Modification of infarct material properties limits adverse ventricular remodeling

Polysaccharide-based hydrogels for medical devices, implants and tissue engineering: A review

Hydrogels are highly biocompatible biomaterials composed of crosslinked three-dimensional networks of hydrophilic polymers. Owing to their natural origin, polysaccharide-based hydrogels (PBHs) possess low toxicity, high biocompatibility and demonstrate in vivo biodegradability, making them great candidates for use in various biomedical devices, implants, and tissue engineering. In addition, many polysaccharides also show additional biological activities such as antimicrobial, anticoagulant, antioxidant, immunomodulatory, hemostatic, and anti-inflammatory, which can provide additional therapeutic benefits. The porous nature of PBHs allows for the immobilization of antibodies, aptamers, enzymes and other molecules on their surface, or within their matrix, potentiating their use in biosensor devices. Specific polysaccharides can be used to produce transparent hydrogels, which have been used widely to fabricate ocular implants. The ability of PBHs to encapsulate drugs and other actives has been utilized for making neural implants and coatings for cardiovascular devices (stents, pacemakers and venous catheters) and urinary catheters. Their high water-absorption capacity has been exploited to make superabsorbent diapers and sanitary napkins. The barrier property and mechanical strength of PBHs has been used to develop gels and films as anti-adhesive formulations for the prevention of post-operative adhesion. Finally, by virtue of their ability to mimic various body tissues, they have been explored as scaffolds and bio-inks for tissue engineering of a wide variety of organs. These applications have been described in detail, in this review.

Personalized biomechanical insights in atrial fibrillation: opportunities & challenges

INTRODUCTION

Atrial fibrillation (AF) is an increasingly prevalent and significant worldwide health problem. Manifested as an irregular atrial electrophysiological activation, it is associated with many serious health complications. AF affects the biomechanical function of the heart as contraction follows the electrical activation, subsequently leading to reduced blood flow. The underlying mechanisms behind AF are not fully understood, but it is known that AF is highly correlated with the presence of atrial fibrosis, and with a manifold increase in risk of stroke.

AREAS COVERED

In this review, we focus on biomechanical aspects in atrial fibrillation, current and emerging use of clinical images, and personalized computational models. We also discuss how these can be used to provide patient-specific care.

EXPERT OPINION

Understanding the connection betweenatrial fibrillation and atrial remodeling might lead to valuable understanding of stroke and heart failure pathophysiology. Established and emerging imaging modalities can bring us closer to this understanding, especially with continued advancements in processing accuracy, reproducibility, and clinical relevance of the associated technologies. Computational models of cardiac electromechanics can be used to glean additional insights on the roles of AF and remodeling in heart function.

Computational models of ventricular mechanics and adaptation in response to right-ventricular pressure overload

Pulmonary arterial hypertension (PAH) is associated with substantial remodeling of the right ventricle (RV), which may at first be compensatory but at a later stage becomes detrimental to RV function and patient survival. Unlike the left ventricle (LV), the RV remains understudied, and with its thin-walled crescent shape, it is often modeled simply as an appendage of the LV. Furthermore, PAH diagnosis is challenging because it often leaves the LV and systemic circulation largely unaffected. Several treatment strategies such as atrial septostomy, right ventricular assist devices (RVADs) or RV resynchronization therapy have been shown to improve RV function and the quality of life in patients with PAH. However, evidence of their long-term efficacy is limited and lung transplantation is still the most effective and curative treatment option. As such, the clinical need for improved diagnosis and treatment of PAH drives a strong need for increased understanding of drivers and mechanisms of RV growth and remodeling (G&R), and more generally for targeted research into RV mechanics pathology. Computational models stand out as a valuable supplement to experimental research, offering detailed analysis of the drivers and consequences of G&R, as well as a virtual test bench for exploring and refining hypotheses of growth mechanisms. In this review we summarize the current efforts towards understanding RV G&R processes using computational approaches such as reduced-order models, three dimensional (3D) finite element (FE) models, and G&R models. In addition to an overview of the relevant literature of RV computational models, we discuss how the models have contributed to increased scientific understanding and to potential clinical treatment of PAH patients.

Biomimetic Strategies to Develop Bioactive Scaffolds for Myocardial Tissue Engineering

The aim of this paper is to provide an overview of the results of the research activity carried out in our laboratories, over the last 10 years, in relation to the development of strategies for the production of biomimetic and bioactive scaffolds for myocardial tissue engineering. Biomimetic and bioactive polymeric scaffolds for cardiac regeneration were designed and manufactured in our laboratories and their morphological, physicochemical, mechanical and biological properties were investigated by different techniques, such as scanning electron microscopy, infrared chemical imaging, swelling test, in vitro degradation assessment, dynamic mechanical analysis, in vitro and in vivo biological tests. Biomimetic scaffolds, able to favor tissue regeneration by mimicking nature, were engineered by different strategies, comprising: (i) the imitation of the composition and interactions among components of the natural extracellular matrix (ECM), by mixing of proteins and polysaccharides; (ii) the material surface modification, using both traditional and innovative techniques, such as molecular imprinting; (iii) the incorporation and release of specific active agents and (iv) the production of scaffolds with a microarchitecture similar to that of native ECM. All the developed strategies were found to be effective in creating materials able to influence cellular behavior and therefore to favor the process of new tissue formation. In particular, the approach based on the combination of different strategies aimed at creating a system capable of communicating with the cells and promoting specific cellular responses, as the ECM does, has appeared particularly promising, in view to favor the formation of a tissue equivalent to the cardiac one.

Influence of Supraphysiologic Biomaterial Stiffness on Ventricular Mechanics and Myocardial Infarct Reinforcement

Injectable intramyocardial biomaterials have promise to limit adverse ventricular remodeling through mechanical and biologic mechanisms. While some success has been observed by injecting materials to regenerate new tissue, optimal biomaterial stiffness to thicken and stiffen infarcted myocardium to limit adverse remodeling has not been determined. In this work, we present an in-vivo study of the impact of biomaterial stiffness over a wide range of stiffness moduli on ventricular mechanics. We utilized injectable methacrylated polyethylene glycol (PEG) hydrogels fabricated at 3 different mechanical moduli: 5 kPa (low), 25 kPa (medium/myocardium), and 250 kPa (high/supraphysiologic). We demonstrate that the supraphysiological high stiffness favorably alters post-infarct ventricular mechanics and prevents negative tissue remodeling. Lower stiffness materials do not alter mechanics and thus to be effective, must instead target biological reparative mechanisms. These results may influence rationale design criteria for biomaterials developed for infarct reinforcement therapy. STATEMENT OF SIGNIFICANCE: Acellular biomaterials for cardiac application can provide benefit via mechanical and biological mechanisms post myocardial infarction. We study the role of biomaterial mechanical characteristics on ventricular mechanics in myocardial infarcts. Previous studies have not measured the influence of injected biomaterials on ventricular mechanics, and consequently rational design criteria is unknown. By utilizing an in-vivo assessment of ventricular mechanics, we demonstrate that low stiffness biomaterial do not alter pathologic ventricular mechanics. Thus, to be effective, low stiffness biomaterials must target biological reparative mechanisms. Physiologic and supra-physiologic biomaterials favorably alter post-infarct mechanics and prevents adverse ventricular remodeling.

Support Pressure Acting on the Epicardial Surface of a Rat Left Ventricle—A Computational Study

The present computational study investigates the effects of an epicardial support pressure mimicking a heart support system without direct blood contact. We chose restrictive cardiomyopathy as a model for a diseased heart. By changing one parameter representing the amount of fibrosis, this model allows us to investigate the impairment in a diseased left ventricle, both during diastole and systole. The aim of the study is to determine the temporal course and value of the support pressure that leads to a normalization of the cardiac parameters in diseased hearts. These are quantified via the end-diastolic pressure, end-diastolic volume, end-systolic volume, and ejection fraction. First, the amount of fibrosis is increased to model diseased hearts at different stages. Second, we determine the difference in the left ventricular pressure between a healthy and diseased heart during a cardiac cycle and apply for the epicardial support as the respective pressure difference. Third, an epicardial support pressure is applied in form of a piecewise constant step function. The support is provided only during diastole, only during systole, or during both phases. Finally, the support pressure is adjusted to reach the corresponding parameters in a healthy rat. Parameter normalization is not possible to achieve with solely diastolic or solely systolic support; for the modeled case with 50% fibrosis, the ejection fraction can be increased by 5% with purely diastolic support and 14% with purely systolic support. However, the ejection fraction reaches the value of the modeled healthy left ventricle (65.6%) using a combination of diastolic and systolic support. The end-diastolic pressure of 13.5 mmHg cannot be decreased with purely systolic support. However, the end-diastolic pressure reaches the value of the modeled healthy left ventricle (7.5 mmHg) with diastolic support as well as with the combination of the diastolic and systolic support. The resulting negative diastolic support pressure is −4.5 mmHg, and the positive systolic support pressure is 90 mmHg. We, thereby, conclude that ventricular support during both diastole and systole is beneficial for normalizing the left ventricular ejection fraction and the end-diastolic pressure, and thus it is a potentially interesting therapy for cardiac insufficiency.

Natural cardiac regeneration conserves native biaxial left ventricular biomechanics after myocardial infarction in neonatal rats

After myocardial infarction (MI), adult mammals exhibit scar formation, adverse left ventricular (LV) remodeling, LV stiffening, and impaired contractility, ultimately resulting in heart failure. Neonatal mammals, however, are capable of natural heart regeneration after MI. We hypothesized that neonatal cardiac regeneration conserves native biaxial LV mechanics after MI. Wistar rat neonates (1 day old, n = 46) and adults (8-10 weeks old, n = 20) underwent sham surgery or permanent left anterior descending coronary artery ligation. At 6 weeks after neonatal MI, Masson's trichrome staining revealed negligible fibrosis. Echocardiography for the neonatal MI (n = 15) and sham rats (n = 14) revealed no differences in LV wall thickness or chamber diameter, and both groups had normal ejection fraction (72.7% vs 77.5%, respectively, p = 0.1946). Biaxial tensile testing revealed similar stress-strain curves along both the circumferential and longitudinal axes across a full range of physiologic stresses and strains. The circumferential modulus (267.9 kPa vs 274.2 kPa, p = 0.7847), longitudinal modulus (269.3 kPa vs 277.1 kPa, p = 0.7435), and maximum shear stress (3.30 kPa vs 3.95 kPa, p = 0.5418) did not differ significantly between the neonatal MI and sham groups, respectively. In contrast, transmural scars were observed at 4 weeks after adult MI. Adult MI hearts (n = 7) exhibited profound LV wall thinning (p < 0.0001), chamber dilation (p = 0.0246), and LV dysfunction (ejection fraction 45.4% vs 79.7%, p < 0.0001) compared to adult sham hearts (n = 7). Adult MI hearts were significantly stiffer than adult sham hearts in both the circumferential (321.5 kPa vs 180.0 kPa, p = 0.0111) and longitudinal axes (315.4 kPa vs 172.3 kPa, p = 0.0173), and also exhibited greater maximum shear stress (14.87 kPa vs 3.23 kPa, p = 0.0162). Our study is the first to show that native biaxial LV mechanics are conserved after neonatal heart regeneration following MI, thus adding biomechanical support for the therapeutic potential of cardiac regeneration in the treatment of ischemic heart disease.

Passive mechanical properties in healthy and infarcted rat left ventricle characterised via a mixture model

During the cardiac cycle, electrical excitation is coupled with mechanical response of the myocardium. Besides the active contraction, passive mechanics plays an important role, and its behaviour differs in healthy and diseased hearts as well as among different animal species. The aim of this study is the characterisation of passive mechanical properties in healthy and infarcted rat myocardium by means of mechanical testing and subsequent parameter fitting. Elasticity assessments via uniaxial extension tests are performed on healthy and infarcted tissue samples from left ventricular rat myocardium. In order to fully characterise the orthotropic cardiac tissue, our experimental data are combined with other previously published tests in rats - shear tests on healthy myocardium and equibiaxial tests on infarcted tissue. In a first step, we calibrate the Holzapfel-Ogden strain energy function in the healthy case. Sa far, this orthotropic constitutive law for the passive myocardium has been fitted to experimental data in several species, however there is a lack of an appropriate parameter set for the rat. With our determined parameters, a finite element simulation of the end-diastolic filling is performed. In a second step, we propose a model for the infarcted tissue. It is represented as a mixture of intact myocardium and a transversely isotropic scar structure. In our mechanical experiments, the tissue after myocardial infarction shows significantly stiffer behaviour than in the healthy case, and the stiffness correlates with the amount of fibrosis. A similar relationship is observed in the computational simulation of the end-diastolic filling. We conclude that our new proposed material model can capture the behaviour of two kinds of tissues - healthy and infarcted rat myocardium, and its calibration with the fitted parameters represents the experimental data well.

Myocardial injection of a thermoresponsive hydrogel with reactive oxygen species scavenger properties improves border zone contractility

The decrease in contractility in myocardium adjacent (border zone; BZ) to a myocardial infarction (MI) is correlated with an increase in reactive oxygen species (ROS). We hypothesized that injection of a thermoresponsive hydrogel, with ROS scavenging properties, into the MI would decrease ROS and improve BZ function. Fourteen sheep underwent antero-apical MI. Seven sheep had a comb-like copolymer synthesized from N-isopropyl acrylamide (NIPAAm) and 1500 MW methoxy poly(ethylene glycol) methacrylate, (NIPAAm-PEG1500), injected (20 × 0.5 mL) into the MI zone 40 min after MI (MI + NIPAAm-PEG1500) and seven sheep were MI controls. Cardiac MRI was performed 2 weeks before and 6 weeks after MI + NIPAAm-PEG1500. BZ wall thickness at end systole was significantly higher for MI + NIPAAm-PEG1500 (12.32 ± 0.51 mm/m² MI + NIPAAm-PEG1500 vs. 9.88 ± 0.30 MI; p = .023). Demembranated muscle force development for BZ myocardium 6 weeks after MI was significantly higher for MI + NIPAAm-PEG1500 (67.67 ± 2.61 mN/m² MI + NIPAAm-PEG1500 vs. 40.53 ± 1.04 MI; p < .0001) but not significantly different from remote myocardium or BZ or non-operated controls. Levels of ROS in BZ tissue were significantly lower in the MI + NIPAAm-PEG1500 treatment group (hydroxyl p = .0031; superoxide p = .0182). We conclude that infarct injection of the NIPAAm-PEG1500 hydrogel with ROS scavenging properties decreased ROS and improved contractile protein function in the border zone 6 weeks after MI.

Multiaxial Lenticular Stress-Strain Relationship of Native Myocardium is Preserved by Infarct-Induced Natural Heart Regeneration in Neonatal Mice

Neonatal mice exhibit natural heart regeneration after myocardial infarction (MI) on postnatal day 1 (P1), but this ability is lost by postnatal day 7 (P7). Cardiac biomechanics intricately affect long-term heart function, but whether regenerated cardiac muscle is biomechanically similar to native myocardium remains unknown. We hypothesized that neonatal heart regeneration preserves native left ventricular (LV) biomechanical properties after MI. C57BL/6J mice underwent sham surgery or left anterior descending coronary artery ligation at age P1 or P7. Echocardiography performed 4 weeks post-MI showed that P1 MI and sham mice (n = 22, each) had similar LV wall thickness, diameter, and ejection fraction (59.6% vs 60.7%, p = 0.6514). Compared to P7 shams (n = 20), P7 MI mice (n = 20) had significant LV wall thinning, chamber enlargement, and depressed ejection fraction (32.6% vs 61.8%, p < 0.0001). Afterward, the LV was explanted and pressurized ex vivo, and the multiaxial lenticular stress-strain relationship was tracked. While LV tissue modulus for P1 MI and sham mice were similar (341.9 kPa vs 363.4 kPa, p = 0.6140), the modulus for P7 MI mice was significantly greater than that for P7 shams (691.6 kPa vs 429.2 kPa, p = 0.0194). We conclude that, in neonatal mice, regenerated LV muscle has similar biomechanical properties as native LV myocardium.

Biomechanics of infarcted left Ventricle-A review of experiments

Myocardial infarction (MI) is one of leading diseases to contribute to annual death rate of 5% in the world. In the past decades, significant work has been devoted to this subject. Biomechanics of infarcted left ventricle (LV) is associated with MI diagnosis, understanding of remodelling, MI micro-structure and biomechanical property characterizations as well as MI therapy design and optimization, but the subject has not been reviewed presently. In the article, biomechanics of infarcted LV was reviewed in terms of experiments achieved in the subject so far. The concerned content includes experimental remodelling, kinematics and kinetics of infarcted LVs. A few important issues were discussed and several essential topics that need to be investigated further were summarized. Microstructure of MI tissue should be observed even carefully and compared between different methods for producing MI scar in the same animal model, and eventually correlated to passive biomechanical property by establishing innovative constitutive laws. More uniaxial or biaxial tensile tests are desirable on MI, border and remote tissues, and viscoelastic property identification should be performed in various time scales. Active contraction experiments on LV wall with MI should be conducted to clarify impaired LV pumping function and supply necessary data to the function modelling. Pressure-volume curves of LV with MI during diastole and systole for the human are also desirable to propose and validate constitutive laws for LV walls with MI.

Bioengineered analog of stromal cell-derived factor 1α preserves the biaxial mechanical properties of native myocardium after infarction

Adverse remodeling of the left ventricle (LV) after myocardial infarction (MI) results in abnormal tissue biomechanics and impaired cardiac function, often leading to heart failure. We hypothesized that intramyocardial delivery of engineered stromal cell-derived factor 1α analog (ESA), our previously-developed supra-efficient pro-angiogenic chemokine, preserves biaxial LV mechanical properties after MI. Male Wistar rats (n = 45) underwent sham surgery (n = 15) or permanent left anterior descending coronary artery ligation. Rats sustaining MI were randomized for intramyocardial injections of either saline (100 μL, n = 15) or ESA (6 μg/kg, n = 15), delivered at four standardized borderzone sites. After 4 weeks, echocardiography was performed, and the hearts were explanted. Tensile testing of the anterolateral LV wall was performed using a displacement-controlled biaxial load frame, and modulus was determined after constitutive modeling. At 4 weeks post-MI, compared to saline controls, ESA-treated hearts had greater wall thickness (1.68 ± 0.05 mm vs 1.42 ± 0.08 mm, p = 0.008), smaller end-diastolic LV internal dimension (6.88 ± 0.29 mm vs 7.69 ± 0.22 mm, p = 0.044), and improved ejection fraction (62.8 ± 3.0% vs 49.4 ± 4.5%, p = 0.014). Histologic analysis revealed significantly reduced infarct size for ESA-treated hearts compared to saline controls (29.4 ± 2.9% vs 41.6 ± 3.1%, p = 0.021). Infarcted hearts treated with ESA exhibited decreased modulus compared to those treated with saline in both the circumferential (211.5 ± 6.9 kPa vs 264.3 ± 12.5 kPa, p = 0.001) and longitudinal axes (194.5 ± 6.5 kPa vs 258.1 ± 14.4 kPa, p < 0.001). In both principal directions, ESA-treated infarcted hearts possessed similar tissue compliance as sham non-infarcted hearts. Overall, intramyocardial ESA therapy improves post-MI ventricular remodeling and function, reduces infarct size, and preserves native LV biaxial mechanical properties.

Left Ventricular Diastolic Myocardial Stiffness and End-Diastolic Myofibre Stress in Human Heart Failure Using Personalised Biomechanical Analysis

Understanding the aetiology of heart failure with preserved (HFpEF) and reduced (HFrEF) ejection fraction requires knowledge of biomechanical factors such as diastolic myocardial stiffness and stress. Cine CMR images and intra-ventricular pressure recordings were acquired in 8 HFrEF, 11 HFpEF and 5 control subjects. Diastolic myocardial stiffness was estimated using biomechanical models and found to be greater in HFrEF (6.4 ± 1.2 kPa) than HFpEF (2.7 ± 0.6 kPa, p < 0.05) and also greater than control (1.2 ± 0.4 kPa, p < 0.005). End-diastolic mid-ventricular myofibre stress derived from the personalised biomechanics model was higher in HFrEF (2.9 ± 0.3 kPa) than control (0.9 ± 0.3 kPa, p < 0.01). Chamber stiffness, measured from the slope of the diastolic pressure-volume relationship, is determined by the intrinsic tissue properties as well as the size and shape of the heart, and was unable to distinguish between any of the three groups (p > 0.05). Personalised biomechanical analysis may provide more specific information about myocardial mechanical behaviour than global chamber indices, which are confounded by variations in ventricular geometry.

In vivo estimation of elastic heterogeneity in an infarcted human heart

In myocardial infarction, muscle tissue of the heart is damaged as a result of ceased or severely impaired blood flow. Survivors have an increased risk of further complications, possibly leading to heart failure. Material properties play an important role in determining post-infarction outcome. Due to spatial variation in scarring, material properties can be expected to vary throughout the tissue of a heart after an infarction. In this study we propose a data assimilation technique that can efficiently estimate heterogeneous elastic material properties in a personalized model of cardiac mechanics. The proposed data assimilation is tested on a clinical dataset consisting of regional left ventricular strains and in vivo pressures during atrial systole from a human with a myocardial infarction. Good matches to regional strains are obtained, and simulated equi-biaxial tests are carried out to demonstrate regional heterogeneities in stress–strain relationships. A synthetic data test shows a good match of estimated versus ground truth material parameter fields in the presence of no to low levels of noise. This study is the first to apply adjoint-based data assimilation to the important problem of estimating cardiac elastic heterogeneities in 3-D from medical images.

Protein/polysaccharide-based scaffolds mimicking native extracellular matrix for cardiac tissue engineering applications

Tissue engineering has emerged as a viable approach to treat disease or repair damage in tissues and organs. One of the key elements for the success of tissue engineering is the use of a scaffold serving as artificial extracellular matrix (ECM). The ECM hosts the cells and improves their survival, proliferation, and differentiation, enabling the formation of new tissue. Here, we propose the development of a class of protein/polysaccharide-based porous scaffolds for use as ECM substitutes in cardiac tissue engineering. Scaffolds based on blends of a protein component, collagen or gelatin, with a polysaccharide component, alginate, were produced by freeze-drying and subsequent ionic and chemical crosslinking. Their morphological, physicochemical, and mechanical properties were determined and compared with those of natural porcine myocardium. We demonstrated that our scaffolds possessed highly porous and interconnected structures, and the chemical homogeneity of the natural ECM was well reproduced in both types of scaffolds. Furthermore, the alginate/gelatin (AG) scaffolds better mimicked the native tissue in terms of interactions between components and protein secondary structure, and in terms of swelling behavior. The AG scaffolds also showed superior mechanical properties for the desired application and supported better adhesion, growth, and differentiation of myoblasts under static conditions. The AG scaffolds were subsequently used for culturing neonatal rat cardiomyocytes, where high viability of the resulting cardiac constructs was observed under dynamic flow culture in a microfluidic bioreactor. We therefore propose our protein/polysaccharide scaffolds as a viable ECM substitute for applications in cardiac tissue engineering. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 769-781, 2018.

An integrated inverse model-experimental approach to determine soft tissue three-dimensional constitutive parameters: application to post-infarcted myocardium

Knowledge of the complete three-dimensional (3D) mechanical behavior of soft tissues is essential in understanding their pathophysiology and in developing novel therapies. Despite significant progress made in experimentation and modeling, a complete approach for the full characterization of soft tissue 3D behavior remains elusive. A major challenge is the complex architecture of soft tissues, such as myocardium, which endows them with strongly anisotropic and heterogeneous mechanical properties. Available experimental approaches for quantifying the 3D mechanical behavior of myocardium are limited to preselected planar biaxial and 3D cuboidal shear tests. These approaches fall short in pursuing a model-driven approach that operates over the full kinematic space. To address these limitations, we took the following approach. First, based on a kinematical analysis and using a given strain energy density function (SEDF), we obtained an optimal set of displacement paths based on the full 3D deformation gradient tensor. We then applied this optimal set to obtain novel experimental data from a 1-cm cube of post-infarcted left ventricular myocardium. Next, we developed an inverse finite element (FE) simulation of the experimental configuration embedded in a parameter optimization scheme for estimation of the SEDF parameters. Notable features of this approach include: (i) enhanced determinability and predictive capability of the estimated parameters following an optimal design of experiments, (ii) accurate simulation of the experimental setup and transmural variation of local fiber directions in the FE environment, and (iii) application of all displacement paths to a single specimen to minimize testing time so that tissue viability could be maintained. Our results indicated that, in contrast to the common approach of conducting preselected tests and choosing an SEDF a posteriori, the optimal design of experiments, integrated with a chosen SEDF and full 3D kinematics, leads to a more robust characterization of the mechanical behavior of myocardium and higher predictive capabilities of the SEDF. The methodology proposed and demonstrated herein will ultimately provide a means to reliably predict tissue-level behaviors, thus facilitating organ-level simulations for efficient diagnosis and evaluation of potential treatments. While applied to myocardium, such developments are also applicable to characterization of other types of soft tissues.

Cell-Extracellular Matrix Mechanobiology: Forceful Tools and Emerging Needs for Basic and Translational Research

Extracellular biophysical cues have a profound influence on a wide range of cell behaviors, including growth, motility, differentiation, apoptosis, gene expression, adhesion, and signal transduction. Cells not only respond to definitively mechanical cues from the extracellular matrix (ECM) but can also sometimes alter the mechanical properties of the matrix and hence influence subsequent matrix-based cues in both physiological and pathological processes. Interactions between cells and materials in vitro can modify cell phenotype and ECM structure, whether intentionally or inadvertently. Interactions between cell and matrix mechanics in vivo are of particular importance in a wide variety of disorders, including cancer, central nervous system injury, fibrotic diseases, and myocardial infarction. Both the in vitro and in vivo effects of this coupling between mechanics and biology hold important implications for clinical applications.

Computational sensitivity investigation of hydrogel injection characteristics for myocardial support

Biomaterial injection is a potential new therapy for augmenting ventricular mechanics after myocardial infarction (MI). Recent in vivo studies have demonstrated that hydrogel injections can mitigate the adverse remodeling due to MI. More importantly, the material properties of these injections influence the efficacy of the therapy. The goal of the current study is to explore the interrelated effects of injection stiffness and injection volume on diastolic ventricular wall stress and thickness. To achieve this, finite element models were constructed with different hydrogel injection volumes (150µL and 300 µL), where the modulus was assessed over a range of 0.1kPa to 100kPa (based on experimental measurements). The results indicate that a larger injection volume and higher stiffness reduce diastolic myofiber stress the most, by maintaining the wall thickness during loading. Interestingly, the efficacy begins to taper after the hydrogel injection stiffness reaches a value of 50kPa. This computational approach could be used in the future to evaluate the optimal properties of the hydrogel.

Ventricular wall biomaterial injection therapy after myocardial infarction: Advances in material design, mechanistic insight and early clinical experiences

Intramyocardial biomaterial injection therapy for myocardial infarction has made significant progress since concept initiation more than 10 years ago. The interim successes and progress in the first 5 years have been extensively reviewed. During the last 5 years, two phase II clinical trials have reported their long term follow up results and many additional biomaterial candidates have reached preclinical and clinical testing. Also in recent years deeper investigations into the mechanisms behind the beneficial effects associated with biomaterial injection therapy have been pursued, and a variety of process and material parameters have been evaluated for their impact on therapeutic outcomes. This review explores the advances made in this biomaterial-centered approach to ischemic cardiomyopathy and discusses potential future research directions as this therapy seeks to positively impact patients suffering from one of the world's most common sources of mortality.

Injectable Bioengineered Hydrogel Therapy in the Treatment of Ischemic Cardiomyopathy

OPINION STATEMENT

Over the past two decades, the field of cardiovascular medicine has seen the rapid development of multiple different modalities for the treatment of ischemic myocardial disease. Most research efforts have focused on strategies aimed at coronary revascularization, with significant technological advances made in percutaneous coronary interventions as well as coronary artery bypass graft surgery. However, recent research efforts have shifted towards ways to address the downstream effects of myocardial infarction on both cellular and molecular levels. To this end, the broad application of injectable hydrogel therapy after myocardial infarction has stimulated tremendous interest. In this article, we will review what hydrogels are, how they can be bioengineered in unique ways to optimize therapeutic potential, and how they can be used as part of a treatment strategy after myocardial infarction.

In vivo assessment of regional mechanics post-myocardial infarction: A focus on the road ahead

Cardiovascular disease, particularly the occurrence of myocardial infarction (MI), remains a leading cause of morbidity and mortality (Go et al., Circulation 127: e6-e245, 2013; Go et al. Circulation 129: e28-e292, 2014). There is growing recognition that a key factor for post-MI outcomes is adverse remodeling and changes in the regional structure, composition, and mechanical properties of the MI region itself. However, in vivo assessment of regional mechanics post-MI can be confounded by the species, temporal aspects of MI healing, as well as size, location, and extent of infarction across myocardial wall. Moreover, MI regional mechanics have been assessed over varying phases of the cardiac cycle, and thus, uniform conclusions regarding the material properties of the MI region can be difficult. This review assesses past studies that have performed in vivo measures of MI mechanics and attempts to provide coalescence on key points from these studies, as well as offer potential recommendations for unifying approaches in terms of regional post-MI mechanics. A uniform approach to biophysical measures of import will allow comparisons across studies, as well as provide a basis for potential therapeutic markers.

Bi-layered polyurethane - Extracellular matrix cardiac patch improves ischemic ventricular wall remodeling in a rat model

As an intervention to abrogate ischemic cardiomyopathy, the concept of applying a temporary, local patch to the surface of the recently infarcted ventricle has been explored from a number of design perspectives. Two important features considered for such a cardiac patch include the provision of appropriate mechanical support and the capacity to influence the remodeling pathway by providing cellular or biomolecule delivery. The objective of this report was to focus on these two features by first evaluating the incorporation of a cardiac extracellular matrix (ECM) component, and second by evaluating the impact of patch anisotropy on the pathological remodeling process initiated by myocardial infarction. The functional outcomes of microfibrous, elastomeric, biodegradable cardiac patches have been evaluated in a rat chronic infarction model. Ten weeks after infarction and 8 wk after patch epicardial placement, echocardiographic function, tissue-level structural remodeling (e.g., biaxial mechanical response and microstructural analysis), and cellular level remodeling were assessed. The results showed that the incorporation of a cardiac ECM altered the progression of several keys aspects of maladaptive remodeling following myocardial infarction. This included decreasing LV global mechanical compliance, inhibiting echocardiographically-measured functional deterioration, mitigating scar formation and LV wall thinning, and promoting angiogenesis. In evaluating the impact of patch anisotropy, no effects from the altered patch mechanics were detected after 8 wk, possibly due to patch fibrous encapsulation. Overall, this study demonstrates the benefit of a cardiac patch design that combines both ventricle mechanical support, through a biodegradable, fibrillary elastomeric component, and the incorporation of ECM-based hydrogel components.

Sonomicrometry-Based Analysis of Post-Myocardial Infarction Regional Mechanics

Following myocardial infarction (MI), detrimental changes to the geometry, composition, and mechanical properties of the left ventricle (LV) are initiated in a process generally termed adverse post-MI remodeling. Cumulatively, these changes lead to a loss of LV function and are deterministic factors in the progression to heart failure. Proposed therapeutic strategies to target aberrant LV mechanics post-MI have shown potential to stabilize LV functional indices throughout the remodeling process. The in vivo quantification of LV mechanics, particularly within the MI region, is therefore essential to the continued development and evaluation of strategies to interrupt the post-MI remodeling process. The present study utilizes a porcine MI model and in vivo sonomicrometry to characterize MI region stiffness at 14 days post-MI. Obtained results demonstrate a significant dependence of mechanical properties on location and direction within the MI region, as well as cardiac phase. While approaches for comprehensive characterization of LV mechanics post-MI still need to be improved and standardized, our findings provide insight into the issues and complexities that must be considered within the MI region itself.

Epicardial infarct repair with bioinductive extracellular matrix promotes vasculogenesis and myocardial recovery

BACKGROUND

Infarcted myocardium can remodel after successful reperfusion, resulting in left ventricular dilation and heart failure. Epicardial infarct repair (EIR) using a bioinductive extracellular matrix (ECM) biomaterial is a novel surgical approach to promote endogenous myocardial repair and functional recovery after myocardial infarction. Using a pre-clinical porcine model of coronary ischemia-reperfusion, we assessed the effects of EIR on regional functional recovery, safety, and possible mechanisms of benefit.

METHODS

An ECM biomaterial (CorMatrix ECM) was applied to the epicardium after 75 minutes of coronary ischemia in a porcine model. Following ischemia-reperfusion injury, animals were randomly assigned in 2:1 fashion to EIR (n = 8) or sham treatment (n = 4). Serial cardiac magnetic resonance imaging was performed on normal (n = 4) and study animals at baseline (1 week) and 6 weeks after treatment. Myocardial function and tissue characteristics were assessed.

RESULTS

Functional myocardial recovery was significantly increased by EIR compared with sham treatment (change in regional myocardial contraction at 6 weeks, 28.6 ± 14.0% vs 4.2 ± 13.5% wall thickening, p < 0.05). Animals receiving EIR had reduced adhesions compared with animals receiving sham treatment (1.44 ± 0.51 vs 3.08 ± 0.89, p < 0.05). Myocardial fibrosis was not increased, and EIR did not cause myocardial constriction, as left ventricular compliance by passive pressure distention at matched volumes was similar between groups (13.9 ± 4.0 mm Hg in EIR group vs 16.0 ± 5.2 mm Hg in sham group, p = 0.61). Animals receiving EIR showed evidence of vasculogenesis in the region of functional recovery.

CONCLUSIONS

In addition to the beneficial effects of successful reperfusion, EIR using a bioinductive ECM enhances myocardial repair and functional recovery. Clinical translation of EIR early after myocardial infarction as an adjunct to surgical revascularization may be warranted in the future.

Injectable shear-thinning hydrogels used to deliver endothelial progenitor cells, enhance cell engraftment, and improve ischemic myocardium

OBJECTIVES

The clinical translation of cell-based therapies for ischemic heart disease has been limited because of low cell retention (<1%) within, and poor targeting to, ischemic myocardium. To address these issues, we developed an injectable hyaluronic acid (HA) shear-thinning hydrogel (STG) and endothelial progenitor cell (EPC) construct (STG-EPC). The STG assembles as a result of interactions of adamantine- and β-cyclodextrin-modified HA. It is shear-thinning to permit delivery via a syringe, and self-heals upon injection within the ischemic myocardium. This directed therapy to the ischemic myocardial border zone enables direct cell delivery to address adverse remodeling after myocardial infarction. We hypothesize that this system will enhance vasculogenesis to improve myocardial stabilization in the context of a clinically translatable therapy.

METHODS

Endothelial progenitor cells (DiLDL(+) VEGFR2(+) CD34(+)) were harvested from adult male rats, cultured, and suspended in the STG. In vitro viability was quantified using a live-dead stain of EPCs. The STG-EPC constructs were injected at the border zone of ischemic rat myocardium after acute myocardial infarction (left anterior descending coronary artery ligation). The migration of the enhanced green fluorescent proteins from the construct to ischemic myocardium was analyzed using fluorescent microscopy. Vasculogenesis, myocardial remodeling, and hemodynamic function were analyzed in 4 groups: control (phosphate buffered saline injection); intramyocardial injection of EPCs alone; injection of the STG alone; and treatment with the STG-EPC construct. Hemodynamics and ventricular geometry were quantified using echocardiography and Doppler flow analysis.

RESULTS

Endothelial progenitor cells demonstrated viability within the STG. A marked increase in EPC engraftment was observed 1-week postinjection within the treated myocardium with gel delivery, compared with EPC injection alone (17.2 ± 0.8 cells per high power field (HPF) vs 3.5 cells ± 1.3 cells per HPF, P = .0002). A statistically significant increase in vasculogenesis was noted with the STG-EPC construct (15.3 ± 5.8 vessels per HPF), compared with the control (P < .0001), EPC (P < .0001), and STG (P < .0001) groups. Statistically significant improvements in ventricular function, scar fraction, and geometry were noted after STG-EPC treatment compared with the control.

CONCLUSIONS

A novel injectable shear-thinning HA hydrogel seeded with EPCs enhanced cell retention and vasculogenesis after delivery to ischemic myocardium. This therapy limited adverse myocardial remodeling while preserving contractility.

Why Is Infarct Expansion Such an Elusive Therapeutic Target?

Myocardial infarct expansion has been associated with an increased risk of infarct rupture and progression to heart failure, motivating therapies such as infarct restraint and polymer injection that aim to limit infarct expansion. However, an exhaustive review of quantitative studies of infarct remodeling reveals that only half found chronic in-plane expansion, and many reported in-plane compaction. Using a finite element model, we demonstrate that the balance between scar stiffening due to collagen accumulation and increased wall stresses due to infarct thinning can produce either expansion or compaction in the pressurized heart-potentially explaining variability in the literature-and that loaded dimensions are much more sensitive to changes in thickness than in stiffness. Our analysis challenges the concept that in-plane expansion is a central feature of post-infarction remodeling; rather, available data suggest that radial thinning is the dominant process during infarct healing and may be an attractive therapeutic target.

Physiological Implications of Myocardial Scar Structure

Once myocardium dies during a heart attack, it is replaced by scar tissue over the course of several weeks. The size, location, composition, structure, and mechanical properties of the healing scar are all critical determinants of the fate of patients who survive the initial infarction. While the central importance of scar structure in determining pump function and remodeling has long been recognized, it has proven remarkably difficult to design therapies that improve heart function or limit remodeling by modifying scar structure. Many exciting new therapies are under development, but predicting their long-term effects requires a detailed understanding of how infarct scar forms, how its properties impact left ventricular function and remodeling, and how changes in scar structure and properties feed back to affect not only heart mechanics but also electrical conduction, reflex hemodynamic compensations, and the ongoing process of scar formation itself. In this article, we outline the scar formation process following a myocardial infarction, discuss interpretation of standard measures of heart function in the setting of a healing infarct, then present implications of infarct scar geometry and structure for both mechanical and electrical function of the heart and summarize experiences to date with therapeutic interventions that aim to modify scar geometry and structure. One important conclusion that emerges from the studies reviewed here is that computational modeling is an essential tool for integrating the wealth of information required to understand this complex system and predict the impact of novel therapies on scar healing, heart function, and remodeling following myocardial infarction.

Bioactive electrospun fibers of poly(glycerol sebacate) and poly(ε-caprolactone) for cardiac patch application

Scaffolds for cardiac patch application must meet stringent requirements such as biocompatibility, biodegradability, and facilitate vascularization in the engineered tissue. Here, a bioactive, biocompatible, and biodegradable electrospun scaffold of poly(glycerol sebacate)-poly(ε-caprolactone) (PGS-PCL) is proposed as a potential scaffold for cardiac patch application. The fibers are smooth bead free with average diameter = 0.8 ± 0.3 μm, mean pore size = 2.2 ± 1.2 μm, porosity = 62 ± 4%, and permeability higher than that of control biological tissue. For the first time, bioactive PGS-PCL fibers functionalized with vascular endothelial growth factor (VEGF) are developed, the approach used being chemical modification of the PGS-PCL fibers followed by subsequent binding of VEGF via amide bonding. The approach results in uniform immobilization of VEGF on the fibers; the concentrations are 1.0 μg cm(-2) for the PGS-PCL (H) and 0.60 μg cm(-2) for the PGS-PCL (L) samples. The bioactive scaffold supports the attachment and growth of seeded myogenic and vasculogenic cell lines. In fact, rat aortic endothelial cells also display angiogenic features indicating potential for the formation of vascular tree in the scaffold. These results therefore demonstrate the prospects of VEGF-functionalized PGS-PCL fibrous scaffold as promising matrix for cardiac patch application.

An integrated electromechanical-growth heart model for simulating cardiac therapies

An emerging class of models has been developed in recent years to predict cardiac growth and remodeling (G&R). We recently developed a cardiac G&R constitutive model that predicts remodeling in response to elevated hemodynamics loading, and a subsequent reversal of the remodeling process when the loading is reduced. Here, we describe the integration of this G&R model to an existing strongly coupled electromechanical model of the heart. A separation of timescale between growth deformation and elastic deformation was invoked in this integrated electromechanical-growth heart model. To test our model, we applied the G&R scheme to simulate the effects of myocardial infarction in a realistic left ventricular (LV) geometry using the finite element method. We also simulate the effects of a novel therapy that is based on alteration of the infarct mechanical properties. We show that our proposed model is able to predict key features that are consistent with experiments. Specifically, we show that the presence of a non-contractile infarct leads to a dilation of the left ventricle that results in a rightward shift of the pressure volume loop. Our model also predicts that G&R is attenuated by a reduction in LV dilation when the infarct stiffness is increased.

MRI evaluation of injectable hyaluronic acid-based hydrogel therapy to limit ventricular remodeling after myocardial infarction

Injectable biomaterials are an attractive therapy to attenuate left ventricular (LV) remodeling after myocardial infarction (MI). Although studies have shown that injectable hydrogels improve cardiac structure and function in vivo, temporal changes in infarct material properties after treatment have not been assessed. Emerging imaging and modeling techniques now allow for serial, non-invasive estimation of infarct material properties. Specifically, cine magnetic resonance imaging (MRI) assesses global LV structure and function, late-gadolinium enhancement (LGE) MRI enables visualization of infarcted tissue to quantify infarct expansion, and spatial modulation of magnetization (SPAMM) tagging provides passive wall motion assessment as a measure of tissue strain, which can all be used to evaluate infarct properties when combined with finite element (FE) models. In this work, we investigated the temporal effects of degradable hyaluronic acid (HA) hydrogels on global LV remodeling, infarct thinning and expansion, and infarct stiffness in a porcine infarct model for 12 weeks post-MI using MRI and FE modeling. Hydrogel treatment led to decreased LV volumes, improved ejection fraction, and increased wall thickness when compared to controls. FE model simulations demonstrated that hydrogel therapy increased infarct stiffness for 12 weeks post-MI. Thus, evaluation of myocardial tissue properties through MRI and FE modeling provides insight into the influence of injectable hydrogel therapies on myocardial structure and function post-MI.

Drug and cell delivery for cardiac regeneration

The spectrum of ischaemic cardiomyopathy, encompassing acute myocardial infarction to congestive heart failure is a significant clinical issue in the modern era. This group of diseases is an enormous source of morbidity and mortality and underlies significant healthcare costs worldwide. Cardiac regenerative therapy, whereby pro-regenerative cells, drugs or growth factors are administered to damaged and ischaemic myocardium has demonstrated significant potential, especially preclinically. While some of these strategies have demonstrated a measure of success in clinical trials, tangible clinical translation has been slow. To date, the majority of clinical studies and a significant number of preclinical studies have utilised relatively simple delivery methods for regenerative therapeutics, such as simple systemic administration or local injection in saline carrier vehicles. Here, we review cardiac regenerative strategies with a particular focus on advanced delivery concepts as a potential means to enhance treatment efficacy and tolerability and ultimately, clinical translation. These include (i) delivery of therapeutic agents in biomaterial carriers, (ii) nanoparticulate encapsulation, (iii) multimodal therapeutic strategies and (iv) localised, minimally invasive delivery via percutaneous transcatheter systems.

Injectable microsphere gel progressively improves global ventricular function, regional contractile strain, and mitral regurgitation after myocardial infarction

BACKGROUND

There is continued need for therapies which reverse or abate the remodeling process after myocardial infarction (MI). In this study, we evaluate the longitudinal effects of calcium hydroxyapatite microsphere gel on regional strain, global ventricular function, and mitral regurgitation (MR) in a porcine MI model.

METHODS

Twenty-five Yorkshire swine were enrolled. Five were dedicated weight-matched controls. Twenty underwent posterolateral infarction by direct ligation of the circumflex artery and its branches. Infarcted animals were randomly divided into the following 4 groups: 1-week treatment; 1-week control; 4-week treatment; and 4-week control. After infarction, animals received either twenty 150 μL calcium hydroxyapatite gel or saline injections within the infarct. At their respective time points, echocardiograms, cardiac magnetic resonance imaging, and tissue were collected for evaluation of MR, regional and global left ventricular function, wall thickness, and collagen content.

RESULTS

Global and regional left ventricular functions were depressed in all infarcted subjects at 1 week compared with healthy controls. By 4-weeks post-infarction, global function had significantly improved in the calcium hydroxyapatite group compared with infarcted controls (ejection fraction 0.485 ± 0.019 vs 0.38 ± 0.017, p < 0.01). Similarly, regional borderzone radial contractile strain (16.3% ± 1.5% vs 11.2% ± 1.5%, p = 0.04), MR grade (0.4 ± 0.2 vs 1.2 ± 0.2, p = 0.04), and infarct thickness (7.8 ± 0.5 mm vs 4.5 ± 0.2 mm, p < 0.01) were improved at this time point in the treatment group compared with infarct controls.

CONCLUSIONS

Calcium hydroxyapatite injection after MI progressively improves global left ventricular function, borderzone function, and mitral regurgitation. Using novel biomaterials to augment infarct material properties is a viable alternative in the current management of heart failure.

Estimating passive mechanical properties in a myocardial infarction using MRI and finite element simulations

Myocardial infarction (MI) triggers a series of maladaptive events that lead to structural and functional changes in the left ventricle. It is crucial to better understand the progression of adverse remodeling, in order to develop effective treatment. In addition, being able to assess changes in vivo would be a powerful tool in the clinic. The goal of the current study is to quantify the in vivo material properties of infarcted and remote myocardium 1 week after MI, as well as the orientation of collagen fibers in the infarct. This will be accomplished by using a combination of magnetic resonance imaging (MRI), catheterization, finite element modeling, and numerical optimization to analyze a porcine model ([Formula: see text]) of posterolateral myocardial infarction. Specifically, properties will be determined by minimizing the difference between in vivo strains and volume calculated from MRI and finite element model predicted strains and volume. The results indicate that the infarct region is stiffer than the remote region and that the infarct collagen fibers become more circumferentially oriented 1 week post-MI. These findings are consistent with previous studies, which employed ex vivo techniques. The proposed methodology will ultimately provide a means of predicting remote and infarct mechanical properties in vivo at any time point post-MI.

Estimating passive mechanical properties in a myocardial infarction using MRI and finite element simulations

Myocardial infarction (MI) triggers a series of maladaptive events that lead to structural and functional changes in the left ventricle. It is crucial to better understand the progression of adverse remodeling, in order to develop effective treatment. In addition, being able to assess changes in vivo would be a powerful tool in the clinic. The goal of the current study is to quantify the in vivo material properties of infarcted and remote myocardium 1 week after MI, as well as the orientation of collagen fibers in the infarct. This will be accomplished by using a combination of magnetic resonance imaging (MRI), catheterization, finite element modeling, and numerical optimization to analyze a porcine model ([Formula: see text]) of posterolateral myocardial infarction. Specifically, properties will be determined by minimizing the difference between in vivo strains and volume calculated from MRI and finite element model predicted strains and volume. The results indicate that the infarct region is stiffer than the remote region and that the infarct collagen fibers become more circumferentially oriented 1 week post-MI. These findings are consistent with previous studies, which employed ex vivo techniques. The proposed methodology will ultimately provide a means of predicting remote and infarct mechanical properties in vivo at any time point post-MI.

Targeted injection of a biocomposite material alters macrophage and fibroblast phenotype and function following myocardial infarction: relation to left ventricular remodeling

A treatment target for progressive left ventricular (LV) remodeling prevention following myocardial infarction (MI) is to affect structural changes directly within the MI region. One approach is through targeted injection of biocomposite materials, such as calcium hydroxyapatite microspheres (CHAM), into the MI region. In this study, the effects of CHAM injections upon key cell types responsible for the MI remodeling process, the macrophage and fibroblast, were examined. MI was induced in adult pigs before randomization to CHAM injections (20 targeted 0.1-ml injections within MI region) or saline. At 7 or 21 days post-MI (n = 6/time point per group), cardiac magnetic resonance imaging was performed, followed by macrophage and fibroblast isolation. Isolated macrophage profiles for monocyte chemotactic macrophage inflammatory protein-1 as measured by real-time polymerase chain reaction increased at 7 days post-MI in the CHAM group compared with MI only (16.3 ± 6.6 versus 1.7 ± 0.6 cycle times values, P < 0.05), and were similar by 21 days post-MI. Temporal changes in fibroblast function and smooth muscle actin (SMA) expression relative to referent control (n = 5) occurred with MI. CHAM induced increases in fibroblast proliferation, migration, and SMA expression-indicative of fibroblast transformation. By 21 days, CHAM reduced LV dilation (diastolic volume: 75 ± 2 versus 97 ± 4 ml) and increased function (ejection fraction: 48 ± 2% versus 38 ± 2%) compared with MI only (both P < 0.05). This study identified that effects on macrophage and fibroblast differentiation occurred with injection of biocomposite material within the MI, which translated into reduced adverse LV remodeling. These unique findings demonstrate that biomaterial injections impart biologic effects upon the MI remodeling process over any biophysical effects.

A model to determine the effect of collagen fiber alignment on heart function post myocardial infarction

BACKGROUND

Adverse remodeling of the left ventricle (LV) following myocardial infarction (MI) leads to heart failure. Recent studies have shown that scar anisotropy is a determinant of cardiac function post-MI, however it remains unclear how changes in extracellular matrix (ECM) organization and structure contribute to changes in LV function. The objective of this study is to develop a model to identify potential mechanisms by which collagen structure and organization affect LV function post-MI.

METHODS

A four-region, multi-scale, cylindrical model of the post-MI LV was developed. The mechanical properties of the infarct region are governed by a constitutive equation based on the uncrimping of collagen fibers. The parameters of this constitutive equation include collagen orientation, angular dispersion, fiber stiffness, crimp angle, and density. Parametric variation of these parameters was used to elucidate the relationship between collagen properties and LV function.

RESULTS

The mathematical model of the LV revealed several factors that influenced cardiac function post-MI. LV function was maximized when collagen fibers were aligned longitudinally. Increased collagen density was also found to improve stroke volume for longitudinal alignments while increased fiber stiffness decreased stroke volume for circumferential alignments.

CONCLUSIONS

The results suggest that cardiac function post-MI is best preserved through increased circumferential compliance. Further, this study identifies several collagen fiber-level mechanisms that could potentially regulate both infarct level and organ level mechanics. Improved understanding of the multi-scale relationships between the ECM and LV function will be beneficial in the design of new diagnostic and therapeutic technologies.

The Therapeutic Effect of Cell Transplantation Versus Noncellular Biomaterial Implantation on Cardiac Structure and Function Following Myocardial Infarction

Although numerous studies demonstrated that localized delivery of either cells or biomaterials improved postinfarction cardiac function, the underlying mechanisms for this effect remain unclear. We performed a comparison of the effects of fetal, neonatal, and human embryonic stem cell-derived cardiac cell as well as mesenchymal stem cell transplantation versus biomaterial (collagen/extracellular matrix) implantation therapy in rat myocardial infarction model in our laboratory, specifically comparing their effects on infarct wall thickness, neovascularization, infarct wall motion, and left ventricular ejection fraction (LVEF). Both cell and biomaterial treatment had similar beneficial effects on cardiac structure (increasing infarct wall thickness and preventing infarct expansion) and function (preventing paradoxical LV systolic bulging and improving LVEF). In this review, we also discussed the underlying mechanisms of cell and biomaterial therapies, their advantages and disadvantages, and future research directions in the field of regenerative cardiology.

Preclinical evaluation of the engineered stem cell chemokine stromal cell-derived factor 1α analog in a translational ovine myocardial infarction model

RATIONALE

After myocardial infarction, there is an inadequate blood supply to the myocardium, and the surrounding borderzone becomes hypocontractile.

OBJECTIVE

To develop a clinically translatable therapy, we hypothesized that in a preclinical ovine model of myocardial infarction, the modified endothelial progenitor stem cell chemokine, engineered stromal cell-derived factor 1α analog (ESA), would induce endothelial progenitor stem cell chemotaxis, limit adverse ventricular remodeling, and preserve borderzone contractility.

METHODS AND RESULTS

Thirty-six adult male Dorset sheep underwent permanent ligation of the left anterior descending coronary artery, inducing an anteroapical infarction, and were randomized to borderzone injection of saline (n=18) or ESA (n=18). Ventricular function, geometry, and regional strain were assessed using cardiac MRI and pressure-volume catheter transduction. Bone marrow was harvested for in vitro analysis, and myocardial biopsies were taken for mRNA, protein, and immunohistochemical analysis. ESA induced greater chemotaxis of endothelial progenitor stem cells compared with saline (P<0.01) and was equivalent to recombinant stromal cell-derived factor 1α (P=0.27). Analysis of mRNA expression and protein levels in ESA-treated animals revealed reduced matrix metalloproteinase 2 in the borderzone (P<0.05), with elevated levels of tissue inhibitor of matrix metalloproteinase 1 and elastin in the infarct (P<0.05), whereas immunohistochemical analysis of borderzone myocardium showed increased capillary and arteriolar density in the ESA group (P<0.01). Animals in the ESA treatment group also had significant reductions in infarct size (P<0.01), increased maximal principle strain in the borderzone (P<0.01), and a steeper slope of the end-systolic pressure-volume relationship (P=0.01).

CONCLUSIONS

The novel, biomolecularly designed peptide ESA induces chemotaxis of endothelial progenitor stem cells, stimulates neovasculogenesis, limits infarct expansion, and preserves contractility in an ovine model of myocardial infarction.

Experimental and computational investigation of altered mechanical properties in myocardium after hydrogel injection

The material properties of myocardium are an important determinant of global left ventricular function. Myocardial infarction results in a series of maladaptive geometric alterations which lead to increased stress and risk of heart failure. In vivo studies have demonstrated that material injection can mitigate these changes. More importantly, the material properties of these injectates can be tuned to minimize wall thinning and ventricular dilation. The current investigation combines experimental data and finite element modeling to correlate how injectate mechanics and volume influence myocardial wall stress. Experimentally, mechanics were characterized with biaxial testing and injected hydrogel volumes were measured with magnetic resonance imaging. Injection of hyaluronic acid hydrogel increased the stiffness of the myocardium/hydrogel composite region in an anisotropic manner, significantly increasing the modulus in the longitudinal direction compared to control myocardium. Increased stiffness, in combination with increased volume from hydrogel injection, reduced the global average fiber stress by ~14% and the transmural average by ~26% in the simulations. Additionally, stiffening in an anisotropic manner enhanced the influence of hydrogel treatment in decreasing stress. Overall, this work provides insight on how injectable biomaterials can be used to attenuate wall stress and provides tools to further optimize material properties for therapeutic applications.

Sustained release of engineered stromal cell-derived factor 1-α from injectable hydrogels effectively recruits endothelial progenitor cells and preserves ventricular function after myocardial infarction

BACKGROUND

Exogenously delivered chemokines have enabled neovasculogenic myocardial repair in models of ischemic cardiomyopathy; however, these molecules have short half-lives in vivo. In this study, we hypothesized that the sustained delivery of a synthetic analog of stromal cell-derived factor 1-α (engineered stromal cell-derived factor analog [ESA]) induces continuous homing of endothelial progenitor cells and improves left ventricular function in a rat model of myocardial infarction.

METHODS AND RESULTS

Our previously designed ESA peptide was synthesized by the addition of a fluorophore tag for tracking. Hyaluronic acid was chemically modified with hydroxyethyl methacrylate to form hydrolytically degradable hydrogels through free-radical-initiated crosslinking. ESA was encapsulated in hyaluronic acid hydrogels during gel formation, and then ESA release, along with gel degradation, was monitored for more than 4 weeks in vitro. Chemotactic properties of the eluted ESA were assessed at multiple time points using rat endothelial progenitor cells in a transwell migration assay. Finally, adult male Wistar rats (n=33) underwent permanent ligation of the left anterior descending (LAD) coronary artery, and 100 µL of saline, hydrogel alone, or hydrogel+25 µg ESA was injected into the borderzone. ESA fluorescence was monitored in animals for more than 4 weeks, after which vasculogenic, geometric, and functional parameters were assessed to determine the therapeutic benefit of each treatment group. ESA release was sustained for 4 weeks in vitro, remained active, and enhanced endothelial progenitor cell chemotaxis. In addition, ESA was detected in the rat heart >3 weeks when delivered within the hydrogels and significantly improved vascularity, ventricular geometry, ejection fraction, cardiac output, and contractility compared with controls.

CONCLUSIONS

We have developed a hydrogel delivery system that sustains the release of a bioactive endothelial progenitor cell chemokine during a 4-week period that preserves ventricular function in a rat model of myocardial infarction.

Localized targeting of biomaterials following myocardial infarction: a foundation to build on

Acute coronary syndromes can give rise to myocardial injury infarction (MI), which in turn promulgates a series of cellular and extracellular events that result in left ventricular (LV) dilation and dysfunction. Localized strategies focused upon interrupting this inexorable process include delivery of bioactive molecules and stem cell derivatives. These localized treatment strategies are often delivered in a biomaterial complex in order to facilitate elution of the bioactive molecules or stem cell engraftment. However, these biomaterials can impart significant and independent effects upon the MI remodeling process. In addition, significant changes in local cell and interstitial biology within the targeted MI region can occur following injection of certain biomaterials, which may hold important considerations when using these materials as matrices for adjuvant drug/cell therapies.

Mathematically engineered stromal cell-derived factor-1α stem cell cytokine analog enhances mechanical properties of infarcted myocardium

OBJECTIVE

The biomechanical response to a myocardial infarction consists of ventricular remodeling that leads to dilatation, loss of contractile function, abnormal stress patterns, and ultimately heart failure. We hypothesized that intramyocardial injection of our previously designed pro-angiogenic chemokine, an engineered stromal cell-derived factor-1α analog (ESA), improves mechanical properties of the heart after infarction.

METHODS

Male rats (n = 54) underwent either sham surgery (n = 17) with no coronary artery ligation or ligation of the left anterior descending artery (n = 37). The rats in the myocardial infarction group were then randomized to receive either saline (0.1 mL, n = 18) or ESA (6 μg/kg, n = 19) injected into the myocardium at 4 predetermined spots around the border zone. Echocardiograms were performed preoperatively and before the terminal surgery. After 4 weeks, the hearts were explanted and longitudinally sectioned. Uniaxial tensile testing was completed using an Instron 5543 Microtester. Optical strain was evaluated using custom image acquisition software, Digi-Velpo, and analyzed in MATLAB.

RESULTS

Compared with the saline control group at 4 weeks, the ESA-injected hearts had a greater ejection fraction (71.8% ± 9.0% vs 55.3% ± 12.6%, P = .0004), smaller end-diastolic left ventricular internal dimension (0.686 ± 0.110 cm vs 0.763 ± 0.160 cm, P = .04), greater cardiac output (36 ± 11.6 mL/min vs 26.9 ± 7.3 mL/min, P = .05), and a lower tensile modulus (251 ± 56 kPa vs 301 ± 81 kPa, P = .04). The tensile modulus for the sham group was 195 ± 56 kPa, indicating ESA injection results in a less stiff ventricle.

CONCLUSIONS

Direct injection of ESA alters the biomechanical response to myocardial infarction, improving the mechanical properties in the postinfarct heart.

Intramyocardial injection of heart tissue-derived extracellular matrix improves postinfarction cardiac function in rats

AIMS

We determined whether implantation of heart tissue-derived decellularized matrix, which contains native biochemical and structural matrix composition, could thicken the infarcted left ventricular (LV) wall and improve LV function in a rat myocardial infarction model.

METHODS AND RESULTS

Myocardial infarction was induced by left coronary ligation in Fischer rats. One week later, saline (75 μL, n = 17) or matrix (75 μL, n = 19) was directly injected into the infarcted area. At 6 weeks after injection, cardiac function was assessed by left ventriculogram, echocardiography, and Millar catheter. The hearts were pressure fixed to measure postmortem LV volume and processed for histology. Left ventriculogram demonstrated that LV ejection fraction (EF) was significantly greater in the matrix-treated (56.7% ± 1.4%) than in the saline-treated group (52.4% ± 1.5%; P = .043), and paradoxical LV systolic bulging was significantly reduced in the matrix-treated group (6.2% ± 1.6% of the LV circumference) compared to the saline-treated group (10.3% ± 1.3%; P = .048). Matrix implantation significantly increased the thickness of infarcted LV wall (0.602 ± 0.029 mm) compared to the saline-treated group (0.484 ± 0.03 mm; P = .0084). Infarct expansion index was significantly lower in the matrix-treated group (1.053 ± 0.051) than in the saline-treated group (1.382 ± 0.096, P = .0058). Blood vessel density and c-kit positive staining cells within the infarct area were comparable between the 2 groups.

CONCLUSIONS

Implantation of heart tissue-derived decellularized matrix thickens the LV infarcted wall, prevents paradoxical LV systolic bulging, and improves LV EF after myocardial infarction in rats. This benefit was not dependent on the enhanced angiogenesis or the recruitment of endogenous stem cells to the injury site.

Tunable hydrogel-microsphere composites that modulate local inflammation and collagen bulking

Injectable biomaterials alone may alter local tissue responses, including inflammatory cascades and matrix production (e.g. stimulatory dermal fillers are used as volumizing agents that induce collagen production). To expand upon the available material compositions and timing of presentation, a tunable hyaluronic acid (HA) and poly(lactide-co-glycolide) (PLGA) microsphere composite system was formulated and assessed in subcutaneous and cardiac tissues. HA functionalized with hydroxyethyl methacrylate (HeMA) was used as a precursor to injectable and degradable hydrogels that carry PLGA microspheres (~50 μm diameter) to tissues, where the HA hydrogel degradation (~20 or 70 days) and quantity of PLGA microspheres (0-300 mgml(-1)) are readily varied. When implanted subcutaneously, faster hydrogel degradation and more microspheres (e.g. 75 mgml(-1)) generally induced more rapid tissue and cellular interactions and a greater macrophage response. In cardiac applications, tissue bulking may be useful to alter stress profiles and to stabilize the tissue after infarction, limiting left ventricular (LV) remodeling. When fast degrading HeMA-HA hydrogels containing 75 mgml(-1) microspheres were injected into infarcted tissue in sheep, LV dilation was limited and the thickness of the myocardial wall and the presence of vessels in the apical infarct region were increased ~35 and ~60%, respectively, compared to empty hydrogels. Both groups decreased volume changes and infarct areas at 8 weeks, compared to untreated controls. This work illustrates the importance of material design in expanding the application of tissue bulking composites to a range of biomedical applications.

Dynamic assessment of mitral annular force profile in an ovine model

BACKGROUND

Limited knowledge exists regarding the forces that act on devices implanted in the mitral annulus. Determining the peak magnitudes, directions, rates, variation throughout the cardiac cycle, and change with left ventricular pressure (LVP) will aid in device development and evaluation.

METHODS

Novel transducers with the ability to measure forces in the septal-lateral and transverse directions were implanted in six healthy ovine subjects. Forces were measured for cardiac cycles reaching a peak LVP of 90, 125, 150, 175, and 200 mm Hg.

RESULTS

The septal-lateral force was observed to significantly increase from 3.9 ± 0.8 N (90) to 5.2 ± 1.0 N (125) p < 0.001, 5.9 ± 0.9 N (150) p < 0.001, 6.4 ± 1.2 N (175) p < 0.001, and 6.7 ± 1.5 N (200 mm Hg) p < 0.001. Similarly, the transverse force was seen to increase from 2.6 ± 0.6 N (90) to 3.8 ± 1.0 N (125) p < 0.01, 4.6 ± 1.3 N (150) p < 0.001, 4.3 ± 1.2 N (175) p < 0.001, and 3.5 ± 0.7 N (200 mm Hg) p < 0.05. In comparison, the septal-lateral force was significantly greater than the transverse force at 90 (p < 0.05), 125 (p < 0.05), 175 (p < 0.001), and 200 mm Hg (p < 0.0005).

CONCLUSIONS

Annular forces and their variations with LVP through the cardiac cycle are described. The results demonstrate differences in force magnitude and rate for increasing levels of LVP between the septal-lateral and transverse directions. These directional differences have strong implications in the development of future mitral devices.

Biomaterials for the treatment of myocardial infarction: a 5-year update

The first review on biomaterials for the treatment of myocardial infarction (MI) was written in 2006. In the last 5 years, the general approaches for biomaterial treatment of MI and subsequent left ventricular remodeling remain the same, namely, left ventricular restraints, epicardial patches, and injectable therapies. Nonetheless, there have been significant developments in this field, including advancement of biomaterial therapies to large animal pre-clinical studies and, more recently, to clinical trials. This review focuses on the progress made in the field of cardiac biomaterial treatments for MI over the last 5 years.