When light interacts with matter it is far from a passive process. We might see it enter and pass through, perhaps not as bright as it entered, but to all intents and purposes, the same.

However, if there was a way to look at the interactions at the scale of photons and molecules, you would see a hive of activity as photons are scattered and absorbed by the constituent components.

Some materials have little effect on the photons, which pass through as if the material was not there. In others, the incident photon can exchange energy and momentum with the material and generate scattered light at slightly different wavelengths. We can observe this using a spectrometer, which measures the frequency shift and spectral broadening caused by these scattering events.

This is the basic principle of Brillouin spectroscopic imaging, which provides a powerful, non-destructive way of studying the mechanical properties of biological samples. Within the sample there are localised vibrations in the constituent components, stimulated by heat, known as thermal phonons. A small fraction of the incoming light interacts with these phonons and is scattered. The Brillouin frequency shift depends on the speed of sound in the material, which increases in stiffer materials, making it a proxy for local mechanical stiffness as described above. Different mechanical environments influence the resulting frequency shift and linewidth, allowing spatial maps of material properties within the sample.

More formally, the shift is related to the longitudinal elastic modulus, which describes resistance to compression. The linewidth provides additional information about how quickly these mechanical vibrations are damped, reflecting viscous or fluid-like behaviour.

Together, these parameters allow Brillouin microscopy to generate high-resolution maps of mechanical properties across cells, hydrogels, and tissues.

Importantly, this enables researchers to study how mechanical properties vary within living systems in a label-free and minimally invasive way, offering new insight into processes such as cell differentiation, tissue development, and disease progression.

This is particularly important for the Mainstream hub, where the focus is on developing optimal conditions for culturing delicate stem cells for therapeutic use. There is a strong relationship between the mechanical properties of a cell’s environment and its differentiation pathway. Brillouin microscopy allows direct characterisation and optimisation of the mechanical properties of both the culture environment and live stem cells simultaneously.