Brillouin Spectroscopy Reveals Mechanical Properties Beyond Hydration

TL;DR

This study demonstrates that Brillouin spectroscopy can reveal complex micromechanical properties of biological samples, influenced by hydration and viscosity, providing a new framework for interpreting spectra beyond simple hydration effects.

Contribution

The paper introduces a comprehensive mechanical interpretation of Brillouin spectra in hydrated biological systems, highlighting the roles of hydration, polymer response, and bulk viscosity.

Findings

Brillouin shifts are modulated by hydration but not solely dominated by water content.

Mechanical response influences Brillouin spectra in complex ways, sometimes positively or negatively correlated with water.

Bulk viscosity significantly contributes to the Brillouin response, affecting interpretation.

Abstract

Characterizing the micromechanical properties of cells and extracellular matrices is critical in mechanobiology. To meet this need, Brillouin light scattering (BLS) has emerged as a noncontact, high-resolution elastography tool that probes the GHz-frequency longitudinal modulus of materials. This longitudinal modulus reflects both elastic and viscous behavior at microscopic scales. However, interpreting Brillouin spectra in biological specimens is challenging: in highly hydrated samples the Brillouin shift is dominated by water dynamics, and the GHz longitudinal modulus does not directly equate to conventional low-frequency stiffness measures (e.g. Young's or shear moduli). Debates remain about how hydration and polymer relaxation influence the Brillouin signal, and how to relate it to macroscopic biomechanics. In particular, the longitudinal viscosity measured by Brillouin scattering includes a contribution from bulk viscosity, which is absent in standard shear rheology and often overlooked. To address these issues, we used different hydrogel systems and solvent mixtures to show Brillouin spectra are modulated but not dominated by hydration. By varying gelatin concentration, we demonstrated that Brillouin shifts can either positively or negatively correlate with water content, depending on the underlying mechanical response. Measurements on ethanol water mixtures further clarify this behavior. Although the Brillouin shift varies strongly with composition, it does not follow water content monotonically and instead reflects changes in the mixture's mechanical properties. By comparing longitudinal, bulk, and shear viscosities, we also showed that bulk viscous dissipation plays a significant role in the Brillouin response. These results established a mechanical framework for interpreting Brillouin spectrum in hydrated and biomolecular systems.