Determining the high strain-rate mechanical properties of soft hydrogels and biological tissues is important for a number of biological and engineering applications but remains challenging due to the high compliance of these materials. Inertial microcavitation rheometry (IMR) is a recently developed experimental technique aimed at addressing this need and requires the optical resolution of cavitation bubble kinematics via high-speed videography. While this approach works well for optically transparent samples of dimensions much larger than the typical micron to sub-millimeter bubble sizes, IMR is challenged in highly light scattering media, such as nearly opaque tissues. One remedy to decrease the light scattering within a tissue is to prepare a thinner sample, which facilitates successful recording of the cavitation bubble dynamics. However, the thickness of the required thin samples can approach the size of the microbubbles, and the resulting confinement of the soft material layer between two boundaries changes the fundamental character of the assumed nearly infinite domain of the IMR theoretical framework, leading to erroneous material property estimates. To address this issue and to facilitate successful application of IMR to thin layers of soft materials under confinement, we developed a modified, thin-layer IMR approach for the accurate determination of high strain-rate viscoelastic material properties of soft solids that utilizes axisymmetric finite-element simulations of bubble dynamics in a thin layer. The approach is applied to two transparent, benchmark gels: 6% and 14% gelatin, and the material parameters estimated using the thin-layer IMR approach are validated against experimental data for isolated, spherical bubbles and oversized bubbles in a thin layer. The thin-layer IMR approach provides a robust methodology for applying IMR to nearly opaque, soft materials, such as tissues.
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