Mechanical stresses across different length scales play a fundamental role in understanding biological systems' functions and engineering soft machines and devices. However, it is challenging to noninvasively probe local mechanical stresses in situ, particularly when the mechanical properties are unknown. We propose an acoustoelastic imaging-based method to infer the local stresses in soft materials by measuring the speeds of shear waves induced by custom-programmed acoustic radiation force. Using an ultrasound transducer to excite and track the shear waves remotely, we demonstrate the application of the method by imaging uniaxial and bending stresses 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. The experiments indicate that our method will find broad applications, ranging from health monitoring of soft structures and machines to diagnosing diseases that alter stresses in soft tissues.
Abstract Few investigations of river evolution have incorporated the series of upper vortex flow disturbances induced by quasi‐knickpoints, and it is unclear whether the regulating ability of riparian vegetation in maintaining meandering channel patterns is valid. In this study, we therefore conducted meander evolution experiments to confirm the effects of vegetation species and vegetation strength on this complex system. Two nonleguminous plants with salinity and alkalinity resistance had non‐uniform degrees of normal seedling growth and different root network lengths. Our findings show that braided swales formed between the upstream developing zone of the vortex flow and the nearest bend and the riparian vegetation generally consolidated the single‐thread channel planform without branching outward under the conditions of flooding flow. Transverse point bars on inner banks were replaced by downstream scroll bars in the overall channel for long time, unless riparian vegetation and flood scour were coupled. Shallow‐rooted plants were inadequate to withstand the inner‐banks being cut by the upper vortex flow. Deep‐rooted plants can significantly stabilize bank lines and thalwegs but are vulnerable to locally low vegetation coverage. Using evolutionary spectral analysis based on thalwegs, we found that the streamwise high‐frequency distribution of bed topography was primarily concentrated downstream of the bifurcation interface when a flood was present in the unvegetated scenario, shrank to the isolated turning interface of the upper reach and the large‐scale spiral swale of the lower reach in the shallow‐rooted scenario and stood out along the area disturbed by bare roots in the deep‐rooted scenario. This experimental study broadens our understanding of vegetation effects in hydro‐bio‐geomorphological system engineering.
Functionally graded soft materials (FGSMs) with microstructures and mechanical properties exhibiting gradients across a spatial volume to satisfy specific functions have received interests in recent years. How to characterize the mechanical properties of these FGSMs in vivo/in situ and/or in a non-destructive manner is a great challenge. This paper investigates the use of ultrasound elastography in the mechanical characterization of FGSMs. An efficient finite-element model was built to calculate the dispersion relation for surface waves in FGSMs. For FGSMs with large elastic gradients, the measured dispersion relation can be used to identify mechanical parameters. In the case where the elastic gradient is smaller than a certain critical value calculated here, our analysis on transient wave motion in FGSMs shows that the group velocities measured at different depths can infer the local mechanical properties. Experiments have been performed on polyvinyl alcohol (PVA) cryogel to demonstrate the usefulness of the method. Our analysis and the results may not only find broad applications in mechanical characterization of FGSMs but also facilitate the use of shear wave elastography in clinics because many diseases change the local elastic properties of soft tissues and lead to different material gradients. This article is part of the theme issue 'Rivlin's legacy in continuum mechanics and applied mathematics'.
Ultrasound elastography enables in vivo measurement of the mechanical properties of living soft tissues in a non-destructive and non-invasive manner and has attracted considerable interest for clinical use in recent years. Continuum mechanics plays an essential role in understanding and improving ultrasound-based elastography methods and is the main focus of this review. In particular, the mechanics theories involved in both static and dynamic elastography methods are surveyed. They may help understand the challenges in and opportunities for the practical applications of various ultrasound elastography methods to characterize the linear elastic, viscoelastic, anisotropic elastic and hyperelastic properties of both bulk and thin-walled soft materials, especially the in vivo characterization of biological soft tissues.
Abstract Visualizing viscoelastic waves in materials and tissues through noninvasive imaging is valuable for analyzing their mechanical properties and detecting internal anomalies. However, traditional elastography techniques have been limited by a maximum wave frequency below 1-10 kHz, which hampers temporal and spatial resolution. Here, we introduce an optical coherence elastography technique that overcomes the limitation by extending the frequency range to MHz. Our system can measure the stiffness of hard materials including bones and extract viscoelastic shear moduli for polymers and hydrogels in conventionally inaccessible ranges between 100 Hz and 1 MHz. The dispersion of Rayleigh surface waves across the ultrawide band allowed us to profile depth-dependent shear modulus in cartilages ex vivo and human skin in vivo with sub-mm anatomical resolution. This technique holds immense potential as a noninvasive measurement tool for material sciences, tissue engineering, and medical diagnostics.
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