Editor's Highlight: Modeling Compound-Induced Fibrogenesis In Vitro Using Three-Dimensional Bioprinted Human Liver Tissues

3D bioprinting of liver models: A systematic scoping review of methods, bioinks, and reporting quality

Background

Effective communication is crucial for broad acceptance and applicability of alternative methods in 3R biomedical research and preclinical testing. 3D bioprinting is used to construct intricate biological structures towards functional liver models, specifically engineered for deployment as alternative models in drug screening, toxicological investigations, and tissue engineering. Despite a growing number of reviews in this emerging field, a comprehensive study, systematically assessing practices and reporting quality for bioprinted liver models is missing.

Methods

In this systematic scoping review we systematically searched MEDLINE (Ovid), EMBASE (Ovid) and BioRxiv for studies published prior to June 2nd, 2022. We extracted data on methodological conduct, applied bioinks, the composition of the printed model, performed experiments and model applications. Records were screened for eligibility and data were extracted from included articles by two independent reviewers from a panel of seven domain experts specializing in bioprinting and liver biology. We used RAYYAN for the screening process and SyRF for data extraction. We used R for data analysis, and R and Graphpad PRISM for visualization.

Results

Through our systematic database search we identified 1042 records, from which 63 met the eligibility criteria for inclusion in this systematic scoping review. Our findings revealed that extrusion-based printing, in conjunction with bioinks composed of natural components, emerged as the predominant printing technique in the bioprinting of liver models. Notably, the HepG2 hepatoma cell line was the most frequently employed liver cell type, despite acknowledged limitations. Furthermore, 51% of the printed models featured co-cultures with non-parenchymal cells to enhance their complexity. The included studies offered a variety of techniques for characterizing these liver models, with their primary application predominantly focused on toxicity testing. Among the frequently analyzed liver markers, albumin and urea stood out. Additionally, Cytochrome P450 (CYP) isoforms, primarily CYP3A and CYP1A, were assessed, and select studies employed nuclear receptor agonists to induce CYP activity.

Conclusion

Our systematic scoping review offers an evidence-based overview and evaluation of the current state of research on bioprinted liver models, representing a promising and innovative technology for creating alternative organ models. We conducted a thorough examination of both the methodological and technical facets of model development and scrutinized the reporting quality within the realm of bioprinted liver models. This systematic scoping review can serve as a valuable template for systematically evaluating the progress of organ model development in various other domains. The transparently derived evidence presented here can provide essential support to the research community, facilitating the adaptation of technological advancements, the establishment of standards, and the enhancement of model robustness. This is particularly crucial as we work toward the long-term objective of establishing new approach methods as reliable alternatives to animal testing, with extensive and versatile applications.

Role of Hepatic Stellate and Liver Sinusoidal Endothelial Cells in a Human Primary Cell Three-Dimensional Model of Nonalcoholic Steatohepatitis

Nonalcoholic steatohepatitis (NASH) is an inflammatory and fibrotic liver disease that has reached epidemic proportions and has no approved pharmacologic therapies. Research and drug development efforts are hampered by inadequate preclinical models. This research describes a three-dimensional bioprinted liver tissue model of NASH built using primary human hepatocytes and nonparenchymal liver cells (hepatic stellate cells, liver sinusoidal endothelial cells, and Kupffer cells) from either healthy or NASH donors. Three-dimensional tissues bioprinted with cells sourced from diseased patients showed a NASH phenotype, including fibrosis. More importantly, this NASH phenotype occurred without the addition of disease-inducing agents. Bioprinted tissues composed entirely of healthy cells exhibited significantly less evidence of disease. The role of individual cell types in driving the NASH phenotype was examined by producing chimeric bioprinted tissues composed of healthy cells together with the addition of one or more diseased nonparenchymal cell types. These experiments reveal a role for both hepatic stellate and liver sinusoidal endothelial cells in the disease process. This model represents a fully human system with potential to detect clinically active targets and eventually therapies.

A comprehensive review on 3D tissue models: Biofabrication technologies and preclinical applications

The limitations of traditional two-dimensional (2D) cultures and animal testing, when it comes to precisely foreseeing the toxicity and clinical effectiveness of potential drug candidates, have resulted in a notable increase in the rate of failure during the process of drug discovery and development. Three-dimensional (3D) in-vitro models have arisen as substitute platforms with the capacity to accurately depict in-vivo conditions and increasing the predictivity of clinical effects and toxicity of drug candidates. It has been found that 3D models can accurately represent complex tissue structure of human body and can be used for a wide range of disease modeling purposes. Recently, substantial progress in biomedicine, materials and engineering have been made to fabricate various 3D in-vitro models, which have been exhibited better disease progression predictivity and drug effects than convention models, suggesting a promising direction in pharmaceutics. This comprehensive review highlights the recent developments in 3D in-vitro tissue models for preclinical applications including drug screening and disease modeling targeting multiple organs and tissues, like liver, bone, gastrointestinal tract, kidney, heart, brain, and cartilage. We discuss current strategies for fabricating 3D models for specific organs with their strengths and pitfalls. We expand future considerations for establishing a physiologically-relevant microenvironment for growing 3D models and also provide readers with a perspective on intellectual property, industry, and regulatory landscape.

The Material World of 3D-Bioprinted and Microfluidic-Chip Models of Human Liver Fibrosis

Biomaterials are extensively used to mimic cell-matrix interactions, which are essential for cell growth, function, and differentiation. This is particularly relevant when developing in vitro disease models of organs rich in extracellular matrix, like the liver. Liver disease involves a chronic wound-healing response with formation of scar tissue known as fibrosis. At early stages, liver disease can be reverted, but as disease progresses, reversion is no longer possible, and there is no cure. Research for new therapies is hampered by the lack of adequate models that replicate the mechanical properties and biochemical stimuli present in the fibrotic liver. Fibrosis is associated with changes in the composition of the extracellular matrix that directly influence cell behavior. Biomaterials could play an essential role in better emulating the disease microenvironment. In this paper, the recent and cutting-edge biomaterials used for creating in vitro models of human liver fibrosis are revised, in combination with cells, bioprinting, and/or microfluidics. These technologies have been instrumental to replicate the intricate structure of the unhealthy tissue and promote medium perfusion that improves cell growth and function, respectively. A comprehensive analysis of the impact of material hints and cell-material interactions in a tridimensional context is provided.

Three Dimensional Bioprinting for Hepatic Tissue Engineering: From In Vitro Models to Clinical Applications

Fabrication of functional organs is the holy grail of tissue engineering and the possibilities of repairing a partial or complete liver to treat chronic liver disorders are discussed in this review. Liver is the largest gland in the human body and plays a responsible role in majority of metabolic function and processes. Chronic liver disease is one of the leading causes of death globally and the current treatment strategy of organ transplantation holds its own demerits. Hence there is a need to develop an in vitro liver model that mimics the native microenvironment. The developed model should be a reliable to understand the pathogenesis, screen drugs and assist to repair and replace the damaged liver. The three-dimensional bioprinting is a promising technology that recreates in vivo alike in vitro model for transplantation, which is the goal of tissue engineers. The technology has great potential due to its precise control and its ability to homogeneously distribute cells on all layers in a complex structure. This review gives an overview of liver tissue engineering with a special focus on 3D bioprinting and bioinks for liver disease modelling and drug screening.

Organ bioprinting: progress, challenges and outlook

Bioprinting, as a groundbreaking technology, enables the fabrication of biomimetic tissues and organs with highly complex structures, multiple cell types, mechanical heterogeneity, and diverse functional gradients. With the growing demand for organ transplantation and the limited number of organ donors, bioprinting holds great promise for addressing the organ shortage by manufacturing completely functional organs. While the bioprinting of complete organs remains a distant goal, there has been considerable progress in the development of bioprinted transplantable tissues and organs for regenerative medicine. This review article recapitulates the current achievements of organ 3D bioprinting, primarily encompassing five important organs in the human body (i.e., the heart, kidneys, liver, pancreas, and lungs). Challenges from cellular techniques, biomanufacturing technologies, and organ maturation techniques are also deliberated for the broad application of organ bioprinting. In addition, the integration of bioprinting with other cutting-edge technologies including machine learning, organoids, and microfluidics is envisioned, which strives to offer the reader the prospect of bioprinting in constructing functional organs.

Liver Injury and Regeneration: Current Understanding, New Approaches, and Future Perspectives

The liver is a complex organ with the ability to regenerate itself in response to injury. However, several factors can contribute to liver damage beyond repair. Liver injury can be caused by viral infections, alcoholic liver disease, non-alcoholic steatohepatitis, and drug-induced liver injury. Understanding the cellular and molecular mechanisms involved in liver injury and regeneration is critical to developing effective therapies for liver diseases. Liver regeneration is a complex process that involves the interplay of various signaling pathways, cell types, and extracellular matrix components. The activation of quiescent hepatocytes that proliferate and restore the liver mass by upregulating genes involved in cell-cycle progression, DNA repair, and mitochondrial function; the proliferation and differentiation of progenitor cells, also known as oval cells, into hepatocytes that contribute to liver regeneration; and the recruitment of immune cells to release cytokines and angiogenic factors that promote or inhibit cell proliferation are some examples of the regenerative processes. Recent advances in the fields of gene editing, tissue engineering, stem cell differentiation, small interfering RNA-based therapies, and single-cell transcriptomics have paved a roadmap for future research into liver regeneration as well as for the identification of previously unknown cell types and gene expression patterns. In summary, liver injury and regeneration is a complex and dynamic process. A better understanding of the cellular and molecular mechanisms driving this phenomenon could lead to the development of new therapies for liver diseases and improve patient outcomes.

Drug-induced oxidative stress in cancer treatments: Angel or devil?

Oxidative stress (OS), defined as redox imbalance in favor of oxidant burden, is one of the most significant biological events in cancer progression. Cancer cells generally represent a higher oxidant level, which suggests a dual therapeutic strategy by regulating redox status (i.e., pro-oxidant therapy and/or antioxidant therapy). Indeed, pro-oxidant therapy exhibits a great anti-cancer capability, attributing to a higher oxidant accumulation within cancer cells, whereas antioxidant therapy to restore redox homeostasis has been claimed to fail in several clinical practices. Targeting the redox vulnerability of cancer cells by pro-oxidants capable of generating excessive reactive oxygen species (ROS) has surfaced as an important anti-cancer strategy. However, multiple adverse effects caused by the indiscriminate attacks of uncontrolled drug-induced OS on normal tissues and the drug-tolerant capacity of some certain cancer cells greatly limit their further applications. Herein, we review several representative oxidative anti-cancer drugs and summarize their side effects on normal tissues and organs, emphasizing that seeking a balance between pro-oxidant therapy and oxidative damage is of great value in exploiting next-generation OS-based anti-cancer chemotherapeutics.

The native liver as inspiration to create superior in vitro hepatic models

Drug induced liver injury (DILI) is one of the major reasons of drug withdrawal during the different phases of drug development. The later in the drug development a drug is discovered to be toxic, the higher the economical as well as the ethical impact will be. In vitro models for early detection of drug liver toxicity are under constant development, however to date a superior model of the liver is still lacking. Ideally, a highly reliable model should be established to maintain the different hepatic cell functionalities to the greatest extent possible, during a period of time long enough to allow for tracking of the toxicity of compounds. In the case of DILI, toxicity can appear even after months of exposure. To reach this goal, an in vitro model should be developed that mimics the in vivo liver environment, function and response to external stimuli. The different approaches for the development of liver models currently used in the field of tissue engineering will be described in this review. Combining different technologies, leading to optimal materials, cells and 3D-constructs will ultimately lead to an ideal superior model that fully recapitulates the liver.

Physiomimetic In Vitro Human Models for Viral Infection in the Liver

Viral hepatitis is a leading cause of liver morbidity and mortality globally. The mechanisms underlying acute infection and clearance, versus the development of chronic infection, are poorly understood. In vitro models of viral hepatitis circumvent the high costs and ethical considerations of animal models, which also translate poorly to studying the human-specific hepatitis viruses. However, significant challenges are associated with modeling long-term infection in vitro. Differentiated hepatocytes are best able to sustain chronic viral hepatitis infection, but standard two-dimensional models are limited because they fail to mimic the architecture and cellular microenvironment of the liver, and cannot maintain a differentiated hepatocyte phenotype over extended periods. Alternatively, physiomimetic models facilitate important interactions between hepatocytes and their microenvironment by incorporating liver-specific environmental factors such as three-dimensional ECM interactions and co-culture with non-parenchymal cells. These physiologically relevant interactions help maintain a functional hepatocyte phenotype that is critical for sustaining viral hepatitis infection. In this review, we provide an overview of distinct, novel, and innovative in vitro liver models and discuss their functionality and relevance in modeling viral hepatitis. These platforms may provide novel insight into mechanisms that regulate viral clearance versus progression to chronic infections that can drive subsequent liver disease.

Large-scale perfused tissues via synthetic 3D soft microfluidics

The vascularization of engineered tissues and organoids has remained a major unresolved challenge in regenerative medicine. While multiple approaches have been developed to vascularize in vitro tissues, it has thus far not been possible to generate sufficiently dense networks of small-scale vessels to perfuse large de novo tissues. Here, we achieve the perfusion of multi-mm³ tissue constructs by generating networks of synthetic capillary-scale 3D vessels. Our 3D soft microfluidic strategy is uniquely enabled by a 3D-printable 2-photon-polymerizable hydrogel formulation, which allows for precise microvessel printing at scales below the diffusion limit of living tissues. We demonstrate that these large-scale engineered tissues are viable, proliferative and exhibit complex morphogenesis during long-term in-vitro culture, while avoiding hypoxia and necrosis. We show by scRNAseq and immunohistochemistry that neural differentiation is significantly accelerated in perfused neural constructs. Additionally, we illustrate the versatility of this platform by demonstrating long-term perfusion of developing neural and liver tissue. This fully synthetic vascularization platform opens the door to the generation of human tissue models at unprecedented scale and complexity.

Preclinical-to-clinical innovations in stem cell therapies for liver regeneration

Acute and chronic liver diseases are the major cause of high morbidity and mortality globally. Liver transplantation is a widely used therapeutic option for liver failure. However, the shortage of availability of liver donors has encouraged research on the alternative approach to liver regeneration. Cell-based regenerative medicine is the best alternative therapy to cater to this need. To date, advanced preclinical approaches have been undertaken on stem cell differentiation and their use in liver tissue engineering for generating efficacious and promising regenerative therapies. Advancements in the bioengineering of stem cells, and organoid generation are the way forward to efficient therapies against liver injury. This review summarizes the recent approaches for stem cell therapy-based liver regeneration and their proof of concepts for clinical application, bioengineering liver organoids to alleviate the liver failure caused due to chronic liver diseases.

Bioprinted Hydrogels for Fibrosis and Wound Healing: Treatment and Modeling

Three-dimensional (3D) printing has been used to fabricate biomaterial scaffolds with finely controlled physical architecture and user-defined patterning of biological ligands. Excitingly, recent advances in bioprinting have enabled the development of highly biomimetic hydrogels for the treatment of fibrosis and the promotion of wound healing. Bioprinted hydrogels offer more accurate spatial recapitulation of the biochemical and biophysical cues that inhibit fibrosis and promote tissue regeneration, augmenting the therapeutic potential of hydrogel-based therapies. Accordingly, bioprinted hydrogels have been used for the treatment of fibrosis in a diverse array of tissues and organs, including the skin, heart, and endometrium. Furthermore, bioprinted hydrogels have been utilized for the healing of both acute and chronic wounds, which present unique biological microenvironments. In addition to these therapeutic applications, hydrogel bioprinting has been used to generate in vitro models of fibrosis in a variety of soft tissues such as the skin, heart, and liver, enabling high-throughput drug screening and tissue analysis at relatively low cost. As biological research begins to uncover the spatial biological features that underlie fibrosis and wound healing, bioprinting offers a powerful toolkit to recapitulate spatially defined pro-regenerative and anti-fibrotic cues for an array of translational applications.

Organ-specific endothelial cell heterogenicity and its impact on regenerative medicine and biomedical engineering applications

Endothelial cells (ECs) are a key cellular component of the vascular system as they form the inner lining of the blood vessels. Recent findings highlight that ECs express extensive phenotypic heterogenicity when following the vascular tree from the major vasculature down to the organ capillaries. However, in vitro models, used for drug development and testing, or to study the role of ECs in health and disease, rarely acknowledge this EC heterogenicity. In this review, we highlight the main differences between different EC types, briefly summarize their different characteristics and focus on the use of ECs in in vitro models. We introduce different approaches on how ECs can be utilized in co-culture test systems in the field of brain, pancreas, and liver research to study the role of the endothelium in health and disease. Finally, we discuss potential improvements to current state-of-the-art in vitro models and future directions.

In vitro long term differentiation and functionality of three-dimensional bioprinted primary human hepatocytes: application for in vivo engraftment

In recent decades, 3D in vitro cultures of primary human hepatocytes (PHH) have been increasingly developed to establish models capable of faithfully mimicking main liver functions. The use of 3D bioprinting, capable of recreating structures composed of cells embedded in matrix with controlled microarchitectures, is an emergent key feature for tissue engineering. In this work, we used an extrusion-based system to print PHH in a methacrylated gelatin matrix (GelMa). PHH bioprinted in GelMa rapidly organized into polarized hollow spheroids and were viable for at least 28 days of culture. These PHH were highly differentiated with maintenance of liver differentiation genes over time, as demonstrated by transcriptomic analysis and functional approaches. The cells were polarized with localization of apico/canalicular regions, and displayed activities of phase I and II biotransformation enzymes that could be regulated by inducers. Furthermore, the implantation of the bioprinted structures in mice demonstrated their capability to vascularize, and their ability to maintain human hepatic specific functions for at least 28 days was illustrated by albumin secretion and debrisoquine metabolism. This model could hold great promise for human liver tissue generation and its use in future biotechnological developments.

MatriGrid® Based Biological Morphologies: Tools for 3D Cell Culturing

Recent trends in 3D cell culturing has placed organotypic tissue models at another level. Now, not only is the microenvironment at the cynosure of this research, but rather, microscopic geometrical parameters are also decisive for mimicking a tissue model. Over the years, technologies such as micromachining, 3D printing, and hydrogels are making the foundation of this field. However, mimicking the topography of a particular tissue-relevant substrate can be achieved relatively simply with so-called template or morphology transfer techniques. Over the last 15 years, in one such research venture, we have been investigating a micro thermoforming technique as a facile tool for generating bioinspired topographies. We call them MatriGrid^®s. In this research account, we summarize our learning outcome from this technique in terms of the influence of 3D micro morphologies on different cell cultures that we have tested in our laboratory. An integral part of this research is the evolution of unavoidable aspects such as possible label-free sensing and fluidic automatization. The development in the research field is also documented in this account.

Microfluidic Organ-on-a-Chip Devices for Liver Disease Modeling In Vitro

Mortality from liver disease conditions continues to be very high. As liver diseases manifest and progress silently, prompt measures after diagnosis are essential in the treatment of these conditions. Microfluidic organs-on-chip platforms have significant potential for the study of the pathophysiology of liver diseases in vitro. Different liver-on-a-chip microphysiological platforms have been reported to study cell-signaling pathways such as those activating stellate cells within liver diseases. Moreover, the drug efficacy for liver conditions might be evaluated on a cellular metabolic level. Here, we present a comprehensive review of microphysiological platforms used for modelling liver diseases. First, we briefly introduce the concept and importance of organs-on-a-chip in studying liver diseases in vitro, reflecting on existing reviews of healthy liver-on-a-chip platforms. Second, the techniques of cell cultures used in the microfluidic devices, including 2D, 3D, and spheroid cells, are explained. Next, the types of liver diseases (NAFLD, ALD, hepatitis infections, and drug injury) on-chip are explained for a further comprehensive overview of the design and methods of developing liver diseases in vitro. Finally, some challenges in design and existing solutions to them are reviewed

In vitro models for non-alcoholic fatty liver disease: Emerging platforms and their applications

Non-alcoholic fatty liver disease (NAFLD) represents a global healthcare challenge, affecting 1 in 4 adults, and death rates are predicted to rise inexorably. The progressive form of NAFLD, non-alcoholic steatohepatitis (NASH), can lead to fibrosis, cirrhosis, and hepatocellular carcinoma. However, no medical treatments are licensed for NAFLD-NASH. Identifying efficacious therapies has been hindered by the complexity of disease pathogenesis, a paucity of predictive preclinical models and inadequate validation of pharmacological targets in humans. The development of clinically relevant in vitro models of the disease will pave the way to overcome these challenges. Currently, the combined application of emerging technologies (e.g., organ-on-a-chip/microphysiological systems) and control engineering approaches promises to unravel NAFLD biology and deliver tractable treatment candidates. In this review, we will describe advances in preclinical models for NAFLD-NASH, the recent introduction of novel technologies in this space, and their importance for drug discovery endeavors in the future.

In vitro models for non-alcoholic fatty liver disease: Emerging platforms and their applications

Non-alcoholic fatty liver disease (NAFLD) represents a global healthcare challenge, affecting 1 in 4 adults, and death rates are predicted to rise inexorably. The progressive form of NAFLD, non-alcoholic steatohepatitis (NASH), can lead to fibrosis, cirrhosis, and hepatocellular carcinoma. However, no medical treatments are licensed for NAFLD-NASH. Identifying efficacious therapies has been hindered by the complexity of disease pathogenesis, a paucity of predictive preclinical models and inadequate validation of pharmacological targets in humans. The development of clinically relevant in vitro models of the disease will pave the way to overcome these challenges. Currently, the combined application of emerging technologies (e.g., organ-on-a-chip/microphysiological systems) and control engineering approaches promises to unravel NAFLD biology and deliver tractable treatment candidates. In this review, we will describe advances in preclinical models for NAFLD-NASH, the recent introduction of novel technologies in this space, and their importance for drug discovery endeavors in the future.

3D Bioprinting at the Frontier of Regenerative Medicine, Pharmaceutical, and Food Industries

3D printing technology has emerged as a key driver behind an ongoing paradigm shift in the production process of various industrial domains. The integration of 3D printing into tissue engineering, by utilizing life cells which are encapsulated in specific natural or synthetic biomaterials (e.g., hydrogels) as bioinks, is paving the way toward devising many innovating solutions for key biomedical and healthcare challenges and heralds' new frontiers in medicine, pharmaceutical, and food industries. Here, we present a synthesis of the available 3D bioprinting technology from what is found and what has been achieved in various applications and discussed the capabilities and limitations encountered in this technology.

Microphysiological systems in absorption, distribution, metabolism, and elimination sciences

The use of microphysiological systems (MPS) to support absorption, distribution, metabolism, and elimination (ADME) sciences has grown substantially in the last decade, in part driven by regulatory demands to move away from traditional animal-based safety assessment studies and industry desires to develop methodologies to efficiently screen and characterize drugs in the development pipeline. The past decade of MPS development has yielded great user-driven technological advances with the collective fine-tuning of cell culture techniques, fluid delivery systems, materials engineering, and performance enhancing modifications. The rapid advances in MPS technology have now made it feasible to evaluate critical ADME parameters within a stand-alone organ system or through interconnected organ systems. This review surveys current MPS developed for liver, kidney, and intestinal systems as stand-alone or interconnected organ systems, and evaluates each system for specific performance criteria recommended by regulatory authorities and MPS leaders that would render each system suitable for evaluating drug ADME. Whereas some systems are more suitable for ADME type research than others, not all system designs were intended to meet the recently published desired performance criteria and are reported as a summary of initial proof-of-concept studies.

The case for cancer-associated fibroblasts: essential elements in cancer drug discovery?

Although cancer-associated fibroblasts (CAFs) have gained increased attention for supporting cancer progression, current CAF-targeted therapeutic options are limited and failing in clinical trials. As the largest component of the tumor microenvironment (TME), CAFs alter the biochemical and physical structure of the TME, modulating cancer progression. Here, we review the role of CAFs in altering drug response, modifying the TME mechanics and the current models for studying CAFs. To provide new perspectives, we highlight key considerations of CAF activity and discuss emerging technologies that can better address CAFs; and therefore, increase the likelihood of therapeutic efficacy. We argue that CAFs are crucial components of the cancer drug discovery pipeline and incorporating these cells will improve drug discovery success rates.

Engineering Modular 3D Liver Culture Microenvironments In Vitro to Parse the Interplay between Biophysical and Biochemical Microenvironment Cues on Hepatic Phenotypes

In vitro models of human liver functions are used across a diverse range of applications in preclinical drug development and disease modeling, with particular increasing interest in models that capture facets of liver inflammatory status. This study investigates how the interplay between biophysical and biochemical microenvironment cues influence phenotypic responses, including inflammation signatures, of primary human hepatocytes (PHH) cultured in a commercially available perfused bioreactor. A 3D printing-based alginate microwell system was designed to form thousands of hepatic spheroids in a scalable manner as a comparator 3D culture modality to the bioreactor. Soft, synthetic extracellular matrix (ECM) hydrogel scaffolds with biophysical properties mimicking features of liver were engineered to replace polystyrene scaffolds, and the biochemical microenvironment was modulated with a defined set of growth factors and signaling modulators. The supplemented media significantly increased tissue density, albumin secretion, and CYP3A4 activity but also upregulated inflammatory markers. Basal inflammatory markers were lower for cells maintained in ECM hydrogel scaffolds or spheroid formats than polystyrene scaffolds, while hydrogel scaffolds exhibited the most sensitive response to inflammation as assessed by multiplexed cytokine and RNA-seq analyses. Together, these engineered 3D liver microenvironments provide insights for probing human liver functions and inflammatory response in vitro.

Emerging Technologies in Multi-Material Bioprinting

Bioprinting, within the emerging field of biofabrication, aims at the fabrication of functional biomimetic constructs. Different 3D bioprinting techniques have been adapted to bioprint cell-laden bioinks. However, single-material bioprinting techniques oftentimes fail to reproduce the complex compositions and diversity of native tissues. Multi-material bioprinting as an emerging approach enables the fabrication of heterogeneous multi-cellular constructs that replicate their host microenvironments better than single-material approaches. Here, bioprinting modalities are reviewed, their being adapted to multi-material bioprinting is discussed, and their advantages and challenges, encompassing both custom-designed and commercially available technologies are analyzed. A perspective of how multi-material bioprinting opens up new opportunities for tissue engineering, tissue model engineering, therapeutics development, and personalized medicine is offered.

Preclinical models of idiosyncratic drug-induced liver injury (iDILI): Moving towards prediction

Idiosyncratic drug-induced liver injury (iDILI) encompasses the unexpected harms that prescription and non-prescription drugs, herbal and dietary supplements can cause to the liver. iDILI remains a major public health problem and a major cause of drug attrition. Given the lack of biomarkers for iDILI prediction, diagnosis and prognosis, searching new models to predict and study mechanisms of iDILI is necessary. One of the major limitations of iDILI preclinical assessment has been the lack of correlation between the markers of hepatotoxicity in animal toxicological studies and clinically significant iDILI. Thus, major advances in the understanding of iDILI susceptibility and pathogenesis have come from the study of well-phenotyped iDILI patients. However, there are many gaps for explaining all the complexity of iDILI susceptibility and mechanisms. Therefore, there is a need to optimize preclinical human in vitro models to reduce the risk of iDILI during drug development. Here, the current experimental models and the future directions in iDILI modelling are thoroughly discussed, focusing on the human cellular models available to study the pathophysiological mechanisms of the disease and the most used in vivo animal iDILI models. We also comment about in silico approaches and the increasing relevance of patient-derived cellular models.

Bioprinting of Human Liver-Derived Epithelial Organoids for Toxicity Studies

There is a need for long-lived hepatic in vitro models to better predict drug induced liver injury (DILI). Human liver-derived epithelial organoids are a promising cell source for advanced in vitro models. Here, organoid technology is combined with biofabrication techniques, which holds great potential for the design of in vitro models with complex and customizable architectures. Here, porous constructs with human hepatocyte-like cells derived from organoids are generated using extrusion-based printing technology. Cell viability of bioprinted organoids remains stable for up to ten days (88-107% cell viability compared to the day of printing). The expression of hepatic markers, transporters, and phase I enzymes increased compared to undifferentiated controls, and is comparable to non-printed controls. Exposure to acetaminophen, a well-known hepatotoxic compound, decreases cell viability of bioprinted liver organoids to 21-51% (p < 0.05) compared to the start of exposure, and elevated levels of damage marker miR-122 are observed in the culture medium, indicating the potential use of the bioprinted constructs for toxicity testing. In conclusion, human liver-derived epithelial organoids can be combined with a biofabrication approach, thereby paving the way to create perfusable, complex constructs which can be used as toxicology- and disease-models.

Modelling human liver fibrosis in the context of non-alcoholic steatohepatitis using a microphysiological system

Non-alcoholic steatohepatitis (NASH) is a common form of chronic liver disease characterised by lipid accumulation, infiltration of immune cells, hepatocellular ballooning, collagen deposition and liver fibrosis. There is a high unmet need to develop treatments for NASH. We have investigated how liver fibrosis and features of advanced clinical disease can be modelled using an in vitro microphysiological system (MPS). The NASH MPS model comprises a co-culture of primary human liver cells, which were cultured in a variety of conditions including+/- excess sugar, fat, exogenous TGFβ or LPS. The transcriptomic, inflammatory and fibrotic phenotype of the model was characterised and compared using a system biology approach to identify conditions that mimic more advanced clinical disease. The transcriptomic profile of the model was shown to closely correlate with the profile of patient samples and the model displayed a quantifiable fibrotic phenotype. The effects of Obeticholic acid and Elafibranor, were evaluated in the model, as wells as the effects of dietary intervention, with all able to significantly reduce inflammatory and fibrosis markers. Overall, we demonstrate how the MPS NASH model can be used to model different aspects of clinical NASH but importantly demonstrate its ability to model advanced disease with a quantifiable fibrosis phenotype.

Drug metabolic stability in early drug discovery to develop potential lead compounds

Knowledge of the metabolic stability of a new drug substance eliminated by biotransformation is essential for envisaging the pharmacokinetic parameters required for deciding drug dosing and frequency. Strategies aimed at modifying lead compounds may improve metabolic stability, thereby reducing the drug dosing frequency. Replacement of selective hydrogens with deuterium can effectively enhance the drug's metabolic stability by increasing the biological half-life. Further, cyclization, change in ring size, and chirality can substantially improve the metabolic stability of drugs. The microsomal t_1/2 approach for measuring drug in vitro intrinsic clearance by automated LC-MS/MS offers sensitive high-throughput screens with reliable data. The obtained in vitro intrinsic clearance from metabolic stability data helps predict the drug's in vivo total clearance using different scaling factors and hepatic clearance models. This review summarizes all the recent approaches and technological advancements in metabolic stability studies for narrowing down the potential lead compounds in drug discovery. Further, we summarized the potential pitfalls and assumptions made during the in vivo intrinsic clearance estimation from in vitro intrinsic clearance.

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.

Bioengineered Liver Models for Investigating Disease Pathogenesis and Regenerative Medicine

Owing to species-specific differences in liver pathways, in vitro human liver models are utilized for elucidating mechanisms underlying disease pathogenesis, drug development, and regenerative medicine. To mitigate limitations with de-differentiated cultures, bioengineers have developed advanced techniques/platforms, including micropatterned cocultures, spheroids/organoids, bioprinting, and microfluidic devices, for perfusing cell cultures and liver slices. Such techniques improve mature functions and culture lifetime of primary and stem-cell human liver cells. Furthermore, bioengineered liver models display several features of liver diseases including infections with pathogens (e.g., malaria, hepatitis C/B viruses, Zika, dengue, yellow fever), alcoholic/nonalcoholic fatty liver disease, and cancer. Here, we discuss features of bioengineered human liver models, their uses for modeling aforementioned diseases, and how such models are being augmented/adapted for fabricating implantable human liver tissues for clinical therapy. Ultimately, continued advances in bioengineered human liver models have the potential to aid the development of novel, safe, and efficacious therapies for liver disease.

Pharmacokinetics-On-a-Chip: In Vitro Microphysiological Models for Emulating of Drugs ADME

Despite many ongoing efforts across the full spectrum of pharmaceutical and biotech industries, drug development is still a costly undertaking that involves a high risk of failure during clinical trials. Animal models played vital roles in understanding the mechanism of human diseases. However, the use of these models has been a subject of heated debate, particularly due to ethical matters and the inevitable pathophysiological differences between animals and humans. Current in vitro models lack the sufficient functionality and predictivity of human pharmacokinetics and toxicity, therefore, are not capable to fully replace animal models. The recent development of micro-physiological systems has shown great potential as indispensable tools for recapitulating key physiological parameters of humans and providing in vitro methods for predicting the pharmacokinetics and pharmacodynamics in humans. Integration of Absorption, Distribution, Metabolism, and Excretion (ADME) processes within one close in vitro system is a paramount development that would meet important unmet pharmaceutical industry needs. In this review paper, synthesis of the ADME-centered organ-on-a-chip technology is systemically presented from what is achieved to what needs to be done, emphasizing the requirements of in vitro models that meet industrial needs in terms of the structure and functions.

Latest impact of engineered human liver platforms on drug development

Drug-induced liver injury (DILI) is a leading cause of drug attrition, which is partly due to differences between preclinical animals and humans in metabolic pathways. Therefore, in vitro human liver models are utilized in biopharmaceutical practice to mitigate DILI risk and assess related mechanisms of drug transport and metabolism. However, liver cells lose phenotypic functions within 1–3 days in two-dimensional monocultures on collagen-coated polystyrene/glass, which precludes their use to model the chronic effects of drugs and disease stimuli. To mitigate such a limitation, bioengineers have adapted tools from the semiconductor industry and additive manufacturing to precisely control the microenvironment of liver cells. Such tools have led to the fabrication of advanced two-dimensional and three-dimensional human liver platforms for different throughput needs and assay endpoints (e.g., micropatterned cocultures, spheroids, organoids, bioprinted tissues, and microfluidic devices); such platforms have significantly enhanced liver functions closer to physiologic levels and improved functional lifetime to >4 weeks, which has translated to higher sensitivity for predicting drug outcomes and enabling modeling of diseased phenotypes for novel drug discovery. Here, we focus on commercialized engineered liver platforms and case studies from the biopharmaceutical industry showcasing their impact on drug development. We also discuss emerging multi-organ microfluidic devices containing a liver compartment that allow modeling of inter-tissue crosstalk following drug exposure. Finally, we end with key requirements for engineered liver platforms to become routine fixtures in the biopharmaceutical industry toward reducing animal usage and providing patients with safe and efficacious drugs with unprecedented speed and reduced cost.

Organ-on-a-chip systems for vascular biology

Organ-on-a-chip (OOC) platforms involve the miniaturization of cell culture systems and enable a variety of novel experimental approaches. These range from modeling the independent effects of biophysical forces on cells to screening novel drugs in multi-organ microphysiological systems, all within microscale devices. As in living systems, the incorporation of vascular structure is a key feature common to almost all organ-on-a-chip systems. In this review we highlight recent advances in organ-on-a-chip technologies with a focus on the vasculature. We first present the developmental process of the blood vessels through which vascular cells assemble into networks and remodel to form complex vascular beds under flow. We then review self-assembled vascular models and flow systems for the study of vascular development and biology as well as pre-patterned vascular models for the generation of perfusable microvessels for modeling vascular and tissue function. We finally conclude with a perspective on developing future OOC approaches for studying different aspects of vascular biology. We highlight the fit for purpose selection of OOC models towards either simple but powerful testbeds for therapeutic development, or complex vasculature to accurately replicate human physiology for specific disease modeling and tissue regeneration.

Intermittent Starvation Extends the Functional Lifetime of Primary Human Hepatocyte Cultures

Primary human hepatocyte (PHH) cultures have become indispensable to mitigate the risk of adverse drug reactions in human patients. In contrast to dedifferentiating monocultures, coculture with nonparenchymal cells maintains PHH functions for 2-4 weeks. However, because the functional lifespan of PHHs in vivo is 200-400 days, it is desirable to further prolong PHH functions in vitro toward modeling chronic drug exposure and disease progression. Fasting has benefits on the longevity of organisms and the health of tissues such as the liver. We hypothesized that a culturing protocol that mimics dynamic fasting/starvation could activate starvation pathways and prolong PHH functional lifetime. To mimic starvation, serum and hormones were intermittently removed from the culture medium of micropatterned cocultures (MPCCs) containing PHHs organized onto collagen domains and surrounded by 3T3-J2 murine fibroblasts. A weekly 2-day starvation optimally prolonged PHH functional lifetime for 6+ weeks in MPCCs versus a decline after 3 weeks in nonstarved controls. The 2-day starvation also enhanced the functions of PHH monocultures for 2 weeks, suggesting direct effects on PHHs. In MPCCs, starvation activated 5' adenosine monophosphate-activated protein kinase (AMPK) and restricted fibroblast overgrowth onto PHH islands, thereby maintaining hepatic polarity. The effects of starvation on MPCCs were partially recapitulated by activating AMPK using metformin or growth arresting fibroblasts via mitomycin-C. Lastly, starved MPCCs demonstrated lower false positives for drug toxicity tests and higher drug-induced cytochrome-P450 activities versus nonstarved controls even after 5 weeks. In conclusion, intermittent serum/hormone starvation extends PHH functional lifetime toward enabling clinically relevant drug screening.

Advances in development and application of human organoids

Innumerable studies associated with cellular differentiation, tissue response and disease modeling have been conducted in two-dimensional (2D) culture systems or animal models. This has been invaluable in deciphering the normal and disease states in cell biology; the key shortcomings of it being suitability for translational or clinical correlations. The past decade has seen several major advances in organoid culture technologies and this has enhanced our understanding of mimicking organ reconstruction. The term organoid has generally been used to describe cellular aggregates derived from primary tissues or stem cells that can self-organize into organotypic structures. Organoids mimic the cellular microenvironment of tissues better than 2D cell culture systems and represent the tissue physiology. Human organoids of brain, thyroid, gastrointestinal, lung, cardiac, liver, pancreatic and kidney have been established from various diseases, healthy tissues and from pluripotent stem cells (PSCs). Advances in patient-derived organoid culture further provides a unique perspective from which treatment modalities can be personalized. In this review article, we have discussed the current strategies for establishing various types of organoids of ectodermal, endodermal and mesodermal origin. We have also discussed their applications in modeling human health and diseases (such as cancer, genetic, neurodegenerative and infectious diseases), applications in regenerative medicine and evolutionary studies.

3D Bioprinting-Based Vascularized Tissue Models Mimicking Tissue-Specific Architecture and Pathophysiology for in vitro Studies

A wide variety of experimental models including 2D cell cultures, model organisms, and 3D in vitro models have been developed to understand pathophysiological phenomena and assess the safety and efficacy of potential therapeutics. In this sense, 3D in vitro models are an intermediate between 2D cell cultures and animal models, as they adequately reproduce 3D microenvironments and human physiology while also being controllable and reproducible. Particularly, recent advances in 3D in vitro biomimicry models, which can produce complex cell structures, shapes, and arrangements, can more similarly reflect in vivo conditions than 2D cell culture. Based on this, 3D bioprinting technology, which enables to place the desired materials in the desired locations, has been introduced to fabricate tissue models with high structural similarity to the native tissues. Therefore, this review discusses the recent developments in this field and the key features of various types of 3D-bioprinted tissues, particularly those associated with blood vessels or highly vascularized organs, such as the heart, liver, and kidney. Moreover, this review also summarizes the current state of the three categories: (1) chemical substance treatment, (2) 3D bioprinting of lesions, and (3) recapitulation of tumor microenvironments (TME) of 3D bioprinting-based disease models according to their disease modeling approach. Finally, we propose the future directions of 3D bioprinting approaches for the creation of more advanced in vitro biomimetic 3D tissues, as well as the translation of 3D bioprinted tissue models to clinical applications.

Quantifying the progression of non-alcoholic fatty liver disease in human biomimetic liver microphysiology systems with fluorescent protein biosensors

Metabolic syndrome is a complex disease that involves multiple organ systems including a critical role for the liver. Non-alcoholic fatty liver disease (NAFLD) is a key component of the metabolic syndrome and fatty liver is linked to a range of metabolic dysfunctions that occur in approximately 25% of the population. A panel of experts recently agreed that the acronym, NAFLD, did not properly characterize this heterogeneous disease given the associated metabolic abnormalities such as type 2 diabetes mellitus (T2D), obesity, and hypertension. Therefore, metabolic dysfunction-associated fatty liver disease (MAFLD) has been proposed as the new term to cover the heterogeneity identified in the NAFLD patient population. Although many rodent models of NAFLD/NASH have been developed, they do not recapitulate the full disease spectrum in patients. Therefore, a platform has evolved initially focused on human biomimetic liver microphysiology systems that integrates fluorescent protein biosensors along with other key metrics, the microphysiology systems database, and quantitative systems pharmacology. Quantitative systems pharmacology is being applied to investigate the mechanisms of NAFLD/MAFLD progression to select molecular targets for fluorescent protein biosensors, to integrate computational and experimental methods to predict drugs for repurposing, and to facilitate novel drug development. Fluorescent protein biosensors are critical components of the platform since they enable monitoring of the pathophysiology of disease progression by defining and quantifying the temporal and spatial dynamics of protein functions in the biosensor cells, and serve as minimally invasive biomarkers of the physiological state of the microphysiology system experimental disease models. Here, we summarize the progress in developing human microphysiology system disease models of NAFLD/MAFLD from several laboratories, developing fluorescent protein biosensors to monitor and to measure NAFLD/MAFLD disease progression and implementation of quantitative systems pharmacology with the goal of repurposing drugs and guiding the creation of novel therapeutics.

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.

Human biomimetic liver microphysiology systems in drug development and precision medicine

Microphysiology systems (MPS), also called organs-on-chips and tissue chips, are miniaturized functional units of organs constructed with multiple cell types under a variety of physical and biochemical environmental cues that complement animal models as part of a new paradigm of drug discovery and development. Biomimetic human liver MPS have evolved from simpler 2D cell models, spheroids and organoids to address the increasing need to understand patient-specific mechanisms of complex and rare diseases, the response to therapeutic treatments, and the absorption, distribution, metabolism, excretion and toxicity of potential therapeutics. The parallel development and application of transdisciplinary technologies, including microfluidic devices, bioprinting, engineered matrix materials, defined physiological and pathophysiological media, patient-derived primary cells, and pluripotent stem cells as well as synthetic biology to engineer cell genes and functions, have created the potential to produce patient-specific, biomimetic MPS for detailed mechanistic studies. It is projected that success in the development and maturation of patient-derived MPS with known genotypes and fully matured adult phenotypes will lead to advanced applications in precision medicine. In this Review, we examine human biomimetic liver MPS that are designed to recapitulate the liver acinus structure and functions to enhance our knowledge of the mechanisms of disease progression and of the absorption, distribution, metabolism, excretion and toxicity of therapeutic candidates and drugs as well as to evaluate their mechanisms of action and their application in precision medicine and preclinical trials.

3D human liver organoids: An in vitro platform to investigate HBV infection, replication and liver tumorigenesis

Hepatitis B Virus (HBV) infection is a leading cause of chronic liver cirrhosis and hepatocellular carcinoma (HCC) with an estimated 400 million people infected worldwide. The precise molecular mechanisms underlying HBV replication and tumorigenesis have remained largely uncharacterized due to the lack of a primary cell model to study HBV, a virus that exhibits stringent host species and cell-type specificity. Organoid technology has recently emerged as a powerful tool to investigate human diseases in a primary 3D cell-culture system that maintains the organisation and functionality of the tissue of origin. In this review, we describe the utilisation of human liver organoid platforms to study HBV. We first present the different categories of liver organoids and their demonstrated ability to support the complete HBV replication cycle. We then discuss the potential applications of liver organoids in investigating HBV infection and replication, related tumorigenesis and novel HBV-directed therapies. Liver organoids can be genetically modified, patient-derived, expanded and biobanked, thereby serving as a clinically-relevant, human, primary cell-derived platform to investigate HBV. Finally, we provide insights into the future applications of this powerful technology in the context of HBV-infection and HCC.

Three-Dimensional Bioprinting of Anatomically Realistic Tissue Constructs for Disease Modeling and Drug Testing

Three-dimensional (3D) bioprinting is an emerging tissue engineering technology, already with several remarkable accomplishments and with more promises to fulfill. Besides the enduring goal of making tissues for implantation, it could also become an essential tool in the worldwide trend to replace animal experimentation with improved in vitro models for disease mechanism studies, or with new high-throughput pharmacological and toxicology assays. All these require the speed, reproducibility, and standardization that bioprinting could easily provide. However, originating from additive manufacturing with its top-down approach of "filling" a virtual volume with a semifluid (hydrogel) material, the finer internal anatomic structure of the tissues, as well as vascularization and innervation, has remained difficult to implement. Thus, the next frontier in bioprinting is the generation of more anatomically realistic models, needed for ascending to the functionality of living tissues. In this study, I discuss the conceptual and practical barriers still hampering the attainment of this goal and suggest solutions to overcome them. In this regard, I introduce two workflows that combine existing methods in new operational sequences: (1) bioprinting guided by images of histological sections assembled in 3D constructs and (2) bioprinting of bidimensional vascular patterns implemented among stackable cellular layers. While more sophisticated methods to capture the tissue structure in 3D constructs certainly exist, I contend that extrusion bioprinting may still offer a simple, practical, and affordable option. Impact statement Paucity of anatomic structural details is one of the limitations of three-dimensional bioprinting toward fulfilling its potential for tissue engineering, drug testing, and toxicological assays. The origins of this problem can be tracked back to derivation of bioprinting from inorganic additive manufacturing, making it more adept to render the shapes of the objects than their content. As solutions, I suggest two simple workflows that can be implemented by most current bioprinters, based on the import into the construct design of anatomically realistic structural information. If more largely adopted, these and similar approaches may significantly improve the applicability of bioprinted constructs.

Cellular extrusion bioprinting improves kidney organoid reproducibility and conformation

Directed differentiation of human pluripotent stem cells to kidney organoids brings the prospect of drug screening, disease modelling and the generation of tissue for renal replacement. Currently, these applications are hampered by organoid variability, nephron immaturity, low throughput and limited scale. Here, we apply extrusion-based three-dimensional cellular bioprinting to deliver rapid and high-throughput generation of kidney organoids with highly reproducible cell number and viability. We demonstrate that manual organoid generation can be replaced by 6- or 96-well organoid bioprinting and evaluate the relative toxicity of aminoglycosides as a proof of concept for drug testing. In addition, three-dimensional bioprinting enables precise manipulation of biophysical properties, including organoid size, cell number and conformation, with modification of organoid conformation substantially increasing nephron yield per starting cell number. This facilitates the manufacture of uniformly patterned kidney tissue sheets with functional proximal tubular segments. Hence, automated extrusion-based bioprinting for kidney organoid production delivers improvements in throughput, quality control, scale and structure, facilitating in vitro and in vivo applications of stem cell-derived human kidney tissue.

Polymeric Bioinks for 3D Hepatic Printing

Three-dimensional (3D) printing techniques have revolutionized the field of tissue engineering. This is especially favorable to construct intricate tissues such as liver, as 3D printing allows for the precise delivery of biomaterials, cells and bioactive molecules in complex geometries. Bioinks made of polymers, of both natural and synthetic origin, have been very beneficial to printing soft tissues such as liver. Using polymeric bioinks, 3D hepatic structures are printed with or without cells and biomolecules, and have been used for different tissue engineering applications. In this review, with the introduction to basic 3D printing techniques, we discuss different natural and synthetic polymers including decellularized matrices that have been employed for the 3D bioprinting of hepatic structures. Finally, we focus on recent advances in polymeric bioinks for 3D hepatic printing and their applications. The studies indicate that much work has been devoted to improvising the design, stability and longevity of the printed structures. Others focus on the printing of tissue engineered hepatic structures for applications in drug screening, regenerative medicine and disease models. More attention must now be diverted to developing personalized structures and stem cell differentiation to hepatic lineage.

Characterization of primary mouse hepatocyte spheroids as a model system to support investigations of drug-induced liver injury

Primary mouse hepatocytes isolated from genetically defined and/or diverse lines and disease models are a valuable resource for studying the impact of genetic and environmental factors on drug response and disease. However, standard monolayer cultures result in a rapid decline in mouse hepatocyte viability and functionality. Therefore, we evaluated 3D spheroid methodology for long-term culture of primary mouse hepatocytes, initially to support investigations of drug-induced liver injury (DILI). Primary hepatocytes isolated from male and female C57BL/6J mice were used to generate spheroids by spontaneous self-aggregation in ultra-low attachment plates. Spheroids with well-defined perimeters were observed within 5 days after seeding and retained morphology, ATP, and albumin levels for an additional 2 weeks in culture. Global microarray profiling and quantitative targeted proteomics assessing 10 important drug metabolizing enzymes and transporters demonstrated maintenance of mRNA and protein levels in spheroids over time. Activities for 5 major P450 enzymes were also stable and comparable to activities previously reported for human hepatocyte spheroids. Time- and concentration-dependent decreases in ATP and albumin were observed in response to the DILI-causing drugs acetaminophen, fialuridine, AMG-009, and tolvaptan. Collectively, our results demonstrate successful long-term culture of mouse hepatocytes as spheroids and their utility to support investigations of DILI.

Tissue Chips and Microphysiological Systems for Disease Modeling and Drug Testing

Tissue chips (TCs) and microphysiological systems (MPSs) that incorporate human cells are novel platforms to model disease and screen drugs and provide an alternative to traditional animal studies. This review highlights the basic definitions of TCs and MPSs, examines four major organs/tissues, identifies critical parameters for organization and function (tissue organization, blood flow, and physical stresses), reviews current microfluidic approaches to recreate tissues, and discusses current shortcomings and future directions for the development and application of these technologies. The organs emphasized are those involved in the metabolism or excretion of drugs (hepatic and renal systems) and organs sensitive to drug toxicity (cardiovascular system). This article examines the microfluidic/microfabrication approaches for each organ individually and identifies specific examples of TCs. This review will provide an excellent starting point for understanding, designing, and constructing novel TCs for possible integration within MPS.

Current Advances on 3D-Bioprinted Liver Tissue Models

The liver, the largest gland in the human body, plays a key role in metabolism, bile production, detoxification, and water and electrolyte regulation. The toxins or drugs that the gastrointestinal system absorbs reach the liver first before entering the bloodstream. Liver disease is one of the leading causes of death worldwide. Therefore, an in vitro liver tissue model that reproduces the main functions of the liver can be a reliable platform for investigating liver diseases and developing new drugs. In addition, the limitations in traditional, planar monolayer cell cultures and animal tests for evaluating the toxicity and efficacy of drug candidates can be overcome. Currently, the newly emerging 3D bioprinting technologies have the ability to construct in vitro liver tissue models both in static scaffolds and dynamic liver-on-chip manners. This review mainly focuses on the construction and applications of liver tissue models based on 3D bioprinting. Special attention is given to 3D bioprinting strategies and bioinks for constructing liver tissue models including the cell sources and hydrogel selection. In addition, the main advantages and limitations and the major challenges and future perspectives are discussed, paving the way for the next generation of in vitro liver tissue models.

Biofabrication of Hepatic Constructs by 3D Bioprinting of a Cell-Laden Thermogel: An Effective Tool to Assess Drug-Induced Hepatotoxic Response

A thermoresponsive Pluronic/alginate semisynthetic hydrogel is used to bioprint 3D hepatic constructs, with the aim to investigate liver-specific metabolic activity of the 3D constructs compared to traditional 2D adherent cultures. The bioprinting method relies on a bioinert hydrogel and is characterized by high-shape fidelity, mild depositing conditions and easily controllable gelation mechanism. Furthermore, the dissolution of the sacrificial Pluronic templating agent significantly ameliorates the diffusive properties of the printed hydrogel. The present findings demonstrate high viability and liver-specific metabolic activity, as assessed by synthesis of urea, albumin, and expression levels of the detoxifying CYP1A2 enzyme of cells embedded in the 3D hydrogel system. A markedly increased sensitivity to a well-known hepatotoxic drug (acetaminophen) is observed for cells in 3D constructs compared to 2D cultures. Therefore, the 3D model developed herein may represent an in vitro alternative to animal models for investigating drug-induced hepatotoxicity.

Emulating Human Tissues and Organs: A Bioprinting Perspective Toward Personalized Medicine

The lack of in vitro tissue and organ models capable of mimicking human physiology severely hinders the development and clinical translation of therapies and drugs with higher in vivo efficacy. Bioprinting allow us to fill this gap and generate 3D tissue analogues with complex functional and structural organization through the precise spatial positioning of multiple materials and cells. In this review, we report the latest developments in terms of bioprinting technologies for the manufacturing of cellular constructs with particular emphasis on material extrusion, jetting, and vat photopolymerization. We then describe the different base polymers employed in the formulation of bioinks for bioprinting and examine the strategies used to tailor their properties according to both processability and tissue maturation requirements. By relating function to organization in human development, we examine the potential of pluripotent stem cells in the context of bioprinting toward a new generation of tissue models for personalized medicine. We also highlight the most relevant attempts to engineer artificial models for the study of human organogenesis, disease, and drug screening. Finally, we discuss the most pressing challenges, opportunities, and future prospects in the field of bioprinting for tissue engineering (TE) and regenerative medicine (RM).