Mechanical Properties of Chondrocytes Estimated from Different Models of Micropipette Aspiration

In this study, two viscoelastic creep expressions for the aspirated length of individual solid-like cells undergoing micropipette aspiration (MPA) were derived based on our previous studies wherein the cell size relative to the micropipette and the cell compressibility were taken into account. Next, three mechanical models of MPA, the half-space model (HSM), incompressible sphere model (ICSM), and compressible sphere model (CSM), were employed to fit the MPA data of chondrocytes. The results indicated that the elastic moduli and viscoelastic parameters of chondrocytes for the ICSM and CSM exhibited significantly higher values than those from the HSM (p < 0.001) because of the considerations of the geometric parameter (ξ) and the compressibility of the cell (ν). For the normal chondrocytes, the elastic moduli obtained from the ICSM and CSM (ν = 0.3) were 47.4 and 78.9% higher than those from the HSM. In the viscoelasticity, the parameters k₁, k₂, and μ for the ICSM were respectively increased by 37.8, 37.9, and 39.0% compared to those from the HSM, whereas for the CSM (ν = 0.3), the above parameters were 135, 314, and 257% higher compared to those from the HSM. And with the increase of ξ and ν, the above mechanical parameters decreased. Furthermore, the thresholds of ξ varying with ν were obtained for the given values of relative errors caused by the HSM in the elastic and viscoelastic parameters. The above findings obviously indicated that the geometric parameter of MPA and the Poisson's ratio of a cell have marked influences on the determination of cellular mechanical parameters by MPA and thus should be considered in the pursuit of more accurate investigations of the mechanical properties of cells.