Brillouin microscopy, what is it really measuring?

TL;DR

Brillouin microscopy primarily measures water content and compressibility in hydrated materials, not stiffness, challenging its use as a direct measure of tissue or cell stiffness.

Contribution

This study clarifies that Brillouin microscopy's signals are dominated by water content, not stiffness, providing a new understanding of what the technique actually measures.

Findings

Brillouin measurements depend strongly on water content.

Correlations with stiffness disappear when hydration is controlled.

Theoretical model supports biphasic compressibility influence.

Abstract

Brillouin microscopy measures compressibility, but is being increasingly used to assess stiffness of cells and tissues. Using hydrogels with tunable properties, we demonstrate that Brillouin microscopy is insensitive to stiffness of hydrated materials, but depends strongly on water content, consistent with a theoretical model of biphasic compressibility. Empirical correlations between Brillouin measurements and stiffness arise due to their mutual dependence on water content, but correlations vanish once hydration is controlled.