Energy-harvesting design aims to turn Wi-Fi signals into usable power

Device for harnessing terahertz radiation might enable self-powering implants, cellphones, other portable electronics.

YAMIL LAGE/AFP via Getty Images
Any device that sends out a Wi-Fi signal also emits terahertz waves — electromagnetic waves with a frequency somewhere between microwaves and infrared light.

These high-frequency radiation waves, known as "T-rays," are also produced by almost anything that registers a temperature, including our own bodies and the inanimate objects around us.

Terahertz waves are pervasive in our daily lives, and if harnessed, their concentrated power could potentially serve as an alternate energy source. Imagine, for instance, a cellphone add-on that passively soaks up ambient T-rays and uses their energy to charge your phone. However, to date, terahertz waves are wasted energy, as there has been no practical way to capture and convert them into any usable form.

Now physicists at MIT have come up with a blueprint for a device they believe would be able to convert ambient terahertz waves into a direct current, a form of electricity that powers many household electronics.

Their design takes advantage of the quantum mechanical, or atomic behavior of the carbon material graphene. They found that by combining graphene with another material, in this case, boron nitride, the electrons in graphene should skew their motion toward a common direction. Any incoming terahertz waves should "shuttle" graphene's electrons, like so many tiny air traffic controllers, to flow through the material in a single direction, as a direct current.

The researchers have published their results in the journal Science Advances, and are working with experimentalists to turn their design into a physical device.

"We are surrounded by electromagnetic waves in the terahertz range," says lead author Hiroki Isobe, a postdoc in MIT's Materials Research Laboratory. "If we can convert that energy into an energy source we can use for daily life, that would help to address the energy challenges we are facing right now."

Isobe's co-authors are Liang Fu, the Lawrence C. and Sarah W. Biedenharn Career Development Associate Professor of Physics at MIT; and Su-yang Xu, a former MIT postdoc who is now an assistant professor chemistry at Harvard University.

Breaking graphene's symmetry

Over the last decade, scientists have looked for ways to harvest and convert ambient energy into usable electrical energy. They have done so mainly through rectifiers, devices that are designed to convert electromagnetic waves from their oscillating (alternating) current to direct current.

Most rectifiers are designed to convert low-frequency waves such as radio waves, using an electrical circuit with diodes to generate an electric field that can steer radio waves through the device as a DC current. These rectifiers only work up to a certain frequency, and have not been able to accommodate the terahertz range.

A few experimental technologies that have been able to convert terahertz waves into DC current do so only at ultracold temperatures — setups that would be difficult to implement in practical applications.

Instead of turning electromagnetic waves into a DC current by applying an external electric field in a device, Isobe wondered whether, at a quantum mechanical level, a material's own electrons could be induced to flow in one direction, in order to steer incoming terahertz waves into a DC current.

Such a material would have to be very clean, or free of impurities, in order for the electrons in the material to flow through without scattering off irregularities in the material. Graphene, he found, was the ideal starting material.

To direct graphene's electrons to flow in one direction, he would have to break the material's inherent symmetry, or what physicists call "inversion." Normally, graphene's electrons feel an equal force between them, meaning that any incoming energy would scatter the electrons in all directions, symmetrically. Isobe looked for ways to break graphene's inversion and induce an asymmetric flow of electrons in response to incoming energy.

Looking through the literature, he found that others had experimented with graphene by placing it atop a layer of boron nitride, a similar honeycomb lattice made of two types of atoms — boron and nitrogen. They found that in this arrangement, the forces between graphene's electrons were knocked out of balance: Electrons closer to boron felt a certain force while electrons closer to nitrogen experienced a different pull. The overall effect was what physicists call "skew scattering," in which clouds of electrons skew their motion in one direction.

Isobe developed a systematic theoretical study of all the ways electrons in graphene might scatter in combination with an underlying substrate such as boron nitride, and how this electron scattering would affect any incoming electromagnetic waves, particularly in the terahertz frequency range.

He found that electrons were driven by incoming terahertz waves to skew in one direction, and this skew motion generates a DC current, if graphene were relatively pure. If too many impurities did exist in graphene, they would act as obstacles in the path of electron clouds, causing these clouds to scatter in all directions, rather than moving as one.

"With many impurities, this skewed motion just ends up oscillating, and any incoming terahertz energy is lost through this oscillation," Isobe explains. "So we want a clean sample to effectively get a skewed motion."

One direction

They also found that the stronger the incoming terahertz energy, the more of that energy a device can convert to DC current. This means that any device that converts T-rays should also include a way to concentrate those waves before they enter the device.

With all this in mind, the researchers drew up a blueprint for a terahertz rectifier that consists of a small square of graphene that sits atop a layer of boron nitride and is sandwiched within an antenna that would collect and concentrate ambient terahertz radiation, boosting its signal enough to convert it into a DC current.

"This would work very much like a solar cell, except for a different frequency range, to passively collect and convert ambient energy," Fu says.

The team has filed a patent for the new "high-frequency rectification" design, and the researchers are working with experimental physicists at MIT to develop a physical device based on their design, which should be able to work at room temperature, versus the ultracold temperatures required for previous terahertz rectifiers and detectors.

"If a device works at room temperature, we can use it for many portable applications," Isobe says.

He envisions that, in the near future, terahertz rectifiers may be used, for instance, to wirelessly power implants in a patient's body, without requiring surgery to change an implant's batteries. Such devices could also convert ambient Wi-Fi signals to charge up personal electronics such as laptops and cellphones.

"We are taking a quantum material with some asymmetry at the atomic scale, that can now be utilized, which opens up a lot of possibilities," Fu says.

This research was funded in part by the U.S. Army Research Laboratory and the U.S. Army Research Office through the Institute for Soldier Nanotechnologies (ISN).

Reprinted with permission of MIT News. Read the original article.

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New research establishes an unexpected connection.

Reactive oxygen species (ROS) accumulate in the gut of sleep-deprived fruit flies, one (left), seven (center) and ten (right) days without sleep.

Image source: Vaccaro et al, 2020/Harvard Medical School
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We don't have to tell you what it feels like when you don't get enough sleep. A night or two of that can be miserable; long-term sleeplessness is out-and-out debilitating. Though we know from personal experience that we need sleep — our cognitive, metabolic, cardiovascular, and immune functioning depend on it — a lack of it does more than just make you feel like you want to die. It can actually kill you, according to study of rats published in 1989. But why?

A new study answers that question, and in an unexpected way. It appears that the sleeplessness/death connection has nothing to do with the brain or nervous system as many have assumed — it happens in your gut. Equally amazing, the study's authors were able to reverse the ill effects with antioxidants.

The study, from researchers at Harvard Medical School (HMS), is published in the journal Cell.

An unexpected culprit

The new research examines the mechanisms at play in sleep-deprived fruit flies and in mice — long-term sleep-deprivation experiments with humans are considered ethically iffy.

What the scientists found is that death from sleep deprivation is always preceded by a buildup of Reactive Oxygen Species (ROS) in the gut. These are not, as their name implies, living organisms. ROS are reactive molecules that are part of the immune system's response to invading microbes, and recent research suggests they're paradoxically key players in normal cell signal transduction and cell cycling as well. However, having an excess of ROS leads to oxidative stress, which is linked to "macromolecular damage and is implicated in various disease states such as atherosclerosis, diabetes, cancer, neurodegeneration, and aging." To prevent this, cellular defenses typically maintain a balance between ROS production and removal.

"We took an unbiased approach and searched throughout the body for indicators of damage from sleep deprivation," says senior study author Dragana Rogulja, admitting, "We were surprised to find it was the gut that plays a key role in causing death." The accumulation occurred in both sleep-deprived fruit flies and mice.

"Even more surprising," Rogulja recalls, "we found that premature death could be prevented. Each morning, we would all gather around to look at the flies, with disbelief to be honest. What we saw is that every time we could neutralize ROS in the gut, we could rescue the flies." Fruit flies given any of 11 antioxidant compounds — including melatonin, lipoic acid and NAD — that neutralize ROS buildups remained active and lived a normal length of time in spite of sleep deprivation. (The researchers note that these antioxidants did not extend the lifespans of non-sleep deprived control subjects.)

fly with thought bubble that says "What? I'm awake!"

Image source: Tomasz Klejdysz/Shutterstock/Big Think

The experiments

The study's tests were managed by co-first authors Alexandra Vaccaro and Yosef Kaplan Dor, both research fellows at HMS.

You may wonder how you compel a fruit fly to sleep, or for that matter, how you keep one awake. The researchers ascertained that fruit flies doze off in response to being shaken, and thus were the control subjects induced to snooze in their individual, warmed tubes. Each subject occupied its own 29 °C (84F) tube.

For their sleepless cohort, fruit flies were genetically manipulated to express a heat-sensitive protein in specific neurons. These neurons are known to suppress sleep, and did so — the fruit flies' activity levels, or lack thereof, were tracked using infrared beams.

Starting at Day 10 of sleep deprivation, fruit flies began dying, with all of them dead by Day 20. Control flies lived up to 40 days.

The scientists sought out markers that would indicate cell damage in their sleepless subjects. They saw no difference in brain tissue and elsewhere between the well-rested and sleep-deprived fruit flies, with the exception of one fruit fly.

However, in the guts of sleep-deprived fruit flies was a massive accumulation of ROS, which peaked around Day 10. Says Vaccaro, "We found that sleep-deprived flies were dying at the same pace, every time, and when we looked at markers of cell damage and death, the one tissue that really stood out was the gut." She adds, "I remember when we did the first experiment, you could immediately tell under the microscope that there was a striking difference. That almost never happens in lab research."

The experiments were repeated with mice who were gently kept awake for five days. Again, ROS built up over time in their small and large intestines but nowhere else.

As noted above, the administering of antioxidants alleviated the effect of the ROS buildup. In addition, flies that were modified to overproduce gut antioxidant enzymes were found to be immune to the damaging effects of sleep deprivation.

The research leaves some important questions unanswered. Says Kaplan Dor, "We still don't know why sleep loss causes ROS accumulation in the gut, and why this is lethal." He hypothesizes, "Sleep deprivation could directly affect the gut, but the trigger may also originate in the brain. Similarly, death could be due to damage in the gut or because high levels of ROS have systemic effects, or some combination of these."

The HMS researchers are now investigating the chemical pathways by which sleep-deprivation triggers the ROS buildup, and the means by which the ROS wreak cell havoc.

"We need to understand the biology of how sleep deprivation damages the body so that we can find ways to prevent this harm," says Rogulja.

Referring to the value of this study to humans, she notes,"So many of us are chronically sleep deprived. Even if we know staying up late every night is bad, we still do it. We believe we've identified a central issue that, when eliminated, allows for survival without sleep, at least in fruit flies."

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