On growth, buckling, and rupture of aneurysms: Cylindrical tube analogy.

Aneurysms are localized bulges of arteries; and they can rupture with fatal consequences. Complex mechanobiological factors preclude in vivo testing to assess the rupture risk of an aneurysm, and size based criteria are often used in clinical practice to guide surgical interventions. It is often found that tortuous and buckled aneurysms can exceed the size recommended for surgical intervention, and yet do not rupture. This study addresses why buckled aneurysms exhibit this intriguing behavior by combining in vitro inflation experiments on hyperelastic cylindrical tubes with finite element calculations. Using a biologically relevant material model for an arterial wall, we show that buckled aneurysms can grow in size without rupture under favorable arterial pre-tensions. Stretch reversal phenomenon exhibited by arteries governs whether buckling or bulging occurs first. Exponential stiffening favors the axial propagation of an aneurysm instead of radial growth or size. The choice of failure criteria based on Ogden's strain energy function, Gent's first stretch invariant, and Cauchy stress are discussed. Failure maps incorporating post-bifurcation (bulging and buckling) response are constructed to delineate the regimes of growth, buckling and rupture of an aneurysm.

Evaluation of aortic tortuosity as a negative predictor of abdominal aortic aneurysm rupture.

OBJECTIVE: The maximal aortic diameter has been used as a key indication for whether to repair abdominal aortic aneurysms (AAAs). Aortic tortuosity has been proposed as another factor to consider. In the present study, we compared the degree of aortic tortuosity in ruptured AAAs with that of unruptured AAAs using computed tomography. METHODS: We performed a retrospective review of a prospectively maintained database of patients who had undergone AAA repair from December 2014 to December 2019. Patients with a ruptured aneurysm (rAAA) were matched with patients with a nonruptured AAA (nrAAA) with the same maximal aneurysm diameter and age. The degree of aortic tortuosity, defined as the maximum lateral deviation from the aortic centerline, was measured on preoperative coronal computed tomography scans. RESULTS: During a 5-year period, 572 AAA cases were identified. The aortic tortuosity of the 25 rAAA cases was compared with that of a matched control group of 31 nrAAAs, selected by the same mean maximum diameter of 8.4 cm and similar patient age. In the rAAA group, the mean age was 74.8 years (84% men). In the nrAAA group, the mean age was 76.3 years (88% men). The mean aortic tortuosity for the rAAA and nrAAA groups was 9.3 ± 7.9 mm and 18.0 ± 11.2 mm, respectively (P < .01). CONCLUSIONS: Greater aortic tortuosity was seen in the nrAAA cases compared with the rAAA cases at the same matched aneurysm size. Thus, aortic tortuosity might confer a reduced rupture risk. Further studies with larger cohorts are needed to verify this observation.

Deformation mechanics of self-expanding venous stents: Modelling and experiments.

Deformation properties of venous stents based on braided design, chevron design, Z design, and diamond design are compared using in vitro experiments coupled with analytical and finite element modelling. Their suitability for deployment in different clinical contexts is assessed based on their deformation characteristics. Self-expanding stainless steel stents possess superior collapse resistance compared to Nitinol stents. Consequently, they may be more reliable to treat diseases like May-Thurner syndrome in which resistance against a concentrated (pinching) force applied on the stent is needed to prevent collapse. Braided design applies a larger radial pressure particularly for vessels of diameter smaller than 75% of its nominal diameter, making it suitable for a long lesion with high recoil. Z design has the least foreshortening, which aids in accurate deployment. Nitinol stents are more compliant than their stainless steel counterparts, which indicates their suitability in veins. The semi-analytical method presented can aid in rapid assessment of topology governed deformation characteristics of stents and their design optimization.

Thermal Transport in Molecular Forests.

Heat propagation in quasi-one-dimensional materials (Q1DMs) often appears puzzling. For example, while an isolated Q1DM, such as a nanowire, a carbon nanotube, or a polymer, can exhibit a high thermal conductivity κ, forests of the same materials can show a reduction in κ. Until now, the complex structures of these assemblies have hindered the emergence of a clear molecular picture for this intriguing phenomenon. We combine coarse-grained simulations with concepts known from polymer physics and thermal transport to unveil a generic microscopic picture of κ reduction in molecular forests. We show that a delicate balance among the persistence length of the Q1DM, the segment orientations, and the flexural vibrations governs the reduction in κ.

Periodic folding of a falling viscoelastic sheet.

A viscoelastic solid sheet fed from a certain height towards a rigid horizontal plane folds on itself provided that there is no slip. This phenomenon commonly occurs in the manufacturing process of textile and paper products. In this paper we apply a particle dynamics model to investigate this phenomenon. At a low feeding velocity and low viscosity, the inertial effect and the viscous dissipation within the sheet are negligible, and our model successfully reproduces the existing quasistatic results in the gravitational regime. As the feeding velocity and the viscosity of the sheet increase, the folding process changes significantly. The length of the folds decrease and the "rolling back" motion of the sheet vanishes. In the inertial regime, a scaling law between the fold length and the feeding velocity is derived by balancing the kinetic energy and the elastic bending energy involved in folding, which is verified by the simulation. It is found that above a critical feeding velocity, the folding morphology transforms from line contact into point contact with the sheet exhibiting a lemniscate-like pattern. Finally, a phase diagram for the folding morphology is constructed. The results presented in this work may offer some insights into the high-speed manufacturing of paper and fabric sheets.

Analyses and design of expansion mechanisms of balloon expandable vascular stents.

Vascular stents are expanded in blood vessels with lumens larger than their cardiac counterparts. Extreme radial expansion significantly reduces the expanded length of some designs, resulting in insufficient lesion coverage and inaccurate placement. It is hypothesized that expansion mechanisms of a balloon-expandable stent, driven by plastic hinges, are controlled by the cell topology. This hypothesis is first tested for stent expansion using kinematic and kinetic analyses, followed by more detailed finite element (FE) calculations. Three balloon-expandable stent designs are laser micro-machined for experimental verification of the length-diameter relations predicted by the analytical and FE models. It is found that stent designs with positive, negative, or zero foreshortening over expansion phase can be designed by tailoring unit cell geometries and hence obtain desired length-diameter and pressure-diameter characteristics.

Local resonance bandgaps in periodic media: theory and experiment.

Periodic composites such as acoustic metamaterials use local resonance phenomenon in designing low frequency sub-Bragg bandgaps. These bandgaps emerge from a resonant scattering interaction between a propagating wave and periodically arranged resonators. This paper develops a receptance coupling technique to combine the dynamics of the resonator with the unit cell dynamics of the background medium to analyze flexural wave transmission in a periodic structure, involving a single degree of freedom coupling between the medium and the resonator. Receptance techniques allow for a straightforward extension to higher dimensional systems with multiple degrees of freedom coupling and for easier experimental measurements. Closed-form expressions for the location and width of sub-Bragg bandgaps are obtained. Rigid body modes of the unit cell of the background medium are shown to set the bounding frequencies for local resonance bandgaps. Results from the receptance analysis compare well with Bloch wave analysis and experiments performed on a finite structural beam with periodic masses and resonators. Stronger coupling and inertia of the resonator increase the local resonance bandgap width. Two-fold periodicity widens the Bragg bandgap, narrowed by local resonators, thus expanding the design space and highlighting the advantages of hierarchical periodicity.

Wave propagation in two-dimensional periodic lattices.

Plane wave propagation in infinite two-dimensional periodic lattices is investigated using Floquet-Bloch principles. Frequency bandgaps and spatial filtering phenomena are examined in four representative planar lattice topologies: hexagonal honeycomb, Kagomé lattice, triangular honeycomb, and the square honeycomb. These topologies exhibit dramatic differences in their long-wavelength deformation properties. Long-wavelength asymptotes to the dispersion curves based on homogenization theory are in good agreement with the numerical results for each of the four lattices. The slenderness ratio of the constituent beams of the lattice (or relative density) has a significant influence on the band structure. The techniques developed in this work can be used to design lattices with a desired band structure. The observed spatial filtering effects due to anisotropy at high frequencies (short wavelengths) of wave propagation are consistent with the lattice symmetries.