Non-invasive measurement of local stress inside soft materials with programmed shear waves

TL;DR

This paper introduces a non-invasive acoustoelastic imaging technique that uses programmed shear waves to measure local stresses inside soft materials, enabling stress mapping without prior knowledge of material properties.

Contribution

The authors develop a novel ultrasound-based method to infer local stresses in soft materials by measuring shear wave speeds, applicable even with unknown material properties.

Findings

Successfully imaged uniaxial and bending stresses in hydrogels

Measured passive uniaxial stress in skeletal muscle

Method operates without knowledge of material constitutive parameters

Abstract

Mechanical stresses in soft materials across different length scales play a fundamental role in understanding the function of biological systems and in the use of artificial materials for engineering soft machines and biomedical devices. Yet it remains a great challenge to probe local mechanical stresses in situ in a non-invasive, non-destructive manner, in particular when the mechanical properties are unknown. To address this challenge, we propose an acoustoelastic imaging-based method to infer the local mechanical stresses in soft materials by measuring the speed of shear waves induced by custom-programmed acoustic radiation force. Using a medical ultrasound transducer to excite and track the shear waves remotely, we demonstrate the application of the method by imaging uniaxial stress and bending stress in an isotropic hydrogel, and the passive uniaxial stress in a skeletal muscle. These measurements were all done without the knowledge of the constitutive parameters of the materials. These examples indicate that our method will find broad applications, ranging from health monitoring of soft structures and machines, to the diagnosis of diseases that alter stresses in soft tissues.