Vascularization of three-dimensional engineered tissues for regenerative medicine applications

A critical review on advances and challenges of bioprinted cardiac patches

Myocardial infarction (MI), which causes irreversible myocardium necrosis, affects 0.25 billion people globally and has become one of the most significant epidemics of our time. Over the past few years, bioprinting has moved beyond a concept of simply incorporating cells into biomaterials, to strategically defining the microenvironment (e.g., architecture, biomolecular signalling, mechanical stimuli, etc.) within which the cells are printed. Among the different bioprinting applications, myocardial repair is a field that has seen some of the most significant advances towards the management of the repaired tissue microenvironment. This review critically assesses the most recent biomedical innovations being carried out in cardiac patch bioprinting, with specific considerations given to the biomaterial design parameters, growth factors/cytokines, biomechanical and bioelectrical conditioning, as well as innovative biomaterial-based "4D" bioprinting (3D scaffold structure + temporal morphology changes) of myocardial tissues, immunomodulation and sustained delivery systems used in myocardium bioprinting. Key challenges include the ability to generate large quantities of cardiac cells, achieve high-density capillary networks, establish biomaterial designs that are comparable to native cardiac extracellular matrix, and manage the sophisticated systems needed for combining cardiac tissue microenvironmental cues while simultaneously establishing bioprinting technologies yielding both high-speed and precision. This must be achieved while considering quality assurance towards enabling reproducibility and clinical translation. Moreover, this manuscript thoroughly discussed the current clinical translational hurdles and regulatory issues associated with the post-bioprinting evaluation, storage, delivery and implantation of the bioprinted myocardial patches. Overall, this paper provides insights into how the clinical feasibility and important regulatory concerns may influence the design of the bioink (biomaterials, cell sources), fabrication and post-fabrication processes associated with bioprinting of the cardiac patches. This paper emphasizes that cardiac patch bioprinting requires extensive collaborations from imaging and 3D modelling technical experts, biomaterial scientists, additive manufacturing experts and healthcare professionals. Further, the work can also guide the field of cardiac patch bioprinting moving forward, by shedding light on the potential use of robotics and automation to increase productivity, reduce financial cost, and enable standardization and true commercialization of bioprinted cardiac patches. STATEMENT OF SIGNIFICANCE: The manuscript provides a critical review of important themes currently pursued for heart patch bioprinting, including critical biomaterial design parameters, physiologically-relevant cardiac tissue stimulations, and newly emerging cardiac tissue bioprinting strategies. This review describes the limited number of studies, to date in the literature, that describe systemic approaches to combine multiple design parameters, including capabilities to yield high-density capillary networks, establish biomaterial composite designs similar to native cardiac extracellular matrix, and incorporate cardiac tissue microenvironmental cues, while simultaneously establishing bioprinting technologies that yield high-speed and precision. New tools such as artificial intelligence may provide the analytical power to consider multiple design parameters and identify an optimized work-flow(s) for enabling the clinical translation of bioprinted cardiac patches.

Biofabrication Strategies for Oral Soft Tissue Regeneration

Gingival recession, a prevalent condition affecting the gum tissues, is characterized by the exposure of tooth root surfaces due to the displacement of the gingival margin. This review explores conventional treatments, highlighting their limitations and the quest for innovative alternatives. Importantly, it emphasizes the critical considerations in gingival tissue engineering leveraging on cells, biomaterials, and signaling factors. Successful tissue-engineered gingival constructs hinge on strategic choices such as cell sources, scaffold design, mechanical properties, and growth factor delivery. Unveiling advancements in recent biofabrication technologies like 3D bioprinting, electrospinning, and microfluidic organ-on-chip systems, this review elucidates their precise control over cell arrangement, biomaterials, and signaling cues. These technologies empower the recapitulation of microphysiological features, enabling the development of gingival constructs that closely emulate the anatomical, physiological, and functional characteristics of native gingival tissues. The review explores diverse engineering strategies aiming at the biofabrication of realistic tissue-engineered gingival grafts. Further, the parallels between the skin and gingival tissues are highlighted, exploring the potential transfer of biofabrication approaches from skin tissue regeneration to gingival tissue engineering. To conclude, the exploration of innovative biofabrication technologies for gingival tissues and inspiration drawn from skin tissue engineering look forward to a transformative era in regenerative dentistry with improved clinical outcomes.

Biomaterials-Based Technologies in Skeletal Muscle Tissue Engineering

For many clinically prevalent severe injuries, the inherent regenerative capacity of skeletal muscle remains inadequate. Skeletal muscle tissue engineering (SMTE) seeks to meet this clinical demand. With continuous progress in biomedicine and related technologies including micro/nanotechnology and 3D printing, numerous studies have uncovered various intrinsic mechanisms regulating skeletal muscle regeneration and developed tailored biomaterial systems based on these understandings. Here, the skeletal muscle structure and regeneration process are discussed and the diverse biomaterial systems derived from various technologies are explored in detail. Biomaterials serve not merely as local niches for cell growth, but also as scaffolds endowed with structural or physicochemical properties that provide tissue regenerative cues such as topographical, electrical, and mechanical signals. They can also act as delivery systems for stem cells and bioactive molecules that have been shown as key participants in endogenous repair cascades. To achieve bench-to-bedside translation, the typical effect enabled by biomaterial systems and the potential underlying molecular mechanisms are also summarized. Insights into the roles of biomaterials in SMTE from cellular and molecular perspectives are provided. Finally, perspectives on the advancement of SMTE are provided, for which gene therapy, exosomes, and hybrid biomaterials may hold promise to make important contributions.

Aggregation of human osteoblasts unlocks self-reliant differentiation and constitutes a microenvironment for 3D-co-cultivation with other bone marrow cells

Skeletal bone function relies on both cells and cellular niches, which, when combined, provide guiding cues for the control of differentiation and remodeling processes. Here, we propose an in vitro 3D model based on human fetal osteoblasts, which eases the study of osteocyte commitment in vitro and thus provides a means to examine the influences of biomaterials, substances or cells on the regulation of these processes. Aggregates were formed from human fetal osteoblasts (hFOB1.19) and cultivated under proliferative, adipo- and osteoinductive conditions. When cultivated under osteoinductive conditions, the vitality of the aggregates was compromised, the expression levels of the mineralization-related gene DMP1 and the amount of calcification and matrix deposition were lower, and the growth of the spheroids stalled. However, within spheres under growth conditions without specific supplements, self-organization processes occur, which promote extracellular calcium deposition, and osteocyte-like cells develop. Long-term cultivated hFOB aggregates were free of necrotic areas. Moreover, hFOB aggregates cultivated under standard proliferative conditions supported the co-cultivation of human monocytes, microvascular endothelial cells and stromal cells. Overall, the model presented here comprises a self-organizing and easily accessible 3D osteoblast model for studying bone marrow formation and in vitro remodeling and thus provides a means to test druggable molecular pathways with the potential to promote life-long bone formation and remodeling.

Evaluation of Osteogenic Potential for Rat Adipose-Derived Stem Cells under Xeno-Free Environment

This study aimed to develop a novel culture method for rat adipose-derived stem cells (rADSC) and evaluate their osteogenic potential. The rADSC cultured in xeno-free culture medium (XF-rADSCs) or conventional culture medium containing fetal bovine serum (FBS-rADSCs) were combined with micropieces of xeno-free recombinant collagen peptide to form 3-dimensional aggregates (XF-rADSC-CellSaic or FBS-rADSC-CellSaic). Both FBS-rADSC and XF-ADSC in CellSaic exhibited multilineage differentiation potential. Compared to FBS-rADSC-CellSaic, XF-rADSC-CellSaic accelerated and promoted osteogenic differentiation in vitro. When transplanted into rat mandibular congenital bone defects, the osteogenically differentiated XF-rADSC-CellSaic induced regeneration of bone tissue with a highly maturated structure compared to FBS-rADSC-CellSaic. In conclusion, XF-rADSC-CellSaic is a feasible 3-dimensional platform for efficient bone formation.

Engineering Organ-on-a-Chip Systems for Vascular Diseases

Vascular diseases, such as atherosclerosis and thrombosis, are major causes of morbidity and mortality worldwide. Traditional in vitro models for studying vascular diseases have limitations, as they do not fully recapitulate the complexity of the in vivo microenvironment. Organ-on-a-chip systems have emerged as a promising approach for modeling vascular diseases by incorporating multiple cell types, mechanical and biochemical cues, and fluid flow in a microscale platform. This review provides an overview of recent advancements in engineering organ-on-a-chip systems for modeling vascular diseases, including the use of microfluidic channels, ECM (extracellular matrix) scaffolds, and patient-specific cells. We also discuss the limitations and future perspectives of organ-on-a-chip for modeling vascular diseases.

Integrating Fused Deposition Modeling and Melt Electrowriting for Engineering Branched Vasculature

We demonstrate for the first time the combination of two additive manufacturing technologies used in tandem, fused deposition modelling (FDM) and melt electrowriting (MEW), to increase the range of possible MEW structures, with a focus on creating branched, hollow scaffolds for vascularization. First, computer-aided design (CAD) was used to design branched mold halves which were then used to FDM print conductive polylactic acid (cPLA) molds. Next, MEW was performed over the top of these FDM cPLA molds using polycaprolactone (PCL), an FDA-approved biomaterial. After the removal of the newly constructed MEW scaffolds from the FDM molds, complementary MEW scaffold halves were heat-melded together by placing the flat surfaces of each half onto a temperature-controlled platform, then pressing the heated halves together, and finally allowing them to cool to create branched, hollow constructs. This hybrid technique permitted the direct fabrication of hollow MEW structures that would otherwise not be possible to achieve using MEW alone. The scaffolds then underwent in vitro physical and biological testing. Specifically, dynamic mechanical analysis showed the scaffolds had an anisotropic stiffness of 1 MPa or 5 MPa, depending on the direction of the applied stress. After a month of incubation, normal human dermal fibroblasts (NHDFs) were seen growing on the scaffolds, which demonstrated that no deleterious effects were exerted by the MEW scaffolds constructed using FDM cPLA molds. The significant potential of our hybrid additive manufacturing approach to fabricate complex MEW scaffolds could be applied to a variety of tissue engineering applications, particularly in the field of vascularization.

Advanced in vitro models for renal cell carcinoma therapy design

Renal cell carcinoma (RCC) and its principal subtype, clear cell RCC, are the most diagnosed kidney cancer. Despite substantial improvement over the last decades, current pharmacological intervention still fails to achieve long-term therapeutic success. RCC is characterized by a high intra- and inter-tumoral heterogeneity and is heavily influenced by the crosstalk of the cells composing the tumor microenvironment, such as cancer-associated fibroblasts, endothelial cells and immune cells. Moreover, multiple physicochemical properties such as pH, interstitial pressure or oxygenation may also play an important role. These elements are often poorly recapitulated in in vitro models used for drug development. This inadequate recapitulation of the tumor is partially responsible for the current lack of an effective and curative treatment. Therefore, there are needs for more complex in vitro or ex vivo drug screening models. In this review, we discuss the current state-of-the-art of RCC models and suggest strategies for their further development.

Spheroid on-demand printing and drug screening of endothelialized hepatocellular carcinoma model at different stages

Hepatocellular carcinoma (HCC) poses a significant threat to human health and medical care. Its dynamic microenvironment and stages of development will influence the treatment strategies in clinics. Reconstructing tumor-microvascular interactions in different stages of the microenvironment is an urgent need forin vitrotumor pathology research and drug screening. However, the absence of tumor aggregates with paracancerous microvascular and staged tumor-endothelium interactions leads to bias in the antitumor drug responses. Herein, a spheroid-on-demand manipulation strategy was developed to construct staged endothelialized HCC models for drug screening. Pre-assembled HepG2 spheroids were directly printed by alternating viscous and inertial force jetting with high cell viability and integrity. A semi-open microfluidic chip was also designed to form a microvascular connections with high density, narrow diameter, and curved morphologies. According to the single or multiple lesions in stages Ⅰ or Ⅰ HCC, endothelialized HCC models from micrometer to millimeter scale with dense tumor cell aggregation and paracancerous endothelial distribution were successively constructed. A migrating stage Ⅰ HCC model was further constructed under TGF-βtreatment, where the spheroids exhibited a more mesenchymal phenotype with a loose cell connection and spheroid dispersion. Finally, the stage ⅠHCC model showed stronger drug resistance compared to the stage Ⅰ model, while the stage III showed a more rapid response. The corresponding work provides a widely applicable method for the reproduction of tumor-microvascular interactions at different stages and holds great promise for the study of tumor migration, tumor-stromal cell interactions, and the development of anti-tumor therapeutic strategies.

Strategies for improving the 3D printability of decellularized extracellular matrix bioink

3D bioprinting is a revolutionary technology capable of replicating native tissue and organ microenvironments by precisely placing cells into 3D structures using bioinks. However, acquiring the ideal bioink to manufacture biomimetic constructs is challenging. A natural extracellular matrix (ECM) is an organ-specific material that provides physical, chemical, biological, and mechanical cues that are hard to mimic using a small number of components. Organ-derived decellularized ECM (dECM) bioink is revolutionary and has optimal biomimetic properties. However, dECM is always "non-printable" owing to its poor mechanical properties. Recent studies have focused on strategies to improve the 3D printability of dECM bioink. In this review, we highlight the decellularization methods and procedures used to produce these bioinks, effective methods to improve their printability, and recent advances in tissue regeneration using dECM-based bioinks. Finally, we discuss the challenges associated with manufacturing dECM bioinks and their potential large-scale applications.

Response of cardiovascular environment to sulfonated hyaluronic acid with higher sulfur content

Sulfonated hyaluronic acid (S-HA) has been shown to promote endothelialization in the treatment of cardiovascular diseases according to amounts of investigations. In this study, two kinds of S-HA with higher sulfur content were obtained successfully. Through a series of cell experiments, it was found that the S-HA with higher sulfur content not only possessed stronger ability of promoting the growth and migration of endothelial cells, regulating the phenotype of smooth muscle cells, but also had stronger anti-inflammatory function. Furthermore, all the S-HA molecules are very compatible with blood.

Bioprinting Technologies and Bioinks for Vascular Model Establishment

Clinically, large diameter artery defects (diameter larger than 6 mm) can be substituted by unbiodegradable polymers, such as polytetrafluoroethylene. There are many problems in the construction of small diameter blood vessels (diameter between 1 and 3 mm) and microvessels (diameter less than 1 mm), especially in the establishment of complex vascular models with multi-scale branched networks. Throughout history, the vascularization strategies have been divided into three major groups, including self-generated capillaries from implantation, pre-constructed vascular channels, and three-dimensional (3D) printed cell-laden hydrogels. The first group is based on the spontaneous angiogenesis behaviour of cells in the host tissues, which also lays the foundation of capillary angiogenesis in tissue engineering scaffolds. The second group is to vascularize the polymeric vessels (or scaffolds) with endothelial cells. It is hoped that the pre-constructed vessels can be connected with the vascular networks of host tissues with rapid blood perfusion. With the development of bioprinting technologies, various fabrication methods have been achieved to build hierarchical vascular networks with high-precision 3D control. In this review, the latest advances in 3D bioprinting of vascularized tissues/organs are discussed, including new printing techniques and researches on bioinks for promoting angiogenesis, especially coaxial printing, freeform reversible embedded in suspended hydrogel printing, and acoustic assisted printing technologies, and freeform reversible embedded in suspended hydrogel (flash) technology.

Anisotropic dense collagen hydrogels with two ranges of porosity to mimic the skeletal muscle extracellular matrix

Despite the crucial role of the extracellular matrix (ECM) in the organotypic organization and function of skeletal muscles, most 3D models do not mimic its specific characteristics, namely its biochemical composition, stiffness, anisotropy, and porosity. Here, a novel 3D in vitro model of muscle ECM was developed reproducing these four crucial characteristics of the native ECM. An anisotropic hydrogel mimicking the muscle fascia was obtained thanks to unidirectional 3D printing of dense collagen with aligned collagen fibrils. The space between the different layers was tuned to generate an intrinsic network of pores (100 μm) suitable for nutrient and oxygen diffusion. By modulating the gelling conditions, the mechanical properties of the construct reached those measured in the physiological muscle ECM. This artificial matrix was thus evaluated for myoblast differentiation. The addition of large channels (600 μm) by molding permitted to create a second range of porosity suitable for cell colonization without altering the physical properties of the hydrogel. Skeletal myoblasts embedded in Matrigel®, seeded within the channels, organized in 3D, and differentiated into multinucleated myotubes. These results show that porous and anisotropic dense collagen hydrogels are promising biomaterials to model skeletal muscle ECM.

Evaluation of Tissue Apposition and Seroma Prevention in an Ovine Model of Surgical Dead Space Using a Novel Air-Purged Vacuum Closure System

BACKGROUND

Postoperative complications associated with seroma formation resulting from surgical dead space continue to present a challenge in modern surgery. There is an unmet need for new technologies that address surgical dead space as well as prevent seroma formation and associated downstream postoperative complications.

METHODS

The novel implantable tissue apposition and drainage system ENIVO was developed and tested in a bilateral ovine external abdominal oblique (EAO) resection model of surgical dead space. The ENIVO system is a portable powered pump and wound interface featuring air-purged vacuum closure (APVC) that delivers a sustained level of vacuum pressure (80 and 100 mmHg) to the treatment site with an intermittent burst of sterile filtered air through the implanted wound interface. Seroma area, seroma volume, and drain migration were assessed at postoperative days 7 and 14, and all animals were euthanized at day 28 with gross assessment of treatment efficacy including the presence of residual seroma and tissue apposition.

RESULTS

The bilateral model created relatively uniform defects of ~120 cm² following excision of ~30 to 50 g of EAO muscle. Median seroma area of ENIVO-treated defects was statistically smaller than standard of care (SoC)-treated defects at days 7 and 14. Median seroma volume at 14 days was significantly reduced in ENIVO-treated defects relative to SoC-treated defects [1.3 (IQR 0.0-79.5) mL and 188.5 (IQR 27.6-342.9) mL, respectively]. At postoperative day 28, 40% (n = 4/10) of SoC defects showed a residual seroma, whereas in contrast, none of the ENIVO-treated defects showed signs of a residual seroma. Median tissue apposition scoring was higher in the ENIVO treatment group [3 (IQR 3-3)] compared with the SoC group [3 (IQR 0-3)].

CONCLUSIONS

The ENIVO system represents a new approach to dead space management and seroma prevention and was shown to outperform a SoC surgical drain in a challenging large defect model of surgical dead space management and seroma prevention.

Strategies to Promote Vascularization in 3D Printed Tissue Scaffolds: Trends and Challenges

Three-dimensional (3D) printing techniques for scaffold fabrication have shown promising advancements in recent years owing to the ability of the latest high-performance printers to mimic the native tissue down to submicron scales. Nevertheless, host integration and performance of scaffolds in vivo have been severely limited owing to the lack of robust strategies to promote vascularization in 3D printed scaffolds. As a result, researchers over the past decade have been exploring strategies that can promote vascularization in 3D printed scaffolds toward enhancing scaffold functionality and ensuring host integration. Various emerging strategies to enhance vascularization in 3D printed scaffolds are discussed. These approaches include simple strategies such as the enhancement of vascular in-growth from the host upon implantation by scaffold modifications to complex approaches wherein scaffolds are fabricated with their own vasculature that can be directly anastomosed or microsurgically connected to the host vasculature, thereby ensuring optimal integration. The key differences among the techniques, their pros and cons, and the future opportunities for utilizing each technique are highlighted here. The Review concludes with the current limitations and future directions that can help 3D printing emerge as an effective biofabrication technique to realize tissues with physiologically relevant vasculatures to ultimately accelerate clinical translation.

Photosynthetic microorganisms for the oxygenation of advanced 3D bioprinted tissues

3D bioprinting technology has emerged as a tool that promises to revolutionize the biomedical field, including tissue engineering and regeneration. Despite major technological advancements, several challenges remain to be solved before 3D bioprinted tissues could be fully translated from the bench to the bedside. As oxygen plays a key role in aerobic metabolism, which allows energy production in the mitochondria; as a consequence, the lack of tissue oxygenation is one of the main limitations of current bioprinted tissues and organs. In order to improve tissue oxygenation, recent approaches have been established for a broad range of clinical applications, with some already applied using 3D bioprinting technologies. Among them, the incorporation of photosynthetic microorganisms, such as microalgae and cyanobacteria, is a promising approach that has been recently explored to generate chimerical plant-animal tissues where, upon light exposure, oxygen can be produced and released in a localized and controlled manner. This review will briefly summarize the state-of-the-art approaches to improve tissue oxygenation, as well as studies describing the use of photosynthetic microorganisms in 3D bioprinting technologies. STATEMENT OF SIGNIFICANCE: 3D bioprinting technology has emerged as a tool for the generation of viable and functional tissues for direct in vitro and in vivo applications, including disease modeling, drug discovery and regenerative medicine. Despite the latest advancements in this field, suboptimal oxygen delivery to cells before, during and after the bioprinting process limits their viability within 3D bioprinted tissues. This review article first highlights state-of-the-art approaches used to improve oxygen delivery in bioengineered tissues to overcome this challenge. Then, it focuses on the emerging roles played by photosynthetic organisms as novel biomaterials for bioink generation. Finally, it provides considerations around current challenges and novel potential opportunities for their use in bioinks, by comparing latest published studies using algae for 3D bioprinting.

Biodegradable Inks in Indirect Three-Dimensional Bioprinting for Tissue Vascularization

Mature vasculature is important for the survival of bioengineered tissue constructs, both in vivo and in vitro; however, the fabrication of fully vascularized tissue constructs remains a great challenge in tissue engineering. Indirect three-dimensional (3D) bioprinting refers to a 3D printing technique that can rapidly fabricate scaffolds with controllable internal pores, cavities, and channels through the use of sacrificial molds. It has attracted much attention in recent years owing to its ability to create complex vascular network-like channels through thick tissue constructs while maintaining endothelial cell activity. Biodegradable materials play a crucial role in tissue engineering. Scaffolds made of biodegradable materials act as temporary templates, interact with cells, integrate with native tissues, and affect the results of tissue remodeling. Biodegradable ink selection, especially the choice of scaffold and sacrificial materials in indirect 3D bioprinting, has been the focus of several recent studies. The major objective of this review is to summarize the basic characteristics of biodegradable materials commonly used in indirect 3D bioprinting for vascularization, and to address recent advances in applying this technique to the vascularization of different tissues. Furthermore, the review describes how indirect 3D bioprinting creates blood vessels and vascularized tissue constructs by introducing the methodology and biodegradable ink selection. With the continuous improvement of biodegradable materials in the future, indirect 3D bioprinting will make further contributions to the development of this field.

Embedded bioprinting for designer 3D tissue constructs with complex structural organization

3D bioprinting has been developed as an effective and powerful technique for the fabrication of living tissue constructs in a well-controlled manner. However, most existing 3D bioprinting strategies face substantial challenges in replicating delicate and intricate tissue-specific structural organizations using mechanically weak biomaterials such as hydrogels. Embedded bioprinting is an emerging bioprinting strategy that can directly fabricate complex structures derived from soft biomaterials within a supporting matrix, which shows great promise in printing large vascularized tissues and organs. Here, we provide a state-of-the-art review on the development of embedded bioprinting including extrusion-based and light-based processes to manufacture complex tissue constructs with biomimetic architectures. The working principles, bioinks, and supporting matrices of embedded printing processes are introduced. The effect of key processing parameters on the printing resolution, shape fidelity, and biological functions of the printed tissue constructs are discussed. Recent innovations in the processes and applications of embedded bioprinting are highlighted, such as light-based volumetric bioprinting and printing of functional vascularized organ constructs. Challenges and future perspectives with regard to translating embedded bioprinting into an effective strategy for the fabrication of functional biological constructs with biomimetic structural organizations are finally discussed. STATEMENT OF SIGNIFICANCE: It is still challenging to replicate delicate and intricate tissue-specific structural organizations using mechanically-weak hydrogels for the fabrication of functional living tissue constructs. Embedded bioprinting is an emerging 3D printing strategy that enables to produce complex tissue structures directly inside a reservoir filled with supporting matrix, which largely widens the choice of bioprinting inks to ECM-like hydrogels. Here we aim to provide a comprehensive review on various embedded bioprinting techniques mainly including extrusion-based and light-based processes. Various bioinks, supporting matrices, key processing parameters as well as their effects on the structures and biological functions of resultant living tissue constructs are discussed. We expect that it can provide an important reference and generate new insights for the bioprinting of large vascularized tissues and organs with biological functions.

hESCs-Derived Early Vascular Cell Spheroids for Cardiac Tissue Vascular Engineering and Myocardial Infarction Treatment

Transplanting functional cells to treat myocardial infarction (MI), a major disease threatening human health, has become the focus of global therapy. However, the efficacy has not been well anticipated, partly due to the lack of microvascular system that supplies nutrients and oxygen. Here, spheroids of early vascular cells (EVCs) derived from human embryonic stem cells (hESCs), rather than single-cell forms, as transplant "seeds" for reconstructing microvascular networks, are proposed. Firstly, EVCs containing CD34⁺ vascular progenitor cells are identified, which effectively differentiate into endothelial cells in situ and form vascular networks in extracellular matrix (ECM) hydrogel. Secondly, cardiac microtissues and cardiac patches with well-organized microvasculature are fabricated by three-dimensional (3D) co-culture or bioprinting with EVCs and cardiomyocytes in hydrogel. Notably, in 3D-bioprinted myocardial models, self-assembly vascularization of EVC spheroids is found to be significantly superior to EVC single cells. EVC spheroids are also injected into ischemic region of MI mouse models to explore its therapeutic potential. These findings uncover hESCs-derived EVC spheroids rather than single cells are more accessible for complex vasculature engineering, which is of great potential for cardiac tissue vascular engineering and MI treatment by cell therapy.

A review on 3D printing in tissue engineering applications

In tissue engineering, 3D printing is an important tool that uses biocompatible materials, cells, and supporting components to fabricate complex 3D printed constructs. This review focuses on the cytocompatibility characteristics of 3D printed constructs, made from different synthetic and natural materials. From the overview of this article, inkjet and extrusion-based 3D printing are widely used methods for fabricating 3D printed scaffolds for tissue engineering. This review highlights that scaffold prepared by both inkjet and extrusion-based 3D printing techniques showed significant impact on cell adherence, proliferation, and differentiation as evidenced by in vitro and in vivo studies. 3D printed constructs with growth factors (FGF-2, TGF-β1, or FGF-2/TGF-β1) enhance extracellular matrix (ECM), collagen I content, and high glycosaminoglycan (GAG) content for cell growth and bone formation. Similarly, the utilization of 3D printing in other tissue engineering applications cannot be belittled. In conclusion, it would be interesting to combine different 3D printing techniques to fabricate future 3D printed constructs for several tissue engineering applications.

Further structural characterization of ovine forestomach matrix and multi-layered extracellular matrix composites for soft tissue repair

Decellularized extracellular matrix (dECM)-based biomaterials are of great clinical utility in soft tissue repair applications due to their regenerative properties. Multi-layered dECM devices have been developed for clinical indications where additional thickness and biomechanical performance are required. However, traditional approaches to the fabrication of multi-layered dECM devices introduce additional laminating materials or chemical modifications of the dECM that may impair the biological functionality of the material. Using an established dECM biomaterial, ovine forestomach matrix, a novel method for the fabrication of multi-layered dECM constructs has been developed, where layers are bonded via a physical interlocking process without the need for additional bonding materials or detrimental chemical modification of the dECM. The versatility of the interlocking process has been demonstrated by incorporating a layer of hyaluronic acid to create a composite material with additional biological functionality. Interlocked composite devices including hyaluronic acid showed improved in vitro bioactivity and moisture retention properties.

Dynamic contrast-enhanced magnetic resonance imaging for monitoring neovascularization during bone regeneration-a randomized in vivo study in rabbits

OBJECTIVES

Micro-computed tomography (μ-CT) and histology, the current gold standard methods for assessing the formation of new bone and blood vessels, are invasive and/or destructive. With that in mind, a more conservative tool, dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI), was tested for its accuracy and reproducibility in monitoring neovascularization during bone regeneration. Additionally, the suitability of blood perfusion as a surrogate of the efficacy of osteoplastic materials was evaluated.

MATERIALS AND METHODS

Sixteen rabbits were used and equally divided into four groups, according to the time of euthanasia (2, 3, 4, and 6 weeks after surgery). The animals were submitted to two 8-mm craniotomies that were filled with blood or autogenous bone. Neovascularization was assessed in vivo through DCE-MRI, and bone regeneration, ex vivo, through μ-CT and histology.

RESULTS

The defects could be consistently identified, and their blood perfusion measured through DCE-MRI, there being statistically significant differences within the blood clot group between 3 and 6 weeks (p = 0.029), and between the former and autogenous bone at six weeks (p = 0.017). Nonetheless, no significant correlations between DCE-MRI findings on neovascularization and μ-CT (r =-0.101, 95% CI [-0.445; 0.268]) or histology (r = 0.305, 95% CI [-0.133; 0.644]) findings on bone regeneration were observed.

CONCLUSIONS

These results support the hypothesis that DCE-MRI can be used to monitor neovascularization but contradict the premise that it could predict bone regeneration as well.

Tissues with Patterned Vessels or Protein Release Induce Vascular Chemotaxis in an In Vitro Platform

Engineered tissues designed for translational applications in regenerative medicine require vascular networks to deliver oxygen and nutrients rapidly to the implanted cells. A limiting factor of in vivo translation is the rapid and successful inosculation, or connection, of host and implanted vascular networks and subsequent perfusion of the implant. An approach gaining favor in vascular tissue engineering is to provide instructive cues from the engineered tissue to enhance host vascular penetration and connection with the implant. Here, we use a novel in vitro platform based on the aortic ring assay to evaluate the impact of patterned, endothelialized vessels or growth factor release from engineered constructs on preinosculative vascular cell outgrowth from surrogate host tissue in a controlled, defined environment, and introduce robust tools for evaluating vascular morphogenesis and chemotaxis. We demonstrate the creation of engineered vessels at the arteriole scale, which develop basement membrane, exhibit tight junctions, and actively sprout into the surrounding bulk hydrogel. Vessel-containing constructs are co-cultured adjacent to rodent aortic rings, and the resulting heterocellular outgrowth is quantified. Cells originating from the aortic ring migrate preferentially toward constructs containing engineered vessels with 1.5-fold faster outgrowth kinetics, 2.5-fold increased cellular density, and 1.6-fold greater network formation versus control (no endothelial cells and growth factor-reduced culture medium). Growth factor release from constructs with nonendothelialized channels and in reduced factor medium equivalently stimulates sustained vascular outgrowth distance, cellular density, and network formation, akin to engineered vessels in endothelial growth medium 2 (EGM-2) medium. In conclusion, we show that three-dimensional endothelialized patterned vessels or growth factor release stimulate a robust, host-derived vascular cell chemotactic response at early time points critical for instructive angiogenic cues. Further, we developed robust, unbiased tools to quantify metrics of vascular morphogenesis and preinosculative heterocellular outgrowth from rat aortic rings and demonstrated the utility of our complex, controlled environment, heterocellular in vitro platform. Impact statement Using a novel in vitro platform, we show that engineered constructs with patterned vessels or angiogenic growth factor release, two methods of instructing host revascularization responses, equivalently improve early host-derived vascular outgrowth. Our platform leverages the aortic ring assay in a tissue engineering context to study preinosculative vascular cell chemotaxis from surrogate host vascular cells in response to paracrine cues from co-cultured engineered tissues using robust, open-source quantification tools. Our accessible and flexible platform enables translationally focused studies in revascularization using implantable therapeutics containing prepatterned vessels with greater environmental control than in vivo studies to advance vascular tissue engineering.

Decellularization Methods of Vagina and Cervix in Tissue Engineering

The vagina is a fibromuscular elastic tubular tract that connects the cervix with the outer genitals and has an important function discharging uterine secretions, sexual intercourse and acts as the passage for the full-term fetus. Currently, a new field of investigation which aims to design tissues and organs similar to their native origin has been developed recently and was named regenerative medicine (tissue engineering and bioengineering). Malformations in cervix tissue represent a hard challenge for medicine. Experts in bioengineering have tried to reconstruct vaginas or cervix with the aim to achieve cervicovaginal disorders, most of them with congenital cause. However, only few research groups have launched themselves upon the decellularization. The aim of this chapter is investigating the decellularization methods for cervix and vaginal tissues.

3D Printing of Microgel Scaffolds with Tunable Void Fraction to Promote Cell Infiltration

Granular, microgel-based materials have garnered interest as promising tissue engineering scaffolds due to their inherent porosity, which can promote cell infiltration. Adapting these materials for 3D bioprinting, while maintaining sufficient void space to enable cell migration, can be challenging, since the rheological properties that determine printability are strongly influenced by microgel packing and void fraction. In this work, a strategy is proposed to decouple printability and void fraction by blending UV-crosslinkable gelatin methacryloyl (GelMA) microgels with sacrificial gelatin microgels to form composite inks. It is observed that inks with an apparent viscosity greater than ≈100 Pa s (corresponding to microgel concentrations ≥5 wt%) have rheological properties that enable extrusion-based printing of multilayered structures in air. By altering the ratio of GelMA to sacrificial gelatin microgels, while holding total concentration constant at 6 wt%, a family of GelMA:gelatin microgel inks is created that allows for tuning of void fraction from 0.20 to 0.57. Furthermore, human umbilical vein endothelial cells (HUVEC) seeded onto printed constructs are observed to migrate into granular inks in a void fraction-dependent manner. Thus, the family of microgel inks holds promise for use in 3D printing and tissue engineering applications that rely upon cell infiltration.

Clip Chemistry: Diverse (Bio)(macro)molecular and Material Function through Breaking Covalent Bonds

In the two decades since the introduction of the "click chemistry" concept, the toolbox of "click reactions" has continually expanded, enabling chemists, materials scientists, and biologists to rapidly and selectively build complexity for their applications of interest. Similarly, selective and efficient covalent bond breaking reactions have provided and will continue to provide transformative advances. Here, we review key examples and applications of efficient, selective covalent bond cleavage reactions, which we refer to herein as "clip reactions." The strategic application of clip reactions offers opportunities to tailor the compositions and structures of complex (bio)(macro)molecular systems with exquisite control. Working in concert, click chemistry and clip chemistry offer scientists and engineers powerful methods to address next-generation challenges across the chemical sciences.

Triblock Copolymer Bioinks in Hydrogel Three-Dimensional Printing for Regenerative Medicine: A Focus on Pluronic F127

Three-dimensional (3D) bioprinting is a novel technique applied to manufacture semisolid or solid objects via deposition of successive thin layers. The widespread implementation of the 3D bioprinting technology encouraged scientists to evaluate its feasibility for applications in human regenerative medicine. 3D bioprinting gained much interest as a new strategy to prepare implantable 3D tissues or organs, tissue and organ evaluation models to test drugs, and cell/material interaction systems. The present work summarizes recent and relevant progress based on the use of hydrogels for the technology of 3D bioprinting and their emerging biomedical applications. An overview of different 3D printing techniques in addition to the nature and properties of bioinks used will be described with a focus on hydrogels as suitable bioinks for 3D printing. A comprehensive overview of triblock copolymers with emphasis on Pluronic F127 (PF127) as a bioink in 3D printing for regenerative medicine will be provided. Several biomedical applications of PF127 in tissue engineering, particularly in bone and cartilage regeneration and in vascular reconstruction, will be also discussed.

Vascularization strategies for tissue engineering for tracheal reconstruction

Tissue engineering technology provides effective alternative treatments for tracheal reconstruction. The formation of a functional microvascular network is essential to support cell metabolism and ensure the long-term survival of grafts. Although several tracheal replacement therapy strategies have been developed in the past, the critical significance of the formation of microvascular networks in 3D scaffolds has not attracted sufficient attention. Here, we review key technologies and related factors of microvascular network construction in tissue-engineered trachea and explore optimized preparation processes of vascularized functional tissues for clinical applications.

Neovascularization of engineered tissues for clinical translation: Where we are, where we should be?

One of the key challenges in engineering three-dimensional tissue constructs is the development of a mature microvascular network capable of supplying sufficient oxygen and nutrients to the tissue. Recent angiogenic therapeutic strategies have focused on vascularization of the constructed tissue, and its integration in vitro; these strategies typically combine regenerative cells, growth factors (GFs) with custom-designed biomaterials. However, the field needs to progress in the clinical translation of tissue engineering strategies. The article first presents a detailed description of the steps in neovascularization and the roles of extracellular matrix elements such as GFs in angiogenesis. It then delves into decellularization, cell, and GF-based strategies employed thus far for therapeutic angiogenesis, with a particularly detailed examination of different methods by which GFs are delivered in biomaterial scaffolds. Finally, interdisciplinary approaches involving advancement in biomaterials science and current state of technological development in fabrication techniques are critically evaluated, and a list of remaining challenges is presented that need to be solved for successful translation to the clinics.

Single-Use Bioreactors for Human Pluripotent and Adult Stem Cells: Towards Regenerative Medicine Applications

Research on human stem cells, such as pluripotent stem cells and mesenchymal stromal cells, has shown much promise in their use for regenerative medicine approaches. However, their use in patients requires large-scale expansion systems while maintaining the quality of the cells. Due to their characteristics, bioreactors have been regarded as ideal platforms to harbour stem cell biomanufacturing at a large scale. Specifically, single-use bioreactors have been recommended by regulatory agencies due to reducing the risk of product contamination, and many different systems have already been developed. This review describes single-use bioreactor platforms which have been used for human stem cell expansion and differentiation, along with their comparison with reusable systems in the development of a stem cell bioprocess for clinical applications.

Construction of chitosan scaffolds with controllable microchannel for tissue engineering and regenerative medicine

Microchannels are effective means of enabling the functional performance of tissue engineering scaffolds. Chitosan, a partial deacetylation derivative of chitin, exhibiting excellent biocompatibility, has been widely used in clinical practice. However, development of chitosan scaffolds with controllable microchannels architecture remains an engineering challenge. Here, we generated chitosan scaffolds with adjustable microchannel by combining a 3D printing microfiber templates-leaching method and a freeze-drying method. We can precisely control the arrangement, diameter and density of microchannel within chitosan scaffolds. Moreover, the integrated bilayer scaffolds with the desired structural parameters in each layer were fabricated and exhibited no delamination. The flow rate and volume of the simulated fluid can be modulated by diverse channels architecture. Additionally, the microchannel structure promoted cell survival, proliferation and distribution in vitro, and improved cell and tissue ingrowth and vascular formation in vivo. This study opens a new road for constructing chitosan scaffolds, and can further extend their application scope across tissue engineering and regenerative medicine.

Engineering Human Cardiac Muscle Patch Constructs for Prevention of Post-infarction LV Remodeling

Tissue engineering combines principles of engineering and biology to generate living tissue equivalents for drug testing, disease modeling, and regenerative medicine. As techniques for reprogramming human somatic cells into induced pluripotent stem cells (iPSCs) and subsequently differentiating them into cardiomyocytes and other cardiac cells have become increasingly efficient, progress toward the development of engineered human cardiac muscle patch (hCMP) and heart tissue analogs has accelerated. A few pilot clinical studies in patients with post-infarction LV remodeling have been already approved. Conventional methods for hCMP fabrication include suspending cells within scaffolds, consisting of biocompatible materials, or growing two-dimensional sheets that can be stacked to form multilayered constructs. More recently, advanced technologies, such as micropatterning and three-dimensional bioprinting, have enabled fabrication of hCMP architectures at unprecedented spatiotemporal resolution. However, the studies working on various hCMP-based strategies for in vivo tissue repair face several major obstacles, including the inadequate scalability for clinical applications, poor integration and engraftment rate, and the lack of functional vasculature. Here, we review many of the recent advancements and key concerns in cardiac tissue engineering, focusing primarily on the production of hCMPs at clinical/industrial scales that are suitable for administration to patients with myocardial disease. The wide variety of cardiac cell types and sources that are applicable to hCMP biomanufacturing are elaborated. Finally, some of the key challenges remaining in the field and potential future directions to address these obstacles are discussed.

On-chip high-definition bioprinting of microvascular structures

'Organ-on-chip' devices which integrate three-dimensional (3D) cell culture techniques with microfluidic approaches have the capacity to overcome the limitations of classical 2D platforms. Although several different strategies have been developed to improve the angiogenesis within hydrogels, one of the main challenges in tissue engineering remains the lack of vascularization in the fabricated 3D models. The present work focuses on the high-definition (HD) bioprinting of microvascular structures directly on-chip using two-photon polymerization (2PP). 2PP is a nonlinear process, where the near-infrared laser irradiation will only lead to the polymerization of a very small volume pixel (voxel), allowing the fabrication of channels in the microvascular range (10-30 µm in diameter). Additionally, 2PP not only enables the fabrication of sub-micrometer resolution scaffolds but also allows the direct embedding of cells within the produced structure. The accuracy of the 2PP printing parameters were optimized in order to achieve high-throughput and HD production of microfluidic vessel-on-chip platforms. The spherical aberrations stemming from the refractive index mismatch and the focusing depth inside the sample were simulated and the effect of the voxel compensation as well as different printing modes were demonstrated. Different layer spacings and their dependency on the applied laser power were compared both in terms of accuracy and required printing time resulting in a 10-fold decrease in structuring time while yielding well-defined channels of small diameters. Finally, the capacity of 2PP to create vascular structures within a microfluidic chip was tested with two different settings, by direct embedding of a co-culture of endothelial- and supporting cells during the printing process and by creating a supporting, cell-containing vascular scaffold barrier where the endothelial cell spheroids can be seeded afterwards. The functionality of the formed vessels was demonstrated with immunostaining of vascular endothelial cadherin (VE-Cadherin) endothelial adhesion molecules in both static and perfused culture.

Decellularization Methods of Ovary in Tissue Engineering

The ovaries or female gonads are situated in the ovarian fossa of the abdominal cavity. These are paired, almond-shaped organs measuring about 3.5 cm long and 1.5 cm thick and exist out of a central medullary zone and a peripheral cortex that are enclosed in a fibrous capsule called the tunica albuginea. The ovaries serve 2 main functions, the first one being the production of female gametes called oocytes (oogenesis). Interestingly, the number of primary oocytes that reside in the ovary is determined at birth. About 400 oocyte-containing follicles successfully go through all the developmental stages from this limited pool during folliculogenesis throughout the female reproductive life. In this process, primordial follicles grow and advance until forming a mature or Graafian follicle; during ovulation, secondary oocytes are released and the remaining follicular wall collapses and forms the highly vascularized corpus luteum or luteal gland. This ovarian cycle is regulated by several hormones secreted from the adenohypophysis and lasts about 28 days. During this cycle, the ovaries also serve as endocrine glands and produce female sex hormones such as estrogens and progesterone (steroidogenesis), influencing the growth and development of tissues sensitive to these hormones such as the endometrium. Hence, the endometrial cycle goes synchronized with the ovarian cycle.

Decellularization Methods of Uterus in Tissue Engineering

A new field of investigation which aims to design tissues and organs similar to their native origin has been developed recently, named as regenerative medicine (tissue engineering and bio-engineering). Uterus is the main organ for regeneration and contributes in the fertility. At an ultimate level, the uterus plays a role in embryo implantation, sperm migration and fetal nutrition. Uterine congenital anomalies, attained uterine lesions and immune system disorders may affect such uterine functions preventing successful pregnancy. Due to following reasons, it is essential to consider regenerative medicine as a new approach for the treatment of uterine dysfunctions to overcome the failures that cannot be treated with clinical medication.

Strategies for re-vascularization and promotion of angiogenesis in trauma and disease

The maintenance of a healthy vascular system is essential to ensure the proper function of all organs of the human body. While macrovessels have the main role of blood transportation from the heart to all tissues, microvessels, in particular capillaries, are responsible for maintaining tissues' functionality by providing oxygen, nutrients and waste exchanges. Occlusion of blood vessels due to atherosclerotic plaque accumulation remains the leading cause of mortality across the world. Autologous vein and artery grafts bypassing are the current gold standard surgical procedures to substitute primarily obstructed vascular structures. Ischemic scenarios that condition blood supply in downstream tissues may arise from blockage phenomena, as well as from other disease or events leading to trauma. The (i) great demand for new vascular substitutes, arising from both the limited availability of healthy autologous vessels, as well as the shortcomings associated with small-diameter synthetic vascular grafts, and (ii) the challenging induction of the formation of adequate and stable microvasculature are current driving forces for the growing interest in the development of bioinspired strategies to ensure the proper function of vasculature in all its dimensional scales. Here, a critical review of well-established technologies and recent biotechnological advances to substitute or regenerate the vascular system is provided.

Syndecan-4 promotes vascular beds formation in tissue engineered liver via thrombospondin 1

Instantaneous blood coagulation after bioengineered liver transplantation is a major issue, and the key process in its prevention is the construction of the endothelial vascular bed on biomimetic scaffolds. However, the specific molecules involved in the regulation of the vascular bed formation remain unclear. Syndecan-4 is a type I transmembrane glycoprotein commonly expressed in the human body; its receptor has been reported as critical for optimal cell adhesion and initiation of intracellular signaling, indicating its promising application in vascular bed formation. In the current study, bioinformatics analysis and in vitro experiments were performed to evaluate whether syndecan-4 promoted endothelial cell migration and functional activation. Exogenous syndecan-4-overexpressing endothelial cells were perfused into the decellularized liver scaffold, which was assessed by Masson’s trichrome staining. Western blotting and qRT-PCR were used to evaluate the effects of syndecan-4 on the thrombospondin 1 (THBS1) stability. We found that syndecan-4 promoted the adhesion of vascular endothelial cells and facilitated cell migration and angiogenesis. Furthermore, syndecan-4 overexpression resulted in a well-aligned endothelium on the decellularized liver scaffolds. Mechanistically, syndecan-4 destabilized THBS1 at the protein level. Therefore, our data revealed that syndecan-4 promoted the biological activity of endothelial cells on the bionic liver vascular bed through THBS1. These findings provide scientific evidences for solving transient blood coagulation after bionic liver transplantation.

Patient and Disease-Specific Induced Pluripotent Stem Cells for Discovery of Personalized Cardiovascular Drugs and Therapeutics

Human induced pluripotent stem cells (iPSCs) have emerged as an effective platform for regenerative therapy, disease modeling, and drug discovery. iPSCs allow for the production of limitless supply of patient-specific somatic cells that enable advancement in cardiovascular precision medicine. Over the past decade, researchers have developed protocols to differentiate iPSCs to multiple cardiovascular lineages, as well as to enhance the maturity and functionality of these cells. Despite significant advances, drug therapy and discovery for cardiovascular disease have lagged behind other fields such as oncology. We speculate that this paucity of drug discovery is due to a previous lack of efficient, reproducible, and translational model systems. Notably, existing drug discovery and testing platforms rely on animal studies and clinical trials, but investigations in animal models have inherent limitations due to interspecies differences. Moreover, clinical trials are inherently flawed by assuming that all individuals with a disease will respond identically to a therapy, ignoring the genetic and epigenomic variations that define our individuality. With ever-improving differentiation and phenotyping methods, patient-specific iPSC-derived cardiovascular cells allow unprecedented opportunities to discover new drug targets and screen compounds for cardiovascular disease. Imbued with the genetic information of an individual, iPSCs will vastly improve our ability to test drugs efficiently, as well as tailor and titrate drug therapy for each patient.

Glycolytic inhibition by resveratrol prevents myoblast cell death caused by glucose deprivation and hypoxia; a possible application to the three-dimensional tissue construction

Decreased cell viability resulting from a severe condition of nutrients deprivation and hypoxia has been the major obstacle in three-dimensional (3D) tissue construction. Therefore, technical improvement which prevents cell death caused by starvation and low oxygen is desired for the development of large, thick tissues. We focused on the anti-glycolytic effect of resveratrol (RSV), a naturally-occurring polyphenol known as a caloric restriction mimetic, and investigated its cytoprotective effect in two-dimensional (2D) and 3D-cell culture using H9c2 rat myoblast cells. Glucose deprivation by culturing with low glucose media caused time- and dose-dependent cell death in H9c2 cells. In contrast, RSV treatment at 100 μM significantly increased the cell viability by preventing cell death. RSV showed anti-glycolytic effect associated with a down-regulation of glycolytic genes (GLUT1, PKM2) and glucose uptake activity, and increased the activation of AMP-activated protein kinase (AMPK), an essential cellular energy sensor activated in the condition of energy deprivation. RSV treatment markedly improved the viability of myoblast cells cultured in a hypoxic, low glucose condition and attenuated the up-regulation of glycolytic genes by hypoxic response. In 3D-cultured model, spheroids constructed with RSV-treated cells showed improved cell viability and intact histological appearance compared with control. These results suggest that glycolytic inhibition by RSV decreases the glucose usage of myoblast cells, therefore, prevents cell death caused by nutrient deprivation and hypoxic condition. Our finding provides useful information to improve cell viability in a condition that nutrients and oxygen are low in supply, and be a possible application to the 3D-tissue construction.

Engineering functional microvessels in synthetic polyurethane random-pore scaffolds by harnessing perfusion flow

Recently reported biomaterial-based approaches toward prevascularizing tissue constructs rely on biologically or structurally complex scaffolds that are complicated to manufacture and sterilize, and challenging to customize for clinical applications. In the current work, a prevascularization method for soft tissue engineering that uses a non-patterned and non-biological scaffold is proposed. Human fibroblasts and HUVECs were seeded on an ionomeric polyurethane-based hydrogel and cultured for 14 days under medium perfusion. A flow rate of 0.05 mL/min resulted in a greater lumen density in the constructs relative to 0.005 and 0.5 mL/min, indicating the critical importance of flow magnitude in establishing microvessels. Constructs generated at 0.05 mL/min perfusion flow were implanted in a mouse subcutaneous model and intravital imaging was used to characterize host blood perfusion through the construct after 2 weeks. Engineered microvessels were functional (i.e. perfused with host blood and non-leaky) and neovascularization of the construct by host vessels was enhanced relative to non-prevascularized constructs. We report on the first strategy toward engineering functional microvessels in a tissue construct using non-bioactive, non-patterned synthetic polyurethane materials.

Development of a Sustained Release Nano-In-Gel Delivery System for the Chemotactic and Angiogenic Growth Factor Stromal-Derived Factor 1α

Stromal-Derived Factor 1α (SDF) is an angiogenic, chemotactic protein with significant potential for applications in a range of clinical areas, including wound healing, myocardial infarction and orthopaedic regenerative approaches. The 26-min in vivo half-life of SDF, however, has limited its clinical translation to date. In this study, we investigate the use of star-shaped or linear poly(glutamic acid) (PGA) polypeptides to produce PGA-SDF nanoparticles, which can be incorporated into a tyramine-modified hyaluronic acid hydrogel (HA-TA) to facilitate sustained localised delivery of SDF. The physicochemical properties and biocompatibility of the PGA-SDF nanoparticle formulations were extensively characterised prior to incorporation into a HA-TA hydrogel. The biological activity of the SDF released from the nano-in-gel system was determined on Matrigel®, scratch and Transwell® migration assays. Both star-shaped and linear PGA facilitated SDF nanoparticle formation with particle sizes from 255-305 nm and almost complete SDF complexation. Star-PGA-SDF demonstrated superior biocompatibility and was incorporated into a HA-TA gel, which facilitated sustained SDF release for up to 35 days in vitro. Released SDF significantly improved gap closure on a scratch assay, produced a 2.8-fold increase in HUVEC Transwell® migration and a 1.5-fold increase in total tubule length on a Matrigel® assay at 12 h compared to untreated cells. Overall, we present a novel platform system for the sustained delivery of bioactive SDF from a nano-in-gel system which could be adapted for a range of biomedical applications.

Regenerative medicine, organ bioengineering and transplantation

Background

Organ transplantation is predicted to increase as life expectancy and the incidence of chronic diseases rises. Regenerative medicine-inspired technologies challenge the efficacy of the current allograft transplantation model.

Methods

A literature review was conducted using the PubMed interface of MEDLINE from the National Library of Medicine. Results were examined for relevance to innovations of organ bioengineering to inform analysis of advances in regenerative medicine affecting organ transplantation. Data reports from the Scientific Registry of Transplant Recipient and Organ Procurement Transplantation Network from 2008 to 2019 of kidney, pancreas, liver, heart, lung and intestine transplants performed, and patients currently on waiting lists for respective organs, were reviewed to demonstrate the shortage and need for transplantable organs.

Results

Regenerative medicine technologies aim to repair and regenerate poorly functioning organs. One goal is to achieve an immunosuppression-free state to improve quality of life, reduce complications and toxicities, and eliminate the cost of lifelong antirejection therapy. Innovative strategies include decellularization to fabricate acellular scaffolds that will be used as a template for organ manufacturing, three-dimensional printing and interspecies blastocyst complementation. Induced pluripotent stem cells are an innovation in stem cell technology which mitigate both the ethical concerns associated with embryonic stem cells and the limitation of other progenitor cells, which lack pluripotency. Regenerative medicine technologies hold promise in a wide array of fields and applications, such as promoting regeneration of native cell lines, growth of new tissue or organs, modelling of disease states, and augmenting the viability of existing ex vivo transplanted organs.

Conclusion

The future of organ bioengineering relies on furthering understanding of organogenesis, in vivo regeneration, regenerative immunology and long-term monitoring of implanted bioengineered organs.