Long-term liver-specific functions of hepatocytes in electrospun chitosan nanofiber scaffolds coated with fibronectin

Reconstructing the hepatocellular carcinoma microenvironment: the current status and challenges of 3D culture technology

Hepatocellular carcinoma (HCC), with high incidence and mortality rates among digestive system diseases, has become a focal point for researchers. However, the more we learn about HCC, the more apparent it becomes that our understanding is still superficial. The successes and failures of numerous studies underscore the urgent need for precision medicine in cancer treatment. A crucial aspect of preclinical research in precision medicine is the experimental model, particularly cell culture models. Among these, 3D cell culture models can effectively integrate and simulate the tumor microenvironment, closely reflecting the in vivo conditions of patients. This capability provides a solid theoretical foundation for personalized treatment approaches. In this review, we first outline the common in vitro 3D cell culture models and examine the essential elements within the tumor microenvironment, followed by insights into the current state and future developments of 3D in vitro cell models for HCC.

The role of chondroitin sulphate as a potential biomaterial for hepatic tissue regeneration: A comprehensive review

Chondroitin sulphate is an anionic hetero-polysaccharide, having numerous structural affinities for building the bio-active components. In addition to biodegradable/biocompatible activities, chondroitin sulphate also possesses anti-coagulant/anti-thrombogenic, anti-inflammatory, anti-oxidant as well as anti-tumor activities. Chondroitin sulphate has an inherited affinity for glycosylation enzymes and receptors, which are overexpressed over degenerated cells and organelles. Because of this affinity, chondroitin sulphate is nominated as an active cellular/subcellular targeted biological macromolecule to assist in site-specific delivery. Chondroitin sulphate is mainly considered a promising biomaterial for drug targeting and tissue engineering due to its specific physicochemical, mechanical, bio-degradation, and biological characteristics. In this review, the fundamental applications of chondroitin sulphate in hepatic tissue engineering are discussed. Chondroitin sulphate along with mesenchymal stem cells (MSCs) based scaffold and hydrogels for biopharmaceuticals' delivery in hepatic tissue engineering are critically discussed. In addition, the manuscript also describes leading features and markers involved in hepatic damage, and the potential role of chondroitin sulphate-based delivery systems in hepatic tissue engineering.

Attenuation of alcohol-induced hepatocyte damage by ginsenoside Rg1 evaluated using atomic force microscopy

Alcoholic liver disease is an important cause of death worldwide. Hepatocyte apoptosis is commonly observed in alcoholic liver disease. In this study, we investigated the effect of ginsenoside Rg1 (G-Rg1), an organic component of ginseng, on the alcohol-induced morphological and biophysical properties of hepatocytes. Human hepatocytes (HL-7702) were treated in vitro with alcohol and G-Rg1. The cell morphology was observed using scanning electron microscopy. Cell height, roughness, adhesion, and elastic modulus were detected using atomic force microscopy. We found that alcohol significantly induced hepatocyte apoptosis, whereas G-Rg1 attenuated the alcohol-induced hepatocyte damage. Scanning electron microscopy revealed that alcohol-induced significant morphological changes in hepatocytes, including decreased cell contraction, roundness, and pseudopods, whereas G-Rg1 inhibited these negative changes. Atomic force microscopy revealed that alcohol increased the cell height and decreased the adhesion and elastic modulus of hepatocytes. Following treatment with G-Rg1, the cell height, adhesion, and elastic modulus of alcohol-injured hepatocytes were all similar to those of normal cells. Thus, G-Rg1 can attenuate the alcohol-induced damage to hepatocytes by modulating the morphology and biomechanics of the cells. RESEARCH HIGHLIGHTS: In this study, the morphological characteristics of hepatocytes were observed using SEM. The changes in hepatocyte three-dimensional images and biomechanical action caused by alcohol and G-Rg1 were examined at the nanoscale using AFM under near-physiological conditions. Alcohol-induced hepatocytes showed abnormal morphology and biophysical properties. G-Rg1 attenuated the alcohol-induced damage to hepatocytes by modulating the morphology and biomechanics of the cells.

Decellularized Liver Nanofibers Enhance and Stabilize the Long-Term Functions of Primary Human Hepatocytes In Vitro

Owing to significant differences across species in liver functions, in vitro human liver models are used for screening the metabolism and toxicity of compounds, modeling diseases, and cell-based therapies. However, the extracellular matrix (ECM) scaffold used for such models often does not mimic either the complex composition or the nanofibrous topography of native liver ECM. Thus, here novel methods are developed to electrospin decellularized porcine liver ECM (PLECM) and collagen I into nano- and microfibers (≈200-1000 nm) without synthetic polymer blends. Primary human hepatocytes (PHHs) on nanofibers in monoculture or in coculture with nonparenchymal cells (3T3-J2 embryonic fibroblasts or primary human liver endothelial cells) display higher albumin secretion, urea synthesis, and cytochrome-P450 1A2, 2A6, 2C9, and 3A4 enzyme activities than on conventionally adsorbed ECM controls. PHH functions are highest on the collagen/PLECM blended nanofibers (up to 34-fold higher CYP3A4 activity relative to adsorbed ECM) for nearly 7 weeks in the presence of the fibroblasts. In conclusion, it is shown for the first time that ECM composition and topography synergize to enhance and stabilize PHH functions for several weeks in vitro. The nanofiber platform can prove useful for the above applications and to elucidate cell-ECM interactions in the human liver.

A review on background, process and application of electrospun nanofibers for tissue regeneration

Electrospinning is a versatile method which is used to synthesize nano/micro sized fibers under the influence of electric field. Electrospun nanoscaffolds are one of the widely accepted platforms for cultivating soft and hard tissues as they create a prefect micro-environment for cell adhesion, proliferation and differentiation. Nanoscaffolds are widely used in the field of tissue engineering due to their versatility in aiding the growth of different types of cells and tissues for varied applications. The composition, molecular weight and structure of polymer used to fabricate nanoscaffold plays an important role in determining the size and strength of the nanofibers prepared. This review gives information about the background, process and different types of polymers used in electrospinning. Recent advances in culturing liver cells, osteoblasts, skin cells, neural cells and coronary artery smooth muscle cells on nanoscaffolds are also elucidated.

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.

Convergence of 3D Bioprinting and Nanotechnology in Tissue Engineering Scaffolds

Three-dimensional (3D) bioprinting has emerged as a promising scaffold fabrication strategy for tissue engineering with excellent control over scaffold geometry and microstructure. Nanobiomaterials as bioinks play a key role in manipulating the cellular microenvironment to alter its growth and development. This review first introduces the commonly used nanomaterials in tissue engineering scaffolds, including natural polymers, synthetic polymers, and polymer derivatives, and reveals the improvement of nanomaterials on scaffold performance. Second, the 3D bioprinting technologies of inkjet-based bioprinting, extrusion-based bioprinting, laser-assisted bioprinting, and stereolithography bioprinting are comprehensively itemized, and the advantages and underlying mechanisms are revealed. Then the convergence of 3D bioprinting and nanotechnology applications in tissue engineering scaffolds, such as bone, nerve, blood vessel, tendon, and internal organs, are discussed. Finally, the challenges and perspectives of convergence of 3D bioprinting and nanotechnology are proposed. This review will provide scientific guidance to develop 3D bioprinting tissue engineering scaffolds by nanotechnology.

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.

Primary Hepatocyte Isolation and Cultures: Technical Aspects, Challenges and Advancements

Hepatocytes are differentiated cells that account for 80% of the hepatic volume and perform all major functions of the liver. In vivo, after an acute insult, adult hepatocytes retain their ability to proliferate and participate in liver regeneration. However, in vitro, prolonged culture and proliferation of viable and functional primary hepatocytes have remained the major and the most challenging goal of hepatocyte-based cell therapies and liver tissue engineering. The first functional cultures of rat primary hepatocytes between two layers of collagen gel, also termed as the "sandwich cultures", were reported in 1989. Since this study, several technical developments including choice of hydrogels, type of microenvironment, growth factors and culture conditions, mono or co-cultures of hepatocytes along with other supporting cell types have evolved for both rat and human primary hepatocytes in recent years. All these improvements have led to a substantial improvement in the number, life-span and hepatic functions of these cells in vitro for several downstream applications. In the current review, we highlight the details, limitations and prospects of different technical strategies being used in primary hepatocyte cultures. We discuss the use of newer biomaterials as scaffolds for efficient culture of primary hepatocytes. We also describe the derivation of mature hepatocytes from other cellular sources such as induced pluripotent stem cells, bone marrow stem cells and 3D liver organoids. Finally, we also explain the use of perfusion-based bioreactor systems and bioengineering strategies to support the long-term function of hepatocytes in 3D conditions.

Rat liver ECM incorporated into electrospun polycaprolactone scaffolds as a platform for hepatocyte culture

Liver disease is expanding across the globe; however, health-care systems still lack approved pharmaceutical treatment strategies to mitigate potential liver failures. Organ transplantation is the only treatment for liver failure and with increasing cases of liver disease, transplant programs increasingly cannot provide timely transplant availability for all patients. The development of pharmaceutical mitigation strategies is clearly necessary and methods to improve drug development processes are considered vital for this purpose. Herein, we present a methodology for incorporating whole organ decellularised rat liver ECM (rLECM) into polycaprolactone (PCL) electrospun scaffolds with the aim of producing biologically relevant liver tissue models. rLECM PCL scaffolds have been produced with 5 w/w% and 10 w/w% rLECM:PCL and were analyzed by SEM imaging, tensile mechanical analyses and FTIR spectroscopy. The hepatocellular carcinoma cell line, HepG2, was cultured upon the scaffolds for 14 days and were analyzed through cell viability assay, DNA quantification, albumin quantification, immunohistochemistry, and RT-qPCR gene expression analysis. Results showed significant increases in proliferative activity of HepG2 on rLECM containing scaffolds alongside maintained key gene expression. This study confirms that rLECM can be utilized to modulate the bioactivity of electrospun PCL scaffolds and has the potential to produce electrospun scaffolds suitable for enhanced hepatocyte cultures and in-vitro liver tissue models.

Mesenchymal Stromal Cell on Liver Decellularised Extracellular Matrix for Tissue Engineering

Background: In end-stage chronic liver disease, transplantation represents the only curative option. However, the shortage of donors results in the death of many patients. To overcome this gap, it is mandatory to develop new therapeutic options. In the present study, we decellularised pig livers and reseeded them with allogeneic porcine mesenchymal stromal cells (pMSCs) to understand whether extracellular matrix (ECM) can influence and/or promote differentiation into hepatocyte-like cells (HLCs). Methods: After decellularisation with SDS, the integrity of ECM-scaffolds was examined by histological staining, immunofluorescence and scanning electron microscope. DNA quantification was used to assess decellularisation. pMSCs were plated on scaffolds by static seeding and maintained in in vitro culture for 21 days. At 3, 7, 14 and 21 days, seeded ECM scaffolds were evaluated for cellular adhesion and growth. Moreover, the expression of specific hepatic genes was performed by RT-PCR. Results: The applied decellularisation/recellularisation protocol was effective. The number of seeded pMSCs increased over the culture time points. Gene expression analysis of seeded pMSCs displayed a weak induction due to ECM towards HLCs. Conclusions: These results suggest that ECM may address pMSCs to differentiate in hepatocyte-like cells. However, only contact with liver-ECM is not enough to induce complete differentiation.

Current strategies with implementation of three-dimensional cell culture: the challenge of quantification

From growing cells in spheroids to arranging them on complex engineered scaffolds, three-dimensional cell culture protocols are rapidly expanding and diversifying. While these systems may often improve the physiological relevance of cell culture models, they come with technical challenges, as many of the analytical methods used to characterize traditional two-dimensional (2D) cells must be modified or replaced to be effective. Here we review the advantages and limitations of quantification methods based either on biochemical measurements or microscopy imaging. We focus on the most basic of parameters that one may want to measure, the number of cells. Precise determination of this number is essential for many analytical techniques where measured quantities are only meaningful when normalized to the number of cells (e.g. cytochrome p450 enzyme activity). Thus, accurate measurement of cell number is often a prerequisite to allowing comparisons across different conditions (culturing conditions or drug and treatment screening) or between cells in different spatial states. We note that this issue is often neglected in the literature with little or no information given regarding how normalization was performed, we highlight the pitfalls and complications of quantification and call for more accurate reporting to improve reproducibility.

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.

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.

Influence of surface topography on PCL electrospun scaffolds for liver tissue engineering

Severe liver disease is one of the most common causes of death globally. Currently, whole organ transplantation is the only therapeutic method for end-stage liver disease treatment, however, the need for donor organs far outweighs demand. Recently liver tissue engineering is starting to show promise for alleviating part of this problem. Electrospinning is a well-known method to fabricate a nanofibre scaffold which mimics the natural extracellular matrix that can support cell growth. This study aims to investigate liver cell responses to topographical features on electrospun fibres. Scaffolds with large surface depression (2 μm) (LSD), small surface depression (0.37 μm) (SSD), and no surface depression (NSD) were fabricated by using a solvent-nonsolvent system. A liver cell line (HepG2) was seeded onto the scaffolds for up to 14 days. The SSD group exhibited higher levels of cell viability and DNA content compared to the other groups. Additionally, the scaffolds promoted gene expression of albumin, with all cases having similar levels, while the cell growth rate was altered. Furthermore, the scaffold with depressions showed 0.8 MPa higher ultimate tensile strength compared to the other groups. These results suggest that small depressions might be preferred by HepG2 cells over smooth and large depression fibres and highlight the potential for tailoring liver cell responses.

A biomimetic orthogonal-bilayer tubular scaffold for the co-culture of endothelial cells and smooth muscle cells

In blood vessels, endothelial cells (ECs) grow along the direction of blood flow, while smooth muscle cells (SMCs) grow circumferentially along the vessel wall. To mimic this structure, a polycaprolactone (PCL) tubular scaffold with orthogonally oriented bilayer nanofibers was prepared via electrospinning and winding. ECs were cultured on the inner layer of the scaffold with axial nanofibers and SMCs were cultured on the outer layer of the scaffold with circumferential nanofibers. Fluorescence images of the F-actin distribution of ECs and SMCs indicated that cells adhered, stretched, and proliferated in an oriented manner on the scaffold. Moreover, layers of ECs and SMCs formed on the scaffold after one month of incubation. The expression levels of platelet-endothelial cell adhesion molecule 1 (PECAM-1) and a contractile SMC phenotype marker in the EC/SMC co-culture system were much higher than those in individual culture systems, thus demonstrating that the proposed biomimetic scaffold promoted the intercellular junction of ECs and preserved the contractile phenotype of SMCs.

To mimic blood vessels, a polycaprolactone tubular scaffold was prepared via electrospinning and winding. Endothelial cells were cultured on the inner layer with axial nanofibers and smooth muscle cells were cultured on the outer layer with circumferential nanofibers.

Novel bone microenvironment model of castration-resistant prostate cancer with chitosan fiber matrix and osteoblasts

The interaction between prostate cancer cells and osteoblasts is essential for the development of bone metastasis. Previously, novel androgen receptor axis-targeted agents (ARATs) were approved for metastatic castration-naïve and non-metastatic castration-resistant prostate cancer (CRPC); both of which are pivotal for investigating the association between the bone microenvironment and tumors. The present study established a novel in vitro 3D microenvironment model that simulated the bone microenvironment of CRPC, and evaluated the drug susceptibility of ARATs and the efficacy of the combination of abiraterone and dutasteride. Green fluorescent protein-transferred C4-2 cells (a CRPC cell line) and red fluorescent protein-transferred human osteoblasts differentiated from human mesenchymal stem cells were co-cultured in chitosan nanofiber matrix-coated culture plates to simulate the 3D scaffold of the bone microenvironment. The growth of C4-2 was quantified using live-cell imaging and the Cell3 iMager duos analysis system. The growth of C4-2 colonies were quantified for a maximum of 30 days. The expression of TGF-β increased and promoted EMT in C4-2 cells co-cultured with osteoblasts, indicating resistance to ARATs. The IC₅₀ of each drug and the combination effect of abiraterone and dutasteride were evaluated using this model. Combination treatment with abiraterone and dutasteride synergistically inhibited the growth of C2-4 colonies compared with individual investigational agents. This could be attributed to the reduction of 3-keto-5α-abiraterone, an androgen receptor agonist. The bone microenvironment model of the present study is unique and useful for evaluating new drug susceptibility testing in prostate cancer cells. This model may help to reveal the unknown mechanisms underlying micro- to clinical bone metastasis in prostate cancer.

Hollow Fiber and Nanofiber Membranes in Bioartificial Liver and Neuronal Tissue Engineering

To date, the creation of biomimetic devices for the regeneration and repair of injured or diseased tissues and organs remains a crucial challenge in tissue engineering. Membrane technology offers advanced approaches to realize multifunctional tools with permissive environments well-controlled at molecular level for the development of functional tissues and organs. Membranes in fiber configuration with precisely controlled, tunable topography, and physical, biochemical, and mechanical cues, can direct and control the function of different kinds of cells toward the recovery from disorders and injuries. At the same time, fiber tools also provide the potential to model diseases in vitro for investigating specific biological phenomena as well as for drug testing. The purpose of this review is to present an overview of the literature concerning the development of hollow fibers and electrospun fiber membranes used in bioartificial organs, tissue engineered constructs, and in vitro bioreactors. With the aim to highlight the main biomedical applications of fiber-based systems, the first part reviews the fibers for bioartificial liver and liver tissue engineering with special attention to their multifunctional role in the long-term maintenance of specific liver functions and in driving hepatocyte differentiation. The second part reports the fiber-based systems used for neuronal tissue applications including advanced approaches for the creation of novel nerve conduits and in vitro models of brain tissue. Besides presenting recent advances and achievements, this work also delineates existing limitations and highlights emerging possibilities and future prospects in this field.

Synthesis and characterization of site selective photo-crosslinkable glycidyl methacrylate functionalized gelatin-based 3D hydrogel scaffold for liver tissue engineering

The presented work outlined the development of a new biocompatible hydrogel material that has potential applications in soft tissue engineering. As a proof of concept, human hepatocytes were used to demonstrate the suitability of this material in providing conducive environment for cellular growth and functional development. Herein, a detailed synthesis of novel gelatin derivatives - photo-crosslinkable glycidyl methacrylate (GMA) functionalized gelatins (Gelatin-GMA), and preparation of three-dimensional (3D) hydrogel scaffolds for the encapsulated Huh-7.5 cells is reported. The Gelatin-GMA biopolymers were synthesized at two different pH values of 3.5 (acidic) and 10.5 (basic) where two different photo-crosslinkable polymers were formed utilizing -COOH & -OH groups in acidic pH, and -NH₂ & -OH groups in basic pH. The hydrogels were prepared using an initiator (Irgacure I2959) in the presence of UV light. The Gelatin-GMA biopolymers were characterized using spectroscopic studies which confirmed the successful preparation of the polymer derivatives. Rheological measurement was carried out to characterize the mechanical properties and derive the mesh sizes of the 3D hydrogels. Subsequently, detailed in vitro hepatocyte compatibility and functionality studies were performed in the 3D cell seeded hydrogel platform. The 3D hydrogel design with larger mesh sizes utilizes the advantage of the excellent diffusion properties of porous platform, and enhanced cell-growth was observed, which in turn elicited favorable Huh-7.5 response. The hydrogels led to improved cellular functions such as differentiation, viability and proliferation. Overall, it showed that the Gelatin-GMA based hydrogels presented better results compared to control sample (GelMA) because of the higher mesh sizes in Gelatin-GMA based hydrogels. Additionally, the functional group studies of the two Gelatin-GMA samples revealed that the cell functionalities are almost unaffected even after the tripeptide - Arg-Gly-Asp (RGD) in Gelatin-GMA synthesized at pH 3.5 is no longer completely available.

Hepatic Differentiation of Stem Cells in 2D and 3D Biomaterial Systems

A critical shortage of donor livers for treating end-stage liver failure signifies the urgent need for alternative treatment options. Hepatocyte-like cells (HLC) derived from various stem cells represent a promising cell source for hepatocyte transplantation, liver tissue engineering, and development of a bioartificial liver assist device. At present, the protocols of hepatic differentiation of stem cells are optimized based on soluble chemical signals introduced in the culture medium and the HLC produced typically retain an immature phenotype. To promote further hepatic differentiation and maturation, biomaterials can be designed to recapitulate cell-extracellular matrix (ECM) interactions in both 2D and 3D configurations. In this review, we will summarize and compare various 2D and 3D biomaterial systems that have been applied to hepatic differentiation, and highlight their roles in presenting biochemical and physical cues to different stem cell sources.

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

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

Regulated differentiation of stem cells into an artificial 3D liver as a transplantable source

End-stage liver disease is one of the leading causes of death around the world. Since insufficient sources of transplantable liver and possible immune rejection severely hinder the wide application of conventional liver transplantation therapy, artificial three-dimensional (3D) liver culture and assembly from stem cells have become a new hope for patients with end-stage liver diseases, such as cirrhosis and liver cancer. However, the induced differentiation of single-layer or 3D-structured hepatocytes from stem cells cannot physiologically support essential liver functions due to the lack of formation of blood vessels, immune regulation, storage of vitamins, and other vital hepatic activities. Thus, there is emerging evidence showing that 3D organogenesis of artificial vascularized liver tissue from combined hepatic cell types derived from differentiated stem cells is practical for the treatment of end-stage liver diseases. The optimization of novel biomaterials, such as decellularized matrices and natural macromolecules, also strongly supports the organogenesis of 3D tissue with the desired complex structure. This review summarizes new research updates on novel differentiation protocols of stem cell-derived major hepatic cell types and the application of new supportive biomaterials. Future biological and clinical challenges of this concept are also discussed.

Tissue Engineering in Liver Regenerative Medicine: Insights into Novel Translational Technologies

Organ and tissue shortage are known as a crucially important public health problem as unfortunately a small percentage of patients receive transplants. In the context of emerging regenerative medicine, researchers are trying to regenerate and replace different organs and tissues such as the liver, heart, skin, and kidney. Liver tissue engineering (TE) enables us to reproduce and restore liver functions, fully or partially, which could be used in the treatment of acute or chronic liver disorders and/or generate an appropriate functional organ which can be transplanted or employed as an extracorporeal device. In this regard, a variety of techniques (e.g., fabrication technologies, cell-based technologies, microfluidic systems and, extracorporeal liver devices) could be applied in tissue engineering in liver regenerative medicine. Common TE techniques are based on allocating stem cell-derived hepatocyte-like cells or primary hepatocytes within a three-dimensional structure which leads to the improvement of their survival rate and functional phenotype. Taken together, new findings indicated that developing liver tissue engineering-based techniques could pave the way for better treatment of liver-related disorders. Herein, we summarized novel technologies used in liver regenerative medicine and their future applications in clinical settings.

Cell-Type-Specific Quantification of a Scaffold-Based 3D Liver Co-Culture

In order to increase the metabolic activity of human hepatocytes and liver cancer cell lines, many approaches have been reported in recent years. The metabolic activity could be increased mainly by cultivating the cells in 3D systems or co-cultures (with other cell lines). However, if the system becomes more complex, it gets more difficult to quantify the number of cells (e.g., on a 3D matrix). Until now, it has been impossible to quantify different cell types individually in 3D co-culture systems. Therefore, we developed a PCR-based method that allows the quantification of HepG2 cells and 3T3-J2 cells separately in a 3D scaffold culture. Moreover, our results show that this method allows better comparability between 2D and 3D cultures in comparison to the often-used approaches based on metabolic activity measurements, such as the conversion of resazurin.

A Hepatic Scaffold from Decellularized Liver Tissue: Food for Thought

Allogeneic liver transplantation is still deemed the gold standard solution for end-stage organ failure; however, donor organ shortages have led to extended waiting lists for organ transplants. In order to overcome the lack of donors, the development of new therapeutic options is mandatory. In the last several years, organ bioengineering has been extensively explored to provide transplantable tissues or whole organs with the final goal of creating a three-dimensional growth microenvironment mimicking the native structure. It has been frequently reported that an extracellular matrix-based scaffold offers a structural support and important biological molecules that could help cellular proliferation during the recellularization process. The aim of the present review is to underline the recent developments in cell-on-scaffold technology for liver bioengineering, taking into account: (1) biological and synthetic scaffolds; (2) animal and human tissue decellularization; (3) scaffold recellularization; (4) 3D bioprinting; and (5) organoid technology. Future possible clinical applications in regenerative medicine for liver tissue engineering and for drug testing were underlined and dissected.

Current progress in hepatic tissue regeneration by tissue engineering

Liver, as a vital organ, is responsible for a wide range of biological functions to maintain homeostasis and any type of damages to hepatic tissue contributes to disease progression and death. Viral infection, trauma, carcinoma, alcohol misuse and inborn errors of metabolism are common causes of liver diseases are a severe known reason for leading to end-stage liver disease or liver failure. In either way, liver transplantation is the only treatment option which is, however, hampered by the increasing scarcity of organ donor. Over the past years, considerable efforts have been directed toward liver regeneration aiming at developing new approaches and methodologies to enhance the transplantation process. These approaches include producing decellularized scaffolds from the liver organ, 3D bio-printing system, and nano-based 3D scaffolds to simulate the native liver microenvironment. The application of small molecules and micro-RNAs and genetic manipulation in favor of hepatic differentiation of distinct stem cells could also be exploited. All of these strategies will help to facilitate the application of stem cells in human medicine. This article reviews the most recent strategies to generate a high amount of mature hepatocyte-like cells and updates current knowledge on liver regenerative medicine.

Hydrogels for Liver Tissue Engineering

Bioengineered livers are promising in vitro models for drug testing, toxicological studies, and as disease models, and might in the future be an alternative for donor organs to treat end-stage liver diseases. Liver tissue engineering (LTE) aims to construct liver models that are physiologically relevant. To make bioengineered livers, the two most important ingredients are hepatic cells and supportive materials such as hydrogels. In the past decades, dozens of hydrogels have been developed to act as supportive materials, and some have been used for in vitro models and formed functional liver constructs. However, currently none of the used hydrogels are suitable for in vivo transplantation. Here, the histology of the human liver and its relationship with LTE is introduced. After that, significant characteristics of hydrogels are described focusing on LTE. Then, both natural and synthetic materials utilized in hydrogels for LTE are reviewed individually. Finally, a conclusion is drawn on a comparison of the different hydrogels and their characteristics and ideal hydrogels are proposed to promote LTE.

A Standardized Collagen-Based Scaffold Improves Human Hepatocyte Shipment and Allows Metabolic Studies over 10 Days

Due to pronounced species differences, hepatotoxicity of new drugs often cannot be detected in animal studies. Alternatively, human hepatocytes could be used, but there are some limitations. The cells are not always available on demand or in sufficient amounts, so far there has been only limited success to allow the transport of freshly isolated hepatocytes without massive loss of function or their cultivation for a long time. Since it is well accepted that the cultivation of hepatocytes in 3D is related to an improved function, we here tested the Optimaix-3D Scaffold from Matricel for the transport and cultivation of hepatocytes. After characterization of the scaffold, we shipped cells on the scaffold and/or cultivated them over 10 days. With the evaluation of hepatocyte functions such as urea production, albumin synthesis, and CYP activity, we showed that the metabolic activity of the cells on the scaffold remained nearly constant over the culture time whereas a significant decrease in metabolic activity occurred in 2D cultures. In addition, we demonstrated that significantly fewer cells were lost during transport. In summary, the collagen-based scaffold allows the transport and cultivation of hepatocytes without loss of function over 10 days.

Growth factor release and enhanced encapsulated periodontal stem cells viability by freeze-dried platelet concentrate loaded thermo-sensitive hydrogel for periodontal regeneration

Periodontium regeneration is a highly challenging process as it requires the regeneration of three different tissues simultaneously. The aim of this study was to develop a composite material that can be easily applied and can sufficiently deliver essential growth factors and progenitor cells for periodontal tissue regeneration.

Freeze-dried platelet concentrate (FDPC) was prepared and incorporated in a thermo-sensitive chitosan/β-glycerol phosphate (β-GP) hydrogel at concentrations of 5, 10, or 15 mg/ml. The viscosity of the hydrogels was investigated as the temperature rises from 25 °C to 37 °C and the release kinetics of transforming growth factor (TGF-β1), platelet-derived growth factor (PDGF-BB) and insulin-like growth factor (IGF-1) were investigated at four time points (1 h, 1 day, 1 week, 2 weeks). Periodontal ligament stem cells (PDLSCs) were isolated from human third molars and encapsulated in the different hydrogel groups. Their viability was investigated after 7 days in culture in comparison to standard culture conditions and non FDPC-loaded hydrogel.

Results showed that loading FDPC in the hydrogel lowered the initial viscosity in comparison to the unloaded control group and did not affect the sol-gel transition in any group. All FDPC-loaded hydrogel groups exhibited sustained release of TGF-β1 and PDGF-BB for two weeks with significant difference between the different concentrations. The loading of 10 and 15 mg/ml of FDPC in the hydrogel increased the PDLSCs viability significantly compared to the unloaded hydrogel and was comparable to the standard culture conditions.

Accordingly, it may be concluded that loading FDPC in a chitosan/β-GP hydrogel can offer enhanced injectability, a sustained release of growth factors and increased viability of encapsulated stem cells which can be beneficial in periodontium tissue regeneration.

A 3D bioinspired highly porous polymeric scaffolding system for in vitro simulation of pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma is an aggressive disease with an extremely low survival rate. This is due to the (i) poor prognosis and (ii) high resistance of the disease to current treatment options. The latter is partly due to the very complex and dense tissue/tumour microenvironment of pancreatic cancer, which contributes to the disease's progression and the inhibition of apoptotic pathways. Over the last years, advances in tissue engineering and the development of three-dimensional (3D) culture systems have shed more light into cancer research by enabling a more realistic recapitulation of the niches and structure of the tumour microenvironment. Herein, for the first time, 3D porous polyurethane scaffolds were fabricated and coated with fibronectin to mimic features of the structure and extracellular matrix present in the pancreatic cancer tumour microenvironment. The developed 3D scaffold could support the proliferation of the pancreatic tumour cells, which was enhanced with the presence of fibronectin, for a month, which is a significantly prolonged in vitro culturing duration. Furthermore, in situ imaging of cellular and biomarker distribution showed the formation of dense cellular masses, the production of collagen-I by the cells and the formation of environmental stress gradients (e.g. HIF-1α) with similar heterogeneity trends to the ones reported in in vivo studies. The results obtained in this study suggest that this bioinspired porous polyurethane based scaffold has great potential for in vitro high throughput studies of pancreatic cancer including drug and treatment screening.

Nanofibrous PLGA electrospun scaffolds modified with type I collagen influence hepatocyte function and support viability in vitro

A major challenge of maintaining primary hepatocytes in vitro is progressive loss of hepatocyte-specific functions, such as protein synthesis and cytochrome P450 (CYP450) catalytic activity. We developed a three-dimensional (3D) nanofibrous scaffold made from poly(l-lactide-co-glycolide) (PLGA) polymer using a newly optimized wet electrospinning technique that resulted in a highly porous structure that accommodated inclusion of primary human hepatocytes. Extracellular matrix (ECM) proteins (type I collagen or fibronectin) at varying concentrations were chemically linked to electrospun PLGA using amine coupling to develop an in vitro culture system containing the minimal essential ECM components of the liver micro-environment that preserve hepatocyte function in vitro. Cell-laden nanofiber scaffolds were tested in vitro to maintain hepatocyte function over a two-week period. Incorporation of type I collagen onto PLGA scaffolds (PLGA-Chigh: 100 µg/mL) led to 10-fold greater albumin secretion, 4-fold higher urea synthesis, and elevated transcription of hepatocyte-specific CYP450 genes (CYP3A4, 3.5-fold increase and CYP2C9, 3-fold increase) in primary human hepatocytes compared to the same cells grown within unmodified PLGA scaffolds over two weeks. These indices, measured using collagen-bonded scaffolds, were also higher than scaffolds coupled to fibronectin or an ECM control sandwich culture composed of type I collagen and Matrigel. Induction of CYP2C9 activity was also higher in these same type I collagen PLGA scaffolds compared to other ECM-modified or unmodified PLGA constructs and was equivalent to the ECM control at 7 days. Together, we demonstrate a minimalist ECM-based 3D synthetic scaffold that accommodates primary human hepatocyte inclusion into the matrix, maintains long-term in vitro survival and stimulates function, which can be attributed to coupling of type I collagen.

STATEMENT OF SIGNIFICANCE

Culturing primary hepatocytes within a three-dimensional (3D) structure that mimics the natural liver environment is a promising strategy for extending the function and viability of hepatocytes in vitro. In the present study we generate porous PLGA nanofibers, that are chemically modified with extracellular matrix proteins, to serve as 3D scaffolds for the in vitro culture of primary human hepatocytes. Our findings demonstrate that the use of ECM proteins, especially type I collagen, in a porous 3D environment helps to improve the synthetic function of primary hepatocytes over time. We believe the work presented within will provide insights to readers for drug toxicity and tissue engineering applications.

3D microfluidic liver cultures as a physiological preclinical tool for hepatitis B virus infection

With more than 240 million people infected, hepatitis B virus (HBV) is a major health concern. The inability to mimic the complexity of the liver using cell lines and regular primary human hepatocyte (PHH) cultures pose significant limitations for studying host/pathogen interactions. Here, we describe a 3D microfluidic PHH system permissive to HBV infection, which can be maintained for at least 40 days. This system enables the recapitulation of all steps of the HBV life cycle, including the replication of patient-derived HBV and the maintenance of HBV cccDNA. We show that innate immune and cytokine responses following infection with HBV mimic those observed in HBV-infected patients, thus allowing the dissection of pathways important for immune evasion and validation of biomarkers. Additionally, we demonstrate that the co-culture of PHH with other non-parenchymal cells enables the identification of the cellular origin of immune effectors, thus providing a valuable preclinical platform for HBV research.