Engineers have created a nano-sized optical fibre that can sense impossibly small forces, from the turbulence generated by swimming bacteria to the sound waves made by the beating of heart cells.

Sensing in biological systems could even allow us to monitor individual cells and alert us to the subtle process of a normal cell turning cancerous.

"This work could open up new doors to track small interactions and changes that couldn't be tracked before," said one of the team, Donald Sirbuly from the University of California San Diego.

Advances in microscope technology have allowed us to delve into the smallest crevices of our physical world, but to truly understand what's going on down there, it takes more than just watching from a distance - we also need to be able to feel what's happening.

Microscopes that can detect incredibly small forces already exist, and the best at doing so - the Atomic Force Microscope (AFM) - is getting better all the time.

Unfortunately, because of the way an atomic force microscopes works, the instrument cannot be scaled down to make it compatible for use in biological systems.

Measuring biological forces in very small vessels required a new approach. The US team created an optical fibre that was made from tin oxide and was 100 times thinner than a human hair - perfect for small sample volumes.

For it to be able to sense, the tin oxide was coated with a thin polymer layer which was then studded with gold nanoparticles.

Using the fibre was very simple: all they had to do was dip the nanoparticle and polymer coated wire into a solution containing live cells or bacteria.

So how does it work?

A light is shone down the optical fibre and interacts with the gold nanoparticles. Biological forces and sounds collide with the gold nanoparticles pushing them ever so slightly into the polymer layer.

Pushing the nanoparticles closer to the fibre causes them to interact more with the light, increasing the intensity of the light detected.

By using this approach, the engineers were able to monitor the small forces caused by the beating of heart cells and by the movement of bacterial flagella.

"We're not just able to pick up these small forces and sounds, we can quantify them using this device. This is a new tool for high-resolution nanomechanical probing," Sirbuly said.

After calibration, the optical fibre technique proved to be 10 times more sensitive than an AFM, and could detect forces less than 160 femtonewtons and sounds less than -30 decibels. That's one thousand time lower than what a human ear can detect.

For reference, the average apple exerts 1 newton of force as it sits here on earth, 100 femtonewtons is ten-trillionths of that force, so a slice of an apple that has been cut into 10 trillion tiny slices.

The technique can detect a range of biological forces and sounds because different polymer coatings can be used to coat the tin oxide fibre.

To measure larger forces, a stiffer polymer coating is required and smaller forces are detected by coating the optical fibre with a really soft material, such as a hydrogel.

In the future, the researchers plan to use the nanofibres to measure bioactivity and the mechanical behaviour of single cells and to improve the fibre's 'listening' capabilities to create ultra-sensitive biological stethoscopes.

Their findings have been published in Nature Photonics.