Liver Tissue Engineering: Challenges and Opportunities

Fabrication of a decellularized liver matrix-based hepatic patch for the repair of CCl4-induced liver injury

This article primarily introduces a new treatment for liver fibrosis/cirrhosis. We developed a hepatic patch by combining decellularized liver matrix (DLM) with the hepatocyte growth factor (HGF)/heparin-complex and evaluated its restorative efficacy. In vitro prophylactic results, the HGF/heparin-DLM patches effectively mitigated CCl4-induced hepatocyte toxicity and restored the cytotoxicity levels to the baseline levels by day 5. Furthermore, these patches restored albumin synthesis of injured hepatocytes to more than 70% of the normal levels within 5 days. In vitro therapeutic results, the urea synthesis of the injured hepatocytes reached 91% of the normal levels after 10 days of culture, indicating successful restoration of hepatic function by the HGF/heparin-DLM patches in both prophylactic and therapeutic models. In vivo results, HGF/heparin-DLM patches attached to the liver and gut exhibited a significant decrease in collagen content (4.44 times and 2.77 times, respectively) and an increase in glycogen content (1.19 times and 1.12 times, respectively) compared to the fibrosis group after 1 week, separately. In summary, liver function was restored and inflammation was inhibited through the combined effects of DLM and the HGF/heparin-complex in fibrotic liver. The newly designed hepatic patch holds promise for both in vitro and in vivo regeneration therapy and preventive health care for liver tissue engineering.

A comprehensive overview of advanced dynamic in vitro intestinal and hepatic cell culture models

Orally administered drugs pass through the gastrointestinal tract before being absorbed in the small intestine and metabolised in the liver. To test the efficacy and toxicity of drugs, animal models are often employed; however, they are not suitable for investigating drug-tissue interactions and making reliable predictions, since the human organism differs drastically from animals in terms of absorption, distribution, metabolism and excretion of substances. Likewise, simple static in vitro cell culture systems currently used in preclinical drug screening often do not resemble the native characteristics of biological barriers. Dynamic models, on the other hand, provide in vivo-like cell phenotypes and functionalities that offer great potential for safety and efficacy prediction. Herein, current microfluidic in vitro intestinal and hepatic models are reviewed, namely single- and multi-tissue micro-bioreactors, which are associated with different methods of cell cultivation, i.e., scaffold-based versus scaffold-free.

Biomedical applications of engineered heparin-based materials

Heparin is a negatively charged polysaccharide with various chain lengths and a hydrophilic backbone. Due to its fascinating chemical and physical properties, nontoxicity, biocompatibility, and biodegradability, heparin has been extensively used in different fields of medicine, such as cardiovascular and hematology. This review highlights recent and future advancements in designing materials based on heparin for various biomedical applications. The physicochemical and mechanical properties, biocompatibility, toxicity, and biodegradability of heparin are discussed. In addition, the applications of heparin-based materials in various biomedical fields, such as drug/gene delivery, tissue engineering, cancer therapy, and biosensors, are reviewed. Finally, challenges, opportunities, and future perspectives in preparing heparin-based materials are summarized.

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.

Recent Advances in Liver Tissue Engineering as an Alternative and Complementary Approach for Liver Transplantation

Acute and chronic liver diseases cause significant morbidity and mortality worldwide, affecting millions of people. Liver transplantation is the primary intervention method, replacing a non-functional liver with a functional one. However, the field of liver transplantation faces challenges such as donor shortage, postoperative complications, immune rejection, and ethical problems. Consequently, there is an urgent need for alternative therapies that can complement traditional transplantation or serve as an alternative method. In this review, we explore the potential of liver tissue engineering as a supplementary approach to liver transplantation, offering benefits to patients with severe liver dysfunctions.

Cellulosic Textiles-An Appealing Trend for Different Pharmaceutical Applications

Cellulose, the most abundant biopolymer in nature, is derived from various sources. The production of pharmaceutical textiles based on cellulose represents a growing sector. In medicated textiles, textile and pharmaceutical sciences are integrated to develop new healthcare approaches aiming to improve patient compliance. Through the possibility of cellulose functionalization, pharmaceutical textiles can broaden the applications of cellulose in the biomedical field. This narrative review aims to illustrate both the methods of extraction and preparation of cellulose fibers, with a particular focus on nanocellulose, and diverse pharmaceutical applications like tissue restoration and antimicrobial, antiviral, and wound healing applications. Additionally, the merging between fabricated cellulosic textiles with drugs, metal nanoparticles, and plant-derived and synthetic materials are also illustrated. Moreover, new emerging technologies and the use of smart medicated textiles (3D and 4D cellulosic textiles) are not far from those within the review scope. In each section, the review outlines some of the limitations in the use of cellulose textiles, indicating scientific research that provides significant contributions to overcome them. This review also points out the faced challenges and possible solutions in a trial to present an overview on all issues related to the use of cellulose for the production of pharmaceutical textiles.

Polysaccharides, proteins, and synthetic polymers based multimodal hydrogels for various biomedical applications: A review

Nature-derived or biologically encouraged hydrogels have attracted considerable interest in numerous biomedical applications owing to their multidimensional utility and effectiveness. The internal architecture of a hydrogel network, the chemistry of the raw materials involved, interaction across the interface of counter ions, and the ability to mimic the extracellular matrix (ECM) govern the clinical efficacy of the designed hydrogels. This review focuses on the mechanistic viewpoint of different biologically driven/inspired biomacromolecules that encourages the architectural development of hydrogel networks. In addition, the advantage of hydrogels by mimicking the ECM and the significance of the raw material selection as an indicator of bioinertness is deeply elaborated in the review. Furthermore, the article reviews and describes the application of polysaccharides, proteins, and synthetic polymer-based multimodal hydrogels inspired by or derived from nature in different biomedical areas. The review discusses the challenges and opportunities in biomaterials along with future prospects in terms of their applications in biodevices or functional components for human health issues. This review provides information on the strategy and inspiration from nature that can be used to develop a link between multimodal hydrogels as the main frame and its utility in biomedical applications as the primary target.

Toxicity of polystyrene nanoplastics to human embryonic kidney cells and human normal liver cells: Effect of particle size and Pb2+ enrichment

Nanoplastics pollution in drinking water has aroused wide concern, but their effects on human health are still poorly understood. Herein we explore the responses of human embryonic kidney 293T cells and human normal liver LO2 cells to polystyrene nanoplastics, mainly focusing on the effects of particle sizes and enrichment of Pb²⁺. When the exposed particle size is higher than 100 nm, there is no obvious death for these two different cell lines. As the particle size decreases from 100 nm, cell mortality goes up. Although the internalization of polystyrene nanoplastics in LO2 cells is at least 5 times higher than that in 293T cells, the mortality of LO2 cells is lower than that of 293T cells, illustrating that LO2 cells are more resistant to polystyrene nanoplastics than 293T cells. Additionally, the Pb²⁺ enrichment on polystyrene nanoplastics in water can further enhance their toxicity, which should be taken seriously. The cytotoxicity of polystyrene nanoplastics to cell lines works through a molecular mechanism involving oxidative stress-induced damage of mitochondria and cell membranes, resulting in a decrease in ATP production and an increase in membrane permeability. Referenced to nanoplastics pollution in drinking water, there is no necessary to panic about the adverse effects of plastic itself on human health, but the enrichment of contaminants should get more attention. This work provides a reference for the risk assessment of nanoplastics in drinking water to human health.

Engineered Fibrous NiTi Scaffolds with Cultured Hepatocytes for Liver Regeneration in Rats

At present, the use of alternative systems to replenish the lost functions of hepatic metabolism and partial replacement of liver organ failure is relevant, due to an increase in the incidence of various liver disorders, insufficiency, and cost of organs for transplantation, as well as the high cost of using the artificial liver systems. The development of low-cost intracorporeal systems for maintaining hepatic metabolism using tissue engineering, as a bridge before liver transplantation or completely replacing liver function, deserves special attention. In vivo applications of intracorporeal fibrous nickel-titanium scaffolds (FNTSs) with cultured hepatocytes are described. Hepatocytes cultured in FNTSs are superior to their injections in terms of liver function, survival time, and recovery in a CCl₄-induced cirrhosis rats' model. 232 animals were divided into 5 groups: control, CCl₄-induced cirrhosis, CCl₄-induced cirrhosis followed by implantation of cell-free FNTSs (sham surgery), CCl₄-induced cirrhosis followed by infusion of hepatocytes (2 mL, 10⁷ cells/mL), and CCl₄-induced cirrhosis followed by FNTS implantation with hepatocytes. Restoration of hepatocyte function in the FNTS implantation with the hepatocytes group was accompanied by a significant decrease in the level of aspartate aminotransferase (AsAT) in blood serum compared to the cirrhosis group. A significant decrease in the level of AsAT was noted after 15 days in the infused hepatocytes group. However, on the 30th day, the AsAT level increased and was close to the cirrhosis group due to the short-term effect after the introduction of hepatocytes without a scaffold. The changes in alanine aminotransferase (AlAT), alkaline phosphatase (AlP), total and direct bilirubin, serum protein, triacylglycerol, lactate, albumin, and lipoproteins were similar to those in AsAT. The survival time of animals was significantly longer in the FNTS implantation with hepatocytes group. The obtained results showed the scaffolds' ability to support hepatocellular metabolism. The development of hepatocytes in FNTS was studied in vivo using 12 animals using scanning electron microscopy. Hepatocytes demonstrated good adhesion to the scaffold wireframe and survival in allogeneic conditions. Mature tissue, including cellular and fibrous, filled the scaffold space by 98% in 28 days. The study shows the extent to which an implantable "auxiliary liver" compensates for the lack of liver function without replacement in rats.

Primary hepatocyte urea assessment in the sodium-alginate patterned hydrogel by electrochemical procedure containing umbilical cord conditioned media

Limitations in liver transplantation and advances in cell therapy methods motivated us to study primary hepatocytes. The main challenge in using primary hepatocytes for liver regeneration is that they lose their functionalities. We aimed to develop a controlled-shape hydrogel and apply the conditioned-media of mesenchymal stromal cells (CM-MSCs) to improve in vitro hepatocyte functions. In this experimental study, following rat hepatocyte isolation by collagenase perfusion and collection of human umbilical cord CM-MSCs, a simple and precise system called electrodeposition was used to produce the patterned alginate hydrogel. To reduce the cytopathic effects, we used an indirect electrodeposition method. For characterizing this structure, mechanical properties, Fourier-transform infrared spectroscopy (FTIR), water uptake, in-vitro degradation, and hydrogel stability were studied. Urea synthesis as a basic function of hepatocytes was assessed in five different groups. Scanning electron microscope (SEM) was utilized to evaluate the primary hepatocyte morphology and their dispersion in the fabricated structure. We observed a significant increase in urea synthesis in the presence of CM-MSCs in patterned hydrogel alginate compared to 2D culture on day 3 (p<0.05). However, there was no significant difference in simple and patterned hydrogel on day 2. We found that the electrodeposition method is appropriate for the rapid fabricating of hydrogel structures with arbitrary patterns for 3D cell culture.

Recent advances of polypyrrole conducting polymer film for biomedical application: Toward a viable platform for cell-microbial interactions

Polypyrrole (PPy) is one of the most studied conductive polymers due to its electrical conductivity and biological properties, which drive the possibility of numerous applications in the biomedical area. The physical-chemical features of PPy allow the manufacture of biocompatible devices, enhancing cell adhesion and proliferation. Furthermore, owing to the electrostatic interactions between the negatively charged bacterial cell wall and the positive charges in the polymer structure, PPy films can perform an effective antimicrobial activity. PPy is also frequently associated with biocompatible agents and antimicrobial compounds to improve the biological response. Thus, this comprehensive review appraised the available evidence regarding the PPy-based films deposited on metallic implanted devices for biomedical applications. We focus on understanding key concepts that could influence PPy attributes regarding antimicrobial effect and cell behavior under in vitro and in vivo settings. Furthermore, we unravel the several agents incorporated into the PPy film and strategies to improve its functionality. Our findings suggest that incorporating other elements into the PPy films, such as antimicrobial agents, biomolecules, and other biocompatible polymers, may improve the biological responses. Overall, the basic properties of PPy, when combined with other composites, electrostimulation techniques, or surface treatment methods, offer great potential in biocompatibility and/or antimicrobial activities. However, challenges in synthesis standardization and potential limitations such as low adhesion and mechanical strength of the film must be overcome to improve and broaden the application of PPy film in biomedical devices.

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.

Bacterial Cellulose-Based Blends and Composites: Versatile Biomaterials for Tissue Engineering Applications

Cellulose of bacterial origin, known as bacterial cellulose (BC), is one of the most versatile biomaterials that has a huge potential in tissue engineering due to its favourable mechanical properties, high hydrophilicity, crystallinity, and purity. Additional properties such as porous nano-fibrillar 3D structure and a high degree of polymerisation of BC mimic the properties of the native extracellular matrix (ECM), making it an excellent material for the fabrication of composite scaffolds suitable for cell growth and tissue development. Recently, the fabrication of BC-based scaffolds, including composites and blends with nanomaterials, and other biocompatible polymers has received particular attention owing to their desirable properties for tissue engineering. These have proven to be promising advanced materials in hard and soft tissue engineering. This review presents the latest state-of-the-art modified/functionalised BC-based composites and blends as advanced materials in tissue engineering. Their applicability as an ideal biomaterial in targeted tissue repair including bone, cartilage, vascular, skin, nerve, and cardiac tissue has been discussed. Additionally, this review briefly summarises the latest updates on the production strategies and characterisation of BC and its composites and blends. Finally, the challenges in the future development and the direction of future research are also discussed.

Evolution of Electrospinning in Liver Tissue Engineering

The major goal of liver tissue engineering is to reproduce the phenotype and functions of liver cells, especially primary hepatocytes ex vivo. Several strategies have been explored in the recent past for culturing the liver cells in the most apt environment using biological scaffolds supporting hepatocyte growth and differentiation. Nanofibrous scaffolds have been widely used in the field of tissue engineering for their increased surface-to-volume ratio and increased porosity, and their close resemblance with the native tissue extracellular matrix (ECM) environment. Electrospinning is one of the most preferred techniques to produce nanofiber scaffolds. In the current review, we have discussed the various technical aspects of electrospinning that have been employed for scaffold development for different types of liver cells. We have highlighted the use of synthetic and natural electrospun polymers along with liver ECM in the fabrication of these scaffolds. We have also described novel strategies that include modifications, such as galactosylation, matrix protein incorporation, etc., in the electrospun scaffolds that have evolved to support the long-term growth and viability of the primary hepatocytes.

Core-shell bioprinting of vascularized in vitro liver sinusoid models

In vitro liver models allow the investigation of the cell behavior in disease conditions or in response to changes in the microenvironment. A major challenge in liver tissue engineering is to mimic the tissue-level complexity: Besides the selection of suitable biomaterial(s) replacing the extracellular matrix (ECM) and cell sources, the three-dimensional (3D) microarchitecture defined by the fabrication method is a critical factor to achieve functional constructs. In this study, coaxial extrusion-based 3D bioprinting has been applied to develop a liver sinusoid-like model that consists of a core compartment containing pre-vascular structures and a shell compartment containing hepatocytes. The shell ink was composed of alginate and methylcellulose (algMC), dissolved in human fresh frozen plasma. The algMC blend conferred high printing fidelity and stability to the core-shell constructs and the plasma as biologically active component enhanced viability and supported cluster formation and biomarker expression of HepG2 embedded in the shell. For the core, a natural ECM-like ink based on angiogenesis-supporting collagen-fibrin (CF) matrices was developed; the addition of gelatin (G) enabled 3D printing in combination with the plasma-algMC shell ink. Human endothelial cells (HUVEC), laden in the CFG core ink together with human fibroblasts as supportive cells, formed a pre-vascular network in the core in the absence and presence of HepG2 in the shell. The cellular interactions occurring in the triple culture model enhanced the albumin secretion. In conclusion, core-shell bioprinting was shown to be a valuable tool to study cell-cell-interactions and to develop complex tissue-like models.

Biomechanical assessment of chronic liver injury using quantitative micro-elastography

Hepatocellular carcinoma is one of the most lethal cancers worldwide, causing almost 700,000 deaths annually. It mainly arises from cirrhosis, which, in turn, results from chronic injury to liver cells and corresponding fibrotic changes. Although it is known that chronic liver injury increases the elasticity of liver tissue, the role of increased elasticity of the microenvironment as a possible hepatocarcinogen is yet to be investigated. One reason for this is the paucity of imaging techniques capable of mapping the micro-scale elasticity variation in liver and correlating that with cancerous mechanisms on the cellular scale. The clinical techniques of ultrasound elastography and magnetic resonance elastography typically do not provide micro-scale resolution, while atomic force microscopy can only assess the elasticity of a limited number of cells. We propose quantitative micro-elastography (QME) for mapping the micro-scale elasticity of liver tissue into images known as micro-elastograms, and therefore, as a technique capable of correlating the micro-environment elasticity of tissue with cellular scale cancerous mechanisms in liver. We performed QME on 13 freshly excised healthy and diseased mouse livers and present micro-elastograms, together with co-registered histology, in four representative cases. Our results indicate a significant increase in the mean (×6.3) and standard deviation (×6.0) of elasticity caused by chronic liver injury and demonstrate that the onset and progression of pathological features such as fibrosis, hepatocyte damage, and immune cell infiltration correlate with localized variations in micro-elastograms.

4D Printing: A Cutting-edge Platform for Biomedical Applications

Nature's materials have evolved over time to be able to respond to environmental stimuli by generating complex structures that can change their functions in response to distance, time, and direction of stimuli. A number of technical efforts are currently being made to improve printing resolution, shape fidelity, and printing speed to mimic the structural design of natural materials with three-dimensional (3D) printing. Unfortunately, this technology is limited by the fact that printed objects are static and cannot be reshaped dynamically in response to stimuli. In recent years, several smart materials have been developed that can undergo dynamic morphing in response to a stimulus, thus resolving this issue. Four-dimensional (4D) printing refers to a manufacturing process involving additive manufacturing, smart materials, and specific geometries. It has become an essential technology for biomedical engineering and has the potential to create a wide range of useful biomedical products. This paper will discuss the concept of 4D bioprinting and the recent developments in smart matrials, which can be actuated by different stimuli and be exploited to develop biomimetic materials and structures, with significant implications for pharmaceutics and biomedical research, as well as prospects for the future.

Review on Multicomponent Hydrogel Bioinks Based on Natural Biomaterials for Bioprinting 3D Liver Tissues

Three-dimensional (3D)-printed in vitro tissue models have been used in various biomedical fields owing to numerous advantages such as enhancements in cell response and functionality. In liver tissue engineering, several studies have been reported using 3D-printed liver tissue models with improved cellular responses and functions in drug screening, liver disease, and liver regenerative medicine. However, the application of conventional single-component bioinks for the printing of 3D in vitro liver constructs remains problematic because of the complex structural and physiological characteristics of the liver. The use of multicomponent bioinks has become an attractive strategy for bioprinting 3D functional in vitro liver tissue models because of the various advantages of multicomponent bioinks, such as improved mechanical properties of the printed tissue construct and cell functionality. Therefore, it is essential to review various 3D bioprinting techniques and multicomponent hydrogel bioinks proposed for liver tissue engineering to suggest future directions for liver tissue engineering. Accordingly, we herein review multicomponent bioinks for 3D-bioprinted liver tissues. We first describe the fabrication methods capable of printing multicomponent bioinks and introduce considerations for bioprinting. We subsequently categorize and evaluate the materials typically utilized for multicomponent bioinks based on their characteristics. In addition, we also review recent studies for the application of multicomponent bioinks to fabricate in vitro liver tissue models. Finally, we discuss the limitations of current studies and emphasize aspects that must be resolved to enhance the future applicability of such bioinks.

Exploiting maleimide-functionalized hyaluronan hydrogels to test cellular responses to physical and biochemical stimuli

Currentin vitrothree-dimensional (3D) models of liver tissue have been limited by the inability to study the effects of specific extracellular matrix (ECM) components on cell phenotypes. This is in part due to limitations in the availability of chemical modifications appropriate for this purpose. For example, hyaluronic acid (HA), which is a natural ECM component within the liver, lacks key ECM motifs (e.g. arginine-glycine-aspartic acid (RGD) peptides) that support cell adhesion. However, the addition of maleimide (Mal) groups to HA could facilitate the conjugation of ECM biomimetic peptides with thiol-containing end groups. In this study, we characterized a new crosslinkable hydrogel (i.e. HA-Mal) that yielded a simplified ECM-mimicking microenvironment supportive of 3D liver cell culture. We then performed a series of experiments to assess the impact of physical and biochemical signaling in the form of RGD peptide incorporation and transforming growth factorß(TGF-ß) supplementation, respectively, on hepatic functionality. Hepatic stellate cells (i.e. LX-2) exhibited increased cell-matrix interactions in the form of cell spreading and elongation within HA-Mal matrices containing RGD peptides, enabling physical adhesions, whereas hepatocyte-like cells (HepG2) had reduced albumin and urea production. We further exposed the encapsulated cells to soluble TGF-ßto elicit a fibrosis-like state. In the presence of TGF-ßbiochemical signals, LX-2 cells became activated and HepG2 functionality significantly decreased in both RGD-containing and RGD-free hydrogels. Altogether, in this study we have developed a hydrogel biomaterial platform that allows for discrete manipulation of specific ECM motifs within the hydrogel to better understand the roles of cell-matrix interactions on cell phenotype and overall liver functionality.

Viscous Fingering as a Rapid 3D Pattering Technique for Engineering Cell-Laden Vascular-Like Constructs

Tissues are much larger than the diffusion limit distance, so rapidly providing blood vessels to supply embedded cells inside tissues with sufficient nutrients and oxygen is regarded as a major strategy for the success of bioengineered large and thick tissue constructs. Here, a patterning technique, viscous fingering, is developed to bioengineer vascularized-like tissues within a few minutes. By controlling viscosity, flow rate, and the volume of photo-cross-linkable prepolymer, macro- and microscale vascular network structures can be precisely engineered using the Hele-Shaw cell that is designed in this study. After cross-linking, a vascular-like gel with fingering structures is formed between the bottom and top base gels, creating a sandwich-like structure. Cells can be incorporated into the fingers, bases, or both gels. The spreading and growth direction of the embedded cells are successfully controlled and guided by manipulating the physical properties of the fingering and base gels individually. Moreover, fingering is generated, connected, and surrounded prepared cell-laden microgels in base prepolymers to form prevascularized tissue-like constructs. Taken together, the 3D cell patterning technique extends the potential for modeling and fabricating large and stackable vascularized tissue-like constructs for both ex vivo and in vivo applications.

4D printing in biomedical applications: emerging trends and technologies

Nature's material systems during evolution have developed the ability to respond and adapt to environmental stimuli through the generation of complex structures capable of varying their functions across direction, distances and time. 3D printing technologies can recapitulate structural motifs present in natural materials, and efforts are currently being made on the technological side to improve printing resolution, shape fidelity, and printing speed. However, an intrinsic limitation of this technology is that printed objects are static and thus inadequate to dynamically reshape when subjected to external stimuli. In recent years, this issue has been addressed with the design and precise deployment of smart materials that can undergo a programmed morphing in response to a stimulus. The term 4D printing was coined to indicate the combined use of additive manufacturing, smart materials, and careful design of appropriate geometries. In this review, we report the recent progress in the design and development of smart materials that are actuated by different stimuli and their exploitation within additive manufacturing to produce biomimetic structures with important repercussions in different but interrelated biomedical areas.

Organoids: a novel modality in disease modeling

Limitations of monolayer culture conditions have motivated scientists to explore new models that can recapitulate the architecture and function of human organs more accurately. Recent advances in the improvement of protocols have resulted in establishing three-dimensional (3D) organ-like architectures called ‘organoids’ that can display the characteristics of their corresponding real organs, including morphological features, functional activities, and personalized responses to specific pathogens. We discuss different organoid-based 3D models herein, which are classified based on their original germinal layer. Studies of organoids simulating the complexity of real tissues could provide novel platforms and opportunities for generating practical knowledge along with preclinical studies, including drug screening, toxicology, and molecular pathophysiology of diseases. This paper also outlines the key challenges, advantages, and prospects of current organoid systems.

Critical signaling pathways governing hepatocellular carcinoma behavior; small molecule-based approaches

Hepatocellular carcinoma (HCC) is the second leading cause of death due to cancer. Although there are different treatment options, these strategies are not efficient in terms of restricting the tumor cell's proliferation and metastasis. The liver tumor microenvironment contains the non-parenchymal cells with supportive or inhibitory effects on the cancerous phenotype of HCC. Several signaling pathways are dis-regulated in HCC and cause uncontrolled cell propagation, metastasis, and recurrence of liver carcinoma cells. Recent studies have established new approaches for the prevention and treatment of HCC using small molecules. Small molecules are compounds with a low molecular weight that usually inhibit the specific targets in signal transduction pathways. These components can induce cell cycle arrest, apoptosis, block metastasis, and tumor growth. Devising strategies for simultaneously targeting HCC and the non-parenchymal population of the tumor could lead to more relevant research outcomes. These strategies may open new avenues for the treatment of HCC with minimal cytotoxic effects on healthy cells. This study provides the latest findings on critical signaling pathways governing HCC behavior and using small molecules in the control of HCC both in vitro and in vivo models.

SARS-CoV-2 and tissue damage: current insights and biomaterial-based therapeutic strategies

The effect of SARS-CoV-2 infection on humanity has gained worldwide attention and importance due to the rapid transmission, lack of treatment options and high mortality rate of the virus. While scientists across the world are searching for vaccines/drugs that can control the spread of the virus and/or reduce the risks associated with infection, patients infected with SARS-CoV-2 have been reported to have tissue/organ damage. With most tissues/organs having limited regenerative potential, interventions that prevent further damage or facilitate healing would be helpful. In the past few decades, biomaterials have gained prominence in the field of tissue engineering, in view of their major role in the regenerative process. Here we describe the effect of SARS-CoV-2 on multiple tissues/organs, and provide evidence for the positive role of biomaterials in aiding tissue repair. These findings are further extrapolated to explore their prospects as a therapeutic platform to address the tissue/organ damage that is frequently observed during this viral outbreak. This study suggests that the biomaterial-based approach could be an effective strategy for regenerating tissues/organs damaged by SARS-CoV-2.

3D bioprinting of hepatocytes: core-shell structured co-cultures with fibroblasts for enhanced functionality

With the aim of understanding and recapitulating cellular interactions of hepatocytes in their physiological microenvironment and to generate an artificial 3D in vitro model, a co-culture system using 3D extrusion bioprinting was developed. A bioink based on alginate and methylcellulose (algMC) was first shown to be suitable for bioprinting of hepatocytes; the addition of Matrigel to algMC enhanced proliferation and morphology of them in monophasic scaffolds. Towards a more complex system that allows studying cellular interactions, we applied core-shell bioprinting to establish tailored 3D co-culture models for hepatocytes. The bioinks were specifically functionalized with natural matrix components (based on human plasma, fibrin or Matrigel) and used to co-print fibroblasts and hepatocytes in a spatially defined, coaxial manner. Fibroblasts acted as supportive cells for co-cultured hepatocytes, stimulating the expression of certain biomarkers of hepatocytes like albumin. Furthermore, matrix functionalization positively influenced both cell types in their respective compartments by enhancing their adhesion, viability, proliferation and function. In conclusion, we established a functional co-culture model with independently tunable compartments for different cell types via core-shell bioprinting. This provides the basis for more complex in vitro models allowing co-cultivation of hepatocytes with other liver-specific cell types to closely resemble the liver microenvironment.

Stem cell therapy and tissue engineering strategies using cell aggregates and decellularized scaffolds for the rescue of liver failure

Liver failure is a lethal condition with hepatocellular dysfunction, and liver transplantation is presently the only effective treatment. However, due to the limited availability of donors and the potential immune rejection, novel therapeutic strategies are actively sought to restore the normal hepatic architectures and functions, especially for livers with inherited metabolic dysfunctions or chronic diseases. Although the conventional cell therapy has shown promising results, the direct infusion of hepatocytes is hampered by limited hepatocyte sources, poor cell viability, and engraftment. Hence, this review mainly highlights the role of stem cells and progenitors as the alternative cell source and summarizes the potential approaches based on tissue engineering to improve the delivery efficiency of cells. Particularly, the underlying mechanisms for cell therapy using stem cells and progenitors are discussed in two main aspects: paracrine effect and cell differentiation. Moreover, tissue-engineering approaches using cell aggregates and decellularized liver scaffolds for bioengineering of functional hepatic constructs are discussed and compared in terms of the potential to replicate liver physiological structures. In the end, a potentially effective strategy combining the premium advantages of stem cell aggregates and decellularized liver scaffolds is proposed as the future direction of liver tissue engineering and regeneration.

Oxygen releasing materials: Towards addressing the hypoxia-related issues in tissue engineering

Manufacturing macroscale cell-laden architectures is one of the biggest challenges faced nowadays in the domain of tissue engineering. Such living constructs, in fact, pose strict requirements for nutrients and oxygen supply that can hardly be addressed through simple diffusion in vitro or without a functional vasculature in vivo. In this context, in the last two decades, a substantial amount of work has been carried out to develop smart materials that could actively provide oxygen-release to contrast local hypoxia in large-size constructs. This review provides an overview of the currently available oxygen-releasing materials and their synthesis and mechanism of action, highlighting their capacities under in vitro tissue cultures and in vivo contexts. Additionally, we also showcase an emerging concept, herein termed as "living materials as releasing systems", which relies on the combination of biomaterials with photosynthetic microorganisms, namely algae, in an "unconventional" attempt to supply the damaged or re-growing tissue with the necessary supply of oxygen. We envision that future advances focusing on tissue microenvironment regulated oxygen-supplying materials would unlock an untapped potential for generating a repertoire of anatomic scale, living constructs with improved cell survival, guided differentiation, and tissue-specific biofunctionality.

Engineering liver microtissues for disease modeling and regenerative medicine

The burden of liver diseases is increasing worldwide, accounting for two million deaths annually. In the past decade, tremendous progress has been made in the basic and translational research of liver tissue engineering. Liver microtissues are small, three-dimensional hepatocyte cultures that recapitulate liver physiology and have been used in biomedical research and regenerative medicine. This review summarizes recent advances, challenges, and future directions in liver microtissue research. Cellular engineering approaches are used to sustain primary hepatocytes or produce hepatocytes derived from pluripotent stem cells and other adult tissues. Three-dimensional microtissues are generated by scaffold-free assembly or scaffold-assisted methods such as macroencapsulation, droplet microfluidics, and bioprinting. Optimization of the hepatic microenvironment entails incorporating the appropriate cell composition for enhanced cell-cell interactions and niche-specific signals, and creating scaffolds with desired chemical, mechanical and physical properties. Perfusion-based culture systems such as bioreactors and microfluidic systems are used to achieve efficient exchange of nutrients and soluble factors. Taken together, systematic optimization of liver microtissues is a multidisciplinary effort focused on creating liver cultures and on-chip models with greater structural complexity and physiological relevance for use in liver disease research, therapeutic development, and regenerative medicine.

Microfibrous Extracellular Matrix Changes the Liver Hepatocyte Energy Metabolism via Integrins

Cell line-based liver models are critical tools for liver-related studies. However, the conventional monolayer culture of hepatocytes, the most widely used in vitro model, does not have the extracellular matrix (ECM), which contributes to the three-dimensional (3D) arrangement of the hepatocytes in the liver. As a result, the metabolic properties of the hepatocytes in the monolayer tissue culture may not accurately reflect those of the hepatocytes in the liver. Here, we developed a modular platform for 3D hepatocyte cultures on fibrous ECMs produced by electrospinning, a technique that can turn a polymer solution to the micro/nanofibers and has been widely used to produce scaffolds for 3D cell cultures. Metabolomics quantitation by liquid chromatography-mass spectrometry (LC-MS) indicated that Huh7 hepatocytes grown in microfibers electrospun from silk fibroin exhibited reduced glycolysis and tricarboxylic acid (TCA) cycle, as compared to the cells cultured as a monolayer. Further mechanistic studies suggested that integrins were correlated to the ECM's effects. This is the first time to report how an ECM scaffold could affect the fundamental metabolism of the hepatocytes via integrins.