Boundary-layer modeling of viscoelastic sphere deformation in acoustic fields: Toward single-cell mechanical...

Acoustically induced cell deformation is receiving increasing attention as a contactless and biocompatible technique for single-cell mechanical phenotyping. Existing studies have extracted cell mechanical properties from acoustic deformation experiments using a thin elastic shell cell model, which describes only membrane elasticity. To address the need to quantify the whole-cell mechanical properties regarding intracellular structures, this work models cells as homogeneous viscoelastic spheres and presents an effective computational model for predicting the cell deformation dynamics in a viscous fluid driven by ultrasound. A perturbation approach is employed to clarify the first-order linear acoustic and second-order nonlinear acoustic effects. To circumvent extreme challenges in numerical simulation due to the separated length scales between acoustic boundary layer thickness and cell radius, the acoustic boundary layer is incorporated into effective fluid-cell coupling boundary conditions via boundary layer analysis. The cell deformation in an ideal standing wave, calculated using the effective model, agrees well with full boundary-layer-resolved simulations while achieving a nearly sixfold reduction in memory consumption. The Young's modulus of Michigan Cancer Foundation-7 (MCF-7) cells is extracted by fitting their deformation to reported experimental data in a focused acoustic beam and an acoustic squeezer. The obtained value of 90 ± 30 Pa is consistent with the accepted range.