Brillouin biomechanics beyond static elasticity. A relaxation-time framework for biological matter

Brillouin light scattering (BLS) has transitioned from condensed matter physics into a premier non-contact, optical modality for the three-dimensional (3D) micromechanical imaging of biological systems. However, the interpretation of hypersonic observables remains obscured by the conceptual conflation of equating GHz longitudinal moduli directly with quasi-static elastic metrics, such as the Young’s modulus (E) from atomic force microscopy (AFM). Here, we demonstrate that this discrepancy reflects not experimental error, but the breakdown of frequency-independent, single-phase elasticity when applied to hydrated soft matter. We present a unified continuum framework that recasts the biomechanical response as a broadband constitutive spectrum governed by dynamically active relaxation channels. Through a tripartite physical model, we map biomaterials onto three coupled sectors: a macroscopic polymer network (frozen α-relaxation), confined molecular micro-environments (β-relaxation and dissipation), and an interstitial fluid phase that hydraulically masks the high-frequency storage modulus. This framework effectively rationalizes the divergent sensitivities of AFM and BLS to cross-linking, hydration, and steric hindrance, while establishing the Brillouin linewidth (ΓB) as a selective micromechanical fingerprint of early pathological vitrification. Ultimately, we argue that BLS operates not as a high-frequency surrogate for conventional elastometry, but as an orthogonal probe of constrained volumetric and dissipative dynamics, outlining the next technological frontiers for multimodal high-frequency mechanobiology.