The pipette aspiration applied to the local stiffness measurement of soft tissues

A simple method of identifying the initial slope of the stress-strain curve (i.e., Young's modulus of the soft tissue) by introducing the pipette aspiration technique is presented. The tissue was assumed to be isotropic and macroscopically homogeneous. Numerical simulations by the linear finite element analysis were performed for the axisymmetric model to survey the effects of friction at the tissue-pipette contact boundary, pipette cross-sectional geometry, relative size of the specimen to the pipette, and the layered inhomogeneity of the specimen tissue. The friction at the contact region had little effect on the measurement of Young's modulus. The configuration of the pipette was shown to affect the measurement for small pipette wall thickness. The measurement also depended on the relative size of the specimen to the pipette for relatively small specimens. The extent of the region contributing to the measurement was roughly twice the inside radius of the pipette. In this region, the maximum stress did not exceed the level of the aspiration pressure, with only minor exceptional locations. Calculation of strain energy components indicated that the major contributions to the deformation under pipette aspiration were by the normal extension and shear deformation in pipette axial direction. Experimental verification of the present method for the isotropic, homogeneous artificial material is also presented.