A modular approach to cardiac tissue engineering

A Review on the Applications of Natural Biodegradable Nano Polymers in Cardiac Tissue Engineering

As cardiac diseases, which mostly result in heart failure, are increasing rapidly worldwide, heart transplantation seems the only solution for saving lives. However, this practice is not always possible due to several reasons, such as scarcity of donors, rejection of organs from recipient bodies, or costly medical procedures. In the framework of nanotechnology, nanomaterials greatly contribute to the development of these cardiovascular scaffolds as they provide an easy regeneration of the tissues. Currently, functional nanofibers can be used in the production of stem cells and in the regeneration of cells and tissues. The small size of nanomaterials, however, leads to changes in their chemical and physical characteristics that could alter their interaction and exposure to stem cells with cells and tissues. This article aims to review the naturally occurring biodegradable nanomaterials that are used in cardiovascular tissue engineering for the development of cardiac patches, vessels, and tissues. Moreover, this article also provides an overview of cell sources used for cardiac tissue engineering, explains the anatomy and physiology of the human heart, and explores the regeneration of cardiac cells and the nanofabrication approaches used in cardiac tissue engineering as well as scaffolds.

Biofabrication of Modular Spheroids as Tumor-Scale Microenvironments for Drug Screening

To streamline the drug discovery pipeline, there is a pressing need for preclinical models which replicate the complexity and scale of native tumors. While there have been advancements in the formation of microscale tumor units, these models are cell-line dependent, time-consuming and have not improved clinical trial success rates. In this study, two methods for generating 3D tumor microenvironments are compared, rapidly fabricated hydrogel microspheres and traditional cell-dense spheroids. These modules are then bioassembled into 3D printed thermoplastic scaffolds, using an automated biofabrication process, to form tumor-scale models. Modules are formed with SKOV3 and HFF cells as monocultures and cocultures, and the fabrication efficiency, cell architecture, and drug response profiles are characterized, both as single modules and as multimodular constructs. Cell-encapsulated Gel-MA microspheres are fabricated with high-reproducibility and dimensions necessary for automated tumor-scale bioassembly regardless of cell type, however, only cocultured spheroids form compact modules suitable for bioassembly. Chemosensitivity assays demonstrate the reduced potency of doxorubicin in coculture bioassembled constructs and a ≈five-fold increase in drug resistance of cocultured cells in 3D modules compared with 2D monolayers. This bioassembly system is efficient and tailorable so that a variety of relevant-sized tumor constructs could be developed to study tumorigenesis and modernize drug discovery.

Effect of Integrin Binding Peptide on Vascularization of Scaffold-Free Microtissue Spheroids

BACKGROUND

Three-dimensional (3D) biomimetic models via various approaches can be used by therapeutic applications of tissue engineering. Creating an optimal vascular microenvironment in 3D model that mimics the extracellular matrix (ECM) and providing an adequate blood supply for the survival of cell transplants are major challenge that need to be overcome in tissue regeneration. However, currently available scaffolds-depended approaches fail to mimic essential functions of natural ECM. Scaffold-free microtissues (SFMs) can successfully overcome some of the major challenges caused by scaffold biomaterials such as low cell viability and high cost.

METHODS

Herein, we investigated the effect of soluble integrin binding peptide of arginine-glycine-aspartic acid (RGD) on vascularization of SFM spheroids of human umbilical vein endothelial cells. In vitro-fabricated microtissue spheroids were constructed and cultivated in 0 mM, 1 mM, 2 mM, and 4 mM of RGD peptide. The dimensions and viability of SFMs were measured.

RESULTS

Maximum dimension and cell viability observed in 2 mM RGD containing SFM. Vascular gene expression of 2 mM RGD containing SFM were higher than other groups, while 4 mM RGD containing SFM expressed minimum vascularization related genes. Immunofluorescent staining results indicating that platelet/endothelial cell adhesion molecule and vascular endothelial growth factor protein expression of 2 mM RGD containing SFM was higher compared to other groups.

CONCLUSION

Collectively, these findings demonstrate that SFM spheroids can be successfully vascularized in determined concentration of RGD peptide containing media. Also, soluble RGD incorporated SFMs can be used as an optimal environment for successful prevascularization strategies.

Nano-porous anodic alumina: fundamentals and applications in tissue engineering

Recently, nanomaterials have been widely utilized in tissue engineering applications due to their unique properties such as the high surface to volume ratio and diversity of morphology and structure. However, most methods used for the fabrication of nanomaterials are rather complicated and costly. Among different nanomaterials, anodic aluminum oxide (AAO) is a great example of nanoporous structures that can easily be engineered by changing the electrolyte type, anodizing potential, current density, temperature, acid concentration and anodizing time. Nanoporous anodic alumina has often been used for mammalian cell culture, biofunctionalization, drug delivery, and biosensing by coating its surface with biocompatible materials. Despite its wide application in tissue engineering, thorough in vivo and in vitro studies of AAO are still required to enhance its biocompatibility and thereby pave the way for its application in tissue replacements. Recognizing this gap, this review article aims to highlight the biomedical potentials of AAO for applications in tissue replacements along with the mechanism of porous structure formation and pore characteristics in terms of fabrication parameters.

Three-dimensional microengineered models of human cardiac diseases

In vitro three-dimensional (3D) microengineered tissue models have been the recent focus of pathophysiological studies, particularly in the field of cardiovascular research. These models, as classified by 3D biomimetic tissues within micrometer-scale platforms, enable precise environmental control on the molecular- and cellular-levels to elucidate biological mechanisms of disease progression and enhance efficacy of therapeutic research. Microengineered models also incorporate directed stem cell differentiation and genome modification techniques that warrant derivation of patient-specific and genetically-edited human cardiac cells for precise recapitulation of diseased tissues. Additionally, integration of added functionalities and/or structures into these models serves to enhance the capability to further extract disease-specific phenotypic, genotypic, and electrophysiological information. This review highlights the recent progress in the development of in vitro 3D microengineered models for study of cardiac-related diseases (denoted as CDs). We will primarily provide a brief overview on currently available 2D assays and animal models for studying of CDs. We will further expand our discussion towards currently available 3D microengineered cardiac tissue models and their implementation for study of specific disease conditions.

Engineered cardiac micromodules for the in vitro fabrication of 3D endogenous macro-tissues

The in vitro fabrication of an endogenous cardiac muscle would have a high impact for both in vitro studies concerning cardiac tissue physiology and pathology, as well as in vivo application to potentially repair infarcted myocardium. To reach this aim, we engineered a new class of cardiac tissue precursor (CTP), specifically conceived in order to promote the synthesis and the assembly of a cardiac extracellular matrix (ECM). The CTPs were obtained by culturing a mixed cardiac cell population, composed of myocyte and non-myocyte cells, into porous gelatin microspheres in a dynamic bioreactor. By engineering the culture conditions, the CTP developed both beating properties and an endogenous immature cardiac ECM. By following a bottom-up approach, a macrotissue was fabricated by molding and packing the engineered tissue precursor in a maturation chamber. During the macrotissue formation, the tissue precursors acted as cardiac tissue depots by promoting the formation of an endogenous and interconnected cardiac network embedding the cells and the microbeads. The myocytes cell fraction pulled on ECM network and induced its compaction against the internal posts represented by the initial porous microbeads. This reciprocal interplay induced ECM consolidation without the use of external biophysical stimuli by leading to the formation of a beating and endogenous macrotissue. We have thus engineered a new class of cardiac micromodules and show its potential for the fabrication of endogenous cardiac tissue models useful for in vitro studies that involve the cardiac tissue remodeling.

A novel high-speed production process to create modular components for the bottom-up assembly of large-scale tissue-engineered constructs

To replace damaged or diseased tissues, large tissue-engineered constructs can be prepared by assembling modular components in a bottom-up approach. However, a high-speed method is needed to produce sufficient numbers of these modules for full-sized tissue substitutes. To this end, a novel production technique is devised, combining air shearing and a plug flow reactor-style design to rapidly produce large quantities of hydrogel-based (here type I collagen) cylindrical modular components with tunable diameters and length. Using this technique, modules containing NIH 3T3 cells show greater than 95% viability while endothelial cell surface attachment and confluent monolayer formation are demonstrated. Additionally, the rapidly produced modules are used to assemble large tissue constructs (>1 cm(3) ) in vitro. Module building blocks containing luciferase-expressing L929 cells are packed in full size adult rat-liver-shaped bioreactors and perfused with cell medium, to demonstrate the capacity to build organ-shaped constructs; bioluminescence demonstrates sustained viability over 3 d. Cardiomyocyte-embedded modules are also used to assemble electrically stimulatable contractile tissue.

Tuning graft- and host-derived vascularization in modular tissue constructs: a potential role of HIF1 activation

A better understanding of the factors governing the vascularization of engineered tissues is crucial for their advancement as therapeutic platforms. Here, we studied the effect of implant volume and cell densities on the in vivo vascularization of modular engineered tissue constructs. Sub-millimeter collagen modules containing adipose-derived mesenchymal stromal cells (adMSC) and enveloped by human umbilical vein endothelial cells (HUVEC) were subcutaneously implanted in severe-combined immunodeficient mice with a beige-mutation (SCID-bg) mice. Implant volume and cell density was varied relative to a base case, defined as a 0.01 mL implant containing 1.5×10(7) adMSC/mL and 3.9×10(6) HUVEC/mL. At 7 and 14 days post-transplantation, the constructs were harvested for immunohistochemical analysis of total (CD31(+)) and graft-derived (UEA1(+)) vessel formation, hypoxia-inducible factor 1-alpha (HIF1α) expression, infiltration of host-derived leukocytes (CD45), and macrophages (F4/80). Implant volume and cell density affected the relative contributions of host- versus graft-derived vascularization, highlighting that different mechanisms underlie the two processes. Graft-derived vessel formation was most rapid and robust in implants with high HIF1α expression, namely large volume implants and implants with high adMSC and HUVEC density (p<0.01 compared to base case at day 7). Many HIF1α(+) cells were vessel-lining HUVEC, suggesting that HIF1 activation may be key to vessel assembly in the graft. Host vessel ingrowth, however, dominated the vascularization of small volume implants (of high and low adMSC density alike), which showed low HIF1α expression at day 7. Host vessels were sustained to day 14 when adMSC density alone was increased, presumably due to increased paracrine secretions. This study points to a potential role of HIF1 activation in the vascularization of tissue constructs, which may be harnessed to engineer robust vessels for therapeutic applications.

Biomechanical regulation of in vitro cardiogenesis for tissue-engineered heart repair

The heart is a continuously pumping organ with an average lifespan of eight decades. It develops from the onset of embryonic cardiogenesis under biomechanical load, performs optimally within a defined range of hemodynamic load, and fails if acutely or chronically overloaded. Unloading of the heart leads to defective cardiogenesis in utero, but can also lead to a desired therapeutic outcome (for example, in patients with heart failure under left ventricular assist device therapy). In light of the well-documented relevance of mechanical loading for cardiac physiology and pathology, it is plausible that tissue engineers have integrated mechanical stimulation regimens into protocols for heart muscle construction. To achieve optimal results, physiological principles of beat-to-beat myocardial loading and unloading should be simulated. In addition, heart muscle engineering, in particular if based on pluripotent stem cell-derived cardiomyocytes, may benefit from staggered tonic loading protocols to simulate viscoelastic properties of the prenatal and postnatal myocardial stroma. This review will provide an overview of heart muscle mechanics, summarize observations on the role of mechanical loading for heart development and postnatal performance, and discuss how physiological loading regimens can be exploited to advance myocardial tissue engineering towards a therapeutic application.

Strategies for directing the structure and function of three-dimensional collagen biomaterials across length scales

Collagen type I is a widely used natural biomaterial that has found utility in a variety of biological and medical applications. Its well-characterized structure and role as an extracellular matrix protein make it a highly relevant material for controlling cell function and mimicking tissue properties. Collagen type I is abundant in a number of tissues, and can be isolated as a purified protein. This review focuses on hydrogel biomaterials made by reconstituting collagen type I from a solubilized form, with an emphasis on in vitro studies in which collagen structure can be controlled. The hierarchical structure of collagen from the nanoscale to the macroscale is described, with an emphasis on how structure is related to function across scales. Methods of reconstituting collagen into hydrogel materials are presented, including molding of macroscopic constructs, creation of microscale modules and electrospinning of nanoscale fibers. The modification of collagen biomaterials to achieve the desired structures and functions is also addressed, with particular emphasis on mechanical control of collagen structure, creation of collagen composite materials and crosslinking of collagenous matrices. Biomaterials scientists have made remarkable progress in rationally designing collagen-based biomaterials and in applying them both to the study of biology and for therapeutic benefit. This broad review illustrates recent examples of techniques used to control collagen structure and thereby to direct its biological and mechanical functions.

Cell therapy, 3D culture systems and tissue engineering for cardiac regeneration

Ischemic Heart Disease (IHD) still represents the "Number One Killer" worldwide accounting for the death of numerous patients. However the capacity for self-regeneration of the adult heart is very limited and the loss of cardiomyocytes in the infarcted heart leads to continuous adverse cardiac-remodeling which often leads to heart-failure (HF). The concept of regenerative medicine comprising cell-based therapies, bio-engineering technologies and hybrid solutions has been proposed as a promising next-generation approach to address IHD and HF. Numerous strategies are under investigation evaluating the potential of regenerative medicine on the failing myocardium including classical cell-therapy concepts, three-dimensional culture techniques and tissue-engineering approaches. While most of these regenerative strategies have shown great potential in experimental studies, the translation into a clinical setting has either been limited or too rapid leaving many key questions unanswered. This review summarizes the current state-of-the-art, important challenges and future research directions as to regenerative approaches addressing IHD and resulting HF.

Del-1 overexpression in endothelial cells increases vascular density in tissue-engineered implants containing endothelial cells and adipose-derived mesenchymal stromal cells

We used a combination of strategies to stimulate the vascularization of tissue-engineered constructs in vivo including a modular approach to build larger tissues from individual building blocks ("modules") mixed together. Each building block included vascular cells by design; modules were submillimeter-sized collagen gels with an outer layer of endothelial cells (ECs), and with embedded adipose-derived mesenchymal stromal cells (adMSCs) to support EC survival and blood vessel maturation in vivo. We transduced the ECs that coat the modules with a lentiviral construct to overexpress the angiogenic extracellular matrix (ECM) protein Developmental endothelial locus-1 (Del-1). Upon injection of modules in a subcutaneous SCID/Bg mouse model, there was an increase in the number of blood vessels for implants with ECs transduced to overexpress Del-1 compared with control implants (with enhanced green fluorescent protein [eGFP]-transduced ECs) over the 21-day duration of the study. The greatest difference between Del-1 and eGFP implants and the highest number of blood vessels were observed 7 days after transplantation. The day-7 Del-1 implants also had increased SMA+ staining compared with control, suggesting increased blood vessel maturation through recruitment of SMA+ smooth muscle cells or pericytes to stabilize the newly formed blood vessels. Perfusion studies (microcomputed tomography, ultrasound imaging, and systemic injection of fluorescent UEA-1 or dextran) showed that some of the newly formed blood vessels (both donor derived and host derived, in both Del-1 and eGFP implants) were perfused and connected to the host vasculature as early as 7 days after transplantation, and at later time points as well. Nevertheless, perfusion of the implants was limited in some cases, suggesting that further improvements are necessary to normalize the vasculature at the implant site.

ADVANCED TISSUE ENGINEERING STRATEGIES FOR VASCULARIZED PARENCHYMAL CONSTRUCTS

The fabrication of vascularized parenchymal organs to alleviate donor shortage in organ transplantation is the holy grail of tissue engineering. However, conventional tissue-engineering strategies have encountered huge challenges in recapitulating complex structural organization of native organs (e.g., orderly arrangement of multiple cell types and vascular network), which plays an important role in engineering functional vascularized parenchymal constructs in vitro. Recent developments of various advanced tissue-engineering strategies have exhibited great promise in replicating organ-specific architectures into artificial constructs. Here, we review the recent advances in top-down and bottom-up strategies for the fabrication of vascularized parenchymal constructs. We highlight the fabrication of microfluidic scaffolds potential for nutrient transport or vascularization as well as the controlled multicellular arrangement. The advantages as well as the limitations associated with these strategies will be discussed. It is envisioned that the combination of microfluidic concept in top-down strategies and multicellular arrangement concept in bottom-up strategies could potentially generate new insights for the fabrication of vascularized parenchymal organs.

A glycosaminoglycan based, modular tissue scaffold system for rapid assembly of perfusable, high cell density, engineered tissues

The limited ability to vascularize and perfuse thick, cell-laden tissue constructs has hindered efforts to engineer complex tissues and organs, including liver, heart and kidney. The emerging field of modular tissue engineering aims to address this limitation by fabricating constructs from the bottom up, with the objective of recreating native tissue architecture and promoting extensive vascularization. In this paper, we report the elements of a simple yet efficient method for fabricating vascularized tissue constructs by fusing biodegradable microcapsules with tunable interior environments. Parenchymal cells of various types, (i.e. trophoblasts, vascular smooth muscle cells, hepatocytes) were suspended in glycosaminoglycan (GAG) solutions (4%/1.5% chondroitin sulfate/carboxymethyl cellulose, or 1.5 wt% hyaluronan) and encapsulated by forming chitosan-GAG polyelectrolyte complex membranes around droplets of the cell suspension. The interior capsule environment could be further tuned by blending collagen with or suspending microcarriers in the GAG solution These capsule modules were seeded externally with vascular endothelial cells (VEC), and subsequently fused into tissue constructs possessing VEC-lined, inter-capsule channels. The microcapsules supported high density growth achieving clinically significant cell densities. Fusion of the endothelialized, capsules generated three dimensional constructs with an embedded network of interconnected channels that enabled long-term perfusion culture of the construct. A prototype, engineered liver tissue, formed by fusion of hepatocyte-containing capsules exhibited urea synthesis rates and albumin synthesis rates comparable to standard collagen sandwich hepatocyte cultures. The capsule based, modular approach described here has the potential to allow rapid assembly of tissue constructs with clinically significant cell densities, uniform cell distribution, and endothelialized, perfusable channels.

Hydrogel based injectable scaffolds for cardiac tissue regeneration

Tissue engineering promises to be an effective strategy that can overcome the lacuna existing in the current pharmacological and interventional therapies and heart transplantation. Heart failure continues to be a major contributor to the morbidity and mortality across the globe. This may be attributed to the limited regeneration capacity after the adult cardiomyocytes are terminally differentiated or injured. Various strategies involving acellular scaffolds, stem cells, and combinations of stem cells, scaffolds and growth factors have been investigated for effective cardiac tissue regeneration. Recently, injectable hydrogels have emerged as a potential candidate among various categories of biomaterials for cardiac tissue regeneration due to improved patient compliance and facile administration via minimal invasive mode that treats complex infarction. This review discusses in detail on the advances made in the field of injectable materials for cardiac tissue engineering highlighting their merits over their preformed counterparts.

3D in vitro tissue models and their potential for drug screening

INTRODUCTION

The development of one standard, simplified in vitro three-dimensional tissue model suitable to biological and pathological investigation and drug-discovery may not yet be feasible, but standardized models for individual tissues or organs are a possibility. Tissue bioengineering, while concerned with finding methods of restoring functionality in disease, is developing technology that can be miniaturized for high throughput screening (HTS) of putative drugs. Through collaboration between biologists, physicists and engineers, cell-based assays are expanding into the realm of tissue analysis. Accordingly, three-dimensional (3D) micro-organoid systems will play an increasing role in drug testing and therapeutics over the next decade. Nevertheless, important hurdles remain before these models are fully developed for HTS.

AREAS COVERED

We highlight advances in the field of tissue bioengineering aimed at enhancing the success of drug candidates through pre-clinical optimization. We discuss models that are most amenable to high throughput screening with emphasis on detection platforms and data modeling.

EXPERT OPINION

Modeling 3D tissues to mimic in-vivo architecture remains a major challenge. As technology advances to provide novel methods of HTS analysis, so do potential pitfalls associated with such models and methods. We remain hopeful that integration of biofabrication with HTS will significantly reduce attrition rates in drug development.

Modeling myocardial growth and hypertrophy in engineered heart muscle

The introduction of biomimetic culture paradigms has advanced myocardial tissue engineering fundamentally, enabling today the provision of engineered rodent and human heart muscle with features characteristically found in postnatal myocardium. This is in marked contrasts to "flat" cardiomyocyte cultures with their typically low degree of organotypic maturation. Here, we discuss the collagen hydrogel-based engineered heart muscle (EHM) technology and provide background information on its use in simulations of myocardial growth and disease.

Materials science and tissue engineering: repairing the heart

Heart failure after a myocardial infarction continues to be a leading killer in the Western world. Currently, there are no therapies that effectively prevent or reverse the cardiac damage and negative left ventricular remodeling process that follows a myocardial infarction. Because the heart has limited regenerative capacity, there has been considerable effort to develop new therapies that could repair and regenerate the myocardium. Although cell transplantation alone was initially studied, more recently, tissue engineering strategies using biomaterial scaffolds have been explored. In this review, we cover the different approaches to engineering the myocardium, including cardiac patches, which are in vitro-engineered constructs of functional myocardium, and injectable scaffolds, which can either encourage endogenous repair and regeneration or act as vehicles to support the delivery of cells and other therapeutics.

Delivery of mesenchymal stem cells in chitosan/collagen microbeads for orthopedic tissue repair

Microencapsulation and delivery of stem cells in biomaterials is a promising approach to repairing damaged tissue in a minimally invasive manner. An appropriate biomaterial niche can protect the embedded cells from the challenging environment in the host tissue, while also directing stem cell differentiation toward the desired lineage. In this study, adult human mesenchymal stem cells (MSC) were embedded in hydrogel microbeads consisting of chitosan and type I collagen using an emulsification process. Glyoxal and β-glycerophosphate were used as chemical and physical crosslinkers to initiate copolymerization of the matrix materials. The average size and size distribution of the microbeads could be varied by controlling the emulsification conditions. Spheroidal microbeads ranging in diameter from 82 ± 19 to 290 ± 78 µm were produced. Viability staining showed that MSC survived the encapsulation process (>90% viability) and spread inside the matrix over a period of 9 days in culture. Induced osteogenic differentiation using medium supplements showed that MSC increased gene expression of osterix and osteocalcin over time in culture, and also deposited calcium mineral. Bone sialoprotein and type I collagen gene expression were not affected. Delivery of microbeads through standard needles at practically relevant flow rates did not adversely affect cell viability, and microbeads could also be easily molded into prescribed geometries for delivery. Such protein-based microbeads may have utility in orthopedic tissue regeneration by allowing minimally invasive delivery of progenitor cells in microenvironments that are both protective and instructive.

Fate of modular cardiac tissue constructs in a syngeneic rat model

Modular cardiac tissues developed both vascular and cardiac structures in vivo, provided that the host response was attenuated by omitting xenoproteins from the modules. Collagen gel modules (with Matrigel(TM)) containing cardiomyocytes (CMs) alone or CMs with surface-seeded endothelial cells (ECs; CM/EC modules) were injected into the peri-infarct zone of the heart in syngeneic Lewis rats. After 3 weeks, donor ECs developed into blood vessel-like structures that also contained erythrocytes. However, no donor CMs were found within the implant sites, presumably because host cells including macrophages and T cells infiltrated extensively into the injection sites. To lessen the host response, Matrigel was omitted from the matrix and the modules were rinsed with serum-free medium prior to implantation. Host cell infiltration was attenuated, resulting in a higher degree of vascularization with CM/EC modules than with CM modules without ECs. Most importantly, donor CMs matured into striated muscle-like structures in Matrigel-free implants.

The expanding world of tissue engineering: the building blocks and new applications of tissue engineered constructs

The field of tissue engineering has been growing in the recent years as more products have made it to the market and as new uses for the engineered tissues have emerged, motivating many researchers to engage in this multidisciplinary field of research. Engineered tissues are now not only considered as end products for regenerative medicine, but also have emerged as enabling technologies for other fields of research ranging from drug discovery to biorobotics. This widespread use necessitates a variety of methodologies for production of tissue engineered constructs. In this review, these methods together with their non-clinical applications will be described. First, we will focus on novel materials used in tissue engineering scaffolds; such as recombinant proteins and synthetic, self assembling polypeptides. The recent advances in the modular tissue engineering area will be discussed. Then scaffold-free production methods, based on either cell sheets or cell aggregates will be described. Cell sources used in tissue engineering and new methods that provide improved control over cell behavior such as pathway engineering and biomimetic microenvironments for directing cell differentiation will be discussed. Finally, we will summarize the emerging uses of engineered constructs such as model tissues for drug discovery, cancer research and biorobotics applications.

Directing the assembly of spatially organized multicomponent tissues from the bottom up

The complexity of the human body derives from numerous modular building blocks assembled hierarchically across multiple length scales. These building blocks, spanning sizes ranging from single cells to organs, interact to regulate development and normal organismal function but become disorganized during disease. Here, we review methods for the bottom-up and directed assembly of modular, multicellular, and tissue-like constructs in vitro. These engineered tissues will help refine our understanding of the relationship between form and function in the human body, provide new models for the breakdown in tissue architecture that accompanies disease, and serve as building blocks for the field of regenerative medicine.

Validation of a high-throughput microtissue fabrication process for 3D assembly of tissue engineered cartilage constructs

Described here is a simple, high-throughput process to fabricate pellets with regular size and shape and the assembly of pre-cultured pellets in a controlled manner into specifically designed 3D plotted porous scaffolds. Culture of cartilage pellets is a well-established process for inducing re-differentiation in expanded chondrocytes. Commonly adopted pellet culture methods using conical tubes are inconvenient, time-consuming and space-intensive. We compared the conventional 15-mL tube pellet culture method with 96-well plate-based methods, examining two different well geometries (round- and v-bottom plates). The high-throughput production method was then used to demonstrate guided placement of pellets within a scaffold of defined pore size and geometry for the 3D assembly of tissue engineered cartilage constructs. While minor differences were observed in tissue quality and size, the chondrogenic re-differentiation capacity of human chondrocytes, as assessed by GAG/DNA, collagen type I and II immunohistochemistry and collagen type I, II and aggrecan mRNA expression, was maintained in the 96-well plate format and pellets of regular size and spheroidal shape were produced. This allowed for simple production of large numbers of reproducible tissue spheroids. Furthermore, the pellet-assembly method successfully allowed fluorescently labelled pellets to be individually visualised in 3D. During subsequent culture of 3D assembled tissue engineered constructs in vitro, pellets fused to form a coherent tissue, promoting chondrogenic differentiation and GAG accumulation.

The reconstruction of lung alveolus-like structure in collagen-matrigel/microcapsules scaffolds in vitro

This study attempted to use collagen–Matrigel as extracellular matrix (ECM) to supply cells with three-dimensional (3D) culture condition and employ alginate-poly-l-lysine-alginate (APA) microcapsules to control the formation of alveolus-like structure in vitro. We tested mice foetal pulmonary cells (FPCs) by immunohistochemistry after 2D culture. The alveolus-like structure was reconstructed by seeding FPCs in collagen–Matrigel mixed with APA microcapsules 1.5 ml. A self-made mould was used to keep the structure from contraction. Meanwhile, it provided static stretch to the structure. After 7, 14 and 21 days of culture, the alveolus-like structure was analysed histologically and immunohistochemically, or by scanning transmission electron microscopy (TEM). We also observed these structures under inverted phase contrast microscope. The expression of pro-surfactant protein C (SpC) was detected by reverse transcription-polymerase chain reaction (RT-PCR). We obtained fibroblasts, epithelial cells and alveolar type II (AE2) cells in FPCs. In the reconstructed structure, seeding cells surrounding the APA microcapsules constructed alveolus-like structures, the size of them ranges from 200 to 300 μm. In each reconstructed lung tissue sheet, microcapsules had integrity. Pan-cytokeratin, vimentin and SpC positive cells were observed in 7- and 14-day cultured structures. TEM showed lamellar bodies of AE2 cells in the reconstructed tissues whereas RT-PCR expressed SpC gene. Primary mice FPCs could form alveolus-like structures in collagen–Matrigel/APA microcapsules engineered scaffolds, which could maintain a differentiated state of AE2 cells.

Long-term culture of HL-1 cardiomyocytes in modular poly(ethylene glycol) microsphere-based scaffolds crosslinked in the phase-separated state

Poly(ethylene glycol) (PEG) microspheres were assembled around HL-1 cardiomyocytes to produce highly porous modular scaffolds. In this study we took advantage of the immiscibility of PEG and dextran to improve upon our previously described modular scaffold fabrication methods. Phase separating the PEG microspheres in dextran solutions caused them to rapidly deswell and crosslink together, eliminating the need for serum protein-based crosslinking. This also led to a dramatic increase in the stiffness of the scaffolds and greatly improved the handling characteristics. HL-1 cardiomyocytes were present during microsphere crosslinking in the cytocompatible dextran solution, exhibiting high cell viability following scaffold formation. Over the course of 2 weeks a 9-fold expansion in cell number was observed. The cardiac functional markers sarcomeric α-actinin and connexin 43 were expressed at 13 and 24 days after scaffold formation. HL-1 cells were spontaneously depolarizing 38 days after scaffold formation, which was visualized by confocal microscopy using a calcium-sensitive dye. Electrical stimulation resulted in synchronization of activation peaks throughout the scaffolds. These findings demonstrate that PEG microsphere scaffolds fabricated in the presence of dextran can support the long-term three-dimensional culture of cells, suggesting applications in cardiovascular tissue engineering.

Formation of an aggregated alginate construct in a tubular perfusion system

Tissue engineering strategies are often limited by in vitro culture techniques of three dimensional scaffolds. Here we develop a method to form an aggregated cell-containing construct in vitro in a bioreactor system. Human mesenchymal stem cells (hMSCs) are cultured in individual alginate beads in a tubular perfusion system (TPS) bioreactor and then aggregated to form a single large construct. Mechanical evaluation of this construct demonstrated that aggregated alginate constructs (AACs) made from beads with 2.15 mm diameters had a Young's modulus of 85.6±15.8 kPa, a tensile strength of 3.24±0.55 kPa and a yield strength of 1.44±0.27 kPa. These mechanical properties were shown to be dependent on the bead size used to fabricate the AACs with smaller bead sizes resulting in stronger constructs. Analysis of metabolic activity revealed that hMSCs encapsulated in alginate exposed to AAC treatment sustained metabolic activity while live dead staining indicated cells remain viable. These results demonstrate the formation of AACs in the TPS bioreactor as an elegant method to create tissue engineering constructs in vitro.

Endothelialized biomaterials for tissue engineering applications in vivo

Rebuilding tissues involves the creation of a vasculature to supply nutrients and this in turn means that the endothelial cells (ECs) of the resulting endothelium must be a quiescent non-thrombogenic blood contacting surface. Such ECs are deployed on biomaterials that are composed of natural materials such as extracellular matrix proteins or synthetic polymers in the form of vascular grafts or tissue-engineered constructs. Because EC function is influenced by their origin, biomaterial surface chemistry and hemodynamics, these issues must be considered to optimize implant performance. In this review, we examine the recent in vivo use of endothelialized biomaterials and discuss the fundamental issues that must be considered when engineering functional vasculature.

Application of an endothelialized modular construct for islet transplantation in syngeneic and allogeneic immunosuppressed rat models

Modular tissue engineering is a novel approach to assemble tissues with an inherent vascularization. In this article, we evaluated whether endothelialized module-driven vascularization enhances islet engraftment in diabetic rats. Two thousand islets were transplanted in the omental pouch of syngeneic and allogeneic immunosuppressed diabetic recipients as free islets, islets in collagen modules, or islets in endothelialized modules. Transplantation of islets in endothelialized modules significantly increased the vessel density compared with controls. Donor green fluorescent protein-positive endothelial cells (ECs) formed vessels in proximity to transplanted islets; donor vessels connected to host vasculature as the vessels included erythrocytes in their lumens and were supported by host smooth muscle cells by 21 days. Transplantation of 2000 islets reversed diabetes in two of five of syngeneic recipients until 60 days, although there was no apparent benefit to islet function of adding ECs relative to collagen modules without EC. However, there was a trend toward increased viability when islets were implanted in endothelialized modules compared with collagen modules at 21 days. Meanwhile, 2000 islets in allogeneic immunosuppressed recipients lowered blood glucose levels short term, but there was graft failure within 1 week. This study explored the simultaneous transplantation of primary ECs with islets in diabetic recipients. The endothelialized modular approach increased vessel density around transplanted islets. Further modulation (i.e., acceleration) of vessel maturation, is presumed necessary to improve islet engraftment.

Vascularization is the key challenge in tissue engineering

The main limitation in engineering in vitro tissues is the lack of a sufficient blood vessel system - the vascularization. In vivo almost all tissues are supplied by these endothelial cell coated tubular networks. Current strategies to create vascularized tissues are discussed in this review. The first strategy is based on the endothelial cells and their ability to form new vessels known as neoangiogenesis. Herein prevascularization techniques are compared to approaches in which biomolecules, such as growth factors, cytokines, peptides and proteins as well as cells are applied to generate new vessels. The second strategy is focused on scaffold-based techniques. Naturally-derived scaffolds, which contain vessels, are distinguished from synthetically manufactured matrices. Advantages and pitfalls of the approaches to create vascularized tissues in vitro are outlined and feasible future strategies are discussed.

Bottom-up tissue engineering

Recapitulating the elegant structures formed during development is an extreme synthetic and biological challenge. Great progress has been made in developing materials to support transplanted cells, yet the complexity of tissues is far beyond that found in even the most advanced scaffolds. Self-assembly is a motif used in development and a route for the production of complex materials. Self-assembly of peptides, proteins and other molecules at the nanoscale is promising, but in addition, intriguing ideas are emerging for self-assembly of micron-scale structures. In this brief review, very recent advances in the assembly of micron-scale cell aggregates and microgels will be described and discussed.