AbstractBackgroundArterial compliance assists the cardiovascular system with three key roles: (i) storing up to 50% of the stroke volume; (ii) ensuring blood flow during diastole; (iii) dampening pressure oscillations through arterial distension. In mock circulation loops (MCLs), arterial compliance was simulated either with membrane, spring, or Windkessel chambers. Although they have been shown to be suitable for cardiac device testing, their passive behavior can limit stress‐based testing of arteries. Here we present an active compliance chamber with a feedback control of variable compliance as part of an MCL designed for biomechanical evaluation of arteries under physiological waveforms.Materials and MethodsThe chamber encloses a piston that changes the volume via a cascaded controller when there is a difference between the real‐time pressure and the physiological reference pressure with the aim to equilibrate both pressures.ResultsThe experimental results showed repeatable physiological waveforms of aortic pressure in health (80–120 mm Hg), systemic hypertension (90–153 mm Hg), and heart failure reduced ejection fraction (78–108 mm Hg). Statistical validation (n = 20) of the function of the chamber is presented against compared raw data.ConclusionWe demonstrate that the active compliance chamber can track the actual pressure of the MCL and balance it in real time (every millisecond) with the reference values in order to shape the given pressure waveform. The active compliance chamber is an advanced tool for MCL applications for biomechanical examination of stented arteries and for preclinical evaluation of vascular implants.

Vascular damage develops with diverging severity during and after percutaneous coronary intervention with stent placement and is the prevailing stimulus for in-stent restenosis. Previous work has failed to link mechanical data obtained in a realistic in vivo or in vitro environment with data collected during imaging processes. We investigated whether specimens of porcine right coronary arteries soften when indented with a stent strut shaped structure, and if the softening results from damage mechanisms inside the fibrillar collagen structure. To simulate the multiaxial loading scenario of a stented coronary artery, we developed the testing device 'LAESIO' that can measure differences in the stress-stretch behavior of the arterial wall before and after the indentation of a strut-like stamp. The testing protocol was optimized according to preliminary experiments, more specifically equilibrium and relaxation tests. After chemical fixation of the specimens and subsequent tissue clearing, we performed three-dimensional surface and second-harmonic generation scans on the deformed specimens. We analyzed and correlated the mechanical response with structural parameters of high-affected tissue located next to the stamp indentation and low-affected tissue beyond the injured area. The results reveal that damage mechanisms, like tissue compression as well as softening, fiber dispersion, and the lesion extent, are direction-dependent, and the severity of them is linked to the strut orientation, indentation pressure, and position. The findings highlight the need for further investigations by applying the proposed methods to human coronary arteries. Additional data and insights might help to incorporate the observed damage mechanisms into material models for finite element analyses to perform more accurate simulations of stent-implantations.

The experimental quantification and modeling of the multiaxial mechanical response of polymer membranes of coronary balloon catheters have not yet been carried out. Due to the lack of insights, it is not shown whether isotropic material models can describe the material response of balloon catheter membranes expanded with nominal or higher, supra-nominal pressures. Therefore, for the first time, specimens of commercial polyamide-12 balloon catheters membranes were investigated during uniaxial and biaxial loading scenarios. Furthermore, the influence of kinematic effects on the material response was observed by comparing results from quasi-static and dynamic biaxial extension tests. Novel clamping techniques are described, which allow to test even tiny specimens taken from the balloon membranes. The results of this study reveal the semi-compliant, nonlinear, and viscoelastic character of polyamide-12 balloon catheter membranes. Above nominal pressure, the membranes show a pronounced anisotropic mechanical behavior with a stiffer response in the circumferential direction. The anisotropic feature intensifies with an increasing strain-rate. A modified polynomial model was applied to represent the realistic mechanical response of the balloon catheter membranes during dynamic biaxial extension tests. This study also includes a compact set of constitutive model parameters for the use of the proposed model in future finite element analyses to perform more accurate simulations of expanding balloon catheters.

Finite element (FE)–based studies of preoperative processes such as folding, pleating, and stent crimping with a comparison with experimental inflation tests are not yet available. Therefore, a novel workflow is presented in which residual stresses of balloon folding and pleating, as well as stent crimping, and the geometries of all contact partners were ultimately implemented in an FE code to simulate stent expansion by using an implicit solver. The numerical results demonstrate that the incorporation of residual stresses and strains experienced during the production step significantly increased the accuracy of the subsequent simulations, especially of the stent expansion model. During the preoperative processes, stresses inside the membrane and the stent material also reached a rather high level. Hence, there can be no presumption that balloon catheters or stents are undamaged before the actual surgery. The implementation of the realistic geometry, in particular the balloon tapers, and the blades of the process devices improved the simulation of the expansion mechanisms, such as dogboning, concave bending, or overexpansion of stent cells. This study shows that implicit solvers are able to precisely simulate the mentioned preoperative processes and the stent expansion procedure without a preceding manipulation of the simulation time or physical mass.

The most challenging complication after coronary stent implantation is in-stent restenosis [1], which is mainly caused by mechanically induced injuries due to overloading. From a biomechanical point of view, the processes occurring inside the arterial tissues during stent implantation (SI) is rather unknown. This study shows a novel approach to quantify vascular damage due to SI a multi-scale examination of coronary arteries with generated injuries using a unique experimental in vitro setup. The setup consists of a biaxial tensile testing stage to apply physiological loads on rectangular specimens of coronary arteries and a triple-axis-unit, which allows the indentation of stent struts into arterial tissues under a specified pressure (Fig. A). In addition, the multi-scale investigation of the mechanical and structural responses of the resulting lesion, following the protocol of Sommer et al. [2], is carried out by calculating Cauchy stresses and analyzing healthy and injured specimens with second harmonic generation (Fig. B) and electron microscopy. The results indicate that the usually wavy collagen fibers straightened, compress and align around the lesion (Fig. B). In addition, the evaluation of the material characteristics reveals a significant softening of injured tissues. Fig. A:Design of the experimental setup, showing a biaxial tensile testing stage (white parts) and the triple-axis-unit for indentation tests (yellow parts). Fig. B:Sectional view through the tissue perpendicular to the lesion. The SHG images show collagen fibers of specimens from a 6-months-old porcine descending aorta responding under different pressures (1 and 4 MPa).