Because SWANS implants are built from passive components, they draw almost no power while listening, extending battery life more than 15 times compared to Bluetooth and NFC. A complete implant with a battery measures 3 by 1.1 by 17 millimeters and fits through a 6-gauge needle.

To test the system, Abramson and his colleagues installed it in chicken breasts, skin-on, bone-in pork bellies, and living rats.

Twitching legs

“Pork belly is a very thick and heterogeneous tissue,” Abramson said. The signal had to get through fat, muscle, bone, and skin. A single 10-volt pulse produced a detectable voltage gradient more than 30 centimeters across the tissue and up to 14 centimeters deep—more than 10 times the coverage of Bluetooth or NFC.

In live rats, the signals reached implants under the skin, in the abdominal cavity, and even in the stomach. Placement didn’t matter much either. “We put our wearable on the stomach of the rat, and then we were able to get actuation in the back of the rat,” Abramson said.

The most advanced experiment used strain sensors placed on the rats’ front legs. When a sensor detected the left or right forelimb moving, the hub sent a limb-specific pulse through the body that reached an implant on the corresponding hind leg, which stimulated the sciatic nerve and made that leg twitch. The team also built an implant-to-implant relay where a device with a temperature sensor passed a signal to a second implant only when it registered a fever above 40° C (104° F).

Abramson notes that pacemakers and neurostimulators already use similar voltages and pulse lengths, so the team did not expect any major safety issues. In a two-month study in rats, scar tissue up to a millimeter thick grew around the implants, but by raising the voltage within safe limits, the team kept communication going throughout. The pulses didn’t stimulate nerves other than the targeted one, didn’t change the heart’s electrical activity, and caused no more cell death or oxidative stress than needles and implants that weren’t electrified.