An elastic, biodegradable cardiac patch induces contractile smooth muscle and improves cardiac remodeling and function in subacute myocardial infarction

Sustained viral gene delivery from a micro-fibrous, elastomeric cardiac patch to the ischemic rat heart

Biodegradable and elastomeric patches have been applied to the surface of infarcted hearts as temporary mechanical supports to effectively alter adverse left ventricular remodeling processes. In this report, recombinant adeno-associated virus (AAV), known for its persistent transgene expression and low pathogenicity, was incorporated into elastomeric polyester urethane urea (PEUU) and polyester ether urethane urea (PEEUU) and processed by electrospinning into two formats (solid fibers and core-sheath fibers) designed to influence the controlled release behavior. The extended release of AAV encoding green fluorescent protein (GFP) was assessed in vitro. Sustained and localized viral particle delivery was achieved over 2 months in vitro. The biodegradable cardiac patches with or without AAV-GFP were implanted over rat left ventricular lesions three days following myocardial infarction to evaluate the transduction effect of released viral vectors. AAV particles were directly injected into the infarcted hearts as a control. Cardiac function and remodeling were significantly improved for 12 weeks after patch implantation compared to AAV injection. More GFP genes was expressed in the AAV patch group than AAV injection group, with both α-SMA positive cells and cardiac troponin T positive cells transduced in the patch group. Overall, the extended release behavior, prolonged transgene expression, and elastomeric mechanical properties make the AAV-loaded scaffold an attractive option for cardiac tissue engineering where both gene delivery and appropriate mechanical support are desired.

Exo-organoplasty interventions: A brief review of past, present and future directions for advance heart failure management

Heart failure (HF) is a debilitating disease in which abnormal function of the heart leads to imbalance of blood demand to tissues and organs. The pathogenesis of HF is very complex and various factors can contribute including myocardial infarction, ischemia, hypertension and genetic cardiomyopathies. HF is the leading cause of death and its prevalence is expected to increase in parallel with the population age. Different kind of therapeutic approaches including lifestyle modification, medication and pacemakers are used for HF patients in NYHA I-III functional class. However, for advance stage HF patient's (NYHA IV), ventricle assist devices are clinically use and stem cells are under active investigation. Most of these therapies leads to modest symptoms relief and have no significant role in long-term survival rate. Currently there is no effective treatment for advance HF except heart transplantation, which is still remain clinically insignificant because of donor pool limitation. As HF is a result of multiple etiologies therefore multi-functional therapeutic platform is needed. Exo-organoplasty interventions are studied from almost one century. The major goals of these interventions are to treat various kind of heart disease from outside the heart muscle without having direct contact with blood. Various kind of interventions (devices and techniques) are developed in this arena with the passage of time. The purpose of this review is to describe the theory behind intervention devices, the devices themselves, their clinical results, advantages and limitations. Furthermore, to present a future multi-functional therapeutic platform (ASD) for advance stage HF management.

Carbon nanotube-composite hydrogels promote intercalated disc assembly in engineered cardiac tissues through β1-integrin mediated FAK and RhoA pathway

Carbon nanotube (CNT)-based hydrogels have been shown to support cardiomyocyte growth and function. However, their role in cellular integrity among cardiomyocytes has not been studied in detail and the mechanisms underlying this process remain unclear. Here, single walled CNTs incorporated into gelatin with methacrylate anhydride (CNT/GelMA) hydrogels were utilized to construct cardiac tissues, which enhanced cardiomyocyte adhesion and maturation. Furthermore, through the use of immunohistochemical staining, transmission electron microscopy and intracellular calcium transient measurement, the incorporation of CNTs into the scaffolds was observed to markedly enhance the assembly and formation in the cardiac constructs. Importantly, we further explored the underlying mechanism behind these effects through the use of immunohistochemical staining and western blotting. The β1-integrin-mediated FAK and RhoA signaling pathways were found to be responsible for CNT-induced upregulation of electrical and mechanical junction proteins respectively. Together, our study provides new insights into the facilitative effects of CNTs on ID formation, which has important significance for improving the quality of engineered cardiac tissue and applying them to cardiac regenerative therapies.

STATEMENT OF SIGNIFICANCE

Currently, the bottleneck to engineering cardiac tissues (ECTs) for cardiac regeneration is the lack of efficient cellular integrity among adjacent cells, especially the insufficient remodeling of intercalated discs (IDs) in ECTs. Recently, carbon nanotube (CNT) hydrogels provide an advantageous supporting microenvironment and therefore benefit greatly the functional performance of ECTs. Although their beneficial effect in modulating ECT performance is evident, the influence of CNTs on structural integrity of ECTs has not been studied in detail, and the mechanisms underlying the process remain to be determined. Here, we utilized carbon nanotube incorporated into gelatin with methacrylate anhydride (CNT/GelMA) hydrogels to construct cardiac tissues, determined the influence of CNTs on intercalated discs (IDs) assembly and formation and explored the underlying mechanisms.

Cardiovascular Tissue Engineering: Preclinical Validation to Bedside Application

Advancements in biomaterial science and available cell sources have spurred the translation of tissue-engineering technology to the bedside, addressing the pressing clinical demands for replacement cardiovascular tissues. Here, the in vivo status of tissue-engineered blood vessels, heart valves, and myocardium is briefly reviewed, illustrating progress toward a tissue-engineered heart for clinical use.

Ascorbic Acid in Polyurethane Systems for Tissue Engineering

The introduction of the paper was devoted to the main items of tissue engineering (TE) and the way of porous structure obtaining as scaffolds. Furthermore, the significant role of the scaffold design in TE was described. It was shown, that properly designed polyurethanes (PURs) find application in TE due to the proper physicochemical, mechanical and biological properties. Then the use of L-ascorbic acid (L-AA) in PUR systems for TE was described. L-AA has been applied in this area due to its suitable biological characteristics and antioxidative properties. Moreover, L-AA influences tissue regeneration due to improving collagen synthesis, which is a primary component of the extracellular matrix (ECM). Modification of PUR with L-AA leads to the materials with higher biocompatibility and such system is promising for TE applications.

Large strain stimulation promotes extracellular matrix production and stiffness in an elastomeric scaffold model

Mechanical conditioning of engineered tissue constructs is widely recognized as one of the most relevant methods to enhance tissue accretion and microstructure, leading to improved mechanical behaviors. The understanding of the underlying mechanisms remains rather limited, restricting the development of in silico models of these phenomena, and the translation of engineered tissues into clinical application. In the present study, we examined the role of large strip-biaxial strains (up to 50%) on ECM synthesis by vascular smooth muscle cells (VSMCs) micro-integrated into electrospun polyester urethane urea (PEUU) constructs over the course of 3 weeks. Experimental results indicated that VSMC biosynthetic behavior was quite sensitive to tissue strain maximum level, and that collagen was the primary ECM component synthesized. Moreover, we found that while a 30% peak strain level achieved maximum ECM synthesis rate, further increases in strain level lead to a reduction in ECM biosynthesis. Subsequent mechanical analysis of the formed collagen fiber network was performed by removing the scaffold mechanical responses using a strain-energy based approach, showing that the denovo collagen also demonstrated mechanical behaviors substantially better than previously obtained with small strain training and comparable to mature collagenous tissues. We conclude that the application of large deformations can play a critical role not only in the quantity of ECM synthesis (i.e. the rate of mass production), but also on the modulation of the stiffness of the newly formed ECM constituents. The improved understanding of the process of growth and development of ECM in these mechano-sensitive cell-scaffold systems will lead to more rational design and manufacturing of engineered tissues operating under highly demanding mechanical environments.

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.

Biomaterial property-controlled stem cell fates for cardiac regeneration

Myocardial infarction (MI) affects more than 8 million people in the United States alone. Due to the insufficient regeneration capacity of the native myocardium, one widely studied approach is cardiac tissue engineering, in which cells are delivered with or without biomaterials and/or regulatory factors to fully regenerate the cardiac functions. Specifically, in vitro cardiac tissue engineering focuses on using biomaterials as a reservoir for cells to attach, as well as a carrier of various regulatory factors such as growth factors and peptides, providing high cell retention and a proper microenvironment for cells to migrate, grow and differentiate within the scaffolds before implantation. Many studies have shown that the full establishment of a functional cardiac tissue in vitro requires synergistic actions between the seeded cells, the tissue culture condition, and the biochemical and biophysical environment provided by the biomaterials-based scaffolds. Proper electrical stimulation and mechanical stretch during the in vitro culture can induce the ordered orientation and differentiation of the seeded cells. On the other hand, the various scaffolds biochemical and biophysical properties such as polymer composition, ligand concentration, biodegradability, scaffold topography and mechanical properties can also have a significant effect on the cellular processes.

•

Cell therapy is an attractive approach for cardiac regeneration after myocardial infarction.

•

Biomaterials are used as cell carriers.

•

This review highlights how biochemical and biophysical properties of biomaterials affect cell fates.

Synthesis and characterization of polycaprolactone urethane hollow fiber membranes as small diameter vascular grafts

The design of bioresorbable synthetic small diameter (<6mm) vascular grafts (SDVGs) capable of sustaining long-term patency and endothelialization is a daunting challenge in vascular tissue engineering. Here, we synthesized a family of biocompatible and biodegradable polycaprolactone (PCL) urethane macromers to fabricate hollow fiber membranes (HFMs) as SDVG candidates, and characterized their mechanical properties, degradability, hemocompatibility, and endothelial development. The HFMs had smooth surfaces and porous internal structures. Their tensile stiffness ranged from 0.09 to 0.11N/mm and their maximum tensile force from 0.86 to 1.03N, with minimum failure strains of approximately 130%. Permeability varied from 1 to 14×10(-6)cm/s, burst pressures from 1158 to 1468mmHg, and compliance from 0.52 to 1.48%/100mmHg. The suture retention forces ranged from 0.55 to 0.81N. HFMs had slow degradation profiles, with 15 to 30% degradation after 8weeks. Human endothelial cells proliferated well on the HFMs, creating stable cell layer coverage. Hemocompatibility studies demonstrated low hemolysis (<2%), platelet activation, and protein adsorption. There were no significant differences in the hemocompatibility of HFMs in the absence and presence of endothelial layers. These encouraging results suggest great promise of our newly developed materials and biodegradable elastomeric HFMs as SDVG candidates.

Timing effect of intramyocardial hydrogel injection for positively impacting left ventricular remodeling after myocardial infarction

Intramyocardial injection of various injectable hydrogel materials has shown benefit in positively impacting the course of left ventricular (LV) remodeling after myocardial infarction (MI). However, since LV remodeling is a complex, time dependent process, the most efficacious time of hydrogel injection is not clear. In this study, we injected a relatively stiff, thermoresponsive and bioabsorbable hydrogel in rat hearts at 3 different time points - immediately after MI (IM), 3 d post-MI (3D), and 2 w post-MI (2W), corresponding to the beginnings of the necrotic, fibrotic and chronic remodeling phases. The employed left anterior descending coronary artery ligation model showed expected infarction responses including functional loss, inflammation and fibrosis with distinct time dependent patterns. Changes in LV geometry and contractile function were followed by longitudinal echocardiography for 10 w post-MI. While all injection times positively affected LV function and wall thickness, the 3D group gave better functional outcomes than the other injection times and also exhibited more local vascularization and less inflammatory markers than the earlier injection time. The results indicate an important role for injection timing in the increasingly explored concept of post-MI biomaterial injection therapy and suggest that for hydrogels with mechanical support as primary function, injection at the beginning of the fibrotic phase may provide improved outcomes.

Synthesis of polyester urethane urea and fabrication of elastomeric nanofibrous scaffolds for myocardial regeneration

Fabrication of bioactive scaffolds is one of the most promising strategies to reconstruct the infarcted myocardium. In this study, we synthesized polyester urethane urea (PEUU), further blended it with gelatin and fabricated PEUU/G nanofibrous scaffolds. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), differential scanning calorimetry (DSC) and X-ray diffraction were used for the characterization of the synthesized PEUU and properties of nanofibrous scaffolds were evaluated using scanning electron microscopy (SEM), ATR-FTIR, contact angle measurement, biodegradation test, tensile strength analysis and dynamic mechanical analysis (DMA). In vitro biocompatibility studies were performed using cardiomyocytes. DMA analysis showed that the scaffolds could be reshaped with cyclic deformations and might remain stable in the frequencies of the physiological activity of the heart. On the whole, our study suggests that aligned PEUU/G 70:30 nanofibrous scaffolds meet the required specifications for cardiac tissue engineering and could be used as a promising construct for myocardial regeneration.

Mechanoactive materials in cardiac science

Mechanically active biomaterials such as shape memory materials, liquid crystal elastomers, dielectric elastomer actuators, and conductive polymers could be used in mechanical devices to augment heart function or condition cardiac cells and artificial tissues for regenerative medicine solutions.

Integration of mesenchymal stem cells with nanobiomaterials for the repair of myocardial infarction

The integration of nanobiomaterials with stem cells represents a promising strategy for the treatment of myocardial infarction. While stem cells and nanobiomaterials each demonstrated partial success in cardiac repair individually, the therapeutic efficacy of the clinical settings for each of these has been low. Hence, a combination of nanobiomaterials with stem cells is vigorously studied to create synergistic effects for treating myocardial infarction. To date, various types of nanomaterials have been incorporated with stem cells to control cell fate, modulate the therapeutic behavior of stem cells, and make them more suitable for cardiac repair. Here, we review the current stem cell therapies for cardiac repair and describe the combinatorial approaches of using nanobiomaterials and stem cells to improve therapeutic efficacy for the treatment of myocardial infarction.

Modifying the mechanics of healing infarcts: Is better the enemy of good?

Myocardial infarction (MI) is a major source of morbidity and mortality worldwide, with over 7 million people suffering infarctions each year. Heart muscle damaged during MI is replaced by a collagenous scar over a period of several weeks, and the mechanical properties of that scar tissue are a key determinant of serious post-MI complications such as infarct rupture, depression of heart function, and progression to heart failure. Thus, there is increasing interest in developing therapies that modify the structure and mechanics of healing infarct scar. Yet most prior attempts at therapeutic scar modification have failed, some catastrophically. This article reviews available information about the mechanics of healing infarct scar and the functional impact of scar mechanical properties, and attempts to infer principles that can better guide future attempts to modify scar. One important conclusion is that collagen structure, mechanics, and remodeling of healing infarct scar vary so widely among experimental models that any novel therapy should be tested across a range of species, infarct locations, and reperfusion protocols. Another lesson from past work is that the biology and mechanics of healing infarcts are sufficiently complex that the effects of interventions are often counterintuitive; for example, increasing infarct stiffness has little effect on heart function, and inhibition of matrix metalloproteases (MMPs) has little effect on scar collagen content. Computational models can help explain such counterintuitive results, and are becoming an increasingly important tool for integrating known information to better identify promising therapies and design experiments to test them. Moving forward, potentially exciting new opportunities for therapeutic modification of infarct mechanics include modulating anisotropy and promoting scar compaction.

Generation and Assessment of Functional Biomaterial Scaffolds for Applications in Cardiovascular Tissue Engineering and Regenerative Medicine

Current clinically applicable tissue and organ replacement therapies are limited in the field of cardiovascular regenerative medicine. The available options do not regenerate damaged tissues and organs, and, in the majority of the cases, show insufficient restoration of tissue function. To date, anticoagulant drug-free heart valve replacements or growing valves for pediatric patients, hemocompatible and thrombus-free vascular substitutes that are smaller than 6 mm, and stem cell-recruiting delivery systems that induce myocardial regeneration are still only visions of researchers and medical professionals worldwide and far from being the standard of clinical treatment. The design of functional off-the-shelf biomaterials as well as automatable and up-scalable biomaterial processing methods are the focus of current research endeavors and of great interest for fields of tissue engineering and regenerative medicine. Here, various approaches that aim to overcome the current limitations are reviewed, focusing on biomaterials design and generation methods for myocardium, heart valves, and blood vessels. Furthermore, novel contact- and marker-free biomaterial and extracellular matrix assessment methods are highlighted.

Myocardial infarction: stem cell transplantation for cardiac regeneration

It is estimated that by 2030, almost 23.6 million people will perish from cardiovascular disease, according to the WHO. The review discusses advances in stem cell therapy for myocardial infarction, including cell sources, methods of differentiation, expansion selection and their route of delivery. Skeletal muscle cells, hematopoietic cells and mesenchymal stem cells (MSCs) and embryonic stem cells (ESCs)-derived cardiomyocytes have advanced to the clinical stage, while induced pluripotent cells (iPSCs) are yet to be considered clinically. Delivery of cells to the sites of injury and their subsequent retention is a major issue. The development of supportive scaffold matrices to facilitate stem cell retention and differentiation are analyzed. The review outlines clinical translation of conjugate stem cell-based cellular therapeutics post-myocardial infarction.

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.

Cardiomyocyte behavior on biodegradable polyurethane/gold nanocomposite scaffolds under electrical stimulation

Following a myocardial infarction (MI), cardiomyocytes are replaced by scar tissue, which decreases ventricular contractile function. Tissue engineering is a promising approach to regenerate such damaged cardiomyocyte tissue. Engineered cardiac patches can be fabricated by seeding a high density of cardiac cells onto a synthetic or natural porous polymer. In this study, nanocomposite scaffolds made of gold nanotubes/nanowires incorporated into biodegradable castor oil-based polyurethane were employed to make micro-porous scaffolds. H9C2 cardiomyocyte cells were cultured on the scaffolds for one day, and electrical stimulation was applied to improve cell communication and interaction in neighboring pores. Cells on scaffolds were examined by fluorescence microscopy and scanning electron microscopy, revealing that the combination of scaffold design and electrical stimulation significantly increased cell confluency of H9C2 cells on the scaffolds. Furthermore, we showed that the gene expression levels of Nkx2.5, atrial natriuretic peptide (ANF) and natriuretic peptide precursor B (NPPB), which are functional genes of the myocardium, were up-regulated by the incorporation of gold nanotubes/nanowires into the polyurethane scaffolds, in particular after electrical stimulation.

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.

Physiologically inspired cardiac scaffolds for tailored in vivo function and heart regeneration

Tissue engineering is well suited for the treatment of cardiac disease due to the limited regenerative capacity of native cardiac tissue and the loss of function associated with endemic cardiac pathologies, such as myocardial infarction and congenital heart defects. However, the physiological complexity of the myocardium imposes extensive requirements on tissue therapies intended for these applications. In recent years, the field of cardiac tissue engineering has been characterized by great innovation and diversity in the fabrication of engineered tissue scaffolds for cardiac repair and regeneration to address these problems. From early approaches that attempted only to deliver cardiac cells in a hydrogel vessel, significant progress has been made in understanding the role of each major component of cardiac living tissue constructs (namely cells, scaffolds, and signaling mechanisms) as they relate to mechanical, biological, and electrical in vivo performance. This improved insight, accompanied by modern material science techniques, allows for the informed development of complex scaffold materials that are optimally designed for cardiac applications. This review provides a background on cardiac physiology as it relates to critical cardiac scaffold characteristics, the degree to which common cardiac scaffold materials fulfill these criteria, and finally an overview of recent in vivo studies that have employed this type of approach.

Carbon nanotubes enhance intercalated disc assembly in cardiac myocytes via the β1-integrin-mediated signaling pathway

Carbon nanotubes (CNTs) offer a new paradigm for constructing functional cardiac patches and repairing myocardial infarction (MI). However, little is known about how CNTs enhance the mechanical integrity and electrophysiological function of cardiac myocytes. To address this issue, we investigated the regularity and precise mechanism of the influence of CNTs on the assembly of intercalated disc (IDs). Here, single walled CNTs incorporated into collagen substrates were utilized as growth supports for neonatal cardiomyocytes, which enhanced cardiomyocyte adhesion and maturation. Furthermore, through the use of immunohistochemical staining, western blotting, transmission electron microscopy, and intracellular calcium transient measurement, we discovered that the addition of CNTs remarkably increased ID-related protein expression and enhanced ID assembly and functionality. On that basis, we further explored the underlying mechanism for how CNTs enhanced ID assembly through the use of immunohistochemical staining and western blotting. We found that the β1-integrin-mediated signaling pathway mediated CNT-induced upregulation of electrical and mechanical junction proteins. Notably, CNTs remarkably accelerated gap junction formation via activation of the β1-integrin-mediated FAK/ERK/GATA4 pathway. These findings provide valuable insight into the mechanistic effects that CNTs have on neonatal cardiomyocyte performance and will have a significant impact on the future of nanomedical research.

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.

Validating whole slide digital morphometric analysis as a microscopy tool

Whole slide imaging (WSI) can be used to quantify multiple responses within tissue sections during histological analysis. Feature Analysis on Consecutive Tissue Sections (FACTS®) allows the investigator to perform digital morphometric analysis (DMA) within specified regions of interest (ROI) across multiple serial sections at faster rates when compared with manual morphometry methods. Using FACTS® in conjunction with WSI is a powerful analysis tool, which allows DMA to target specific ROI across multiple tissue sections stained for different biomarkers. DMA may serve as an appropriate alternative to classic, manual, histologic morphometric measures, which have historically relied on the selection of high-powered fields of views and manual scoring (e.g., a gold standard). In the current study, existing preserved samples were used to determine if DMA would provide similar results to manual counting methods. Rodent hearts (n=14, left ventricles) were stained with Masson's trichrome, and reacted for cluster of differentiation 68 (CD-68). This study found no statistical significant difference between a classic, manual method and the use of digital algorithms to perform the similar counts (p=0.38). DMA offers researchers the ability to accurately evaluate morphological characteristics in a reproducible fashion without investigator bias and with higher throughput.

Development and evaluation of elastomeric hollow fiber membranes as small diameter vascular graft substitutes

Engineering of small diameter (<6mm) vascular grafts (SDVGs) for clinical use remains a significant challenge. Here, elastomeric polyester urethane (PEU)-based hollow fiber membranes (HFMs) are presented as an SDVG candidate to target the limitations of current technologies and improve tissue engineering designs. HFMs are fabricated by a simple phase inversion method. HFM dimensions are tailored through adjustments to fabrication parameters. The walls of HFMs are highly porous. The HFMs are very elastic, with moduli ranging from 1-4MPa, strengths from 1-5MPa, and max strains from 300-500%. Permeability of the HFMs varies from 0.5-3.5×10(-6)cm/s, while burst pressure varies from 25 to 35psi. The suture retention forces of HFMs are in the range of 0.8 to 1.2N. These properties match those of blood vessels. A slow degradation profile is observed for all HFMs, with 71 to 78% of the original mass remaining after 8weeks, providing a suitable profile for potential cellular incorporation and tissue replacement. Both human endothelial cells and human mesenchymal stem cells proliferate well in the presence of HFMs up to 7days. These results demonstrate a promising customizable PEU HFMs for small diameter vascular repair and tissue engineering applications.

A novel polyurethane/cellulose fibrous scaffold for cardiac tissue engineering

A high mechanical strength and biomimetic scaffold is electrospun from a blend of polyurethane and ethyl cellulose, being promising in applications for therapeutic purposes as a cardiac graft for reconstructing or regeneration of damaged myocardium.

Electrospun biodegradable elastic polyurethane scaffolds with dipyridamole release for small diameter vascular grafts

Acellular biodegradable small diameter vascular grafts (SDVGs) require antithrombosis, intimal hyperplasia inhibition and rapid endothelialization to improve the graft patency. However, current antithrombosis and antiproliferation approaches often conflict with endothelial cell layer formation on SDVGs. To address this limitation, biodegradable elastic polyurethane urea (BPU) and the drug dipyridamole (DPA) were mixed and then electrospun into a biodegradable fibrous scaffold. The BPU would provide the appropriate mechanical support, while the DPA in the scaffold would offer biofunctions as required above. We found that the resulting scaffolds had tensile strengths and strains comparable with human coronary artery. The DPA in the scaffolds was continuously released up to 91 days in phosphate buffer solution at 37 °C, with a low burst release within the first 3 days. Compared to BPU alone, improved non-thrombogenicity of the DPA-loaded BPU scaffolds was evidenced with extended human blood clotting time, lower TAT complex concentration, lower hemolysis and reduced human platelet deposition. The scaffolds with a higher DPA content (5 and 10%) inhibited proliferation of human aortic smooth muscle cell significantly. Furthermore, the DPA-loaded scaffolds had no adverse effect on human aortic endothelial cell growth, yet it improved their proliferation. The attractive mechanical properties and biofunctions of the DPA-loaded BPU scaffold indicate its potential as an acellular biodegradable SDVG for vascular replacement.

Cardiac tissue slice transplantation as a model to assess tissue-engineered graft thickness, survival, and function

BACKGROUND

Cell therapies offer the potential to improve cardiac function after myocardial infarction. Although injection of single-cell suspensions has proven safe, cell retention and survival rates are low. Tissue-engineered grafts allow cell delivery with minimal initial cell loss and mechanical support to the heart. However, graft performance cannot be easily compared, and optimal construct thickness, vascularization, and survival kinetics are unknown.

METHODS AND RESULTS

Cardiac tissue slices (CTS) were generated by sectioning mouse hearts (n=40) expressing firefly luciferase and green fluorescent protein into slices of defined size and thickness using a vibrating blade microtome. Bioluminescence imaging of CTS transplanted onto hearts of immunodeficient mice demonstrated survival of ≤30% of transplanted cells. Cardiac slice perfusion was re-established within 3 days, likely through anastomosis of pre-existing vessels with the host vasculature and invasion of vessels from the host. Immunofluorescence showed a peak in cell death 3 days after transplantation and a gradual decline thereafter. MRI revealed preservation of contractile function and an improved ejection fraction 1 month after transplantation of CTS (28±2% CTS versus 22±2% control; P=0.05). Importantly, this effect was specific to CTS because transplantation of skeletal muscle tissue slices led to faster dilative remodeling and higher animal mortality.

CONCLUSIONS

In summary, this is the first study to use CTS as a benchmark to validate and model tissue-engineered graft studies. CTS transplantation improved cell survival, established reperfusion, and enhanced cardiac function after myocardial infarction. These findings also confirm that dilative remodeling can be attenuated by topical transplantation of CTS but not skeletal muscle tissue grafts.

The role of tissue engineering and biomaterials in cardiac regenerative medicine

In recent years, the development of 3-dimensional engineered heart tissue (EHT) has made large strides forward because of advances in stem cell biology, materials science, prevascularization strategies, and nanotechnology. As a result, the role of tissue engineering in cardiac regenerative medicine has become multifaceted as new applications become feasible. Cardiac tissue engineering has long been established to have the potential to partially or fully restore cardiac function after cardiac injury. However, EHTs may also serve as surrogate human cardiac tissue for drug-related toxicity screening. Cardiotoxicity remains a major cause of drug withdrawal in the pharmaceutical industry. Unsafe drugs reach the market because preclinical evaluation is insufficient to weed out cardiotoxic drugs in all their forms. Bioengineering methods could provide functional and mature human myocardial tissues, ie, physiologically relevant platforms, for screening the cardiotoxic effects of pharmaceutical agents and facilitate the discovery of new therapeutic agents. Finally, advances in induced pluripotent stem cells have made patient-specific EHTs possible, which opens up the possibility of personalized medicine. Herein, we give an overview of the present state of the art in cardiac tissue engineering, the challenges to the field, and future perspectives.

Plasma-functionalized electrospun matrix for biograft development and cardiac function stabilization

Cardiac tissue engineering approaches can deliver large numbers of cells to the damaged myocardium and have thus increasingly been considered as a possible curative treatment to counteract the high prevalence of progressive heart failure after myocardial infarction (MI). Optimal scaffold architecture and mechanical and chemical properties, as well as immune- and bio-compatibility, need to be addressed. We demonstrated that radio-frequency plasma surface functionalized electrospun poly(ɛ-caprolactone) (PCL) fibres provide a suitable matrix for bone-marrow-derived mesenchymal stem cell (MSC) cardiac implantation. Using a rat model of chronic MI, we showed that MSC-seeded plasma-coated PCL grafts stabilized cardiac function and attenuated dilatation. Significant relative decreases of 13% of the ejection fraction (EF) and 15% of the fractional shortening (FS) were observed in sham treated animals; respective decreases of 20% and 25% were measured 4 weeks after acellular patch implantation, whereas a steadied function was observed 4 weeks after MSC-patch implantation (relative decreases of 6% for both EF and FS).

Stem cell-loaded nanofibrous patch promotes the regeneration of infarcted myocardium with functional improvement in rat model

Myocardial infarction (MI) leads to the loss of cardiomyocytes, followed by left ventricular (LV) remodeling and cardiac dysfunction. The authors hypothesize that an elastic, biodegradable nanofibrous cardiac patch loaded with mesenchymal stem cells (MSC) could restrain LV remodeling and improve cardiac function after MI. Poly(ε-caprolactone)/gelatin (PG) nanofibers were fabricated by electrospinning, and the nanofibers displayed a porous and uniform nanofibrous structure with a diameter of 244±51nm. An MI model was established by ligation of the left anterior descending coronary artery of female Sprague-Dawley rats. The PG nanofibrous patch seeded with MSC, isolated from rat bone marrow, was implanted on the epicardium of the infarcted region of the LV wall of the heart. After transplantation, the PG-cell patch restricted the expansion of the LV wall effectively and reduced the scar size, and the density of the microvessels increased. Cells within the patch were able to migrate towards the scar tissue, and promoted new blood vessel formation at the infarct site. Angiogenesis and the cardiac functions improved significantly after 4weeks of implantation. The MSC-seeded PG nanofibrous patches are demonstrated to provide sufficient mechanical support, to induce angiogenesis and to accelerate cardiac repair in a rat model of MI. The study highlights the positive impact of implantation of an MSC-seeded PG nanofibrous patch as a novel constituent for MI repair.

A comparison of electrospun polymers reveals poly(3-hydroxybutyrate) fiber as a superior scaffold for cardiac repair

The development of biomaterials for myocardial tissue engineering requires a careful assessment of their performance with regards to functionality and biocompatibility, including the immune response. Poly(3-hydroxybutyrate) (PHB), poly(e-caprolactone) (PCL), silk, poly-lactic acid (PLA), and polyamide (PA) scaffolds were generated by electrospinning, and cell compatibility in vitro, and immune response and cardiac function in vitro and in vivo were compared with a noncrosslinked collagen membrane (Col) control material. Results showed that cell adhesion and growth of mesenchymal stem cells, cardiomyocytes, and cardiac fibroblasts in vitro was dependent on the polymer substrate, with PHB and PCL polymers permitting the greatest adhesion/growth of cells. Additionally, polymer substrates triggered unique expression profiles of anti- and pro-inflammatory cytokines in human peripheral blood mononuclear cells. Implantation of PCL, silk, PLA, and PA patches on the epicardial surface of healthy rats induced a classical foreign body reaction pattern, with encapsulation of polymer fibers and induction of the nonspecific immune response, whereas Col and PHB patches were progressively degraded. When implanted on infarcted rat heart, Col, PCL, and PHB reduced negative remodeling, but only PHB induced significant angiogenesis. Importantly, Col and PHB modified the inflammatory response to an M2 macrophage phenotype in cardiac tissue, indicating a more beneficial reparative process and remodeling. Collectively, these results identify PHB as a superior substrate for cardiac repair.

Engineering the heart: evaluation of conductive nanomaterials for improving implant integration and cardiac function

Recently, carbon nanotubes together with other types of conductive materials have been used to enhance the viability and function of cardiomyocytes in vitro. Here we demonstrated a paradigm to construct ECTs for cardiac repair using conductive nanomaterials. Single walled carbon nanotubes (SWNTs) were incorporated into gelatin hydrogel scaffolds to construct three-dimensional ECTs. We found that SWNTs could provide cellular microenvironment in vitro favorable for cardiac contraction and the expression of electrochemical associated proteins. Upon implantation into the infarct hearts in rats, ECTs structurally integrated with the host myocardium, with different types of cells observed to mutually invade into implants and host tissues. The functional measurements showed that SWNTs were essential to improve the performance of ECTs in inhibiting pathological deterioration of myocardium. This work suggested that conductive nanomaterials hold therapeutic potential in engineering cardiac tissues to repair myocardial infarction.

Preparation of acellular myocardial scaffolds with well-preserved cardiomyocyte lacunae, and method for applying mechanical and electrical simulation to tissue construct

Cardiac tissue engineering/regeneration using decellularized myocardium has attracted great research attention due to its potential benefit for myocardial infarction (MI) treatment. Here we describe an optimal decellularization protocol to generate 3D porcine myocardial scaffolds with well-preserved cardiomyocyte lacunae and a multi-stimulation bioreactor that is able to provide coordinated mechanical and electrical stimulation for facilitating cardiac construct development.

The effect of bioengineered acellular collagen patch on cardiac remodeling and ventricular function post myocardial infarction

Regeneration of the damaged myocardium is one of the most challenging fronts in the field of tissue engineering due to the limited capacity of adult heart tissue to heal and to the mechanical and structural constraints of the cardiac tissue. In this study we demonstrate that an engineered acellular scaffold comprising type I collagen, endowed with specific physiomechanical properties, improves cardiac function when used as a cardiac patch following myocardial infarction. Patches were grafted onto the infarcted myocardium in adult murine hearts immediately after ligation of left anterior descending artery and the physiological outcomes were monitored by echocardiography, and by hemodynamic and histological analyses four weeks post infarction. In comparison to infarcted hearts with no treatment, hearts bearing patches preserved contractility and significantly protected the cardiac tissue from injury at the anatomical and functional levels. This improvement was accompanied by attenuated left ventricular remodeling, diminished fibrosis, and formation of a network of interconnected blood vessels within the infarct. Histological and immunostaining confirmed integration of the patch with native cardiac cells including fibroblasts, smooth muscle cells, epicardial cells, and immature cardiomyocytes. In summary, an acellular biomaterial with specific biomechanical properties promotes the endogenous capacity of the infarcted myocardium to attenuate remodeling and improve heart function following myocardial infarction.

In situ tissue regeneration through host stem cell recruitment

The field of tissue engineering has made steady progress in translating various tissue applications. Although the classical tissue engineering strategy, which involves the use of culture-expanded cells and scaffolds to produce a tissue construct for implantation, has been validated, this approach involves extensive cell expansion steps, requiring a lot of time and laborious effort before implantation. To bypass this ex vivo process, a new approach has been introduced. In situ tissue regeneration utilizes the body's own regenerating capacity by mobilizing host endogenous stem cells or tissue-specific progenitor cells to the site of injury. This approach relies on development of a target-specific biomaterial scaffolding system that can effectively control the host microenvironment and mobilize host stem/progenitor cells to target tissues. An appropriate microenvironment provided by implanted scaffolds would facilitate recruitment of host cells that can be guided to regenerating structural and functional tissues.

The effect of polymer degradation time on functional outcomes of temporary elastic patch support in ischemic cardiomyopathy

Biodegradable polyurethane patches have been applied as temporary mechanical supports to positively alter the remodeling and functional loss following myocardial infarction. How long such materials need to remain in place is unclear. Our objective was to compare the efficacy of porous onlay support patches made from one of three types of biodegradable polyurethane with relatively fast (poly(ester urethane)urea; PEUU), moderate (poly(ester carbonate urethane)urea; PECUU), and slow (poly(carbonate urethane)urea; PCUU) degradation rates in a rat model of ischemic cardiomyopathy. Microporous PEUU, PECUU or PCUU (n = 10 each) patches were implanted over left ventricular lesions 2 wk following myocardial infarction in rat hearts. Infarcted rats without patching and age-matched healthy rats (n = 10 each) were controls. Echocardiography was performed every 4 wk up to 16 wk, at which time hemodynamic and histological assessments were performed. The end-diastolic area for the PEUU group at 12 and 16 wk was significantly larger than for the PECUU or PCUU groups. Histological analysis demonstrated greater vascular density in the infarct region for the PECUU or PCUU versus PEUU group at 16 wk. Improved left ventricular contractility and diastolic performance in the PECUU group was observed at 16 wk compared to infarction controls. The results indicate that the degradation rate of an applied elastic patch influences the functional benefits associated patch placement, with a moderately slow degrading PECUU patch providing improved outcomes.

Natural ECM as biomaterial for scaffold based cardiac regeneration using adult bone marrow derived stem cells

Cellular therapy using stem cells for cardiac diseases has recently gained much interest in the scientific community due to its potential in regenerating damaged and even dead tissue and thereby restoring the organ function. Stem cells from various sources and origin are being currently used for regeneration studies directly or along with differentiation inducing agents. Long term survival and minimal side effects can be attained by using autologous cells and reduced use of inducing agents. Cardiomyogenic differentiation of adult derived stem cells has been previously reported using various inducing agents but the use of a potentially harmful DNA demethylating agent 5-azacytidine (5-azaC) has been found to be critical in almost all studies. Alternate inducing factors and conditions/stimulant like physical condition including electrical stimulation, chemical inducers and biological agents have been attempted by numerous groups to induce cardiac differentiation. Biomaterials were initially used as artificial scaffold in in vitro studies and later as a delivery vehicle. Natural ECM is the ideal biological scaffold since it contains all the components of the tissue from which it was derived except for the living cells. Constructive remodeling can be performed using such natural ECM scaffolds and stem cells since, the cells can be delivered to the site of infraction and once delivered the cells adhere and are not "lost". Due to the niche like conditions of ECM, stem cells tend to differentiate into tissue specific cells and attain several characteristics similar to that of functional cells even in absence of any directed differentiation using external inducers. The development of niche mimicking biomaterials and hybrid biomaterial can further advance directed differentiation without specific induction. The mechanical and electrical integration of these materials to the functional tissue is a problem to be addressed. The search for the perfect extracellular matrix for therapeutic applications including engineering cardiac tissue structures for post ischemic cardiac tissue regeneration continues.

Myocardial scaffold-based cardiac tissue engineering: application of coordinated mechanical and electrical stimulations

Recently, we developed an optimal decellularization protocol to generate 3D porcine myocardial scaffolds, which preserve the natural extracellular matrix structure, mechanical anisotropy, and vasculature templates and also show good cell recellularization and differentiation potential. In this study, a multistimulation bioreactor was built to provide coordinated mechanical and electrical stimulation for facilitating stem cell differentiation and cardiac construct development. The acellular myocardial scaffolds were seeded with mesenchymal stem cells (10(6) cells/mL) by needle injection and subjected to 5-azacytidine treatment (3 μmol/L, 24 h) and various bioreactor conditioning protocols. We found that after 2 days of culturing with mechanical (20% strain) and electrical stimulation (5 V, 1 Hz), high cell density and good cell viability were observed in the reseeded scaffold. Immunofluorescence staining demonstrated that the differentiated cells showed a cardiomyocyte-like phenotype by expressing sarcomeric α-actinin, myosin heavy chain, cardiac troponin T, connexin-43, and N-cadherin. Biaxial mechanical testing demonstrated that positive tissue remodeling took place after 2 days of bioreactor conditioning (20% strain + 5 V, 1 Hz); passive mechanical properties of the 2 day and 4 day tissue constructs were comparable to those of the tissue constructs produced by stirring reseeding followed by 2 weeks of static culturing, implying the effectiveness and efficiency of the coordinated simulations in promoting tissue remodeling. In short, the synergistic stimulations might be beneficial not only for the quality of cardiac construct development but also for patients by reducing the waiting time in future clinical scenarios.

The pharmacology of regenerative medicine

Regenerative medicine is a rapidly evolving multidisciplinary, translational research enterprise whose explicit purpose is to advance technologies for the repair and replacement of damaged cells, tissues, and organs. Scientific progress in the field has been steady and expectations for its robust clinical application continue to rise. The major thesis of this review is that the pharmacological sciences will contribute critically to the accelerated translational progress and clinical utility of regenerative medicine technologies. In 2007, we coined the phrase "regenerative pharmacology" to describe the enormous possibilities that could occur at the interface between pharmacology, regenerative medicine, and tissue engineering. The operational definition of regenerative pharmacology is "the application of pharmacological sciences to accelerate, optimize, and characterize (either in vitro or in vivo) the development, maturation, and function of bioengineered and regenerating tissues." As such, regenerative pharmacology seeks to cure disease through restoration of tissue/organ function. This strategy is distinct from standard pharmacotherapy, which is often limited to the amelioration of symptoms. Our goal here is to get pharmacologists more involved in this field of research by exposing them to the tools, opportunities, challenges, and interdisciplinary expertise that will be required to ensure awareness and galvanize involvement. To this end, we illustrate ways in which the pharmacological sciences can drive future innovations in regenerative medicine and tissue engineering and thus help to revolutionize the discovery of curative therapeutics. Hopefully, the broad foundational knowledge provided herein will spark sustained conversations among experts in diverse fields of scientific research to the benefit of all.