Sound-powered sensors stand to save millions of batteries

Researchers at ETH Zurich have developed a sensor that utilises energy from sound waves to control electronic devices. This could one day save millions of batteries.

Sensors that monitor infrastructures such as bridges or buildings or that are used in medical devices such as hearing prostheses require constant power. The energy for this usually comes from batteries, which are replaced as soon as they are empty. This creates a huge waste problem. An EU study assumes that 78 million batteries will end up as waste every day in 2025.

A new type of mechanical sensor developed by researchers led by Marc Serra-Garcia and ETH geophysics professor Johan Robertsson could now provide a remedy. They have already applied for a patent for their invention and have now presented the principle in the specialist journal Advanced Functional Materials.

Certain sound waves cause the sensor to vibrate

"The sensor works purely mechanically and does not need an external energy source. It simply uses the vibrational energy contained in sound waves," says Johan Robertsson.

When a certain word is spoken or a tone or noise is heard, the sound waves emitted - and only these - cause the sensor to vibrate. This energy is then sufficient to generate a tiny electrical pulse that switches on an electronic device that is switched off.

The prototype that the researchers developed in Robertsson's laboratory at the Innovation Park in Dübendorf has already been patented. He can distinguish between the spoken words "three" and "four". The word "four" has more sound energy than the word "three", which causes the sensor to resonate. This causes the sensor to vibrate. The word three, on the other hand, does not generate any resonance in the sensor. The word "four" could therefore switch on a device or trigger further processes. Nothing would happen with "three".

Newer variants of the sensor should be able to distinguish between up to twelve different words, such as standard machine commands like "on", "off", "up" or "down". They are also much smaller than the prototype: whereas the latter was the size of the palm of your hand, the new ones are about the size of a thumbnail, and the researchers are aiming for further miniaturization.

Metamaterial without problematic substances

The sensor is a so-called meta-material. It is not the material used that gives it its special properties, but the structure. "The sensor consists only of silicone and contains neither toxic heavy metals nor any rare earths like conventional electronic sensors," emphasizes Serra-Garcia.

The sensor is made up of dozens of identical or similarly structured plates that are connected to each other via tiny bars. These connecting bars act like springs. The researchers used computer models and algorithms to develop the special design of these microstructured platelets and how they are attached to each other. These springs are also decisive as to whether a particular sound source sets the sensor in motion or not.

Monitoring infrastructure

The battery-free sensors can be used in earthquake or building monitoring, for example. Among other things, the sensor could register when a building develops a crack that has the right sound or wave energy.

There is also interest in battery-free sensors for monitoring decommissioned oil wells. Gas can escape from leaks in boreholes, producing a characteristic hissing sound. Such a mechanical sensor could detect this hissing and trigger an alarm without constantly consuming electricity. This would require significantly less maintenance and be cheaper.

Sensor for medical implants

Serra-Garcia also sees applications in medical devices, such as cochlear implants. These prostheses for the deaf require a permanent power supply for signal processing from batteries located behind the ear, where there is no room for large battery packs. The wearers of such devices must therefore replace the batteries every 12 hours. Such sensors could also be used for continuous measurement of eye pressure. "There is not enough space in the eye for a sensor with a battery," says the researcher.

"The industry is also very interested in zero-energy sensors," explains Serra-Garcia. He no longer works at ETH, but is constantly developing mechanical sensors together with his team at the Amolf public research center in the Netherlands. The aim is to launch a solid prototype by 2027. "If we haven't found anyone interested by then, we might found our own start-up."

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