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.
After revision: Brillouin microscopy is a new tool to characterize the stiffness of cells and tissues . Using hydrogels with tunable properties, we demonstrate that Brillouin microscopy is insensitive to stiffness of hydrated materials , however the underlying relationship between stiffness and Brillouin measurements remains unknown.