Engineering pulmonary vasculature in decellularized rat and human lungs

Advancing Organoid Engineering for Tissue Regeneration and Biofunctional Reconstruction

Tissue damage and functional abnormalities in organs have become a considerable clinical challenge. Organoids are often applied as disease models and in drug discovery and screening. Indeed, several studies have shown that organoids are an important strategy for achieving tissue repair and biofunction reconstruction. In contrast to established stem cell therapies, organoids have high clinical relevance. However, conventional approaches have limited the application of organoids in clinical regenerative medicine. Engineered organoids might have the capacity to overcome these challenges. Bioengineering-a multidisciplinary field that applies engineering principles to biomedicine-has bridged the gap between engineering and medicine to promote human health. More specifically, bioengineering principles have been applied to organoids to accelerate their clinical translation. In this review, beginning with the basic concepts of organoids, we describe strategies for cultivating engineered organoids and discuss the multiple engineering modes to create conditions for breakthroughs in organoid research. Subsequently, studies on the application of engineered organoids in biofunction reconstruction and tissue repair are presented. Finally, we highlight the limitations and challenges hindering the utilization of engineered organoids in clinical applications. Future research will focus on cultivating engineered organoids using advanced bioengineering tools for personalized tissue repair and biofunction reconstruction.

Orthotopic transplantation of the bioengineered lung using a mouse-scale perfusion-based bioreactor and human primary endothelial cells

Whole lung engineering and the transplantation of its products is an ambitious goal and ultimately a viable solution for alleviating the donor-shortage crisis for lung transplants. There are several limitations currently impeding progress in the field with a major obstacle being efficient revascularization of decellularized scaffolds, which requires an extremely large number of cells when using larger pre-clinical animal models. Here, we developed a simple but effective experimental pulmonary bioengineering platform by utilizing the lung as a scaffold. Revascularization of pulmonary vasculature using human umbilical cord vein endothelial cells was feasible using a novel in-house developed perfusion-based bioreactor. The endothelial lumens formed in the peripheral alveolar area were confirmed using a transmission electron microscope. The quality of engineered lung vasculature was evaluated using box-counting analysis of histological images. The engineered mouse lungs were successfully transplanted into the orthotopic thoracic cavity. The engineered vasculature in the lung scaffold showed blood perfusion after transplantation without significant hemorrhage. The mouse-based lung bioengineering system can be utilized as an efficient ex-vivo screening platform for lung tissue engineering.

Advances in the differentiation of pluripotent stem cells into vascular cells

Blood vessels constitute a closed pipe system distributed throughout the body, transporting blood from the heart to other organs and delivering metabolic waste products back to the lungs and kidneys. Changes in blood vessels are related to many disorders like stroke, myocardial infarction, aneurysm, and diabetes, which are important causes of death worldwide. Translational research for new approaches to disease modeling and effective treatment is needed due to the huge socio-economic burden on healthcare systems. Although mice or rats have been widely used, applying data from animal studies to human-specific vascular physiology and pathology is difficult. The rise of induced pluripotent stem cells (iPSCs) provides a reliable in vitro resource for disease modeling, regenerative medicine, and drug discovery because they carry all human genetic information and have the ability to directionally differentiate into any type of human cells. This review summarizes the latest progress from the establishment of iPSCs, the strategies for differentiating iPSCs into vascular cells, and the in vivo transplantation of these vascular derivatives. It also introduces the application of these technologies in disease modeling, drug screening, and regenerative medicine. Additionally, the application of high-tech tools, such as omics analysis and high-throughput sequencing, in this field is reviewed.

Decellularization of Massive Bone Allografts By Perfusion: A New Protocol for Tissue Engineering

In terms of large bone defect reconstructions, massive bone allografts may sometimes be the only solution. However, they are still burdened with a high postoperative complication rate. Our hypothesis is that the immunogenicity of residual cells in the graft is involved in this issue. Decellularization by perfusion might therefore be the answer to process and create more biologically effective massive bone allografts. Seventy-two porcine bones were used to characterize the efficiency of our sodium hydroxide-based decellularization protocol. A sequence of solvent perfusion through each nutrient artery was set up to ensure the complete decellularization of whole long bones. Qualitative (histology and immunohistochemistry [IHC]) and quantitative (fluoroscopic absorbance and enzyme-linked immunosorbent assay) evaluations were performed to assess the decellularization and the preservation of the extracellular matrix in the bone grafts. Cytotoxicity and compatibility were also tested. Comparatively to nontreated bones, our experiments showed a very high decellularization quality, demonstrating that perfusion is mandatory to achieve an entire decellularization. Moreover, results showed a good preservation of the bone composition and microarchitecture, Haversian systems and vascular network included. This protocol reduces the human leukocyte antigen antigenic load of the graft by >50%. The majority of measured growth factors is still present in the same amount in the decellularized bones compared to the nontreated bones. Histology and IHC show that the bones were cell compatible, noncytotoxic, and capable of inducing osteoblastic differentiation of mesenchymal stem cells. Our decellularization/perfusion protocol allowed to create decellularized long bone graft models, thanks to their inner vascular network, ready for in vivo implantation or to be further used as seeding matrices.

Biomedical Application of Decellularized Scaffolds

Tissue loss and end-stage organ failure are serious health problems across the world. Natural and synthetic polymer scaffold material based artificial organs play an important role in the field of tissue engineering and organ regeneration, but they are not from the body and may cause side effects such as rejection. In recent years, the biomimetic decellularized scaffold based materials have drawn great attention in the tissue engineering field for their good biocompatibility, easy modification, and excellent organism adaptability. Therefore, in this review, we comprehensively summarize the application of decellularized scaffolds in tissue engineering and biomedicine in recent years. The preparation methods, modification strategies, construction of artificial tissues, and application in biomedical applications are discussed. We hope that this review will provide a useful reference for research on decellularized scaffolds and promote their application tissue engineering.

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.

Pushing the boundaries of innovation: the potential of ex vivo organ perfusion from an interdisciplinary point of view

Ex vivo machine perfusion (EVMP) is an emerging technique for preserving explanted solid organs with primary application in allogeneic organ transplantation. EVMP has been established as an alternative to the standard of care static-cold preservation, allowing for prolonged preservation and real-time monitoring of organ quality while reducing/preventing ischemia-reperfusion injury. Moreover, it has paved the way to involve expanded criteria donors, e.g., after circulatory death, thus expanding the donor organ pool. Ongoing improvements in EVMP protocols, especially expanding the duration of preservation, paved the way for its broader application, in particular for reconditioning and modification of diseased organs and tumor and infection therapies and regenerative approaches. Moreover, implementing EVMP for in vivo-like preclinical studies improving disease modeling raises significant interest, while providing an ideal interface for bioengineering and genetic manipulation. These approaches can be applied not only in an allogeneic and xenogeneic transplant setting but also in an autologous setting, where patients can be on temporary organ support while the diseased organs are treated ex vivo, followed by reimplantation of the cured organ. This review provides a comprehensive overview of the differences and similarities in abdominal (kidney and liver) and thoracic (lung and heart) EVMP, focusing on the organ-specific components and preservation techniques, specifically on the composition of perfusion solutions and their supplements and perfusion temperatures and flow conditions. Novel treatment opportunities beyond organ transplantation and limitations of abdominal and thoracic EVMP are delineated to identify complementary interdisciplinary approaches for the application and development of this technique.

Current status and future prospects of decellularized kidney tissue

End-stage renal disease (ESRD) is characterized by progressive loss of kidney function, which can result in damage to various tissues and organs. Dialysis therapy and kidney transplantation are currently the only therapeutic options available for patients with ESRD. In the case of kidney transplantation, organ shortage and high organ rejection have increased the need for novel treatment modalities. Therefore, organ regeneration employing decellularization technology has emerged as a viable alternative to conventional organ transplantation. In this technology, organs are decellularized using physical, chemical, or biological means to create a natural scaffold and foundation for cell seeding. After in vivo transplantation, this scaffold can be recellularized using stem cells or adult differentiated cells, resulting in a functional organ devoid of immune response. This review focuses on the primary agents used for renal decellularization and the current status of kidney regeneration using decellularization.

Human Lung Organoids-A Novel Experimental and Precision Medicine Approach

The global burden of respiratory diseases is very high and still on the rise, prompting the need for accurate models for basic and translational research. Several model systems are currently available ranging from simple airway cell cultures to complex tissue-engineered lungs. In recent years, human lung organoids have been established as highly transferrable three-dimensional in vitro model systems for lung research. For acute infectious and chronic inflammatory diseases as well as lung cancer, human lung organoids have opened possibilities for precise in vitro research and a deeper understanding of mechanisms underlying lung injury and regeneration. Human lung organoids from induced pluripotent stem cells or from adult stem cells of patients' samples introduce tools for understanding developmental processes and personalized medicine approaches. When further state-of-the-art technologies and protocols come into use, the full potential of human lung organoids can be harnessed. High-throughput assays in drug development, gene therapy, and organoid transplantation are current applications of organoids in translational research. In this review, we emphasize novel approaches in translational and personalized medicine in lung research focusing on the use of human lung organoids.

Intratracheally injected human-induced pluripotent stem cell-derived pneumocytes and endothelial cells engraft in the distal lung and ameliorate emphysema in a rat model

OBJECTIVES

Pulmonary emphysema is characterized by the destruction of alveolar units and reduced gas exchange capacity. In the present study, we aimed to deliver induced pluripotent stem cell-derived endothelial cells and pneumocytes to repair and regenerate distal lung tissue in an elastase-induced emphysema model.

METHODS

We induced emphysema in athymic rats via intratracheal injection of elastase as previously reported. At 21 and 35 days after elastase treatment, we suspended 80 million induced pluripotent stem cell-derived endothelial cells and 20 million induced pluripotent stem cell-derived pneumocytes in hydrogel and injected the mixture intratracheally. On day 49 after elastase treatment, we performed imaging, functional analysis, and collected lungs for histology.

RESULTS

Using immunofluorescence detection of human-specific human leukocyte antigen 1, human-specific CD31, and anti--green fluorescent protein for the reporter labeled pneumocytes, we found that transplanted cells engrafted in 14.69% ± 0.95% of the host alveoli and fully integrated to form vascularized alveoli together with host cells. Transmission electron microscopy confirmed the incorporation of the transplanted human cells and the formation of a blood-air barrier. Human endothelial cells formed perfused vasculature. Computed tomography scans revealed improved vascular density and decelerated emphysema progression in cell-treated lungs. Proliferation of both human and rat cell was higher in cell-treated versus nontreated controls. Cell treatment reduced alveolar enlargement, improved dynamic compliance and residual volume, and improved diffusion capacity.

CONCLUSIONS

Our findings suggest that human induced pluripotent stem cell-derived distal lung cells can engraft in emphysematous lungs and participate in the formation of functional distal lung units to ameliorate the progression of emphysema.

A new paradigm in transplant immunology: At the crossroad of synthetic biology and biomaterials

Solid organ transplant (SOT) recipients require meticulously tailored immunosuppressive regimens to minimize graft loss and mortality. Traditional approaches focus on inhibiting effector T cells, while the intricate and dynamic immune responses mediated by other components remain unsolved. Emerging advances in synthetic biology and material science have provided novel treatment modalities with increased diversity and precision to the transplantation community. This review investigates the active interface between these two fields, highlights how living and non-living structures can be engineered and integrated for immunomodulation, and discusses their potential application in addressing the challenges in SOT clinical practice.

An electrical analog permeability model assessing fluid flow in a decellularized organ

BACKGROUND AND OBJECTIVE

In recellularization, cell-seeding efficiency refers to the uniform distribution of cells across the decellularized organ, which should be enhanced to ensure effective functioning. During cell seeding, flow dynamics influence the distribution of cells because the driving force of cell movement is the fluid force. However, after decellularization, because of flow permeability through the vessel wall, the fluid pressure and velocity in the vessels of vascular trees are significantly reduced compared with those in the native organ, which might affect cell seeding efficiency. Therefore, it is necessary to assess the flow characteristics in the vessels of decellularized organs to select appropriate seeding conditions. Although electrical analog models have been widely used to investigate the flow distribution in organs, current models do not reflect the permeable conditions. This study proposes a model to extend the conventional electrical analog model to simulate the flow characteristics in decellularized organs.

METHODS

A resistor reflecting permeable flow was added to the original electrical analog model to describe the permeable conditions in the decellularized organs. Decellularization and pressure drop measurements of the kidney were also conducted for model development, calibration, and validation. The developed model was then applied to a decellularized kidney to reveal changes in flow characteristics.

RESULTS

The resistance calculation of permeable flow was determined for each generation of vascular trees. The coefficient of permeability can be indicated by the measured flow exiting through the outlet or the pressure drop across the decellularized organ. The developed permeability model had a qualitative and quantitative agreement with the experimental data without calibration. The results of the permeability model for the decellularized kidney indicated significant reductions of up to 70% in the flow rate and pressure of the organ compared to the native kidney.

CONCLUSIONS

The developed model can simulate the flow characteristics in each individual vessel of decellularized organs. The results from the model can be used to assess the optimal flow rate condition for the cell seeding process.

Rational engineering of lung alveolar epithelium

Engineered whole lungs may one day expand therapeutic options for patients with end-stage lung disease. However, the feasibility of ex vivo lung regeneration remains limited by the inability to recapitulate mature, functional alveolar epithelium. Here, we modulate multimodal components of the alveolar epithelial type 2 cell (AEC2) niche in decellularized lung scaffolds in order to guide AEC2 behavior for epithelial regeneration. First, endothelial cells coordinate with fibroblasts, in the presence of soluble growth and maturation factors, to promote alveolar scaffold population with surfactant-secreting AEC2s. Subsequent withdrawal of Wnt and FGF agonism synergizes with tidal-magnitude mechanical strain to induce the differentiation of AEC2s to squamous type 1 AECs (AEC1s) in cultured alveoli, in situ. These results outline a rational strategy to engineer an epithelium of AEC2s and AEC1s contained within epithelial-mesenchymal-endothelial alveolar-like units, and highlight the critical interplay amongst cellular, biochemical, and mechanical niche cues within the reconstituting alveolus.

Bioengineering and Clinical Translation of Human Lung and its Components

Clinical lung transplantation has rapidly established itself as the gold standard of treatment for end-stage lung diseases in a restricted group of patients since the first successful lung transplant occurred. Although significant progress has been made in lung transplantation, there are still numerous obstacles on the path to clinical success. The development of bioartificial lung grafts using patient-derived cells may serve as an alternative treatment modality; however, challenges include developing appropriate scaffold materials, advanced culture strategies for lung-specific multiple cell populations, and fully matured constructs to ensure increased transplant lifetime following implantation. This review highlights the development of tissue-engineered tracheal and lung equivalents over the past two decades, key problems in lung transplantation in a clinical environment, the advancements made in scaffolds, bioprinting technologies, bioreactors, organoids, and organ-on-a-chip technologies. The review aims to fill the lacuna in existing literature toward a holistic bioartificial lung tissue, including trachea, capillaries, airways, bifurcating bronchioles, lung disease models, and their clinical translation. Herein, the efforts are on bridging the application of lung tissue engineering methods in a clinical environment as it is thought that tissue engineering holds enormous promise for overcoming the challenges associated with the clinical translation of bioengineered human lung and its components.

Decellularized Extracellular Matrix for Remodeling Bioengineering Organoid's Microenvironment

Over the past decade, stem cell- and tumor-derived organoids are the most promising models in developmental biology and disease modeling, respectively. The matrix is one of three main elements in the construction of an organoid and the most important module of its extracellular microenvironment. However, the source of the currently available commercial matrix, Matrigel, limits the application of organoids in clinical medicine. It is worth investigating whether the original decellularized extracellular matrix (dECM) can be exploited as the matrix of organoids and improving organoid construction are very important. In this review, tissue decellularization protocols and the characteristics of decellularization methods, the mechanical support and biological cues of extraccellular matrix (ECM), methods for construction of multifunctional dECM and responsive dECM hydrogel, and the potential applications of functional dECM are summarized. In addition, some expectations are provided for dECM as the matrix of organoids in clinical applications.

Future of Lung Transplantation: Xenotransplantation and Bioengineering Lungs

Xenotransplantation promises to alleviate the issue of donor organ shortages and to decrease waiting times for transplantation. Recent advances in genetic engineering have allowed for the creation of pigs with up to 16 genetic modifications. Several combinations of genetic modifications have been associated with extended graft survival and life-supporting function in experimental heart and kidney xenotransplants. Lung xenotransplantation carries specific challenges related to the large surface area of the lung vascular bed, its innate immune system's intrinsic hyperreactivity to perceived 'danger', and its anatomic vulnerability to airway flooding after even localized loss of alveolocapillary barrier function. This article discusses the current status of lung xenotransplantation, and challenges related to immunology, physiology, anatomy, and infection. Tissue engineering as a feasible alternative to develop a viable lung replacement solution is discussed.

Repopulation of decellularized organ scaffolds with human pluripotent stem cell-derived pancreatic progenitor cells

Diabetes is an emerging global epidemic that affects more that 285 million people worldwide. Engineering of endocrine pancreas tissue holds great promise for the future of diabetes therapy. Here we demonstrate the feasibility of re-engineering decellularized organ scaffolds using regenerative cell source. We differentiated human pluripotent stem cells (hPSC) toward pancreatic progenitor (PP) lineage and repopulated decellularized organ scaffolds with these hPSC-PP cells. We observed that hPSCs cultured and differentiated as aggregates are more suitable for organ repopulation than isolated single cell suspension. However, recellularization with hPSC-PP aggregates require a more extensive vascular support, which was found to be superior in decellularized liver over the decellularized pancreas scaffolds. Upon continued culture for nine days with chemical induction in the bioreactor, the seeded hPSC-PP aggregates demonstrated extensive and uniform cellular repopulation and viability throughout the thickness of the liver scaffolds. Furthermore, the decellularized liver scaffolds was supportive of the endocrine cell fate of the engrafted cells. Our novel strategy to engineer endocrine pancreas construct is expected to find potential applications in preclinical testing, drug discovery and diabetes therapy.

Directed self-assembly of a xenogeneic vascularized endocrine pancreas for type 1 diabetes

Intrahepatic islet transplantation is the standard cell therapy for β cell replacement. However, the shortage of organ donors and an unsatisfactory engraftment limit its application to a selected patients with type 1 diabetes. There is an urgent need to identify alternative strategies based on an unlimited source of insulin producing cells and innovative scaffolds to foster cell interaction and integration to orchestrate physiological endocrine function. We previously proposed the use of decellularized lung as a scaffold for β cell replacement with the final goal of engineering a vascularized endocrine organ. Here, we prototyped this technology with the integration of neonatal porcine islet and healthy subject-derived blood outgrowth endothelial cells to engineer a xenogeneic vascularized endocrine pancreas. We validated ex vivo cell integration and function, its engraftment and performance in a preclinical model of diabetes. Results showed that this technology not only is able to foster neonatal pig islet maturation in vitro, but also to perform in vivo immediately upon transplantation and for over 18 weeks, compared to normal performance within 8 weeks in various state of the art preclinical models. Given the recent progress in donor pig genetic engineering, this technology may enable the assembly of immune-protected functional endocrine organs.

Tissue engineering of decellularized pancreas scaffolds for regenerative medicine in diabetes

Diabetes mellitus is a global disease requiring long-term treatment and monitoring. At present, pancreas or islet transplantation is the only reliable treatment for achieving stable euglycemia in Type I diabetes patients. However, the shortage of viable pancreata for transplantation limits the use of this therapy for the majority of patients. Organ decellularization and recellularization is emerging as a promising solution to overcome the shortage of viable organs for transplantation by providing a potential alternative source of donor organs. Several studies on decellularization and recellularization of rodent, porcine, and human pancreata have been performed, and show promise for generating usable decellularized pancreas scaffolds for subsequent recellularization and transplantation. In this state-of-the-art review, we provide an overview of the latest advances in pancreas decellularization, recellularization, and revascularization. We also discuss clinical considerations such as potential transplantation sites, donor source, and immune considerations. We conclude with an outlook on the remaining work that needs to be done in order to realize the goal of using this technology to create bioengineered pancreata for transplantation in diabetes patients. STATEMENT OF SIGNIFICANCE: Pancreas or islet transplantation is a means of providing insulin-independence in diabetes patients. However, due to the shortage of viable pancreata, whole-organ decellularization and recellularization is emerging as a promising solution to overcome organ shortage for transplantation. Several studies on decellularization and recellularization of rodent, porcine, and human pancreata have shown promise for generating usable decellularized pancreas scaffolds for subsequent recellularization and transplantation. In this state-of-the-art review, we highlight the latest advances in pancreas decellularization, recellularization, and revascularization. We also discuss clinical considerations such as potential transplantation sites, donor source, and immune considerations. We conclude with future work that needs to be done in order to realize clinical translation of bioengineered pancreata for transplantation in diabetes patients.

Decellularized vascularized bone grafts: A preliminary in vitro porcine model for bioengineered transplantable bone shafts

Introduction: Durable reconstruction of critical size bone defects is still a surgical challenge despite the availability of numerous autologous and substitute bone options. In this paper, we have investigated the possibility of creating a living bone allograft, using the perfusion/decellularization/recellularization (PDR) technique, which was applied to an original model of vascularized porcine bone graft. Materials and Methods: 11 porcine bone forelimbs, including radius and ulna, were harvested along with their vasculature including the interosseous artery and then decellularized using a sequential detergent perfusion protocol. Cellular clearance, vasculature, extracellular matrix (ECM), and preservation of biomechanical properties were evaluated. The cytocompatibility and in vitro osteoinductive potential of acellular extracellular matrix were studied by static seeding of NIH-3T3 cells and porcine adipose mesenchymal stem cells (pAMSC), respectively. Results: The vascularized bone grafts were successfully decellularized, with an excellent preservation of the 3D morphology and ECM microarchitecture. Measurements of DNA and ECM components revealed complete cellular clearance and preservation of ECM's major proteins. Bone mineral density (BMD) acquisitions revealed a slight, yet non-significant, decrease after decellularization, while biomechanical testing was unmodified. Cone beam computed tomography (CBCT) acquisitions after vascular injection of barium sulphate confirmed the preservation of the vascular network throughout the whole graft. The non-toxicity of the scaffold was proven by the very low amount of residual sodium dodecyl sulfate (SDS) in the ECM and confirmed by the high live/dead ratio of fibroblasts seeded on periosteum and bone ECM-grafts after 3, 7, and 16 days of culture. Moreover, cell proliferation tests showed a significant multiplication of seeded cell populations at the same endpoints. Lastly, the differentiation study using pAMSC confirmed the ECM graft's potential to promote osteogenic differentiation. An osteoid-like deposition occurred when pAMSC were cultured on bone ECM in both proliferative and osteogenic differentiation media. Conclusion: Fully decellularized bone grafts can be obtained by perfusion decellularization, thereby preserving ECM architecture and their vascular network, while promoting cell growth and differentiation. These vascularized decellularized bone shaft allografts thus present a true potential for future in vivo reimplantation. Therefore, they may offer new perspectives for repairing large bone defects and for bone tissue engineering.

Alternative lung cell model systems for toxicology testing strategies: Current knowledge and future outlook

Due to the current relevance of pulmonary toxicology (with focus upon air pollution and the inhalation of hazardous materials), it is important to further develop and implement physiologically relevant models of the entire respiratory tract. Lung model development has the aim to create human relevant systems that may replace animal use whilst balancing cost, laborious nature and regulatory ambition. There is an imperative need to move away from rodent models and implement models that mimic the holistic characteristics important in lung function. The purpose of this review is therefore, to describe and identify the various alternative models that are being applied towards assessing the pulmonary toxicology of inhaled substances, as well as the current and potential developments of various advanced models and how they may be applied towards toxicology testing strategies. These models aim to mimic various regions of the lung, as well as implementing different exposure methods with the addition of various physiologically relevent conditions (such as fluid-flow and dynamic movement). There is further progress in the type of models used with focus on the development of lung-on-a-chip technologies and bioprinting, as well as and the optimization of such models to fill current knowledge gaps within toxicology.

Angiogenesis and Re-endothelialization in decellularized scaffolds: Recent advances and current challenges in tissue engineering

Decellularization of tissues and organs has recently become a promising approach in tissue engineering and regenerative medicine to circumvent the challenges of organ donation and complications of transplantations. However, one main obstacle to reaching this goal is acellular vasculature angiogenesis and endothelialization. Achieving an intact and functional vascular structure as a vital pathway for supplying oxygen and nutrients remains the decisive challenge in the decellularization/re-endothelialization procedure. In order to better understand and overcome this issue, complete and appropriate knowledge of endothelialization and its determining variables is required. Decellularization methods and their effectiveness, biological and mechanical characteristics of acellular scaffolds, artificial and biological bioreactors, and their possible applications, extracellular matrix surface modification, and different types of utilized cells are factors affecting endothelialization consequences. This review focuses on the characteristics of endothelialization and how to optimize them, as well as discussing recent developments in the process of re-endothelialization.

Organ Regeneration Through Stem Cells and Tissue Engineering

Loss of organ and tissue due to injuries or diseases led to the development of regenerative therapies to decrease reliance on organ transplantations. It deals with employing the self-renewal ability of stem cells to differentiate into numerous lineages to assist in providing effective treatment for a range of various injuries and diseases. Regenerative engineering of organs or tissues represents an ever-expanding field that is aimed at developing biological replacements for dysfunctional organs or injured tissues. The critical issue, however, with the engineering of organs outside the human body is the insufficient availability of human cells, the absence of a suitable matrix with the same architecture and composition as the target tissue, and the maintenance of organ viability in the absence of the blood supply. The issue regarding the maintenance of the engineered organ viability can be solved using bioreactors consisting of mediums with defined chemical composition, i.e., nutrients, cofactors, and growth factors that can successively sustain the target cell's viability. Engineered extracellular matrices and stem cells to regenerate organs outside the human body are also being used. Clinically, various adult stem cell therapies are readily under practice. This review will focus on the regeneration of organs through various types of stem cells and tissue engineering techniques.

Clinical Translation of Engineered Pulmonary Vascular Models

Diseases in pulmonary vasculature remain a major cause of morbidity and mortality worldwide. Numerous pre-clinical animal models were developed to understand lung vasculature during diseases and development. However, these systems are typically limited in their ability to represent human pathophysiology for the study of disease and drug mechanisms. In recent years, a growing number of studies have focused on developing in vitro experimental platforms that mimic human tissues/organs. In this chapter, we discuss the key components involved in developing engineered pulmonary vascular modeling systems and provide perspectives on ways to improve the translational potential of existing models.

Lung Tissue Engineering: Toward a More Deliberate Approach

Chronic lung disease remains a leading cause of morbidity and mortality. Given the dearth of definitive therapeutic options, there is an urgent need to augment the pool of donor organs for transplantation. One strategy entails building a lung ex vivo in the laboratory. The past decade of whole lung tissue engineering has laid a foundation of systems and strategies for this approach. Meanwhile, tremendous progress in lung stem cell biology is elucidating cues contributing to alveolar repair, and speaks to the potential of whole lung regeneration in the future. This perspective discusses the key challenges facing the field and highlights opportunities to combine insights from biology with engineering strategies to adopt a more deliberate, and ultimately successful, approach to lung engineering.

Organoid research on human early development and beyond

The organoid field has been developing rapidly during the last decade. Organoids for human pre-, peri- and post-implantation development have opened an avenue to study these biological processes in vitro, which have been hampered by lack of accessible research models for long term. The technologies of four fields, single cell omics sequencing, genome editing and lineage tracing, microfluidics and tissue engineering, have fueled the rapid development of the organoid field. In this review, we will discuss the organoid research on human early development as well as future directions of the organoid field combining with other powerful technologies.

Regenerative medicine technologies applied to transplant medicine.an update

Regenerative medicine (RM) is changing how we think and practice transplant medicine. In regenerative medicine, the aim is to develop and employ methods to regenerate, restore or replace damaged/diseased tissues or organs. Regenerative medicine investigates using tools such as novel technologies or techniques, extracellular vesicles, cell-based therapies, and tissue-engineered constructs to design effective patient-specific treatments. This review illustrates current advancements in regenerative medicine that may pertain to transplant medicine. We highlight progress made and various tools designed and employed specifically for each tissue or organ, such as the kidney, heart, liver, lung, vasculature, gastrointestinal tract, and pancreas. By combing both fields of transplant and regenerative medicine, we can harbor a successful collaboration that would be beneficial and efficacious for the repair and design of de novo engineered whole organs for transplantations.

Lessons learned from pre-clinical testing of xenogeneic decellularized esophagi in a rabbit model

Decellularization of esophagi from several species for tissue engineering is well described, but successful implantation in animal models of esophageal replacement has been challenging. The purpose of this study was to assess feasibility and applicability of esophageal replacement using decellularized porcine esophageal scaffolds in a new pre-clinical model. Following surgical replacement in rabbits with a vascularizing muscle flap, we observed successful anastomoses of decellularized scaffolds, cues of early neovascularization, and prevention of luminal collapse by the use of biodegradable stents. However, despite the success of the surgical procedure, the long-term survival was limited by the fragility of the animal model. Our results indicate that transplantation of a decellularized porcine scaffold is possible and vascular flaps may be useful to provide a vascular supply, but long-term outcomes require further pre-clinical testing in a different large animal model.

Regenerative medicine: postnatal approaches

Paper 2 of the paediatric regenerative medicine Series focuses on recent advances in postnatal approaches. New gene, cell, and niche-based technologies and their combinations allow structural and functional reconstitution and simulation of complex postnatal cell, tissue, and organ hierarchies. Organoid and tissue engineering advances provide human disease models and novel treatments for both rare paediatric diseases and common diseases affecting all ages, such as COVID-19. Preclinical studies for gastrointestinal disorders are directed towards oesophageal replacement, short bowel syndrome, enteric neuropathy, biliary atresia, and chronic end-stage liver failure. For respiratory diseases, beside the first human tracheal replacement, more complex tissue engineering represents a promising solution to generate transplantable lungs. Genitourinary tissue replacement and expansion usually involve application of biocompatible scaffolds seeded with patient-derived cells. Gene and cell therapy approaches seem appropriate for rare paediatric diseases of the musculoskeletal system such as spinal muscular dystrophy, whereas congenital diseases of complex organs, such as the heart, continue to challenge new frontiers of regenerative medicine.

Right lung transplantation with a left-to-right inverted anastomosis in a rat model

Objective

Right lung transplantation in rats has been attempted occasionally, but the technical complexity makes it challenging to apply routinely. Additionally, basic research on inverted lobar lung transplantation is scarce because of the lack of a cost-effective experimental model. We first reported right lung transplantation in a rat model using left-to-right inverted anastomosis to imitate the principle of clinically inverted lung transplantation.

Methods

Right lung transplantation was performed in 10 consecutive rats. By using a 3-cuff technique, the left lung of the donor rat was implanted into the right thoracic cavity of the recipient rat. The rat lung graft was rotated 180° along the vertical axis to achieve anatomic matching of right hilar structures. Another 10 consecutive rats had received orthotopic left lung transplantation as a control.

Results

All lung transplantation procedures were technically successful without intraoperative failure. One rat (10%) died of full pulmonary atelectasis after right lung transplantation, whereas all rats survived after left lung transplantation. No significant difference was observed in heart-lung block retrieval (8.6 ± 0.8 vs 8.4 ± 0.9 minutes), cuff preparation (8.3 ± 0.9 vs 8.7 ± 0.9 minutes), or total procedure time (58.2 ± 2.6 vs 56.6 ± 2.1 minutes) between the right lung transplantation and standard left lung transplantation groups (P > .05), although the cold ischemia time (14.2 ± 0.9 vs 25.5 ± 1.7 minutes) and warm ischemia time (19.8 ± 1.5 vs 13.7 ± 1.8 minutes) were different (P < .001).

Conclusions

Right lung transplantation with a left-to-right inverted anastomosis in a rat model is technically easy to master, expeditious, and reproducible. It can potentially imitate the principle of clinically inverted lung transplantation and become an alternative to standard left lung transplantation.

Recellularization of Bioengineered Scaffolds for Vascular Composite Allotransplantation

Traumatic injuries or cancer resection resulting in large volumetric soft tissue loss requires surgical reconstruction. Vascular composite allotransplantation (VCA) is an emerging reconstructive option that transfers multiple, complex tissues as a whole subunit from donor to recipient. Although promising, VCA is limited due to side effects of immunosuppression. Tissue-engineered scaffolds obtained by decellularization and recellularization hold great promise. Decellularization is a process that removes cellular materials while preserving the extracellular matrix architecture. Subsequent recellularization of these acellular scaffolds with recipient-specific cells can help circumvent adverse immune-mediated host responses and allow transplantation of allografts by reducing and possibly eliminating the need for immunosuppression. Recellularization of acellular tissue scaffolds is a technique that was first investigated and reported in whole organs. More recently, work has been performed to apply this technique to VCA. Additional work is needed to address barriers associated with tissue recellularization such as: cell type selection, cell distribution, and functionalization of the vasculature and musculature. These factors ultimately contribute to achieving tissue integration and viability following allotransplantation. The present work will review the current state-of-the-art in soft tissue scaffolds with specific emphasis on recellularization techniques. We will discuss biological and engineering process considerations, technical and scientific challenges, and the potential clinical impact of this technology to advance the field of VCA and reconstructive surgery.

Emerging Paradigms in Bioengineering the Lungs

In end-stage lung diseases, the shortage of donor lungs for transplantation and long waiting lists are the main culprits in the significantly increasing number of patient deaths. New strategies to curb this issue are being developed with the help of recent advancements in bioengineering technology, with the generation of lung scaffolds as a steppingstone. There are various types of lung scaffolds, namely, acellular scaffolds that are developed via decellularization and recellularization techniques, artificial scaffolds that are synthesized using synthetic, biodegradable, and low immunogenic materials, and hybrid scaffolds which combine the advantageous properties of materials in the development of a desirable lung scaffold. There have also been advances in the design of bioreactors in terms of providing an optimal regenerative environment for the maturation of functional lung tissue over time. In this review, the emerging paradigms in the field of lung tissue bioengineering will be discussed.

Transplantation of bioengineered liver capable of extended function in a preclinical liver failure model

Unlimited organ availability would represent a paradigm shift in transplantation. Long-term in vivo engraftment and function of scaled-up bioengineered liver grafts have not been previously reported. In this study, we describe a human-scale transplantable liver graft engineered on a porcine liver-derived scaffold. We repopulated the scaffold parenchyma with primary hepatocytes and the vascular system with endothelial cells. For in vivo functional testing, we performed auxiliary transplantation of the repopulated scaffold in pigs with induced liver failure. It was observed that the auxiliary bioengineered liver graft improved liver function for 28 days and exhibited upregulation of liver-specific genes. This study is the first of its kind to present 28 days of posttransplant evaluation of a bioengineered liver graft using a preclinical large animal model. Furthermore, it provides definitive evidence for the feasibility of engineering human-scale transplantable liver grafts for clinical applications.

Cell-Seeded Biomaterial Scaffolds: The Urgent Need for Unanswered Accelerated Angiogenesis

One of the most arduous challenges in tissue engineering is neovascularization, without which there is a lack of nutrients delivered to a target tissue. Angiogenesis should be completed at an optimal density and within an appropriate period of time to prevent cell necrosis. Failure to meet this challenge brings about poor functionality for the tissue in comparison with the native tissue, extensively reducing cell viability. Prior studies devoted to angiogenesis have provided researchers with some biomaterial scaffolds and cell choices for angiogenesis. For example, while most current angiogenesis approaches require a variety of stimulatory factors ranging from biomechanical to biomolecular to cellular, some other promising stimulatory factors have been underdeveloped (such as electrical, topographical, and magnetic). When it comes to choosing biomaterial scaffolds in tissue engineering for angiogenesis, key traits rush to mind including biocompatibility, appropriate physical and mechanical properties (adhesion strength, shear stress, and malleability), as well as identifying the appropriate biomaterial in terms of stability and degradation profile, all of which may leave essential trace materials behind adversely influencing angiogenesis. Nevertheless, the selection of the best biomaterial and cells still remains an area of hot dispute as such previous studies have not sufficiently classified, integrated, or compared approaches. To address the aforementioned need, this review article summarizes a variety of natural and synthetic scaffolds including hydrogels that support angiogenesis. Furthermore, we review a variety of cell sources utilized for cell seeding and influential factors used for angiogenesis with a concentrated focus on biomechanical factors, with unique stimulatory factors. Lastly, we provide a bottom-to-up overview of angiogenic biomaterials and cell selection, highlighting parameters that need to be addressed in future studies.

Advanced In Vitro Lung Models for Drug and Toxicity Screening: The Promising Role of Induced Pluripotent Stem Cells

The substantial socioeconomic burden of lung diseases, recently highlighted by the disastrous impact of the coronavirus disease 2019 (COVID-19) pandemic, accentuates the need for interventive treatments capable of decelerating disease progression, limiting organ damage, and contributing to a functional tissue recovery. However, this is hampered by the lack of accurate human lung research models, which currently fail to reproduce the human pulmonary architecture and biochemical environment. Induced pluripotent stem cells (iPSCs) and organ-on-chip (OOC) technologies possess suitable characteristics for the generation of physiologically relevant in vitro lung models, allowing for developmental studies, disease modeling, and toxicological screening. Importantly, these platforms represent potential alternatives for animal testing, according to the 3Rs (replace, reduce, refine) principle, and hold promise for the identification and approval of new chemicals under the European REACH (registration, evaluation, authorization and restriction of chemicals) framework. As such, this review aims to summarize recent progress made in human iPSC- and OOC-based in vitro lung models. A general overview of the present applications of in vitro lung models is presented, followed by a summary of currently used protocols to generate different lung cell types from iPSCs. Lastly, recently developed iPSC-based lung models are discussed.

A Pulmonary Vascular Model From Endothelialized Whole Organ Scaffolds

The development of an in vitro system for the study of lung vascular disease is critical to understanding human pathologies. Conventional culture systems fail to fully recapitulate native microenvironmental conditions and are typically limited in their ability to represent human pathophysiology for the study of disease and drug mechanisms. Whole organ decellularization provides a means to developing a construct that recapitulates structural, mechanical, and biological features of a complete vascular structure. Here, we developed a culture protocol to improve endothelial cell coverage in whole lung scaffolds and used single-cell RNA-sequencing analysis to explore the impact of decellularized whole lung scaffolds on endothelial phenotypes and functions in a biomimetic bioreactor system. Intriguingly, we found that the phenotype and functional signals of primary pulmonary microvascular revert back—at least partially—toward native lung endothelium. Additionally, human induced pluripotent stem cell-derived endothelium cultured in decellularized lung systems start to gain various native human endothelial phenotypes. Vascular barrier function was partially restored, while small capillaries remained patent in endothelial cell-repopulated lungs. To evaluate the ability of the engineered endothelium to modulate permeability in response to exogenous stimuli, lipopolysaccharide (LPS) was introduced into repopulated lungs to simulate acute lung injury. After LPS treatment, proinflammatory signals were significantly increased and the vascular barrier was impaired. Taken together, these results demonstrate a novel platform that recapitulates some pulmonary microvascular functions and phenotypes at a whole organ level. This development may help pave the way for using the whole organ engineering approach to model vascular diseases.

Surface modification of decellularized bovine carotid arteries with human vascular cells significantly reduces their thrombogenicity

BACKGROUND

Since autologous veins are unavailable when needed in more than 20% of cases in vascular surgery, the production of personalized biological vascular grafts for implantation has become crucial. Surface modification of decellularized xenogeneic grafts with vascular cells to achieve physiological luminal coverage and eventually thromboresistance is an important prerequisite for implantation. However, ex vivo thrombogenicity testing remains a neglected area in the field of tissue engineering of vascular grafts due to a multifold of reasons.

METHODS

After seeding decellularized bovine carotid arteries with human endothelial progenitor cells and umbilical cord-derived mesenchymal stem cells, luminal endothelial cell coverage (LECC) was correlated with glucose and lactate levels on the cell supernatant. Then a closed loop whole blood perfusion system was designed. Recellularized grafts with a LECC > 50% and decellularized vascular grafts were perfused with human whole blood for 2 h. Hemolysis and complete blood count evaluation was performed on an hourly basis, followed by histological and immunohistochemical analysis.

RESULTS

While whole blood perfusion of decellularized grafts significantly reduced platelet counts, platelet depletion from blood resulting from binding to re-endothelialized grafts was insignificant (p = 0.7284). Moreover, macroscopic evaluation revealed thrombus formation only in the lumen of unseeded grafts and histological characterization revealed lack of CD41 positive platelets in recellularized grafts, thus confirming their thromboresistance.

CONCLUSION

In the present study we were able to demonstrate the effect of surface modification of vascular grafts in their thromboresistance in an ex vivo whole blood perfusion system. To our knowledge, this is the first study to expose engineered vascular grafts to human whole blood, recirculating at high flow rates, immediately after seeding.

High-Throughput Culture Method of Induced Pluripotent Stem Cell Derived Alveolar Epithelial Cells

Lung regeneration is dependent on the availability of progenitor lung cells. Large numbers of self-renewing, patient-specific induced pluripotent stem cell derived alveolar epithelial cells (iPSC-AECs) are needed to adequately recellularize whole organ constructs. Prior methods to generated functional iPSC-AECs are not feasible for large-scale cell production. We present a novel protocol to produce iPSC-AECs which is scalable for whole organ regeneration. Differentiation of iPSCs was performed with genetically modified iPSCs with fluorescent reporters which underwent differentiation in a stepwise protocol mimicking lung development. Cells were purified, sorted, and embedded in a liquid Matrigel precursor to form either adherent droplets or to form cell-laden Matrigel spheroids which were subsequently transferred to spinner flasks with media as floating droplets. After culture, monolayer spheres of iPSC-AECs were isolated to form single cell suspensions. Equal numbers of iPSC-AECs from the two culture conditions were seeded into decellularized lung scaffolds. IPSC-AECs cultured in floating droplets were significantly more proliferative than those in adherent droplets, with significantly higher total cell counts and Ki67 expression. Equivalent expression of the distal lung markers was observed for both culture conditions. Lungs recellularized from both culture groups had similar histologic appearance. Media changes took significantly less time with the floating droplet method and was more cost effective. The floating droplet culture method demonstrated enhanced proliferative capacity, stable distal lung epithelial phenotype, and reduced resources compared to prior culture methods. Here we provide a means for iPSC-AEC production for regeneration of whole lung constructs.

Engineering a 3D Vascularized Adipose Tissue Construct Using a Decellularized Lung Matrix

Critically sized defects in subcutaneous white adipose tissue result in extensive disfigurement and dysfunction and remain a reconstructive challenge for surgeons; as larger defect sizes are correlated with higher rates of complications and failure due to insufficient vascularization following implantation. Our study demonstrates, for the first time, a method to engineer perfusable, pre-vascularized, high-density adipose grafts that combine patient-derived adipose cells with a decellularized lung matrix (DLM). The lung is one of the most vascularized organs with high flow, low resistance, and a large blood-alveolar interface separated by a thin basement membrane. For our work, the large volume capacity within the alveolar compartment was repurposed for high-density adipose cell filling, while the acellular vascular bed provided efficient graft perfusion throughout. Both adipocytes and hASCs were successfully delivered and remained in the alveolar space even after weeks of culture. While adipose-derived cells maintained their morphology and functionality in both static and perfusion DLM cultures, perfusion culture offered enhanced outcomes over static culture. Furthermore, we demonstrate that endothelial cells seamlessly integrate into the acellular vascular tree of the DLM with adipocytes. These results support that the DLM is a unique platform for creating vascularized adipose tissue grafts for large defect filling.

A Two-Step Bioreactor for Decellularized Lung Epithelialization

Bioreactors for the reseeding of decellularized lung scaffolds have evolved with various advancements, including biomimetic mechanical stimulation, constant nutrient flow, multi-output monitoring, and large mammal scaling. Although dynamic bioreactors are not new to the field of lung bioengineering, ideal conditions during cell seeding have not been extensively studied or controlled. To address the lack of cell dispersal in traditional seeding methods, we have designed a two-step bioreactor. The first step is a novel system that rotates a seeded lung every 20 min at different angles in a sequence designed to anchor 20% of cells to a particular location based on the known rate of attachment. The second step involves perfusion-ventilation culture to ensure nutrient dispersion and cellular growth. Compared to statically seeded lungs, rotationally seeded lungs had significantly increased dsDNA content and more uniform cellular distribution after perfusion and ventilation had been administered. The addition of this novel seeding system before traditional culture methods will aid in recellularizing the lung and other geometrically complex organs for tissue engineering.

Engineering Functional Vasculature in Decellularized Lungs Depends on Comprehensive Endothelial Cell Tropism

Tissue engineering using decellularized whole lungs as matrix scaffolds began as a promise for creating autologous transplantable lungs for patients with end-stage lung disease and can also be used to study strategies for lung regeneration. Vascularization remains a critical component for all solid organ bioengineering, yet there has been limited success in generating functional re-endothelialization of most pulmonary vascular segments. We evaluated recellularization of the blood vessel conduits of acellular mouse scaffolds with highly proliferating, rat pulmonary microvascular endothelial progenitor cells (RMEPCs), pulmonary arterial endothelial cells (PAECs) or microvascular endothelial cells (MVECs). After 8 days of pulsatile perfusion, histological analysis showed that PAECs and MVECs possessed selective tropism for larger vessels or microvasculature, respectively. In contrast, RMEPCs lacked site preference and repopulated all vascular segments. RMEPC-derived endothelium exhibited thrombomodulin activity, expression of junctional genes, ability to synthesize endothelial signaling molecules, and formation of a restrictive barrier. The RMEPC phenotype described here could be useful for identifying endothelial progenitors suitable for efficient vascular organ and tissue engineering, regeneration and repair.

Bioengineering platforms for cell therapeutics derived from pluripotent and direct reprogramming

Pluripotent and direct reprogramming technologies hold great potential for tissue repair and restoration of tissue and organ function. The implementation of induced pluripotent stem cells and directly reprogrammed cells in biomedical research has resulted in a significant leap forward in the highly promising area of regenerative medicine. While these therapeutic strategies are promising, there are several obstacles to overcome prior to the introduction of these therapies into clinical settings. Bioengineering technologies, such as biomaterials, bioprinting, microfluidic devices, and biostimulatory systems, can enhance cell viability, differentiation, and function, in turn the efficacy of cell therapeutics generated via pluripotent and direct reprogramming. Therefore, cellular reprogramming technologies, in combination with tissue-engineering platforms, are poised to overcome current bottlenecks associated with cell-based therapies and create new ways of producing engineered tissue substitutes.

Cell-based therapies for vascular regeneration: Past, present and future

Tissue vascularization remains one of the outstanding challenges in regenerative medicine. Beyond its role in circulating oxygen and nutrients, the vasculature is critical for organ development, function and homeostasis. Importantly, effective vascular regeneration is key in generating large 3D tissues for regenerative medicine applications to enable the survival of cells post-transplantation, organ growth, and integration into the host system. Therefore, the absence of clinically applicable means of (re)generating vessels is one of the main obstacles in cell replacement therapy. In this review, we highlight cell-based vascularization strategies which demonstrate clinical potential, discuss their strengths and limitations and highlight the main obstacles hindering cell-based therapeutic vascularization.

Development of alginate and gelatin-based pleural and tracheal sealants

Pleural and tracheal injuries remain significant problems, and an easy to use, effective pleural or tracheal sealant would be a significant advance. The major challenges are requirements for adherence, high strength and elasticity, dynamic durability, appropriate biodegradability, and lack of cell or systemic toxicity. We designed and evaluated two sealant materials comprised respectively of alginate methacrylate and of gelatin methacryloyl, each functionalized by conjugation with dopamine HCl. Both compounds are cross-linked into easily applied as pre-formed hydrogel patches or as in situ hydrogels formed at the wound site utilizing FDA-approved photo-initiators and oxidants. Material testing demonstrates appropriate adhesiveness, tensile strength, burst pressure, and elasticity with no significant cell toxicity in vitro assessments. Air-leak was absent after sealant application to experimentally-induced injuries in ex-vivo rat lung and tracheal models and in ex vivo pig lungs. Sustained repair of experimentally-induced pleural injury was observed for up to one month in vivo rat models and for up to 2 weeks in vivo rat tracheal injury models without obvious air leak or obvious toxicities. The alginate-based sealant worked best in a pre-formed hydrogel patch whereas the gelatin-based sealant worked best in an in situ formed hydrogel at the wound site thus providing two potential approaches. These studies provide a platform for further pre-clinical and potential clinical investigations. STATEMENT OF SIGNIFICANCE: Pneumothorax and pleural effusions resulting from trauma and a range of lung diseases and critical illnesses can result in lung collapse that can be immediately life-threatening or result in chronic leaking (bronchopleural fistula) that is currently difficult to manage. This leads to significantly increased morbidity, mortality, hospital stays, health care costs, and other complications. We have developed sealants originating from alginate and gelatin biomaterials, each functionalized by methacryloylation and by dopamine conjugation to have desired mechanical characteristics for use in pleural and tracheal injuries. The sealants are easily applied, non-cytotoxic, and perform well in vitro and in vivo model systems of lung and tracheal injuries. These initial proof of concept investigations provide a platform for further studies.

Decellularized human-sized pulmonary scaffolds for lung tissue engineering: a comprehensive review

The ultimate goal of lung bioengineering is to produce transplantable lungs for human beings. Therefore, large-scale studies are of high importance. In this paper, we review the investigations on decellularization and recellularization of human-sized lung scaffolds. First, studies that introduce new ways to enhance the decellularization of large-scale lungs are reviewed, followed by the investigations on the xenogeneic sources of lung scaffolds. Then, decellularization and recellularization of diseased lung scaffolds are discussed to assess their usefulness for tissue engineering applications. Next, the use of stem cells in recellularizing acellular lung scaffolds is reviewed, followed by the case studies on the transplantation of bioengineered lungs. Finally, the remaining challenges are discussed, and future directions are highlighted.

Generation of vascular chimerism within donor organs

Whole organ perfusion decellularization has been proposed as a promising method to generate non-immunogenic organs from allogeneic and xenogeneic donors. However, the ability to recellularize organ scaffolds with multiple patient-specific cells in a spatially controlled manner remains challenging. Here, we propose that replacing donor endothelial cells alone, while keeping the rest of the organ viable and functional, is more technically feasible, and may offer a significant shortcut in the efforts to engineer transplantable organs. Vascular decellularization was achieved ex vivo, under controlled machine perfusion conditions, in various rat and porcine organs, including the kidneys, liver, lungs, heart, aorta, hind limbs, and pancreas. In addition, vascular decellularization of selected organs was performed in situ, within the donor body, achieving better control over the perfusion process. Human placenta-derived endothelial progenitor cells (EPCs) were used as immunologically-acceptable human cells to repopulate the luminal surface of de-endothelialized aorta (in vitro), kidneys, lungs and hind limbs (ex vivo). This study provides evidence that artificially generating vascular chimerism is feasible and could potentially pave the way for crossing the immunological barrier to xenotransplantation, as well as reducing the immunological burden of allogeneic grafts.

A Robust Protocol for Decellularized Human Lung Bioink Generation Amenable to 2D and 3D Lung Cell Culture

Decellularization efforts must balance the preservation of the extracellular matrix (ECM) components while eliminating the nucleic acid and cellular components. Following effective removal of nucleic acid and cell components, decellularized ECM (dECM) can be solubilized in an acidic environment with the assistance of various enzymes to develop biological scaffolds in different forms, such as sheets, tubular constructs, or three-dimensional (3D) hydrogels. Each organ or tissue that undergoes decellularization requires a distinct and optimized protocol to ensure that nucleic acids are removed, and the ECM components are preserved. The objective of this study was to optimize the decellularization process for dECM isolation from human lung tissues for downstream 2D and 3D cell culture systems. Following protocol optimization and dECM isolation, we performed experiments with a wide range of dECM concentrations to form human lung dECM hydrogels that were physically stable and biologically responsive. The dECM based-hydrogels supported the growth and proliferation of primary human lung fibroblast cells in 3D cultures. The dECM is also amenable to the coating of polyester membranes in Transwell™ Inserts to improve the cell adhesion, proliferation, and barrier function of primary human bronchial epithelial cells in 2D. In conclusion, we present a robust protocol for human lung decellularization, generation of dECM substrate material, and creation of hydrogels that support primary lung cell viability in 2D and 3D culture systems

Research progress in decellularized extracellular matrix-derived hydrogels

Decellularized extracellular matrix (dECM) is widely used in regenerative medicine as a scaffold material due to its unique biological activity and good biocompatibility. Hydrogel is a three-dimensional network structure polymer with high water content and high swelling that can simulate the water environment of human tissues, has good biocompatibility, and can exchange nutrients, oxygen, and waste with cells. At present, hydrogel is the ideal biological material for tissue engineering. In recent years, rapid development of the hydrogel theory and technology and progress in the use of dECM to form hydrogels have attracted considerable attention to dECM hydrogels as an innovative method for tissue engineering and regenerative medicine. This article introduces the classification of hydrogels, and focuses on the history and formation of dECM hydrogels, the source of dECM, the application of dECM hydrogels in tissue engineering and the commercial application of dECM materials.

Development of novel apoptosis-assisted lung tissue decellularization methods

Decellularized tissues hold great potential for both regenerative medicine and disease modeling applications. The acellular extracellular matrix (ECM)-enriched scaffolds can be recellularized with patient-derived cells prior to transplantation, or digested to create thermally-gelling ECM hydrogels for 3D cell culture. Current methods of decellularization clear cellular components using detergents, which can result in loss of ECM proteins and tissue architectural integrity. Recently, an alternative approach utilizing apoptosis to decellularize excised murine sciatic nerves resulted in superior ECM preservation, cell removal, and immune tolerance in vivo. However, this apoptosis-assisted decellularization approach has not been optimized for other tissues with a more complex geometry, such as lungs. To this end, we developed an apoptosis-assisted lung tissue decellularization method using a combination of camptothecin and sulfobetaine-10 (SB-10) to induce apoptosis and facilitate gentle and effective removal of cell debris, respectively. Importantly, combination of the two agents resulted in superior cell removal and ECM preservation compared to either of the treatments alone, presumably because of pulmonary surfactants. In addition, our method was superior in cell removal compared to a previously established detergent-based decellularization protocol. Furthermore, thermally-gelling lung ECM hydrogels supported high viability of rat lung epithelial cells for up to 2 weeks in culture. This work demonstrates that apoptosis-based lung tissue decellularization is a superior technique that warrants further utilization for both regenerative medicine and disease modeling purposes.

Assembly and Function of a Bioengineered Human Liver for Transplantation Generated Solely from Induced Pluripotent Stem Cells

The availability of an autologous transplantable auxiliary liver would dramatically affect the treatment of liver disease. Assembly and function in vivo of a bioengineered human liver derived from induced pluripotent stem cells (iPSCs) has not been previously described. By improving methods for liver decellularization, recellularization, and differentiation of different liver cellular lineages of human iPSCs in an organ-like environment, we generated functional engineered human mini livers and performed transplantation in a rat model. Whereas previous studies recellularized liver scaffolds largely with rodent hepatocytes, we repopulated not only the parenchyma with human iPSC-hepatocytes but also the vascular system with human iPS-endothelial cells, and the bile duct network with human iPSC-biliary epithelial cells. The regenerated human iPSC-derived mini liver containing multiple cell types was tested in vivo and remained functional for 4 days after auxiliary liver transplantation in immunocompromised, engineered (IL2rg^-/-) rats.