NutriChip: nutrition analysis meets microfluidics

Patient-Derived Microphysiological Systems for Precision Medicine

Patient-derived microphysiological systems (P-MPS) have emerged as powerful tools in precision medicine that provide valuable insight into individual patient characteristics. This review discusses the development of P-MPS as an integration of patient-derived samples, including patient-derived cells, organoids, and induced pluripotent stem cells, into well-defined MPSs. Emphasizing the necessity of P-MPS development, its significance as a nonclinical assessment approach that bridges the gap between traditional in vitro models and clinical outcomes is highlighted. Additionally, guidance is provided for engineering approaches to develop microfluidic devices and high-content analysis for P-MPSs, enabling high biological relevance and high-throughput experimentation. The practical implications of the P-MPS are further examined by exploring the clinically relevant outcomes obtained from various types of patient-derived samples. The construction and analysis of these diverse samples within the P-MPS have resulted in physiologically relevant data, paving the way for the development of personalized treatment strategies. This study describes the significance of the P-MPS in precision medicine, as well as its unique capacity to offer valuable insights into individual patient characteristics.

Lab-on-chip technologies for exploring the gut-immune axis in metabolic disease

The continued rise in metabolic diseases such as obesity and type 2 diabetes mellitus poses a global health burden, necessitating further research into factors implicated in the onset and progression of these diseases. Recently, the gut-immune axis, with diet as a main regulator, has been identified as a possible role player in their development. Translation of conventional 2D in vitro and animal models is however limited, while human studies are expensive and preclude individual mechanisms from being investigated. Lab-on-chip technology therefore offers an attractive new avenue to study gut-immune interactions. This review provides an overview of the influence of diet on gut-immune interactions in metabolic diseases and a critical analysis of the current state of lab-on-chip technology to study this axis. While there has been progress in the development of "immuno-competent" intestinal lab-on-chip models, with studies showing the ability of the technology to provide mechanical cues, support longer-term co-culture of microbiota and maintain in vivo-like oxygen gradients, platforms which combine all three and include intestinal and immune cells are still lacking. Further, immune cell types and inclusion of microenvironment conditions which enable in vivo-like immune cell dynamics as well as host-microbiome interactions are limited. Future model development should focus on combining these conditions to create an environment capable of hosting more complex microbiota and immune cells to allow further study into the effects of diet and related metabolites on the gut-immune ecosystem and their role in the prevention and development of metabolic diseases in humans.

Advancements in Nanoparticle Characterization

Nanotechnology for drug delivery has made significant advancements over the last two decades. Innovations have been made in cancer research and development, including chemotherapies, imaging agents, and vaccine strategies, as well as other therapeutic areas, e.g., the recent commercialization of mRNA lipid nanoparticles as vaccines against the SARS-CoV-2 virus. The field has also seen technological advancements to aid in addressing the complex questions posed by these novel therapies. In this latest edition of protocols and methods for nanoparticle characterization, we highlight both old and new methodologies for defining physicochemical properties, present both in vitro and in vivo methods to test for a variety of immunotoxicities, and describe assays used for pharmacological studies to assess drug release and tissue distribution.

A generic pump-free organ-on-a-chip platform for assessment of intestinal drug absorption

Organ-on-a-chip technology has shown great potential in disease modeling and drug evaluation. However, traditional organ-on-a-chip devices are mostly pump-dependent with low throughput, which makes it difficult to leverage their advantages. In this study, we have developed a generic, pump-free organ-on-a-chip platform consisting of a 32-unit chip and an adjustable rocker, facilitating high-throughput dynamic cell culture with straightforward operation. By utilizing the rocker to induce periodic fluid forces, we can achieve fluidic conditions similar to those obtained with traditional pump-based systems. Through constructing a gut-on-a-chip model, we observed remarkable enhancements in the expression of barrier-associated proteins and the spatial distribution of differentiated intestinal cells compared to static culture. Furthermore, RNA sequencing analysis unveiled enriched pathways associated with cell proliferation, lipid transport, and drug metabolism, indicating the ability of the platform to mimic critical physiological processes. Additionally, we tested seven drugs that represent a range of high, medium, and low in vivo permeability using this model and found a strong correlation between their Papp values and human Fa, demonstrating the capability of this model for drug absorption evaluation. Our findings highlight the potential of this pump-free organ-on-a-chip platform as a valuable tool for advancing drug development and enabling personalized medicine.

Immunity-on-a-Chip: Integration of Immune Components into the Scheme of Organ-on-a-Chip Systems

Studying the immune system in vitro aims to understand how, when, and where the immune cells migrate/differentiate and respond to the various triggering events and the decision points along the immune response journey. It becomes evident that organ-on-a-chip (OOC) technology has a superior capability to recapitulate the cell-cell and tissue-tissue interaction in the body, with a great potential to provide tools for tracking the paracrine signaling with high spatial-temporal precision and implementing in situ real-time, non-destructive detection assays, therefore, enabling extraction of mechanistic information rather than phenotypic information. However, despite the rapid development in this technology, integration of the immune system into OOC devices stays among the least navigated tasks, with immune cells still the major missing components in the developed models. This is mainly due to the complexity of the immune system and the reductionist methodology of the OOC modules. Dedicated research in this field is demanded to establish the understanding of mechanism-based disease endotypes rather than phenotypes. Herein, we systemically present a synthesis of the state-of-the-art of immune-cantered OOC technology. We comprehensively outlined what is achieved and identified the technology gaps emphasizing the missing components required to establish immune-competent OOCs and bridge these gaps.

Gut-on-a-Chip Research for Drug Development: Implications of Chip Design on Preclinical Oral Bioavailability or Intestinal Disease Studies

The gut plays a key role in drug absorption and metabolism of orally ingested drugs. Additionally, the characterization of intestinal disease processes is increasingly gaining more attention, as gut health is an important contributor to our overall health. The most recent innovation to study intestinal processes in vitro is the development of gut-on-a-chip (GOC) systems. Compared to conventional in vitro models, they offer more translational value, and many different GOC models have been presented over the past years. Herein, we reflect on the almost unlimited choices in designing and selecting a GOC for preclinical drug (or food) development research. Four components that largely influence the GOC design are highlighted, namely (1) the biological research questions, (2) chip fabrication and materials, (3) tissue engineering, and (4) the environmental and biochemical cues to add or measure in the GOC. Examples of GOC studies in the two major areas of preclinical intestinal research are presented: (1) intestinal absorption and metabolism to study the oral bioavailability of compounds, and (2) treatment-orientated research for intestinal diseases. The last section of this review presents an outlook on the limitations to overcome in order to accelerate preclinical GOC research.

Organoids/organs-on-a-chip: new frontiers of intestinal pathophysiological models

Organoids/organs-on-a-chip open up new frontiers for basic and clinical research of intestinal diseases. Species-specific differences hinder research on animal models, while organoids are emerging as powerful tools due to self-organization from stem cells and the reproduction of the functional properties in vivo. Organs-on-a-chip is also accelerating the process of faithfully mimicking the intestinal microenvironment. And by combining organoids and organ-on-a-chip technologies, they further are expected to serve as innovative preclinical tools and could outperform traditional cell culture models or animal models in the future. Above all, organoids/organs-on-a-chip with other strategies like genome editing, 3D printing, and organoid biobanks contribute to modeling intestinal homeostasis and disease. Here, the current challenges and future trends in intestinal pathophysiological models will be summarized.

Gut-on-a-Chip for the Analysis of Bacteria-Bacteria Interactions in Gut Microbial Community: What Would Be Needed for Bacterial Co-Culture Study to Explore the Diet-Microbiota Relationship?

Bacterial co-culture studies using synthetic gut microbiomes have reported novel research designs to understand the underlying role of bacterial interaction in the metabolism of dietary resources and community assembly of complex microflora. Since lab-on-a-chip mimicking the gut (hereafter "gut-on-a-chip") is one of the most advanced platforms for the simulative research regarding the correlation between host health and microbiota, the co-culture of the synthetic bacterial community in gut-on-a-chip is expected to reveal the diet-microbiota relationship. This critical review analyzed recent research on bacterial co-culture with perspectives on the ecological niche of commensals, probiotics, and pathogens to categorize the experimental approaches for diet-mediated management of gut health as the compositional and/or metabolic modulation of the microbiota and the control of pathogens. Meanwhile, the aim of previous research on bacterial culture in gut-on-a-chip has been mainly limited to the maintenance of the viability of host cells. Thus, the integration of study designs established for the co-culture of synthetic gut consortia with various nutritional resources into gut-on-a-chip is expected to reveal bacterial interspecies interactions related to specific dietary patterns. This critical review suggests novel research topics for co-culturing bacterial communities in gut-on-a-chip to realize an ideal experimental platform mimicking a complex intestinal environment.

The translational roadmap of the gut models, focusing on gut-on-chip

It is difficult to model in vitro the intestine when seeking to include crosstalk with the gut microbiota, immune and neuroendocrine systems. Here we present a roadmap of the current models to facilitate the choice in preclinical and translational research with a focus on gut-on-chip. These micro physiological systems (MPS) are microfluidic devices that recapitulate in vitro the physiology of the intestine. We reviewed the gut-on-chips that had been developed in academia and industries as single chip and that have three main purpose: replicate the intestinal physiology, the intestinal pathological features, and for pharmacological tests.

The translational roadmap of the gut models, focusing on gut-on-chip

It is difficult to model in vitro the intestine when seeking to include crosstalk with the gut microbiota, immune and neuroendocrine systems. Here we present a roadmap of the current models to facilitate the choice in preclinical and translational research with a focus on gut-on-chip. These micro physiological systems (MPS) are microfluidic devices that recapitulate in vitro the physiology of the intestine. We reviewed the gut-on-chips that had been developed in academia and industries as single chip and that have three main purpose: replicate the intestinal physiology, the intestinal pathological features, and for pharmacological tests.

Microfluidic system for immune cell activation and inflammatory cytokine profiling: Application to screening of dietary supplements for anti-inflammatory properties

A versatile and reconfigurable microfluidic chip has been fully in-house fabricated and tested for immune cell culture, activation, and quantification of multi-cytokine secretion. The chip comprises three vertically stacked fluidic layers for perfusion, cell culture and cytokine capture, and quantification, respectively. The perfused media were separated from the cell culture by employing a biomimetic membrane as a model of the intestinal epithelial layer. Time-resolved detection and quantification of several secreted cytokines were enabled by an array of parallel channels, which are interfaced with the cell culture by a porous membrane. Each channel hosts magnetic beads conjugated with a specific antibody against the cytokine of interest. Magnetic bead-assisted agitation enables homogenization of the cell culture supernatant and perfusion of the cytokines through the bottom immune assay channels. As a proof of concept, THP-1 monocytic cells and their induced macrophages were used as a model of immune-responsive cells. The cells were sequentially stimulated by lipopolysaccharides and two dietary supplements, namely, docosahexaenoic acid (DHA) and curcumin, which are known to possess inflammasome-modulating activity. Both DHA and curcumin have shown anti-inflammatory effects by downregulating the secretion of TNFα, IL-6, IL-1β, and IL-10. Treatment of the cells with DHA and curcumin together lowered the TNFα secretion by ∼54%. IL-6 secretion was lowered upon cell treatment with curcumin, DHA, or DHA and curcumin co-treatment by 69%, 78%, or 67%, respectively. IL-1β secretion was lowered by 67% upon curcumin treatment and 70% upon curcumin and DHA co-treatment. IL-10 secretion was also lowered upon treating the cells with DHA, curcumin, or DHA and curcumin together by 7%, 53%, or 54%, respectively. The limit of the detection of the assay was determined as 25 pg/ml. Four cytokine profiling was demonstrated, but the design of the chip can be improved to allow a larger number of cytokines to be simultaneously detected from the same set of cells.

Dual-Material 3D-Printed Intestinal Model Devices with Integrated Villi-like Scaffolds

In vitro small intestinal models aim to mimic the in vivo intestinal function and structure, including the villi architecture of the native tissue. Accurate models in a scalable format are in great demand to advance, for example, the development of orally administered pharmaceutical products. Widely used planar intestinal cell monolayers for compound screening applications fail to recapitulate the three-dimensional (3D) microstructural characteristics of the intestinal villi arrays. This study employs stereolithographic 3D printing to manufacture biocompatible hydrogel-based scaffolds with villi-like micropillar arrays of tunable dimensions in poly(ethylene glycol) diacrylates (PEGDAs). The resulting 3D-printed microstructures are demonstrated to support a month-long culture and induce apicobasal polarization of Caco-2 epithelial cell layers along the villus axis, similar to the native intestinal microenvironment. Transport analysis requires confinement of compound transport to the epithelial cell layer within a compound diffusion-closed reservoir compartment. We meet this challenge by sequential printing of PEGDAs of different molecular weights into a monolithic device, where a diffusion-open villus-structured hydrogel bottom supports the cell culture and mass transport within the confines of a diffusion-closed solid wall. As a functional demonstrator of this scalable dual-material 3D micromanufacturing technology, we show that Caco-2 cells seeded in villi-wells form a tight epithelial barrier covering the villi-like micropillars and that compound-induced challenges to the barrier integrity can be monitored by standard high-throughput analysis tools (fluorescent tracer diffusion and transepithelial electrical resistance).

Design and Fabrication of Low-Cost Microfluidic Chips and Microfluidic Routing System for Reconfigurable Multi-(Organ-on-a-Chip) Assembly

A low-cost, versatile, and reconfigurable fluidic routing system and chip assembly have been fabricated and tested. The platform and its accessories were fabricated in-house without the need for costly and specialized equipment nor specific expertise. An agarose-based artificial membrane was integrated into the chips and employed to test the chip-to-chip communication in various configurations. Various chip assemblies were constructed and tested which demonstrate the versatile utility of the fluidic routing system that enables the custom design of the chip-to-chip communication and the possibility of fitting a variety of (organ-on-a-chip)-based biological models with multicell architectures. The reconfigurable chip assembly would enable selective linking/isolating the desired chip/compartment, hence allowing the study of the contribution of specific cell/tissue within the in vitro models.

Relevance of organ(s)-on-a-chip systems to the investigation of food-gut microbiota-host interactions

The ever greater understanding of the composition and function of the gut microbiome has provided new opportunities with respect to understanding and treating human disease. However, the models employed for in vitro and in vivo animal studies do not always provide the required insights. As a result, one such alternative in vitro cell culture based system, organ-on-a-chip technology, has recently attracted attention as a means of obtaining data that is representative of responses in humans. Organ-on-a-chip systems are designed to mimic the interactions of different tissue elements that were missing from traditional two-dimensional tissue culture. While they do not traditionally include a microbiota component, organ-on-a-chip systems provide a potentially valuable means of characterising the interactions between the microbiome and human tissues with a view to providing even greater accuracy. From a dietary perspective, these microbiota-organ-on-a-chip combinations can help researchers to predict how the consumption of specific foods and ingredients can impact on human health and disease. We provide an overview of the relevance and interactions of the gut microbiota and the diet in human health, we summarise the components involved in the organ-on-a-chip systems, how these systems have been employed for microbiota based studies and their potential relevance to study the interplay between food-gut microbiota-host interactions.

Evaluation of Hybrid Vesicles in an Intestinal Cell Model Based on Structured Paper Chips

Cell culture-based intestinal models are important to evaluate nanoformulations intended for oral drug delivery. We report the use of a floating structured paper chip as a scaffold for Caco-2 cells and HT29-MTX-E12 cells that are two established cell types used in intestinal cell models. The formation of cell monolayers for both mono- and cocultures in the paper chip are confirmed and the level of formed cell-cell junctions is evaluated. Further, cocultures show first mucus formation between 6-10 days with the mucus becoming more pronounced after 19 days. Hybrid vesicles (HVs) made from phospholipids and the amphiphilic block copolymer poly(cholesteryl methacrylate)-block-poly(2-carboxyethyl acrylate) in different ratios are used as a representative soft nanoparticle to assess their mucopenetration ability in paper chip-based cell cultures. The HV assembly is characterized, and it is illustrated that these HVs cross the mucus layer and are found intracellularly within 3 h when the cells are grown in the paper chips. Taken together, the moist three-dimensional cellulose environment of structured paper chips offers an interesting cell culture-based intestinal model that can be further integrated with fluidic systems or online read-out opportunities.

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

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

Modeling Immunity In Vitro: Slices, Chips, and Engineered Tissues

Modeling immunity in vitro has the potential to be a powerful tool for investigating fundamental biological questions, informing therapeutics and vaccines, and providing new insight into disease progression. There are two major elements to immunity that are necessary to model: primary immune tissues and peripheral tissues with immune components. Here, we systematically review progress made along three strategies to modeling immunity: ex vivo cultures, which preserve native tissue structure; microfluidic devices, which constitute a versatile approach to providing physiologically relevant fluid flow and environmental control; and engineered tissues, which provide precise control of the 3D microenvironment and biophysical cues. While many models focus on disease modeling, more primary immune tissue models are necessary to advance the field. Moving forward, we anticipate that the expansion of patient-specific models may inform why immunity varies from patient to patient and allow for the rapid comprehension and treatment of emerging diseases, such as coronavirus disease 2019.

Glioma-on-a-Chip Models

Glioma, as an aggressive type of cancer, accounts for virtually 80% of malignant brain tumors. Despite advances in therapeutic approaches, the long-term survival of glioma patients is poor (it is usually fatal within 12-14 months). Glioma-on-chip platforms, with continuous perfusion, mimic in vivo metabolic functions of cancer cells for analytical purposes. This offers an unprecedented opportunity for understanding the underlying reasons that arise glioma, determining the most effective radiotherapy approach, testing different drug combinations, and screening conceivable side effects of drugs on other organs. Glioma-on-chip technologies can ultimately enhance the efficacy of treatments, promote the survival rate of patients, and pave a path for personalized medicine. In this perspective paper, we briefly review the latest developments of glioma-on-chip technologies, such as therapy applications, drug screening, and cell behavior studies, and discuss the current challenges as well as future research directions in this field.

Defined gut microbial communities: promising tools to understand and combat disease

Defined gut microbial communities are emerging tools that allow detailed studies of microbial ecosystems and their interactions with the host. In this article, we review strategies underlying the design of defined consortia and summarize the efforts to introduce simplified communities into in vitro and in vivo models. We conclude by highlighting the potential of defined microbial ecosystems as effective modulation strategies for health benefits.

Oral fate and stabilization technologies of lactoferrin: a systematic review

Lactoferrin (Lf), a bioactive protein initially found in many biological secretions including milk, is regarded as the nutritional supplement or therapeutic ligand due to its multiple functions. Research on its mode of action reveals that intact Lf or its active peptide (i.e., lactoferricin) shows an important multifunctional performance. Oral delivery is considered as the most convenient administration route for this bioactive protein. Unfortunately, Lf is sensitive to the gastrointestinal (GI) physicochemical stresses and lactoferricin is undetectable in GI digesta. This review introduces the functionality of Lf at the molecular level and its degradation behavior in GI tract is discussed in detail. Subsequently, the absorption and transport of Lf from intestine into the blood circulation, which is pivotal to its health promoting effects in various tissues, and some assisting labeling methods are discussed. Stabilization technologies aiming at preserving the structural integrity and functional properties of orally administrated Lf are summarized and compared. Altogether, this work comprehensively reviews the structure-function relationship of Lf, its oral fate and the development of stabilization technologies for the enhancement of the oral bioavailability of Lf. The existing limitations and scope for future research are also discussed.

Refining Host-Pathogen Interactions: Organ-on-Chip Side of the Coin

Bioinspired organ-level in vitro platforms that recapitulate human organ physiology and organ-specific responses have emerged as effective technologies for infectious disease research, drug discovery, and personalized medicine. A major challenge in tissue engineering for infectious diseases has been the reconstruction of the dynamic 3D microenvironment reflecting the architectural and functional complexity of the human body in order to more accurately model the initiation and progression of host-microbe interactions. By bridging the gap between in vitro experimental models and human pathophysiology and providing alternatives for animal models, organ-on-chip microfluidic devices have so far been implemented in multiple research areas, contributing to major advances in the field. Given the emergence of the recent pandemic, plug-and-play organ chips may hold the key for tackling an unmet clinical need in the development of effective therapeutic strategies. In this review, latest studies harnessing organ-on-chip platforms to unravel host-pathogen interactions are presented to highlight the prospects for the microfluidic technology in infectious diseases research.

Advances of microfluidic intestine-on-a-chip for analyzing anti-inflammation of food

Microfluidic intestine-on-a-chip enables novel means of emulating human intestinal pathophysiology in vitro, which can potentially reduce animal testing and substitute simple 2D culture system. Though a great deal of work has been done in the development of microfluidic platforms for intestinal disease modeling and drug screening, potential investigation of the effect of bioactive food compounds on intestinal inflammation remains largely unexplored. In this review, different biomaterials and chip designs have been explored in the fabrication of intestine-on-a-chip. Other key parameters must be carefully controlled and selected, including shear stress, cell type and cell co-culture spatial configuration, etc. Appropriate techniques to quantify the barrier integrity including trans-epithelial electric resistance, specific tight junction markers and permeability measurements should be standardized and compared with in vivo data. Integration of the gut microbiome and the provision of intestinal-specific environment are the key parameters to realize the in vivo intestinal model simulation and accelerate the screening efficiency of bioactive food compounds.

Gut bioengineering strategies for regenerative medicine

Gastrointestinal disease burden continues to rise in the United States and worldwide. The development of bioengineering strategies to model gut injury or disease and to reestablish functional gut tissue could expand therapeutic options and improve clinical outcomes. Current approaches leverage a rapidly evolving gut bioengineering toolkit aimed at 1) de novo generation of gutlike tissues at multiple scales for microtissue models or implantable grafts and 2) regeneration of functional gut in vivo. Although significant progress has been made in intestinal organoid cultures and engineered tissues, development of predictive in vitro models and effective regenerative therapies remains challenging. In this review, we survey emerging bioengineering tools and recent methodological advances to identify current challenges and future opportunities in gut bioengineering for disease modeling and regenerative medicine.

Immunocompetent Human Intestinal Models in Preclinical Drug Development

The intestinal epithelial barrier, together with the microbiome and local immune system, is a critical component that maintains intestinal homeostasis. Dysfunction may lead to chronic inflammation, as observed in inflammatory bowel diseases. Animal models have historically been used in preclinical research to identify and validate new drug targets in intestinal inflammatory diseases. Yet, limitations about their biological relevance to humans and advances in tissue engineering have forced the development of more complex three-dimensional reconstructed intestinal epithelium. By introducing immune and commensal microbial cells, these models more accurately mimic the gut's physiology and the pathophysiological changes occurring in vivo in the inflamed intestine. Specific advantages and limitations of two-dimensional (2D) and three-dimensional (3D) intestinal models such as coculture systems, organoids, and microfluidic devices to study inflammatory and immune-related responses are highlighted. While current cell culture models lack the cellular and molecular complexity observed in vivo, the emphasis is put on how these models can be used to improve preclinical drug development for inflammatory diseases of the intestine.

A chip device to determine surface charge properties of confluent cell monolayers by measuring streaming potential

Cell surface charge is an important element of the function of biological barriers, but no chip device has been described to measure cell surface charge properties of confluent barrier cell monolayers. The aim of this study was the design and fabrication of a dynamic lab-on-a-chip (LOC) device which is suitable to monitor transcellular electrical resistance, as well as streaming potential parallel to the surface of cell layers. We successfully measured the streaming potential of a biological barrier culture model with the help of our previously published versatile lab-on-a-chip device equipped with two Ag/AgCl electrodes. The inclusion of these "zeta electrodes", a voltage preamplifier and an oscilloscope in our set-up made it possible to successfully record signals describing the surface charge properties of brain endothelial cell monolayers, used as a barrier model in our experiments. Data obtained on the new chip device were verified by comparing streaming potential results measured in the LOC device and zeta potential results by the commonly used laser-Doppler velocimetry (LDv) method and model simulations. Changes in the negative surface charge of the barrier model by treatments with neuraminidase enzyme modifying the cell membrane glycocalyx or lidocaine altering the lipid membrane charge could be measured by both the upgraded LOC device and LDv. The new chip device can help to gain meaningful new information on how surface charge is linked to barrier function in both physiological and pathological conditions.

Gut-on-a-chip: Current progress and future opportunities

Organ-on-a-chip technology tries to mimic the complexity of native tissues in vitro. Important progress has recently been made in using this technology to study the gut with and without microbiota. These in vitro models can serve as an alternative to animal models for studying physiology, pathology, and pharmacology. While these models have greater physiological relevance than two-dimensional (2D) cell systems in vitro, endocrine and immunological functions in gut-on-a-chip models are still poorly represented. Furthermore, the construction of complex models, in which different cell types and structures interact, remains a challenge. Generally, gut-on-a-chip models have the potential to advance our understanding of the basic interactions found within the gut and lay the foundation for future applications in understanding pathophysiology, developing drugs, and personalizing medical treatments.

Organ-on-a-chip engineering: Toward bridging the gap between lab and industry

Organ-on-a-chip (OOC) is a very ambitious emerging technology with a high potential to revolutionize many medical and industrial sectors, particularly in preclinical-to-clinical translation in the pharmaceutical arena. In vivo, the function of the organ(s) is orchestrated by a complex cellular structure and physiochemical factors within the extracellular matrix and secreted by various types of cells. The trend in in vitro modeling is to simplify the complex anatomy of the human organ(s) to the minimal essential cellular structure "micro-anatomy" instead of recapitulating the full cellular milieu that enables studying the absorption, metabolism, as well as the mechanistic investigation of drug compounds in a "systemic manner." However, in order to reflect the human physiology in vitro and hence to be able to bridge the gap between the in vivo and in vitro data, simplification should not compromise the physiological relevance. Engineering principles have long been applied to solve medical challenges, and at this stage of organ-on-a-chip technology development, the work of biomedical engineers, focusing on device engineering, is more important than ever to accelerate the technology transfer from the academic lab bench to specialized product development institutions and to the increasingly demanding market. In this paper, instead of presenting a narrative review of the literature, we systemically present a synthesis of the best available organ-on-a-chip technology from what is found, what has been achieved, and what yet needs to be done. We emphasized mainly on the requirements of a "good in vitro model that meets the industrial need" in terms of the structure (micro-anatomy), functions (micro-physiology), and characteristics of the device that hosts the biological model. Finally, we discuss the biological model-device integration supported by an example and the major challenges that delay the OOC technology transfer to the industry and recommended possible options to realize a functional organ-on-a-chip system.

Recapitulation of the accessible interface of biopsy-derived canine intestinal organoids to study epithelial-luminal interactions

Recent advances in canine intestinal organoids have expanded the option for building a better in vitro model to investigate translational science of intestinal physiology and pathology between humans and animals. However, the three-dimensional geometry and the enclosed lumen of canine intestinal organoids considerably hinder the access to the apical side of epithelium for investigating the nutrient and drug absorption, host-microbiome crosstalk, and pharmaceutical toxicity testing. Thus, the creation of a polarized epithelial interface accessible from apical or basolateral side is critical. Here, we demonstrated the generation of an intestinal epithelial monolayer using canine biopsy-derived colonic organoids (colonoids). We optimized the culture condition to form an intact monolayer of the canine colonic epithelium on a nanoporous membrane insert using the canine colonoids over 14 days. Transmission and scanning electron microscopy revealed a physiological brush border interface covered by the microvilli with glycocalyx, as well as the presence of mucin granules, tight junctions, and desmosomes. The population of stem cells as well as differentiated lineage-dependent epithelial cells were verified by immunofluorescence staining and RNA in situ hybridization. The polarized expression of P-glycoprotein efflux pump was confirmed at the apical membrane. Also, the epithelial monolayer formed tight- and adherence-junctional barrier within 4 days, where the transepithelial electrical resistance and apparent permeability were inversely correlated. Hence, we verified the stable creation, maintenance, differentiation, and physiological function of a canine intestinal epithelial barrier, which can be useful for pharmaceutical and biomedical researches.

Microphysiological Engineering of Immune Responses in Intestinal Inflammation

The epithelial barrier in the gastrointestinal (GI) tract is a protective interface that endures constant exposure to the external environment while maintaining its close contact with the local immune system. Growing evidence has suggested that the intercellular crosstalk in the GI tract contributes to maintaining the homeostasis in coordination with the intestinal microbiome as well as the tissue-specific local immune elements. Thus, it is critical to map the complex crosstalks in the intestinal epithelial-microbiome-immune (EMI) axis to identify a pathological trigger in the development of intestinal inflammation, including inflammatory bowel disease. However, deciphering a specific contributor to the onset of pathophysiological cascades has been considerably hindered by the challenges in current in vivo and in vitro models. Here, we introduce various microphysiological engineering models of human immune responses in the EMI axis under the healthy conditions and gut inflammation. As a prospective model, we highlight how the human “gut inflammation-on-a-chip” can reconstitute the pathophysiological immune responses and contribute to understanding the independent role of inflammatory factors in the EMI axis on the initiation of immune responses under barrier dysfunction. We envision that the microengineered immune models can be useful to build a customizable patient's chip for the advance in precision medicine.

Microfluidic methods for precision diagnostics in food allergy

Food allergy has reached epidemic proportions and has become a significant source of healthcare burden. Oral food challenge, the gold standard for food allergy assessment, often is not performed because it places the patient at risk of developing anaphylaxis. However, conventional alternative food allergy tests lack a sufficient predictive value. Therefore, there is a critical need for better diagnostic tests that are both accurate and safe. Microfluidic methods have the potential of helping one to address such needs and to personalize the diagnostics. This article first reviews conventional diagnostic approaches used in food allergy. Second, it reviews recent efforts to develop novel biomarkers and in vitro diagnostics. Third, it summarizes the microfluidic methods developed thus far for food allergy diagnosis. The article concludes with a discussion of future opportunities for using microfluidic methods for achieving precision diagnostics in food allergy, including multiplexing the detection of multiple biomarkers, sampling of tissue-resident cytokines and immune cells, and multi-organ-on-a-chip technology.

Engineering Microphysiological Immune System Responses on Chips

Tissues- and organs-on-chips are microphysiological systems (MPSs) that model the architectural and functional complexity of human tissues and organs that is lacking in conventional cell monolayer cultures. While substantial progress has been made in a variety of tissues and organs, chips recapitulating immune responses have not advanced as rapidly. This review discusses recent progress in MPSs for the investigation of immune responses. To illustrate recent developments, we focus on two cases in point: immunocompetent tumor microenvironment-on-a-chip devices that incorporate stromal and immune cell components and pathomimetic modeling of human mucosal immunity and inflammatory crosstalk. More broadly, we discuss the development of systems immunology-on-a-chip devices that integrate microfluidic engineering approaches with high-throughput omics measurements and emerging immunological applications of MPSs.

Dynamic in vitro intestinal barrier model coupled to chip-based liquid chromatography mass spectrometry for oral bioavailability studies

In oral bioavailability studies, evaluation of the absorption and transport of drugs and food components across the intestinal barrier is crucial. Advances in the field of organ-on-a-chip technology have resulted in a dynamic gut-on-a-chip model that better mimics the in vivo microenvironment of the intestine. Despite a few recent integration attempts, ensuring a biologically relevant microenvironment while coupling with a fully online detection system still represents a major challenge. Herein, we designed an online technique to measure drug permeability and analyse unknown product formation across an intestinal epithelial layer of Caco-2 and HT29-MTX cells cultured on a flow-through Transwell system, while ensuring the quality and relevance of the biological model. Chip-based ultra-performance liquid chromatography quadrupole time-of-flight mass spectrometry (UPLC-QTOF-MS) was coupled to the dynamic Transwell system via a series of switching valves, thus allowing alternating measurements of the apical and basolateral sides of the in vitro model. Two trap columns were integrated for online sample pre-treatment and compatibility enhancement. Temporal analysis of the intestinal permeability was successfully demonstrated using verapamil as a model drug and ergotamine epimers as a model for natural toxins present in foods. Evidence was obtained that our newly developed dynamic system provided reliable results versus classical static in vitro models, and moreover, for the first time, epimer-specific transport is shown for ergotamine. Finally, initial experiments with the drug granisetron suggest that metabolic activity can be studied as well, thus highlighting the versatility of the bio-integrated online analysis system developed. Graphical abstract.

Gut-on-chip: Recreating human intestine in vitro

The human gut is important for food digestion and absorption, as well as a venue for a large number of microorganisms that coexist with the host. Although numerous in vitro models have been proposed to study intestinal pathology or interactions between intestinal microbes and host, they are far from recapitulating the real intestinal microenvironment in vivo. To assist researchers in further understanding gut physiology, the intestinal microbiome, and disease processes, a novel technology primarily based on microfluidics and cell biology, called "gut-on-chip," was developed to simulate the structure, function, and microenvironment of the human gut. In this review, we first introduce various types of gut-on-chip systems, then highlight their applications in drug pharmacokinetics, host-gut microbiota crosstalk, and nutrition metabolism. Finally, we discuss challenges in this field and prospects for better understanding interactions between intestinal flora and human hosts, and then provide guidance for clinical treatment of related diseases.

Models of the Gut for Analyzing the Impact of Food and Drugs

Models of the human gastrointestinal tract (GIT) can be powerful tools for examining the biological interactions of food products and pharmaceuticals. This can be done under normal healthy conditions or using models of disease-many of which have no curative therapy. This report outlines the field of gastrointestinal modeling, with a particular focus on the intestine. Traditional in vivo animal models are compared to a range of in vitro models. In vitro systems are elaborated over time, recently culminating with microfluidic intestines-on-chips (IsOC) and 3D bioengineered models. Macroscale models are also reviewed for their important contribution in the microbiota studies. Lastly, it is discussed how in silico approaches may have utility in predicting and interpreting experimental data. The various advantages and limitations of the different systems are contrasted. It is posited that only through complementary use of these models will salient research questions be able to be addressed.

Fish-gut-on-chip: development of a microfluidic bioreactor to study the role of the fish intestine in vitro

In this study we present the first fish-gut-on-chip model. This model is based on the reconstruction of the intestinal barrier by culturing two intestinal cell lines from rainbow trout, namely epithelial RTgutGC and fibroblastic RTgutF, in an artificial microenvironment. For a realistic mimicry of the interface between the intestinal lumen and the interior of the organism we i) developed ultrathin and highly porous silicon nitride membranes that serve as basement membrane analogues and provide a culture interface for the fish cells; ii) constructed a unique micro-well plate-based microfluidic bioreactor that enables parallelization of experiments and creates realistic fluid flow exposure scenarios for the cells; iii) integrated electrodes in the reactor for non-invasive impedance sensing of cellular well-being. In a first approach, we used this reactor to investigate the response of epithelial fish cells to in vivo-like shear stress rates of 0.002-0.06 dyne per cm2, resulting from fluid flow within the intestinal lumen. Moreover, we investigated the interplay of epithelial and fibroblast cells under optimal flow conditions to carefully evaluate the benefits and drawbacks of the more complex reconstruction of the intestinal architecture. With our fish-gut-on-chip model we open up new strategies for a better understanding of basic fish physiology, for the refinement of fish feed in aquaculture and for predicting chemical uptake and bioaccumulation in fish for environmental risk assessment. The basic principles of our reactor prototype, including the use of ultrathin membranes, an open microfluidic circuit for perfusion and the micro-well plate-based format for simplified handling and avoidance of air-bubbles, will as well be of great value for other barrier-on-chip models.

Digestion-on-a-chip: a continuous-flow modular microsystem recreating enzymatic digestion in the gastrointestinal tract

In vitro digestions are essential for determining the bioavailability of compounds, such as nutrients. We have developed a cell-free, miniaturized enzymatic digestive system, employing three micromixers connected in series to mimic the digestive functions of the mouth, stomach and small intestine. This system continuously processes samples, e.g. containing nutrients, to provide a constant flow of digested materials which may be presented to a subsequent gut-on-a-chip absorption module, containing living human intestinal cells. Our system incorporates three-compartment enzymatic digestion, one of the key functions of the gastrointestinal tract. In each of these compartments, we modify the chemical environment, including pH, buffer, and mineral composition, to closely mimic the local physiological environment and create optimal conditions for digestive processes to take place. It will therefore provide an excellent addition to existing gut-on-a-chip systems, providing the next step in determining the bio-availability of orally administered compounds in a fast and continuous-flow ex vivo system. In this paper, we demonstrate enzymatic digestion in each separate compartment using compounds, starch and casein, as model nutrients. The use of transparent, microfluidic micromixers based on chaotic advection, which can be probed directly with a microscope, enabled enzyme kinetics to be monitored from the very start of a reaction. Furthermore, we have digested lactoferrin in our system, demonstrating complete digestion of this milk protein in much shorter times than achievable with standard in vitro digestions using batch reactors.

Bioinspired Engineering of Organ-on-Chip Devices

The human body can be viewed as an organism consisting of a variety of cellular and non-cellular materials interacting in a highly ordered manner. Its complex and hierarchical nature inspires the multi-level recapitulation of the human body in order to gain insights into the inner workings of life. While traditional cell culture models have led to new insights into the cellular microenvironment and biological control in vivo, deeper understanding of biological systems and human pathophysiology requires the development of novel model systems that allow for analysis of complex internal and external interactions within the cellular microenvironment in a more relevant organ context. Engineering organ-on-chip systems offers an unprecedented opportunity to unravel the complex and hierarchical nature of human organs. In this chapter, we first highlight the advances in microfluidic platforms that enable engineering of the cellular microenvironment and the transition from cells-on-chips to organs-on-chips. Then, we introduce the key features of the emerging organs-on-chips and their proof-of-concept applications in biomedical research. We also discuss the challenges and future outlooks of this state-of-the-art technology.

Gut-microbiota-on-a-chip: an enabling field for physiological research

Overwhelming scientific evidence today confirms that the gut microbiota is a central player in human health. Knowledge about interactions between human gut microbiota and human health has evolved rapidly in the last decade, based on experimental work involving analysis of human fecal samples or animal models (mainly rodents). A more detailed and cost-effective description of this interplay is now being enabled by the use of in vitro systems (i.e., gut-microbiota-on-chip systems) that recapitulate key aspects of the interaction between microbiota and human cells. Here, we review recent examples of the design and use of pioneering on-chip platforms for the study of the cross-talk between representative members of human microbiota and human microtissues. In these systems, the combined use of state-of-the-art microfluidics, biomaterials, cell culture techniques, classical microbiology, and a touch of genetic expression profiling have converged for the development of gut-on-chip platforms capable of recreating key features of the interplay between human microbiota and host human tissues. We foresee that the integration of novel microfabrication techniques and stem cell technologies will further accelerate the development of more complex and physiologically relevant microbiota-on-chip platforms. In turn, this will foster the faster acquisition of knowledge regarding human microbiota and will enable important advances in the understanding of how to control or prevent disease.

In vitro Models and On-Chip Systems: Biomaterial Interaction Studies With Tissues Generated Using Lung Epithelial and Liver Metabolic Cell Lines

In vitro models are very important in medicine and biology, because they provide an insight into cells' and microorganisms' behavior. Since these cells and microorganisms are isolated from their natural environment, these models may not completely or precisely predict the effects on the entire organism. Improvement in this area is secured by organ-on-a-chip development. The organ-on-a-chip assumes cells cultured in a microfluidic chip. The chip simulates bioactivities, mechanics and physiological behavior of organs or organ systems, generating artificial organs in that way. There are several cell lines used so far for each tested artificial organ. For lungs, mostly used cell lines are 16HBE, A549, Calu-3, NHBE, while mostly used cell lines for liver are HepG2, Hep 3B, TPH1, etc. In this paper, state of the art for lung and liver organ-on-a-chip is presented, together with the established in vitro testing on lung and liver cell lines, with the emphasis on Calu-3 (for lung cell lines) and Hep-G2 (for liver cell lines). Primary focus in this review is to discuss different researches on the topics of lung and liver cell line models, approaches in determining fate and transport, cell partitioning, cell growth and division, as well as cell dynamics, meaning toxicity and effects. The review is finalized with current research gaps and problems, stating potential future developments in the field.

Barriers-on-chips: Measurement of barrier function of tissues in organs-on-chips

Disruption of tissue barriers formed by cells is an integral part of the pathophysiology of many diseases. Therefore, a thorough understanding of tissue barrier function is essential when studying the causes and mechanisms of disease as well as when developing novel treatments. In vitro methods play an integral role in understanding tissue barrier function, and several techniques have been developed in order to evaluate barrier integrity of cultured cell layers, from microscopy imaging of cell-cell adhesion proteins to measuring ionic currents, to flux of water or transport of molecules across cellular barriers. Unfortunately, many of the current in vitro methods suffer from not fully recapitulating the microenvironment of tissues and organs. Recently, organ-on-chip devices have emerged to overcome this challenge. Organs-on-chips are microfluidic cell culture devices with continuously perfused microchannels inhabited by living cells. Freedom of changing the design of device architecture offers the opportunity of recapitulating the in vivo physiological environment while measuring barrier function. Assessment of barriers in organs-on-chips can be challenging as they may require dedicated setups and have smaller volumes that are more sensitive to environmental conditions. But they do provide the option of continuous, non-invasive sensing of barrier quality, which enables better investigation of important aspects of pathophysiology, biological processes, and development of therapies that target barrier tissues. Here, we discuss several techniques to assess barrier function of tissues in organs-on-chips, highlighting advantages and technical challenges.

Comparison of conventional and advanced in vitro models in the toxicity testing of nanoparticles

Humans are exposed to a wide variety of nanoparticles (NPs) present in the environment, in consumer, health and medical products, and in food. Conventional cytotoxicity testing compared to animal testing is less expensive, faster and avoids ethical problems at the expense of a lower predictive value. New cellular models and exposure conditions have been developed to overcome the limitations of conventional cell culture and obtain more predictive data. The use of three-dimensional culture, co-culture and inclusion of mechanical stimulation can provide physiologically more relevant culture conditions. These systems are particularly relevant for oral, respiratory and intravenous exposure to NPs and it may be assumed that physiologically relevant application of the NPs can improve the predictive value of in vitro testing. Various groups have used advanced culture and exposure systems, but few direct comparisons between data from conventional cultures and from advanced systems exist. In silico models may present another option to predict human health risk by NPs without using animal studies. In the absence of validation, the question whether these alternative models provide more predictive data than conventional testing remains elusive.

Fitting tissue chips and microphysiological systems into the grand scheme of medicine, biology, pharmacology, and toxicology

Microphysiological systems (MPS), which include engineered organoids (EOs), single organ/tissue chips (TCs), and multiple organs interconnected to create miniature in vitro models of human physiological systems, are rapidly becoming effective tools for drug development and the mechanistic understanding of tissue physiology and pathophysiology. The second MPS thematic issue of Experimental Biology and Medicine comprises 15 articles by scientists and engineers from the National Institutes of Health, the IQ Consortium, the Food and Drug Administration, and Environmental Protection Agency, an MPS company, and academia. Topics include the progress, challenges, and future of organs-on-chips, dissemination of TCs into Pharma, children's health protection, liver zonation, liver chips and their coupling to interconnected systems, gastrointestinal MPS, maturation of immature cardiomyocytes in a heart-on-a-chip, coculture of multiple cell types in a human skin construct, use of synthetic hydrogels to create EOs that form neural tissue models, the blood-brain barrier-on-a-chip, MPS models of coupled female reproductive organs, coupling MPS devices to create a body-on-a-chip, and the use of a microformulator to recapitulate endocrine circadian rhythms. While MPS hardware has been relatively stable since the last MPS thematic issue, there have been significant advances in cell sourcing, with increased reliance on human-induced pluripotent stem cells, and in characterization of the genetic and functional cell state in MPS bioreactors. There is growing appreciation of the need to minimize perfusate-to-cell-volume ratios and respect physiological scaling of coupled TCs. Questions asked by drug developers are followed by an analysis of the potential value, costs, and needs of Pharma. Of highest value and lowest switching costs may be the development of MPS disease models to aid in the discovery of disease mechanisms; novel compounds including probes, leads, and clinical candidates; and mechanism of action of drug candidates. Impact statement Microphysiological systems (MPS), which include engineered organoids and both individual and coupled organs-on-chips and tissue chips, are a rapidly growing topic of research that addresses the known limitations of conventional cellular monoculture on flat plastic - a well-perfected set of techniques that produces reliable, statistically significant results that may not adequately represent human biology and disease. As reviewed in this article and the others in this thematic issue, MPS research has made notable progress in the past three years in both cell sourcing and characterization. As the field matures, currently identified challenges are being addressed, and new ones are being recognized. Building upon investments by the Defense Advanced Research Projects Agency, National Institutes of Health, Food and Drug Administration, Defense Threat Reduction Agency, and Environmental Protection Agency of more than $200 million since 2012 and sizable corporate spending, academic and commercial players in the MPS community are demonstrating their ability to meet the translational challenges required to apply MPS technologies to accelerate drug development and advance toxicology.

Microscale Bioreactors for in situ characterization of GI epithelial cell physiology

The development of in vitro artificial small intestines that realistically mimic in vivo systems will enable vast improvement of our understanding of the human gut and its impact on human health. Synthetic in vitro models can control specific parameters, including (but not limited to) cell types, fluid flow, nutrient profiles and gaseous exchange. They are also “open” systems, enabling access to chemical and physiological information. In this work, we demonstrate the importance of gut surface topography and fluid flow dynamics which are shown to impact epithelial cell growth, proliferation and intestinal cell function. We have constructed a small intestinal bioreactor using 3-D printing and polymeric scaffolds that mimic the 3-D topography of the intestine and its fluid flow. Our results indicate that TEER measurements, which are typically high in static 2-D Transwell apparatuses, is lower in the presence of liquid sheer and 3-D topography compared to a flat scaffold and static conditions. There was also increased cell proliferation and discovered localized regions of elevated apoptosis, specifically at the tips of the villi, where there is highest sheer. Similarly, glucose was actively transported (as opposed to passive) and at higher rates under flow.