Mini-antennas could power brain-computer interfaces, medical devices

Engineers have figured out how to make antennas for wireless communication 100 times smaller than their current size, an advance that could lead to tiny brain implants, micro–medical devices, or phones you can wear on your finger. The brain implants in particular are “like science fiction,” says study author Nian Sun, an electrical engineer and materials scientist at Northeastern University in Boston. But that hasn’t stopped him from trying to make them a reality.

The new mini-antennas play off the difference between electromagnetic (EM) waves, such as light and radio waves, and acoustic waves, such as sound and inaudible vibrations. EM waves are fluctuations in an electromagnetic field, and they travel at light speed—an astounding 300,000,000 meters per second. Acoustic waves are the jiggling of matter, and they travel at the much slower speed of sound—in a solid, typically a few thousand meters per second. So, at any given frequency, an EM wave has a much longer wavelength than an acoustic wave.

Antennas receive information by resonating with EM waves, which they convert into electrical voltage. For such resonance to occur, a traditional antenna's length must roughly match the wavelength of the EM wave it receives, meaning that the antenna must be relatively big. However, like a guitar string, an antenna can also resonate with acoustic waves. The new antennas take advantage of this fact. They will pick up EM waves of a given frequency if its size matches the wavelength of the much shorter acoustic waves of the same frequency. That means that that for any given signal frequency, the antennas can be much smaller.

The trick is, of course, to quickly turn the incoming EM waves into acoustic waves. To do that, the two-part antenna employs a thin sheet of a so-called piezomagnetic material, which expands and contracts when exposed to a magnetic field. If it's the right size and shape, the sheet efficiently converts the incoming EM wave to acoustic vibrations. That piezomagnetic material is then attached to a piezoelectric material, which converts the vibrations to an oscillating electrical voltage. When the antenna sends out a signal, information travels in the reverse direction, from electrical voltage to vibrations to EM waves. The biggest challenge, Sun says, was finding the right piezomagnetic material—he settled on a combination of iron, gallium, and boron—and then producing it at high quality.

The team created two kinds of acoustic antennas. One has a circular membrane, which works for frequencies in the gigahertz range, including those for WiFi. The other has a rectangular membrane, suitable for megahertz frequencies used for TV and radio. Each is less than a millimeter across, and both can be manufactured together on a single chip. When researchers tested one of the antennas in a specially insulated room, they found that compared to a conventional ring antenna of the same size, it sent and received 2.5 gigahertz signals about 100,000 times more efficiently, they report today in Nature Communications .

“This work has brought the original concept one big step closer to reality,” says Y. Ethan Wang, an electrical engineer at the University of California, Los Angeles, who helped develop the idea, but did not work on the new study. Rudy Diaz, an electrical engineer at Arizona State University in Tempe, likes the concept and execution, but he suspects that in a consumer device or inside the body the antennas will give off too much heat because of their high energy density. Wang notes that the acoustic antennas are tricky to manufacture, and in many cases larger conventional antennas will do just fine.

Still, Sun is pursuing practical applications. Tiny antennas could reduce the size of cellphones, shrink satellites, connect tiny objects to the so-called internet of things, or be swallowed or implanted for medical monitoring or personal identification. He’s shrinking kilohertz-frequency antennas—good for communicating through the ground or water—from cables thousands of meters long to palm-sized devices. Such antennas could link people on Earth’s surface to submarines or miners. With a neurosurgeon at Massachusetts General Hospital, he’s also creating brain implants for reading or controlling neural activity—helpful for diagnosing and treating people with epilepsy, or eventually for building those sci-fi brain-computer interfaces.