Cardiac tissue engineering using stem cells

Trends and Challenges in the Development of 3D-Printed Heart Valves and Other Cardiac Implants: A Review of Current Advances

This article provides a comprehensive review of the current trends and challenges in the development of 3D-printed heart valves and other cardiac implants. By providing personalized solutions and pushing the limits of regenerative medicine, 3D printing technology has revolutionized the field of cardiac healthcare. The use of several organic and synthetic polymers in 3D printing heart valves is explored in this article, with emphasis on both their benefits and drawbacks. In cardiac tissue engineering, stem cells are essential, and their potential to lessen immunological rejection and thrombogenic consequences is highlighted. In the clinical applications section, the article emphasizes the importance of 3D printing in preoperative planning. Surgery results are enhanced when surgeons can visualize and assess the size and placement of implants using patient-specific anatomical models. Customized implants that are designed to match the anatomy of a particular patient reduce the likelihood of complications and enhance postoperative results. The development of physiologically active cardiac implants, made possible by 3D bioprinting, shows promise by eliminating the need for artificial valves. In conclusion, this paper highlights cutting-edge research and the promise of 3D-printed cardiac implants to improve patient outcomes and revolutionize cardiac treatment.

Latest Advances in 3D Bioprinting of Cardiac Tissues

Cardiovascular diseases (CVDs) are known as the major cause of death worldwide. In spite of tremendous advancements in medical therapy, the gold standard for CVD treatment is still transplantation. Tissue engineering, on the other hand, has emerged as a pioneering field of study with promising results in tissue regeneration using cells, biological cues, and scaffolds. Three-dimensional (3D) bioprinting is a rapidly growing technique in tissue engineering because of its ability to create complex scaffold structures, encapsulate cells, and perform these tasks with precision. More recently, 3D bioprinting has made its debut in cardiac tissue engineering, and scientists are investigating this technique for development of new strategies for cardiac tissue regeneration. In this review, the fundamentals of cardiac tissue biology, available 3D bioprinting techniques and bioinks, and cells implemented for cardiac regeneration are briefly summarized and presented. Afterwards, the pioneering and state-of-the-art works that have utilized 3D bioprinting for cardiac tissue engineering are thoroughly reviewed. Finally, regulatory pathways and their contemporary limitations and challenges for clinical translation are discussed.

Investigating the Transient Regenerative Potential of Cardiac Muscle Using a Neonatal Pig Partial Apical Resection Model

Researchers have shown that adult zebrafish have the potential to regenerate 20% of the ventricular muscle within two months of apex resection, and neonatal mice have the capacity to regenerate their heart after apex resection up until day 7 after birth. The goal of this study was to determine if large mammals (porcine heart model) have the capability to fully regenerate a resected portion of the left ventricular apex during the neonatal stage, and if so, how long the regenerative potential persists. A total of 36 piglets were divided into the following groups: 0-day control and surgical groups and seven-day control and surgical groups. For the apex removal groups, each piglet was subjected to a partial wall thickness resection (~30% of the ventricular wall thickness). Heart muscle function was assessed via transthoracic echocardiograms; the seven-day surgery group experienced a decrease in ejection fraction and fractional shortening. Upon gross necropsy, for piglets euthanized four weeks post-surgery, all 0-day-old hearts showed no signs of scarring or any indication of the induced injury. Histological analysis confirmed that piglets in the 0-day surgery group exhibited various degrees of regeneration, with half of the piglets showing full regeneration and the other half showing partial regeneration. However, each piglet in the seven-day surgery group demonstrated epicardial fibrosis along with moderate to severe dissecting interstitial fibrosis, which was accompanied by an abundant collagenous extracellular matrix as the result of a scar formation in the resection site. Histology of one 0-day apex resection piglet (briefly lain on and accidentally killed by the mother sow three days post-surgery) revealed dense, proliferative mesenchymal cells bordering the fibrin and hemorrhage zone and differentiating toward immature cardiomyocytes. We further examined the heart explants at 5-days post-surgery (5D PO) and 1-week post-surgery (1W PO) to assess the repair progression. For the 0-day surgery piglets euthanized at 5D PO and 1W PO, half had abundant proliferating mesenchymal cells, suggesting active regeneration, while the other half showed increased extracellular collagen. The seven-day surgery piglets euthanized at 5D PO, and 1W PO showed evidence of greatly increased extracellular collagen, while some piglets had proliferating mesenchymal cells, suggesting a regenerative effort is ongoing while scar formation seems to predominate. In short, our qualitative findings suggest that the piglets lose the full myocardial regenerative potential by 7 days after birth, but greatly preserve the regenerative potential within 1 day post-partum.

3D Bioprinting of cardiac tissue and cardiac stem cell therapy

Cardiovascular tissue engineering endeavors to repair or regenerate damaged or ineffective blood vessels, heart valves, and cardiac muscle. Current strategies that aim to accomplish such a feat include the differentiation of multipotent or pluripotent stem cells on appropriately designed biomaterial scaffolds that promote the development of mature and functional cardiac tissue. The advent of additive manufacturing 3D bioprinting technology further advances the field by allowing heterogenous cell types, biomaterials, and signaling factors to be deposited in precisely organized geometries similar to those found in their native counterparts. Bioprinting techniques to fabricate cardiac tissue in vitro include extrusion, inkjet, laser-assisted, and stereolithography with bioinks that are either synthetic or naturally-derived. The article further discusses the current practices for postfabrication conditioning of 3D engineered constructs for effective tissue development and stability, then concludes with prospective points of interest for engineering cardiac tissues in vitro. Cardiovascular three-dimensional bioprinting has the potential to be translated into the clinical setting and can further serve to model and understand biological principles that are at the root of cardiovascular disease in the laboratory.

In Vivo Tracking and 1H/19F Magnetic Resonance Imaging of Biodegradable Polyhydroxyalkanoate/Polycaprolactone Blend Scaffolds Seeded with Labeled Cardiac Stem Cells

Medium-chain length polyhydroxyalkanoates (MCL-PHAs) have demonstrated exceptional properties for cardiac tissue engineering (CTE) applications. Despite prior work on MCL-PHA/polycaprolactone (PCL) blends, optimal scaffold production and use as an alternative delivery route for controlled release of seeded cardiac progenitor cells (CPCs) in CTE applications in vivo has been lacking. We present herein applicability of MCL-PHA/PCL (95/5 wt %) blends fabricated as thin films with an improved performance compared to the neat MCL-PHA. Polymer characterization confirmed the chemical structure and composition of the synthesized scaffolds, while thermal, wettability, and mechanical properties were also investigated and compared in neat and porous counterparts. In vitro cytocompatibility studies were performed using perfluorocrown-ether-nanoparticle-labeled murine CPCs and studied using confocal microscopy and ¹⁹F magnetic resonance spectroscopy and magnetic resonance imaging (MRI). Seeded scaffolds were implanted and studied in the postmortem murine heart in situ and in two additional C57BL/6 mice in vivo (using single-layered and double-layered scaffolds) and imaged immediately after and at 7 days postimplantation. Superior MCL-PHA/PCL scaffold performance has been demonstrated compared to MCL-PHA through experimental comparisons of (a) morphological data using scanning electron microscopy and (b) contact angle measurements attesting to improved CPC adhesion, (c) in vitro confocal microscopy showing increased SC proliferative capacity, and (d) mechanical testing that elicited good overall responses. In vitro MRI results justify the increased seeding density, increased in vitro MRI signal, and improved MRI visibility in vivo, in the double-layered compared to the single-layered scaffolds. Histological evaluations [bright-field, cytoplasmic (Atto647) and nuclear (4',6-diamidino-2-phenylindole) stains] performed in conjunction with confocal microscopy imaging attest to CPC binding within the scaffold, subsequent release and migration to the neighboring myocardium, and increased retention in the murine myocardium in the case of the double-layered scaffold. Thus, MCL-PHA/PCL blends possess tremendous potential for controlled delivery of CPCs and for maximizing possible regeneration in myocardial infarction.

Surface-modified polymers for cardiac tissue engineering

Cardiovascular disease (CVD), leading to myocardial infarction and heart failure, is one of the major causes of death worldwide. The physiological system cannot significantly regenerate the capabilities of a damaged heart. The current treatment involves pharmacological and surgical interventions; however, less invasive and more cost-effective approaches are sought. Such new approaches are developed to induce tissue regeneration following injury. Hence, regenerative medicine plays a key role in treating CVD. Recently, the extrinsic stimulation of cardiac regeneration has involved the use of potential polymers to stimulate stem cells toward the differentiation of cardiomyocytes as a new therapeutic intervention in cardiac tissue engineering (CTE). The therapeutic potentiality of natural or synthetic polymers and cell surface interactive factors/polymer surface modifications for cardiac repair has been demonstrated in vitro and in vivo. This review will discuss the recent advances in CTE using polymers and cell surface interactive factors that interact strongly with stem cells to trigger the molecular aspects of the differentiation or formulation of cardiomyocytes for the functional repair of heart injuries or cardiac defects.

Stem Cell Therapies in Cardiovascular Disease

Despite considerable advances in medicine, cardiovascular disease is still rising, with ischemic heart disease being the leading cause of death and disability worldwide. Thus extensive efforts are continuing to establish effective therapeutic modalities that would improve both quality of life and survival in this patient population. Novel therapies are being investigated not only to protect the myocardium against ischemia-reperfusion injury but also to regenerate the heart. Stem cell therapy, such as potential use of human mesenchymal stem cells and induced pluripotent stem cells and their exosomes, will make it possible not only to address molecular mechanisms of cardiac conditioning, but also to develop new therapies for ischemic heart disease. Despite the studies and progress made over the last 15 years on the use of stem cell therapy for cardiovascular disease, the efforts are still in their infancy. Even though the expectations have been high, the findings indicate that most of the clinical trials generally have been small and the results inconclusive. Because of many negative findings, there is certain pessimism that cardiac cell therapy is likely to yield any meaningful results over the next decade or so. Similar to other new technologies, early failures are not unusual and they may be followed by impressive success. Nevertheless, there has been considerable attention to safety by the clinical investigators because the adverse events of stem cell therapy have been impressively rare. In summary, although regenerative biology might not help the cardiovascular patient in the near term, it is destined to do so over the next several decades.

Nano-Enabled Approaches for Stem Cell-Based Cardiac Tissue Engineering

Cardiac diseases are the most prevalent causes of mortality in the world, putting a major economic burden on global healthcare system. Tissue engineering strategies aim at developing efficient therapeutic approaches to overcome the current challenges in prolonging patients survival upon cardiac diseases. The integration of advanced biomaterials and stem cells has offered enormous promises for regeneration of damaged myocardium. Natural or synthetic biomaterials have been extensively used to deliver cells or bioactive molecules to the site of injury in heart. Additionally, nano-enabled approaches (e.g., nanomaterials, nanofeatured surfaces) have been instrumental in developing suitable scaffolding biomaterials and regulating stem cells microenvironment to achieve functional therapeutic outcomes. This review article explores tissue engineering strategies, which have emphasized on the use of nano-enabled approaches in combination with stem cells for regeneration and repair of injured myocardium upon myocardial infarction (MI). Primarily a wide range of biomaterials, along with different types of stem cells, which have utilized in cardiac tissue engineering will be presented. Then integration of nanomaterials and surface nanotopographies with biomaterials and stem cells for myocardial regeneration will be presented. The advantages and challenges of these approaches will be reviewed and future perspective will be discussed.

Engineered Biomaterials to Enhance Stem Cell-Based Cardiac Tissue Engineering and Therapy

Cardiovascular disease is a leading cause of death worldwide. Since adult cardiac cells are limited in their proliferation, cardiac tissue with dead or damaged cardiac cells downstream of the occluded vessel does not regenerate after myocardial infarction. The cardiac tissue is then replaced with nonfunctional fibrotic scar tissue rather than new cardiac cells, which leaves the heart weak. The limited proliferation ability of host cardiac cells has motivated investigators to research the potential cardiac regenerative ability of stem cells. Considerable progress has been made in this endeavor. However, the optimum type of stem cells along with the most suitable matrix-material and cellular microenvironmental cues are yet to be identified or agreed upon. This review presents an overview of various types of biofunctional materials and biomaterial matrices, which in combination with stem cells, have shown promises for cardiac tissue replacement and reinforcement. Engineered biomaterials also have applications in cardiac tissue engineering, in which tissue constructs are developed in vitro by combining stem cells and biomaterial scaffolds for drug screening or eventual implantation. This review highlights the benefits of using biomaterials in conjunction with stem cells to repair damaged myocardium and give a brief description of the properties of these biomaterials that make them such valuable tools to the field.

A DIC Based Technique to Measure the Contraction of a Skeletal Muscle Engineered Tissue

Tissue engineering is a multidisciplinary science based on the application of engineering approaches to biologic tissue formation. Engineered tissue internal organization represents a key aspect to increase biofunctionality before transplant and, as regarding skeletal muscles, the potential of generating contractile forces is dependent on the internal fiber organization and is reflected by some macroscopic parameters, such as the spontaneous contraction. Here we propose the application of digital image correlation (DIC) as an independent tool for an accurate and noninvasive measurement of engineered muscle tissue spontaneous contraction. To validate the proposed technique we referred to the X-MET, a promising 3-dimensional model of skeletal muscle. The images acquired through a high speed camera were correlated with a custom-made algorithm and the longitudinal strain predictions were employed for measuring the spontaneous contraction. The spontaneous contraction reference values were obtained by studying the force response. The relative error between the spontaneous contraction frequencies computed in both ways was always lower than 0.15%. In conclusion, the use of a DIC based system allows for an accurate and noninvasive measurement of biological tissues' spontaneous contraction, in addition to the measurement of tissue strain field on any desired region of interest during electrical stimulation.

Autologous tissue patch rich in stem cells created in the subcutaneous tissue

AIM: To investigate whether we could create natural autologous tissue patches in the subcutaneous space for organ repair.

METHODS: We implanted the following three types of inert foreign bodies in the subcutaneous tissue of rats to produce autologous tissue patches of different geometries: (1) a large-sized polyvinyl tube (L = 25 mm, internal diameter = 7 mm) sealed at both ends by heat application for obtaining a large flat piece of tissue patch for organ repair; (2) a fine polyvinyl tubing (L = 25 mm, internal diameter = 3 mm) for creating cylindrically shaped grafts for vascular or nerve repair; and (3) a slurry of polydextran particle gel for inducing a bladder-like tissue. Implantation of inert materials was carried out by making a small incision on one or either side of the thoracic-lumbar region of rats. Subcutaneous pockets were created by blunt dissection around the incision into which the inert bodies were inserted (1 or 2 per rat). The incisions were closed with silk sutures, and the animals were allowed to recover. In case of the polydextran gel slurry 5 mL of the slurry was injected in the subcutaneous space using an 18 gauge needle. After implanting the foreign bodies a newly regenerated encapsulating tissue developed around the foreign bodies. The tissues were harvested after 4-42 d of implantation and studied by gross examination, histology, and histochemistry for organization, vascularity, and presence of mesenchymal stem cells (MSCs) (CD271+CD34+ cells).

RESULTS: Implanting a large cylindrically shaped polyvinyl tube resulted in a large flat sheet of tissue that could be tailored to a specific size and shape for use as a tissue patch for repairing large organs. Implanting a smaller sized polyvinyl tube yielded a cylindrical tissue that could be useful for repairing nerves and blood vessels. This type of patch could be obtained in different lengths by varying the length of the implanted tube. Implanting a suspension of inert polydextran suspension gave rise to a bladder-like tissue that could be potentially used for repairing heart valves. Histologically, the three different types of tissue patches generated were organized similarly, consisting of three layers, increasing in thickness until day 14. The inner layer in contact with the inert material was avascular; a middle layer that was highly vascular and filled with matrix, and an outer layer consisting of loose connective tissue. MSCs identified as CD271+CD34+ cells were present in the medial layer and around major blood vessels at day 4 but absent at later time points. The early-harvested tissues, endowed with MSCs, could be used for tissue repair, while the later-harvested tissues, being less vascular but thicker and tougher, could be used as filler tissue for cosmetic purposes.

CONCLUSION: An autologous, vascularized tissue patch of desired shape and size can be created in the subcutaneous space by implanting different types of inert bodies.

Application of novel anodized titanium for enhanced recruitment of H9C2 cardiac myoblast

OBJECTIVES

Anodized treated titanium surfaces, have been proposed as potential surfaces with better cell attachment capacities. We have investigated the adhesion and proliferation properties of H9C2 cardiac myoblasts on anodized treated titanium surface.

MATERIALS AND METHODS

Surface topography and anodized tubules were examined by high-resolution scanning electron microscopy (SEM). Control and test substrates were inserted to the bottom of 24-well tissue culture plates. Culture media including H9C2 cells were loaded on the surface of substrate and control wells at the second passage. Evaluation of cell growth, proliferation, viability and surface cytotoxicity was performed using MTT test. After 48 hr, some samples were inspected by SEM. DAPI-staining was used to count attached cells.

RESULTS

MTT results for cells cultured on anodized titanium and unanodized titanium surfaces was equal to 1.56 and 0.55 fold change compared to tissue culture polystyrene (TCPS). The surface had no cytotoxic effects on cells. The average cell attachment to TCPS, unanodized and anodized titanium surface was 2497±40.16, 1250±20.11 and 4859.5±54.173, respectively. Cell adhesion to anodized titanium was showed 1.95 and 3.89 fold increase compared to TCPS and unanodized titanium, respectively (P<0.05).

CONCLUSION

Anodized titanium surfaces can be potentially applied for enhanced recruitment of H9C2 cells. This unique property makes these inexpensive anodized surfaces as a candidate surface for attachment of cardiac cells and consequently for cardiac regeneration purposes.

Evaluation of an established pericardium patch for delivery of mesenchymal stem cells to cardiac tissue

The present study has evaluated a commercial pericardial material for its capacity to assist as a natural extracellular matrix (ECM) patch for the delivery and retention of mesenchymal stem cells for cardiac repair. The repair of cardiac tissue with cells delivered by an appropriate bioscaffold is expected to offer a superior, long-lasting treatment strategy. The present material, CardioCel®, is based on acellular pericardium that has been stabilized by treatments, including a low concentration of glutaraldehyde, that eliminate calcification after implantation. In the present study, we have assessed this material using human bone marrow mesenchymal stem cells at various cell densities under standard, static cell culture conditions. The initial seeding densities were monitored to evaluate the extent of cell attachment and cell viability, with subsequent cell proliferation assessed up to 4 weeks using an MTS assay. Cell morphology, infiltration, and spreading were tracked using scanning electron microscopy and phalloidin staining. The efficacy of long-term cell survival was further assessed by examining the extent and type of new tissue formation on seeded scaffolds at 70 days; both type I and type III collagens were present in fibrillar structures on these scaffolds indicating that the seeded stem cells had the capacity to differentiate into collagen-producing cells necessary to repair damaged ECM. These data show that the CardioCel® scaffold is an appropriate substrate for the stem cells and has the potential to both retain seeded stem cells and to act as a template for cell propagation and new tissue formation.

Cardiac differentiation of cardiosphere-derived cells in scaffolds mimicking morphology of the cardiac extracellular matrix

Stem cell therapy has the potential to regenerate heart tissue after myocardial infarction (MI). The regeneration is dependent upon cardiac differentiation of the delivered stem cells. We hypothesized that timing of the stem cell delivery determines the extent of cardiac differentiation as cell differentiation is dependent on matrix properties such as biomechanics, structure and morphology, and these properties in cardiac extracellular matrix (ECM) continuously vary with time after MI. In order to elucidate the relationship between ECM properties and cardiac differentiation, we created an in vitro model based on ECM-mimicking fibers and a type of cardiac progenitor cell, cardiosphere-derived cells (CDCs). A simultaneous fiber electrospinning and cell electrospraying technique was utilized to fabricate constructs. By blending a highly soft hydrogel with a relatively stiff polyurethane and modulating fabrication parameters, tissue constructs with similar cell adhesion property but different global modulus, single fiber modulus, fiber density and fiber alignment were achieved. The CDCs remained alive within the constructs during a 1week culture period. CDC cardiac differentiation was dependent on the scaffold modulus, fiber volume fraction and fiber alignment. Two constructs with relatively low scaffold modulus, ∼50-60kPa, most significantly directed the CDC differentiation into mature cardiomyocytes as evidenced by gene expressions of cardiac troponin T (cTnT), calcium channel (CACNA1c) and cardiac myosin heavy chain (MYH6), and protein expressions of cardiac troponin I (cTnI) and connexin 43 (CX43). Of these two low-modulus constructs, the extent of differentiation was greater for lower fiber alignment and higher fiber volume fraction. These results suggest that cardiac ECM properties may have an effect on cardiac differentiation of delivered stem cells.

Cardiac Tissue Vascularization: From Angiogenesis to Microfluidic Blood Vessels

Myocardial infarction results from a blockage of a major coronary artery that shuts the delivery of oxygen and nutrients to a region of the myocardium, leading to massive cardiomyocytes death and regression of microvasculature. Growth factor and cell delivery methods have been attempted to revascularize the ischemic myocardium and prevent further cell death. Implantable cardiac tissue patches were engineered to directly revascularize as well as remuscularize the affected muscle. However, inadequate vascularization in vitro and in vivo limits the efficacy of these new treatment options. Breakthroughs in cardiac tissue vascularization will profoundly impact ischemic heart therapies. In this review, we discuss the full spectrum of vascularization approaches ranging from biological angiogenesis to microfluidic blood vessels as related to cardiac tissue engineering.

Controlling the structural and functional anisotropy of engineered cardiac tissues

The ability to control the degree of structural and functional anisotropy in 3D engineered cardiac tissues would have high utility for both in vitro studies of cardiac muscle physiology and pathology as well as potential tissue engineering therapies for myocardial infarction. Here, we applied a high aspect ratio soft lithography technique to generate network-like tissue patches seeded with neonatal rat cardiomyocytes. Fabricating longer elliptical pores within the patch networks increased the overall cardiomyocyte and extracellular matrix alignment within the patch. Improved uniformity of cell and matrix alignment yielded an increase in anisotropy of action potential propagation and faster longitudinal conduction velocity (LCV). Cardiac tissue patches with a higher degree of cardiomyocyte alignment and electrical anisotropy also demonstrated greater isometric twitch forces. After two weeks of culture, specific measures of electrical and contractile function (LCV = 26.8 ± 0.8 cm s(-1), specific twitch force = 8.9 ± 1.1 mN mm(-2) for the longest pores studied) were comparable to those of neonatal rat myocardium. We have thus described methodology for engineering of highly functional 3D engineered cardiac tissues with controllable degree of anisotropy.

Electronic "expression" of the inward rectifier in cardiocytes derived from human-induced pluripotent stem cells

BACKGROUND

Human-induced pluripotent stem cell (h-iPSC)-derived cardiac myocytes are a unique model in which human myocyte function and dysfunction are studied, especially those from patients with genetic disorders. They are also considered a major advance for drug safety testing. However, these cells have considerable unexplored potential limitations when applied to quantitative action potential (AP) analysis. One major factor is spontaneous activity and resulting variability and potentially anomalous behavior of AP parameters.

OBJECTIVE

To demonstrate the effect of using an in silico interface on electronically expressed I(K1), a major component lacking in h-iPSC-derived cardiac myocytes.

METHODS

An in silico interface was developed to express synthetic I(K1) in cells under whole-cell voltage clamp.

RESULTS

Electronic I(K1) expression established a physiological resting potential, eliminated spontaneous activity, reduced spontaneous early and delayed afterdepolarizations, and decreased AP variability. The initiated APs had the classic rapid upstroke and spike and dome morphology consistent with data obtained with freshly isolated human myocytes as well as the readily recognizable repolarization attributes of ventricular and atrial cells. The application of 1 µM of BayK-8644 resulted in anomalous AP shortening in h-iPSC-derived cardiac myocytes. When I(K1) was electronically expressed, BayK-8644 prolonged the AP, which is consistent with the existing results on native cardiac myocytes.

CONCLUSIONS

The electronic expression of I(K1) is a simple and robust method to significantly improve the physiological behavior of the AP and electrical profile of h-iPSC-derived cardiac myocytes. Increased stability enables the use of this preparation for a controlled quantitative analysis of AP parameters, for example, drug responsiveness, genetic disorders, and dynamic behavior restitution profiles.

Functional cardiac tissue engineering

Heart attack remains the leading cause of death in both men and women worldwide. Stem cell-based therapies, including the use of engineered cardiac tissues, have the potential to treat the massive cell loss and pathological remodeling resulting from heart attack. Specifically, embryonic and induced pluripotent stem cells are a promising source for generation of therapeutically relevant numbers of functional cardiomyocytes and engineering of cardiac tissues in vitro. This review will describe methodologies for successful differentiation of pluripotent stem cells towards the cardiovascular cell lineages as they pertain to the field of cardiac tissue engineering. The emphasis will be placed on comparing the functional maturation in engineered cardiac tissues and developing heart and on methods to quantify cardiac electrical and mechanical function at different spatial scales.

Alginate scaffolds for mesenchymal stem cell cardiac therapy: influence of alginate composition

Despite the success of alginate scaffolds and mesenchymal stem cells (MSCs) therapy in cardiac failure treatment, the impact of the physicochemical environment provided by alginate matrices on cell behavior has never been investigated. The purpose of this work was double: to determine the alginate composition influence on (1) encapsulated rat MSC viability, paracrine activity, and phenotype in vitro and (2) cardiac implantability and in vivo biocompatibility of patch shape scaffolds. Two alginates, differing in composition and thus presenting different mechanical properties when hydrogels, were characterized. In both cases, encapsulated MSC viability was maintained at around 75%, and their secretion characteristics were retained 28 days postencapsulation. In vivo study revealed a high cardiac compatibility of the tested alginates: cardiac parameters were maintained, and rats did not present any sign of infection. Moreover, explanted hydrogels appeared surrounded by a vascularized tissue. However, scaffold implantability was highly dependent on alginate composition. G-type alginate patches, presenting higher elastic and Young moduli than M-type alginate patches, showed a better implantation easiness and were the only ones that maintained their shape and morphology in vivo. As a consequence of alginate chemical composition and resulting hydrogel structuration, G-type alginate hydrogels appear to be more adapted for cardiac implantation.

Biomimetic perfusion and electrical stimulation applied in concert improved the assembly of engineered cardiac tissue

Maintenance of normal myocardial function depends intimately on synchronous tissue contraction, driven by electrical activation and on adequate nutrient perfusion in support thereof. Bioreactors have been used to mimic aspects of these factors in vitro to engineer cardiac tissue but, due to design limitations, previous bioreactor systems have yet to simultaneously support nutrient perfusion, electrical stimulation and unconstrained (i.e. not isometric) tissue contraction. To the best of our knowledge, the bioreactor system described herein is the first to integrate these three key factors in concert. We present the design of our bioreactor and characterize its capability in integrated experimental and mathematical modelling studies. We then cultured cardiac cells obtained from neonatal rats in porous, channelled elastomer scaffolds with the simultaneous application of perfusion and electrical stimulation, with controls excluding either one or both of these two conditions. After 8 days of culture, constructs grown with simultaneous perfusion and electrical stimulation exhibited substantially improved functional properties, as evidenced by a significant increase in contraction amplitude (0.23 ± 0.10% vs 0.14 ± 0.05%, 0.13 ± 0.08% or 0.09 ± 0.02% in control constructs grown without stimulation, without perfusion, or either stimulation or perfusion, respectively). Consistently, these constructs had significantly improved DNA contents, cell distribution throughout the scaffold thickness, cardiac protein expression, cell morphology and overall tissue organization compared to control groups. Thus, the simultaneous application of medium perfusion and electrical conditioning enabled by the use of the novel bioreactor system may accelerate the generation of fully functional, clinically sized cardiac tissue constructs.

Post-ischaemic angiogenic therapy using in vivo prevascularized ascorbic acid-enriched myocardial artificial grafts improves heart function in a rat model

Angiogenesis plays a key role in post-ischaemic myocardial repair. We hypothesized that epicardial implantation of an ascorbic acid (AA)-enriched myocardial artificial graft (MAG), which has been prevascularized in the recipients' own body, promotes restoration of the ischaemic heart. Gelatin patches were seeded with GFP-luciferase-expressing rat cardiomyoblasts and enriched with 5 μm AA. Grafts were prevascularized in vivo for 3 days, using a renal pouch model in rats. The MAG patch was then implanted into the same rat's ischaemic heart following myocardial infarction (MI). MAG-treated animals (MAG group, n = 6) were compared to untreated infarcted animals as injury controls (MI group, n = 6) and sham-operated rats as healthy controls (healthy group, n = 7). In vivo bioluminescence imaging indicated a decrease in donor cell survival by 83% during the first week post-implantation. Echocardiographic and haemodynamic assessment 4 weeks after MI revealed that MAG treatment attenuated left ventricular (LV) remodelling (LV end-systolic volume, 0.31 ± 0.13 vs 0.81 ± 0.01 ml, p < 0.05; LV end-diastolic volume 0.79 ± 0.33 vs 1.83 ± 0.26 ml, p < 0.076) and preserved LV wall thickness (0.21 ± 0.03 vs 0.09 ± 0.005 cm, p < 0.05) compared to the MI group. Cardiac output was higher in MAG than MI (51.59 ± 6.5 vs 25.06 ± 4.24 ml/min, p < 0.01) and comparable to healthy rats (47.08 ± 1.9 ml/min). Histology showed decreased fibrosis, and a seven-fold increase in blood vessel density in the scar area of MAG compared to MI group (15.3 ± 1.1 vs 2.1 ± 0.3 blood vessels/hpf, p < 0.0001). Implantation of AA-enriched prevascularized grafts enhanced vascularity in ischaemic rat hearts, attenuated LV remodelling and preserved LV function.

Engineered approaches to the stem cell microenvironment for cardiac tissue regeneration

Micro- and nanoscale engineering approaches in medicine have the potential to recreate physiologically relevant stem cell microenvironments to enhance our understanding of stem cell behaviour and bring stem cell therapy closer to fruition. The realization of such advancements will impact a number of therapeutic applications, the most immediate of which may be the repair of heart tissue. Despite profound advances in creating physiologically relevant in vivo stem cell niches through the control of biochemical regulatory factors, further synergism of innovative techniques promise to elucidate the impact of a number of physical cues such as stem cell differentiation into cardiac cells, the electromechanical coupling among these cells, and the formation of bioengineered cardiac tissue grafts. This review examines the recent physiologically relevant micro- and nanoengineering efforts that have been made to address these factors. In Sections II and III, we introduce the traditional focuses of stem cell derived cardiac tissue: differentiation directed by transcription factors and structural cues within the stem cell niche. However, the majority of this review, Sections IV-VII, endeavours to highlight innovative and unconventional microscale engineering techniques that have employed topographic, biomaterial, microfluidic, mechanical, electrical, and optical stimulation for stem cell based cardiac tissue engineering.

Scaffold stiffness affects the contractile function of three-dimensional engineered cardiac constructs

We investigated the effects of the initial stiffness of a three-dimensional elastomer scaffold--highly porous poly(glycerol sebacate)--on functional assembly of cardiomyocytes cultured with perfusion for 8 days. The polymer elasticity varied with the extent of polymer cross-links, resulting in three different stiffness groups, with compressive modulus of 2.35 ± 0.03 (low), 5.28 ± 0.36 (medium), and 5.99 ± 0.40 (high) kPa. Laminin coating improved the efficiency of cell seeding (from 59 ± 15 to 90 ± 21%), resulting in markedly increased final cell density, construct contractility, and matrix deposition, likely because of enhanced cell interaction and spreading on scaffold surfaces. Compact tissue was formed in the low and medium stiffness groups, but not in the high stiffness group. In particular, the low stiffness group exhibited the greatest contraction amplitude in response to electric field pacing, and had the highest compressive modulus at the end of culture. A mathematical model was developed to establish a correlation between the contractile amplitude and the cell distribution within the scaffold. Taken together, our findings suggest that the contractile function of engineered cardiac constructs positively correlates with low compressive stiffness of the scaffold.

Cell therapy for heart failure: the need for a new therapeutic strategy

Improvements in the treatment of ischemic heart disease have led to a significant growth in the numbers of patients with systolic heart failure secondary to myocardial injury. Current therapies fail to address the loss of contractile tissue due to myocardial injury. Cell therapy is singular in its promise of primarily treating this underlying issue through salvage of viable myocardium or generation of new contractile tissue. Multiple cell types have been used to target acute myocardial infarction, chronic ischemic heart disease and heart failure due to infarction. Bone marrow mononuclear cells have been used to increase myocardial salvage after acute infarction. Randomized trials of over 800 patients have demonstrated no safety issues, and meta-analyses have suggested an improvement in left ventricular function in treated patients with trends toward improvements in hard cardiac end points. Cell therapy for chronic ischemic heart disease with bone marrow angiogenic progenitors has shown similar safety and trends toward improvement in function. While these therapies have targeted patients with viable myocardium, myoblasts have been used to treat patients with left ventricular dysfunction secondary to transmural infarction. Cell types with cardiomyogenic potential, including induced pluripotent stem cells and cardiac progenitor cells, offer the promise of true myocardial regeneration. Future studies with these cells may open the door for true myocardial regeneration.

Design and characterization of a modified T-flask bioreactor for continuous monitoring of engineered tissue stiffness

Controlling environmental conditions, such as mechanical stimuli, is critical for directing cells into functional tissue. This study reports on the development of a bioreactor capable of controlling the mechanical environment and continuously measuring force-displacement in engineered tissue. The bioreactor was built from off the shelf components, modified off the shelf components, and easily reproducible custom built parts to facilitate ease of setup, reproducibility and experimental flexibility. A T-flask was modified to allow for four tissue samples, mechanical actuation via a LabView controlled stepper motor and transduction of force from inside the T-flask to an external sensor. In vitro bench top testing with instrumentation springs and tissue culture experiments were performed to validate system performance. Force sensors were highly linear (R(2) > 0.998) and able to maintain force readings for extended periods of time. Tissue culture experiments involved cyclic loading of polyurethane scaffolds seeded with and without (control) human foreskin fibroblasts for 8 h/day for 14 days. After supplementation with TGF-beta, tissue constructs showed an increase in stiffness between consecutive days and from the acellular controls. These experiments confirmed the ability of the bioreactor to distinguish experimental groups and monitor tissue stiffness during tissue development.