Broadband Rheometry of Native Soft Tissues via i-Rheo-Indent

Abstract

Characterising the frequency-dependent viscoelasticity of soft biological tissues is important for mechanobiology, disease phenotyping, and the design of biomimetic materials. However, conventional oscillatory rheometry becomes increasingly compromised by instrumental inertia at high frequencies and often requires specimens to be cut, reshaped, or compressed to conform to predefined measuring geometries, while local indentation methods probe only limited tissue volumes. Here, we apply i-Rheo-Indent, a macroscale indentation approach that transforms the measured force-relaxation and indentation histories from a single experiment directly into the broadband viscoelastic spectrum, without fitting to a predefined constitutive model. Measurements on a 3D-printed porcine liver analogue and murine quadriceps and brain showed good agreement with conventional oscillatory rheometry over the frequency range in which the latter remained reliable, while extending the accessible response up to 500 rad/s, beyond the regime increasingly dominated by instrument inertia. The method also enabled direct characterisation of intact tubular murine colon, yielding an elastic response consistent with published indentation measurements while additionally resolving the frequency-dependent viscous contribution. Intact murine livers exhibited substantially larger viscoelastic moduli than biopsy-punched specimens prepared for parallel-plate rheometry, indicating that specimen preparation can materially influence the measured mechanical response. By combining rapid broadband acquisition, macroscale probing, minimal specimen manipulation, and comparatively simple instrumentation, i-Rheo-Indent provides a practical route to characterising soft tissues closer to their native mechanical state, with potential applications in mechanobiology, disease phenotyping, and the development of tissue-engineered and biomimetic materials.