Engineered materials for organoid systems

Cadherin-dependent adhesion modulated 3D cell-assembly

The emergence of synthetic biology has opened new avenues in constructing cell-assembly biosystems with specific gene expression and function. The phenomena of cell spreading and detachment during tissue development and cancer metastasis are caused by surface tension, which in turn results from differences in cell-cell adhesion mediated by the dimerization of cadherin expressed on the cell surface. In this study, E- and P-cadherin plasmids were first constructed based on the differential adhesion hypothesis, then they were electroporated into K562 cells and HEK293T cells, respectively, to explore the process of cell migration and assembly regulated by cadherins. Using this approach, some special 3D cell functional components with a phase separation structure were fabricated successfully. Our work will be of potential application in the construction of self-assembling synthetic tissues and organoids.

Encapsulation of Cells in a Collagen Matrix Surrounded by an Alginate Hydrogel Shell for 3D Cell Culture

Numerous techniques for mammalian cell culture have been developed to mimic the complex in vivo three-dimensional structure of tissues and organs. Among them, the sole use of proteins to create a matrix where cells are embedded already gives rise to self-organized multicellular assemblies. Loading cells in a controlled extracellular matrix along with cell culture and monitoring through a strategy that is compatible with pipetting tools would be beneficial for high throughput screening applications or simply for a standardized method. Here, we design submillimeter compartments having a thin alginate hydrogel shell and a core made of a collagen matrix where cells are embedded. The process, using a microfluidic device, is based on a high speed co-extrusion in air, leading to a compound jet whose fragmentation is controlled. The resulting core-shell liquid drops are then collected in a gelling bath that triggers a fast hardening of the shell and is followed by a slower self-assembly of collagen molecules into fibers. We show how to formulate the core solution in order to maintain cell viability at physiological conditions that otherwise induce tropocollagen molecules to self-assemble, while being able to prevent flow disturbances that are detrimental for this jetting method. Encapsulated Caco-2 cells, mainly used to model the intestinal barrier, proliferate and form a closed polarized epithelial cell monolayer where the apical membrane faces the continuous medium.

Automated high-speed 3D imaging of organoid cultures with multi-scale phenotypic quantification

Current imaging approaches limit the ability to perform multi-scale characterization of three-dimensional (3D) organotypic cultures (organoids) in large numbers. Here, we present an automated multi-scale 3D imaging platform synergizing high-density organoid cultures with rapid and live 3D single-objective light-sheet imaging. It is composed of disposable microfabricated organoid culture chips, termed JeWells, with embedded optical components and a laser beam-steering unit coupled to a commercial inverted microscope. It permits streamlining organoid culture and high-content 3D imaging on a single user-friendly instrument with minimal manipulations and a throughput of 300 organoids per hour. We demonstrate that the large number of 3D stacks that can be collected via our platform allows training deep learning-based algorithms to quantify morphogenetic organizations of organoids at multi-scales, ranging from the subcellular scale to the whole organoid level. We validated the versatility and robustness of our approach on intestine, hepatic, neuroectoderm organoids and oncospheres.

Technologies to Assess Drug Response and Heterogeneity in Patient-Derived Cancer Organoids

Patient-derived cancer organoids (PDCOs) are organotypic 3D cultures grown from patient tumor samples. PDCOs provide an exciting opportunity to study drug response and heterogeneity within and between patients. This research can guide new drug development and inform clinical treatment planning. We review technologies to assess PDCO drug response and heterogeneity, discuss best practices for clinically relevant drug screens, and assert the importance of quantifying single-cell and organoid heterogeneity to characterize response. Autofluorescence imaging of PDCO growth and metabolic activity is highlighted as a compelling method to monitor single-cell and single-organoid response robustly and reproducibly. We also speculate on the future of PDCOs in clinical practice and drug discovery.Future development will require standardization of assessment methods for both morphology and function in PDCOs, increased throughput for new drug development, prospective validation with patient outcomes, and robust classification algorithms.

Engineering multiscale structural orders for high-fidelity embryoids and organoids

Embryoids and organoids hold great promise for human biology and medicine. Herein, we discuss conceptual and technological frameworks useful for developing high-fidelity embryoids and organoids that display tissue- and organ-level phenotypes and functions, which are critically needed for decoding developmental programs and improving translational applications. Through dissecting the layers of inputs controlling mammalian embryogenesis, we review recent progress in reconstructing multiscale structural orders in embryoids and organoids. Bioengineering tools useful for multiscale, multimodal structural engineering of tissue- and organ-level cellular organization and microenvironment are also discussed to present integrative, bioengineering-directed approaches to achieve next-generation, high-fidelity embryoids and organoids.

Spatial confinement toward creating artificial living systems

Lifeforms are regulated by many physicochemical factors, and these factors could be controlled to play a role in the construction of artificial living systems. Among these factors, spatial confinement is an important one, which mediates biological behaviors at multiscale levels and participates in the biomanufacturing processes accordingly. This review describes how spatial confinement, as a fundamental biological phenomenon, provides cues for the construction of artificial living systems. Current knowledge about the role of spatial confinement in mediating individual cell behavior, collective cellular behavior, and tissue-level behavior are categorized. Endeavors on the synthesis of biomacromolecules, artificial cells, engineered tissues, and organoids in spatially confined bioreactors are then emphasized. After that, we discuss the cutting-edge applications of spatially confined artificial living systems in biomedical fields. Finally, we conclude by assessing the remaining challenges and future trends in the context of fundamental science, technical improvement, and practical applications.

Three-dimensional models of the lung: past, present and future: a mini review

Respiratory diseases are a major reason for death in both men and women worldwide. The development of therapies for these diseases has been slow and the lack of relevant human models to understand lung biology inhibits therapeutic discovery. The lungs are structurally and functionally complex with many different cell types which makes designing relevant lung models particularly challenging. The traditional two-dimensional (2D) cell line cultures are, therefore, not a very accurate representation of the in vivo lung tissue. The recent development of three-dimensional (3D) co-culture systems, popularly known as organoids/spheroids, aims to bridge the gap between 'in-dish' and 'in-tissue' cell behavior. These 3D cultures are modeling systems that are widely divergent in terms of culturing techniques (bottom-up/top-down) that can be developed from stem cells (adult/embryonic/pluripotent stem cells), primary cells or from two or more types of cells, to build a co-culture system. Lung 3D models have diverse applications including the understanding of lung development, lung regeneration, disease modeling, compound screening, and personalized medicine. In this review, we discuss the different techniques currently being used to generate 3D models and their associated cellular and biological materials. We further detail the potential applications of lung 3D cultures for disease modeling and advances in throughput for drug screening.

In Situ Super-Resolution Imaging of Organoids and Extracellular Matrix Interactions via Phototransfer by Allyl Sulfide Exchange-Expansion Microscopy (PhASE-ExM)

3D organoid models have recently seen a boom in popularity, as they can better recapitulate the complexity of multicellular organs compared to other in vitro culture systems. However, organoids are difficult to image because of the limited penetration depth of high-resolution microscopes and depth-dependent light attenuation, which can limit the understanding of signal transduction pathways and characterization of intimate cell-extracellular matrix (ECM) interactions. To overcome these challenges, phototransfer by allyl sulfide exchange-expansion microscopy (PhASE-ExM) is developed, enabling optical clearance and super-resolution imaging of organoids and their ECM in 3D. PhASE-ExM uses hydrogels prepared via photoinitiated polymerization, which is advantageous as it decouples monomer diffusion into thick organoid cultures from the hydrogel fabrication. Apart from compatibility with organoids cultured in Matrigel, PhASE-ExM enables 3.25× expansion and super-resolution imaging of organoids cultured in synthetic poly(ethylene glycol) (PEG) hydrogels crosslinked via allyl-sulfide groups (PEG-AlS) through simultaneous photopolymerization and radical-mediated chain-transfer reactions that complete in <70 s. Further, PEG-AlS hydrogels can be in situ softened to promote organoid crypt formation, providing a super-resolution imaging platform both for pre- and post-differentiated organoids. Overall, PhASE-ExM is a useful tool to decipher organoid behavior by enabling sub-micrometer scale, 3D visualization of proteins and signal transduction pathways.

3D-bioprinted cancer-on-a-chip: level-up organotypic in vitro models

Combinatorial conjugation of organ-on-a-chip platforms with additive manufacturing technologies is rapidly emerging as a disruptive approach for upgrading cancer-on-a-chip systems towards anatomic-sized dynamic in vitro models. This valuable technological synergy has potential for giving rise to truly physiomimetic 3D models that better emulate tumor microenvironment elements, bioarchitecture, and response to multidimensional flow dynamics. Herein, we showcase the most recent advances in bioengineering 3D-bioprinted cancer-on-a-chip platforms and provide a comprehensive discussion on design guidelines and possibilities for high-throughput analysis. Such hybrid platforms represent a new generation of highly sophisticated 3D tumor models with improved biomimicry and predictability of therapeutics performance.

Recent advance of <i>in vitro</i> models in natural phytochemicals absorption and metabolism

Natural phytochemicals absorption and metabolic process are mainly in the human gut. Simulating the absorption and metabolism of natural phytochemicals in vitro to predict the rate and degree of absorption of natural phytochemicals provides convenience for many researchers. However, in this process, many physiological factors <i>in vitro</i> are affected, such as stomach and intestinal juice composition, pH, intestinal transmission rate and so on. In recent years, the research methods have gradually improved to make these models more suitable for the natural phytochemicals absorption process, <i>in vitro</i> simulation models have become an essential means to study natural phytochemicals absorption. Therefore, this paper introduces the advantages and disadvantages of commonly used <i>in vitro</i> simulation models of natural phytochemicals absorption and metabolism, as well as briefly introduces the working principle of each model. To provide a theoretical basis for simulating natural phytochemicals absorption <i>in vitro</i> and development and utilization of natural phytochemicals.

Supramolecular Biomaterials in the Netherlands

Synthetically designed biomaterials strive to recapitulate and mimic the complex environment of natural systems. Using natural materials as a guide, the ability to create high performance biomaterials that control cell fate, and support the next generation of cell and tissue-based therapeutics, is starting to emerge. Supramolecular chemistry takes inspiration from the wealth of non-covalent interactions found in natural materials that are inherently complex, and using the skills of synthetic and polymer chemistry, recreates simple systems to imitate their features. Within the past decade, supramolecular biomaterials have shown utility in tissue engineering and the progress predicts a bright future. On this 30th anniversary of the Netherlands Biomaterials and Tissue Engineering society, we will briefly recount the state of supramolecular biomaterials in the Dutch academic and industrial research and development context. This review will provide the background, recent advances, industrial successes and challenges, as well as future directions of the field, as we see it. Throughout this work, we notice the intricate interplay between simplicity and complexity in creating more advanced solutions. We hope that the interplay and juxtaposition between these two forces can propel the field forward.

Retina organoids: Window into the biophysics of neuronal systems

With a kind of magnetism, the human retina draws the eye of neuroscientist and physicist alike. It is attractive as a self-organizing system, which forms as a part of the central nervous system via biochemical and mechanical cues. The retina is also intriguing as an electro-optical device, converting photons into voltages to perform on-the-fly filtering before the signals are sent to our brain. Here, we consider how the advent of stem cell derived in vitro analogs of the retina, termed retina organoids, opens up an exploration of the interplay between optics, electrics, and mechanics in a complex neuronal network, all in a Petri dish. This review presents state-of-the-art retina organoid protocols by emphasizing links to the biochemical and mechanical signals of in vivo retinogenesis. Electrophysiological recording of active signal processing becomes possible as retina organoids generate light sensitive and synaptically connected photoreceptors. Experimental biophysical tools provide data to steer the development of mathematical models operating at different levels of coarse-graining. In concert, they provide a means to study how mechanical factors guide retina self-assembly. In turn, this understanding informs the engineering of mechanical signals required to tailor the growth of neuronal network morphology. Tackling the complex developmental and computational processes in the retina requires an interdisciplinary endeavor combining experiment and theory, physics, and biology. The reward is enticing: in the next few years, retina organoids could offer a glimpse inside the machinery of simultaneous cellular self-assembly and signal processing, all in an in vitro setting.

Engineering Hydrogels for the Development of Three-Dimensional In Vitro Models

The superiority of in vitro 3D cultures over conventional 2D cell cultures is well recognized by the scientific community for its relevance in mimicking the native tissue architecture and functionality. The recent paradigm shift in the field of tissue engineering toward the development of 3D in vitro models can be realized with its myriad of applications, including drug screening, developing alternative diagnostics, and regenerative medicine. Hydrogels are considered the most suitable biomaterial for developing an in vitro model owing to their similarity in features to the extracellular microenvironment of native tissue. In this review article, recent progress in the use of hydrogel-based biomaterial for the development of 3D in vitro biomimetic tissue models is highlighted. Discussions of hydrogel sources and the latest hybrid system with different combinations of biopolymers are also presented. The hydrogel crosslinking mechanism and design consideration are summarized, followed by different types of available hydrogel module systems along with recent microfabrication technologies. We also present the latest developments in engineering hydrogel-based 3D in vitro models targeting specific tissues. Finally, we discuss the challenges surrounding current in vitro platforms and 3D models in the light of future perspectives for an improved biomimetic in vitro organ system.

Next-generation cancer organoids

Organotypic models of patient-specific tumours are revolutionizing our understanding of cancer heterogeneity and its implications for personalized medicine. These advancements are, in part, attributed to the ability of organoid models to stably preserve genetic, proteomic, morphological and pharmacotypic features of the parent tumour in vitro, while also offering unprecedented genomic and environmental manipulation. Despite recent innovations in organoid protocols, current techniques for cancer organoid culture are inherently uncontrolled and irreproducible, owing to several non-standardized facets including cancer tissue sources and subsequent processing, medium formulations, and animal-derived three-dimensional matrices. Given the potential for cancer organoids to accurately recapitulate the intra- and intertumoral biological heterogeneity associated with patient-specific cancers, eliminating the undesirable technical variability accompanying cancer organoid culture is necessary to establish reproducible platforms that accelerate translatable insights into patient care. Here we describe the current challenges and recent multidisciplinary advancements and opportunities for standardizing next-generation cancer organoid systems.

Advances in vascularization and innervation of constructs for neural tissue engineering

A growing number of technologies are being developed to promote vascularization and innervation in engineered tissues. Organ-on-a-chip, organoid and 3D printing technologies, as well as pre-vascularized and oriented scaffolds, have been employed for vascularization and innervation of engineered tissues both in vivo and in vitro. Both vascularization and innervation are critical for neural tissue engineering, as these complex tissues require provision of both blood and nerves. As such, this review will have particular focus on neural tissue engineering. We examine state-of-the-art approaches for tissue vascularization and innervation and identify promising methods for developing vascularized and innervated engineered neural constructs.

Hydrogel Mechanics Influence the Growth and Development of Embedded Brain Organoids

Brain organoids are three-dimensional, tissue-engineered neural models derived from induced pluripotent stem cells that enable studies of neurodevelopmental and disease processes. Mechanical properties of the microenvironment are known to be critical parameters in tissue engineering, but the mechanical consequences of the encapsulating matrix on brain organoid growth and development remain undefined. Here, Matrigel was modified with an interpenetrating network (IPN) of alginate, to tune the mechanical properties of the encapsulating matrix. Brain organoids grown in IPNs were viable, with the characteristic formation of neuroepithelial buds. However, organoid growth was significantly restricted in the stiffest matrix tested. Moreover, stiffer matrixes skewed cell populations toward mature neuronal phenotypes, with fewer and smaller neural rosettes. These findings demonstrate that the mechanics of the culture environment are important parameters in brain organoid development and show that the self-organizing capacity and subsequent architecture of brain organoids can be modulated by forces arising from growth-induced compression of the surrounding matrix. This study therefore suggests that carefully designing the mechanical properties of organoid encapsulation materials is a potential strategy to direct organoid growth and maturation toward desired structures.

A microenvironment-inspired synthetic three-dimensional model for pancreatic ductal adenocarcinoma organoids

Experimental in vitro models that capture pathophysiological characteristics of human tumours are essential for basic and translational cancer biology. Here, we describe a fully synthetic hydrogel extracellular matrix designed to elicit key phenotypic traits of the pancreatic environment in culture. To enable the growth of normal and cancerous pancreatic organoids from genetically engineered murine models and human patients, essential adhesive cues were empirically defined and replicated in the hydrogel scaffold, revealing a functional role of laminin-integrin α3/α6 signalling in establishment and survival of pancreatic organoids. Altered tissue stiffness-a hallmark of pancreatic cancer-was recapitulated in culture by adjusting the hydrogel properties to engage mechano-sensing pathways and alter organoid growth. Pancreatic stromal cells were readily incorporated into the hydrogels and replicated phenotypic traits characteristic of the tumour environment in vivo. This model therefore recapitulates a pathologically remodelled tumour microenvironment for studies of normal and pancreatic cancer cells in vitro.

Machine Learning-Driven Biomaterials Evolution

Biomaterials is an exciting and dynamic field, which uses a collection of diverse materials to achieve desired biological responses. While there is constant evolution and innovation in materials with time, biomaterials research has been hampered by the relatively long development period required. In recent years, driven by the need to accelerate materials development, the applications of machine learning in materials science has progressed in leaps and bounds. The combination of machine learning with high-throughput theoretical predictions and high-throughput experiments (HTE) has shifted the traditional Edisonian (trial and error) paradigm to a data-driven paradigm. In this review, each type of biomaterial and their key properties and use cases are systematically discussed, followed by how machine learning can be applied in the development and design process. The discussions are classified according to various types of materials used including polymers, metals, ceramics, and nanomaterials, and implants using additive manufacturing. Last, the current gaps and potential of machine learning to further aid biomaterials discovery and application are also discussed.

Recent advances of 3D hydrogel culture systems for mesenchymal stem cell-based therapy and cell behavior regulation

Mesenchymal stem cells (MSCs) have been increasingly recognized as resources for disease treatments and regenerative medicine. Meanwhile, the unique chemical and physical properties of hydrogels provide innate advantages to achieve...

Construction of 3D hierarchical tissue platforms for modeling diabetes

Diabetes mellitus (DM) is one of the most serious systemic diseases worldwide, and the majority of DM patients face severe complications. However, many of underlying disease mechanisms related to these complications are difficult to understand with the use of currently available animal models. With the urgent need to fundamentally understand DM pathology, a variety of 3D biomimetic platforms have been generated by the convergence of biofabrication and tissue engineering strategies for the potent drug screening platform of pre-clinical research. Here, we suggest key requirements for the fabrication of physiomimetic tissue models in terms of recapitulating the cellular organization, creating native 3D microenvironmental niches for targeted tissue using biomaterials, and applying biofabrication technologies to implement tissue-specific geometries. We also provide an overview of various in vitro DM models, from a cellular level to complex living systems, which have been developed using various bioengineering approaches. Moreover, we aim to discuss the roadblocks facing in vitro tissue models and end with an outlook for future DM research.

In Vitro Recapitulation of Neuropsychiatric Disorders with Pluripotent Stem Cells-Derived Brain Organoids

Adolescent neuropsychiatric disorders have been recently increasing due to genetic and environmental influences. Abnormal brain development before and after birth contribute to the pathology of neuropsychiatric disorders. However, it is difficult to experimentally investigate because of the complexity of brain and ethical constraints. Recently generated human brain organoids from pluripotent stem cells are considered as a promising in vitro model to recapitulate brain development and diseases. To better understand how brain organoids could be applied to investigate neuropsychiatric disorders, we analyzed the key consideration points, including how to generate brain organoids from pluripotent stem cells, the current application of brain organoids in recapitulating neuropsychiatric disorders and the future perspectives. This review covered what have been achieved on modeling the cellular and neural circuit deficits of neuropsychiatric disorders and those challenges yet to be solved. Together, this review aims to provide a fundamental understanding of how to generate brain organoids to model neuropsychiatric disorders, which will be helpful in improving the mental health of adolescents.

Cortical spheroids display oscillatory network dynamics

Three-dimensional brain cultures can facilitate the study of central nervous system function and disease, and one of the most important components that they present is neuronal activity on a network level. Here we demonstrate network activity in rodent cortical spheroids while maintaining the networks intact in their 3D state. Networks developed by nine days in culture and became more complex over time. To measure network activity, we imaged neurons in rat and mouse spheroids labelled with a calcium indicator dye, and in mouse spheroids expressing GCaMP. Network activity was evident when we electrically stimulated spheroids, was abolished with glutamatergic blockade, and was altered by GABAergic blockade or partial glutamatergic blockade. We quantified correlations and distances between somas with micron-scale spatial resolution. Spheroids seeded at as few as 4000 cells gave rise to emergent network events, including oscillations. These results are the first demonstration that self-assembled rat and mouse spheroids exhibit network activity consistent with in vivo network events. These results open the door to experiments on neuronal networks that require fewer animals and enable high throughput experiments on network-perturbing alterations in neurons and glia.

Chemically defined stem cell microniche engineering by microfluidics compatible with iPSCs' growth in 3D culture

The development of induced pluripotent stem cell (iPSCs) has opened unprecedented opportunities for biomedical applications, but poorly defined animal-derived matrices yield cells with limited therapeutic value. Considerable challenges remain in improving cell-culturing approaches to create the conditions for iPSCs' reliable expansion. Herein we report the development of a chemically defined, artificial three-dimensional (3D) microniche for iPSCs' growth and reliable expansion, constructed with degradable polyethyleneglycol-co-polycaprolactone and RGDfk-functionalized dendritic polyglycerol precursors according to bioorthogonal strain-promoted azide-alkyne cycloaddition by droplet-based microfluidics. This compatible microniche can allow for the robust production of iPSCs that maintain high pluripotency expression and excellent viability without pathogen or immunogen transfer risks. This microniche technology shows great promise in enabling iPSCs to achieve their full therapeutic potential.

Stem Cell-Based Embryo Models: En Route to a Programmable Future

Early-stage human embryogenesis, such as implantation, gastrulation, and neurulation, are critical for successful pregnancy. For decades, our knowledge about these stages has been limited by the inaccessibility to such embryo specimens in vivo and the difficulty in rebuilding them in vitro. Although human embryos could be cultured in vitro beyond implantation, it remains challenging for the cultured embryos to recapitulate the continuous, coordinated morphogenesis and cytodifferentiation as seen in vivo. Stem cell-based embryo models, mainly derived from human pluripotent stem cells, are organized structures mimicking essential developmental processes in the early-stage human embryos. Despite their invaluable potentials, most embryo models are based on the self-organization of human pluripotent stem cells, which are limited in controllability, reproducibility, and developmental fidelity. Recently, the integration of bioengineered tools and stem cell biology has fueled a technological transformation towards programmable, highly complex, high-fidelity stem cell-based embryo models. Given its scientific and clinical significance, we present an overview of recent paradigm-shifting advances as well as historical perspectives regarding the past, present, and future of synthetic human embryology. Following the developmental roadmap of human embryogenesis, we critically review existing stem cell-based models for implantation, gastrulation, and neurulation, respectively. We highlight the limitations encountered by autonomous self-organization strategy and discuss the concept and application of guided cell organization as a game-changer for innovating next-generation embryo models. Future endeavors in synthetic human embryology should rationally leverage both the self-organizing power and programmable microenvironmental guidance to secure faithful reconstructions of the hierarchical orders of human embryogenesis in vitro.

Bridging the explanatory gaps: What can we learn from a biological agency perspective?

We begin this article by delineating the explanatory gaps left by prevailing gene-focused approaches in our understanding of phenotype determination, inheritance, and the origin of novel traits. We aim not to diminish the value of these approaches but to highlight where their implementation, despite best efforts, has encountered persistent limitations. We then discuss how each of these explanatory gaps can be addressed by expanding research foci to take into account biological agency-the capacity of living systems at various levels to participate in their own development, maintenance, and function by regulating their structures and activities in response to conditions they encounter. Here we aim to define formally what agency and agents are and-just as importantly-what they are not, emphasizing that agency is an empirical property connoting neither intention nor consciousness. Lastly, we discuss how incorporating agency helps to bridge explanatory gaps left by conventional approaches, highlight scientific fields in which implicit agency approaches are already proving valuable, and assess the opportunities and challenges of more systematically incorporating biological agency into research programs.

Bioengineering Approaches for the Advanced Organoid Research

Recent advances in 3D cell culture technology have enabled scientists to generate stem cell derived organoids that recapitulate the structural and functional characteristics of native organs. Current organoid technologies have been striding toward identifying the essential factors for controlling the processes involved in organoid development, including physical cues and biochemical signaling. There is a growing demand for engineering dynamic niches characterized by conditions that resemble in vivo organogenesis to generate reproducible and reliable organoids for various applications. Innovative biomaterial-based and advanced engineering-based approaches have been incorporated into conventional organoid culture methods to facilitate the development of organoid research. The recent advances in organoid engineering, including extracellular matrices and genetic modulation, are comprehensively summarized to pinpoint the parameters critical for organ-specific patterning. Moreover, perspective trends in developing tunable organoids in response to exogenous and endogenous cues are discussed for next-generation developmental studies, disease modeling, and therapeutics.

Advance in Human Epithelial-Derived Organoids Research

Organoids have complex three-dimensional structures that exhibit functionalities and feature architectures similar to those of in vivo organs and are developed from adult stem cells, embryonic stem cells, and pluripotent stem cells through a self-organization process. Organoids derived from adult epithelial stem cells are the most mature and extensive. In recent years, using organoid culture techniques, researchers have established various adult human tissue-derived epithelial organoids, including intestinal, colon, lung, liver, stomach, breast, and oral mucosal organoids, all of which exhibit strong research and application prospects. Studies have shown that epithelial organoids are mainly applied in drug discovery, personalized drug response testing, disease mechanism research, and regenerative medicine. In this review, we mainly discuss current organoid culture systems and potential applications of this technique with human epithelial tissue.

A review on biomaterials for ovarian tissue engineering

Considerable challenges in engineering the female reproductive tissue are the follicle's unique architecture, the need to recapitulate the extracellular matrix, and tissue vascularization. Over the years, various strategies have been developed for preserving fertility in women diagnosed with cancer, such as embryo, oocyte, or ovarian tissue cryopreservation. While autotransplantation of cryopreserved ovarian tissue is a viable choice to restore fertility in prepubertal girls and women who need to begin chemo- or radiotherapy soon after the cancer diagnosis, it is not suitable for all patients due to the risk of having malignant cells present in the ovarian fragments in some types of cancer. Advances in tissue engineering such as 3D printing and ovary-on-a-chip technologies have the potential to be a translational strategy for precisely recapitulating normal tissue in terms of physical structure, vascularization, and molecular and cellular spatial distribution. This review first introduces the ovarian tissue structure, describes suitable properties of biomaterials for ovarian tissue engineering, and highlights recent advances in tissue engineering for developing an artificial ovary. STATEMENT OF SIGNIFICANCE: The increase of survival rates in young cancer patients has been accompanied by a rise in infertility/sterility in cancer survivors caused by the gonadotoxic effect of some chemotherapy regimens or radiotherapy. Such side-effect has a negative impact on these patients' quality of life as one of their main concerns is generating biologically related children. To aid female cancer patients, several research groups have been resorting to tissue engineering strategies to develop an artificial ovary. In this review, we discuss the numerous biomaterials cited in the literature that have been tested to encapsulate and in vitro culture or transplant isolated preantral follicles from human and different animal models. We also summarize the recent advances in tissue engineering that can potentially be optimal strategies for developing an artificial ovary.

Plant cell adhesion and growth on artificial fibrous scaffolds as an in vitro model for plant development

Mechanistic studies of plant development would benefit from an in vitro model that mimics the endogenous physical interactions between cells and their microenvironment. Here, we present artificial scaffolds to which both solid- and liquid-cultured tobacco BY-2 cells adhere without perturbing cell morphology, division, and cortical microtubule organization. Scaffolds consisting of polyvinylidene tri-fluoroethylene (PVDF-TrFE) were prepared to mimic the cell wall’s fibrillar structure and its relative hydrophobicity and piezoelectric property. We found that cells adhered best to scaffolds consisting of nanosized aligned fibers. In addition, poling of PVDF-TrFE, which orients the fiber dipoles and renders the scaffold more piezoelectric, increased cell adhesion. Enzymatic treatments revealed that the plant cell wall polysaccharide, pectin, is largely responsible for cell adhesion to scaffolds, analogous to pectin-mediated cell adhesion in plant tissues. Together, this work establishes the first plant biomimetic scaffolds that will enable studies of how cell-cell and cell-matrix interactions affect plant developmental pathways.

Efficient fabrication of monodisperse hepatocyte spheroids and encapsulation in hybrid hydrogel with controllable extracellular matrix effect

Three-dimensional (3D) culture techniques, such as spheroid and organoid cultures, have gained increasing interest in biomedical research. However, the understanding and control of extracellular matrix (ECM) effect in spheroid and organoid culture has been limited. Here, we report a biofabrication approach to efficiently form uniform-sized 3D hepatocyte spheroids and encapsulate them in a hybrid hydrogel composed of alginate and various ECM molecules. Cells were seeded in a microwell platform to form spheroid before being encapsulated directly in a hybrid hydrogel containing various ECM molecules, including collagen type I (COL1), collagen type IV (COL4), fibronectin (FN), and laminin (LM). A systematic analysis of the effect of ECM molecules on the primary mouse hepatocyte phenotype was then performed. Our results showed that hydrogel encapsulation of hepatocyte spheroid promoted hepatic marker expression and secretory functions. In addition, different ECM molecules elicited distinct effects on hepatic functions in 3D encapsulated hepatocyte spheroids, but not in 2D hepatocyte and 3D non-encapsulated spheroids. When encapsulated in hybrid hydrogel containing LM alone or COL1 alone, hepatocyte spheroids exhibited improved hepatic functions overall. Analysis of gene and protein expression showed an upregulation of integrinα1 and integrinα6 when LM was introduced in the hybrid hydrogel, suggesting a possible role of integrin signaling involved in the ECM effect. Finally, a combinatorial screening was performed to demonstrate the potential to screen a multitude of 3D microenvironments of varying ECM combinations that exhibited synergistic influence, indicating a strong positive effect of COL1 and a negative interaction effect of COL1·LM on both albumin and urea secretion. These findings illustrate the broad application potential of this biofabrication approach in identifying optimal ECM composition(s) for engineering 3D tissue, and elucidating defined ECM cues for tissue engineering and regenerative medicine.

Viscoelasticity and Adhesion Signaling in Biomaterials Control Human Pluripotent Stem Cell Morphogenesis in 3D Culture

Organoids are lumen-containing multicellular structures that recapitulate key features of the organs, and are increasingly used in models of disease, drug testing, and regenerative medicine. Recent work has used 3D culture models to form organoids from human induced pluripotent stem cells (hiPSCs) in reconstituted basement membrane (rBM) matrices. However, rBM matrices offer little control over the microenvironment. More generally, the role of matrix viscoelasticity in directing lumen formation remains unknown. Here, viscoelastic alginate hydrogels with independently tunable stress relaxation (viscoelasticity), stiffness, and arginine-glycine-aspartate (RGD) ligand density are used to study hiPSC morphogenesis in 3D culture. A phase diagram that shows how these properties control hiPSC morphogenesis is reported. Higher RGD density and fast stress relaxation promote hiPSC viability, proliferation, apicobasal polarization, and lumen formation, while slow stress relaxation at low RGD densities triggers hiPSC apoptosis. Notably, hiPSCs maintain pluripotency in alginate hydrogels for much longer times than is reported in rBM matrices. Lumen formation is regulated by actomyosin contractility and is accompanied by translocation of Yes-associated protein (YAP) from the nucleus to the cytoplasm. The results reveal matrix viscoelasticity as a potent factor regulating stem cell morphogenesis and provide new insights into how engineered biomaterials may be leveraged to build organoids.

Advancing models of neural development with biomaterials

Human pluripotent stem cells have emerged as a promising in vitro model system for studying the brain. Two-dimensional and three-dimensional cell culture paradigms have provided valuable insights into the pathogenesis of neuropsychiatric disorders, but they remain limited in their capacity to model certain features of human neural development. Specifically, current models do not efficiently incorporate extracellular matrix-derived biochemical and biophysical cues, facilitate multicellular spatio-temporal patterning, or achieve advanced functional maturation. Engineered biomaterials have the capacity to create increasingly biomimetic neural microenvironments, yet further refinement is needed before these approaches are widely implemented. This Review therefore highlights how continued progression and increased integration of engineered biomaterials may be well poised to address intractable challenges in recapitulating human neural development.

Exosomes in the Tumor Microenvironment: From Biology to Clinical Applications

Cancer is one of the most important health problems and the second leading cause of death worldwide. Despite the advances in oncology, cancer heterogeneity remains challenging to therapeutics. This is because the exosome-mediated crosstalk between cancer and non-cancer cells within the tumor microenvironment (TME) contributes to the acquisition of all hallmarks of cancer and leads to the formation of cancer stem cells (CSCs), which exhibit resistance to a range of anticancer drugs. Thus, this review aims to summarize the role of TME-derived exosomes in cancer biology and explore the clinical potential of mesenchymal stem-cell-derived exosomes as a cancer treatment, discussing future prospects of cell-free therapy for cancer treatment and challenges to be overcome.

Organoid Technology: A Reliable Developmental Biology Tool for Organ-Specific Nanotoxicity Evaluation

Engineered nanomaterials are bestowed with certain inherent physicochemical properties unlike their parent materials, rendering them suitable for the multifaceted needs of state-of-the-art biomedical, and pharmaceutical applications. The log-phase development of nano-science along with improved "bench to beside" conversion carries an enhanced probability of human exposure with numerous nanoparticles. Thus, toxicity assessment of these novel nanoscale materials holds a key to ensuring the safety aspects or else the global biome will certainly face a debacle. The toxicity may span from health hazards due to direct exposure to indirect means through food chain contamination or environmental pollution, even causing genotoxicity. Multiple ways of nanotoxicity evaluation include several in vitro and in vivo methods, with in vitro methods occupying the bulk of the "experimental space." The underlying reason may be multiple, but ethical constraints in in vivo animal experiments are a significant one. Two-dimensional (2D) monoculture is undoubtedly the most exploited in vitro method providing advantages in terms of cost-effectiveness, high throughput, and reproducibility. However, it often fails to mimic a tissue or organ which possesses a defined three-dimensional structure (3D) along with intercellular communication machinery. Instead, microtissues such as spheroids or organoids having a precise 3D architecture and proximate in vivo tissue-like behavior can provide a more realistic evaluation than 2D monocultures. Recent developments in microfluidics and bioreactor-based organoid synthesis have eased the difficulties to prosper nano-toxicological analysis in organoid models surpassing the obstacle of ethical issues. The present review will enlighten applications of organoids in nanotoxicological evaluation, their advantages, and prospects toward securing commonplace nano-interventions.

Biomaterials for intestinal organoid technology and personalized disease modeling

Recent advances in intestinal organoid technologies have paved the way for in vitro recapitulation of the homeostatic renewal of adult tissues, tissue or organ morphogenesis during development, and pathogenesis of many disorders. In vitro modelling of individual patient diseases using organoid systems have been considered key in establishing rational design of personalized treatment strategies and in improving therapeutic outcomes. In addition, the transplantation of organoids into diseased tissues represents a novel approach to treat currently incurable diseases. Emerging evidence from intensive studies suggests that organoid systems' development and functional maturation depends on the presence of an extracellular matrix with suitable biophysical properties, where advanced synthetic hydrogels open new avenues for theoretical control of organoid phenotypes and potential applications of organoids in therapeutic purposes. In this review, we discuss the status, applications, challenges and perspectives of intestinal organoid systems emphasising on hydrogels and their properties suitable for intestinal organoid culture. We provide an overview of hydrogels used for intestinal organoid culture and key factors regulating their biological activity. The comparison of different hydrogels would be a theoretical basis for establishing design principles of synthetic niches directing intestinal cell fates and functions. STATEMENT OF SIGNIFICANCE: Intestinal organoid is an in vitro recapitulation of the gut, which self-organizes from intestinal stem cells and maintains many features of the native tissue. Since the development of this technology, intestinal organoid systems have made significant contribution to rapid progress in intestinal biology. Prevailing methodology for organoid culture, however, depends on animal-derived matrices and suffers from variability and potential risk for contamination of pathogens, limiting their therapeutic application. Synthetic scaffold matrices, hydrogels, might provide solutions to these issues and deepen our understanding on how intestinal cells sense and respond to key biophysical properties of the surrounding matrices. This review provides an overview of developing intestinal models and biomaterials, thereby leading to better understanding of current intestinal organoid systems for both biologists and materials scientists.

Organoid engineering with microfluidics and biomaterials for liver, lung disease, and cancer modeling

As life expectancy improves and the number of people suffering from various diseases increases, the need for developing effective personalized disease models is rapidly rising. The development of organoid technology has led to better recapitulation of the in vivo environment of organs, and can overcome the constraints of existing disease models. However, for more precise disease modeling, engineering approaches such as microfluidics and biomaterials, that aid in mimicking human physiology, need to be integrated with the organoid models. In this review, we introduce key elements for disease modeling and recent engineering advances using both liver and lung organoids. Due to the importance of personalized medicine, we also emphasize patient-derived cancer organoid models and their engineering approaches. These organoid-based disease models combined with microfluidics, biomaterials, and co-culture systems will provide a powerful research platform for understanding disease mechanisms and developing precision medicine; enabling preclinical drug screening and drug development. STATEMENT OF SIGNIFICANCE: The development of organoid technology has led to better recapitulation of the in vivo environment of organs, and can overcome the constraints of existing disease models. However, for more precise disease modeling, engineering approaches such as microfluidics and biomaterials, that aid in mimicking human physiology, need to be integrated with the organoid models. In this review, we introduce liver, lung, and cancer organoids integrated with various engineering approaches as a novel platform for personalized disease modeling. These engineered organoid-based disease models will provide a powerful research platform for understanding disease mechanisms and developing precision medicine.

Biomaterial-guided stem cell organoid engineering for modeling development and diseases

Organoids are miniature models of organs to recapitulate spatiotemporal cellular organization and tissue functionality. The production of organoids has revolutionized the field of developmental biology, providing the possibility to study and guide human development and diseases in a dish. More recently, novel biomaterial-based culture systems demonstrated the feasibility and versatility to engineer and produce the organoids in a consistent and reproducible manner. By engineering proper tissue microenvironment, functional organoids have been able to exhibit spatial-distinct tissue patterning and morphogenesis. This review focuses on enabling technologies in the field of organoid engineering, including the control of biochemical and biophysical cues via hydrogels, as well as size and geometry control via microwell and microfabrication techniques. In addition, this review discusses the enhancement of organoid systems for therapeutic applications using biofabrication and organoid-on-chip platforms, which facilitate the assembly of complex organoid systems for in vitro modeling of development and diseases. STATEMENT OF SIGNIFICANCE: Stem cell organoids have revolutionized the fields of developmental biology and tissue engineering, providing the opportunity to study human organ development and disease progression in vitro. Various works have demonstrated that organoids can be generated using a wide variety of engineering tools, materials, and systems. Specific culture microenvironment is tailored to support the formation, function, and physiology of the organ of interest. This review highlights the importance of cellular microenvironment in organoid culture, the versatility of organoid engineering techniques, and future perspectives to build better organoid systems.

Biomaterials by design: Harnessing data for future development

Biomaterials is an interdisciplinary field of research to achieve desired biological responses from new materials, regardless of material type. There have been many exciting innovations in this discipline, but commercialization suffers from a lengthy discovery to product pipeline, with many failures along the way. Success can be greatly accelerated by harnessing machine learning techniques to comb through large amounts of data. There are many potential benefits of moving from an unstructured empirical approach to a development strategy that is entrenched in data. Here, we discuss the recent work on the use of machine learning in the discovery and design of biomaterials, including new polymeric, metallic, ceramics, and nanomaterials, and how machine learning can interface with emerging use cases of 3D printing. We discuss the steps for closer integration of machine learning to make this exciting possibility a reality.

Engineering the Dynamics of Cell Adhesion Cues in Supramolecular Hydrogels for Facile Control over Cell Encapsulation and Behavior

The extracellular matrix (ECM) forms through hierarchical assembly of small and larger polymeric molecules into a transient, hydrogel-like fibrous network that provides mechanical support and biochemical cues to cells. Synthetic, fibrous supramolecular networks formed via non-covalent assembly of various molecules are therefore potential candidates as synthetic mimics of the natural ECM, provided that functionalization with biochemical cues is effective. Here, combinations of slow and fast exchanging molecules that self-assemble into supramolecular fibers are employed to form transient hydrogel networks with tunable dynamic behavior. Obtained results prove that modulating the ratio between these molecules dictates the extent of dynamic behavior of the hydrogels at both the molecular and the network level, which is proposed to enable effective incorporation of cell-adhesive functionalities in these materials. Excitingly, the dynamic nature of the supramolecular components in this system can be conveniently employed to formulate multicomponent supramolecular hydrogels for easy culturing and encapsulation of single cells, spheroids, and organoids. Importantly, these findings highlight the significance of molecular design and exchange dynamics for the application of supramolecular hydrogels as synthetic ECM mimics.

2D biointerfaces to study stem cell-ligand interactions

Stem cells have great potential in the field of tissue engineering and regenerative medicine due to their inherent regenerative capabilities. However, an ongoing challenge within their clinical translation is to elicit or predict the desired stem cell behavior once transplanted. Stem cell behavior and function are regulated by their interaction with biophysical and biochemical signals present in their natural environment (i.e., stem cell niches). To increase our understanding about the interplay between stem cells and their resident microenvironments, biointerfaces have been developed as tools to study how these substrates can affect stem cell behaviors. This article aims to review recent developments on fabricating cell-instructive interfaces to control cell adhesion processes towards directing stem cell behavior. After an introduction on stem cells and their natural environment, static surfaces exhibiting predefined biochemical signals to probe the effect of chemical features on stem cell behaviors are discussed. In the third section, we discuss more complex dynamic platforms able to display biochemical cues with spatiotemporal control using on-off ligand display, reversible ligand display, and ligand mobility. In the last part of the review, we provide the reader with an outlook on future designs of biointerfaces. STATEMENT OF SIGNIFICANCE: Stem cells have great potential as treatments for many degenerative disorders prevalent in our aging societies. However, an ongoing challenge within their clinical translation is to promote stem cell mediated regeneration once they are transplanted in the body. Stem cells reside within our bodies where their behavior and function are regulated by interactions with their natural environment called the stem cell niche. To increase our understanding about the interplay between stem cells and their niche, 2D materials have been developed as tools to study how specific signals can affect stem cell behaviors. This article aims to review recent developments on fabricating cell-instructive interfaces to control cell adhesion processes towards directing stem cell behavior.

Simultaneous 2D and 3D cell culture array for multicellular geometry, drug discovery and tumor microenvironment reconstruction

Cell culture systems are indispensablein vitrotools for biomedical research. Although conventional two-dimensional (2D) cell cultures are still used for most biomedical and biological studies, the three-dimensional (3D) cell culture technology attracts increasing attention from researchers, especially in cancer and stem cell research. Due to the different spatial structures, cells in 2D and 3D cultures exhibit different biochemical and biophysical phenotypes. Therefore, a new platform with both 2D and 3D cell cultures is needed to bridge the gap between 2D and 3D cell-based assays. Here, a simultaneous 2D and 3D cell culture array system was constructed by microprinting technology, in which cancer cells exhibited heterozygous geometry structures with both 2D monolayers and 3D spheroids. Cells grown in 3D spheroids showed higher proliferation ability and stronger cell-cell adhesion. Spheroids derived from various types of cancer cell lines exhibited distinct morphologies through a geometrical confinement stimulated biomechanical transduction. Z-projected images of cancer cell aggregates were used to analyze 3D multicellular architecture features. Notably, by using a support vector machine classifier, we distinguished tumor cells from normal cells with an accuracy greater than 95%, according to the geometrical features of multicellular spheroids in phase contrast microscopy images. Cancer cells in multicellular spheroid arrays exhibited higher drug resistance of anticancer drug cisplatin than cells grown in 2D cultures. Finally, we developed a co-culture system composed of tumor spheroid arrays, fibroblast cells and photo-crosslinkable gelatin methacryloyl hydrogel to mimic tumor microenvironment which consisted of solid tumor massed, surrounding stromal cells and extracellular matrix. Together, our newly developed simultaneous 2D and 3D cell culture array has great potential in comprehensive evaluation of cellular events in both 2D and 3D, rapid production of spheroid arrays and multicellular geometry-based tumor cell detection.

The anti-scarring role of Lycium barbarum polysaccharide on cornea epithelial-stromal injury

PURPOSE

Cornea epithelial-stromal scarring is related to the differentiation of fibroblasts into opaque myofibroblasts. Our study aims to assess the effectiveness of Lycium barbarum polysaccharide (LBP) solution as a pre-treatment in minimizing corneal scarring.

METHODS

Human corneal fibroblasts were cultured in a three-dimensional collagen type I-based hydrogel in an eye-on-a-chip model. Fibroblasts were pre-treated with 2 mg/mL LBP for 24 h, followed by another 24-h incubation with 10 ng/mL transforming growth factor-beta 1 (TGF-β1) to induce relevant physiological events after stromal injury. Intracellular pro-fibrotic proteins, extracellular matrix proteins, and pro-inflammatory cytokines that involved in fibrosis, were assessed using immunocytochemistry and enzyme-linked immunosorbent assays.

RESULTS

Compared to the positive control TGF-β1 group, LBP pre-treated cells had a significantly lower expression of alpha-smooth muscle actin, marker of myofibroblasts, vimentin (p < 0.05), and also extracellular matrix proteins both collagen type II and type III (p < 0.05) that can be found in scar tissues. Moreover, LBP pre-treated cells had a significantly lower secretion of pro-inflammatory cytokines interleukin-6 and interleukin-8 (p < 0.05). The cell-laden hydrogel contraction and stiffness showed no significant difference between LBP pre-treatment and control groups. Fibroblasts pretreated with LBP as well had reduced angiogenic factors expression and suppression of undesired proliferation (p < 0.05).

CONCLUSION

Our results showed that LBP reduced both pro-fibrotic proteins and pro-inflammatory cytokines on corneal injury in vitro. We suggest that LBP, as a natural Traditional Chinese Medicine, may potentially be a novel topical pre-treatment option prior to corneal refractive surgeries with an improved prognosis.

Magnetic Stiffening in 3D Cell Culture Matrices

The mechanical environment of a cell is not constant. This dynamic behavior is exceedingly difficult to capture in (synthetic) in vitro matrices. This paper describes a novel, highly adaptive hybrid hydrogel composed of magnetically sensitive magnetite nanorods and a stress-responsive synthetic matrix. Nanorod rearrangement after application of (small) magnetic fields induces strain in the network, which results in a strong (over 10-fold) stiffening even at minimal (2.5 wt %) nanorod concentrations. Moreover, the stiffening mechanism yields a fast and fully reversible response. In the manuscript, we quantitatively analyze that forces generated by the particles are comparable to cellular forces. We demonstrate the value of magnetic stiffening in a 3D MCF10A epithelial cell experiment, where simply culturing on top of a permanent magnet gives rise to changes in the cell morphology. This work shows that our hydrogels are uniquely suited as 3D cell culture systems with on-demand adaptive mechanical properties.

Deciphering Organoids: High-Dimensional Analysis of Biomimetic Cultures

Organoids are self-organising stem cell-derived ex vivo cultures widely adopted as biomimetic models of healthy and diseased tissues. Traditional low-dimensional experimental methods such as microscopy and bulk molecular analysis have generated remarkable biological insights from organoids. However, as complex heterocellular systems, organoids are especially well-positioned to take advantage of emerging high-dimensional technologies. In particular, single-cell methods offer considerable opportunities to analyse organoids at unprecedented scale and depth, enabling comprehensive characterisation of cellular processes and spatial organisation underpinning organoid heterogeneity. This review evaluates state-of-the-art analytical methods applied to organoids, discusses the latest advances in single-cell technologies, and speculates on the integration of these two rapidly developing fields.

Brain organoid: a 3D technology for investigating cellular composition and interactions in human neurological development and disease models in vitro

Abstract

The study of human brain physiology, including cellular interactions in normal and disease conditions, has been a challenge due to its complexity and unavailability. Induced pluripotent stem cell (iPSC) study is indispensable in the study of the pathophysiology of neurological disorders. Nevertheless, monolayer systems lack the cytoarchitecture necessary for cellular interactions and neurological disease modeling. Brain organoids generated from human pluripotent stem cells supply an ideal environment to model both cellular interactions and pathophysiology of the human brain. This review article discusses the composition and interactions among neural lineage and non-central nervous system cell types in brain organoids, current studies, and future perspectives in brain organoid research. Ultimately, the promise of brain organoids is to unveil previously inaccessible features of neurobiology that emerge from complex cellular interactions and to improve our mechanistic understanding of neural development and diseases.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02369-8.

The Translational Application of Hydrogel for Organoid Technology: Challenges and Future Perspectives

Human organoids mimic the physiology and tissue architecture of organs and are of great significance for promoting the study of human diseases. Traditionally, organoid cultures rely predominantly on animal or tumor-derived extracellular matrix (ECM), resulting in poor reproducibility. This limits their utility in for large-scale drug screening and application for regenerative medicine. Recently, synthetic polymeric hydrogels, with high biocompatibility and biodegradability, stability, uniformity of compositions, and high throughput properties, have emerged as potential materials for achieving 3D architectures for organoid cultures. Compared to conventional animal or tumor-derived organoids, these newly engineered hydrogel-based organoids more closely resemble human organs, as they are able to mimic native structural and functional properties observed in-situ. In this review, recent developments in hydrogel-based organoid culture will be summarized, emergent hydrogel technology will be highlighted, and future challenges in applying them to organoid culture will be discussed.

Extracellular matrix requirements for gastrointestinal organoid cultures

Organoids are a new class of biological model systems that have garnered significant interest in the life sciences. When provided with the proper 3D matrix and biochemical factors, stem cells can self-organize and form tissue-specific organoids. Thus far, there has been a substantial effort to identify soluble niche components essential for organoid culture; however, the role of the solid extracellular matrix (ECM) as an essential element of the niche is still largely lacking. In this review, we discuss the importance of the ECM in intestinal, hepatic, and pancreatic organoid culture and how biomaterial-based approaches can be used to probe different ECM properties required for more physiologically and translationally relevant organoid models.