Application of subcutaneous extracellular matrix to prepare bilayer heparin-coated polycaprolactone/decellularized small-diameter vascular graft for tissue regeneration

In clinical practice, the demand for functional small-diameter vascular grafts continues to increase. In this study, a decellularized aorta artery was inserted into a poly(caprolactone) (PCL) vascular scaffold for self-assembly in-vitro to create a hybrid scaffold. The hybrid scaffold was then implanted subcutaneously into the dorsal flanks and the subcutaneous extracellular matrix was applied for bilayer adhesion. After decellularization, the hybrid scaffold was coated with heparin to prepare a bilayer tissue-engineered vascular graft (BTEVG). The BTEVG exhibited enhanced biomechanical properties compared with those of decellularized tissue. The bilayer scaffold remained patent and displayed no expansion or aneurysm after implantation at 2 months. Endothelial cell formation was observed on the neointimal surface. In the neointimal, decellularized tissue in the inner layer inhibited smooth muscle cells proliferation and neointimal hyperplasia of BTEVG. M2 macrophage cell proliferation in the neointimal may inhibit vascular smooth muscle cell proliferation. Although the PCL-H scaffold demonstrated calcification formation, no calcification was found in the BTEVG. Therefore, BTEVGs can be applied for rapid remodeling of small-diameter blood vessels.

Pericytes Promote More Vascularization than Stromal Cells via an Interleukin-6-Dependent Mechanism in Microfluidic Chips

Pericytes are a key player in vascularization, protecting endothelial cells from external harm and promoting the formation of new vessels when necessary. However, pericytic identity and its relationship with other cell types, such as mesenchymal stromal/stem cells, is highly debated. This study compares the role of pericytes and unselected stromal cells in vascularization using multichannel microfluidic chips. In both angiogenesis and vasculogenesis, pericytes promote more vessel formation than stromal cells. Pericytes can wrap around endothelial vessels acting as mural cells, while stromal cells remain separated. Whole-transcriptome sequencing confirms an upregulation of pro-vascularization genes in endothelial cell-pericyte co-cultures, while metabolism increases and inflammation decreases in stromal cell co-cultures. Treatment of stromal-endothelial cell co-cultures with either conditioned media or isolated extracellular vesicles from pericytes replicates the increase in vasculogenesis of the direct co-cultures. Cytokine quantification reveals that interleukin 6 (IL-6) is significantly increased in pericyte conditions. Blocking it with siltuximab results in a reduction of pericyte vasculogenic potential comparable to stromal cell levels, revealing that pericyte pro-vascularization is mediated by IL-6. This study provides new insights into the relationship between pericytes and endothelial cells and the elusive identity of mesenchymal stromal cells. These findings are relevant for both vascular biology and tissue engineering.

Template-Assisted Electrospinning and 3D Printing of Multilayered Hierarchical Vascular Grafts

Fabricating complex hierarchical structures mimicking natural vessels and arteries is pivotal for addressing problems of cardiovascular diseases. Various fabrication strategies have been explored to achieve this goal, each contributing unique advantages and challenges to the development of functional vascular grafts. In this study, a three-layered tubular structure resembling vascular grafts was fabricated using biocompatible and biodegradable copolymers of poly(butylene succinate) (PBS) using advanced manufacturing techniques. The outer layer was fabricated by template-assisted electrospinning utilizing a 3D-printed scaffold with a precise hexagonal pore design as the template, and the inner layer was coated with gelatin through perfusion. Cellulose nanocrystals (CNCs) were incorporated into electrospun fibers to enhance mechanical properties. The gelatin coating was applied to the lumen using perfusion coating, resembling the inner layer. Integration of 3D-printed structures with electrospun fibers via template-assisted electrospinning and gelatin coating resulted in a seamless multilayered scaffold. Mechanical testing demonstrated robustness, surpassing natural arteries in some aspects, while the gelatin coating significantly reduced liquid leakage, ensuring leak-free functionality. Cytotoxicity assessment confirmed the biocompatibility of processed materials with fibroblast cells, supporting potential for medical applications.

Engineered Microfibers for Tissue Engineering

Hydrogel microfibers are hydrogel materials engineered into fiber structures. Techniques such as wet spinning, microfluidic spinning, and 3D bioprinting are often used to prepare microfibers due to their ability to precisely control the size, morphology, and structure of the microfibers. Microfibers with different structural morphologies have different functions; they provide a flow-through culture environment for cells to improve viability, and can also be used to induce the differentiation of cells such as skeletal muscle and cardiac muscle cells to eventually form functional organs in vitro through special morphologies. This Review introduces recent advances in microfluidics, 3D bioprinting, and wet spinning in the preparation of microfibers, focusing on the materials and fabrication methods. The applications of microfibers in tissue engineering are highlighted by summarizing their contributions in engineering biomimetic blood vessels, vascularized tissues, bone, heart, pancreas, kidney, liver, and fat. Furthermore, applications of engineered fibers in tissue repair and drug screening are also discussed.

Microvascular network based on the Hilbert curve for nutrient transport in thick tissue

To address the uneven nutrient distribution within three-dimensional (3D) tissue models and organoids currently used in medical research, this study introduces a microvascular network based on the Hilbert curve. Our aim was to develop innovative solutions for enhancing nutrient supply in thick tissue models in vitro. By using 3D bioprinting, we engineered microvascular networks of varying Hilbert orders and validated their efficacy in enhancing nutrient uniformity through numerical simulations and experiments. These networks facilitated broader and more uniform nutrient distribution throughout the thick tissue models, particularly the 2° Hilbert microvascular structure, which occupies less space and significantly reduces regions of cellular death. Furthermore, we explored the potential of assembling larger tissue constructs using the 2° Hilbert microvascular network, showcasing its applicability in constructing large-scale biological models. The findings suggest that the 2° Hilbert microvascular structure is particularly effective in ensuring adequate nutrient delivery, thus enhancing the viability and functionality of large-volume tissue models. These innovations hold significant promise for advancing the fields of tissue engineering and regenerative medicine by improving nutrient delivery to in vitro thick tissue block models. This provides a robust foundation for future in vitro research and clinical applications, potentially leading to more effective treatments and interventions in the medical field. The development of these microvascular networks represents a crucial step forward in overcoming the limitations of current 3D tissue models and organoids, paving the way for more sophisticated and reliable biomedical research tools.

Microvascular Fragment-Loaded Platelet-Rich Plasma Dressing Promotes Cutaneous Wound Healing

Objective: Chronic wounds represent a considerable burden for the affected patients and the health care system. To overcome this problem, effective treatment strategies are urgently required. In this study, we tested a novel approach by combining platelet-rich plasma (PRP) and microvascular fragments (MVF) to create a prevascularized gel dressing. Approach: MVF were enzymatically isolated from the epididymal fat pads of transgenic green fluorescent protein (GFP)+ C57BL/6J donor mice. Subsequently, 5,000 MVF were suspended in 10 μL murine PRP as carrier and transferred into full-thickness skin wounds within dorsal skinfold chambers of C57BL/6J wild-type mice (PRP+MVF). Wound healing in comparison to empty wounds (control) and wounds filled with PRP alone was repeatedly analyzed throughout 14 days by means of stereomicroscopy, histology, and immunohistochemistry. Results: Planimetric assessment of the wound size over time revealed a significantly accelerated and improved healing of PRP+MVF-treated wounds when compared with PRP-treated and empty control wounds. These wounds also exhibited a significantly higher density of blood and lymph vessels, which originated from the GFP+ MVF isolates and effectively promoted granulation tissue formation inside the skin defects. Innovation: This study is the first to combine PRP and MVF for the improvement of wound healing. Conclusion: The combination of PRP and MVF represents a promising approach for the future treatment of wounds that do not heal spontaneously due to poor wound-healing conditions.

Advanced Cardiac Patches for the Treatment of Myocardial Infarction

Myocardial infarction is a cardiovascular disease characterized by a high incidence rate and mortality. It leads to various cardiac pathophysiological changes, including ischemia/reperfusion injury, inflammation, fibrosis, and ventricular remodeling, which ultimately result in heart failure and pose a significant threat to global health. Although clinical reperfusion therapies and conventional pharmacological interventions improve emergency survival rates and short-term prognoses, they are still limited in providing long-lasting improvements in cardiac function or reversing pathological progression. Recently, cardiac patches have gained considerable attention as a promising therapy for myocardial infarction. These patches consist of scaffolds or loaded therapeutic agents that provide mechanical reinforcement, synchronous electrical conduction, and localized delivery within the infarct zone to promote cardiac restoration. This review elucidates the pathophysiological progression from myocardial infarction to heart failure, highlighting therapeutic targets and various cardiac patches. The review considers the primary scaffold materials, including synthetic, natural, and conductive materials, and the prevalent fabrication techniques and optimal properties of the patch, as well as advanced delivery strategies. Last, the current limitations and prospects of cardiac patch research are considered, with the goal of shedding light on innovative products poised for clinical application.

Development and characterization of Polycaprolactone/chitosan-based scaffolds for tissue engineering of various organs: A review

Research in creating 3D structures mirroring the extracellular matrix (ECM) with accurate environmental cues holds paramount significance in biological applications.Biomaterials that replicate ECM properties-mechanical, physicochemical, and biological-emerge as pivotal tools in mimicking ECM behavior.Incorporating synthetic and natural biomaterials is widely used to produce scaffolds suitable for the intended organs.Polycaprolactone (PCL), a synthetic biomaterial, boasts commendable mechanical properties, albeit with relatively modest biological attributes due to its hydrophobic nature.Chitosan (CTS) exhibits strong biological traits but lacks mechanical resilience for complex tissue regeneration.Notably, both PCL and CTS have demonstrated their application in tissue engineering for diverse types of tissues.Their combination across varying PCL:CTS ratios has increased the likelihood of fabricating scaffolds to address defects in sturdy and pliable tissues.This comprehensive analysis aspires to accentuate their distinct attributes within tissue engineering across different organs.The central focus resides in the role of PCL:CTS-based scaffolds, elucidating their contribution to the evolution of advanced functional 3D frameworks tailored for tissue engineering across diverse organs.Moreover, this discourse delves into the considerations pertinent to each organ.

Progress in cardiac tissue engineering and regeneration: Implications of gelatin-based hybrid scaffolds

Cardiovascular diseases, particularly myocardial infarction (MI), remain a leading cause of morbidity and mortality worldwide. Current treatments for MI, more palliative than curative, have limitations in reversing the disease completely. Tissue engineering (TE) has emerged as a promising strategy to address this challenge and may lead to improved therapeutic approaches for MI. Gelatin-based scaffolds, including gelatin and its derivative, gelatin methacrylate (GelMA), have attracted significant attention in cardiac tissue engineering (CTE) due to their optimal physical and biochemical properties and capacity to mimic the native extracellular matrix (ECM). CTE mainly recruits two classes of gelatin/GelMA-based scaffolds: hydrogels and nanofibrous. This article reviews state-of-the-art gelatin/GelMA-based hybrid scaffolds currently applied for CTE and regenerative therapy. Hybrid scaffolds, fabricated by combining gelatin/GelMA hydrogel or nanofibrous scaffolds with other materials such as natural/synthetic polymers, nanoparticles, protein-based biomaterials, etc., are explored for enhanced cardiac tissue regeneration functionality. The engraftment of stem/cardiac cells, bioactive molecules, or drugs into these hybrid systems shows great promise in cardiac tissue repair and regeneration. Finally, the role of gelatin/GelMA scaffolds combined with the 3D bioprinting strategy in CTE will also be briefly highlighted.

Recent Advances in Cell Sheet-Based Tissue Engineering for Bone Regeneration

In conventional bone tissue engineering, cells are seeded onto scaffolds to create three-dimensional (3D) tissues, but the cells on the scaffolds are unable to effectively perform their physiological functions due to their low density and viability. Cell sheet (CS) engineering is expected to be free from this limitation. CS engineering uses the principles of self-assembly and self-organization of endothelial and mesenchymal stem cells to prepare CSs as building blocks for engineering bone grafts. This process recapitulates the native tissue development, thus attracting significant attention in the field of bone regeneration. However, the method is still in the prebasic experimental stage in bone defect repair. To make the method clinically applicable and valuable in personalized and precision medicine, current research is focused on the preparation of multifunctionalized building blocks using CS technologies, such as 3D layered CSs containing microvascular structures. Considering the great potential of CS engineering in repairing bone defects, in this review, the types of cell technologies are first outlined. We then summarize the various types of CSs as building blocks for engineering bone grafts. Furthermore, the specific applications of CSs in bone repair are discussed. Finally, we present specific suggestions for accelerating the application of CS engineering in the clinical treatment of bone defects.

PVA-Based Electrospun Materials-A Promising Route to Designing Nanofiber Mats with Desired Morphological Shape-A Review

Poly(vinyl alcohol) is one of the most attractive polymers with a wide range of uses because of its water solubility, biocompatibility, low toxicity, good mechanical properties, and relatively low cost. This review article focuses on recent advances in poly(vinyl alcohol) electrospinning and summarizes parameters of the process (voltage, distance, flow rate, and collector), solution (molecular weight and concentration), and ambient (humidity and temperature) in order to comprehend the influence on the structural, mechanical, and chemical properties of poly(vinyl alcohol)-based electrospun matrices. The importance of poly(vinyl alcohol) electrospinning in biomedical applications is emphasized by exploring a literature review on biomedical applications including wound dressings, drug delivery, tissue engineering, and biosensors. The study also highlights a new promising area of particles formation through the electrospraying of poly(vinyl alcohol). The limitations and advantages of working with different poly(vinyl alcohol) matrices are reviewed, and some recommendations for the future are made to advance this field of study.

Cardiovascular Tissue Engineering Models for Atherosclerosis Treatment Development

In the early years of tissue engineering, scientists focused on the generation of healthy-like tissues and organs to replace diseased tissue areas with the aim of filling the gap between organ demands and actual organ donations. Over time, the realization has set in that there is an additional large unmet need for suitable disease models to study their progression and to test and refine different treatment approaches. Increasingly, researchers have turned to tissue engineering to address this need for controllable translational disease models. We review existing and potential uses of tissue-engineered disease models in cardiovascular research and suggest guidelines for generating adequate disease models, aimed both at studying disease progression mechanisms and supporting the development of dedicated drug-delivery therapies. This involves the discussion of different requirements for disease models to test drugs, nanoparticles, and drug-eluting devices. In addition to realistic cellular composition, the different mechanical and structural properties that are needed to simulate pathological reality are addressed.

Vascular reconstruction of the decellularized biomatrix for whole-organ engineering-a critical perspective and future strategies

Whole-organ re-engineering is the most challenging goal yet to be achieved in tissue engineering and regenerative medicine. One essential factor in any transplantable and functional tissue engineering is fabricating a perfusable vascular network with macro- and micro-sized blood vessels. Whole-organ development has become more practical with the use of the decellularized organ biomatrix (DOB) as it provides a native biochemical and structural framework for a particular organ. However, reconstructing vasculature and re-endothelialization in the DOB is a highly challenging task and has not been achieved for constructing a clinically transplantable vascularized organ with an efficient perfusable capability. Here, we critically and articulately emphasized factors that have been studied for the vascular reconstruction in the DOB. Furthermore, we highlighted the factors used for vasculature development studies in general and their application in whole-organ vascular reconstruction. We also analyzed in detail the strategies explored so far for vascular reconstruction and angiogenesis in the DOB for functional and perfusable vasculature development. Finally, we discussed some of the crucial factors that have been largely ignored in the vascular reconstruction of the DOB and the future directions that should be addressed systematically.

Histological assessment of nanostructured fibrin-agarose skin substitutes grafted in burnt patients. A time-course study

A previously developed fibrin-agarose skin model-UGRSKIN-showed promising clinical results in severely burnt patients. To determine the histological parameters associated to the biocompatibility and therapeutic effects of this model, we carried out a comprehensive structural and ultrastructural study of UGRSKIN grafted in severely burnt patients after 3 months of follow-up. The grafted epidermis was analogue to native human skin from day 30th onward, revealing well-structured strata with well-differentiated keratinocytes expressing CK5, CK8, CK10, claudin, plakoglobin, filaggrin, and involucrin in a similar way to controls, suggesting that the epidermis was able to mature and differentiate very early. Melanocytes and Langerhans cells were found from day 30th onward, together with a basement membrane, abundant hemidesmosomes and lack of rete ridges. At the dermal layer, we found an interface between the grafted skin and the host tissue at day 30th, which tended to disappear with time. The grafted superficial dermis showed a progressive increase in properly-oriented collagen fibers, elastic fibers and proteoglycans, including decorin, similarly to control dermis at day 60-90th of in vivo follow-up. Blood vessels determined by CD31 and SMA expression were more abundant in grafted skin than controls, whereas lymphatic vessels were more abundant at day 90th. These results contribute to shed light on the histological parameters associated to biocompatibility and therapeutic effect of the UGRSKIN model grafted in patients and demonstrate that the bioengineered skin grafted in patients is able to mature and differentiate very early at the epithelial level and after 60-90 days at the dermal level.

Intelligent Vascularized 3D/4D/5D/6D-Printed Tissue Scaffolds

Blood vessels are essential for nutrient and oxygen delivery and waste removal. Scaffold-repairing materials with functional vascular networks are widely used in bone tissue engineering. Additive manufacturing is a manufacturing technology that creates three-dimensional solids by stacking substances layer by layer, mainly including but not limited to 3D printing, but also 4D printing, 5D printing and 6D printing. It can be effectively combined with vascularization to meet the needs of vascularized tissue scaffolds by precisely tuning the mechanical structure and biological properties of smart vascular scaffolds. Herein, the development of neovascularization to vascularization to bone tissue engineering is systematically discussed in terms of the importance of vascularization to the tissue. Additionally, the research progress and future prospects of vascularized 3D printed scaffold materials are highlighted and presented in four categories: functional vascularized 3D printed scaffolds, cell-based vascularized 3D printed scaffolds, vascularized 3D printed scaffolds loaded with specific carriers and bionic vascularized 3D printed scaffolds. Finally, a brief review of vascularized additive manufacturing-tissue scaffolds in related tissues such as the vascular tissue engineering, cardiovascular system, skeletal muscle, soft tissue and a discussion of the challenges and development efforts leading to significant advances in intelligent vascularized tissue regeneration is presented.

Application of stem cells in engineered vascular graft and vascularized organs

Recent studies report that stem cell therapies have been applied successfully to patients, This has increased anticipations that this regeneration strategy could be a potential method to treat a wide range of intractable diseases some day. Stem cells offer new prospects for the treatment of incurable diseases and for tissue regeneration and repairation because of their unique biological properties. Angiogenesis a key process in tissue regeneration and repairation. Vascularization of organs is one of the main challenges hindering the clinical application of engineered tissues. Efficient production of engineered vascular grafts and vascularized organs is of critical importance for regenerative medicine. In this review, we focus on the types of stem cells that are widely used in tissue engineering and regeneration, as well as their application of these stem cells in the construction of tissue-engineered vascular grafts and vascularization of tissue-engineered organs.

Role of Matrix Metalloproteinase-2 in the Development of Atherosclerosis among Patients with Coronary Artery Disease

Coronary artery disease (CAD) is a caused by atherosclerotic plaque buildup in the coronary arteries that supply blood and oxygen to the heart. Matrix metalloproteinase (MMP) is a family of zinc-dependent endopeptidase that is involved in various stages of atherosclerosis as demonstrated in in vitro and in vivo studies. MMP-2 is associated with both stable and unstable atherosclerotic plaque formation. The current review aimed to identify the role of MMP-2 in atherosclerosis development among CAD patients. Literature search was conducted through four online databases and only studies that were published from 2018 until February 2023 were included. The risk of bias was assessed by using the Newcastle-Ottawa Scale. A total of 10,622 articles were initially identified, and only eight studies that fulfilled the selection criteria were included in this review. The results showed that MMP-2 levels and activity were higher in patients with unstable CAD than those with stable CAD and healthy subjects. There was a significant association between MMP-2 levels and cardiovascular disease with MMP-14 levels, which is a pro-MMP-2 activator. In addition, two single nucleotide polymorphisms of the MMP-2 gene (rs243865 and rs243866) were significantly associated with the development of atherosclerosis. In conclusion, MMP-2 plays a crucial role in the development of atherosclerosis among patients with CAD and could be a potential target for CAD therapy.

Hydrogel coating based on dopamine-modified hyaluronic acid and gelatin with spatiotemporal drug release capacity for quick endothelialization and long-term anticoagulation

Due to the vital roles of vascular intima in preventing thrombus generation and maintaining vascular patency, methods to promote quick endothelialization on vascular grafts have drawn much attention. In this study, we novelly applied a double-layered hydrogel coating with spatiotemporal drug release capacity on a polycaprolactone (PCL) fibrous scaffold. The composite coating consisted of an inner dopamine-modified hyaluronic acid (HA) hydrogel and an outer gelatin hydrogel, which were generated via different crosslinking methods. Especially, heparin and chondroitin sulfate were introduced to the HA and gelatin hydrogels during the processing, thus endowing the vascular scaffold spatiotemporal drug release behavior. The composite coating developed surface hydrophilicity and mechanical properties of the PCL scaffold meanwhile stimulating the proliferation and angiogenesis behaviors of endothelial cells. Long-term anticoagulation property of the modified scaffold was also demonstrated in vitro. This investigation provides a universal strategy for quick endothelialization and long-term anticoagulation promotion of vascular grafts, which may be potentially used in treating cardiovascular diseases.

Strategies to counteract adverse remodeling of vascular graft: A 3D view of current graft innovations

Vascular grafts are widely used for vascular surgeries, to bypass a diseased artery or function as a vascular access for hemodialysis. Bioengineered or tissue-engineered vascular grafts have long been envisioned to take the place of bioinert synthetic grafts and even vein grafts under certain clinical circumstances. However, host responses to a graft device induce adverse remodeling, to varied degrees depending on the graft property and host's developmental and health conditions. This in turn leads to invention or failure. Herein, we have mapped out the relationship between the design constraints and outcomes for vascular grafts, by analyzing impairment factors involved in the adverse graft remodeling. Strategies to tackle these impairment factors and counteract adverse healing are then summarized by outlining the research landscape of graft innovations in three dimensions-cell technology, scaffold technology and graft translation. Such a comprehensive view of cell and scaffold technological innovations in the translational context may benefit the future advancements in vascular grafts. From this perspective, we conclude the review with recommendations for future design endeavors.

Identification of histological threshold concepts in health sciences curricula: Students' perception

Students' metacognitive skills and perceptions are considered important variables for high-quality learning. In this study, students' perceptions were used to identify histological threshold concepts (integrative, irreversible, transformative, and troublesome) in three health sciences curricula. A specific questionnaire was developed and validated to characterize students' perceptions of histological threshold concepts. A sample of 410 undergraduate students enrolled in the dentistry, medicine, and pharmacy degree programs participated in the study. Concepts assessed in the study were clustered to ten categories (factors) by exploratory and confirmatory factor analysis. Concepts linked to tissue organization and tissue functional states received the highest scores from students in all degree programs, suggesting that the process of learning histology requires the integration of both static concepts related to the constituent elements of tissues and dynamic concepts such as stem cells as a tissue renewal substrate, or the euplasic, proplasic and retroplasic states of tissues. The complexity of integrating static and dynamic concepts may pose a challenging barrier to the comprehension of histology. In addition, several differences were detected among the students in different degree programs. Dentistry students more often perceived morphostructural concepts as threshold concepts, whereas medical students highlighted concepts related to two-dimensional microscopic identification. Lastly, pharmacy students identified concepts related to tissue general activity as critical for the comprehension and learning of histology. The identification of threshold concepts through students' perceptions is potentially useful to improve the teaching and learning process in health sciences curricula.

One-Step Bioprinting of Multi-Channel Hydrogel Filaments Using Chaotic Advection: Fabrication of Pre-Vascularized Muscle-Like Tissues

The biofabrication of living constructs containing hollow channels is critical for manufacturing thick tissues. However, current technologies are limited in their effectiveness in the fabrication of channels with diameters smaller than hundreds of micrometers. It is demonstrated that the co-extrusion of cell-laden hydrogels and sacrificial materials through printheads containing Kenics static mixing elements enables the continuous and one-step fabrication of thin hydrogel filaments (1 mm in diameter) containing dozens of hollow microchannels with widths as small as a single cell. Pre-vascularized skeletal muscle-like filaments are bioprinted by loading murine myoblasts (C2C12 cells) in gelatin methacryloyl - alginate hydrogels and using hydroxyethyl cellulose as a sacrificial material. Higher viability and metabolic activity are observed in filaments with hollow multi-channels than in solid constructs. The presence of hollow channels promotes the expression of Ki67 (a proliferation biomarker), mitigates the expression of hypoxia-inducible factor 1-alpha , and markedly enhances cell alignment (i.e., 82% of muscle myofibrils aligned (in ±10°) to the main direction of the microchannels after seven days of culture). The emergence of sarcomeric α-actin is verified through immunofluorescence and gene expression. Overall, this work presents an effective and practical tool for the fabrication of pre-vascularized engineered tissues.

Decellularization of Porcine Carotid Arteries: From the Vessel to the High-Quality Scaffold in Five Hours

The use of biologically derived vessels as small-diameter vascular grafts in vascular diseases is currently intensely studied. Vessel decellularization provides a biocompatible scaffold with very low immunogenicity that avoids immunosuppression after transplantation. Good scaffold preservation is important as it facilitates successful cell repopulation. In addition, mechanical characteristics have to be carefully evaluated when the graft is intended to be used as an artery due to the high pressures the vessel is subjected to. Here, we present a new and fast decellularization protocol for porcine carotid arteries, followed by investigation of the quality of obtained vessel scaffolds in terms of maintenance of important extracellular matrix components, mechanical resistance, and compatibility with human endothelial cells. Our results evidence that our decellularization protocol minimally alters both the presence of scaffold proteins and their mechanical behavior and human endothelial cells could adhere to the scaffold in vitro. We conclude that if a suitable protocol is used, a high-quality decellularized arterial scaffold of non-human origin can be promptly obtained, having a great potential to be recellularized and used as an arterial graft in transplantation medicine.

Construction of multifunctional cell aggregates in angiogenesis and osteogenesis through incorporating hVE-cad-Fc-modified PLGA/β-TCP microparticles for enhancing bone regeneration

Multicellular aggregates have been widely utilized for regenerative medicine; however, the heterogeneous structure and undesired bioactivity of cell-only aggregates hinder their clinical translation. In this study, we fabricated an innovative kind of microparticle-integrated cellular aggregate with multifunctional activities in angiogenesis and osteogenesis, by combining stem cells from human exfoliated deciduous teeth (SHEDs) and bioactive composite microparticles. The poly(lactide-co-glycolide) (PLGA)-based bioactive microparticles (PTV microparticles) were ∼15 μm in diameter, with dispersed β-tricalcium phosphate (β-TCP) nanoparticles and surface-modified vascular endothelialcadherin fusion protein (hVE-cad-Fc). After co-culturing with microparticles in U-bottomed culture plates, SHEDs could firmly attach to the microparticles with a homogeneous distribution. The PTV microparticle-integrated SHED aggregates (PTV/SHED aggregates) showed significant positive CD31 and ALP expression, as well as the significantly upregulated osteogenesis makers (Runx2, ALP, and OCN) and angiogenesis makers (Ang-1 and CD31), compared with PLGA, PLGA/β-TCP (PT) and PLGA/hVE-cad-Fc (PV) microparticle-integrated SHED aggregates. Finally, in mice, 3 mm calvarial defects filled with the PTV microparticle-integrated SHED aggregates achieved abundant vascularized neo-bone regeneration within 4 weeks. Overall, we believe that these multifunctional PTV/SHED aggregates could be used as modules for bottom-up regenerative medicine, and provide a promising method for vascularized bone regeneration.

An encapsulated fibrin-based bioartificial tissue construct with integrated macrovessels, microchannels and capillary tubes

Facilitating sufficient nutrient and oxygen supply in large-scale bioartificial constructs is a critical step in organ bioengineering. Immediate perfusion not only depends on a dense capillary network, but also requires integrated large-diameter vessels that allow vascular anastomoses during implantation. These requirements set high demands for matrix generation as well as for in vitro cultivation techniques and remain mostly unsolved challenges up until today. Additionally, bioartificial constructs must have sufficient biomechanical stability to withstand mechanical stresses during and after implantation. We developed a bioartificial tissue construct with a fibrin matrix containing human umbilical vein endothelial cells and adipose tissue-derived stem cells facilitating capillary-like network formation. This core matrix was surrounded by a dense acellular fibrin capsule providing biomechanical stability. Two fibrin-based macrovessels were integrated on each side of the construct and interconnected via four 1.2 mm thick microchannels penetrating the cellularized core matrix. After four days of perfusion in a custom-built bioreactor, homogenous capillary-like network formation throughout the core matrix was observed. The fibrin capsule stabilized the core matrix and facilitated the generation of a self-supporting construct. Thus, the encapsulated fibrin tissue construct could provide a universal pre-vascularized matrix for seeding with different cell types in various tissue engineering approaches. This article is protected by copyright. All rights reserved.

Microvascular development in the rat arteriovenous loop model in vivo-A step by step intravital microscopy analysis

The arteriovenous (AV) loop model is a key technique to solve one of the major problems of tissue engineering-providing adequate vascular support for a tissue construct of significant size. However, the molecular and cellular mechanisms of vascularization and factors influencing the generation of new tissue in the AV loop are still poorly understood. We previously established a novel intravital microscopy approach to study these events. In this study, we implanted our observation chamber filled with two types of hydrogels such as fibrin and methacrylate gelatin (GelMA) and performed intravital microscopy (IVM) on days 7, 14, and 21. Initial microvessel formation was observed in GelMA on day 14, while the vessel network showed clear indicators of network rearrangement and maturation on day 21. No visible microvessels were observed in fibrin. The chambers were explanted on day 21. Histological examination revealed higher numbers of microvessels in GelMA compared to fibrin, while the AV loop was thrombosed in all fibrin constructs, possibly due to matrix degradation. GelMA proved to be an ideal matrix for IVM studies in the AV loop model due to its slow degradation and transparency. This IVM model can be employed as a novel tool for live and thus faster comprehension of crucial events in the tissue regeneration process, which can improve tissue engineering application.

In Vivo Evaluation of Gamma-Irradiated and Heparin-Immobilized Small-Diameter Polycaprolactone Vascular Grafts with VEGF in Aged Rats

The effectiveness of small-diameter vascular grafts depends on their antithrombogenic properties and ability to undergo accelerated endothelialization. The extreme hydrophobic nature of poly(ε-caprolactone) (PCL) hinders vascular tissue integration, limiting its use in medical implants. To enhance the antithrombogenicity of PCL as a biomaterial, we grafted 2-aminoethyl methacrylate (AEMA) hydrochloride onto the PCL surface using gamma irradiation; developed a biodegradable heparin-immobilized PCL nanofibrous scaffold using gamma irradiation and N-(3-dimethylaminopropyl)-N′-ethyl carbodiimide hydrochloride/N-hydroxysuccinimide reaction chemistry; and incorporated vascular endothelial growth factor (VEGF) into the scaffold to promote vascular endothelial cell proliferation and prevent thrombosis on the vascular grafts. We assessed the physicochemical properties of PCL, heparin-AEMA-PCL (H-PCL), and VEGF-loaded heparin-AEMA-PCL (VH-PCL) vascular grafts using scanning electron microscopy, attenuated total reflection–Fourier transform infrared spectroscopy, toluidine blue O staining, and fibrinogen adsorption and surface wettability measurement. In addition, we implanted the vascular grafts into 24-month-old Sprague Dawley rats and evaluated them for 3 months. The H-PCL and VH-PCL vascular grafts improved the recovery of blood vessel function by promoting the proliferation of endothelial cells and preventing thrombosis in clinical and histological evaluation, indicating their potential to serve as functional vascular grafts in vascular tissue engineering.

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 Bilayered, Electrospun Poly(Glycerol-Sebacate)/Polyurethane-Polyurethane Scaffold for Engineering of Endothelial Basement Membrane

In the cardiovascular system, heart valves and vessels are subjected to continuous cyclic mechanical loadings due to the pulsatile nature of blood flow. Hence, in leveraging tissue engineering (TE) strategies to regenerate such a system, the candidate scaffold should not only be biocompatible with the desired biodegradation rate, but it should also be mechanically competent to provide a supportive structure for facilitating stem cells retention, growth, and differentiation. To this end, herein, we introduced a novel scaffold composed of poly(glycerol-sebacate) (PGS) and polyurethane (PU), which comprises of two layers: an electrospun pure PU layer beneath another electrospun PGS/PU layer with a different ratio of PGS to PU (3:2, 1:1, 2:3 Wt:Wt). The electrospun PGS/PU-PU scaffold was mechanically competent and showed intended hydrophilicity and a good biodegradation rate. Moreover, the PGS/PU-PU scaffold indicated cell viability and proliferation within ten days of in vitro cell culture and upon 7 day vascular endothelial growth factor (VEGF) stimulation, supported endothelial differentiation of mesenchymal stem cells by significant overexpression of platelet-endothelial cell adhesion molecule-1, von Willebrand factor, and VEGF receptor 2. The results of this study could be implemented in cardiovascular TE strategies when regeneration of blood vessel or heart valve is desired.

Vessel grafts for tissue engineering revisited-Vessel segments show location-specific vascularization patterns in ex vivo ring assay

OBJECTIVE

Transplantation of prefabricated tissue-engineered flaps can be a potential alternative for healing large tissue defects. Providing adequate vascular supply for an engineered tissue construct is one of the key points in establishing successful tissue engineering-based treatment approaches. In tissue engineering-based vascularization techniques like the arteriovenous loop, vascular grafts with high angiogenic potential can help to enhance neovascularization and tissue formation. Therefore, our study aimed to compare the angiogenic potential of vascular grafts from different locations in the rat.

METHODS

The angiogenic activity was investigated by an ex vivo vessel outgrowth ring assay using 1-mm height vascular segments embedded in fibrin for 2 weeks.

RESULTS

Maximum vessel outgrowth was observed on Days 10-12. Upper extremity vessels exhibited stronger outgrowth than lower extremity vessels. Moreover, arterial vessels demonstrated higher angiogenic potential compared with venous vessels.

CONCLUSION

Collectively, our ex vivo findings suggest that upper extremity arterial vessels have a higher angiogenic capacity, which could be used to improve neovascularization and tissue formation in tissue engineering.

PLGA/β-TCP composite scaffold incorporating cucurbitacin B promotes bone regeneration by inducing angiogenesis

Objectives

Vascularization is an essential step in successful bone tissue engineering. The induction of angiogenesis in bone tissue engineering can be enhanced through the delivery of therapeutic agents that stimulate vessel and bone formation. In this study, we show that cucurbitacin B (CuB), a tetracyclic terpene derived from Cucurbitaceae family plants, facilitates the induction of angiogenesis in vitro.

Methods

We incorporated CuB into a biodegradable poly (lactide-co-glycolide) (PLGA) and β-tricalcium phosphate (β-TCP) biomaterial scaffold (PT/CuB) Using 3D low-temperature rapid prototyping (LT-RP) technology. A rat skull defect model was used to verify whether the drug-incorporated scaffold has the effects of angiogenesis and osteogenesis in vivo for the regeneration of bone defect. Cytotoxicity assay was performed to determine the safe dose range of the CuB. Tube formation assay and western blot assay were used to analyze the angiogenesis effect of CuB.

Results

PT/CuB scaffold possessed well-designed bio-mimic structure and improved mechanical properties. CuB was linear release from the composite scaffold without affecting pH value. The results demonstrated that the PT/CuB scaffold significantly enhanced neovascularization and bone regeneration in a rat critical size calvarial defect model compared to the scaffold implants without CuB. Furthermore, CuB stimulated angiogenic signaling via up-regulating VEGFR2 and VEGFR-related signaling pathways.

Conclusion

CuB can serve as promising candidate compound for promoting neovascularization and osteogenesis, especially in tissue engineering for repair of bone defects.

The translational potential of this article

This study highlights the potential use of CuB as a therapeutic agent and strongly support its adoption as a component of composite scaffolds for tissue-engineering of bone repair.

Engineering Tissue-Specific, Multiscale Microvasculature with a Capillary Network for Prevascularized Tissue

Although there are various pre-existing technologies for engineering vasculatures, multiscale modeling of the architecture of human vasculature at a capillary scale remains a challenge. In this study, a novel technology is developed for the production of a functional, multiscale microvasculature comprising of endothelialized channels and tissue-specific capillary networks. Perfusable, endothelialized channels are bioprinted, after which angiogenic sprouts are grown into user-designed capillary networks. The induction of branched and liver-lobule-like capillary networks confirm that the technology can produce various types of tissue-specific multiscale microvasculatures. Further, the channels and capillaries are deemed to be functional when evaluated in vitro. An ex vivo assay demonstrates that the microvasculature can induce neovessel ingrowth, integrate with host vessels, and facilitate blood flow. Remarkably, blood flows through the implanted capillary network without any change in its morphology. Finally, the technology is applied to produce a vascularized liver tissue; it significantly improves its hepatic function. It is believed that this new technology will create new possibilities in the development of highly vascularized and functional tissues/organs on a clinically relevant scale.

Generation of a novel model of bioengineered human oral mucosa with increased vascularization potential

Objective

The aim of this study was to generate novel models of bioartificial human oral mucosa with increased vascularization potential for future use as an advanced therapies medicinal product, by using different vascular and mesenchymal stem cell sources.

Background

Oral mucosa substitutes could contribute to the clinical treatment of complex diseases affecting the oral cavity. Although several models of artificial oral mucosa have been described, biointegration is a major issue that could be favored by the generation of novel substitutes with increased vascularization potential once grafted in vivo.

Methods

Three types of mesenchymal stem cells (MSCs) were obtained from adipose tissue, bone marrow, and dental pulp, and their in vitro potential was evaluated by inducing differentiation to the endothelial lineage using conditioning media. Then, 3D models of human artificial oral mucosa were generated using biocompatible fibrin‐agarose biomaterials combined with human oral mucosa fibroblasts and each type of MSC before and after induction to the endothelial lineage, using human umbilical vein endothelial cells (HUVEC) as controls. The vascularization potential of each oral mucosa substitute was assessed in vitro and in vivo in nude mice.

Results

In vitro induction of MSCs kept in culture was able to increase the expression of VEGF, CD31, and vWF endothelial markers, especially in bone marrow and dental pulp‐MSCs, and numerous proteins with a role in vasculogenesis become overexpressed. Then, in vivo grafting resulted in a significant increase in blood vessels formation at the interface area between the graft and the host tissues, with significantly positive expression of VEGF, CD31, vWF, and CD34 as compared to negative controls, especially when pre‐differentiated MSCs derived from bone marrow and dental pulp were used. In addition, a significantly higher number of cells committed to the endothelial lineage expressing the same endothelial markers were found within the bioartificial tissue.

Conclusion

Our results suggest that the use of pre‐differentiated MSCs could contribute to a rapid generation of a vascular network that may favor in vivo biointegration of bioengineered human oral mucosa substitutes.

Synthetic extracellular matrices with tailored adhesiveness and degradability support lumen formation during angiogenic sprouting

A major deficit in tissue engineering strategies is the lack of materials that promote angiogenesis, wherein endothelial cells from the host vasculature invade the implanted matrix to form new blood vessels. To determine the material properties that regulate angiogenesis, we have developed a microfluidic in vitro model in which chemokine-guided endothelial cell sprouting into a tunable hydrogel is followed by the formation of perfusable lumens. We show that long, perfusable tubes only develop if hydrogel adhesiveness and degradability are fine-tuned to support the initial collective invasion of endothelial cells and, at the same time, allow for matrix remodeling to permit the opening of lumens. These studies provide a better understanding of how cell-matrix interactions regulate angiogenesis and, therefore, constitute an important step towards optimal design criteria for tissue-engineered materials that require vascularization.

Current tissue engineering strategies lack materials that promote angiogenesis. Here the authors develop a microfluidic in vitro model in which chemokine-guided endothelial cell sprouting into a tunable hydrogel is followed by the formation of perfusable lumens to determine the material properties that regulate angiogenesis.

Stepwise immobilization of keratin-dopamine conjugates and gold nanoparticles on PET sheets for potential vascular graft with the catalytic generation of nitric oxide

Gold nanoparticles(AuNPs) are capable to catalyze the nitric oxide (NO) generation from endogenous and exogenous donors, thereby promoting re-endothelialization and inhibiting intimal hyperplasia and thrombosis. Herein, keratin-dopamine conjugates were synthesized and then immobilized on the surface of the pre-aminolyzed poly(ethylene terephthalate) (PET) via self-polymerization of dopamine residue, following by the formation of AuNPs in situ without extra reductant. The modified PET sheets(PET-AuNPs) could promote the growth of HUVECs while inhibit the proliferation of HUASMCs due to their catalytic generation of NO from GSNO. In addition, these sheets exhibited antibacterial properties and good blood compatibility without hemolysis. Taken together, this strategy for designing prosthetic vascular grafts to treat cardiovascular diseases has great potential.

Lotus seedpod-inspired internal vascularized 3D printed scaffold for bone tissue repair

In the field of bone defect repair, 3D printed scaffolds have the characteristics of personalized customization and accurate internal structure. However, how to construct a well-structured vascular network quickly and effectively inside the scaffold is essential for bone repair after transplantation. Herein, inspired by the unique biological structure of "lotus seedpod", hydrogel microspheres encapsulating deferoxamine (DFO) liposomes were prepared through microfluidic technology as "lotus seeds", and skillfully combined with a three-dimensional (3D) printed bioceramic scaffold with biomimetic "lotus" biological structure which can internally grow blood vessels. In this composite scaffold system, DFO was effectively released by 36% in the first 6 h, which was conducive to promote the growth of blood vessels inside the scaffold quickly. In the following 7 days, the release rate of DFO reached 69%, which was fundamental in the formation of blood vessels inside the scaffold as well as osteogenic differentiation of bone mesenchymal stem cells (BMSCs). It was confirmed that the composite scaffold could significantly promote the human umbilical vein endothelial cells (HUVECs) to form the vascular morphology within 6 h in vitro. In vivo, the composite scaffold increased the expression of vascularization and osteogenic related proteins Hif1-α, CD31, OPN, and OCN in the rat femoral defect model, significantly cutting down the time of bone repair. To sum up, this "lotus seedpod" inspired porous bioceramic 3D printed scaffold with internal vascularization functionality has broad application prospects in the future.

Coupling Osteogenesis and Vasculogenesis in Engineered Orthopedic Tissues

Inadequate vascularization of engineered tissue constructs is a main challenge in developing a clinically impactful therapy for large, complex, and recalcitrant bone defects. It is well established that bone and blood vessels form concomitantly during development, as well as during repair after injury. Endothelial cells (ECs) and mesenchymal stromal cells (MSCs) are known to be key players in orthopedic tissue regeneration and vascularization, and these cell types have been used widely in tissue engineering strategies to create vascularized bone. Coculture studies have demonstrated that there is crosstalk between ECs and MSCs that can lead to synergistic effects on tissue regeneration. At the same time, the complexity in fabricating, culturing, and characterizing engineered tissue constructs containing multiple cell types presents a challenge in creating multifunctional tissues. In particular, the timing, spatial distribution, and cell phenotypes that are most conducive to promoting concurrent bone and vessel formation are not well understood. This review describes the processes of bone and vascular development, and how these have been harnessed in tissue engineering strategies to create vascularized bone. There is an emphasis on interactions between ECs and MSCs, and the culture systems that can be used to understand and control these interactions within a single engineered construct. Developmental engineering strategies to mimic endochondral ossification are discussed as a means of generating vascularized orthopedic tissues. The field of tissue engineering has made impressive progress in creating tissue replacements. However, the development of larger, more complex, and multifunctional engineered orthopedic tissues will require a better understanding of how osteogenesis and vasculogenesis are coupled in tissue regeneration. Impact statement Vascularization of large engineered tissue volumes remains a challenge in developing new and more biologically functional bone grafts. A better understanding of how blood vessels develop during bone formation and regeneration is needed. This knowledge can then be applied to develop new strategies for promoting both osteogenesis and vasculogenesis during the creation of engineered orthopedic tissues. This article summarizes the processes of bone and blood vessel development, with a focus on how endothelial cells and mesenchymal stromal cells interact to form vascularized bone both during development and growth, as well as tissue healing. It is meant as a resource for tissue engineers who are interested in creating vascularized tissue, and in particular to those developing cell-based therapies for large, complex, and recalcitrant bone defects.

Vascularization strategies in tissue engineering approaches for soft tissue repair

Insufficient vascularization during tissue repair is often associated with poor clinical outcomes. This is a concern especially when patients have critical-sized injuries, where the size of the defect restricts vascularity, or even in small defects that have to be treated under special conditions, such as after radiation therapy (relevant to tumor resection) that hinders vascularity. In fact, poor vascularization is one of the major obstacles for clinical application of tissue engineering methods in soft tissue repair. As a key issue, lack of graft integration, caused by inadequate vascularization after implantation, can lead to graft failure. Moreover, poor vascularization compromises the viability of cells seeded in deep portions of scaffolds/graft materials, due to hypoxia and insufficient nutrient supply. In this article we aim to review vascularization strategies employed in tissue engineering techniques to repair soft tissues. For this purpose, we start by providing a brief overview of the main events during the physiological wound healing process in soft tissues. Then, we discuss how tissue repair can be achieved through tissue engineering, and considerations with regards to the choice of scaffold materials, culture conditions, and vascularization techniques. Next, we highlight the importance of vascularization, along with strategies and methods of prevascularization of soft tissue equivalents, particularly cell-based prevascularization. Lastly, we present a summary of commonly used in vitro methods during the vascularization of tissue-engineered soft tissue constructs.

Microvascular Tissue Engineering-A Review

Tissue engineering and regenerative medicine have come a long way in recent decades, but the lack of functioning vasculature is still a major obstacle preventing the development of thicker, physiologically relevant tissue constructs. A large part of this obstacle lies in the development of the vessels on a microscale-the microvasculature-that are crucial for oxygen and nutrient delivery. In this review, we present the state of the art in the field of microvascular tissue engineering and demonstrate the challenges for future research in various sections of the field. Finally, we illustrate the potential strategies for addressing some of those challenges.

Rebuilding the Vascular Network: In vivo and in vitro Approaches

As the material transportation system of the human body, the vascular network carries the transportation of materials and nutrients. Currently, the construction of functional microvascular networks is an urgent requirement for the development of regenerative medicine and in vitro drug screening systems. How to construct organs with functional blood vessels is the focus and challenge of tissue engineering research. Here in this review article, we first introduced the basic characteristics of blood vessels in the body and the mechanism of angiogenesis in vivo, summarized the current methods of constructing tissue blood vessels in vitro and in vivo, and focused on comparing the functions, applications and advantages of constructing different types of vascular chips to generate blood vessels. Finally, the challenges and opportunities faced by the development of this field were discussed.

Review: Tissue Engineering of Small-Diameter Vascular Grafts and Their In Vivo Evaluation in Large Animals and Humans

To date, a wide range of materials, from synthetic to natural or a mixture of these, has been explored, modified, and examined as small-diameter tissue-engineered vascular grafts (SD-TEVGs) for tissue regeneration either in vitro or in vivo. However, very limited success has been achieved due to mechanical failure, thrombogenicity or intimal hyperplasia, and improvements of the SD-TEVG design are thus required. Here, in vivo studies investigating novel and relative long (10 times of the inner diameter) SD-TEVGs in large animal models and humans are identified and discussed, with emphasis on graft outcome based on model- and graft-related conditions. Only a few types of synthetic polymer-based SD-TEVGs have been evaluated in large-animal models and reflect limited success. However, some polymers, such as polycaprolactone (PCL), show favorable biocompatibility and potential to be further modified and improved in the form of hybrid grafts. Natural polymer- and cell-secreted extracellular matrix (ECM)-based SD-TEVGs tested in large animals still fail due to a weak strength or thrombogenicity. Similarly, native ECM-based SD-TEVGs and in-vitro-developed hybrid SD-TEVGs that contain xenogeneic molecules or matrix seem related to a harmful graft outcome. In contrast, allogeneic native ECM-based SD-TEVGs, in-vitro-developed hybrid SD-TEVGs with allogeneic banked human cells or isolated autologous stem cells, and in-body tissue architecture (IBTA)-based SD-TEVGs seem to be promising for the future, since they are suitable in dimension, mechanical strength, biocompatibility, and availability.

Scaffold-free tissue-engineered arterial grafts derived from human skeletal myoblasts

Tissue-engineered vascular grafts (TEVGs) are in urgent demand for both adult and pediatric patients. Although several approaches have utilized vascular smooth muscle cells (SMCs) and endothelial cells as cell sources for TEVGs, these cell sources have a limited proliferative capacity that results in an inability to reconstitute neotissues. Skeletal myoblasts are attractive cell sources as they possess high proliferative capacity, and they are already being tested in clinical trials for patients with ischemic cardiomyopathy. Our previous study demonstrated that periodic hydrostatic pressurization (PHP) promoted fibronectin fibrillogenesis in vascular SMCs, and that PHP-induced extracellular matrix (ECM) arrangements enabled the fabrication of implantable arterial grafts derived from SMCs without using a scaffold material. We assessed the molecular response of human skeletal myoblasts to PHP exposure, and aimed to fabricate arterial grafts from the myoblasts by exposure to PHP. To examine the PHP-response genes, human skeletal myoblasts were subjected to bulk RNA-sequencing after PHP exposure. Gene-set enrichment analysis revealed significant positive correlations between PHP exposure and vascular development-related genes. Real-time polymerase chain reaction (RT-PCR) demonstrated that PHP significantly upregulated collagen and elastic fiber formation-related gene expression, such as fibronectin, lysyl oxidase, collagen type I α1, collagen type IV α1, and tropoelastin. Based on these findings showing the potential role of PHP in vessel formation, we fabricated arterial grafts by repeated cell seeding and exposure to PHP every 24 hours. The resultant 15-layered myoblast grafts had high collagen content, which provided a tensile rupture strength of 899 ± 104 mm Hg. Human skeletal myoblast grafts were implanted as patch grafts in the aorta of immunosuppressed rats and found to be endothelialized and completely patent until the endpoint of 60 postoperative days. Implanted human myoblasts were gradually replaced by host-derived cells, which successfully formed vascular neotissues with layered elastic fibers. These findings suggest that human skeletal myoblasts have the potential to be a feasible cell source for scaffold-free implantable arterial grafts under PHP culture conditions.

Challenges and Possibilities of Cell-Based Tissue-Engineered Vascular Grafts

There is urgent demand for biologically compatible vascular grafts for both adult and pediatric patients. The utility of conventional nonbiodegradable materials is limited because of their thrombogenicity and inability to grow, while autologous vascular grafts involve considerable disadvantages, including the invasive procedures required to obtain these healthy vessels from patients and insufficient availability in patients with systemic atherosclerosis. All of these issues could be overcome by tissue-engineered vascular grafts (TEVGs). A large body of evidence has recently emerged in support of TEVG technologies, introducing diverse cell sources (e.g., somatic cells and stem cells) and novel fabrication methods (e.g., scaffold-guided and self-assembled approaches). Before TEVG can be applied in a clinical setting, however, several aspects of the technology must be improved, such as the feasibility of obtaining cells, their biocompatibility and mechanical properties, and the time needed for fabrication, while the safety of supplemented materials, the patency and nonthrombogenicity of TEVGs, their growth potential, and the long-term influence of implanted TEVGs in the body must be assessed. Although recent advances in TEVG fabrication have yielded promising results, more research is needed to achieve the most feasible methods for generating optimal TEVGs. This article reviews multiple aspects of TEVG fabrication, including mechanical requirements, extracellular matrix components, cell sources, and tissue engineering approaches. The potential of periodic hydrostatic pressurization in the production of scaffold-free TEVGs with optimal elasticity and stiffness is also discussed. In the future, the integration of multiple technologies is expected to enable improved TEVG performance.

Evaluation of Marine Agarose Biomaterials for Tissue Engineering Applications

Five agarose types (D1LE, D2LE, LM, MS8 and D5) were evaluated in tissue engineering and compared for the first time using an array of analysis methods. Acellular and cellular constructs were generated from 0.3–3%, and their biomechanical properties, in vivo biocompatibility (as determined by LIVE/DEAD, WST-1 and DNA release, with n = 6 per sample) and in vivo biocompatibility (by hematological and biochemical analyses and histology, with n = 4 animals per agarose type) were analyzed. Results revealed that the biomechanical properties of each hydrogel were related to the agarose concentration (p < 0.001). Regarding the agarose type, the highest (p < 0.001) Young modulus, stress at fracture and break load were D1LE, D2LE and D5, whereas the strain at fracture was higher in D5 and MS8 at 3% (p < 0.05). All agaroses showed high biocompatibility on human skin cells, especially in indirect contact, with a correlation with agarose concentration (p = 0.0074 for LIVE/DEAD and p = 0.0014 for WST-1) and type, although cell function tended to decrease in direct contact with highly concentrated agaroses. All agaroses were safe in vivo, with no systemic effects as determined by hematological and biochemical analysis and histology of major organs. Locally, implants were partially encapsulated and a pro-regenerative response with abundant M2-type macrophages was found. In summary, we may state that all these agarose types can be safely used in tissue engineering and that the biomechanical properties and biocompatibility were strongly associated to the agarose concentration in the hydrogel and partially associated to the agarose type. These results open the door to the generation of specific agarose-based hydrogels for definite clinical applications such as the human skin, cornea or oral mucosa.

Engineering pericyte-supported microvascular capillaries in cell-laden hydrogels using stem cells from the bone marrow, dental pulp and dental apical papilla

Engineered tissue constructs require the fabrication of highly perfusable and mature vascular networks for effective repair and regeneration. In tissue engineering, stem cells are widely employed to create mature vascularized tissues in vitro. Pericytes are key to the maturity of these vascular networks, and therefore the ability of stem cells to differentiate into pericyte-like lineages should be understood. To date, there is limited information regarding the ability of stem cells from the different tissue sources to differentiate into pericytes and form microvascular capillaries in vitro. Therefore, here we tested the ability of the stem cells derived from bone marrow (BMSC), dental pulp (DPSC) and dental apical papilla (SCAP) to engineer pericyte-supported vascular capillaries when encapsulated along with human umbilical vein endothelial cells (HUVECs) in gelatin methacrylate (GelMA) hydrogel. Our results show that the pericyte differentiation capacity of BMSC was greater with high expression of α-SMA and NG2 positive cells. DPSC had α-SMA positive cells but showed very few NG2 positive cells. Further, SCAP cells were positive for α-SMA while they completely lacked NG2 positive cells. We found the pericyte differentiation ability of these stem cells to be different, and this significantly affected the vasculogenic ability and quality of the vessel networks. In summary, we conclude that, among stem cells from different craniofacial regions, BMSCs appear more suitable for engineering of mature vascularized networks than DPSCs or SCAPs.

3D Composite Bioprinting for Fabrication of Artificial Biological Tissues

Three-dimensional (3D) bioprinting is an important technology for fabricating artificial tissue. To effectively reconstruct the multiscale structure and multi-material gradient of natural tissues and organs, 3D bioprinting has been increasingly developed into multi-process composite mode. The current 3D composite bioprinting is a combination of two or more printing processes, and oftentimes, physical field regulation that can regulate filaments or cells during or after printing may be involved. Correspondingly, both path planning strategy and process control all become more complex. Hence, the computer-aided design and computer-aided manufacturing (CAD/CAM) system that is traditionally used in 3D printing system is now facing challenges. Thus, the scale information that cannot be modeled in the CAD process should be considered in the design of CAM by adding a process management module in the traditional CAD/CAM system and add more information reflecting component gradient in the path planning strategy.