Body Talk With Magnets

Photo: UC San Diego Jacobs School of Engineering

Want to send your total number of steps from your Fitbit without eating up battery life, or communicate between your iPhone and your Apple watch with no fear of eavesdropping? Engineers in California say the best way for wearable devices to talk amongst themselves is by sending magnetic fields through the wearer’s body.

The shortest path for data to travel from one wearable device to another is a straight line, but generally that means going through the body. Because radio doesn’t travel easily through tissue, Bluetooth or WiFi signals have to take a longer path around the body, and that requires more power. A signal carried by a magnetic field, on the other hand, could travel from, say, one hand to the other without interference from the torso in between, says Patrick Mercier, associate director of the Center for Wearable Sensors at the University of California, San Diego.

The difference is significant. Path loss in a Bluetooth device operating at 2.4 GHz is approximately 70 dB. Mercier and his team designed a prototype magnetic transmitter that had a path loss of only about 10 dB. Because decibels are measured on a logarithmic scale, that’s a huge reduction. “The loss with this is up to 10 million times lower than in Bluetooth,” says Mercier, who presented the research at the conference of the IEEE Engineering in Medicine and Biology Society, in Milan, Italy, in August.

And while a Class 2 Bluetooth headset transmits a signal out to about 10 meters away, the magnetic field barely extends beyond the body, making it difficult for someone to tap into the signal.

To demonstrate their idea, the researchers wrapped insulated copper wires around the heads, arms, and legs of lab members and measured the loss. It generally takes a coil to generate a magnetic field, so devices such as smartwatches, which are already designed to encircle a body part, could easily be built with such a system, Mercier says. Other devices, such as smartphones and headphones, already have coils in them. But with those, engineers would have to deal with the fact that they are generally worn flat against the body rather than around it, and the magnetic field would be oriented differently.

As for whether people would object to having magnetic signals passed through their bodies, Mercier points out that that the transmission would be far weaker than the Earth’s magnetic field, which we live in without harm. “If there’s any sort of field that you want to put in your body, it’s going to be a magnetic field, because biological tissues don’t respond to magnetic fields in any significant way,” he says.

There might be a problem with interference between this system and a medical implant, such as a pacemaker or insulin pump, but Mercier says that’s just a design issue. “We could engineer it so that there is no effect,” he says.

Mercier hopes to build a practical version of his transmitter within a couple of years.



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