Medical implants sometimes have to be taken out of the body to be recharged, but with this approach, that problem could be solved.

Wireless charging has become increasingly popular in recent years, especially for devices like smartphones — not having to deal with the hassle of cables is a much-appreciated improvement for millions of users. But wireless charging comes with its own set of problems.

Most notably, these chargers only work at a small distance and have poor efficiency as well. There’s another inconvenience for medical devices as well. Most commonly, wireless chargers use metallic (copper) coils to produce an electromagnetic field. This field is charged and transfers energy to a coil in the device that’s being charged, converting the magnetic field into electrical current. But with this method, the charger and the device need to be in contact with each other — which is not an option when the device is inside the body.

To bypass these issues, researchers working at the Korea Institute of Science and Technology (KIST) used ultrasonic waves instead of electromagnetic waves. They created a generator that transmits ultrasonic waves, and a component that converts these waves into electricity. The charging works through water, air, and of course, human tissues.

With this setup, they were able to transfer the energy of eight megawatt-hours to a device at an efficiency of 4%, enough to power up a device like a pacemaker, a sensor, or a drug-delivery tool. For comparison, the efficiency of conventional sonic chargers is around 1%, although the efficiency of regular chargers is over 90%.

“This technology has shown that it is possible for implants to be operated with the acoustic charging method,” KIST researcher Song Hyun-Cheol said in a statement. While more progress is still required before the devices can be used effectively, the team is confident. “Therefore, if we improve the stability and efficiency of the device further, we expect that we will be able to apply the technology to wireless implantable sensors.”

The charging doesn’t generate too much heat and can be used safely inside the human body, the researchers also add. Another advantage of the approach is that ultrasonic waves have been used in various medical scenarios for over a century and it’s a safe and widespread method.

In addition to medical implants, researchers envision that the technique could be used in deep-sea sensors as well — though the efficacy would have to be significantly improved for that.

This isn’t the first time something like this has been attempted. Studies from 2014 and 2016 demonstrated the same technology, but the approach hasn’t picked up too much steam since then.

The study was published in Royal Society of Chemistry.