attract the hydrogen—

and a stronger change in electrical resistance.

“This is an exact analog to biological systems,”

says Hai (Helen) Li, Ph.D., a professor of electri-

cal and computer engineering at Duke University,

who was a Ph.D. student of one of the co-authors

but did not work on the study. “[Humans] have

sensors and receive the external signals, and then

we process it immediately; it doesn’t have to pass

through something else.” Right now, machines

use different systems to receive information, then

process it—like how a camera takes in light and

then processes it into a photo. But in this study,

the changes in the nickel oxide’s electrical resis-

tance were a direct reaction to the gases in its

environment. Neuromorphic computing could

similarly enhance a computer’s ability to receive

and process information—when applied to devices

like cameras, these machines could become

smaller and more efficient. More generally, says

Li, our brains use very little power in comparison

to computers; successfully mimicking the brain’s

processes could therefore save energy.

Other devices could use the technology to self-

tailor to the consumers’ needs. Medical implants

with neuromorphic computing, says Li, could send

signals along an injured nerve to help people regain

use of paralyzed fingers or toes. Independent from

human programmers, such a device would make

its own decisions based on biological f luctuations.

Consumer products will require greater

advancement in the neuromorphic computing field

than this sole study, however. Here, habituation

and sensitization appeared as chemical reactions

to hydrogen and ozone, but computers run on an

electric current rather than hydrogen or ozone gas.

“Being able to do all of this by electrical stimulus

will be fascinating,” Ramanathan says. “Then you

can start to use traditional stimulus that is histor-

ically used in electronics.”

Shriram Ramanathan

and his colleagues

modeled their nickel

oxide experiments

after experiments pre-

viously conducted with

sea slugs in the genus

Aplysia. For centuries,

scientists have used

these slugs to study

learning, behavior,

and memory.

The slugs can

retain information for

weeks—a killer “long-

term” memory for an

organism with a year-

long life span—and

display a relatively high

level of neural plas-

ticity. It may not make

them “intelligent,” but

it does make them the

perfect subject for

researchers teasing

out the origins of

fundamental learning

behaviors like habitua-

tion and sensitization.

Aplysia’s colossal

neurons, the largest in

the animal kingdom,

can grow to 1 mm in

length. The neurons’

gargantuan size make

it easier for scientists

to physically see and

study physiological

responses to stimuli.

WHAT WE’RE

LEARNING

FROM SEA

SLUGS’ GIANT

NEURONS