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Koukas E. et al., 2023: Numerical investigation of shock-induced bubble collapse dynamics and fluid-solid interactions during shock-wave lithotripsy.

Department of Mechanical Engineering and Aeronautics, School of Mathematics, Computer Science and Engineering, City University of London, Northampton Square, EC1V 0HB London, UK.
Department of Engineering, School of Physics Engineering and Computer Science (SPECS), University of Hertfordshire, College Lane Campus, AL10 9AB Hatfield, UK.
Department of Mechanical Engineering and Aeronautics, School of Mathematics, Computer Science and Engineering, City University of London, Northampton Square, EC1V 0HB London, UK.

Abstract

In this paper we investigate the bubble collapse dynamics under shock-induced loading near soft and rigid bio-materials, during shock wave lithotripsy. A novel numerical framework was developed, that employs a Diffuse Interface Method (DIM) accounting for the interaction across fluid-solid-gas interfaces. For the resolution of the extended variety of length scales, due to the dynamic and fine interfacial structures, an Adaptive Mesh Refinement (AMR) framework for unstructured grids was incorporated. This multi-material multi-scale approach aims to reduce the numerical diffusion and preserve sharp interfaces. The presented numerical framework is validated for cases of bubble dynamics, under high and low ambient pressure ratios, shock-induced collapses, and wave transmission problems across a fluid-solid interface, against theoretical and numerical results. Three different configurations of shock-induced collapse applications near a kidney stone and soft tissue have been simulated for different stand-off distances and bubble attachment configurations. The obtained results reveal the detailed collapse dynamics, jet formation, solid deformation, rebound, primary and secondary shock wave emissions, and secondary collapse that govern the near-solid collapse and penetration mechanisms. Significant correlations of the problem configuration to the overall collapse mechanisms were found, stemming from the contact angle/attachment of the bubble and from the properties of solid material. In general, bubbles with their center closer to the kidney stone surface produce more violent collapses. For the soft tissue, the bubble movement prior to the collapse is of great importance as new structures can emerge which can trap the liquid jet into induced crevices. Finally, the tissue penetration is examined for these cases and a novel tension-driven tissue injury mechanism is elucidated, emanating from the complex interaction of the bubble/tissue interaction during the secondary collapse phase of an entrapped bubble in an induced crevice with the liquid jet.

Ultrason Sonochem. 2023 Mar 31;95:106393. doi: 10.1016/j.ultsonch.2023.106393. Online ahead of print.PMID: 37031534

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Comments 1

Othmar Wess on Saturday, 28 October 2023 10:45

Cavitation is considered to play an important role in shock wave lithotripsy and other medical applications. The fragmentation process of kidney stones and other body concrements is still under discussion. Coarse fragmentation is attributed to constructive interference of several pressure waves generated by the impinging shock wave with the result of spallation due to the Hopkinson`s effect. Following this these ideas, only coarse fragments are can be expected depending on the wavelength of the waves which are in the range of several millimetres. Fine fragments in the range of millimetres, however, require a different mechanism such as cavitation as usually considered.

The paper investigates the impact of shock waves on gas bubbles in the vicinity of solid and soft surfaces mimicking soft tissue and typical kidney stone material. Bubbles adherent or close to the surfaces, collapse violently forming micro jets directed towards the surface and are considered the mechanism of fragmentation and possible tissue. Excessive calculation displays interesting colour visualization of the jet formation and pressure distribution generated by shock wave induced bubble collapse.
This interesting and excellent study is worth reading, but requires a solid physical background. Nevertheless, a couple of remarks may be allowed:

1. It is not obvious, that gas bubbles are still existent when a shock wave pulse arrives after 500-1000 ms (usual repetition frequency of a lithotripsy treatment 01 1-2 Hz) after a previous shock wave. Shock wave generated cavitation bubbles vanish completely after 1 ms, long before the following shock wave arrives.
2. The forces created by the collapsing bubble are not proven being strong enough to fracture stone material more than slight surface erosions.
3. The theory of Momentum transfer (1) offers a mechanism for coarse and fine fragmentation without the need of cavitation.

1 Ref.: Wess, O.J., Mayer, J. Fragmentation of brittle material by shock wave lithotripsy. Momentum transfer and inertia: a novel view on fragmentation mechanisms. Urolithiasis 48, 137–149 (2020). https://doi.org/10.1007/s00240-018-1102-6

Othmar Wess

Cavitation is considered to play an important role in shock wave lithotripsy and other medical applications. The fragmentation process of kidney stones and other body concrements is still under discussion. Coarse fragmentation is attributed to constructive interference of several pressure waves generated by the impinging shock wave with the result of spallation due to the Hopkinson`s effect. Following this these ideas, only coarse fragments are can be expected depending on the wavelength of the waves which are in the range of several millimetres. Fine fragments in the range of millimetres, however, require a different mechanism such as cavitation as usually considered. The paper investigates the impact of shock waves on gas bubbles in the vicinity of solid and soft surfaces mimicking soft tissue and typical kidney stone material. Bubbles adherent or close to the surfaces, collapse violently forming micro jets directed towards the surface and are considered the mechanism of fragmentation and possible tissue. Excessive calculation displays interesting colour visualization of the jet formation and pressure distribution generated by shock wave induced bubble collapse. This interesting and excellent study is worth reading, but requires a solid physical background. Nevertheless, a couple of remarks may be allowed: 1. It is not obvious, that gas bubbles are still existent when a shock wave pulse arrives after 500-1000 ms (usual repetition frequency of a lithotripsy treatment 01 1-2 Hz) after a previous shock wave. Shock wave generated cavitation bubbles vanish completely after 1 ms, long before the following shock wave arrives. 2. The forces created by the collapsing bubble are not proven being strong enough to fracture stone material more than slight surface erosions. 3. The theory of Momentum transfer (1) offers a mechanism for coarse and fine fragmentation without the need of cavitation. 1 Ref.: Wess, O.J., Mayer, J. Fragmentation of brittle material by shock wave lithotripsy. Momentum transfer and inertia: a novel view on fragmentation mechanisms. Urolithiasis 48, 137–149 (2020). https://doi.org/10.1007/s00240-018-1102-6 Othmar Wess
Tuesday, 23 July 2024