Biaxial mechanical properties of the natural and glutaraldehyde treated aortic valve cusp--Part I: Experimental results

Planar biaxial testing of CXL strengthening effects

The stiffening effect of corneal crosslinking (CXL) treatment, a therapeutic approach for managing the progression of keratoconus, has been primarily investigated using uniaxial tensile experiments. However, this testing technique has several drawbacks and is unable to measure the mechanical response of cornea under a multiaxial loading state. In this work, we used biaxial mechanical testing method to characterize biomechanical properties of porcine cornea before and after CXL treatment. We also investigated the influence of preconditioning on measured properties and used TEM images to determine microstructural characteristics of the extracellular matrix. The conventional method of CXL treatment was used for crosslinking the porcine cornea. The biaxial experiments were done by an ElectroForce TestBench system at a stretch ratio of 1:1 and a displacement rate of 2 mm/min with and without preconditioning. The experimental measurements showed no significant difference in the mechanical properties of porcine cornea along the nasal temporal (NT) and superior inferior (SI) direction. Furthermore, the CXL therapy significantly enhanced the mechanical properties along both directions without creating anisotropic response. The samples tested with preconditioning showed significantly stiffer response than those tested without preconditioning. The TEM images showed that the CXL therapy did not increase the diameter of collagen fibers but significantly decreased their interfibrillar spacing, consistent with the mechanical property improvement of CXL treated samples.

Physical and Computational Modeling for Transcatheter Structural Heart Interventions

Structural heart disease interventions rely heavily on preprocedural planning and simulation to improve procedural outcomes and predict and prevent potential procedural complications. Modeling technologies, namely 3-dimensional (3D) printing and computational modeling, are nowadays increasingly used to predict the interaction between cardiac anatomy and implantable devices. Such models play a role in patient education, operator training, procedural simulation, and appropriate device selection. However, current modeling is often limited by the replication of a single static configuration within a dynamic cardiac cycle. Recognizing that health systems may face technical and economic limitations to the creation of "in-house" 3D-printed models, structural heart teams are pivoting to the use of computational software for modeling purposes.

Assessing turbulent effects in ascending aorta in presence of bicuspid aortic valve

Aortic valves with bicuspids have two rather than three leaflets, which is a congenital heart condition. About 0.5-2% of people have a bicuspid aortic valve. Blood flow through the aorta is commonly believed to be laminar, although aortic valve disorders can cause turbulent transitions. Understanding the impact of turbulence is crucial for foreseeing how the disease will progress. The study's objective was use large eddy simulation to provide a thorough analysis of the turbulence in bicuspid aortic valve dysfunction. Using a large eddy simulation, the blood flow patterns of the bicuspid and tricuspid aortic valves were compared, and significant discrepancies were found. The velocity field in flow in bicuspid configurations was asymmetrically distributed toward the ascending aorta. In tricuspid aortic valve (TAV) the flow, on the other hand, was symmetrical within the same aortic segment. Moreover, we looked into standard deviation, Q-criterion, viscosity ratio and wall shear stresses for each cases to understand transition to turbulence. Our findings indicate that in the bicuspid aortic valve (BAV) case, the fluid-dynamic abnormalities increase. The global turbulent kinetic energy and time-averaged wall shear stress for the TAV and BAV scenarios were also examined. We discovered that the global turbulent kinetic energy was higher in the BAV case compared to TAV, in addition to the increased wall shear stress induced by the BAV in the ascending aorta.

Patient-Specific Quantitative In-Vivo Assessment of Human Mitral Valve Leaflet Strain Before and After MitraClip Repair

PURPOSE

Mitral regurgitation (MR) is a highly prevalent and deadly cardiac disease characterized by improper mitral valve (MV) leaflet coaptation. Among the plethora of available treatment strategies, the MitraClip is an especially safe option, but optimizing its long-term efficacy remains an urgent challenge.

METHODS

We applied our noninvasive image-based strain computation pipeline [1] to intraoperative transesophageal echocardiography datasets taken from ten patients undergoing MitraClip repair, spanning a range of MR etiologies and MitraClip configurations. We then analyzed MV leaflet strains before and after MitraClip implementation to develop a better understanding of (1) the pre-operative state of human regurgitant MV, and (2) the MitraClip's impact on the MV leaflet deformations.

RESULTS

The MV pre-operative strain fields were highly variable, underscoring both the heterogeneity of the MR in the patient population and the need for patient-specific treatment approaches. Similarly, there were no consistent overall post-operative strain patterns, although the average A2 segment radial strain difference between pre- and post-operative states was consistently positive. In contrast, the post-operative strain fields were better correlated to their respective pre-operative strain fields than to the inter-patient post-operative strain fields. This quantitative result implies that the patient specific pre-operative state of the MV guides its post-operative deformation, which suggests that the post-operative state can be predicted using pre-operative data-derived modelling alone.

CONCLUSIONS

The pre-operative MV leaflet strain patterns varied considerably across the range of MR disease states and after MitraClip repair. Despite large inter-patient heterogeneity, the post-operative deformation appears principally dictated by the pre-operative deformation state. This novel finding suggests that though the variation in MR functional state and MitraClip-induced deformation were substantial, the post-operative state can be predicted from the pre-operative data alone. This study suggests that, with use of larger patient cohort and corresponding long-term outcomes, quantitative predictive factors of MitraClip durability can be identified.

Functional mechanical behavior of the murine pulmonary heart valve

Genetically modified mouse models provide a versatile and efficient platform to extend our understanding of the underlying disease processes and evaluate potential treatments for congenital heart valve diseases. However, applications have been limited to the gene and molecular levels due to the small size of murine heart valves, which prohibits the use of standard mechanical evaluation and in vivo imaging methods. We have developed an integrated imaging/computational mechanics approach to evaluate, for the first time, the functional mechanical behavior of the murine pulmonary heart valve (mPV). We utilized extant mPV high resolution µCT images of 1-year-old healthy C57BL/6J mice, with mPVs loaded to 0, 10, 20 or 30 mmHg then chemically fixed to preserve their shape. Individual mPV leaflets and annular boundaries were segmented and key geometric quantities of interest defined and quantified. The resulting observed inter-valve variations were small and consistent at each TVP level. This allowed us to develop a high fidelity NURBS-based geometric model. From the resultant individual mPV geometries, we developed a mPV shape-evolving geometric model (SEGM) that accurately represented mPV shape changes as a continuous function of transvalvular pressure. The SEGM was then integrated into an isogeometric finite element based inverse model that estimated the individual leaflet and regional mPV mechanical behaviors. We demonstrated that the mPV leaflet mechanical behaviors were highly anisotropic and nonlinear, with substantial leaflet and regional variations. We also observed the presence of strong axial mechanical coupling, suggesting the important role of the underlying collagen fiber architecture in the mPV. When compared to larger mammalian species, the mPV exhibited substantially different mechanical behaviors. Thus, while qualitatively similar, the mPV exhibited important functional differences that will need to accounted for in murine heart valve studies. The results of this novel study will allow detailed murine tissue and organ level investigations of semi-lunar heart valve diseases.

The effects of strain history on aortic valve interstitial cell activation in a 3D hydrogel environment

Aortic valves (AVs) undergo unique stretch histories that include high rates and magnitudes. While major differences in deformation patterns have been observed between normal and congenitally defective bicuspid aortic valves (BAVs), the relation to underlying mechanisms of rapid disease onset in BAV patients remains unknown. To evaluate how the variations in stretch history affect AV interstitial cell (AVIC) activation, high-throughput methods were developed to impart varied cyclical biaxial stretch histories into 3D poly(ethylene) glycol hydrogels seeded with AVICs for 48 h. Specifically, a physiologically mimicking stretch history was compared to two stretch histories with varied peak stretch and stretch rate. Post-conditioned AVICs were imaged for nuclear shape, alpha smooth muscle actin (αSMA) and vimentin (VMN) polymerization, and small mothers against decapentaplegic homologs 2 and 3 (SMAD 2/3) nuclear activity. The results indicated that bulk gel deformations were accurately transduced to the AVICs. Lower peak stretches lead to increased αSMA polymerization. In contrast, VMN polymerization was a function of stretch rate, with SMAD 2/3 nuclear localization and nuclear shape also trending toward stretch rate dependency. Lower than physiological levels of stretch rate led to higher SMAD 2/3 activity, higher VMN polymerization around the nucleus, and lower nuclear elongation. αSMA polymerization did not correlate with VMN polymerization, SMAD 2/3 activity, nor nuclear shape. These results suggest that a negative feedback loop may form between SMAD 2/3, VMN, and nuclear shape to maintain AVIC homeostatic nuclear deformations, which is dependent on stretch rate. These novel results suggest that AVIC mechanobiological responses are sensitive to stretch history and provide insight into the mechanisms of AV disease.

Recent Advances in Tissue-Engineered Cardiac Scaffolds-The Progress and Gap in Mimicking Native Myocardium Mechanical Behaviors

Heart failure is the leading cause of death in the US and worldwide. Despite modern therapy, challenges remain to rescue the damaged organ that contains cells with a very low proliferation rate after birth. Developments in tissue engineering and regeneration offer new tools to investigate the pathology of cardiac diseases and develop therapeutic strategies for heart failure patients. Tissue -engineered cardiac scaffolds should be designed to provide structural, biochemical, mechanical, and/or electrical properties similar to native myocardium tissues. This review primarily focuses on the mechanical behaviors of cardiac scaffolds and their significance in cardiac research. Specifically, we summarize the recent development of synthetic (including hydrogel) scaffolds that have achieved various types of mechanical behavior-nonlinear elasticity, anisotropy, and viscoelasticity-all of which are characteristic of the myocardium and heart valves. For each type of mechanical behavior, we review the current fabrication methods to enable the biomimetic mechanical behavior, the advantages and limitations of the existing scaffolds, and how the mechanical environment affects biological responses and/or treatment outcomes for cardiac diseases. Lastly, we discuss the remaining challenges in this field and suggestions for future directions to improve our understanding of mechanical control over cardiac function and inspire better regenerative therapies for myocardial restoration.

Lengthwise regional mechanics of the human aneurysmal ascending thoracic aorta

The prognosis of patients undergoing emergency endovascular repair of ascending thoracic aortic aneurysm (ATAA) depends on defect location, with root disease bearing worse outcomes than proximal or distal aortopathy. We speculate that a spatial gradient in aneurysmal tissue mechanics through the length of the ascending thoracic aorta may fuel noted survival discrepancies. To this end, we performed planar biaxial testing on 153 root, proximal, and distal segments of ATAA samples collected from 80 patients receiving elective open surgical repair. Following data averaging via surface fitting-based interpolation of strain-controlled protocols, we combined in-vitro and in-vivo measurements of loads and geometry to resolve inflation-extension kinematics and evaluate mechanical metrics of stress, stiffness, and energy at consistent deformation levels. Representative (averaged) experimental data and simulated in-vivo conditions revealed significantly larger biaxial stiffness at the root compared to either proximal or distal tissues, which persisted as the entire aorta stiffened during aging. Advancing age further reduced biaxial stretch and energy storage, a measure of aortic function, across all ATAA segments. Importantly, age emerged as a stronger predictor of tissue mechanics in ATAA disease than either bicuspid aortic valve or connective tissue disorders. Besides strengthening the general understanding of aneurysmal disease, our findings provide specifications to customize the design of stent-grafts for the treatment of ATAA disease. Optimization of deployment and interaction of novel endovascular devices with the local native environment is expected to carry significant potential for improving clinical outcomes. STATEMENT OF SIGNIFICANCE: Elucidating the lengthwise regional mechanics of ascending thoracic aortic aneurysms (ATAAs) is critical for the design of endovascular devices tailored to the ascending aorta. Stent-grafts provide a less invasive alternative to support the long-term survival of ATAA patients ineligible for open surgical repair. In this study, we developed a numerical framework that combines semi-inverse constitutive and forward modeling with in-vitro and in-vivo data to extract mechanical descriptors of ATAA tissue behavior at physiologically meaningful deformation. Moving distally from the aortic root to the first ascending aortic branch, we observed a progressive decline in biaxial stiffness. Furthermore, we showed that aging leads to reduced aortic function and is a stronger predictor of mechanics than either valve morphology or underlying syndromic disorder.

Development of an ultra-low-cost planar biaxial tester for soft tissue characterization

Nowadays, the research in the arena of biomedical engineering or specifically soft tissue characterization is rapidly increasing. Due to the complex properties of soft tissues such as, anisotropy and viscoelasticity, it is difficult to predict the deformation behaviour. Hence, soft tissue characterization is essential to analyze these metrics. Soft tissue characterization, specifically, can be done by implementing a planar biaxial tester. Currently, available biaxial testers are mostly developed with respect to other mechanical components such as metals, and not for the soft tissues. Also, these devices are very costly, which makes it difficult for the low and middle income countries to perform this characterization. To solve this problem, in this work, an extremely low-cost biaxial tester was designed and developed. The design of the biaxial tester was simple and modular to allow device modifications according to the applications. The device has a force capability of less than 0.4 kN and a variable speed of 18 mm min^-1to 300 mm min^-1. The biaxial tester was validated using a standard test material with mechanical testing machine and was further tested on several wound geometries including circular, square, diamond shaped, L-Plasty, and elliptical. The developed fully automated device exhibited high accuracy with real-time monitoring. Furthermore, test results on the wounds showed the device's capability to differentiate amongst the considered wound geometries. This device can be helpful to medical students and doctors in understanding the mechanical behaviour of soft tissues during injury induced damage, disease, wounds healing and also for plethora of applications such as expansion testing of skin grafts.

Future prospects in the tissue engineering of heart valves: a focus on the role of stem cells

INTRODUCTION

Heart valve disease is a growing burden on the healthcare system. Current solutions are insufficient for young patients and do not offer relief from reintervention. Tissue engineered heart valves (TEHVs) offer a solution that grows and responds to the native environment in a similar way to a healthy valve. Stem cells hold potential to populate these valves as a malleable source that can adapt to environmental cues.

AREAS COVERED

This review covers current methods of recapitulating features of native heart valves with tissue engineering through use of stem cell populations with in situ and in vitro methods.

EXPERT OPINION

In the field of TEHVs, we see a variety of approaches in cell source, biomaterial, and maturation methods. Choosing appropriate cell populations may be very patient specific; consistency and predictability will be key to long-term success. In situ methods are closer to translation but struggle with consistent cellularization. In vitro culture requires specialized methods but may recapitulate native valve cell populations with higher fidelity. Understanding how cell populations react to valve conditions and immune response is vital for success. Detrimental valve pathologies have proven to be difficult to avoid in early translation attempts.

Engineering Heart Valve Interfaces Using Melt Electrowriting: Biomimetic Design Strategies from Multi-Modal Imaging

Interfaces within biological tissues not only connect different regions but also contribute to the overall functionality of the tissue. This is especially true in the case of the aortic heart valve. Here, melt electrowriting (MEW) is used to engineer complex, user-defined, interfaces for heart valve scaffolds. First, a multi-modal imaging investigation into the interfacial regions of the valve reveals differences in collagen orientation, density, and recruitment in previously unexplored regions including the commissure and inter-leaflet triangle. Overlapping, suturing, and continuous printing methods for interfacing MEW scaffolds are then investigated for their morphological, tensile, and flexural properties, demonstrating the superior performance of continuous interfaces. G-codes for MEW scaffolds with complex interfaces are designed and generated using a novel software and graphical user interface. Finally, a singular MEW scaffold for the interfacial region of the aortic heart valve is presented incorporating continuous interfaces, gradient porosities, variable layer numbers across regions, and tailored fiber orientations inspired by the collagen distribution and orientation from the multi-modal imaging study. The scaffold exhibits similar yield strain, hysteresis, and relaxation behavior to porcine heart valves. This work demonstrates the ability of a bioinspired approach for MEW scaffold design to address the functional complexity of biological tissues.

Structural and biomechanical characterizations of acellular porcine mitral valve scaffolds: anterior leaflets, posterior leaflets, and chordae tendineae

Mitral valve (MV) tissue engineering is still in its early stage, and one major challenge in MV tissue engineering is to identify appropriate scaffold materials. With the potential of acellular MV scaffolds being demonstrated recently, it is important to have a full understanding of the biomechanics of the native MV components and their acellular scaffolds. In this study, we have successfully characterized the structural and mechanical properties of porcine MV components, including anterior leaflet (AL), posterior leaflet (PL), strut chordae, and basal chordae, before and after decellularization. Quantitative DNA assay showed more than 90% reduction in DNA content, and Griffonia simplicifolia (GS) lectin immunohistochemistry confirmed the complete lack of porcine α-Gal antigen in the acellular MV components. In the acellular AL and PL, the atrialis, spongiosa, and fibrosa trilayered structure, along with its ECM constitutes, i.e., collagen fibers, elastin fibers, and portion of GAGs, were preserved. Nevertheless, the ECM of both AL and PL experienced a certain degree of disruption, exhibiting a less dense, porous ECM morphology. The overall anatomical morphology of the strut and basal chordae were also maintained after decellularization, with longitudinal morphology experiencing minimum disruption, but the cross-sectional morphology exhibiting evenly-distributed porous structure. In the acellular AL and PL, the nonlinear anisotropic biaxial mechanical behavior was overall preserved; however, uniaxial tensile tests showed that the removal of cellular content and the disruption of structural ECM did result in small decreases in maximum tensile modulus, tissue extensibility, failure stress, and failure strain for both MV leaflets and chordae.

HIF1A inhibitor PX-478 reduces pathological stretch-induced calcification and collagen turnover in aortic valve

HIF1A is significantly upregulated in calcified human aortic valves (AVs). Furthermore, HIF1A inhibitor PX-478 was shown to inhibit AV calcification under static and disturbed flow conditions. Since elevated stretch is one of the major mechanical stimuli for AV calcification, we investigated the effect of PX-478 on AV calcification and collagen turnover under a pathophysiological cyclic stretch (15%) condition. Porcine aortic valve (PAV) leaflets were cyclically (1 Hz) stretched at 15% for 24 days in osteogenic medium with or without PX-478. In addition, PAV leaflets were cyclically stretched at a physiological (10%) and 15% for 3 days in regular medium to assess its effect of on HIF1A mRNA expression. It was found that 100 μM (high concentration) PX-478 could significantly inhibit PAV calcification under 15% stretch, whereas 50 μM (moderate concentration) PX-478 showed a modest inhibitory effect on PAV calcification. Nonetheless, 50 μM PX-478 significantly reduced PAV collagen turnover under 15% stretch. Surprisingly, it was observed that cyclic stretch (15% vs. 10%) did not have any significant effect on HIF1A mRNA expression in PAV leaflets. These results suggest that HIF1A inhibitor PX-478 may impart its anti-calcific and anti-matrix remodeling effect in a stretch-independent manner.

Controlled Comparison of Simulated Hemodynamics Across Tricuspid and Bicuspid Aortic Valves

Bicuspid aortic valve is the most common congenital heart defect, affecting 1-2% of the global population. Patients with bicuspid valves frequently develop dilation and aneurysms of the ascending aorta. Both hemodynamic and genetic factors are believed to contribute to dilation, yet the precise mechanism underlying this progression remains under debate. Controlled comparisons of hemodynamics in patients with different forms of bicuspid valve disease are challenging because of confounding factors, and simulations offer the opportunity for direct and systematic comparisons. Using fluid-structure interaction simulations, we simulate flows through multiple aortic valve models in a patient-specific geometry. The aortic geometry is based on a healthy patient with no known aortic or valvular disease, which allows us to isolate the hemodynamic consequences of changes to the valve alone. Four fully-passive, elastic model valves are studied: a tricuspid valve and bicuspid valves with fusion of the left- and right-, right- and non-, and non- and left-coronary cusps. The resulting tricuspid flow is relatively uniform, with little secondary or reverse flow, and little to no pressure gradient across the valve. The bicuspid cases show localized jets of forward flow, excess streamwise momentum, elevated secondary and reverse flow, and clinically significant levels of stenosis. Localized high flow rates correspond to locations of dilation observed in patients, with the location related to which valve cusps are fused. Thus, the simulations support the hypothesis that chronic exposure to high local flow contributes to localized dilation and aneurysm formation.

Benchtop characterization of the tricuspid valve leaflet pre-strains

The pre-strains of biological soft tissues are important when relating their in vitro and in vivo mechanical behaviors. In this study, we present the first-of-its-kind experimental characterization of the tricuspid valve leaflet pre-strains. We use 3D photogrammetry and the reproducing kernel method to calculate the pre-strains within the central 10×10 mm region of the tricuspid valve leaflets from n=8 porcine hearts. In agreement with previous pre-strain studies for heart valve leaflets, our results show that all the three tricuspid valve leaflets shrink after explant from the ex vivo heart. These calculated strains are leaflet-specific and the septal leaflet experiences the most compressive changes. Furthermore, the strains observed after dissection of the central 10×10 mm region of the leaflet are smaller than when the valve is explanted, suggesting that our computed pre-strains are mainly due to the release of in situ annulus and chordae connections. The leaflets are then mounted on a biaxial testing device and preconditioned using force-controlled equibiaxial loading. We show that the employed preconditioning protocol does not 100% restore the leaflet pre-strains as removed during tissue dissection, and future studies are warranted to explore alternative preconditioning methods. Finally, we compare the calculated biomechanically oriented metrics considering five stress-free reference configurations. Interestingly, the radial tissue stretches and material anisotropies are significantly smaller compared to the post-preconditioning configuration. Extensions of this work can further explore the role of this unique leaflet-specific leaflet pre-strains on in vivo valve behavior via high-fidelity in-silico models.

Combining Unique Planar Biaxial Testing with Full-Field Thickness and Displacement Measurement for Spatial Characterization of Soft Tissues

Soft tissues rely on the incredible complexity of their microstructure for proper function. Local variations in material properties arise as tissues develop and adapt, often in response to changes in loading. A barrier to investigating the heterogeneous nature of soft tissues is the difficulty of developing experimental protocols and analysis tools that can accurately capture spatial variations in mechanical behavior. In this article, we detail protocols enabling mechanical characterizations of anisotropic, heterogeneous soft tissues or tissue analogs. We present a series of mechanical tests designed to maximize inhomogeneous strain fields and in-plane shear forces. A customized, 3D-printable gripping system reduces tissue handling and enhances shear. High-resolution imaging and laser micrometry capture full-field displacement and thickness, respectively. As the equipment necessary to conduct these protocols is commercially available, the experimental methods presented offer an accessible route toward addressing heterogeneity. © 2022 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Unique biaxial testing of soft tissues and tissue analogs Basic Protocol 2: Full-field thickness measurement of soft tissues and tissue analogs Support Protocol 1: Creating and speckling cruciform-shaped samples for mechanical testing Support Protocol 2: Creating custom gripping system to minimize sample handling.

Computational Modeling of Aortic Stenosis With a Reduced Degree-of-Freedom Fluid-Structure Interaction Valve Model

In this study, a novel reduced degree-of-freedom (rDOF) aortic valve model is employed to investigate the fluid-structure interaction (FSI) and hemodynamics associated with aortic stenosis. The dynamics of the valve leaflets are determined by an ordinary differential equation with two parameters and this rDOF model is shown to reproduce key features of more complex valve models. The hemodynamics associated with aortic stenosis is studied for three cases: a healthy case and two stenosed cases. The focus of the study is to correlate the hemodynamic features with the source generation mechanism of systolic murmurs associated with aortic stenosis. In the healthy case, extremely weak flow fluctuations are observed. However, in the stenosed cases, simulations show significant turbulent fluctuations in the ascending aorta, which are responsible for the generation of strong wall pressure fluctuations after the aortic root mostly during the deceleration phase of the systole. The intensity of the murmur generation increases with the severity of the stenosis, and the source locations for the two diseased cases studied here lie around 1.0 inlet duct diameters (Do) downstream of the ascending aorta.

Scaffold Geometry-Imposed Anisotropic Mechanical Loading Guides the Evolution of the Mechanical State of Engineered Cardiovascular Tissues in vitro

Cardiovascular tissue engineering is a promising approach to develop grafts that, in contrast to current replacement grafts, have the capacity to grow and remodel like native tissues. This approach largely depends on cell-driven tissue growth and remodeling, which are highly complex processes that are difficult to control inside the scaffolds used for tissue engineering. For several tissue engineering approaches, adverse tissue growth and remodeling outcomes were reported, such as aneurysm formation in vascular grafts, and leaflet retraction in heart valve grafts. It is increasingly recognized that the outcome of tissue growth and remodeling, either physiological or pathological, depends at least partly on the establishment of a homeostatic mechanical state, where one or more mechanical quantities in a tissue are maintained in equilibrium. To design long-term functioning tissue engineering strategies, understanding how scaffold parameters such as geometry affect the mechanical state of a construct, and how this state guides tissue growth and remodeling, is therefore crucial. Here, we studied how anisotropic versus isotropic mechanical loading—as imposed by initial scaffold geometry—influences tissue growth, remodeling, and the evolution of the mechanical state and geometry of tissue-engineered cardiovascular constructs in vitro. Using a custom-built bioreactor platform and nondestructive mechanical testing, we monitored the mechanical and geometric changes of elliptical and circular, vascular cell-seeded, polycaprolactone-bisurea scaffolds during 14 days of dynamic loading. The elliptical and circular scaffold geometries were designed using finite element analysis, to induce anisotropic and isotropic dynamic loading, respectively, with similar maximum stretch when cultured in the bioreactor platform. We found that the initial scaffold geometry-induced (an)isotropic loading of the engineered constructs differentially dictated the evolution of their mechanical state and geometry over time, as well as their final structural organization. These findings demonstrate that controlling the initial mechanical state of tissue-engineered constructs via scaffold geometry can be used to influence tissue growth and remodeling and determine tissue outcomes.

Local tissue heterogeneity may modulate neuronal responses via altered axon strain fields: insights about innervated joint capsules from a computational model

In innervated collagenous tissues, tissue scale loading may contribute to joint pain by transmitting force through collagen fibers to the embedded mechanosensitive axons. However, the highly heterogeneous collagen structures of native tissues make understanding this relationship challenging. Recently, collagen gels with embedded axons were stretched and the resulting axon signals were measured, but these experiments were unable to measure the local axon strain fields. Computational discrete fiber network models can directly determine axon strain fields due to tissue scale loading. Therefore, this study used a discrete fiber network model to identify how heterogeneous collagen networks (networks with multiple collagen fiber densities) change axon strain due to tissue scale loading. In this model, a composite cylinder (axon) was embedded in a Delaunay network (collagen). Homogeneous networks with a single collagen volume fraction and two types of heterogeneous networks with either a sparse center or dense center were created. Measurements of fiber forces show higher magnitude forces in sparse regions of heterogeneous networks and uniform force distributions in homogeneous networks. The average axon strain in the sparse center networks decreases when compared to homogeneous networks with similar collagen volume fractions. In dense center networks, the average axon strain increases compared to homogeneous networks. The top 1% of axon strains are unaffected by network heterogeneity. Based on these results, the interaction of tissue scale loading, collagen network heterogeneity, and axon strains in native musculoskeletal tissues should be considered when investigating the source of joint pain.

A design-based model of the aortic valve for fluid-structure interaction

This paper presents a new method for modeling the mechanics of the aortic valve and simulates its interaction with blood. As much as possible, the model construction is based on first principles, but such that the model is consistent with experimental observations. We require that tension in the leaflets must support a pressure, then derive a system of partial differential equations governing its mechanical equilibrium. The solution to these differential equations is referred to as the predicted loaded configuration; it includes the loaded leaflet geometry, fiber orientations and tensions needed to support the prescribed load. From this configuration, we derive a reference configuration and constitutive law. In fluid-structure interaction simulations with the immersed boundary method, the model seals reliably under physiological pressures and opens freely over multiple cardiac cycles. Further, model closure is robust to extreme hypo- and hypertensive pressures. Then, exploiting the unique features of this model construction, we conduct experiments on reference configurations, constitutive laws and gross morphology. These experiments suggest the following conclusions: (1) The loaded geometry, tensions and tangent moduli primarily determine model function. (2) Alterations to the reference configuration have little effect if the predicted loaded configuration is identical. (3) The leaflets must have sufficiently nonlinear material response to function over a variety of pressures. (4) Valve performance is highly sensitive to free edge length and leaflet height. These conclusions suggest appropriate gross morphology and material properties for the design of prosthetic aortic valves. In future studies, our aortic valve modeling framework can be used with patient-specific models of vascular or cardiac flow.

Linking collagen fiber architecture to tissue-level biaxial mechanical behaviors of porcine semilunar heart valve cusps

The semilunar heart valves regulate the blood flow from the ventricles to the major arteries through the opening and closing of the scallop shaped cusps. These cusps are composed of collagen fibers that act as the primary loading-bearing component. The load-dependent collagen fiber architecture has been previously examined in the existing literature; however, these studies relied on chemical clearing and tissue modifications to observe the underlying changes in response to mechanical loads. In the present study, we address this gap in knowledge by quantifying the collagen fiber orientations and alignments of the aortic and pulmonary cusps through a multi-scale, non-destructive experimental approach. This opto-mechanical approach, which combines polarized spatial frequency domain imaging and biaxial mechanical testing, provides a greater field of view (10-25mm) and faster imaging time (45-50s) than other traditional collagen imaging techniques. The birefringent response of the collagen fibers was fit with a von Mises distribution, while the biaxial mechanical testing data was implemented into a modified full structural model for further analysis. Our results showed that the semilunar heart valve cusps are more extensible in the tissue's radial direction than the circumferential direction under all the varied biaxial testing protocols, together with greater material anisotropy among the pulmonary valve cusps compared to the aortic valve cusps. The collagen fibers were shown to reorient towards the direction of the greatest applied loading and incrementally realign with the increased applied stress. The collagen fiber architecture within the aortic valve cusps were found to be more homogeneous than the pulmonary valve counterparts, reflecting the differences in the physiological environments experienced by these two semilunar heart valves. Further, the von Mises distribution fitting highlighted the presence and contribution of two distinct fiber families for each of the two semilunar heart valves. The results from this work would provide valuable insight into connecting tissue-level mechanics to the underlying collagen fiber architecture-an essential information for the future development of high-fidelity aortic/pulmonary valve computational models.

Parameterization, geometric modeling, and isogeometric analysis of tricuspid valves

Approximately 1.6 million patients in the United States are affected by tricuspid valve regurgitation, which occurs when the tricuspid valve does not close properly to prevent backward blood flow into the right atrium. Despite its critical role in proper cardiac function, the tricuspid valve has received limited research attention compared to the mitral and aortic valves on the left side of the heart. As a result, proper valvular function and the pathologies that may cause dysfunction remain poorly understood. To promote further investigations of the biomechanical behavior and response of the tricuspid valve, this work establishes a parameter-based approach that provides a template for tricuspid valve modeling and simulation. The proposed tricuspid valve parameterization presents a comprehensive description of the leaflets and the complex chordae tendineae for capturing the typical three-cusp structural deformation observed from medical data. This simulation framework develops a practical procedure for modeling tricuspid valves and offers a robust, flexible approach to analyze the performance and effectiveness of various valve configurations using isogeometric analysis. The proposed methods also establish a baseline to examine the tricuspid valve's structural deformation, perform future investigations of native valve configurations under healthy and disease conditions, and optimize prosthetic valve designs.

Effect of Valve Height on the Opening and Closing Performance of the Aortic Valve Under Aortic Root Dilatation

Patients with aortic valve disease can suffer from valve insufficiency after valve repair surgery due to aortic root dilatation. The paper investigates the effect of valve height (Hv) on the aortic valve opening and closing in order to select the appropriate range of Hv for smoother blood flow through the aortic valve and valve closure completely in the case of continuous aortic root dilatation. A total of 20 parameterized three-dimensional models of the aortic root were constructed following clinical surgical guidance. Aortic annulus diameter (DAA) was separately set to 26, 27, 28, 29, and 30 mm to simulate aortic root dilatation. H_V value was separately set to 13.5, 14, 14.5, and 15 mm to simulate aortic valve alterations in surgery. Time-varying pressure loads were applied to the valve, vessel wall of the ascending aorta, and left ventricle. Then, finite element analysis software was employed to simulate the movement and mechanics of the aortic root. The feasible design range of the valve size was evaluated using maximum stress, geometric orifice area (GOA), and leaflet contact force. The results show that the valve was incompletely closed when H_V was 13.5 mm and D_AA was 29 or 30 mm. The GOA of the valve was small when H_V was 15 mm and D_AA was 26 or 27 mm. The corresponding values of the other models were within the normal range. Compared with the model with an H_V of 14 mm, the model with an H_V of 14.5 mm could effectively reduce maximum stress and had relatively larger GOA and less change in contact force. As a result, valve height affects the performance of aortic valve opening and closing. Smaller H_V is adapted to smaller D_AA and vice versa. When H_V is 14.5 mm, the valve is well adapted to the dilatation of the aortic root to enhance repair durability. Therefore, more attention should be paid to H_V in surgical planning.

Incorporating nanocrystalline cellulose into a multifunctional hydrogel for heart valve tissue engineering applications

Functional tissue engineered heart valves (TEHV) have been an elusive goal for nearly 30 years. Among the persistent challenges are the requirements for engineered valve leaflets that possess nonlinear elastic tissue biomechanical properties, support quiescent fibroblast phenotype, and resist osteogenic differentiation. Nanocellulose is an attractive tunable biological material that has not been employed to this application. In this study, we fabricated a series of photocrosslinkable composite hydrogels mNCC-MeGel (mNG) by conjugating TEMPO-modified nanocrystalline cellulose (mNCC) onto the backbone of methacrylated gelatin (MeGel). Their structures were characterized by FTIR, ¹ HNMR and uniaxial compression testing. Human adipose-derived mesenchymal stem cells (HADMSC) were encapsulated within the material and evaluated for valve interstitial cell phenotypes over 14 days culture in both normal and osteogenic media. Compared to the MeGel control group, the HADMSC encapsulated within mNG showed decreased alpha smooth muscle actin (αSMA) expression and increased vimentin and aggrecan expression, suggesting the material supports a quiescent fibroblastic phenotype. Under osteogenic media conditions, HADMSC within mNG hydrogels showed lower expression of osteogenic genes, including Runx2 and osteocalcin, indicating resistance toward calcification. As a proof of principle, the mNG hydrogel, combined with a viscosity enhancing agent, was used to 3D bioprint a tall, self-standing tubular structure that sustained cell viability. Together, these results identify mNG as an attractive biomaterial for TEHV applications.

A Novel Pulmonary Valve Replacement Surgery Strategy Using Contracting Band for Patients With Repaired Tetralogy of Fallot: An MRI-Based Multipatient Modeling Study

Patients with repaired Tetralogy of Fallot (ToF), a congenital heart defect which includes a ventricular septal defect and severe right ventricular outflow obstruction, account for the majority of cases with late-onset right ventricle (RV) failure. Current surgery procedures, including pulmonary valve replacement (PVR) with right ventricle remodeling, yield mixed results. PVR with active band insertion was hypothesized to be of clinical usage on improving RV function measured by ejection fraction (EF). In lieu of risky open-heart surgeries and experiments on animal and human, computational biomechanical models were adapted to study the impact of PVR with five band insertion options. Cardiac magnetic resonance (CMR) images were acquired from seven TOF patients before PVR surgery for model construction. For each patient, five different surgery plans combined with passive and active contraction band with contraction ratio of 20, 15, and 10% were studied. Those five plans include three single-band plans with different band locations; one plan with two bands, and one plan with three bands. Including the seven no-band models, 147 computational bi-ventricle models were constructed to simulate RV cardiac functions and identify optimal band plans. Patient variations with different band plans were investigated. Surgery plan with three active contraction bands and band active contraction ratio of 20% had the best performance on improving RV function. The mean ± SD RV ejection fraction value from the seven patients was 42.90 ± 5.68%, presenting a 4.19% absolute improvement or a 10.82% relative improvement, when compared with the baseline models (38.71 ± 5.73%, p = 0.016). The EF improvements from the seven patients varied from 2.87 to 6.01%. Surgical procedures using active contraction bands have great potential to improve RV function measured by ejection fraction for patients with repaired ToF. It is possible to have higher right ventricle ejection fraction improvement with more bands and higher band active contraction ratio. Our findings with computational models need to be further validated by animal experiments before clinical trial could become possible.

Feasibility of two-dimensional speckle-tracking echocardiography of aortic valve in patients with calcific aortic valve disease

Calcific aortic valve (AV) disease is associated with increased stiffness and reduced motion of AV leaflets, has a progressive course, and can develop into aortic stenosis (AS). Our aim was to evaluate whether two-dimensional speckle-tracking echocardiography (STE) may be used for the assessment of AV stiffness. We applied STE to AV leaflets in patients with various degrees of degenerative changes of AV and measured strain as an absolute value of the radial deformation of AV leaflets. Deformation of AV expressed as averaged AV strain was greatest in patients with a normal AV (23.4 ± 6.4%), compared to those with aortic sclerosis (12.9 ± 3.2%), moderate-to-severe AS (11.9 ± 4%), and severe AS (10.9 ± 3.5%) (p < 0.01). A non-linear relationship and moderate correlation of AV strain with transvalvular hemodynamic parameters was observed. In patients with mild-to-moderate AS, the strain of AV leaflets also correlated negatively with AV calcification (r = -0.59, p = 0.008). Good inter-observer agreement was obtained for averaged AV strain with a coefficient of variation of 0.15 and an interclass correlation coefficient of 0.94 (p < 0.0001). In this study we demonstrated that deformation of AV leaflets as assessed by STE might be a potential method for a non-invasive evaluation of AV biomechanical properties and of the progression of calcific aortic disease. Further development of the two-dimensional speckle tracking technique specifically for valve structures is needed to enable a better quantification of leaflet deformation.

Subject-specific multiscale modeling of aortic valve biomechanics

A Finite Element workflow for the multiscale analysis of the aortic valve biomechanics was developed and applied to three physiological anatomies with the aim of describing the aortic valve interstitial cells biomechanical milieu in physiological conditions, capturing the effect of subject-specific and leaflet-specific anatomical features from the organ down to the cell scale. A mixed approach was used to transfer organ-scale information down to the cell-scale. Displacement data from the organ model were used to impose kinematic boundary conditions to the tissue model, while stress data from the latter were used to impose loading boundary conditions to the cell level. Peak of radial leaflet strains was correlated with leaflet extent variability at the organ scale, while circumferential leaflet strains varied over a narrow range of values regardless of leaflet extent. The dependency of leaflet biomechanics on the leaflet-specific anatomy observed at the organ length-scale is reflected, and to some extent emphasized, into the results obtained at the lower length-scales. At the tissue length-scale, the peak diastolic circumferential and radial stresses computed in the fibrosa correlated with the leaflet surface area. At the cell length-scale, the difference between the strains in two main directions, and between the respective relationships with the specific leaflet anatomy, was even more evident; cell strains in the radial direction varied over a relatively wide range (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$0.36-0.87$$\end{document}0.36-0.87) with a strong correlation with the organ length-scale radial strain (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$R^{2}= 0.95$$\end{document}R2=0.95); conversely, circumferential cell strains spanned a very narrow range (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$0.75-0.88$$\end{document}0.75-0.88) showing no correlation with the circumferential strain at the organ level (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$R^{2}= 0.02$$\end{document}R2=0.02). Within the proposed simulation framework, being able to account for the actual anatomical features of the aortic valve leaflets allowed to gain insight into their effect on the structural mechanics of the leaflets at all length-scales, down to the cell scale.

A pilot study on biaxial mechanical, collagen microstructural, and morphological characterizations of a resected human intracranial aneurysm tissue

Intracranial aneurysms (ICAs) are focal dilatations that imply a weakening of the brain artery. Incidental rupture of an ICA is increasingly responsible for significant mortality and morbidity in the American’s aging population. Previous studies have quantified the pressure-volume characteristics, uniaxial mechanical properties, and morphological features of human aneurysms. In this pilot study, for the first time, we comprehensively quantified the mechanical, collagen fiber microstructural, and morphological properties of one resected human posterior inferior cerebellar artery aneurysm. The tissue from the dome of a right posterior inferior cerebral aneurysm was first mechanically characterized using biaxial tension and stress relaxation tests. Then, the load-dependent collagen fiber architecture of the aneurysm tissue was quantified using an in-house polarized spatial frequency domain imaging system. Finally, optical coherence tomography and histological procedures were used to quantify the tissue’s microstructural morphology. Mechanically, the tissue was shown to exhibit hysteresis, a nonlinear stress-strain response, and material anisotropy. Moreover, the unloaded collagen fiber architecture of the tissue was predominantly aligned with the testing Y-direction and rotated towards the X-direction under increasing equibiaxial loading. Furthermore, our histological analysis showed a considerable damage to the morphological integrity of the tissue, including lack of elastin, intimal thickening, and calcium deposition. This new unified characterization framework can be extended to better understand the mechanics-microstructure interrelationship of aneurysm tissues at different time points of the formation or growth. Such specimen-specific information is anticipated to provide valuable insight that may improve our current understanding of aneurysm growth and rupture potential.

A Cell-free Biodegradable Synthetic Artificial Ligament for the Reconstruction of Anterior Cruciate Ligament in a Rat Model

Traditional Anterior Cruciate Ligament (ACL) reconstruction is commonly performed using an allograft or autograft and possesses limitations such as donor site morbidity, decreased range of motion, and potential infection. However, a biodegradable synthetic graft could greatly assist in the prevention of such restrictions after ACL reconstruction. In this study, artificial grafts were generated using "wet" and "dry" electrospinning processes with a biodegradable elastomer, poly (ester urethane) urea (PEUU), and were evaluated in vitro and in vivo in a rat model. Four groups were established: (1) Wet PEUU artificial ligament, (2) Dry PEUU artificial ligament, (3) Dry polycaprolactone artificial ligament (PCL), and (4) autologous flexor digitorum longus tendon graft. Eight weeks after surgery, the in vivo tensile strength of wet PEUU ligaments had significantly increased compared to the other synthetic ligaments. These results aligned with increased infiltration of host cells and decreased inflammation within the wet PEUU grafts. In contrast, very little cellular infiltration was observed in PCL and dry PEUU grafts. Micro-computed tomography analysis performed at 4 and 8 weeks postoperatively revealed significantly smaller bone tunnels in the tendon autograft and wet PEUU groups. The Wet PEUU grafts served as an adequate functioning material and allowed for the creation of tissues that closely resembled the ACL.

Poly(ethylene glycol)-Based Coatings for Bioprosthetic Valve Tissues: Toward Restoration of Physiological Behavior

Bioprosthetic valves (BPVs) have a limited lifespan in the body necessitating repeated surgeries to replace the failed implant. Early failure of these implants has been linked to various surface properties of the valve. Surface properties of BPVs are significantly different from physiological valves because of the fixation process used when processing the xenograft tissue. To improve the longevity of BPVs, efforts need to be taken to improve the surface properties and shield the implant from the bodily interactions that degrade it. Toward this goal, we evaluated the use of hydrogel coatings to attach to the BPV tissue and impart surface properties that are close to physiological. Hydrogels are well characterized for their biocompatibility and highly tunable surface characteristics. Using a previously published coating method, we deposited hydrogel coatings of poly(ethylene glycol)diacrylate (PEGDA) and poly(ethylene glycol)diacrylamide (PEGDAA) atop BPV samples. Coated samples were evaluated against the physiological tissue and uncoated glutaraldehyde-fixed tissue for deposition of hydrogel, surface adherence, mechanical properties, and fixation properties. Results showed both PEGDA- and PEGDAA-deposited coatings were nearly continuous across the valve leaflet surface. Further, the PEGDA- and PEGDAA-coated samples showed restoration of physiological levels of protein adhesion and mechanical stiffness. Interestingly, the coating process rather than the coating itself altered the material behavior yet did not alter the cross-linking from fixation. These results show that the PEG-based coatings for BPVs can successfully alter surface properties of BPVs and help promote physiological characteristics without interfering with the necessary fixation.

Biology and Biomechanics of the Heart Valve Extracellular Matrix

Heart valves are dynamic structures that, in the average human, open and close over 100,000 times per day, and 3 × 10⁹ times per lifetime to maintain unidirectional blood flow. Efficient, coordinated movement of the valve structures during the cardiac cycle is mediated by the intricate and sophisticated network of extracellular matrix (ECM) components that provide the necessary biomechanical properties to meet these mechanical demands. Organized in layers that accommodate passive functional movements of the valve leaflets, heart valve ECM is synthesized during embryonic development, and remodeled and maintained by resident cells throughout life. The failure of ECM organization compromises biomechanical function, and may lead to obstruction or leaking, which if left untreated can lead to heart failure. At present, effective treatment for heart valve dysfunction is limited and frequently ends with surgical repair or replacement, which comes with insuperable complications for many high-risk patients including aged and pediatric populations. Therefore, there is a critical need to fully appreciate the pathobiology of biomechanical valve failure in order to develop better, alternative therapies. To date, the majority of studies have focused on delineating valve disease mechanisms at the cellular level, namely the interstitial and endothelial lineages. However, less focus has been on the ECM, shown previously in other systems, to be a promising mechanism-inspired therapeutic target. Here, we highlight and review the biology and biomechanical contributions of key components of the heart valve ECM. Furthermore, we discuss how human diseases, including connective tissue disorders lead to aberrations in the abundance, organization and quality of these matrix proteins, resulting in instability of the valve infrastructure and gross functional impairment.

Mechanical testing of glutaraldehyde cross-linked mitral valves. Part two: Elastic and viscoelastic properties of chordae tendineae

The aim of this study was to assess whether the mechanical properties of mitral valve chordae tendineae are sensitive to being cross-linked under load. A total 64 chordae were extracted from eight porcine hearts. Two chordae (posterior basal) from each heart were subjected to uniaxial ramp testing and six chordae (two strut, two anterior basal and two posterior basal) were subjected to dynamic mechanical analysis over frequencies between 0.5 and 10 Hz. Chordae were either cross-linked in tension or cross-linked in the absence of loading. Chordae cross-linked under load transitioned from high to low extension at a lower strain than cross-linked unloaded chordae (0.07 cf. 0.22), with greater pre-transitional (30.8 MPa cf. 5.78 MPa) and post-transitional (139 MPa cf. 74.1 MPa) moduli. The mean storage modulus of anterior strut chordae ranged from 48 to 54 MPa for cross-linked unloaded chordae, as compared to 53-61 MPa cross-linked loaded chordae. The mean loss modulus of anterior strut chordae ranged from 2.3 to 2.9 MPa for cross-linked unloaded chordae, as compared to 3.8-4.8 MPa cross-linked loaded chordae. The elastic and viscoelastic properties of chordae following glutaraldehyde cross-linking are dependent on the inclusion/exclusion of loading during the cross-linking process; with loading increasing the magnitude of the material properties measured.

Mechanical testing of glutaraldehyde cross-linked mitral valves. Part one: In vitro mechanical behaviour

The aim of this study was to perform an initial assessment, in vitro, of the feasibility of using a glutaraldehyde cross-linked porcine mitral valve to retain acute functionality, focusing on assessing mitral regurgitation. Six porcine hearts were tested using an in vitro simulator. Testing was repeated following cross-linking of mitral valves; where cross-linking was achieved by placing them in a glutaraldehyde solution. The simulator enabled systolic pressure on the ventricular side of the valve to be mimicked. Following testing, mitral valve leaflets underwent Scanning Electron Microscopy of the ventricular surface of both the anterior and posterior leaflets (1 cm² samples). The peak pressure withstood by cross-linked valves was significantly lower than for untreated valves (108 mmHg cf. 128 mmHg for untreated valves; p  < 0.05). The peak pressure was typically reached 0.5 s later than for the untreated valve. While both cross-linked and untreated valves exhibited endothelium denudation, the unfixed valve had less endothelial loss. Glutaraldehyde cross-linking of porcine mitral valves may be of potential value in assessing improved bioprosthetic mitral valve replacements. However, a more immobile valve exhibiting endothelial denudation (i.e. sclerosis) was a possible concerns identified following in vitro acute assessment.

A versatile biaxial testing platform for soft tissues

Uniaxial testing remains the most common modality of mechanical analysis for biological and other soft materials; however, biaxial testing enables a more comprehensive understanding of these materials' mechanical behavior. In recent years, a number of commercially available biaxial testing systems designed for biological materials have been produced; however, there are common limitations that are often associated with using these systems. For example, the range of allowable sample geometries are relatively constrained, the clamping systems are relatively limited with respect to allowable configurations, the load and displacement ranges are relatively small, and the software and control elements offer relatively limited options. Due to these constraints, there are significant benefits associated with designing custom biaxial testing systems that meet the technical requirements for testing a broad range of materials. Herein we present a design for a biaxial testing system with capabilities that extend beyond those associated with typical commercially available systems. Our design is capable of performing uniaxial tests, traditional biaxial tests, and double lap shear (simple shear) tests, in either a displacement or load control mode. Testing protocols have been developed and proof-of-concept experiments have been performed on commercially available silicone membranes and rat abdominal skin samples.

3D Printing, Computational Modeling, and Artificial Intelligence for Structural Heart Disease

Structural heart disease (SHD) is a new field within cardiovascular medicine. Traditional imaging modalities fall short in supporting the needs of SHD interventions, as they have been constructed around the concept of disease diagnosis. SHD interventions disrupt traditional concepts of imaging in requiring imaging to plan, simulate, and predict intraprocedural outcomes. In transcatheter SHD interventions, the absence of a gold-standard open cavity surgical field deprives physicians of the opportunity for tactile feedback and visual confirmation of cardiac anatomy. Hence, dependency on imaging in periprocedural guidance has led to evolution of a new generation of procedural skillsets, concept of a visual field, and technologies in the periprocedural planning period to accelerate preclinical device development, physician, and patient education. Adaptation of 3-dimensional (3D) printing in clinical care and procedural planning has demonstrated a reduction in early-operator learning curve for transcatheter interventions. Integration of computation modeling to 3D printing has accelerated research and development understanding of fluid mechanics within device testing. Application of 3D printing, computational modeling, and ultimately incorporation of artificial intelligence is changing the landscape of physician training and delivery of patient-centric care. Transcatheter structural heart interventions are requiring in-depth periprocedural understanding of cardiac pathophysiology and device interactions not afforded by traditional imaging metrics.

A structural-based computational model of tendon-bone insertion tissues

Tendon-to-bone insertion provides a gradual transition from soft tendon to hard bone tissue, functioning to alleviate stress concentrations at the junction of these tissues. Such macroscopic mechanical properties are achieved due to the internal structure in which collagen fibers and mineralization levels are key ingredients. We develop a structural-based model of tendon-to-bone insertion incorporating such details as fiber preferred orientation, fiber directional dispersion, mineralization level, and their inhomogeneous spatial distribution. A python script is developed to alter the tapered tendon-bone transition zone and to provide spatial grading of material properties, which may be rather complex as experiments suggest. A simple linear interpolation between tendon and bone material properties is first used to describe the graded property within the insertion region. Stress distributions are obtained and compared for spatially graded and various piece-wise materials properties. It is observed that spatial grading results in more smooth stress distributions and significantly reduces maximum stresses. The geometry of the tissue model is optimized by minimizing the peak stress to mimic in-vivo tissue remodeling. The in-silico elastic models constructed in this work are verified and modified by comparing to our in-situ biaxial mechanical testing results, thereby serving as translational tools for accurately predicting the material behavior of the tendon-to-bone insertions. This model will be useful for understanding how tendon-to-bone insertion develops during tissue remodeling, as well as for developing orthopedic implants.

General method for classification of fiber families in fiber-reinforced materials: application to in-vivo human skin images

Fiber structures play a major role for the function of fiber-reinforced materials such as biological tissue. An objective classification of the fiber orientations into fiber families is crucial to understand its mechanical properties. We introduce the Fiber Image Network Evaluation Algorithm (FINE algorithm) to classify and quantify the number of fiber families in scientific images. Each fiber family is characterized by an amplitude, a mean orientation, and a dispersion. A new alignment index giving the averaged fraction of aligned fibers is defined. The FINE algorithm is validated by realistic grayscale Monte-Carlo fiber images. We apply the algorithm to an in-vivo depth scan of second harmonic generation images of dermal collagen in human skin. The derived alignment index exhibits a crossover at a critical depth where two fiber families with a perpendicular orientation around the main tension line arise. This strongly suggests the presence of a transition from the papillary to the reticular dermis. Hence, the FINE algorithm provides a valuable tool for a reliable classification and a meaningful interpretation of in-vivo collagen fiber networks and general fiber reinforced materials.

The role of stress concentration in calcified bicuspid aortic valve

Calcific aortic valve disease (CAVD) is the most common valvular heart disease in the aging population, and is now believed to be a slow, progressive, yet actively regulated process. The disease progression can be divided into two phases: initiation phase associated with lipid deposition and inflammation response, and the later propagation phase with active calcification growth. It has been hypothesized that elevated mechanical stress plays a major role in both phases of disease progression. In order to identify a direct link between leaflet stress and calcification development, we performed patient-specific finite-element (FE) analyses of six bicuspid aortic valves (BAV), where the leaflets, raphe and calcifications were all considered. The results showed that during the initiation phase, calcium buildup is likely to occur along the leaflet-root attachment curve (ATC), and the commissures, which are subject to the most drastic changes in stress during the cardiac cycle. During the propagation phase, the presence of calcification would lead to local stress concentration along its boundary, hence further calcification growth. Three patterns of calcification formation were identified on BAV leaflets: 'radial', which extended radially from ATC into the leaflet belly region; 'commissure to commissure', which extended circumferentially along the coaptation; and 'raphe', which located in the vicinity of the raphe. Furthermore, we found a strong correlation between regions with a high risk of calcium buildup and regions with elevated mechanical stress. The high-risk regions predicted at diastole on the non-calcified leaflet from FE models agreed reasonably well with the in vivo calcification locations, which indicates that patient-specific FE modelling could help us to evaluate the potential risk of calcification formation in the early stage of CAVD.

Fluid-Structure Interaction Models of Bioprosthetic Heart Valve Dynamics in an Experimental Pulse Duplicator

Computer modeling and simulation is a powerful tool for assessing the performance of medical devices such as bioprosthetic heart valves (BHVs) that promises to accelerate device design and regulation. This study describes work to develop dynamic computer models of BHVs in the aortic test section of an experimental pulse-duplicator platform that is used in academia, industry, and regulatory agencies to assess BHV performance. These computational models are based on a hyperelastic finite element extension of the immersed boundary method for fluid-structure interaction (FSI). We focus on porcine tissue and bovine pericardial BHVs, which are commonly used in surgical valve replacement. We compare our numerical simulations to experimental data from two similar pulse duplicators, including a commercial ViVitro system and a custom platform related to the ViVitro pulse duplicator. Excellent agreement is demonstrated between the computational and experimental results for bulk flow rates, pressures, valve open areas, and the timing of valve opening and closure in conditions commonly used to assess BHV performance. In addition, reasonable agreement is demonstrated for quantitative measures of leaflet kinematics under these same conditions. This work represents a step towards the experimental validation of this FSI modeling platform for evaluating BHVs.

Multi-Band Surgery for Repaired Tetralogy of Fallot Patients With Reduced Right Ventricle Ejection Fraction: A Pilot Study

Introduction

Right ventricle (RV) failure is one of the most common symptoms among patients with repaired tetralogy of Fallot (TOF). The current surgery treatment approach including pulmonary valve replacement (PVR) showed mixed post-surgery outcomes. A novel PVR surgical strategy using active contracting bands is proposed to improve the post-PVR outcome. In lieu of testing the risky surgical procedures on real patients, computational simulations (virtual surgery) using biomechanical ventricle models based on patient-specific cardiac magnetic resonance (CMR) data were performed to test the feasibility of the PVR procedures with active contracting bands. Different band combination and insertion options were tested to identify optimal surgery designs.

Method

Cardiac magnetic resonance data were obtained from one TOF patient (male, age 23) whose informed consent was obtained. A total of 21 finite element models were constructed and solved following our established procedures to investigate the outcomes of the band insertion surgery. The non-linear anisotropic Mooney–Rivlin model was used as the material model. Five different band insertion plans were simulated (three single band models with different band locations, one model with two bands, and one model with three bands). Three band contraction ratios (10, 15, and 20%) and passive bands (0% contraction ratio) were tested. RV ejection fraction was used as the measure for cardiac function.

Results

The RV ejection fraction from the three-band model with 20% contraction increased to 41.58% from the baseline of 37.38%, a 4.20% absolute improvement. The RV ejection fractions from the other four band models with 20% contraction rate were 39.70, 39.45, and 40.70% (two-band) and 39.17%, respectively. The mean RV stress and strain values from all of the 21 models showed only modest differences (5–11%).

Conclusion

This pilot study demonstrated that the three-band model with 20% band contraction ratio led to 4.20% absolute improvement in the RV ejection fraction, which is considered as clinically significant. The passive elastic bands led to the reduction of the RV ejection fractions. The modeling results and surgical strategy need to be further developed and validated by a multi-patient study and animal experiments before clinical trial could become possible. Tissue regeneration techniques are needed to produce materials for the contracting bands.

Skin Structure-Function Relationships and the Wound Healing Response to Intrinsic Aging

Significance: Chronic wounds, such as diabetic foot ulcers, venous stasis ulcers, and pressure ulcers affect millions of Americans each year, and disproportionately afflict our increasingly older population. Older individuals are predisposed to wound infection, repeated trauma, and the development of chronic wounds. However, a complete understanding of how the attributes of aging skin affect the wound healing process has remained elusive. Recent Advances: A variety of studies have demonstrated that the dermal matrix becomes thinner, increasingly crosslinked, and fragmented with advanced age. These structural changes, as well as an increase in cell senescence, result in altered collagen fiber remodeling and increased stiffness. Studies combining mechanical testing with advanced imaging techniques are providing new insights into the relationships between these age-related changes. Emerging research into the mechanobiology of aging and the wound healing process indicate that the altered mechanical environment of aged skin may have a significant effect on age-related delays in healing. Critical Issues: The interpretation and synthesis of clinical studies is confounded by the effects of common comorbidities that also contribute to the development of chronic wounds. A lack of quantitative biomarkers of wound healing and age-related changes makes understanding structure-function relationships during the wound healing process challenging. Future Directions: Additional work is needed to establish quantitative and mechanistic relationships among age-related changes in the skin microstructure, mechanical function, and the cellular responses to wound healing.

Modeling risk of coronary obstruction during transcatheter aortic valve replacement

OBJECTIVE

In this study we aimed to evaluate risk of coronary obstruction during transcatheter aortic valve replacement and develop improved criteria based on computational modeling.

METHODS

Patient specific 3-dimensional models were constructed and validated for 28 patients out of 600 patients who were flagged as high risk for coronary obstruction (defined as meeting coronary ostium height < 14 mm and/or sinus of Valsalva diameter [SOVd] < 30 mm). The models consisted finite element analysis to predict the post- transcatheter aortic valve replacement native cusp apposition relative to the coronary ostium and were validated in vitro. The distance from cusp to coronary ostium (DLC) was derived from the 3-dimensional models and indexed with the coronary artery diameter to yield a fractional obstruction measure (DLC/d).

RESULTS

Twenty-two out of 28 high-risk patients successfully underwent transcatheter aortic valve replacement without coronary obstruction and 6 did not. DLC/d between the 2 groups was significantly different (P < .00078), whereas neither coronary ostium height nor SOVd were significantly different (P > .32). A cutoff of DLC/d < 0.7 was predictive with 100% sensitivity and 95.7% specificity. The optimal sensitivity and specificity of coronary ostium height and SOVd in this high-risk group was only 60% and 40%, respectively, for cutoff coronary ostium height of 10 mm and SOVd of 30.5 mm.

CONCLUSIONS

Three-dimensional modeling has the potential to enable more patients to be safely treated with transcatheter aortic valve replacement who have a low-lying coronary ostium or small SOVd. DLC/d is more predictive of obstruction than coronary ostium height and SOVd.

Mechanical considerations for polymeric heart valve development: Biomechanics, materials, design and manufacturing

The native human heart valve leaflet contains a layered microstructure comprising a hierarchical arrangement of collagen, elastin, proteoglycans and various cell types. Here, we review the various experimental methods that have been employed to probe this intricate microstructure and which attempt to elucidate the mechanisms that govern the leaflet's mechanical properties. These methods include uniaxial, biaxial, and flexural tests, coupled with microstructural characterization techniques such as small angle X-ray scattering (SAXS), small angle light scattering (SALS), and polarized light microscopy. These experiments have revealed complex elastic and viscoelastic mechanisms that are highly directional and dependent upon loading conditions and biochemistry. Of all engineering materials, polymers and polymer-based composites are best able to mimic the tissue-level mechanical behavior of the native leaflet. This similarity to native tissue permits the fabrication of polymeric valves with physiological flow patterns, reducing the risk of thrombosis compared to mechanical valves and in some cases surpassing the in vivo durability of bioprosthetic valves. Earlier work on polymeric valves simply assumed the mechanical properties of the polymer material to be linear elastic, while more recent studies have considered the full hyperelastic stress-strain response. These material models have been incorporated into computational models for the optimization of valve geometry, with the goal of minimizing internal stresses and improving durability. The latter portion of this review recounts these developments in polymeric heart valves, with a focus on mechanical testing of polymers, valve geometry, and manufacturing methods.

Mechanical and finite element evaluation of a bioprinted scaffold following recellularization in a rat subcutaneous model

Tissue engineered heart valves (TEHV) provide several advantages over currently available aortic heart valve replacements. Bioprinting provides a patient-specific means of developing a TEHV scaffold from imaging data, and the capability to embed the patient's own cells within the scaffold. In this work we investigated the remodeling capacity of a collagen-based bio-ink by implanting bioprinted disks in a rat subcutaneous model for 2, 4 and 12 weeks and evaluating the mechanical response using biaxial testing and subsequent finite element (FE) modeling. Samples explanted after 2 and 4 weeks showed inferior mechanical properties compared to native tissues while 12 week explants showed a mechanical response of similar magnitude but did not demonstrate the anisotropy present in native tissues. In the FE analysis, the model utilizing mechanical properties from samples explanted after 12 weeks showed the closest mechanical behavior to the native tissues. However, in diastole native tissues showed higher stress in the leaflet belly and lower strain at the commissures compared to 12 week explants, likely due to the anisotropy present in the native tissues. Thus, either further remodeling is required in situ in the aortic valve position or by in vitro preconditioning in an environment such as a bioreactor. Regardless, these results demonstrate the utility of FE analysis to optimize bioprinting process parameters for the most favorable in vivo mechanical performance.

Quantification of Material Constants for a Phenomenological Constitutive Model of Porcine Tricuspid Valve Leaflets for Simulation Applications

The tricuspid valve is a one-way valve on the pulmonary side of the heart, which prevents backflow of blood during ventricular contractions. Development of computational models of the tricuspid valve is important both in understanding the normal valvular function and in the development/improvement of surgical procedures and medical devices. A key step in the development of such models is quantification of the mechanical properties of the tricuspid valve leaflets. In this study, after examining previously measured five-loading-protocol biaxial stress-strain response of porcine tricuspid valves, a phenomenological constitutive framework was chosen to represent this response. The material constants were quantified for all three leaflets, which were shown to be highly anisotropic with average anisotropy indices of less than 0.5 (an anisotropy index value of 1 indicates a perfectly isotropic response, whereas a smaller value of the anisotropy index indicates an anisotropic response). To obtain mean values of material constants, stress-strain responses of the leaflet samples were averaged and then fitted to the constitutive model (average R2 over 0.9). Since the sample thicknesses were not hugely different, averaging the data using the same tension levels and stress levels produced similar average material constants for each leaflet.