Multilayered soft material characterization using surface wave elastography and laser Doppler vibrometry

To characterize mechanical properties in layered media, we present a broadband surface-wave elastography approach for estimating shear wave speeds in a three-layer soft sample using a scanning laser Doppler vibrometer (LDV). Surface waves were generated using a piezoelectric transducer and measured on a layered phantom mimicking skin, including an embedded stiff inclusion representing a skin abnormality. By extending the axial displacement profile of Rayleigh waves in a single-layer half-space to a multilayered medium, we enabled forward modeling and inversion to estimate the shear wave speeds of each layer. The inversion assumed elastic layers, with estimated speeds compared to reference values from single-layer wave elastography. While overall layer characterization showed higher error, mainly due to viscoelastic effects not captured in the model, the inclusion was accurately identified. The embedded inclusion, with a shear wave speed about 1.7 times that of surrounding material, was estimated with only 1.5% error, and the surrounding layer with approximately 10% error. In addition, in vivo measurements were performed on the human forearm skin, demonstrating the feasibility of the proposed method under realistic physiological conditions and strengthening the clinical relevance of the study. The approach is adaptable to alternative excitation and measurement methods and offers high sensitivity, as demonstrated by LDV's ability to detect low-amplitude displacements. This methodology is suitable for skin tissues and holds promise for diagnosing abnormalities and monitoring treatment response.