Before revision: Brillouin microscopy is a new tool to characterize the stiffness of cells and tissues  , however the underlying relationship between stiffness and Brillouin measurements remains unknown. As the stiffness of biological materialsis influenced by hydration, we performed Brillouin microscopy on hydrogels whilst independently examining the role of stiffness and water content. Brillouin measurements were independent of stiffness , but correlated strongly with hydration , indicating that Brillouin microscopy really measures water content .
After revision: Brillouin microscopy is a new tool to characterize the stiffness of cells and tissues , 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 .