On a gray, drizzling winter day in 1896, within the vine-carpeted walls of Toynbee Hall, the London press witnessed what seemed like an acoustic miracle.
Spread out on a table was a curious modular device. It had a towering antenna powered by an induction coil and a telegraph key to switch it on and off. Every time the inventor, a young Italian man with slicked-back hair, pressed the key, a bell would ring across the room. Depending on how long the device stayed on—long press, tap, long press—the bell, in essence, transmitted Morse code.
“All without wires!” the press would later report.
They had witnessed what was one of the first public demonstrations of radio transmission. Although its inventor, Guglielmo Marconi, was hardly the first to experiment with radio waves, he nevertheless pushed the technology from a lab curiosity into a societal necessity.
Thanks to the technology, we’re able to talk across vast distances with our phones, network computers, and gape at the season finale of Game of Thrones—in color! with sound!—on an otherwise black mirror.
But that’s just the beginning. According to Dr. Nian Xiang Sun, an engineer at Northeastern University, radio antennas could soon allow us to communicate with our own bodies.
This week, Sun and his team reported antennas roughly 100 times smaller than their current size. Based on thin films just a few hundred micrometers across—roughly the width of your hair—the ultra-compact antennas “are expected to have great impacts” on tiny brain implants, micro medical devices, or pills with wireless capability that report your health from within.
Miniaturizing traditional antennas has baffled engineers for decades, so the new work is a “huge deal,” agrees Dr. John Domann at Virginia Tech, who was not involved in the work.
How do antennas work, anyways?
It’s all about vibrations.
Traditional antennas, often made of metal, are built to receive and transmit electromagnetic (EM) waves. As these oscillations wash through the metal cable, they shake up electrons within the antenna and produce an electric current.
The current generates a voltage readout, which essentially “translates” EM information into an electrical language.
But here’s the kicker: antennas only work if their length roughly matches up with the wavelength of the EM it picks up. In physics parlance, this is called “resonance.”
As a rule of thumb, a conventional antenna needs to be at least one-tenth the length of the EM wavelength that it’s picking up for a given frequency. The radio part of the spectrum encompasses relatively long wavelengths, anywhere from centimeters to meters. For the most commonly-used part of the radio-wave spectrum—say, cell phones—the antennas need to be at least a few centimeters long to reliably convey the message.
“A lot of people have tried hard to reduce the size of antennas. This has been an open challenge for the whole society,” he says. “We looked into this problem and thought, ‘why don’t we use a new mechanism?’”
A sounding success
Rather than slamming their heads against the limits of physics, Sun’s team turned their attention to a different type of vibrations, acoustic waves.
When matter jiggles, it produces these (sometimes inaudible) “sound waves.” Compared to their EM counterparts, acoustic waves are on a completely different spectrum—sound, not light. Because they travel much slower than light, their wavelengths can be orders of magnitude shorter for any given frequency (frequency = speed/wavelength).
The hack, then, is to efficiently turn EM waves into acoustic ones, which are then converted into voltages. The new antenna does this double-conversion—both ways—via two layers of special material.
When EM waves wash over a thin magnetic film, it jiggles its atoms to switch their magnetic alignments back and forth, which causes it to expand and contract.
Depending on the size and shape of the film, it can efficiently convert different incoming EM wavelengths into corresponding acoustic vibrations.
These acoustic waves then trigger an attached layer of piezoelectric material—stuff that generates electrical voltages when bent or stretched—to change its shape.
Because the resulting acoustic waves oscillate at the same frequency as the incoming EM wave but have a much shorter wavelength, the film only budges a few hundred nanometers to convey the original message.
To emit radio waves the process occurs in reverse: an electric current applied to the piezoelectric film causes it to vibrate, which in turn massages the magnetic layer to generate an oscillating magnetic field—and boom, EM waves!
The biggest challenge, says Sun, was in finding the right materials to make the magnetic film and produce it at high quality. “We needed to ensure high-fidelity, on-chip conversion,” the authors say.
As a proof of concept, the team fabricated two types of their “nanoelectromechanical systems.” One antenna, shaped like a rectangle, efficiently picked up and transmitted megahertz frequencies that underlie TV and radio transmissions. The other—a circular membrane—reacted to resonance frequencies in the gigahertz range, which includes those used for WiFi.
When researchers pitted their system against a traditional loop-shaped antenna of the same size, it transmitted 2.5 gigahertz signals roughly 100,000 times more efficiently.
The antennas are completely passive, without needing an external power source or batteries, the authors noted. What’s more, arrays of similar antennas ranging from tens of megahertz to tens of gigahertz could be manufactured onto the same chip, allowing a single chip to pick up and send out a shockingly broad band of frequencies.
A radiohead future?
While the original concept of a miniature antenna was proposed two years ago, Sun’s prototype brought it “one big step closer to reality,” says Dr. Y. Ethan Wang at UCLA, one of the engineers who helped develop the underlying theory.
While promising, Wang warns of several problems that could trip up the newfangled antenna. For one, it’s not yet clear if the device outperforms conventional metal antennas in every aspect. For another, the device may generate too much heat to be safely used in consumer wearables or smartphones, potentially triggering another exploding phone fiasco.
But Sun has high hopes. “These are the first magnetoelectric antennas that have been demonstrated, which are not perfect,” he says. “We see a lot of room for improvement.”
The team is working with companies to commercialize the technology, and hopes to bring the mini-antennas to use “within two to three years.” While Sun sees “enormous potential” in his invention—smartphones, GPS, WiFi, the Internet of Things—he is particularly excited about using the device for biomedical implants.
Together with neurosurgeons at the Massachusetts General Hospital, Sun is creating bio-compatible brain implants that can read and control neural activity using his tiny antennas.
Transcranial magnetic stimulation, which uses a giant magnetic wand over the head to induce electrical currents within large areas of the brain, has already shown promise for depression, migraines and certain learning disorders. But targeting specific neural networks is close to impossible, and the treatment risks jostling other normally-functioning brain regions.
While deep brain stimulation can overcome the limitation by implanting electrodes directly into the brain, it inevitably causes tissue scarring around the implantation site and renders the device useless.
Sun expects his antenna and future iterations to help.
“Something that’s millimeters or even micrometers in size would make biomedical implantation much easier to achieve, and the tissue damage would be much less,” he says.