In 1945, a hand-carved wooden copy of the Great Seal of the United States was hung in the residential study of the U.S. ambassador to the Soviet Union. The detailing was remarkable—each feather on the eagle’s wings delicately defined, its beak curved just so. But this wasn’t what made the piece astounding.

It was also a bug.

The seal was presented to the ambassador as a peaceful gesture by a delegation from the Young Pioneer Organization of the Soviet Union. But tucked within the artful work, just under the bird’s beak, was a listening device. It had no battery or active electronics, making it virtually undetectable. Indeed, it wasn’t found for seven years.

Now, a team of researchers at the University of Washington in Seattle are once again harnessing similar technology for an equally astounding, but less covert, operation: battery-free cell phones.

Though it’s unlikely the design will ever replace our power-hungry smart phones, the technology has the promise to help in a pinch. “Let's say you're stranded somewhere and your phone runs out of battery,” says Vamsi Talla, who worked on the project while he was a research associate at the University of Washington. “You still could make a 911 call, which could be a lifesaver.”

With a regular cell phone, there are two things that consume a lot of power, says Talla. First, your phone has to convert your voice (an analog signal) into a digital signal. Second, it transmits that signal in the form of radio frequencies to a tower.

So the team stripped these components from their redesigned phone. Instead, they used the basic principals of the Soviet-era technology to rework how the device functions.

In the case of the Great Seal Bug, voices within the room caused a small diaphragm hidden in the structure to vibrate and change the resonance of the internal cavity. To listen in, eager Soviet agents on the street would just need to focus the right frequency of radio waves in the direction of the seal, which activated an antenna inside the bug. Then they could collect the reflections bounced back—also known as backscatter.

In the case of the battery-free phone, a radio signal is constantly emitted from a remote base station. When a person speaks into the device, these frequencies are reflected back to the base station. It is then the base station’s task to connect to the cellular network and transmit the call.

“You can't break the laws of physics,” says Talla. “If something has to do a lot of work then it needs a lot of power.” So instead of the phone doing most of the work, the base station becomes a communication powerhouse. The phone itself consumes just a few microwatts of power, harvesting its energy either from ambient radio waves or light. For solar power, the device uses a tiny (1 cm square) photodiode.

For now, the system is fairly rudimentary—built completely with off-the-shelf components. The user wears headphones to listen into the call. It has no screen and looks a bit like a computer board with a touch-sensitive number pad. It’s also more like a walkie-talkie than a phone, says Talla. “You can either talk or receive at a single point of time.” To talk, the user presses a button and speaks into the gadget.

For these early prototypes, which the researchers have applied for a patent for, the sound quality isn’t great and the user must be within 50 feet of the base station for the phone to work. Though the researchers hope to improve the range, the phone is likely not something that people in remote locations will easily be able to turn to if trouble strikes.

“Overall what they are doing is very interesting and they are pursuing a goal that everyone would love and kill for,” says David Blaauw, an electrical engineer whose research at the University of Michigan emphasizes low-power design. “It’s a very nice implementation and it’s a very complete system that they are demonstrating, but there are some issues that appear to be somewhat fundamental to the design.”

For one, says Blaauw, who was not involved in the research, the range of the phone is going to be difficult to scale up because signal strength strongly degrades with distance. And using backscattering, the signal has to travel pretty far. “The signal has to go from the basestation to the cell phone and then get reflected passively and then travel back to the base station,” he says. “That’s why they get tens of meters not hundreds of meters or kilometers, which is what modern cell phones get.” In fact, telecommunications experts say that a modern cell phone can connect to a tower more than 20 miles away.

The other issue, says Blaauw, could be interference. Since the base station is working to pick out the small change in passively reflected radio waves, multiple phones working simultaneously could make it challenging to pick out the correct signal in all the noise. But Talla says that they are working to solve this issue in future prototypes, using the base station for additional processing and filtering.

The team is continuing their work on the battery-free phone in cooperation with Jeeva Wireless, a spin-off company from the lab’s research focused on commercializing ultra-low power Wi-Fi. Talla, currently the CTO of Jeeva, says that they plan to incorporate a camera for pictures and texting capabilities using an E-ink screen—the same technology currently seen in Kindle and other E-readers. Another possible future for the gadget would be to incorporate it in a regular smart phone, says Talla. That way, if the battery dies, the user still could make emergency calls. Either way, they hope to improve the quality of calls by developing a microphone specifically designed for backscatter.

According to Talla, the device has a bright future with next-generation technologies. “An option of 5G would make our tech more practical,” he says. With 5G, companies are planning to move cellular base stations much closer together to allow for higher data rates. For the battery-free cell phone, the shift to closer cellular towers would increase coverage, particularly in urban environments.

Though the idea is an intriguing one, it will be a long time before battery-free cell phones make it to market. For now, at least, you’ll still have to keep that extra charging cable on hand.