Construction of three-dimensional vascularized functional human liver tissue using a layer-by-layer cell coating technique

Bioengineering vascularized liver tissue for biomedical research and application

The liver performs a wide range of biological functions that are essential to body homeostasis. Damage to liver tissue can result in reduced organ function, and if chronic in nature can lead to organ scarring and progressive disease. Currently, donor liver transplantation is the only longterm treatment for end-stage liver disease. However, orthotopic organ transplantation suffers from several drawbacks that include organ scarcity and lifelong immunosuppression. Therefore, new therapeutic strategies are required. One promising strategy is the engineering of implantable and vascularized liver tissue. This resource could also be used to build the next generation of liver tissue models to better understand human health, disease and aging in vitro. This article reviews recent progress in the field of liver tissue bioengineering, including microfluidic-based systems, bio-printed vascularized tissue, liver spheroids and organoid models, and the induction of angiogenesis in vivo.

Recent Developments in Layer-by-Layer Assembly for Drug Delivery and Tissue Engineering Applications

Surfaces with biological functionalities are of great interest for biomaterials, tissue engineering, biophysics, and for controlling biological processes. The layer-by-layer (LbL) assembly is a highly versatile methodology introduced 30 years ago, which consists of assembling complementary polyelectrolytes or biomolecules in a stepwise manner to form thin self-assembled films. In view of its simplicity, compatibility with biological molecules, and adaptability to any kind of supporting material carrier, this technology has undergone major developments over the past decades. Specific applications have emerged in different biomedical fields owing to the possibility to load or immobilize biomolecules with preserved bioactivity, to use an extremely broad range of biomolecules and supporting carriers, and to modify the film's mechanical properties via crosslinking. In this review, the focus is on the recent developments regarding LbL films formed as 2D or 3D objects for applications in drug delivery and tissue engineering. Possible applications in the fields of vaccinology, 3D biomimetic tissue models, as well as bone and cardiovascular tissue engineering are highlighted. In addition, the most recent technological developments in the field of film construction, such as high-content liquid handling or machine learning, which are expected to open new perspectives in the future developments of LbL, are presented.

Characterization of Gelatin-Polycaprolactone Membranes by Electrospinning

New advances in materials science and medicine have enabled the development of new and increasingly sophisticated biomaterials. One of the most widely used biopolymers is polycaprolactone (PCL) because it has properties suitable for biomedical applications, tissue engineering scaffolds, or drug delivery systems. However, PCL scaffolds do not have adequate bioactivity, and therefore, alternatives have been studied, such as mixing PCL with bioactive polymers such as gelatin, to promote cell growth. Thus, this work will deal with the fabrication of nanofiber membranes by means of the electrospinning technique using PCL-based solutions (12 wt.% and 20 wt.%) and PCL with gelatin (12 wt.% and 8 wt.%, respectively). Formic acid and acetic acid, as well as mixtures of both in different proportions, have been used to prepare the preliminary solutions, thus supporting the electrospinning process by controlling the viscosity of the solutions and, therefore, the size and uniformity of the fibers. The physical properties of the solutions and the morphological, mechanical, and thermal properties of the membranes were evaluated. Results demonstrate that it is possible to achieve the determined properties of the samples with an appropriate selection of polymer concentrations as well as solvents.

Microphysiological Constructs and Systems: Biofabrication Tactics, Biomimetic Evaluation Approaches, and Biomedical Applications

In recent decades, microphysiological constructs and systems (MPCs and MPSs) have undergone significant development, ranging from self-organized organoids to high-throughput organ-on-a-chip platforms. Advances in biomaterials, bioinks, 3D bioprinting, micro/nanofabrication, and sensor technologies have contributed to diverse and innovative biofabrication tactics. MPCs and MPSs, particularly tissue chips relevant to absorption, distribution, metabolism, excretion, and toxicity, have demonstrated potential as precise, efficient, and economical alternatives to animal models for drug discovery and personalized medicine. However, current approaches mainly focus on the in vitro recapitulation of the human anatomical structure and physiological-biochemical indices at a single or a few simple levels. This review highlights the recent remarkable progress in MPC and MPS models and their applications. The challenges that must be addressed to assess the reliability, quantify the techniques, and utilize the fidelity of the models are also discussed.

Advancing cell surface modification in mammalian cells with synthetic molecules

Biological cells, being the fundamental entities of life, are widely acknowledged as intricate living machines. The manipulation of cell surfaces has emerged as a progressively significant domain of investigation and advancement in recent times. Particularly, the alteration of cell surfaces using meticulously crafted and thoroughly characterized synthesized molecules has proven to be an efficacious means of introducing innovative functionalities or manipulating cells. Within this realm, a diverse array of elegant and robust strategies have been recently devised, including the bioorthogonal strategy, which enables selective modification. This review offers a comprehensive survey of recent advancements in the modification of mammalian cell surfaces through the use of synthetic molecules. It explores a range of strategies, encompassing chemical covalent modifications, physical alterations, and bioorthogonal approaches. The review concludes by addressing the present challenges and potential future opportunities in this rapidly expanding field.

Cultivation and Transplantation of Three-Dimensional Skins with Laser-Processed Biodegradable Membranes

For the treatment of irreversible, extensive skin damage, artificial skins or cultured skins are useful when allogeneic skins are unavailable. However, most of them lack vasculature, causing delayed perfusion and hence delay or failure in engraftment of the tissues. We previously developed a prevascularized three-dimensional (3D) cultured skin based on the layer-by-layer cell coating technique (LbL-3D skin), in which cells are seeded and laminated on a porous polymer membrane for medium supply to the thick cultured tissue. Recent animal studies have demonstrated that LbL-3D skin can achieve rapid perfusion and high graft survival after transplantation. However, there were practical issues with separating LbL-3D skins from the membranes before transplantation and the handling separated LbL-3D skins for transplantation. To address these problems, in this study, we examined the use of biodegradable porous polymer membranes that enabled the transplantation of LbL-3D skins together with the membranes, which could be decomposed after transplantation. Thin films made from poly (lactic-co-glycolic acid) (PLGA) were irradiated with femtosecond laser pulses to create micro through-holes, producing porous membranes. We designed and fabricated culture inserts with the PLGA membranes and cultivated LbL-3D skins with 2 × 106 neonatal normal human dermal fibroblasts and 1 × 104 human umbilical vein endothelial cells in the dermis of 20 cell layers and 1 × 105 neonatal human epidermal keratinocytes in the epidermis. Histological analyses revealed that the skins cultured on the PLGA membranes had thickness of about 400 μm and that there were no defects in the quality of the skins cultured on the PLGA membranes when compared with those cultured on the conventional (nonbiodegradable) commercial membranes. The cultured LbL-3D skins were then transplanted together with the PLGA membranes onto full-thickness excisional wounds in mice. At 7 days posttransplantation onto a mouse, the tissues above and below the membrane were connected through the holes with collagen-positive fibers that appeared to migrate from both the host and donor sides, and favorable reepithelization was observed throughout the transplanted skin region. However, insufficient engraftment was observed in some cases. Thus, further optimization of the membrane conditions would be needed to improve the transplantation outcome.

Construction of Human Three-Dimensional Lung Model Using Layer-by-Layer Method

The respiratory tract is one of the frontline barriers for biological defense. Lung epithelial intercellular adhesions provide protection from bacterial and viral infections and prevent invasion into deep tissues by pathogens. Dysfunction of lung epithelial intercellular adhesion caused by pathogens is associated with development of several diseases, such as acute respiratory distress syndrome, pneumonia, and asthma. To elucidate the pathological mechanism of respiratory infections, two-dimensional cell cultures and animal models are commonly used, although are not useful for evaluating host specificity or human biological response. With the rapid progression and worldwide spread of severe acute respiratory syndrome coronavirus-2, there is increasing interest in the development of a three-dimensional (3D) in vitro lung model for analyzing interactions between pathogens and hosts. However, some models possess unclear epithelial polarity or insufficient barrier functions and need the use of complex technologies, have high cost, and long cultivation terms. We previously reported about the fabrication of 3D cellular multilayers using a layer-by-layer (LbL) cell coating technique with extracellular matrix protein, fibronectin (FN), and gelatin (G). In the present study, such a LbL cell coating technique was utilized to construct a human 3D lung model in which a monolayer of the human lower airway epithelial adenocarcinoma cell line Calu-3 cells was placed on 3D-cellular multilayers composed of FN-G-coated human primary pulmonary fibroblast cells. The 3D lung model thus constructed demonstrated an epithelial-fibroblast layer that maintained uniform thickness until 7 days of incubation. Moreover, expressions of E-cadherin, ZO-1, and mucin in the epithelial layer were observed by immunohistochemical staining. Epithelial barrier integrity was evaluated using transepithelial electrical resistance values. The results indicate that the present constructed human 3D lung model is similar to human lung tissues and also features epithelial polarity and a barrier function, thus is considered useful for evaluating infection and pathological mechanisms related to pneumonia and several pathogens. Impact statement A novel in vitro model of lung tissue was established. Using a layer-by-layer cell coating technique, a three-dimensional cultured lung model was constructed. The present novel model was shown to have epithelial polarity and chemical barrier functions. This model may be useful for investigating interaction pathogens and human biology.

Generation of multilineage liver organoids with luminal vasculature and bile ducts from human pluripotent stem cells via modulation of Notch signaling

BACKGROUND

The generation of liver organoids recapitulating parenchymal and non-parenchymal cell interplay is essential for the precise in vitro modeling of liver diseases. Although different types of multilineage liver organoids (mLOs) have been generated from human pluripotent stem cells (hPSCs), the assembly and concurrent differentiation of multiple cell types in individual mLOs remain a major challenge. Particularly, most studies focused on the vascularization of mLOs in host tissue after transplantation in vivo. However, relatively little information is available on the in vitro formation of luminal vasculature in mLOs themselves.

METHODS

The mLOs with luminal blood vessels and bile ducts were generated by assembling hepatic endoderm, hepatic stellate cell-like cells (HscLCs), and endothelial cells derived entirely from hPSCs using 96-well ultra-low attachment plates. We analyzed the effect of HscLC incorporation and Notch signaling modulation on the formation of both bile ducts and vasculature in mLOs using immunofluorescence staining, qRT-PCR, ELISA, and live-perfusion imaging. The potential use of the mLOs in fibrosis modeling was evaluated by histological and gene expression analyses after treatment with pro-fibrotic cytokines.

RESULTS

We found that hPSC-derived HscLCs are crucial for generating functional microvasculature in mLOs. HscLC incorporation and subsequent vascularization substantially reduced apoptotic cell death and promoted the survival and growth of mLOs with microvessels. In particular, precise modulation of Notch signaling during a specific time window in organoid differentiation was critical for generating both bile ducts and vasculature. Live-cell imaging, a series of confocal scans, and electron microscopy demonstrated that blood vessels were well distributed inside mLOs and had perfusable lumens in vitro. In addition, exposure of mLOs to pro-fibrotic cytokines induced early fibrosis-associated events, including upregulation of genes associated with fibrotic induction and endothelial cell activation (i.e., collagen I, α-SMA, and ICAM) together with destruction of tissue architecture and organoid shrinkage.

CONCLUSION

Our results demonstrate that mLOs can reproduce parenchymal and non-parenchymal cell interactions and suggest that their application can advance the precise modeling of liver diseases in vitro.

Extracellular Microenvironmental Control for Organoid Assembly

Organoids, which are multicellular clusters with similar physiological functions to living organs, have gained increasing attention in bioengineering. As organoids become more advanced, methods to form complex structures continue to develop. There is evidence that the extracellular microenvironment can regulate organoid quality. The extracellular microenvironment consists of soluble bioactive molecules, extracellular matrix, and biofluid flow. However, few efforts have been made to discuss the microenvironment optimal to engineer specific organoids. Therefore, this review article examines the extent to which engineered extracellular microenvironments regulate organoid quality. First, we summarize the natural tissue and organ's unique chemical and mechanical properties, guiding researchers to design an extracellular microenvironment used for organoid engineering. Then, we summarize how the microenvironments contribute to the formation and growth of the brain, lung, intestine, liver, retinal, and kidney organoids. The approaches to forming and evaluating the resulting organoids are also discussed in detail. Impact statement Organoids, which are multicellular clusters with similar physiological function to living organs, have been gaining increasing attention in bioengineering. As organoids become more advanced, methods to form complex structures continue to develop. This review article focuses on recent efforts to engineer the extracellular microenvironment in organoid research. We summarized the natural organ's microenvironment, which informs researchers of key factors that can influence organoid formation. Then, we summarize how these microenvironmental controls significantly contribute to the formation and growth of the corresponding brain, lung, intestine, liver, retinal, and kidney organoids. The approaches to forming and evaluating the resulting organoids are discussed in detail, including extracellular matrix choice and properties, culture methods, and the evaluation of the morphology and functionality through imaging and biochemical analysis.

Human Vascular Endothelial Cells Promote the Secretion of Vascularization Factors and Migration of Human Skin Fibroblasts under Co-Culture and Its Preliminary Application

The good treatment of skin defects has always been a challenge in the medical field, and the emergence of tissue engineering skin provides a new idea for the treatment of injured skin. However, due to the single seed cells, the tissue engineering skin has the problem of slow vascularization at the premonitory site after implantation into the human body. Cell co-culture technology can better simulate the survival and communication environment of cells in the human body. The study of multicellular co-culture hopes to bring a solution to the problem of tissue engineering. In this paper, human skin fibroblasts (HSFs) and human vascular endothelial cells (HVECs) were co-cultured in Transwell. The Cell Counting Kit 8 (CCK8), Transwell migration chamber, immunofluorescence, Western blot (WB), and real time quantitative PCR (RT-qPCR) were used to study the effects of HVECs on cell activity, migration factor (high mobility group protein 1, HMGB1) and vascularization factor (vascular endothelial growth factor A, VEGFA and fibroblast growth factor 2, FGF2) secretion of HSFs after co-cultured with HVECs in the Transwell. The biological behavior of HSFs co-cultured with HVECs was studied. The experimental results are as follows: (1) The results of cck8 showed that HVECS could promote the activity of HSFs. (2) HVECs could significantly promote the migration of HSFs and promote the secretion of HMGB1. (3) HVECs could promote the secretion of VEGFA and FGF2 of HSFs. (4) The HVECs and HSFs were inoculated on tissue engineering scaffolds at the ratio of 1:4 and were co-cultured and detected for 7 days. The results showed that from the third day, the number of HSFs was significantly higher than that of the control group without HVECs.

A Transwell-Based Vascularized Model to Investigate the Effect of Interstitial Flow on Vasculogenesis

Interstitial flow plays a significant role in vascular system development, mainly including angiogenesis and vasculogenesis. However, compared to angiogenesis, the effect of interstitial flow on vasculogenesis is less explored. Current in vitro models for investigating the effect of interstitial flow on vasculogenesis heavily rely on microfluidic chips, which require microfluidic expertise and facilities, and may not be accessible to biological labs. Here, we proposed a facile approach to building perfusable vascular networks through the self-assembly of endothelial cells in a modified transwell format and investigated the effect of interstitial flow on vasculogenesis. We found that the effect of interstitial flow on vasculogenesis was closely related to the existence of VEGF and fibroblasts in the developed model: (1) In the presence of fibroblasts, interstitial flow (within the range of 0.1-0.6 μm/s) facilitated the perfusability of the engineered vasculatures. Additional VEGF in the culture medium further worked synergically with interstitial flow to develop longer, wider, denser, and more perfusable vasculatures than static counterparts; (2) In the absence of fibroblasts, vasculatures underwent severe regression within 7 days under static conditions. However, interstitial flow greatly inhibited vessel regression and enhanced vascular perfusability and morphogenesis without the need for additional VEGF. These results revealed that the effect of interstitial flow might vary depending on the existence of VEGF and fibroblasts, and would provide some guidelines for constructing in vitro self-assembled vasculatures. The established transwell-based vascularized model provides a simple method to build perfusable vasculatures and could also be utilized for creating functional tissues in regenerative medicine.

In Vitro Generation of Luminal Vasculature in Liver Organoids: From Basic Vascular Biology to Vascularized Hepatic Organoids

Liver organoids have gained much attention in recent years for their potential applications to liver disease modeling and pharmacologic drug screening. Liver organoids produced in vitro reflect some aspects of the in vivo physiological and pathological conditions of the liver. However, the generation of liver organoids with perfusable luminal vasculature remains a major challenge, hindering precise and effective modeling of liver diseases. Furthermore, vascularization is required for large organoids or assembloids to closely mimic the complexity of tissue architecture without cell death in the core region. A few studies have successfully generated liver organoids with endothelial cell networks, but most of these vascular networks produced luminal structures after being transplanted into tissues of host animals. Therefore, formation of luminal vasculature is an unmet need to overcome the limitation of liver organoids as an in vitro model investigating different acute and chronic liver diseases. Here, we provide an overview of the unique features of hepatic vasculature under pathophysiological conditions and summarize the biochemical and biophysical cues that drive vasculogenesis and angiogenesis in vitro. We also highlight recent progress in generating vascularized liver organoids in vitro and discuss potential strategies that may enable the generation of perfusable luminal vasculature in liver organoids.

Physiologically relevant microsystems to study viral infection in the human liver

Viral hepatitis is a leading cause of liver disease and mortality. Infection can occur acutely or chronically, but the mechanisms that govern the clearance of virus or lack thereof are poorly understood and merit further investigation. Though cures for viral hepatitis have been developed, they are expensive, not readily accessible in vulnerable populations and some patients may remain at an increased risk of developing hepatocellular carcinoma (HCC) even after viral clearance. To sustain infection in vitro, hepatocytes must be fully mature and remain in a differentiated state. However, primary hepatocytes rapidly dedifferentiate in conventional 2D in vitro platforms. Physiologically relevant or physiomimetic microsystems, are increasingly popular alternatives to traditional two-dimensional (2D) monocultures for in vitro studies. Physiomimetic systems reconstruct and incorporate elements of the native cellular microenvironment to improve biologic functionality in vitro. Multiple elements contribute to these models including ancillary tissue architecture, cell co-cultures, matrix proteins, chemical gradients and mechanical forces that contribute to increased viability, longevity and physiologic function for the tissue of interest. These microsystems are used in a wide variety of applications to study biological phenomena. Here, we explore the use of physiomimetic microsystems as tools for studying viral hepatitis infection in the liver and how the design of these platforms is tailored for enhanced investigation of the viral lifecycle when compared to conventional 2D cell culture models. Although liver-based physiomimetic microsystems are typically applied in the context of drug studies, the platforms developed for drug discovery purposes offer a solid foundation to support studies on viral hepatitis. Physiomimetic platforms may help prolong hepatocyte functionality in order to sustain chronic viral hepatitis infection in vitro for studying virus-host interactions for prolonged periods.

Recombinant Spidroin Microgel as the Base of Cell-Engineered Constructs Mediates Liver Regeneration in Rats

Aim: In this study, we seek to check if recombinant spidroin rS1/9 is applicable for cell-engineering construct development. Novel technologies of cell and tissue engineering are relevant for chronic liver failure management. Liver regeneration may represent one of the possible treatment options if a cell-engineered construct (CEC) is used. Nowadays, one can see the continuous study of various matrices to create an appropriate CEC. Materials and Methods: We have adhered allogenic liver cells and multipotent mesenchymal bone marrow stem cells (MMSC BM) to a microgel with recombinant spidroin rS1/9. Then we have studied the developed implantable CEC in a rat model (n = 80) of chronic liver failure achieved by prolonged poisoning with carbon tetrachloride. Results: Our results demonstrate that the CECs change the values of biochemical tests and morphological parameters in chronic liver failure in rats. Conclusion: We consider there to be a positive effect from the microgel-based CECs with recombinant spidroin rS1/9 in the treatment of chronic liver failure.

In vitro grafting of hepatic spheroids and organoids on a microfluidic vascular bed

With recent progress in modeling liver organogenesis and regeneration, the lack of vasculature is becoming the bottleneck in progressing our ability to model human hepatic tissues in vitro. Here, we introduce a platform for routine grafting of liver and other tissues on an in vitro grown microvascular bed. The platform consists of 64 microfluidic chips patterned underneath a 384-well microtiter plate. Each chip allows the formation of a microvascular bed between two main lateral vessels by inducing angiogenesis. Chips consist of an open-top microfluidic chamber, which enables addition of a target tissue by manual or robotic pipetting. Upon grafting a liver microtissue, the microvascular bed undergoes anastomosis, resulting in a stable, perfusable vascular network. Interactions with vasculature were found in spheroids and organoids upon 7 days of co-culture with space of Disse-like architecture in between hepatocytes and endothelium. Veno-occlusive disease was induced by azathioprine exposure, leading to impeded perfusion of the vascularized spheroid. The platform holds the potential to replace animals with an in vitro alternative for routine grafting of spheroids, organoids, or (patient-derived) explants.

Syngeneic implantation of mouse hepatic progenitor cell-derived three-dimensional liver tissue with dense collagen fibrils

BACKGROUND

Liver transplantation is a therapy for irreversible liver failure; however, at present, donor organs are in short supply. Cell transplantation therapy for liver failure is still at the developmental stage and is critically limited by a shortage of human primary hepatocytes.

AIM

To investigate the possibility that hepatic progenitor cells (HPCs) prepared from the portal branch-ligated hepatic lobe may be used in regenerative medicine, we attempted to enable the implantation of extracellular matrices containing organoids consisting of HPC-derived hepatocytes and non-parenchymal cells.

METHODS

In vitro liver organoid tissue has been generated by accumulating collagen fibrils, fibroblasts, and HPCs on a mesh of polylactic acid fabric using a bioreactor; this was subsequently implanted into syngeneic wild-type mice.

RESULTS

The in vitro liver organoid tissues generated transplantable tissues in the condensed collagen fibril matrix and were obtained from the mouse through partial hepatectomy.

CONCLUSION

Liver organoid tissue was produced from expanded HPCs using an originally designed bioreactor system. This tissue was comparable to liver lobules, and with fibroblasts embedded in the network collagen fibrils of this artificial tissue, it is useful for reconstructing the hepatic interstitial structure.

Development of digital organ-on-a-chip to assess hepatotoxicity and extracellular vesicle-based anti-liver cancer immunotherapy

Organ-on-a-chip systems have been increasingly recognized as attractive platforms to assess toxicity and to develop new therapeutic agents. However, current organ-on-a-chip platforms are limited by a “single pot” design, which inevitably requires holistic analysis and limits parallel processing. Here, we developed a digital organ-on-a-chip by combining a microwell array with cellular microspheres, which significantly increased the parallelism over traditional organ-on-a-chip for drug development. Up to 127 uniform liver cancer microspheres in this digital organ-on-a-chip format served as individual analytical units, allowing for analysis with high consistency and quick response. Our platform displayed evident anti-cancer efficacy at a concentration of 10 μM for sorafenib, and had greater alignment than the “single pot” organ-on-a-chip with a previous in vivo study. In addition, this digital organ-on-a-chip demonstrated the treatment efficacy of natural killer cell-derived extracellular vesicles for liver cancer at 50 μg/mL. The successful development of this digital organ-on-a-chip platform provides high-parallelism and a low-variability analytical tool for toxicity assessment and the exploration of new anticancer modalities, thereby accelerating the joint endeavor to combat cancer.

Graphic abstract

Mimicking Native Liver Lobule Microarchitecture In Vitro with Parenchymal and Non-parenchymal Cells Using 3D Bioprinting for Drug Toxicity and Drug Screening Applications

Bioengineering an in vitro liver model recapitulating the native liver microarchitecture consisting of parenchymal and non-parenchymal cells is crucial in achieving cellular crosstalk and hepatic metabolic functions for accurate hepatotoxicity prediction. Bioprinting holds the promise of engineering constructs with precise control over the spatial distribution of multiple cells. Two distinct tissue-specific liver extracellular matrix (ECM)-based bioinks with excellent printability and rheological attributes are formulated for supporting parenchymal and non-parenchymal cells. A physiologically relevant human vascularized liver model is bioprinted with a novel liver ECM-based bioink laden with human adipose mesenchymal stem cell-derived hepatocyte-like cells (HLCs), human umbilical vein endothelial cells (HUVECs), and human hepatic stellate cells (HHSCs) using an extrusion-based bioprinting approach and validated for hepatotoxicity assessment. The HLC/HUVEC/HHSC-laden liver model resembles native alternate cords of hepatocytes with a functional sinusoidal lumen-like network in both horizontal and vertical directions, demonstrating enhanced albumin production, urea synthesis, and cytochrome P450 (CPR) activity. Furthermore, the liver model is evaluated for drug toxicity assessment following 24 h exposure to different concentrations of (i) non-hepatotoxicants aspirin and dexamethasone, (ii) idiosyncratic hepatotoxicant trovafloxacin mesylate, and (iii) clinical hepatotoxicant acetaminophen and troglitazone. A follow-up cell viability and metabolic competence evaluation by estimating DNA concentration, lactate dehydrogenase activity, and CPR activity revealed a dose-dependent clinically relevant hepatotoxic response. These results corroborated that the developed clinically relevant vascularized liver model is affordable and would aid pharmaceutical companies in speeding up the drug development and provide a robust platform for hepatotoxicity screening.

Constructing vascularized hepatic tissue by cell-assembled viscous tissue sedimentation method and its application for vascular toxicity assessment

In vitro Construction of the liver sinusoidal structure using artificial tissue is an important but worthwhile challenge, particularly for assessing the risk of diseases such as sinusoidal obstruction syndrome (SOS). Current models are unsuitable for evaluating the toxicity because of lacking sinusoidal capillary. In this study, we developed a vascularized hepatic tissue (VHT) using a unique tissue engineering technique, the cell assembled viscous tissue by sedimentation (CAViTs) method. The "viscous bodies" created using the CAViTs method exhibited significant self-assembly within 6 h after seeding, promoting cell-cell interaction. The level of albumin secreted by the VHT was four times higher than that of 2D-coculture and maintained for 1 month. The gene expression pattern of the VHT was closer to that of total human liver, compared with the 2D system. Quantitative evaluations of the vascular structure of VHT treated with two typical SOS-inducing compounds, monocrotaline and retrorsine, revealed higher sensitivity (IC₅₀ = 40.35 µM), 19.92 times higher than the cell-viability assay. Thus, VHT represents an innovative in vitro model that mimics the vessel network structure and could become a useful tool for the early screening of compounds associated with a risk of vascular toxicity. STATEMENT OF SIGNIFICANCE: Mimicking sinusoidal structures in in vitro liver model is important to consider from the perspective of predicting hepatotoxicity such like sinusoidal obstruction syndrome (SOS). However, it was difficult to reconstruct the vascular structure within the hepatocyte-rich environment. In this study, we constructed a vascularized hepatic tissue in a high-throughput manner by a unique method using collagen and heparin, and evaluated its applicability to toxicity assessment. Vessel morphology analysis of the model treated by monocrotaline, which is a well-known SOS-inducing compound, could predict the toxicity with higher sensitivity. To the best of our knowledge, this is the first report to provide vascularized hepatic tissues using sinusoidal endothelial cells at least for demonstrating applicability to the evaluation of SOS induction risk.

Three-dimensional Vascularized β-cell Spheroid Tissue Derived From Human Induced Pluripotent Stem Cells for Subcutaneous Islet Transplantation in a Mouse Model of Type 1 Diabetes

BACKGROUND

Islet transplantation is an effective replacement therapy for type 1 diabetes (T1D) patients. However, shortage of donor organ for allograft is obstacle for further development of the treatment. Subcutaneous transplantation with stem cell-derived β-cells might overcome this, but poor vascularity in the site is burden for success in the transplantation. We investigated the effect of subcutaneous transplantation of vascularized β-cell spheroid tissue constructed 3-dimensionally using a layer-by-layer (LbL) cell-coating technique in a T1D model mouse.

METHODS

We used MIN6 cells to determine optimal conditions for the coculture of β-cell spheroids, normal human dermal fibroblasts, and human umbilical vein endothelial cells, and then, under those conditions, we constructed vascularized spheroid tissue using human induced pluripotent stem cell-derived β-cells (hiPS β cells). The function of insulin secretion of the vascularized hiPS β-cell spheroid tissue was evaluated in vitro. Furthermore, the function was investigated in T1D model NOD/SCID mice subcutaneously transplanted with the tissue.

RESULTS

In vitro, the vascularized hiPS β-cell spheroid tissue exhibited enhanced insulin secretion. The vascularized hiPS β-cell spheroid tissue also significantly decreased blood glucose levels in diabetic immunodeficient mice when transplanted subcutaneously. Furthermore, host mouse vessels were observed in the explanted vascularized hiPS β-cell spheroid tissue.

CONCLUSIONS

Vascularized hiPS β-cell spheroid tissue decreased blood glucose levels in the diabetic mice. This therapeutic effect was suggested due to host angiogenesis in the graft. This method could lead to a promising regenerative treatment for T1D patients.

Recent Advances on Cell-Based Co-Culture Strategies for Prevascularization in Tissue Engineering

Currently, the fabrication of a functional vascular network to maintain the viability of engineered tissues is a major bottleneck in the way of developing a more advanced engineered construct. Inspired by vasculogenesis during the embryonic period, the in vitro prevascularization strategies have focused on optimizing communications and interactions of cells, biomaterial and culture conditions to develop a capillary-like network to tackle the aforementioned issue. Many of these studies employ a combination of endothelial lineage cells and supporting cells such as mesenchymal stem cells, fibroblasts, and perivascular cells to create a lumenized endothelial network. These supporting cells are necessary for the stabilization of the newly developed endothelial network. Moreover, to optimize endothelial network development without impairing biomechanical properties of scaffolds or differentiation of target tissue cells, several other factors, including target tissue, endothelial cell origins, the choice of supporting cell, culture condition, incorporated pro-angiogenic factors, and choice of biomaterial must be taken into account. The prevascularization method can also influence the endothelial lineage cell/supporting cell co-culture system to vascularize the bioengineered constructs. This review aims to investigate the recent advances on standard cells used in in vitro prevascularization methods, their co-culture systems, and conditions in which they form an organized and functional vascular network.

Engineering mammalian living materials towards clinically relevant therapeutics

Engineered living materials represent a new generation of human-made biotherapeutics that are highly attractive for a myriad of medical applications. In essence, such cell-rich platforms provide encodable bioactivities with extended lifetimes and environmental multi-adaptability currently unattainable in conventional biomaterial platforms. Emerging cell bioengineering tools are herein discussed from the perspective of materializing living cells as cooperative building blocks that drive the assembly of multiscale living materials. Owing to their living character, pristine cellular units can also be imparted with additional therapeutically-relevant biofunctionalities. On this focus, the most recent advances on the engineering of mammalian living materials and their biomedical applications are herein outlined, alongside with a critical perspective on major roadblocks hindering their realistic clinical translation. All in all, transposing the concept of leveraging living materials as autologous tissue-building entities and/or self-regulated biotherapeutics opens new realms for improving precision and personalized medicine strategies in the foreseeable future.

Three-Dimensional Liver Culture Systems to Maintain Primary Hepatic Properties for Toxicological Analysis In Vitro

Drug-induced liver injury (DILI) is the major reason for failures in drug development and withdrawal of approved drugs from the market. Two-dimensional cultures of hepatocytes often fail to reliably predict DILI: hepatoma cell lines such as HepG2 do not reflect important primary-like hepatic properties and primary human hepatocytes (pHHs) dedifferentiate quickly in vitro and are, therefore, not suitable for long-term toxicity studies. More predictive liver in vitro models are urgently required in drug development and compound safety evaluation. This review discusses available human hepatic cell types for in vitro toxicology analysis and their usage in established and emerging three-dimensional (3D) culture systems. Generally, 3D cultures maintain or improve primary hepatic functions (including expression of drug-metabolizing enzymes) of different liver cells for several weeks of culture, thus allowing long-term and repeated-dose toxicity studies. Spheroid cultures of pHHs have been comprehensively tested, but also other cell types such as HepaRG benefit from 3D culture systems. Emerging 3D culture techniques include usage of induced pluripotent stem-cell-derived hepatocytes and primary-like upcyte cells, as well as advanced culture techniques such as microfluidic liver-on-a-chip models. In-depth characterization of existing and emerging 3D hepatocyte technologies is indispensable for successful implementation of such systems in toxicological analysis.

Emerging liver organoid platforms and technologies

Building human organs in a dish has been a long term goal of researchers in pursue of physiologically relevant models of human disease and for replacement of worn out and diseased organs. The liver has been an organ of interest for its central role in regulating body homeostasis as well as drug metabolism. An accurate liver replica should contain the multiple cell types found in the organ and these cells should be spatially organized to resemble tissue structures. More importantly, the in vitro model should recapitulate cellular and tissue level functions. Progress in cell culture techniques and bioengineering approaches have greatly accelerated the development of advance 3-dimensional (3D) cellular models commonly referred to as liver organoids. These 3D models described range from single to multiple cell type containing cultures with diverse applications from establishing patient-specific liver cells to modeling of chronic liver diseases and regenerative therapy. Each organoid platform is advantageous for specific applications and presents its own limitations. This review aims to provide a comprehensive summary of major liver organoid platforms and technologies developed for diverse applications.

A Comprehensive Review on the Applications of Exosomes and Liposomes in Regenerative Medicine and Tissue Engineering

Tissue engineering and regenerative medicine are generally concerned with reconstructing cells, tissues, or organs to restore typical biological characteristics. Liposomes are round vesicles with a hydrophilic center and bilayers of amphiphiles which are the most influential family of nanomedicine. Liposomes have extensive research, engineering, and medicine uses, particularly in a drug delivery system, genes, and vaccines for treatments. Exosomes are extracellular vesicles (EVs) that carry various biomolecular cargos such as miRNA, mRNA, DNA, and proteins. As exosomal cargo changes with adjustments in parent cells and position, research of exosomal cargo constituents provides a rare chance for sicknesses prognosis and care. Exosomes have a more substantial degree of bioactivity and immunogenicity than liposomes as they are distinctly chiefly formed by cells, which improves their steadiness in the bloodstream, and enhances their absorption potential and medicinal effectiveness in vitro and in vivo. In this review, the crucial challenges of exosome and liposome science and their functions in disease improvement and therapeutic applications in tissue engineering and regenerative medicine strategies are prominently highlighted.

Effect of 3D-Fibroblast Dermis Constructed by Layer-by-Layer Cell Coating Technique on Tight Junction Formation and Function in Full-Thickness Skin Equivalent

Human skin equivalents (HSEs) consisting of an epidermis and dermis have been used as promising tools for drug evaluation and for clinical applications in regenerative medicine. Normal human dermal fibroblasts (NHDFs) are essential for the fabrication of HSEs because they play an important role in the maturation of the epidermis. Recently, epidermal tight junctions (TJs), which are complex cell-cell junctions, have attracted much attention as a second barrier and regulator for other barrier functions. In a previous study, we revealed the expression of TJ-related proteins and the time course of formation of TJ structure in the HSE (layer-by-layer (LbL)-three-dimensional (3D) Skin) constructed by layer-by-layer (LbL) cell coating technique that have a unique dermis consisting of NHDFs only (3D-fibroblast dermis). However, the effect of the 3D-fibroblast dermis on the formation of functional epidermal TJs is unknown. In this study, we investigated the effect of the 3D-fibroblast dermis on the expression of TJ-related proteins and TJ function in LbL-3D Skin. We demonstrated that the 3D-fibroblast dermis affects the long-term expression of TJ-related proteins and the formation of TJ with barrier function in the epidermis. These results show that the 3D-fibroblast dermis in LbL-3D Skin contributes to the formation and maintenance of functional TJs as in native human skin by direct contact with KCs.

Observation of a tight junction structure generated in LbL-3D skin reconstructed by layer-by-layer cell coating technique

Tissue-engineered skin equivalents are reconstructed the functions of human skin and can be used as an alternative to animal experiments in basic study or as cultured skin for regenerative medicine. Recent studies confirmed that epidermal tight junctions (TJs), which are complex intercellular junctions formed in the stratum granulosum of human skin, play an important part in the formation of the skin barrier function. In well-formed reconstructed human skin models, there are several reports on the expression of TJ proteins and their localization in epidermal layer, however, the morphological features of TJ, showing tight junctional contacts and the process of TJ formation have yet to be investigated. In this study, we systematically examined and identified TJ-related proteins and TJ structure in three-dimensional (3D) human skin equivalents reconstructed by layer-by-layer (LbL) cell coating technique (LbL-3D Skin). We demonstrate localization of TJ-related proteins and time course of formation of TJ structure with typical junctional morphology in LbL-3D Skin. These data provide evidence that the LbL-3D Skin is an in vitro model with structure and function extremely similar to living skin.

Three-Dimensional Idiopathic Pulmonary Fibrosis Model Using a Layer-by-Layer Cell Coating Technique

Idiopathic pulmonary fibrosis (IPF) is a severe health problem characterized by progressive fibroblast proliferation and aberrant vascular remodeling. However, the lack of a suitable in vitro model that replicates cell-specific changes in IPF tissue is a crucial issue. Three-dimensional (3D) cell cultures allow the mimicking of cell-specific functions, facilitating development of novel antifibrosis drugs. We have established a layer-by-layer (LbL) cell coating technique that enables the construction of 3D tissue and also vascularized 3D tissue. This study evaluated whether this technique is beneficial for constructing an in vitro IPF-3D model using human lung fibroblasts and microvascular endothelial cells. We fabricated an in vitro IPF-3D model to provide IPF-derived fibroblasts-specific function and aberrant microvascular structure using the LbL cell coating technique. We also found that this in vitro IPF-3D model showed drug responsiveness to two antifibrosis drugs that have recently been approved worldwide. This in vitro IPF-3D model constructed by a LbL cell coating technique would help in the understanding of fibroblast function and the microvascular environment in IPF and could also be used to predict the efficacy of novel antifibrosis drugs. Impact statement We established a novel in vitro model mimicking idiopathic pulmonary fibrosis. Three-dimensional culture was constructed by layer-by-layer cell coating technique. This novel model provides a visualization of fibroblast-specific function. This assay allows for the assessment of pulmonary microvascular environment. Our model may be useful for predicting the efficacy of novel antifibrosis drugs.

Liver Organoids: Recent Developments, Limitations and Potential

Liver cell types derived from induced pluripotent stem cells (iPSCs) share the potential to investigate development, toxicity, as well as genetic and infectious disease in ways currently limited by the availability of primary tissue. With the added advantage of patient specificity, which can play a role in all of these areas. Many iPSC differentiation protocols focus on 3 dimensional (3D) or organotypic differentiation, as these offer the advantage of more closely mimicking in vivo systems including; the formation of tissue like architecture and interactions/crosstalk between different cell types. Ultimately such models have the potential to be used clinically and either with or more aptly, in place of animal models. Along with the development of organotypic and micro-tissue models, there will be a need to co-develop imaging technologies to enable their visualization. A variety of liver models termed "organoids" have been reported in the literature ranging from simple spheres or cysts of a single cell type, usually hepatocytes, to those containing multiple cell types combined during the differentiation process such as hepatic stellate cells, endothelial cells, and mesenchymal cells, often leading to an improved hepatic phenotype. These allow specific functions or readouts to be examined such as drug metabolism, protein secretion or an improved phenotype, but because of their relative simplicity they lack the flexibility and general applicability of ex vivo tissue culture. In the liver field these are more often constructed rather than developed together organotypically as seen in other organoid models such as brain, kidney, lung and intestine. Having access to organotypic liver like surrogates containing multiple cell types with in vivo like interactions/architecture, would provide vastly improved models for disease, toxicity and drug development, combining disciplines such as microfluidic chip technology with organoids and ultimately paving the way to new therapies.

Engineering Vascularized Organoid-on-a-Chip Models

Recreating human organ-level function in vitro is a rapidly evolving field that integrates tissue engineering, stem cell biology, and microfluidic technology to produce 3D organoids. A critical component of all organs is the vasculature. Herein, we discuss general strategies to create vascularized organoids, including common source materials, and survey previous work using vascularized organoids to recreate specific organ functions and simulate tumor progression. Vascularization is not only an essential component of individual organ function but also responsible for coupling the fate of all organs and their functions. While some success in coupling two or more organs together on a single platform has been demonstrated, we argue that the future of vascularized organoid technology lies in creating organoid systems complete with tissue-specific microvasculature and in coupling multiple organs through a dynamic vascular network to create systems that can respond to changing physiological conditions.

Engineered Liver Tissue Culture in an In Vitro Tubular Perfusion System

Liver disease and the subsequent loss of liver function is an enormous clinical challenge. A severe shortage of donor liver tissue greatly limits patients' options for a timely transplantation. Tissue engineering approaches offer a promising alternative to organ transplantation by engineering artificial implantable tissues. We have established a platform of cell-laden microbeads as basic building blocks to assemble macroscopic tissues via different mechanisms. This modular fabrication strategy possesses great potential for liver tissue engineering in a bottom-up manner. In this study, we encapsulated human hepatocytes into microbeads presenting a favorable microenvironment consisting of collagen and mesenchymal stem cells, and then we perfused the beads in a three-dimensional printed tubular perfusion bioreactor that promoted oxygen and medium diffusion to the impregnated cells. We noted high cell vitality and retention of parenchymal cell functionality for up to 30 days in this culture system. Our engineering-based approach led to the advancement in tissue size and long-term functionality of an artificial liver tissue in vitro.

Modular assembly-based approach of loosely packing co-cultured hepatic tissue elements with endothelialization for liver tissue engineering

Background

In liver tissue engineering, co-culturing hepatocytes with typical non-parenchymal hepatic cells to form cell aggregates is available to mimic the in vivo microenvironment and promote cell biological functions. With a modular assembly approach, endothelialized hepatic cell aggregates can be packed for perfusion culture, which enables the construction of large-scale liver tissues. Since tightly packed aggregates tend to fuse with each other and block perfusion flows, a loosely packed mode was introduced in our study.

Methods

Using an oxygen-permeable polydimethylsiloxane (PDMS)-based microwell device, highly dense endothelialized hepatic cell aggregates were generated as hepatic tissue elements by co-culturing hepatocellular carcinoma (HepG2) cells, Swiss 3T3 cells, and human umbilical vein endothelial cells (HUVECs). The co-cultured aggregates were then harvested and applied in a PDMS-fabricated bioreactor for 10 days of perfusion culture. To maintain appropriate interstitial spaces for stable perfusion, biodegradable poly-L-lactic acid (PLLA) scaffold fibers were used and mixed with the aggregates, forming a loosely packed mode.

Results

In a microwell co-culture, Swiss 3T3 cells significantly contributed to the formation of hepatic cell aggregates. HUVECs developed a peripheral distribution in aggregates for endothelialization. In the perfusion culture, compared with pure HepG2 aggregates, HepG2/Swiss 3T3/HUVECs co-cultured aggregates exhibited a higher level of cell proliferation and liver-specific function expression (i.e., glucose consumption and albumin secretion). Under the loosely packed mode, co-cultured aggregates showed a characteristic histological morphology with cell migration and adhesion to fibers. The assembled hepatic tissue elements were obtained with 32% of in vivo cell density.

Conclusions

In a co-culture of HepG2, Swiss 3T3, and HUVECs, Swiss 3T3 cells were observed to be beneficial for the formation of endothelialized hepatic cell aggregates. Loosely packed aggregates enabled long-term perfusion culture with high viability and biological function. This study will guide us in constructing large-scale liver tissue models by way of aggregate-based modular assembly.

Fabrication and characterization of a thick, viable bi-layered stem cell-derived surrogate for future myocardial tissue regeneration

Cardiac tissue surrogates show promise for restoring mechanical and electrical function in infarcted left ventricular (LV) myocardium. For these cardiac surrogates to be useful in vivo, they are required to support synchronous and forceful contraction over the infarcted region. These design requirements necessitate a thickness sufficient to produce a useful contractile force, an area large enough to cover an infarcted region, and prevascularization to overcome diffusion limitations. Attempts to meet these requirements have been hampered by diffusion limits of oxygen and nutrients (100-200 μm) leading to necrotic regions.This study demonstrates a novel layer-by-layer (LbL) fabrication method used to produce tissue surrogates that meet these requirements and mimic normal myocardium in form and function. Thick (1.5-2 mm) LbL cardiac tissues created from human induced pluripotent stem cell-derived cardiomyocytes and endothelial cells were assessed, in vitro, over a four week period for viability (< 5.6 ± 1.4 % nectrotic cells), cell morphology, viscoelastic properties and functionality. Viscoelastic properties of the cardiac surrogates were determined via stress relaxation response modeling and compared to native murine LV tissue. Viscoelastic characterization showed that the generalized Maxwell model of order 4 described the samples well (0.7 < R2 < 0.98). Functional performance assessment showed enhanced t-tubule network development, gap junction communication as well as conduction velocity (16.9 ± 2.3 cm s-1). These results demonstrate that LbL fabrication can be utilized successfully in creating complex, functional cardiac surrogates for therapeutic applications.

Supersensitive Layer-by-Layer 3D Cardiac Tissues Fabricated on a Collagen Culture Vessel Using Human-Induced Pluripotent Stem Cells

Background: The fabrication of artificial cardiac tissue is an active area of research due to the shortage of donors for heart transplantation and for drug development. In our previous study, we fabricated vascularized three-dimensional (3D) cardiac tissue by layer-by-layer (LbL) and cell accumulation technique. However, it was not able to develop sufficient function because it was cultured on a hard plastic substrate. Experiment: Herein, we report the fabrication of high-performance 3D cardiac tissue by LbL and cell accumulation technique using a collagen culture vessel. Results: By using a collagen culture vessel, 3D cardiac tissue could be fabricated on a collagen culture vessel and this tissue showed high functionality due to improved interaction with the vessel. In the case of the plastic culture insert, 3D cardiac tissue was found to be peeled off, but this did not occur on the collagen culture vessel. In addition, the 3D cardiac tissue fabricated on a collagen culture vessel showed contraction that was 20 times larger than the tissue fabricated on a plastic culture insert. As a result of evaluation of cardiotoxicity using E-4031, the sensitivity of arrhythmia detection was increased by using collagen culture vessel. Conclusions: These results are expected to contribute to transplantation and drug discovery research as a 3D cardiac tissue model with a function similar to that of the living heart.

Construction of Vascularized Oral Mucosa Equivalents Using a Layer-by-Layer Cell Coating Technology

There have been many advances in tissue engineering with respect to in vitro and in vivo models of oral mucosa equivalents (OMEs). To apply in vitro reconstructed oral mucosa models to regenerative medicine and alternatives to animal testing, it is necessary to develop the technology of reconstructing different types of oral tissues, such as control of epithelial differentiation and introduction of appendages. We previously reported that functional three-dimensional (3D) tissue models could be quickly constructed by using a layer-by-layer (LbL) cell coating technique that assembles extracellular matrix (ECM) nanofilms to a cell surface. In this study, 3D human OMEs composed of lamina-propria, keratinized or non-keratinized epithelium, and blood capillaries were constructed by using the LbL cell coating technology. Human oral mucosal fibroblasts (HOMFs) were coated with ECM nanofilms and accumulated for the construction of oral mucosal lamina-propria. To construct OMEs with keratinized or non-keratinized epithelium, human oral keratinocytes isolated from gingiva (human oral gingival keratinocytes: HOGKs) or human oral keratinocytes isolated from oral mucosa (human oral mucosal keratinocytes: HOMKs) were used in this study. We further studied the construction of epithelialized OMEs with density- and size-controlled blood capillary networks by using human umbilical vein endothelial cells (HUVECs). It was revealed that these constructions had barrier functions in accordance with their histological characterization. The OMEs with keratinization (K-OMEs) showed higher transepithelial electrical resistance (TEER) values compared with OMEs with non-keratinization (N-OMEs). The constructed epithelialized OMEs with blood capillaries are useful for in vitro/ex vivo research models and regenerative medicine as in oral tissue regeneration. The results suggest that OMEs with oral tissue appendages are more promising alternatives to animal testing and can be applied to the design of in vitro oral models that mimic human tissue organs.

Noninvasive optical coherence tomography imaging of three-dimensional cardiac tissues derived from human induced pluripotent stem cells

Artificial three-dimensional (3D) tissues have the potential to be used in regenerative medicine or in vitro screening. In particular, the fabrication of 3-D cardiac tissues is greatly anticipated. However, hierarchical organization of 3-D tissues is still unknown. In regenerative medicine and drug discovery, noninvasive evaluation methods of 3-D tissues including inside of it play a key role. In this study, we report on noninvasive methods of analyzing bio-fabricated 3-D cardiac tissues using optical coherence tomography (OCT) and image analysis. Three-dimensional cardiac tissues were fabricated by coating of extracellular matrix nanofilms onto a cell surface using a layer-by-layer (LbL) technique. At first, we investigated the relationship between surface beating and its thickness to assess the value of internal analysis. The results showed that the surface beating was influenced by the thickness. Next, we tried to quantitatively evaluate the internal beating of 3-D cardiac tissues. We also confirmed the methods by changing the beating properties through the administration of isoproterenol. Our results demonstrated that the beating properties of 3-D cardiac tissues differed by depth. The results of this study suggest that information on the internal properties of 3-D cardiac tissue was necessary to understand how it functions. The combination of OCT and image analysis can be used to evaluate the internal beating properties, including changes in beating induced by a drug. It is suggested that OCT and image analysis have the potential to be used as noninvasive methods in regenerative medicine and pharmaceutical applications.

Novel alternative transplantation therapy for orthotopic liver transplantation in liver failure: A systematic review

BACKGROUND

Orthotopic liver transplantation (OLT) is the only treatment for end-stage liver failure; however, graft shortage impedes its applicability. Therefore, studies investigating alternative therapies are plenty. Nevertheless, no study has comprehensively analyzed these therapies from different perspectives.

AIM

To summarize the current status of alternative transplantation therapies for OLT and to support future research.

METHODS

A systematic literature search was performed using PubMed, Cochrane Library and EMBASE for articles published between January 2010 and 2018, using the following MeSH terms: [(liver transplantation) AND cell] OR [(liver transplantation) AND differentiation] OR [(liver transplantation) AND organoid] OR [(liver transplantation) AND xenotransplantation]. Various types of studies describing therapies to replace OLT were retrieved for full-text evaluation. Among them, we selected articles including in vivo transplantation.

RESULTS

A total of 89 studies were selected. There are three principle forms of treatment for liver failure: Xeno-organ transplantation, scaffold-based transplantation, and cell transplantation. Xeno-organ transplantation was covered in 14 articles, scaffold-based transplantation was discussed in 22 articles, and cell transplantation was discussed in 53 articles. Various types of alternative therapies were discussed: Organ liver, 25 articles; adult hepatocytes, 31 articles; fetal hepatocytes, three articles; mesenchymal stem cells (MSCs), 25 articles; embryonic stem cells, one article; and induced pluripotent stem cells, three articles and other sources. Clinical applications were discussed in 12 studies: Cell transplantation using hepatocytes in four studies, five studies using umbilical cord-derived MSCs, three studies using bone marrow-derived MSCs, and two studies using hematopoietic stem cells.

CONCLUSION

The clinical applications are present only for cell transplantation. Scaffold-based transplantation is a comprehensive treatment combining organ and cell transplantations, which warrants future research to find relevant clinical applications.

Vascularized cardiac tissue construction with orientation by layer-by-layer method and 3D printer

Herein, we report the fabrication of native organ-like three-dimensional (3D) cardiac tissue with an oriented structure and vascular network using a layer-by-layer (LbL), cell accumulation and 3D printing technique for regenerative medicine and pharmaceutical applications. We firstly evaluated the 3D shaping ability of hydroxybutyl chitosan (HBC), a thermoresponsive polymer, by using a robotic dispensing 3D printer. Next, we tried to fabricate orientation-controlled 3D cardiac tissue using human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM) and normal human cardiac fibroblasts (NHCF) coated with extracellular matrix (ECM) nanofilms by layer-by-layer technique. These cells were seeded in the fabricated rectangular shape HBC gel frame. After cultivation of the fabricated tissue, fluorescence staining of the cytoskeleton revealed that hiPSC-CM and NHCF were aligned in one direction. Moreover, we were able to measure its contractile behavior using a video image analysis system. These results indicate that orientation-controlled cardiac tissue has more remarkable contractile function than uncontrolled cardiac tissue. Finally, co-culture with human cardiac microvascular endothelial cells (HMVEC) successfully provided a vascular network in orientation-controlled 3D cardiac tissue. The constructed 3D cardiac tissue with an oriented structure and vascular network would be a useful tool for regenerative medicine and pharmaceutical applications.

Preparation of multifunctional poly(l-lactic acid) film using heparin-mimetic polysaccharide multilayers: Hemocompatibility, cytotoxicity, antibacterial and drug loading/releasing properties

Poly(l-lactic acid) (PLLA) has been the most commonly used polymer for making bioresorbable vascular scaffolds (BVS). Despite owning remarkable properties, BVS made from PLLA are facing higher rates of early thrombosis compared with permanent metallic scaffolds. To solve this issue, we modified the PLLA film surface with heparin-mimetic polysaccharide multilayers consisting of sulfated Chinese yam polysaccharide (SCYP) and chitosan (CS) through layer-by-layer (LBL) assembly. The surface chemical compositions, morphologies and growth manner of SCYP/CS multilayers were investigated using X-ray photoelectron spectroscopy, scanning electron microscopy, atomic force microscopy and UV-vis spectroscopy. The relevant hemocompatibility results showed that multilayer-modified PLLA could effectively resist protein adsorption, suppress the platelet adhesion, prolong clotting time, prevent contact and complement activation as well as reduce hemolysis rate. Moreover, the multilayer-modified PLLA exhibited non-cytotoxicity, good antibacterial ability against E. coli and S. aureus, and drug loading/sustained releasing behavior. Overall, the multifunctional PLLA film with integrated properties of hemocompatibility, non-cytotoxicity, antibacterial and drug loading/releasing behavior could be successfully achieved by deposition of SCYP/CS multilayers, which will have potential application in blood-contacting biomedical materials.

Advanced Biomaterials and Processing Methods for Liver Regeneration: State-of-the-Art and Future Trends

Liver diseases contribute markedly to the global burden of mortality and disease. The limited organ disposal for orthotopic liver transplantation results in a continuing need for alternative strategies. Over the past years, important progress has been made in the field of tissue engineering (TE). Many of the early trials to improve the development of an engineered tissue construct are based on seeding cells onto biomaterial scaffolds. Nowadays, several TE approaches have been developed and are applied to one vital organ: the liver. Essential elements must be considered in liver TE-cells and culturing systems, bioactive agents or growth factors (GF), and biomaterials and processing methods. The potential of hepatocytes, mesenchymal stem cells, and others as cell sources is demonstrated. They need engineered biomaterial-based scaffolds with perfect biocompatibility and bioactivity to support cell proliferation and hepatic differentiation as well as allowing extracellular matrix deposition and vascularization. Moreover, they require a microenvironment provided using conventional or advanced processing technologies in order to supply oxygen, nutrients, and GF. Herein the biomaterials and the conventional and advanced processing technologies, including cell-sheets process, 3D bioprinting, and microfluidic systems, as well as the future trends in these major fields are discussed.

Advanced Bottom-Up Engineering of Living Architectures

Bottom-up tissue engineering is a promising approach for designing modular biomimetic structures that aim to recapitulate the intricate hierarchy and biofunctionality of native human tissues. In recent years, this field has seen exciting progress driven by an increasing knowledge of biological systems and their rational deconstruction into key core components. Relevant advances in the bottom-up assembly of unitary living blocks toward the creation of higher order bioarchitectures based on multicellular-rich structures or multicomponent cell-biomaterial synergies are described. An up-to-date critical overview of long-term existing and rapidly emerging technologies for integrative bottom-up tissue engineering is provided, including discussion of their practical challenges and required advances. It is envisioned that a combination of cell-biomaterial constructs with bioadaptable features and biospecific 3D designs will contribute to the development of more robust and functional humanized tissues for therapies and disease models, as well as tools for fundamental biological studies.

One-Step Rapid Fabrication of Cell-Only Living Fibers

Cellular aggregates are used as relevant regenerative building blocks, tissue models, and cell delivery platforms. Biomaterial-free structures are often assembled either as 2D cell sheets or spherical microaggregates, both incompatible with free-form deposition, and dependent on challenging processes for macroscale 3D upscaling. The continuous and elongated nature of fiber-shaped materials enables their deposition in unrestricted multiple directions. Cellular fiber fabrication has often required exogenously provided support proteins and/or the use of biomaterial-based sacrificial templates. Here, the rapid (<24 h) assembly of fiberoids is reported: living centimeter-long scaffold-free fibers of cells produced in the absence of exogenous materials or supplements. Adipose-derived mesenchymal stem cell fiberoids can be easily modulated into complex multidimensional geometries and show tissue-invasive properties while keeping the secretion of trophic factors. Proangiogenic properties studied on a chick chorioallantoic membrane in an ovo model are observed for heterotypic fiberoids containing endothelial cells. These micro-to-macrotissues may find application as morphogenic therapeutic and tissue-mimetic building blocks, with the ability to integrate 3D and 4D full biological materials.

Engineered Liver-on-a-Chip Platform to Mimic Liver Functions and Its Biomedical Applications: A Review

Hepatology and drug development for liver diseases require in vitro liver models. Typical models include 2D planar primary hepatocytes, hepatocyte spheroids, hepatocyte organoids, and liver-on-a-chip. Liver-on-a-chip has emerged as the mainstream model for drug development because it recapitulates the liver microenvironment and has good assay robustness such as reproducibility. Liver-on-a-chip with human primary cells can potentially correlate clinical testing. Liver-on-a-chip can not only predict drug hepatotoxicity and drug metabolism, but also connect other artificial organs on the chip for a human-on-a-chip, which can reflect the overall effect of a drug. Engineering an effective liver-on-a-chip device requires knowledge of multiple disciplines including chemistry, fluidic mechanics, cell biology, electrics, and optics. This review first introduces the physiological microenvironments in the liver, especially the cell composition and its specialized roles, and then summarizes the strategies to build a liver-on-a-chip via microfluidic technologies and its biomedical applications. In addition, the latest advancements of liver-on-a-chip technologies are discussed, which serve as a basis for further liver-on-a-chip research.

A Layer-by-Layer Single-Cell Coating Technique To Produce Injectable Beating Mini Heart Tissues via Microfluidics

Human induced pluripotent stem cells (hiPSCs) are used as an alternative for human embryonic stem cells. Cardiomyocytes derived from hiPSCs are employed in cardiac tissue regeneration constructs due to the heart's low regeneration capacity after infarction. A coculture of hiPSC-CM and primary dermal fibroblasts is encapsulated in injectable poly(ethylene glycol)-based microgels via microfluidics to enhance the efficiency of regenerative cell transplantations. The microgels are prepared via Michael-type addition of multi-arm PEG-based molecules with an enzymatically degradable peptide as a cross-linker and modified with a cell-adhesive peptide. Cell-cell interactions and, consequently, cell viability are improved by a thin extracellular matrix (ECM) coating formed on the cell surfaces via layer-by-layer (LbL) deposition. The beating strength of encapsulated cardiomyocytes (∼60 BPM) increases by 2-fold compared to noncoated cells. The combination of microfluidics with the LbL technique offers a new technology to fabricate functional cardiac mini tissues for cell transplantation therapies.

Engineering tissue-specific blood vessels

Vascular diversity among organs has recently become widely recognized. Several studies using mouse and human fetal tissues revealed distinct characteristics of organ-specific vasculature in molecular and functional levels. Thorough understanding of vascular heterogeneities in human adult tissues is significant for developing novel strategies for targeted drug delivery and tissue regeneration. Recent advancements in microfabrication techniques, biomaterials, and differentiation protocols allowed for incorporation of microvasculature into engineered organs. Such vascularized organ models represent physiologically relevant platforms that may offer innovative tools for dissecting the effects of the organ microenvironment on vascular development and expand our present knowledge on organ-specific human vasculature. In this article, we provide an overview of the current structural and molecular evidence on microvascular diversity, bioengineering methods used to recapitulate the microenvironmental cues, and recent vascularized three-dimensional organ models from the perspective of tissue-specific vasculature.

A novel strategy to engineer pre-vascularized 3-dimensional skin substitutes to achieve efficient, functional engraftment

Autologous split-thickness skin grafts are the preferred treatment for excised burn wounds, but donor sites for autografting are often limited in patients with extensive burns. A number of alternative treatments are already in use to treat large burns and ulcers. Despite intense efforts to develop tissue-engineered skin, delayed or absent vascularization is one of the major reasons for tissue-engineered skin engraftment failure. To overcome these problems, we developed a scaffold-free 3-dimensional (3D) skin substitute containing vascular networks that combine dermal fibroblasts, endothelial cells, and epidermal keratinocytes based on our layer-by-layer cell coating technique. We transplanted the pre-vascularized 3D skin substitutes onto full-thickness skin defects on severe combined immunodeficiency mice to assess their integration with the host tissue and effects on wound healing. We used non-vascularized 3D skin substitutes as a control. Vessels containing red blood cells were evident in the non-vascularized control by day 14. However, blood perfusion of the human-derived vasculature could be detected within 7 days of grafting. Moreover, the pre-vascularized 3D skin substitutes had high graft survival and their epidermal layers were progressively replaced by mouse epidermis. We propose that a novel dermo-epidermal 3D skin substitute containing blood vessels can promote efficient reconstruction of full-thickness skin defects.

The influence of the ratio of liver cells and bone marrow in the implantable cell-engineering structures of the liver on the recovery efficiency of functional and morphological parameters in chronic liver failure

Aim:to determinate the most effective liver cells and multipotent mesenchymal stromal cells of bone marrow (MMSC BM) ratio into implantable cell engineering constructions (CECs) used for chronic liver failure (CLF) correcting.

Materials and methods.For creating liver CECs it was used a biopolymer implant – a composition of a heterogeneous collagen-containing gel (BMCG) (Sphero®GEL trademark) containing viable liver cells and MMSC BM in the following ratios – 1 : 1; 5 : 1 and 10 : 1 respectively. CECs with different ratios of liver cells and MMSC BM were implanted into liver of rats in which chronic liver failure (CLF), was modeled by using CCl4. The effectiveness of the regulatory effects of CECs (with different cell ratios) on regenerative processes in livers were assessed by using biochemical, morphological and morphometric methods at different periods after their implantation.

Results.Corrective effect of CECs with different cell composition on biochemical and morphological parameters of livers at chronic liver failure was established. During studying the liver CECs with various cell ratios of liver cells and MMSC BM (1 : 1; 5 : 1 and 10 : 1 respectively), it was found that the most optimal ratio of cells into the CECs is 5 : 1, because at this ratio of cells, there were a more distinct normalization of the morphological and functional liver parameters within 365 days after modeling CLF and maintenance of the structural homeostasis into the CECs. Themselves, which allows predicting their long-term regulatory effect on the liver tissue in CLF and maintaining its normal structural and functional state.

Conclusion.The effective correction of chronic liver failure can be carried out by using the implanted liver CECs, in which donor liver cells and MMSC BM where presented in ratios – 1 : 1; 5 : 1 and 10 : 1. But analysis of prolonged correction of liver morphological and functional parameters at CECs using it was allow to recommend the preferences using of CECs with ratio 5 : 1, because prolonged preservation of structural homeostasis into these CECs makes possible to prognosticate their prolonged regulatory action on the liver tissue at CLF, especially for recipients on a waiting list for liver transplantation.