Residual Stress Effects on Needle-Induced Cavitation*

Needle-induced cavitation (NIC) measures the local stiffness of soft solids through pressurized fluid injection and can be performed in vivo. The critical pressure observed in NIC has been connected to the elasticity, surface tension, and fracture properties of the gel. Current elasticity models of NIC assume a fully relaxed initial deformation state, but deep penetration and puncture by the needle prior to injection can present significant residual stresses that have the potential to alter both the critical pressure and deformation mechanism observed. The effect of puncture process mechanics, and resulting residual stresses, was experimentally measured by monitoring the force on the needle during the cavitation process. Residual stress was found to alter the observed critical pressure, and this effect was interpreted through a simple, analytical model of the deep insertion process. Understanding the impact of the needle insertion process on NIC will enable confident measurement of local mechanical properties in soft gels and tissues.