scholarly journals On biological flow networks: Antagonism between hydrodynamic and metabolic stimuli as driver of topological transitions.

2021 ◽  
Author(s):  
Felix Kramer ◽  
Carl D Modes
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Computational Models ◽  
Spanning Trees ◽  
Biological Fluid ◽  
Space Filling ◽  
Network Adaptation ◽  
Optimal Filtration ◽  
Hydrodynamic Stimuli ◽  
Optimization Schemes

A plethora of computational models have been developed in recent decades to account for the morphogenesis of complex biological fluid networks, such as capillary beds. Contemporary adaptation models are based on optimization schemes where networks react and adapt toward given flow patterns. Doing so, a system reduces dissipation and network volume, thereby altering its final form. Yet, recent numeric studies on network morphogenesis, incorporating uptake of metabolites by the embedding tissue, have indicated the conventional approach to be insufficient. Here, we systematically study a hybrid-model which combines the network adaptation schemes intended to generate space-filling perfusion as well as optimal filtration of metabolites. As a result, we find hydrodynamic stimuli (wall-shear stress) and filtration based stimuli (uptake of metabolites) to be antagonistic as hydrodynamically optimized systems have suboptimal uptake qualities and vice versa. We show that a switch between different optimization regimes is typically accompanied with a complex transition between topologically redundant meshes and spanning trees. Depending on the metabolite demand and uptake capabilities of the adaptating networks, we are further able to demonstrate the existence of nullity re-entrant behavior and the development of compromised phenotypes such as dangling non-perfused vessels and bottlenecks.

Axisymmetric Model of an Intracranial Saccular Aneurysm: Theory and Computation

2013 ◽  
Author(s):  
Colin W. Curtis ◽  
Michael L. Calvisi
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Computational Models ◽  
Stokes Equations ◽  
Spherical Geometry ◽  
Element Model ◽  
Saccular Aneurysm ◽  
Vortical Flow ◽  
Axisymmetric Model ◽  
Wall Shear

An axisymmetric model of an intracranial saccular aneurysm is presented and analyzed. The model assumes a simplified spherical geometry for the aneurysm in order to develop insight into the mechanisms that effect wall shear stress and deformation of the membrane. A theoretical model is first developed based on Stokes’ equations for viscous flow in order to derive a stream function that describes vortical flow inside a sphere representative of flow inside a real aneurysm. This flow pattern is implemented in a finite element model of a spherical aneurysm using the software COMSOL Multiphysics. The results indicate close agreement between the theoretical and computational models in terms of the flow streamlines and location of the maximum wall shear stress. Furthermore, the computational model accounts for the deformation and stress of the membrane, showing regions of maximum tension and compression at opposite poles of the saccular membrane. This work elucidates many important results regarding the mechanics of saccular aneurysms and provides a basis for developing more physiologically realistic analyses.

Characterizing Flow in the Vertebro-Basilar System Using MR in Conjunction With Subject-Specific Computational Models

2010 ◽  
Author(s):  
Amanda K. Wake ◽  
John C. Gore ◽  
J. Christopher Gatenby
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Vascular Graft ◽  
Graft Failure ◽  
Computational Models ◽  
Artery Bypass ◽  
Bypass Graft ◽  
Arterial Graft ◽  
Vessel Patency ◽  
Wall Shear

Coronary artery bypass graft failure is often a consequence of intimal hyperplasia (IH), which correlates with hemodynamic factors (e.g., wall shear stress); this relationship has been used to evaluate arterial graft design [e.g., 1–4]. The vertebro-basilar system is a native arterial merge (i.e., two arteries, the vertebrals, converge into a single artery, the basilar artery); thus, characterizing the flow field of this system in healthy subjects could be useful for early detection of anomalies (e.g., aneurysms) or for vascular graft design improvements to ensure graft/vessel patency. This study uses high field MR and phase contrast MR (PCMR) to investigate the hemodynamics of the vertebro-basilar system in a healthy, adult subject for predicting pathophysiologically-relevant flow patterns (e.g., low wall shear stress) that are related to IH and subsequent graft failure.

A Model of Airflow in the Nasal Cavities: Implications for Nasal Air Conditioning and Epistaxis

2009 ◽  
Vol 23 (3) ◽  
pp. 244-249 ◽  
Author(s):  
Neil Bailie ◽  
Brendan Hanna ◽  
John Watterson ◽  
Geraldine Gallagher
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Nasal Cavity ◽  
Air Conditioning ◽  
Computational Models ◽  
Middle Turbinate ◽  
Inferior Turbinate ◽  
Nasal Airflow ◽  
Wall Shear ◽  
Little’S Area

Background A friction force is generated when moving air contacts the nasal walls, referred to as wall shear stress. This interaction facilitates heat and mass transfer between the mucosa and air, i.e., air-conditioning. The objective of this research was to study the distribution of wall shear stress within the nasal cavity to identify areas that contribute significantly to air-conditioning within the nasal cavity. Methods Three-dimensional computational models of the nasal airways of five healthy subjects (three male and two female subjects) were constructed from nasal CT scans. Numerical simulations of nasal airflow were conducted using the commercial computational fluid dynamics code Fluent 6 (Ansys, Inc., Canonsburg, PA). Wall shear stress was derived from the numerical simulation. Air-conditioning was simulated to confirm the relationship with wall shear stress. Results Nasal airflow simulations predicted high wall shear stress along the anterior aspect of the inferior turbinate, the anteroinferior aspect of the middle turbinate, and within Little's area. Conclusion The airflow simulations indicate that the inferior and middle turbinates and Little's area on the anterior nasal septum contribute significantly to nasal air-conditioning. The concentration of wall shear stress within Little's area indicates a desiccating and potentially traumatic effect of inhaled air that may explain the predilection for spontaneous epistaxis at this site.

scholarly journals Biomechanical impact of provisional stenting and balloon dilatation on coronary bifurcation: clinical implications

2017 ◽  
Vol 123 (1) ◽  
pp. 221-226 ◽  
Author(s):  
Henry Y. Chen ◽  
Khalid Al-Saadon ◽  
Yves Louvard ◽  
Ghassan S. Kassab
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Balloon Dilatation ◽  
Computational Models ◽  
Stress Ratio ◽  
Solid Mechanics ◽  
Side Branch ◽  
Wall Shear ◽  
Provisional Stenting

In-stent restenosis (ISR) and stent thrombosis remain clinically significant problems for bifurcations. Although the role of wall shear stress (WSS) has been well investigated, the role of circumferential wall stresses (CWS) has not been well studied in provisional stenting with and without final kissing balloon (FKB). We hypothesized that the perturbation of CWS at the SB in provisional stenting and balloon dilatation is an important factor in addition to WSS, and, hence, may affect restenosis rates (i.e., higher CWS correlates with higher restenosis). To test this hypothesis, we developed computational models of stent, FKB at bifurcation, and finite element simulations that considered both fluid and solid mechanics of the vessel wall. We computed the stress ratio (CWS/WSS) to show potential correlation with restenosis in clinical studies (i.e., higher stress ratio correlates with higher restenosis). Our simulation results show that stenting in the main branch (MB) increases the maximum CWS in the side branch (SB) and, hence, yields a higher stress ratio in the SB, as compared with the MB. FKB dilatation decreases the CWS and increases WSS, which collectively lowers the stress ratio in the SB. The changes of stress ratio were correlated positively with clinical data in provisional stenting and FKB. Both fluid and solid mechanics need to be evaluated when considering various stenting techniques at bifurcations, as solid stresses also play an important role in clinical outcome. An integrative index of bifurcation mechanics is the stress ratio that considers both CWS and WSS. NEW & NOTEWORTHY Although the role of wall shear stress (WSS) has been well investigated, the role of circumferential wall stresses (CWS) has not been well studied in provisional stenting with and without final kissing balloon. Both fluid and solid mechanics need to be evaluated when considering various stenting techniques at bifurcations. An integrative index of bifurcation mechanics is the stress ratio that considers both CWS and WSS.

Computational models for wall shear stress estimation in scaffolds: A comparative study of two complete geometries

2012 ◽  
Vol 45 (9) ◽  
pp. 1586-1592 ◽  
Author(s):  
F. Maes ◽  
T. Claessens ◽  
M. Moesen ◽  
H. Van Oosterwyck ◽  
P. Van Ransbeeck ◽  
...  
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Comparative Study ◽  
Computational Models ◽  
Stress Estimation ◽  
Wall Shear

On the creation of wall shear stress by helical flow structures in stented coronary vessels

2013 ◽  
Vol 14 (1-2) ◽  
pp. 109-115
Author(s):  
Michael Stiehm ◽  
Martin Brede ◽  
Daniel Quosdorf ◽  
Alfred Leder
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Computational Models ◽  
Coronary Vessels ◽  
Helical Flow ◽  
Flow Structures ◽  
Flow Topology ◽  
Flow Deceleration ◽  
Induced Flow ◽  
Wall Shear

AbstractThe post-operative situation in a stented vessel is characterised by struts which extend into the vessel lumen. These barriers on the surface provoke a topological change of the blood flow inducing flow deceleration and stagnation zones. Low values of wall shear stress (WSS) especially up- and downstream of the struts are found accordingly. Clinical studies correlate the occurrence of complications like restenosis and thrombosis with the alteration of the spatial WSS distribution. In this study 3D computational models were used to characterise the flow topology of three different stent types. For this purpose steady state simulations of the flow field within a simplified stented coronary artery were performed. The stent types differ in their strut patterns so that the variation of the induced flow structures can be observed. The aim of these investigations is to evaluate the effect of a purposeful flow control by altering the design of the struts. An improved alignment of the struts will be able to guide the flow to benefit the spatial WSS distribution. To compare the performance of the different stent types the size of the area charged with a WSS value below 0.5 Pa is used as a criterion. We will demonstrate that those strut pattern which generate helical flow structures significantly reduce the critical region of low WSS values.

scholarly journals Impact of main branch stenting on endothelial shear stress: role of side branch diameter, angle and lesion

2011 ◽  
Vol 9 (71) ◽  
pp. 1187-1193 ◽  
Author(s):  
Henry Y. Chen ◽  
Issam D. Moussa ◽  
Charles Davidson ◽  
Ghassan S. Kassab
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Computational Models ◽  
Stress Gradient ◽  
Haemodynamic Effect ◽  
Side Branch ◽  
Negative Effects ◽  
Stent Restenosis ◽  
Negative Effect ◽  
Wall Shear

In-stent restenosis and stent thrombosis remain clinically significant problems for bifurcation lesions. The objective of this study is to determine the haemodynamic effect of the side branch (SB) on main branch (MB) stenting. We hypothesize that the presence of a SB has a negative effect on MB wall shear stress (WSS), wall shear stress gradient (WSSG) and oscillatory shear index (OSI); and that the bifurcation diameter ratio (SB diameter/MB diameter) and angle are important contributors. We further hypothesized that stent undersizing exaggerates the negative effects on WSS, WSSG and OSI. To test these hypotheses, we developed computational models of stents and non-Newtonian blood. The models were then interfaced, meshed and solved in a validated finite-element package. Stents at bifurcation models were created with 30° and 70° bifurcation angles and bifurcations with diameter ratios of SB/MB = 1/2 and 3/4. It was found that stents placed in the MB at a bifurcation lowered WSS dramatically, while elevating WSSG and OSI. Undersizing the stent exaggerated the decrease in WSS, increase in WSSG and OSI, and disturbed the flow between the struts and the vessel wall. Stenting the MB at bifurcations with larger SB/MB ratios or smaller SB angles (30°) resulted in lower WSS, higher WSSG and OSI. Stenosis at the SB lowered WSS and elevated WSSG and OSI. These findings highlight the effects of major biomechanical factors in MB stenting on endothelial WSS, WSSG, OSI and suggests potential mechanisms for the potentially higher adverse clinical events associated with bifurcation stenting.

Relating Wall Shear Stress, Bleb Formation and Rupture of Cerebral Aneurysms: Image-Based Modeling and Clinical Observations

2008 ◽  
Author(s):  
Juan R. Cebral ◽  
Christopher M. Putman
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Computational Models ◽  
Cerebral Aneurysms ◽  
Clinical Observations ◽  
History Of ◽  
Wall Shear ◽  
Blood Flow Patterns

Cerebral aneurysms are widely believed to form and grow as a result of the interactions of hemodynamics and wall mechano-biology. Researchers have used a variety of tools to study these complex multi-factorial mechanisms including animal, in vitro, and computational models. The goal of these experiments has been to approximate the in vivo environment so that theories about the natural history of brain aneurysms can be developed and tested in realistic systems. Studying the link between hemodynamics and clinical observations of aneurysm progression is necessary to reach an understanding of the relative importance of the different mechanisms involved in these processes [1]. The objective of our research is to investigate the possible relationship between wall shear stress (WSS) — which is known to regulate mechano-biological processes at the arterial wall — produced by different blood flow patterns and the evolution and rupture of cerebral aneurysms.

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