Characterization of drug permeability in Caco-2 monolayers by mass spectrometry on a membrane-based microfluidic device

Membrane-based microfluidic systems for medical and biological applications

Microfluidic devices with integrated membranes that enable control of mass transport in constrained environments have shown considerable growth over the last decade. Membranes are a key component in several industrial processes such as chemical, pharmaceutical, biotechnological, food, and metallurgy separation processes as well as waste management applications, allowing for modular and compact systems. Moreover, the miniaturization of a process through microfluidic devices leads to process intensification together with reagents, waste and cost reduction, and energy and space savings. The combination of membrane technology and microfluidic devices allows therefore magnification of their respective advantages, providing more valuable solutions not only for industrial processes but also for reproducing biological processes. This review focuses on membrane-based microfluidic devices for biomedical science with an emphasis on microfluidic artificial organs and organs-on-chip. We provide the basic concepts of membrane technology and the laws governing mass transport. The role of the membrane in biomedical microfluidic devices, along with the required properties, available materials, and current challenges are summarized. We believe that the present review may be a starting point and a resource for researchers who aim to replicate a biological phenomenon on-chip by applying membrane technology, for moving forward the biomedical applications.

Parathyroid-on-a-chip simulating parathyroid hormone secretion in response to calcium concentration

The parathyroid gland is an endocrine organ that plays a crucial role in regulating calcium levels in blood serum through the secretion of parathyroid hormone (PTH). Hypoparathyroidism is a chronic disease that can occur due to parathyroid defects, but due to the difficulty of creating animal models of this disease or obtaining human normal parathyroid cells, the evaluation of parathyroid functionality for drug development is limited. Although parathyroid-like cells that secrete PTH have recently been reported, their functionality may be overestimated using traditional culture methods that lack in vivo similarities, particularly vascularization. To overcome these limitations, we obtained parathyroid organoids from tonsil-derived mesenchymal stem cells (TMSCs) and fabricated a parathyroid-on-a-chip, capable of simulating PTH secretion based on calcium concentration. This chip exhibited differences in PTH secretion according to calcium concentration and secreted PTH within the range of normal serum levels. In addition, branches of organoids, which are difficult to observe in animal models, were observed in this chip. This could serve as a guideline for successful engraftment in implantation therapies in the future.

Three-Dimensional Scaffolds for Intestinal Cell Culture: Fabrication, Utilization, and Prospects

The intestine is a visceral organ that integrates absorption, metabolism, and immunity, which is vulnerable to external stimulus. Researchers in the fields such as food science, immunology, and pharmacology have committed to developing appropriate in vitro intestinal cell models to study the intestinal absorption and metabolism mechanisms of various nutrients and drugs, or pathogenesis of intestinal diseases. In the past three decades, the intestinal cell models have undergone a significant transformation from conventional two-dimensional cultures to three-dimensional (3D) systems, and the achievements of 3D cell culture have been greatly contributed by the fabrication of different scaffolds. In this review, we first introduce the developing trend of existing intestinal models. Then, four types of scaffolds, including Transwell, hydrogel, tubular scaffolds, and intestine-on-a-chip, are discussed for their 3D structure, composition, advantages, and limitations in the establishment of intestinal cell models. Excitingly, some of the in vitro intestinal cell models based on these scaffolds could successfully mimic the 3D structure, microenvironment, mechanical peristalsis, fluid system, signaling gradients, or other important aspects of the original human intestine. Furthermore, we discuss the potential applications of the intestinal cell models in drug screening, disease modeling, and even regenerative repair of intestinal tissues. This review presents an overview of state-of-the-art scaffold-based cell models within the context of intestines, and highlights their major advances and applications contributing to a better knowledge of intestinal diseases. Impact statement The intestine tract is crucial in the absorption and metabolism of nutrients and drugs, as well as immune responses against external pathogens or antigens in a complex microenvironment. The appropriate experimental cell model in vitro is needed for in-depth studies of intestines, due to the limitation of animal models in dynamic control and real-time assessment of key intestinal physiological and pathological processes, as well as the "R" principles in laboratory animal experiments. Three-dimensional (3D) scaffold-based cell cultivation has become a developing tendency because of the superior cell proliferation and differentiation and more physiologically relevant environment supported by the customized 3D scaffolds. In this review, we summarize four types of up-to-date 3D cell culture scaffolds fabricated by various materials and techniques for a better recapitulation of some essential physiological and functional characteristics of original intestines compared to conventional cell models. These emerging 3D intestinal models have shown promising results in not only evaluating the pharmacokinetic characteristics, security, and effectiveness of drugs, but also studying the pathological mechanisms of intestinal diseases at cellular and molecular levels. Importantly, the weakness of the representative 3D models for intestines is also discussed.

Trends in 3D models of inflammatory bowel disease

Inflammatory bowel disease (IBD) encompasses a set of chronic inflammatory conditions, namely Crohn's disease and ulcerative colitis. Despite all advances in the management of IBD, a definitive cure is not available, largely due to a lack of a holistic understanding of its etiology and pathophysiology. Several in vitro, in vivo, and ex vivo models have been developed over the past few decades in order to abbreviate remaining gaps. The establishment of reliable and predictable in vitro intestinal inflammation models may indeed provide valuable tools to expedite and validate the development of therapies for IBD. Three-dimensional (3D) models provide a more accurate representation of the different layers of the intestine, contributing to a stronger impact on drug screening and research on intestinal inflammation, and bridging the gap between in vitro and in vivo research. This work provides a critical overview on the state-of-the-art on existing 3D models of intestinal inflammation and discusses the remaining challenges, providing insights on possible pathways towards achieving IBD mimetic models. We also address some of the main challenges faced by implementing cell culture models in IBD research while bearing in mind clinical translational aspects.

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.

Technological advances for analyzing the content of organ-on-a-chip by mass spectrometry

Three-dimensional (3D) cell cultures, including organ-on-a-chip (OOC) devices, offer the possibility to mimic human physiology conditions better than 2D models. The organ-on-a-chip devices have a wide range of applications, including mechanical studies, functional validation, and toxicology investigations. Despite many advances in this field, the major challenge with the use of organ-on-a-chips relies on the lack of online analysis methods preventing the real-time observation of cultured cells. Mass spectrometry is a promising analytical technique for real-time analysis of cell excretes from organ-on-a-chip models. This is due to its high sensitivity, selectivity, and ability to tentatively identify a large variety of unknown compounds, ranging from metabolites, lipids, and peptides to proteins. However, the hyphenation of organ-on-a-chip with MS is largely hampered by the nature of the media used, and the presence of nonvolatile buffers. This in turn stalls the straightforward and online connection of organ-on-a-chip outlet to MS. To overcome this challenge, multiple advances have been made to pre-treat samples right after organ-on-a-chip and just before MS. In this review, we summarised these technological advances and exhaustively evaluated their benefits and shortcomings for successful hyphenation of organ-on-a-chip with MS.

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.

Simulating the Effect of Gut Microbiome on Cancer Cell Growth Using a Microfluidic Device

The imbalance in the gut microbiome plays a vital role in the progression of many diseases, including cancer, due to increased inflammation in the body. Since gut microbiome-induced inflammation can serve as a novel therapeutic strategy, there is an increasing need to identify novel approaches to investigate the effect of inflammation instigated by gut microbiome on cancer cells. However, there are limited biomimetic co-culture systems that allow testing of the causal relationship of the microbiome on cancer cells. Here we developed a microfluidic chip that can simulate the interaction of the gut microbiome and cancer cells to investigate the effects of bacteria and inflammatory stress on cancer cells in vitro. To test the microfluidic chip, we used colorectal cancer cells, as an increased microbiome abundance has been associated with poor outcomes in colorectal cancer. We cultured colorectal cancer cells with Bacillus bacteria or lipopolysaccharide (LPS), a purified bacterial membrane that induces a significant inflammatory response, in the microfluidic device. Our results showed that both LPS and Bacillus significantly accelerated the growth of colorectal cancer cells, therefore supporting that the increased presence of certain bacteria promotes cancer cell growth. The microfluidic device included in this study may have significant implications in identifying new treatments for various cancer types in the future.

Engineered Biomimetic Membranes for Organ-on-a-Chip

Organ-on-a-chip (OOC) systems are engineered nanobiosystems to mimic the physiochemical environment of a specific organ in the body. Among various components of OOC systems, biomimetic membranes have been regarded as one of the most important key components to develop controllable biomimetic bioanalysis systems. Here, we review the preparation and characterization of biomimetic membranes in comparison with the features of the extracellular matrix. After that, we review and discuss the latest applications of engineered biomimetic membranes to fabricate various organs on a chip, such as liver, kidney, intestine, lung, skin, heart, vasculature and blood vessels, brain, and multiorgans with perspectives for further biomedical applications.

Rapid Customization and Manipulation Mechanism of Micro-Droplet Chip for 3D Cell Culture

A full PDMS micro-droplet chip for 3D cell culture was prepared by using SLA light-curing 3D printing technology. This technology can quickly customize various chips required for experiments, saving time and capital costs for experiments. Moreover, an injection molding method was used to prepare the full PDMS chip, and the convex mold was prepared by light-curing 3D printing technology. Compared with the traditional preparation process of micro-droplet chips, the use of 3D printing technology to prepare micro-droplet chips can save manufacturing and time costs. The different ratios of PDMS substrate and cover sheet and the material for making the convex mold can improve the bonding strength and power of the micro-droplet chip. Use the prepared micro-droplet chip to carry out micro-droplet forming and manipulation experiments. Aimed to the performance of the full PDMS micro-droplet chip in biological culture was verified by using a solution such as chondrocyte suspension, and the control of the micro-droplet was achieved by controlling the flow rate of the dispersed phase and continuous phase. Experimental verification shows that the designed chip can meet the requirements of experiments, and it can be observed that the micro-droplets of sodium alginate and the calcium chloride solution are cross-linked into microspheres with three-dimensional (3D) structures. These microspheres are fixed on a biological scaffold made of calcium silicate and polyvinyl alcohol. Subsequently, the state of the cells after different time cultures was observed, and it was observed that the chondrocytes grew well in the microsphere droplets. The proposed method has fine control over the microenvironment and accurate droplet size manipulation provided by fluid flow compared to existing studies.

Organotypic and Microphysiological Human Tissue Models for Drug Discovery and Development-Current State-of-the-Art and Future Perspectives

The number of successful drug development projects has been stagnant for decades despite major breakthroughs in chemistry, molecular biology, and genetics. Unreliable target identification and poor translatability of preclinical models have been identified as major causes of failure. To improve predictions of clinical efficacy and safety, interest has shifted to three-dimensional culture methods in which human cells can retain many physiologically and functionally relevant phenotypes for extended periods of time. Here, we review the state of the art of available organotypic culture techniques and critically review emerging models of human tissues with key importance for pharmacokinetics, pharmacodynamics, and toxicity. In addition, developments in bioprinting and microfluidic multiorgan cultures to emulate systemic drug disposition are summarized. We close by highlighting important trends regarding the fabrication of organotypic culture platforms and the choice of platform material to limit drug absorption and polymer leaching while supporting the phenotypic maintenance of cultured cells and allowing for scalable device fabrication. We conclude that organotypic and microphysiological human tissue models constitute promising systems to promote drug discovery and development by facilitating drug target identification and improving the preclinical evaluation of drug toxicity and pharmacokinetics. There is, however, a critical need for further validation, benchmarking, and consolidation efforts ideally conducted in intersectoral multicenter settings to accelerate acceptance of these novel models as reliable tools for translational pharmacology and toxicology. SIGNIFICANCE STATEMENT: Organotypic and microphysiological culture of human cells has emerged as a promising tool for preclinical drug discovery and development that might be able to narrow the translation gap. This review discusses recent technological and methodological advancements and the use of these systems for hit discovery and the evaluation of toxicity, clearance, and absorption of lead compounds.

The Progress of Intestinal Epithelial Models from Cell Lines to Gut-On-Chip

Over the past years, several preclinical in vitro and ex vivo models have been developed that helped to understand some of the critical aspects of intestinal functions in health and disease such as inflammatory bowel disease (IBD). However, the translation to the human in vivo situation remains problematic. The main reason for this is that these approaches fail to fully reflect the multifactorial and complex in vivo environment (e.g., including microbiota, nutrition, and immune response) in the gut system. Although conventional models such as cell lines, Ussing chamber, and the everted sac are still used, increasingly more sophisticated intestinal models have been developed over the past years including organoids, InTESTine™ and microfluidic gut-on-chip. In this review, we gathered the most recent insights on the setup, advantages, limitations, and future perspectives of most frequently used in vitro and ex vivo models to study intestinal physiology and functions in health and disease.

The Inhibitory Activity of Curcumin on P-Glycoprotein and Its Uptake by and Efflux from LS180 Cells Is Not Affected by Its Galenic Formulation

The biological activities of curcumin in humans, including its antioxidative and anti-inflammatory functions, are limited by its naturally low bioavailability. Different formulation strategies have been developed, but the uptake of curcumin from these galenic formulations into and efflux from intestinal cells, which may be critical processes limiting bioavailability, have not been directly compared. Furthermore, little is known about their effect on P-glycoprotein activity, an important determinant of the pharmacokinetics of potentially co-administered drugs. P-glycoprotein activity was determined in LS180 cells, incubated with 30 or 60 µmol/L of curcumin in the form of seven different formulations or native curcuma extract for 1 h. All formulations inhibited P-glycoprotein activity at both concentrations. Curcumin uptake, after 1 h incubation of LS180 cells with the formulations (60 µmol/L), showed significant variability but no consistent effects. After 1 h pre-treatment with the formulations and further 8 h with curcumin-free medium, curcumin in cell culture supernatants, reflecting the efflux, differed between individual formulations, again without a clear effect. In conclusion, curcumin inhibits P-glycoprotein activity independently of its formulation. Its uptake by and efflux from intestinal cells was not significantly different between formulations, indicating that these processes are not important regulatory points for its bioavailability.

In vitro models to evaluate ingestible devices: Present status and current trends

Orally ingestible medical devices offer significant opportunity in the diagnosis and treatment of gastrointestinal conditions. Their development necessitates the use of models that simulate the gastrointestinal environment on both a macro and micro scale. An evolution in scientific technology has enabled a wide range of in vitro, ex vivo and in vivo models to be developed that replicate the gastrointestinal tract. This review describes the landscape of the existing range of in vitro tools that are available to characterize ingestible devices. Models are presented with details on their benefits and limitations with regards to the evaluation of ingestible devices and examples of their use in the evaluation of such devices is presented where available. The multitude of models available provides a suite of tools that can be used in the evaluation of ingestible devices that should be selected on the functionality of the device and the mechanism of its function.

Increasing Post-Digestive Solubility of Curcumin Is the Most Successful Strategy to Improve its Oral Bioavailability: A Randomized Cross-Over Trial in Healthy Adults and In Vitro Bioaccessibility Experiments

SCOPE

Different mechanistic approaches to improve the low oral bioavailability of curcumin have been developed, but not yet directly compared in humans.

METHODS AND RESULTS

In a randomized, double-blind, cross-over trial with 12 healthy adults, the 24 h pharmacokinetics of a single dose of 207 mg curcumin is compared from the following formulations: native, liposomes, with turmeric oils, with adjuvants (including piperine), submicron-particles, phytosomes, γ-cyclodextrin complexes, and micelles. No free, but only conjugated curcumin is detected in all subjects. Compared to native curcumin, a significant increase in the area under the plasma concentration-time curve is observed for micellar curcumin (57-fold) and the curcumin-γ-cyclodextrin complex (30-fold) only. In vitro digestive stability, solubility, and micellization efficiency of micellar curcumin (100%, 80%, and 55%) and curcumin-γ-cyclodextrin complex (73%, 33%, and 23%) are higher compared to all other formulations (<72%, <8%, and <4%). The transport efficiencies through Caco-2 cell monolayers of curcumin from the digested mixed-micellar fractions did not differ significantly.

CONCLUSION

The improved oral bioavailability of micellar curcumin, and to a lesser extent of γ-cyclodextrin curcumin complexes, appears to be facilitated by increased post-digestive stability and solubility, whereas strategies targeting post-absorptive processes, including inhibition of biotransformation, appear ineffective.

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.

In vitro models replicating the human intestinal epithelium for absorption and metabolism studies: A systematic review

Absorption, distribution, metabolism and excretion (ADME) studies represent a fundamental step in the early stages of drug discovery. In particular, the absorption of orally administered drugs, which occurs at the intestinal level, has gained attention since poor oral bioavailability often led to failures for new drug approval. In this context, several in vitro preclinical models have been recently developed and optimized to better resemble human physiology in the lab and serve as an animal alternative to accomplish the 3Rs principles. However, numerous models are ineffective in recapitulating the key features of the human small intestine epithelium and lack of prediction potential for drug absorption and metabolism during the preclinical stage. In this review, we provide an overview of in vitro models aimed at mimicking the intestinal barrier for pharmaceutical screening. After briefly describing how the human small intestine works, we present i) conventional 2D synthetic and cell-based systems, ii) 3D models replicating the main features of the intestinal architecture, iii) micro-physiological systems (MPSs) reproducing the dynamic stimuli to which cells are exposed in the native microenvironment. In this review, we will highlight the benefits and drawbacks of the leading intestinal models used for drug absorption and metabolism studies.

Multi-organ-on-a-chip for pharmacokinetics and toxicokinetic study of drugs

Introduction: Accurate prediction of pharmacokinetic (PK) and toxicokinetics (TK) of drugs is imperative for successful development of new pharmaceutics. Although conventional in vitro methods for predicting the PK and TK of drugs are well established, limitations still exist and more advanced chip-based in vitro platforms combined with mathematical models can help researchers overcome the limitations. Areas covered: We will review recent progress in the development of multi-organ-on-a-chip platforms for predicting PK and TK of drugs, as well as mathematical approaches that can be combined with these platforms for experiment design, data analysis and in vitro-in vivo extrapolation (IVIVE) for application to humans. Expert opinion: Although there remain some challenges to be addressed, the remarkable progress in the area of multi-organ-on-a-chip in recent years indicate that we will see tangible outcomes that can be utilized in the pharmaceutical industry in near future.

A versatile, compartmentalised gut-on-a-chip system for pharmacological and toxicological analyses

A novel, integrated, in vitro gastrointestinal (GI) system is presented to study oral bioavailability parameters of small molecules. Three compartments were combined into one hyphenated, flow-through set-up. In the first compartment, a compound was exposed dynamically to enzymatic digestion in three consecutive microreactors, mimicking the processes of the mouth, stomach, and intestine. The resulting solution (chyme) continued to the second compartment, a flow-through barrier model of the intestinal epithelium allowing absorption of the compound and metabolites thereof. The composition of the effluents from the barrier model were analysed either offline by electrospray-ionisation-mass spectrometry (ESI-MS), or online in the final compartment using chip-based ESI-MS. Two model drugs, omeprazole and verapamil, were used to test the integrated model. Omeprazole was shown to be broken down upon treatment with gastric acid, but reached the cell barrier unharmed when introduced to the system in a manner emulating an enteric-coated formulation. In contrast, verapamil was unaffected by digestion. Finally, a reduced uptake of verapamil was observed when verapamil was introduced to the system dissolved in apple juice, a simple food matrix. It is envisaged that this integrated, compartmentalised GI system has potential for enabling future research in the fields of pharmacology, toxicology, and nutrition.

Cell Analysis on Microfluidics Combined with Mass Spectrometry

Cell analysis is of great significance for the exploration of human diseases and health. However, there are not many techniques for high-throughput cell analysis in the simulated cell microenvironment. The high designability of the microfluidic chip enables multiple kinds of cells to be co-cultured on the chip, with other functions such as sample preprocessing and cell manipulation. Mass spectrometry (MS) can detect a large number of biomolecules without labelling. Therefore, the application of the microfluidic chip coupled with MS has represented a major branch of cell analysis over the past decades. Here, we concisely introduce various microfluidic devices coupled with MS used for cell analysis. The main functions of microfluidic devices are described first, followed by introductions of different interfaces with different types of MS. Then, their various applications in cell analysis are highlighted, with an emphasis on cell metabolism, drug screening, and signal transduction. Current limitations and prospective trends of microfluidics coupled with MS are discussed at the end.

A Fluidic Isolation-Assisted Homogeneous-Flow-Pressure Chip-Solid Phase Extraction-Mass Spectrometry System for Online Dynamic Monitoring of 25-Hydroxyvitamin D3 Biotransformation in Cells

It is well known that cell can response to various chemical and mechanical stimuli. Therefore, flow pressure variation induced by sample loading and elution should be small enough to ignore the physical impact on cells when we use a Chip-SPE-MS system for cells. However, most existent Chip-SPE-MS systems ignored the pressure alternation because it is extremely difficult to develop a homogeneous-flow-pressure hyphenated module. Herein, we developed an interesting fluidic isolation-assisted homogeneous-flow-pressure Chip-SPE-MS system and demonstrated that it is adequate for online high-throughput determination and quantification of the 25-hydroxyvitamin D₃ (25(OH)D₃) biotransformation in different cells. Briefly, the homogeneous ambient flow pressure is achieved by fluidic isolation between the cell culture channel and the SPE column, and an automatic sampling probe could accomplish the sample loading and dispensing to fulfill online pretreatment of the sample. Through this new system, the expression levels of 24,25-dihydroxyvitamin D₃ (24,25(OH)₂D₃) can be determined in real time with a detection limit of 2.54 nM. In addition, the results revealed that 25(OH)D₃ metabolic activity differed significantly between normal L-02 cells and cancerous HepG2 cells. Treatment of L-02 cells with a high dose of 25(OH)D₃ was found to increase significant formation of 24,25(OH)₂D₃, but this change was not apparent in HepG2 cells. The presented system promises to be a versatile tool for online accurate molecule biotransformation investigation and drug screening processes.

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-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.

Blood-Brain Barrier Permeable Chitosan Oligosaccharides Interfere with β-Amyloid Aggregation and Alleviate β-Amyloid Protein Mediated Neurotoxicity and Neuroinflammation in a Dose- and Degree of Polymerization-Dependent Manner

It is proven that β-amyloid (Aβ) aggregates containing cross-β-sheet structures led to oxidative stress, neuroinflammation, and neuronal loss via multiple pathways. Therefore, reduction of Aβ neurotoxicity via inhibiting aggregation of Aβ or dissociating toxic Aβ aggregates into nontoxic forms might be effective therapeutic methods for Alzheimer's disease (AD) treatment. This study was designed to explore interference of chitosan oligosaccharides (COS) on β-(1-42)-amyloid protein (Aβ42) aggregation and Aβ42-induced cytotoxicity. Here it was demonstrated that COS showed good blood-brain barrier (BBB) penetration ability in vitro and in vivo. The experimental results showed that COS efficiently interfered with Aβ42 aggregation in dose- and degree of polymerization (DP)-dependent manners, and COS monomer with DP6 showed the best effect on preventing conformational transition into β-sheet-rich structures. Based on the binding affinity analysis by microscale thermophoresis (MST), it was confirmed that COS could directly bind with Aβ42 in a DP-dependent manner. Our findings demonstrated that different performance of COS monomers with different DPs against Aβ42 assembly was, to some extent, attributable to their different binding capacities with Aβ42. As a result, COS significantly ameliorated Aβ42-induced cytotoxicity. Taken together, our studies would point towards a potential role of COS in treatment of AD.

Microfluidic lumen-based systems for advancing tubular organ modeling

Microfluidic lumen-based systems are microscale models that recapitulate the anatomy and physiology of tubular organs. These technologies can mimic human pathophysiology and predict drug response, having profound implications for drug discovery and development. Herein, we review progress in the development of microfluidic lumen-based models from the 2000s to the present. The core of the review discusses models for mimicking blood vessels, the respiratory tract, the gastrointestinal tract, renal tubules, and liver sinusoids, and their application to modeling organ-specific diseases. We also highlight emerging application areas, such as the lymphatic system, and close the review discussing potential future directions.

Rapid Fabrication of Membrane-Integrated Thermoplastic Elastomer Microfluidic Devices

Leveraging the advantageous material properties of recently developed soft thermoplastic elastomer materials, this work presents the facile and rapid fabrication of composite membrane-integrated microfluidic devices consisting of Flexdym^TM polymer and commercially available porous polycarbonate membranes. The three-layer devices can be fabricated in under 2.5 h, consisting of a 2-min hot embossing cycle, conformal contact between device layers and a low-temperature baking step. The strength of the Flexdym^TM-polycarbonate seal was characterized using a specialized microfluidic delamination device and an automated pressure controller configuration, offering a standardized and high-throughput method of microfluidic burst testing. Given a minimum bonding distance of 200 μm, the materials showed bonding that reliably withstood pressures of 500 mbar and above, which is sufficient for most microfluidic cell culture applications. Bonding was also stable when subjected to long term pressurization (10 h) and repeated use (10,000 pressure cycles). Cell culture trials confirmed good cell adhesion and sustained culture of human dermal fibroblasts on a polycarbonate membrane inside the device channels over the course of one week. In comparison to existing porous membrane-based microfluidic platforms of this configuration, most often made of polydimethylsiloxane (PDMS), these devices offer a streamlined fabrication methodology with materials having favourable properties for cell culture applications and the potential for implementation in barrier model organ-on-chips.

The Past, Present and Future of Intestinal In Vitro Cell Systems for Drug Absorption Studies

The intestinal epithelium acts as a selective barrier for the absorption of water, nutrients and orally administered drugs. To evaluate the gastrointestinal permeability of a candidate molecule, scientists and drug developers have a multitude of cell culture models at their disposal. Static transwell cultures constitute the most extensively characterized intestinal in vitro system and can accurately categorize molecules into low, intermediate and high permeability compounds. However, they lack key aspects of intestinal physiology, including the cellular complexity of the intestinal epithelium, flow, mechanical strain, or interactions with intestinal mucus and microbes. To emulate these features, a variety of different culture paradigms, including microfluidic chips, organoids and intestinal slice cultures have been developed. Here, we provide an updated overview of intestinal in vitro cell culture systems and critically review their suitability for drug absorption studies. The available data show that these advanced culture models offer impressive possibilities for emulating intestinal complexity. However, there is a paucity of systematic absorption studies and benchmarking data and it remains unclear whether the increase in model complexity and costs translates into improved drug permeability predictions. In the absence of such data, conventional static transwell cultures remain the current gold-standard paradigm for drug absorption studies.

Establishment and Application of Peristaltic Human Gut-Vessel Microsystem for Studying Host-Microbial Interaction

Intestinal floras influence a lot of biological functions of the organism. Although animal model are strong tools for researches on the relationship between host and microbe, a physiologically relevant in vitro human gut model was still required. Here, a novel human gut-vessel microfluidic system was established to study the host–microbial interaction. Peristaltic motion of the cells on the chip was driven by a pneumatic pump. When intestinal epithelial cells (Caco2) were co-cultured with vascular endothelial cells (HUVECs) on the peristaltic microfluidic chip, Caco2 showed normal barrier and absorption functions after 5 days cultivation, which generally took 21 days in static Transwell models. Intestinal microvilli and glycocalyx layer were seen after 4 days cultivation, and Lactobacillus casei was successfully co-cultured for a week in the intestinal cavity. A model for intestinal damage and inflammatory responses caused by E. coli was set up on this chip, which were successfully suppressed by Lactobacillus casei or antibiotic. In summary, this human gut-vessel microfluidic system showed a good potential for investigating the host–microbial interaction and the effect and mechanism of microbiome on intestinal diseases in vitro.

Microphysiological systems for ADME-related applications: current status and recommendations for system development and characterization

Over the last decade, progress has been made on the development of microphysiological systems (MPS) for absorption, distribution, metabolism, and excretion (ADME) applications. Central to this progress has been proof of concept data generated by academic and industrial institutions followed by broader characterization studies, which provide evidence for scalability and applicability to drug discovery and development. In this review, we describe some of the advances made for specific tissue MPS and outline the desired functionality for such systems, which are likely to make them applicable for practical use in the pharmaceutical industry. Single organ MPS platforms will be valuable for modelling tissue-specific functions. However, dynamic organ crosstalk, especially in the context of disease or toxicity, can only be obtained with the use of inter-linked MPS models which will enable scientists to address questions at the intersection of pharmacokinetics (PK) and efficacy, or PK and toxicity. In the future, successful application of MPS platforms that closely mimic human physiology may ultimately reduce the need for animal models to predict ADME outcomes and decrease the overall risk and cost associated with drug development.

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.

Microphysiological Systems: Next Generation Systems for Assessing Toxicity and Therapeutic Effects of Nanomaterials

Microphysiological systems, also known as organ-on-a-chip platforms, show promise for the development of new testing methods that can be more accurate than both conventional two-dimensional cultures and costly animal studies. The development of more intricate microphysiological systems can help to better mimic the human physiology and highlight the systemic effects of different drugs and materials. Nanomaterials are among a technologically important class of materials used for diagnostic, therapeutic, and monitoring purposes; all of which and can be tested using new organ-on-a-chip systems. In addition, the toxicity of nanomaterials which have entered the body from ambient air or diet can have deleterious effects on various body systems. This in turn can be studied in newly developed microphysiological systems. While organ-on-a-chip models can be useful, they cannot pick up secondary and systemic toxicity. Thus, the utilization of multi-organ-on-a-chip systems for advancing nanotechnology will largely be reflected in the future of drug development, toxicology studies and precision medicine. Various aspects of related studies, current challenges, and future perspectives are discussed in this paper.

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.

Flat and microstructured polymeric membranes in organs-on-chips

In recent years, organs-on-chips (OOCs) have been developed to meet the desire for more realistic in vitro cell culture models. These systems introduce microfluidics, mechanical stretch and other physiological stimuli to in vitro models, thereby significantly enhancing their descriptive power. In most OOCs, porous polymeric membranes are used as substrates for cell culture. The polymeric material, morphology and shape of these membranes are often suboptimal, despite their importance for achieving ideal cell functionality such as cell-cell interaction and differentiation. The currently used membranes are flat and thus do not account for the shape and surface morphology of a tissue. Moreover, the polymers used for fabrication of these membranes often lack relevant characteristics, such as mechanical properties matching the tissue to be developed and/or cytocompatibility. Recently, innovative techniques have been reported for fabrication of porous membranes with suitable porosity, shape and surface morphology matching the requirements of OOCs. In this paper, we review the state of the art for developing these membranes and discuss their application in OOCs.

Inhibition of Liver Tumor Cell Metastasis by Partially Acetylated Chitosan Oligosaccharide on A Tumor-Vessel Microsystem

Chitooligosaccharides (COS), the only cationic oligosaccharide in nature, have been demonstrated to have anti-tumor activity. However, the inhibitory effects of COS on different stages of tumor metastasis are still unknown, and it is not clear what stage(s) of tumor metastasis COS targeted. To study the inhibitory effects of a new partially acetylated chitooligosaccharide (paCOS) with fraction of acetylation (FA) 0.46 on each phase of liver cancer cell metastasis, a dynamic tumor-vessel microsystem undergoing physiological flow was leveraged. paCOS (FA = 0.46) significantly inhibited proliferation of HepG2 cells through vascular absorption on the chip, and inhibited migration of HepG2 cells by inhibiting the formation of pseudopod in liver tumor cells. It was also found that paCOS at 10 μg/mL had a stronger inhibitory effect on liver tumor cells invading blood vessels than that of paCOS at 100 μg/mL, and paCOS at 100 μg/mL, which had a significant destructive effect on tumor vascular growth and barrier function. Moreover, paCOS reduced the number of liver tumor cells adhering onto the surface of HUVECs layer after 3 h of treatment. Therefore, the results revealed that paCOS had considerable potential as drugs for anti-tumor metastasis.

Establishment and application of a dynamic tumor-vessel microsystem for studying different stages of tumor metastasis and evaluating anti-tumor drugs

Tumor metastasis is one of the main causes of cancer-related death, and it is difficult to study the whole process of tumor metastasis in vivo due to the complex physiological environment in the body. Therefore, it's crucial to develop simple and physiologically relevant in vitro cancer models to study the metastasis process, especially different phases of tumor metastasis. A novel microfluidic tumor-vessel co-culture system was established to reproduce the different phases of cancer metastasis (proliferation, migration, intravasation and adherence) individually in vitro for the first time. It was observed that blood vessels with fluid flow had big impact on metastasis of liver cancer cells HepG2 and breast ones MDA-MB-231. In particular, it was found that both HepG2 and MDA-MB-231 cells migrated in the direction of “blood flow”. Furthermore, MDA-MB-231 cells invaded through paracellular mode disrupting the intercellular endothelial junctions, whereas HepG2 cells engaged in transcellular intravasation through transcellular process. Compared with traditional assays, much more potent inhibition of 5-fluorouracil (5-Fu) on different phases of tumor metastasis was observed on the microsystem. In summary, the microfluidic device yielded abundant information about each phase of tumor metastasis, and would provide a powerful platform for use in drug screening, toxicology studies, and personalized medicine in future.

The different stages of the cancer metastasis were reproduced individually on a novel tumor-vessel co-culture microsystem.