Engineered materials for organoid systems

Advances in Hybrid Fabrication toward Hierarchical Tissue Constructs

The diversity of manufacturing processes used to fabricate 3D implants, scaffolds, and tissue constructs is continuously increasing. This growing number of different applicable fabrication technologies include electrospinning, melt electrowriting, volumetric-, extrusion-, and laser-based bioprinting, the Kenzan method, and magnetic and acoustic levitational bioassembly, to name a few. Each of these fabrication technologies feature specific advantages and limitations, so that a combination of different approaches opens new and otherwise unreachable opportunities for the fabrication of hierarchical cell-material constructs. Ongoing challenges such as vascularization, limited volume, and repeatability of tissue constructs at the resolution required to mimic natural tissue is most likely greater than what one manufacturing technology can overcome. Therefore, the combination of at least two different manufacturing technologies is seen as a clear and necessary emerging trend, especially within biofabrication. This hybrid approach allows more complex mechanics and discrete biomimetic structures to address mechanotransduction and chemotactic/haptotactic cues. Pioneering milestone papers in hybrid fabrication for biomedical purposes are presented and recent trends toward future manufacturing platforms are analyzed.

Soft-Hard Composites for Bioelectric Interfaces

Bioelectric devices can probe fundamental biological dynamics and improve the lives of human beings. However, direct application of traditional rigid electronics onto soft tissues can cause signal transduction and biocompatibility issues. One common mitigation strategy is the use of soft-hard composites to form more biocompatible interfaces with target cells or tissues. Here, we identify several soft-hard composite designs in naturally occurring systems. We use these designs to categorize the existing bioelectric interfaces and to suggest future opportunities. We discuss the utility of soft-hard composites for a variety of interfaces, such as in vitro and in vivo electronic or optoelectronic sensing and genetic and non-genetic modulation. We end the review by proposing new soft-hard composites for future bioelectric studies.

Fully synthetic matrices for in vitro culture of primary human intestinal enteroids and endometrial organoids

Epithelial organoids derived from human donor tissues are important tools in fields ranging from regenerative medicine to drug discovery. Organoid culture requires expansion of stem/progenitor cells in Matrigel, a tumor-derived extracellular matrix (ECM). An alternative completely synthetic ECM could improve reproducibility, clarify mechanistic phenomena, and enable human implantation of organoids. We designed synthetic ECMs with tunable biomolecular and biophysical properties to identify gel compositions supporting human tissue-derived stem/progenitor epithelial cells as enteroids and organoids starting with single cells rather than tissue fragments. The synthetic ECMs consist of 8-arm PEG-macromers modified with ECM-binding peptides and different combinations of integrin-binding peptides, crosslinked with peptides susceptible to matrix metalloprotease (MMP) degradation, and tuned to exhibit a range of biophysical properties. A gel containing an α2β1 integrin-binding peptide (GFOGER) and matrix binder peptides grafted to a 20 kDa 8-arm PEG macromer showed the most robust support of human duodenal and colon enteroids and endometrial organoids. In this synthetic ECM, human intestinal enteroids and endometrial organoids emerge from single cells and show cell-specific and apicobasal polarity markers upon differentiation. Intestinal enteroids, in addition, retain their proliferative capacity, are functionally responsive to basolateral stimulation, express canonical markers of intestinal crypt cells including Paneth cells, and can be serially passaged. The success of this synthetic ECM in supporting human postnatal organoid culture from multiple different donors and from both the intestine and endometrium suggests it may be broadly useful for other epithelial organoid culture.

Tools for probing host-bacteria interactions in the gut microenvironment: From molecular to cellular levels

Healthy function of the gut microenvironment is dependent on complex interactions between the bacteria of the microbiome, epithelial and immune (host) cells, and the surrounding tissue. Misregulation of these interactions is implicated in disease. A range of tools have been developed to study these interactions, from mechanistic studies to therapeutic evaluation. In this Digest, we highlight select tools at the cellular and molecular level for probing specific cell-microenvironment interactions. Approaches are overviewed for controlling and probing cell-cell interactions, from transwell and microfluidic devices to engineered bacterial peptidoglycan fragments, and cell-matrix interactions, from three-dimensional scaffolds to chemical handles for in situ modifications.

100th Anniversary of Macromolecular Science Viewpoint: Synthetic Protein Hydrogels

Our bodies are composed of soft tissues made of various proteins. In contrast, most hydrogels designed for biological applications are made of synthetic polymers. Recently, it is increasingly recognized that genetically synthesized proteins can be tailored as building blocks of hydrogels with biological, chemical, and mechanical properties similar to native soft tissues. In this Viewpoint, we summarize recent progress in synthetic protein hydrogels. We compare the structural and mechanical properties of different protein building blocks. We discuss various biocompatible cross-linking strategies based on covalent chemical reactions and noncovalent physical interactions. We introduce how stimulus-responsive conformational changes or intermolecular interactions at the molecular level can be used to engineer responsive hydrogels. We highlight that hydrogel network structures are as important as the protein sequences for the properties and functions of protein hydrogels and should be carefully designed. Despite great progress and potentials of synthetic protein hydrogels, there are still quite a few unsettled challenges and unexploited opportunities, providing abundant room for future investigation and development, particularly as this field is quickly expanding beyond its initial stage. We discuss a number of possible directions, including optimizing protein production and reducing cost, engineering anisotropic hydrogels to better mimic native tissues, rationally designing hydrogel mechanical properties, investigating interplays of hydrogels and residing cells for 3D cell culture and organoid construction, and evaluating long-term cytotoxicity and immune response.

Biomaterials and biosensors in intestinal organoid culture, a progress review

As an emerging technology, intestinal organoids are promising new tools for basic and translational research in gastroenterology. Currently, culture of intestinal organoids relies mostly on a type of tumor-derived scaffolds, namely Matrigel, which may pose tumorigenic risks to organoid implantation. Apart from the traditional detection methods, such as tissue slicing and fluorescence staining, the monitoring of intestinal organoids requires real-time biosensors that can adapt to their three-dimensional dynamic growth patterns. In this review, we summarized the recent advances in developing definite hydrogel scaffolds for intestinal organoid culture and identified key parameters for scaffold design. In addition, classified by different substrate compositions like pH, electrolytes, and functional proteins, we concluded the existing live-imaging biosensors and elucidated their underlying mechanisms. We hope this review enhances the understanding of intestinal organoid culture and provides more practical approaches to investigate them.

Effect of Laminin Derived Peptides IKVAV and LRE Tethered to Hyaluronic Acid on hiPSC Derived Neural Stem Cell Morphology, Attachment and Neurite Extension

Low neural tissue extracellular matrix (ECM) content has led to the understudy of its effects on neural cells and tissue. Hyaluronic acid (HA) and laminin are major neural ECM components, but direct comparisons of their cellular effects could not be located in the literature. The current study uses human-induced pluripotent stem-cell-derived neural stem cells to assess the effects of HA, laminin, and HA with laminin-derived peptides IKVAV and LRE on cellular morphology, attachment, neurite extension and ECM remodeling. Increased attachment was observed on HA with and without IKVAV and LRE compared to laminin. Cellular morphology and neurite extension were similar on all surfaces. Using a direct binding inhibitor of Cav2.2 voltage gated calcium channel activity, a known binding partner of LRE, reduced attachment on HA with and without IKVAV and LRE and altered cellular morphology on surfaces with laminin or IKVAV and LRE. HA with IKVAV and LRE reduced the fluorescent intensity of fibronectin staining, but did not alter the localization of ECM remodeling enzymes matrix metalloprotease 2 and 9 staining compared to HA. Overall, the data indicate HA, IKVAV and LRE have complementary effects on human-induced pluripotent stem-cell-derived neural stem cell behavior.

Topographic Cues Impact on Embryonic Stem Cell Zscan4-Metastate

The extracellular microenvironment proved to exert a potent regulatory effect over different aspects of Embryonic Stem Cells (ESCs) behavior. In particular, the employment of engineered culture surfaces aimed at modulating ESC self-organization resulted effective in directing ESCs toward specific fate decision. ESCs fluctuate among different levels of functional potency and in this context the Zscan4 gene marks the so-called "metastate," a cellular state in which ESCs retain both self-renewal and pluripotency capabilities. Here we investigated the impact of topographic cues on ESCs pluripotency, differentiation and organization capabilities. To this aim, we engineered culturing platforms of nanograted surfaces with different features size and we investigated their impact on ESCs multicellular organization and Zscan4 gene expression. We showed that the morphology of ESC-derived aggregates and Zscan4 expression are strictly intertwined. Our data suggest that ESC Zscan4 metastate can be promoted if the adhesive surface conditions guide cellular self-aggregation into 3D dome-like structure, in which both cell-material interactions and cell-cell contact are supportive for Zscan4 expression.

Recreating Physiological Environments In Vitro: Design Rules for Microfluidic-Based Vascularized Tissue Constructs

Vascularization of engineered tissue constructs remains one of the greatest unmet challenges to mimicking the native tissue microenvironment in vitro. The main obstacle is recapitulating the complexity of the physiological environment while providing simplicity in operation and manipulation of the model. Microfluidic technology has emerged as a promising tool that enables perfusion of the tissue constructs through engineered vasculatures and precise control of the vascular microenvironment cues in vitro. The tunable microenvironment includes i) biochemical cues such as coculture, supporting matrix, and growth factors and ii) engineering aspects such as vasculature engineering methods, fluid flow, and shear stress. In this systematic review, the design considerations of the microfluidic-based in vitro model are discussed, with an emphasis on microenvironment control to enhance the development of next-generation vascularized engineered tissues.

Advanced Bottom-Up Engineering of Living Architectures

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

Gastrointestinal tract modeling using organoids engineered with cellular and microbiota niches

The recent emergence of organoid technology has attracted great attention in gastroenterology because the gastrointestinal (GI) tract can be recapitulated in vitro using organoids, enabling disease modeling and mechanistic studies. However, to more precisely emulate the GI microenvironment in vivo, several neighboring cell types and types of microbiota need to be integrated into GI organoids. This article reviews the recent progress made in elucidating the crosstalk between GI organoids and components of their microenvironment. We outline the effects of stromal cells (such as fibroblasts, neural cells, immune cells, and vascular cells) on the gastric and intestinal epithelia of organoids. Because of the important roles that microbiota play in the physiology and function of the GI tract, we also highlight interactions between organoids and commensal, symbiotic, and pathogenic microorganisms and viruses. GI organoid models that contain niche components will provide new insight into gastroenterological pathophysiology and disease mechanisms.

Bioengineered microenvironment to culture early embryos

The abnormalities of early post-implantation embryos can lead to early pregnancy loss and many other syndromes. However, it is hard to study embryos after implantation due to the limited accessibility. The success of embryo culture in vitro can avoid the challenges of embryonic development in vivo and provide a powerful research platform for research in developmental biology. The biophysical and chemical cues of the microenvironments impart significant spatiotemporal effects on embryonic development. Here, we summarize the main strategies which enable researchers to grow embryos outside of the body while overcoming the implantation barrier, highlight the roles of engineered microenvironments in regulating early embryonic development, and finally discuss the future challenges and new insights of early embryo culture.

Engineered Plant-Based Nanocellulose Hydrogel for Small Intestinal Organoid Growth

Organoids are three-dimensional self-renewing and organizing clusters of cells that recapitulate the behavior and functionality of developed organs. Referred to as "organs in a dish," organoids are invaluable biological models for disease modeling or drug screening. Currently, organoid culture commonly relies on an expensive and undefined tumor-derived reconstituted basal membrane which hinders its application in high-throughput screening, regenerative medicine, and diagnostics. Here, we introduce a novel engineered plant-based nanocellulose hydrogel is introduced as a well-defined and low-cost matrix that supports organoid growth. Gels containing 0.1% nanocellulose fibers (99.9% water) are ionically crosslinked and present mechanical properties similar to the standard animal-based matrix. The regulation of the osmotic pressure is performed by a salt-free strategy, offering conditions for cell survival and proliferation. Cellulose nanofibers are functionalized with fibronectin-derived adhesive sites to provide the required microenvironment for small intestinal organoid growth and budding. Comparative transcriptomic profiling reveals a good correlation with transcriptome-wide gene expression pattern between organoids cultured in both materials, while differences are observed in stem cells-specific marker genes. These hydrogels are tunable and can be combined with laminin-1 and supplemented with insulin-like growth factor (IGF-1) to optimize the culture conditions. Nanocellulose hydrogel emerges as a promising matrix for the growth of organoids.

Vertically-Aligned Functionalized Silicon Micropillars for 3D Culture of Human Pluripotent Stem Cell-Derived Cortical Progenitors

Silicon is a promising material for tissue engineering since it allows to produce micropatterned scaffolding structures resembling biological tissues. Using specific fabrication methods, it is possible to build aligned 3D network-like structures. In the present study, we exploited vertically-aligned silicon micropillar arrays as culture systems for human iPSC-derived cortical progenitors. In particular, our aim was to mimic the radially-oriented cortical radial glia fibres that during embryonic development play key roles in controlling the expansion, radial migration and differentiation of cortical progenitors, which are, in turn, pivotal to the establishment of the correct multilayered cerebral cortex structure. Here we show that silicon vertical micropillar arrays efficiently promote expansion and stemness preservation of human cortical progenitors when compared to standard monolayer growth conditions. Furthermore, the vertically-oriented micropillars allow the radial migration distinctive of cortical progenitors in vivo. These results indicate that vertical silicon micropillar arrays can offer an optimal system for human cortical progenitors' growth and migration. Furthermore, similar structures present an attractive platform for cortical tissue engineering.

Engineered 3D Polymer and Hydrogel Microenvironments for Cell Culture Applications

The realization of biomimetic microenvironments for cell biology applications such as organ-on-chip, in vitro drug screening, and tissue engineering is one of the most fascinating research areas in the field of bioengineering. The continuous evolution of additive manufacturing techniques provides the tools to engineer these architectures at different scales. Moreover, it is now possible to tailor their biomechanical and topological properties while taking inspiration from the characteristics of the extracellular matrix, the three-dimensional scaffold in which cells proliferate, migrate, and differentiate. In such context, there is therefore a continuous quest for synthetic and nature-derived composite materials that must hold biocompatible, biodegradable, bioactive features and also be compatible with the envisioned fabrication strategy. The structure of the current review is intended to provide to both micro-engineers and cell biologists a comparative overview of the characteristics, advantages, and drawbacks of the major 3D printing techniques, the most promising biomaterials candidates, and the trade-offs that must be considered in order to replicate the properties of natural microenvironments.

iPSCs-Based Neural 3D Systems: A Multidimensional Approach for Disease Modeling and Drug Discovery

Induced pluripotent stem cells (iPSCs)-based two-dimensional (2D) protocols have offered invaluable insights into the pathophysiology of neurological diseases. However, these systems are unable to reproduce complex cytoarchitectural features, cell-cell and tissue-tissue interactions like their in vivo counterpart. Three-dimensional (3D)-based culture protocols, though in their infancy, have offered new insights into modeling human diseases. Human neural organoids try to recapitulate the cellular diversity of complex tissues and can be generated from iPSCs to model the pathophysiology of a wide spectrum of pathologies. The engraftment of iPSCs into mice models and the improvement of differentiation protocols towards 3D cultures has enabled the generation of more complex multicellular systems. Consequently, models of neuropsychiatric disorders, infectious diseases, brain cancer and cerebral hypoxic injury can now be investigated from new perspectives. In this review, we consider the advancements made in modeling neuropsychiatric and neurological diseases with iPSC-derived organoids and their potential use to develop new drugs.