Sensors used to monitor infrastructures such as bridges and buildings, or in medical devices such as prosthetics for the hearing impaired, require a constant power source. Typically this power is provided by batteries that are discarded and replaced when discharged, leading to a major waste problem.

An EU study predicts 78 million batteries will be used every day by 2025 will be thrown into the trash.

A new type of mechanical sensor developed by researchers led by Marc Serra-Garcia and ETH geophysics professor Johan Robertsson may now be the solution. Its creators have already filed a patent for their invention and have presented the principle in a journal. Advanced Functional Materials.

Some sound waves cause the sensor to vibrate

“The sensor works completely mechanically and does not require an external energy source. It only uses the vibration energy contained in sound waves,” says Robertsson.

Every time a particular word is spoken or a particular tone or noise is produced, the sound waves emitted (and only those) cause the sensor to vibrate. This energy is enough to create a small electrical pulse that turns on an electronic device that is then turned off.

The prototype developed by the researchers in Robertsson’s laboratory at the Zurich Swiss Innovation Park in Dübendorf has already been patented. Can distinguish the spoken words “three” and “four”. The word “four” has more sound energy resonating with the sensor compared to the word “three”, causing the sensor to vibrate, while the word “three” does not. This means that the word “four” can turn on the device or start further processes. Nothing will happen to the “three”.

Newer versions of the sensor should be able to distinguish up to twelve different words, such as standard machine commands such as “on”, “off”, “up” and “down”. Compared to the palm-sized prototype, the new versions are much smaller (about the size of a miniature) and the researchers aim to miniaturize them even further.

Metamaterial free of problematic substances

The sensor is something called a metamaterial: The special properties of the sensor are given by the structure, not by the material used. “Our sensor is made entirely of silicon and contains neither toxic heavy metals nor rare earth elements like traditional electronic sensors,” says Serra-Garcia.

The sensor consists of dozens of identical or similar build plates that are connected together with the help of small rods. These connecting rods act as springs. The researchers used computer modeling and algorithms to design the specific design of these microstructured plates and determine how they would be connected together. They are springs that determine whether the sensor will be activated by a particular sound source.

Monitoring infrastructure

Potential uses for these battery-free sensors include earthquake or building monitoring. For example, they can record when a crack with the right sound or wave energy occurs in a building.

There is also interest in battery-free sensors for monitoring decommissioned oil wells. Gas can escape through leaks in wells, creating a characteristic hissing sound. Such a mechanical sensor could detect this hiss and trigger the alarm without constantly consuming electricity, making it much cheaper and requiring much less maintenance.

Sensor for medical implants

Serra-Garcia also sees applications in medical devices such as cochlear implants. These prosthetics for the deaf require constant power to process signals from batteries. The power units are located behind the ear, where there is no room for large batteries. This means that owners of such devices need to change batteries every twelve hours. The new sensors can also be used to continuously measure eye pressure. “There isn’t enough room in the eye for a battery-powered sensor,” she says.

“There is also a lot of interest in zero energy sensors in the industry,” adds Serra-Garcia. He no longer works at ETH but at AMOLF, a public research institute in the Netherlands, where he and his team develop mechanical sensors. Their goal is to launch a reliable prototype by 2027. “If we don’t get anyone’s attention by then, we can start our own initiative.”