The effect of hydrogel injection on cardiac function and myocardial mechanics in a computational post-infarction model

In silico Mechanics of Stem Cells Intramyocardially Transplanted with a Biomaterial Injectate for Treatment of Myocardial Infarction

PURPOSE

Biomaterial and stem cell delivery are promising approaches to treating myocardial infarction. However, the mechanical and biochemical mechanisms underlying the therapeutic benefits require further clarification. This study aimed to assess the deformation of stem cells injected with the biomaterial into the infarcted heart.

METHODS

A microstructural finite element model of a mid-wall infarcted myocardial region was developed from ex vivo microcomputed tomography data of a rat heart with left ventricular infarct and intramyocardial biomaterial injectate. Nine cells were numerically seeded in the injectate of the microstructural model. The microstructural and a previously developed biventricular finite element model of the same rat heart were used to quantify the deformation of the cells during a cardiac cycle for a biomaterial elastic modulus (Einj) ranging between 4.1 and 405,900 kPa.

RESULTS

The transplanted cells' deformation was largest for Einj = 7.4 kPa, matching that of the cells, and decreased for an increase and decrease in Einj. The cell deformation was more sensitive to Einj changes for softer (Einj ≤ 738 kPa) than stiffer biomaterials.

CONCLUSIONS

Combining the microstructural and biventricular finite element models enables quantifying micromechanics of transplanted cells in the heart. The approach offers a broader scope for in silico investigations of biomaterial and cell therapies for myocardial infarction and other cardiac pathologies.

Improvement in cardiac function and regional LV strain following intramyocardial injection of a theranostic hydrogel early postmyocardial infarction in a porcine model

Myocardial infarction (MI) is often complicated by left ventricular (LV) remodeling and heart failure. We evaluated the feasibility of a multimodality imaging approach to guide delivery of an imageable hydrogel and assessed LV functional changes with therapy. Yorkshire pigs underwent surgical occlusions of branches of the left anterior descending and/or circumflex artery to create an anterolateral MI. We evaluated the hemodynamic and mechanical effects of intramyocardial delivery of an imageable hydrogel in the central infarct area (Hydrogel group, n = 8) and a Control group (n = 5) early post-MI. LV and aortic pressure and ECG were measured and contrast cineCT angiography was performed at baseline, 60 min post-MI, and 90 min post-hydrogel delivery. LV hemodynamic indices, pressure-volume measures, and normalized regional and global strains were measured and compared. Both Control and Hydrogel groups demonstrated a decline in heart rate, LV pressure, stroke volume, ejection fraction, and pressure-volume loop area, and an increase in myocardial performance (Tei) index and supply/demand (S/D) ratio. After hydrogel delivery, Tei index and S/D ratio were reduced to baseline levels, diastolic and systolic functional indices either stabilized or improved, and radial strain and circumferential strain increased significantly in the MI regions (ENrr: +52.7%, ENcc: +44.1%). However, the Control group demonstrated a progressive decline in all functional indices to levels significantly below those of Hydrogel group. Thus, acute intramyocardial delivery of a novel imageable hydrogel to MI region resulted in rapid stabilization or improvement in LV hemodynamics and function.NEW & NOTEWORTHY Our study demonstrates that contrast cineCT imaging can be used to evaluate the acute effects of intramyocardial delivery of a therapeutic hydrogel to the central MI region early post MI, which resulted in a rapid stabilization of LV hemodynamics and improvement in regional and global LV function.

Effect of biomaterial stiffness on cardiac mechanics in a biventricular infarcted rat heart model with microstructural representation of in situ intramyocardial injectate

Intramyocardial delivery of biomaterials is a promising concept for treating myocardial infarction. The delivered biomaterial provides mechanical support and attenuates wall thinning and elevated wall stress in the infarct region. This study aimed at developing a biventricular finite element model of an infarcted rat heart with a microstructural representation of an in situ biomaterial injectate, and a parametric investigation of the effect of the injectate stiffness on the cardiac mechanics. A three-dimensional subject-specific biventricular finite element model of a rat heart with left ventricular infarct and microstructurally dispersed biomaterial delivered 1 week after infarct induction was developed from ex vivo microcomputed tomography data. The volumetric mesh density varied between 303 mm-3 in the myocardium and 3852 mm-3 in the injectate region due to the microstructural intramyocardial dispersion. Parametric simulations were conducted with the injectate's elastic modulus varying from 4.1 to 405,900 kPa, and myocardial and injectate strains were recorded. With increasing injectate stiffness, the end-diastolic median myocardial fibre and cross-fibre strain decreased in magnitude from 3.6% to 1.1% and from -6.0% to -2.9%, respectively. At end-systole, the myocardial fibre and cross-fibre strain decreased in magnitude from -20.4% to -11.8% and from 6.5% to 4.6%, respectively. In the injectate, the maximum and minimum principal strains decreased in magnitude from 5.4% to 0.001% and from -5.4% to -0.001%, respectively, at end-diastole and from 38.5% to 0.06% and from -39.0% to -0.06%, respectively, at end-systole. With the microstructural injectate geometry, the developed subject-specific cardiac finite element model offers potential for extension to cellular injectates and in silico studies of mechanotransduction and therapeutic signalling in the infarcted heart with an infarct animal model extensively used in preclinical research.

CineCT platform for in vivo and ex vivo measurement of 3D high resolution Lagrangian strains in the left ventricle following myocardial infarction and intramyocardial delivery of theranostic hydrogel

Myocardial infarction (MI) produces acute changes in strain and stiffness within the infarct that can affect remote areas of the left ventricle (LV) and drive pathological remodeling. We hypothesized that intramyocardial delivery of a hydrogel within the MI region would lower wall stress and reduce adverse remodeling in Yorkshire pigs (n = 5). ^99mTc-Tetrofosmin SPECT imaging defined the location and geometry of induced MI and border regions in pigs, and in vivo and ex vivo contrast cine computed tomography (cineCT) quantified deformations of the LV myocardium. Serial in vivo cineCT imaging provided data in hearts from control pigs (n = 3) and data from pigs (n = 5) under baseline conditions before MI induction, post-MI day 3, post-MI day 7, and one hour after intramyocardial delivery of a hyaluronic acid (HA)-based hydrogel with shear-thinning and self-healing properties to the central infarct area. Isolated, excised hearts underwent similar cineCT imaging using an ex vivo perfused heart preparation with cyclic LV pressurization. Deformations were evaluated using nonlinear image registration of cineCT volumes between end-diastole (ED) and end-systole (ES), and 3D Lagrangian strains were calculated from the displacement gradients. Post-MI day 3, radial, circumferential, maximum principal, and shear strains were reduced within the MI region (p < 0.04) but were unchanged in normal regions (p > 0.6), and LV end diastolic volume (LV EDV) increased (p = 0.004), while ejection fraction (EF) and stroke volume (SV) decreased (p < 0.02). Post-MI day 7, radial strains in MI border zones increased (p = 0.04) and dilation of LV EDV continued (p = 0.052). There was a significant negative linear correlation between regional radial and maximum principal/shear strains and percent infarcted tissue in all hearts (R² > 0.47, p < 0.004), indicating that cineCT strain measures could predict MI location and degree of injury. Post-hydrogel day 7 post-MI, LV EDV was significantly reduced (p = 0.009), EF increased (p = 0.048), and radial (p = 0.021), maximum principal (p = 0.051), and shear strain (p = 0.047) increased within regions bordering the infarct. A smaller strain improvement within the infarct and normal regions was also noted on average along with an improvement in SV in 4 out of 5 hearts. CineCT provides a reliable method to assess regional changes in strains post-MI and the therapeutic effects of intramyocardial hydrogel delivery.

From the Matrix to the Nucleus and Back: Mechanobiology in the Light of Health, Pathologies, and Regeneration of Oral Periodontal Tissues

Among oral tissues, the periodontium is permanently subjected to mechanical forces resulting from chewing, mastication, or orthodontic appliances. Molecularly, these movements induce a series of subsequent signaling processes, which are embedded in the biological concept of cellular mechanotransduction (MT). Cell and tissue structures, ranging from the extracellular matrix (ECM) to the plasma membrane, the cytosol and the nucleus, are involved in MT. Dysregulation of the diverse, fine-tuned interaction of molecular players responsible for transmitting biophysical environmental information into the cell's inner milieu can lead to and promote serious diseases, such as periodontitis or oral squamous cell carcinoma (OSCC). Therefore, periodontal integrity and regeneration is highly dependent on the proper integration and regulation of mechanobiological signals in the context of cell behavior. Recent experimental findings have increased the understanding of classical cellular mechanosensing mechanisms by both integrating exogenic factors such as bacterial gingipain proteases and newly discovered cell-inherent functions of mechanoresponsive co-transcriptional regulators such as the Yes-associated protein 1 (YAP1) or the nuclear cytoskeleton. Regarding periodontal MT research, this review offers insights into the current trends and open aspects. Concerning oral regenerative medicine or weakening of periodontal tissue diseases, perspectives on future applications of mechanobiological principles are discussed.

A comparison of two quasi-static computational models for assessment of intra-myocardial injection as a therapeutic strategy for heart failure

Myocardial infarction, or heart attack, is the leading cause of mortality globally. Although the treatment of myocardial infarct has improved significantly, scar tissue that persists can often lead to increased stress and adverse remodeling of surrounding tissue and ultimately to heart failure. Intra-myocardial injection of biomaterials represents a potential treatment to attenuate remodeling, mitigate degeneration, and reverse the disease process in the tissue. In vivo experiments on animal models have shown functional benefits of this therapeutic strategy. However, a poor understanding of the optimal injection pattern, volume, and material properties has acted as a barrier to its widespread clinical adoption. In this study, we developed two quasistatic finite element simulations of the left ventricle to investigate the mechanical effect of intra-myocardial injection. The first model employed an idealized left ventricular geometry with rule-based cardiomyocyte orientation. The second model employed a subject-specific left ventricular geometry with cardiomyocyte orientation from diffusion tensor magnetic resonance imaging. Both models predicted cardiac parameters including ejection fraction, systolic wall thickening, and ventricular twist that matched experimentally reported values. All injection simulations showed cardiomyocyte stress attenuation, offering an explanation for the mechanical reinforcement benefit associated with injection. The study also enabled a comparison of injection location and the corresponding effect on cardiac performance at different stages of the cardiac cycle. While the idealized model has lower fidelity, it predicts cardiac function and differentiates the effects of injection location. Both models represent versatile in silico tools to guide optimal strategy in terms of injection number, volume, site, and material properties.

A Preliminary Computational Investigation Into the Flow of PEG in Rat Myocardial Tissue for Regenerative Therapy

Myocardial infarction (MI), a type of cardiovascular disease, affects a significant proportion of people around the world. Traditionally, non-communicable chronic diseases were largely associated with aging populations in higher income countries. It is now evident that low- to middle-income countries are also affected and in these settings, younger individuals are at high risk. Currently, interventions for MI prolong the time to heart failure. Regenerative medicine and stem cell therapy have the potential to mitigate the effects of MI and to significantly improve the quality of life for patients. The main drawback with these therapies is that many of the injected cells are lost due to the vigorous motion of the heart. Great effort has been directed toward the development of scaffolds which can be injected alongside stem cells, in an attempt to improve retention and cell engraftment. In some cases, the scaffold alone has been seen to improve heart function. This study focuses on a synthetic polyethylene glycol (PEG) based hydrogel which is injected into the heart to improve left ventricular function following MI. Many studies in literature characterize PEG as a Newtonian fluid within a specified shear rate range, on the macroscale. The aim of the study is to characterize the flow of a 20 kDa PEG on the microscale, where the behavior is likely to deviate from macroscale flow patterns. Micro particle image velocimetry (μPIV) is used to observe flow behavior in microchannels, representing the gaps in myocardial tissue. The fluid exhibits non-Newtonian, shear-thinning behavior at this scale. Idealized two-dimensional computational fluid dynamics (CFD) models of PEG flow in microchannels are then developed and validated using the μPIV study. The validated computational model is applied to a realistic, microscopy-derived myocardial tissue model. From the realistic tissue reconstruction, it is evident that the myocardial flow region plays an important role in the distribution of PEG, and therefore, in the retention of material.

Kinematic boundary conditions substantially impact in silico ventricular function

Computational cardiac mechanical models, individualized to the patient, have the potential to elucidate the fundamentals of cardiac (patho-)physiology, enable non-invasive quantification of clinically significant metrics (eg, stiffness, active contraction, work), and anticipate the potential efficacy of therapeutic cardiovascular intervention. In a clinical setting, however, the available imaging resolution is often limited, which limits cardiac models to focus on the ventricles, without including the atria, valves, and proximal arteries and veins. In such models, the absence of surrounding structures needs to be accounted for by imposing realistic kinematic boundary conditions, which, for prognostic purposes, are preferably generic and thus non-image derived. Unfortunately, the literature on cardiac models shows no consistent approach to kinematically constrain the myocardium. The impact of different approaches (eg, fully constrained base, constrained epi-ring) on the predictive capacity of cardiac mechanical models has not been thoroughly studied. For that reason, this study first gives an overview of current approaches to kinematically constrain (bi) ventricular models. Next, we developed a patient-specific in silico biventricular model that compares well with literature and in vivo recorded strains. Alternative constraints were introduced to assess the influence of commonly used mechanical boundary conditions on both the predicted global functional behavior of the in-silico heart (cavity volumes, stroke volume, ejection fraction) and local strain distributions. Meaningful differences in global functioning were found between different kinematic anchoring strategies, which brought forward the importance of selecting appropriate boundary conditions for biventricular models that, in the near future, may inform clinical intervention. However, whilst statistically significant differences were also found in local strain distributions, these differences were minor and mostly confined to the region close to the applied boundary conditions.

Personalised computational cardiology: Patient-specific modelling in cardiac mechanics and biomaterial injection therapies for myocardial infarction

Predictive computational modelling in biomedical research offers the potential to integrate diverse data, uncover biological mechanisms that are not easily accessible through experimental methods and expose gaps in knowledge requiring further research. Recent developments in computing and diagnostic technologies have initiated the advancement of computational models in terms of complexity and specificity. Consequently, computational modelling can increasingly be utilised as enabling and complementing modality in the clinic—with medical decisions and interventions being personalised. Myocardial infarction and heart failure are amongst the leading causes of death globally despite optimal modern treatment. The development of novel MI therapies is challenging and may be greatly facilitated through predictive modelling. Here, we review the advances in patient-specific modelling of cardiac mechanics, distinguishing specificity in cardiac geometry, myofibre architecture and mechanical tissue properties. Thereafter, the focus narrows to the mechanics of the infarcted heart and treatment of myocardial infarction with particular attention on intramyocardial biomaterial delivery.

Fibrotic infarction on the LV free wall may alter the mechanics of healthy septal wall during passive filling

The effect of myocardial infarction on the global functioning of the heart is well known. Less is understood regarding the effect of LV fibrotic infarction on the cardiac mechanics of the septal wall. To determine this unknown, the stress and strain of septal wall on the healthy and infarcted rat heart model is measured by using finite element models of rat heart geometries. The main objective of this study was to utilized computational methods to study the effect of LV free wall fibrotic infarction on the healthy septal wall. Three-dimensional biventricular rat heart geometries were developed from cardiac magnetic resonance images of a healthy heart and a heart with left ventricular (LV) fibrotic infarction after infarct induction. From these geometries, FE models were established. Three-dimensional biventricular rat heart geometries developed from cardiac magnetic resonance images were used in creating FE models of healthy and infarcted rat hearts. The average radial strain percentage change of the healthy septal wall on the epicardium, mid-wall and endocardium was 61%, 52% and 14% higher than the infarcted septal wall, respectively. It was concluded that the fibrotic infarction has a potential cause the malfunction of the heart due to high myocardial stress and strain that the septal wall experiences.

Cellular mechanosensitivity to substrate stiffness decreases with increasing dissimilarity to cell stiffness

Computational modelling has received increasing attention to investigate multi-scale coupled problems in micro-heterogeneous biological structures such as cells. In the current study, we investigated for a single cell the effects of (1) different cell-substrate attachment (2) and different substrate modulus [Formula: see text] on intracellular deformations. A fibroblast was geometrically reconstructed from confocal micrographs. Finite element models of the cell on a planar substrate were developed. Intracellular deformations due to substrate stretch of [Formula: see text], were assessed for: (1) cell-substrate attachment implemented as full basal contact (FC) and 124 focal adhesions (FA), respectively, and [Formula: see text]140 KPa and (2) [Formula: see text], 140, 1000, and 10,000 KPa, respectively, and FA attachment. The largest strains in cytosol, nucleus and cell membrane were higher for FC (1.35[Formula: see text], 0.235[Formula: see text] and 0.6[Formula: see text]) than for FA attachment (0.0952[Formula: see text], 0.0472[Formula: see text] and 0.05[Formula: see text]). For increasing [Formula: see text], the largest maximum principal strain was 4.4[Formula: see text], 5[Formula: see text], 5.3[Formula: see text] and 5.3[Formula: see text] in the membrane, 9.5[Formula: see text], 1.1[Formula: see text], 1.2[Formula: see text] and 1.2[Formula: see text] in the cytosol, and 4.5[Formula: see text], 5.3[Formula: see text], 5.7[Formula: see text] and 5.7[Formula: see text] in the nucleus. The results show (1) the importance of representing FA in cell models and (2) higher cellular mechanical sensitivity for substrate stiffness changes in the range of cell stiffness. The latter indicates that matching substrate stiffness to cell stiffness, and moderate variation of the former is very effective for controlled variation of cell deformation. The developed methodology is useful for parametric studies on cellular mechanics to obtain quantitative data of subcellular strains and stresses that cannot easily be measured experimentally.

Excessive volume of hydrogel injectates may compromise the efficacy for the treatment of acute myocardial infarction

Biomaterial injectates are promising as a therapy for myocardial infarction to inhibit the adverse ventricular remodeling. The current study explored interrelated effects of injectate volume and infarct size on treatment efficacy. A finite element model of a rat heart was utilized to represent ischemic infarcts of 10%, 20%, and 38% of left ventricular wall volume and polyethylene glycol hydrogel injectates of 25%, 50%, and 75% of the infarct volume. Ejection fraction was 49.7% in the healthy left ventricle and 44.9%, 46.4%, 47.4%, and 47.3% in the untreated 10% infarct and treated with 25%, 50%, and 75% injectate, respectively. Maximum end-systolic infarct fiber stress was 41.6, 53.4, 44.7, 44.0, and 45.3 kPa in the healthy heart, the untreated 10% infarct, and when treated with the three injectate volumes, respectively. Treating the 10% and 38% infarcts with the 25% injectate volume reduced the maximum end-systolic fiber stress by 16.3% and 34.7% and the associated strain by 30.2% and 9.8%, respectively. The results indicate the existence of a threshold for injectate volume above which efficacy does not further increase but may decrease. The efficacy of an injectate in reducing infarct stress and strain changes with infarct size. Copyright © 2016 John Wiley & Sons, Ltd.

Biomechanics of Cardiac Function

The heart pumps blood to maintain circulation and ensure the delivery of oxygenated blood to all the organs of the body. Mechanics play a critical role in governing and regulating heart function under both normal and pathological conditions. Biological processes and mechanical stress are coupled together in regulating myocyte function and extracellular matrix structure thus controlling heart function. Here, we offer a brief introduction to the biomechanics of left ventricular function and then summarize recent progress in the study of the effects of mechanical stress on ventricular wall remodeling and cardiac function as well as the effects of wall mechanical properties on cardiac function in normal and dysfunctional hearts. Various mechanical models to determine wall stress and cardiac function in normal and diseased hearts with both systolic and diastolic dysfunction are discussed. The results of these studies have enhanced our understanding of the biomechanical mechanism in the development and remodeling of normal and dysfunctional hearts. Biomechanics provide a tool to understand the mechanism of left ventricular remodeling in diastolic and systolic dysfunction and guidance in designing and developing new treatments.

Influence of matrix and bulk behaviour of an injectable hydrogel on the survival of encapsulated cardiac cells

The influence of physicochemical, morphological and mechanical behaviour of an injectable poly(propylene fumarate-co-ethylene glycol)/PEGDA hydrogel on the viability and proliferation of encapsulated cardiac cells was investigated.

Studying the influence of hydrogel injections into the infarcted left ventricle using the element-free Galerkin method

Myocardial infarction is an increasing health problem worldwide. Because of an under-supply of blood, the cardiomyocytes in the affected region permanently lose their ability to contract. This in turn gradually weakens the overall heart function. A new therapeutic approach based on the injection of a gel into the infarcted area aims to support the healing and to inhibit adverse remodelling that can lead to heart failure. A computational model is the basis for obtaining a better understanding of the heart mechanics, in particular, how myocardial infarction and gel injections affect its pumping performance. A strain invariant-based stored energy function is proposed to account for the passive mechanical behaviour of the model, which also makes provision for active contraction. To incorporate injections an additive homogenization approach is introduced. The numerical framework is developed using an in-house code based on the element-free Galerkin method. The main focus of this contribution is to investigate the influence of gel injections on the mechanics of the left ventricle during the diastolic filling and systolic isovolumetric (isochoric) contraction phases. It is found that gel injections are able to reduce the elevated fibre stresses caused by an infarct.

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.

Micro-structurally detailed model of a therapeutic hydrogel injectate in a rat biventricular cardiac geometry for computational simulations

Biomaterial injection-based therapies have showed cautious success in restoration of cardiac function and prevention of adverse remodelling into heart failure after myocardial infarction (MI). However, the underlying mechanisms are not well understood. Computational studies utilised simplified representations of the therapeutic myocardial injectates. Wistar rats underwent experimental infarction followed by immediate injection of polyethylene glycol hydrogel in the infarct region. Hearts were explanted, cryo-sectioned and the region with the injectate histologically analysed. Histological micrographs were used to reconstruct the dispersed hydrogel injectate. Cardiac magnetic resonance imaging data from a healthy rat were used to obtain an end-diastolic biventricular geometry which was subsequently adjusted and combined with the injectate model. The computational geometry of the injectate exhibited microscopic structural details found the in situ. The combination of injectate and cardiac geometry provides realistic geometries for multiscale computational studies of intra-myocardial injectate therapies for the rat model that has been widely used for MI research.