Drug permeability assay using microhole-trapped cells in a microfluidic device

A novel membrane-on-chip guides morphogenesis for the reconstruction of the intestinal crypt-villus axis

Reconstructing the microscale villous organisation and functionality of the small intestine is essential for developingin vitroplatforms tailored for absorption studies as well as for investigating intestinal morphogenesis in development and disease. However, the current fabrication techniques able to mimic the villus-crypt axis poses significant challenges in terms of reconstruction of the complex 3D microarchitecture. These challenges extend beyond mere structural intricacies to encompass the incorporation of diverse cell types and the management of intricate fluid dynamics within the system. Here, we introduce a novel microfluidic device calledIn-Crypts, which integrates a cell-instructive membrane aimed at inducing and guiding Caco-2 cells morphogenesis. Patterned topographical cues embossed onto the porous membrane induce the formation of a well-organized intestinal epithelium, characterized by proliferating crypt-like domains and differentiated villus-like regions. Notably, our cell-instructive porous membrane effectively sustains stem cells development, faithfully replicating the niche environment ofin vivointestinal crypts thus mirroring the cell biogeography observedin vivo. Moreover, by introducing dynamic fluid flow, we provide a faithful recapitulation of the native microenvironmental shear stress experienced by the intestinal epithelium. This stress plays a crucial role in influencing cell behaviour, differentiation, and overall functionality, thus offering a highly realistic model for studying intestinal physiology and pathology. The resulting intestinal epithelium exhibits significantly denser regions of mucus and microvilli, characteristic typically absent in static cultures, upregulating more than 1.5 of the amount expressed in the classical flattened configuration, enhanced epithelial cell differentiation and increased adsorptive surface area. Hence, the innovative design ofIn-Cryptsproves the critical role of employing a cell-instructive membrane in argument the physiological relevance of organs-on-chips. This aspect, among others, will contribute to a more comprehensive understanding of organism function, directly impacting drug discovery and development.

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.

Advances and challenges in organ-on-chip technology: toward mimicking human physiology and disease in vitro

Organs-on-chips have been tissues or three-dimensional (3D) mini-organs that comprise numerous cell types and have been produced on microfluidic chips to imitate the complicated structures and interactions of diverse cell types and organs under controlled circumstances. Several morphological and physiological distinctions exist between traditional 2D cultures, animal models, and the growing popular 3D cultures. On the other hand, animal models might not accurately simulate human toxicity because of physiological variations and interspecies metabolic capability. The on-chip technique allows for observing and understanding the process and alterations occurring in metastases. The present study aimed to briefly overview single and multi-organ-on-chip techniques. The current study addresses each platform's essential benefits and characteristics and highlights recent developments in developing and utilizing technologies for single and multi-organs-on-chips. The study also discusses the drawbacks and constraints associated with these models, which include the requirement for standardized procedures and the difficulties of adding immune cells and other intricate biological elements. Finally, a comprehensive review demonstrated that the organs-on-chips approach has a potential way of investigating organ function and disease. The advancements in single and multi-organ-on-chip structures can potentially increase drug discovery and minimize dependency on animal models, resulting in improved therapies for human diseases.

Recent advances in cell-based in vitro models for predicting drug permeability across brain, intestinal, and pulmonary barriers

INTRODUCTION

Recent years have witnessed remarkable progress in the development of cell-based in vitro models aimed at predicting drug permeability, particularly focusing on replicating the barrier properties of the blood-brain barrier (BBB), intestinal epithelium, and lung epithelium.

AREA COVERED

This review provides an overview of 2D in vitro platforms, including monocultures and co-culture systems, highlighting their respective advantages and limitations. Additionally, it discusses tools and techniques utilized to overcome these limitations, paving the way for more accurate predictions of drug permeability. Furthermore, this review delves into emerging technologies, particularly microphysiological systems (MPS), encompassing static platforms such as organoids and dynamic platforms like microfluidic devices. Literature searches were performed using PubMed and Google Scholar. We focus on key terms such as in vitro permeability models, MPS, organoids, intestine, BBB, and lungs.

EXPERT OPINION

The potential of these MPS to mimic physiological conditions more closely offers promising avenues for drug permeability assessment. However, transitioning these advanced models from bench to industry requires rigorous validation against regulatory standards. Thus, there is a pressing need to validate MPS to industry and regulatory agency standards to exploit their potential in drug permeability prediction fully. This review underscores the importance of such validation processes to facilitate the translation of these innovative technologies into routine pharmaceutical practice.

Advances in the Evaluation of Gastrointestinal Absorption Considering the Mucus Layer

Because of the increasing sophistication of formulation technology and the increasing polymerization of compounds directed toward undruggable drug targets, the influence of the mucus layer on gastrointestinal drug absorption has received renewed attention. Therefore, understanding the complex structure of the mucus layer containing highly glycosylated glycoprotein mucins, lipids bound to the mucins, and water held by glycans interacting with each other is critical. Recent advances in cell culture and engineering techniques have led to the development of evaluation systems that closely mimic the ecological environment and have been applied to the evaluation of gastrointestinal drug absorption while considering the mucus layer. This review provides a better understanding of the mucus layer components and the gastrointestinal tract's biological defense barrier, selects an assessment system for drug absorption in the mucus layer based on evaluation objectives, and discusses the overview and features of each assessment system.

Challenges in Permeability Assessment for Oral Drug Product Development

Drug permeation across the intestinal epithelium is a prerequisite for successful oral drug delivery. The increased interest in oral administration of peptides, as well as poorly soluble and poorly permeable compounds such as drugs for targeted protein degradation, have made permeability a key parameter in oral drug product development. This review describes the various in vitro, in silico and in vivo methodologies that are applied to determine drug permeability in the human gastrointestinal tract and identifies how they are applied in the different stages of drug development. The various methods used to predict, estimate or measure permeability values, ranging from in silico and in vitro methods all the way to studies in animals and humans, are discussed with regard to their advantages, limitations and applications. A special focus is put on novel techniques such as computational approaches, gut-on-chip models and human tissue-based models, where significant progress has been made in the last few years. In addition, the impact of permeability estimations on PK predictions in PBPK modeling, the degree to which excipients can affect drug permeability in clinical studies and the requirements for colonic drug absorption are addressed.

Journey of organ on a chip technology and its role in future healthcare scenario

Organ on a chip refers to microengineered biomimetic system which reflects structural and functional characteristics of human tissue. It involves biomaterial technology, cell biology and engineering combined together in a miniaturized platform. Several models using different organs such as lungs on a chip, liver on a chip, kidney on a chip, heart on a chip, intestine on a chip and skin on a chip have been successfully developed. Food and Drug administration (FDA) has also shown confidence in this technology and has partnered with industries/institutes which are working with this technology. In this review, the concepts and applications of Organ on a chip model in different scientific domains including disease model development, drug screening, toxicology, pathogenesis study, efficacy testing and virology is discussed. It is envisaged that amalgamation of various organs on chip modules into a unified body on chip device is of utmost importance for diagnosis and treatment, especially considering the complications due to the ongoing COVID-19 pandemic. It is expected that the market demand for developing organ on chip devices to skyrocket in the near future.

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.

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.

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.

Microflow-Based Device for In Vitro and Ex Vivo Drug Permeability Studies

This paper presents a novel microflow-based concept for studying the permeability of in vitro cell models or ex vivo tissues. Using the proposed concept, we demonstrate how to maintain physiologically relevant test conditions and produce highly reproducible permeability values for a range (31) of drug compounds. The apparent permeability coefficients (Papp) showed excellent correlation (0.89) with the values from experiments performed with a conventional Ussing chamber. Additionally, the microflow-based concept produces notably more concentrated samples than the conventional Ussing chamber-based approach, despite the fact that more than 10 times smaller quantities of test compounds and biological membranes are needed in the microflow-based concept.

Oral delivery of protein-based therapeutics: Gastroprotective strategies, physiological barriers and in vitro permeability prediction

The number of biological molecules emerging as therapeutics is growing exponentially due to their higher specificity and tolerability profiles compared to small molecules. Despite this, their traditionally parenteral delivery often results in poor patient compliance and incomplete treatment. Current research is focussed on developing effective oral delivery strategies to facilitate administration of these biomolecules, however no universal method exists to simultaneously provide gastric protection as well as enhance transport across the gastrointestinal epithelium. Furthermore, for efficient formulation development it is imperative that we can reliably analyse permeability of biomolecules through the gastrointestinal tract, highlighting the importance of the continual development and ongoing evaluation of in vitro predictive permeability tools. Here, we review the physiological obstacles associated with peptide and protein delivery throughout the gastrointestinal tract. Furthermore, we highlight methods utilised to circumvent these barriers and promote improved intestinal permeability. Lastly, we explore in vitro models employed to predict epithelial transport. Key findings highlight the need to carefully understand gastrointestinal physiology, allowing specific engineering of oral delivery systems for biomolecules. Significant importance is placed upon understanding enzymatic degradation susceptibility as well as uptake mechanisms for particulate and protein-based therapeutics for the development of successful oral protein delivery platforms.

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.

Pharmacokinetic and pharmacodynamic insights from microfluidic intestine-on-a-chip models

Introduction: After administration, a drug undergoes absorption, distribution, metabolism, and elimination (ADME) before exerting its effect on the body. The combination of these process yields the pharmacokinetic (PK) and pharmacodynamic (PD) profiles of a drug. Although accurate prediction of PK and PD profiles is essential for drug development, conventional in vitro models are limited by their lack of physiological relevance. Recently, microtechnology-based in vitro model systems, termed 'organ-on-a-chip,' have emerged as a potential solution.Areas covered: Orally administered drugs are absorbed through the intestinal wall and transported to the liver before entering systemic circulation, which plays an important role in the PK and PD profiles. Recently developed, chip-based in vitro models can be useful models for simulating such processes and will be covered in this paper.Expert opinion: The potential of intestine-on-a-chip models combined with conventional PK-PD modeling has been demonstrated with promising preliminary results. However, there are several challenges to overcome. Development of the intestinal wall, integration of the gut microbiome, and the provision of an intestine-specific environment must be achieved to realize in vivo-like intestinal model and enhance the efficiency of drug development.

A Biomimetic Human Gut-on-a-Chip for Modeling Drug Metabolism in Intestine

Drug metabolism in the intestine is considered to substantially contribute to the overall first-pass metabolism, which has been neglected for a long time. It is highly desirable to develop a reliable model to evaluate drug metabolism in the intestine in vitro. In this work, we made the first attempt to develop a biomimetic human gut-on-a-chip for modeling drug metabolism in intestine. In this chip, constant flow, together with porous nitrocellulose membrane and collagen I, mimics an in vivo-like intestinal microenvironment. The Caco-2 cells grown in the chip formed a compact intestinal epithelial layer with continuous expression of the tight junction protein, ZO-1. Furthermore, higher gene expression of villin, sucrase-isomaltase, and alkaline phosphatase demonstrated that cells in the biomimetic human gut-on-a-chip device were more mature with near-physiological functions compared to the control on planar substrate. In particular, cellular metabolic activity was assessed on different substrates, indicating higher metabolic efficiency of ifosfamide and verapamil in the biomimetic human gut-on-a-chip model. Taken together, our results suggested that this biomimetic human gut-on-a-chip promoted the differentiation of intestinal cells with enhanced functionality by creating a biomimetic 3D microenvironment in vitro. It might offer a bioactive, low-cost, and flexible in vitro platform for studies on intestinal metabolism as well as preclinical drug development.

Implementation of a dynamic intestinal gut-on-a-chip barrier model for transport studies of lipophilic dioxin congeners

Novel microfluidic technologies allow the manufacture of in vitro organ-on-a-chip systems that hold great promise to adequately recapitulate the biophysical and functional complexity of organs found in vivo. In this study, a gut-on-a-chip model was developed aiming to study the potential cellular association and transport of food contaminants. Intestinal epithelial cells (Caco-2) were cultured on a porous polyester membrane that was tightly clamped between two glass slides to form two separate flow chambers. Glass syringes, polytetrafluoroethylene tubing and glass microfluidic chips were selected to minimize surface adsorption of the studied compounds (i.e. highly lipophilic dioxins), during the transport studies. Confocal microscopy studies revealed that, upon culturing under constant flow for 7 days, Caco-2 cells formed complete and polarized monolayers as observed after culturing for 21 days under static conditions in Transwells. We exposed Caco-2 monolayers in the chip and Transwell to a mixture of 17 dioxin congeners (7 polychlorinated dibenzo-p-dioxins and 10 polychlorinated dibenzofurans) for 24 h. Gas chromatography-high resolution mass spectrometry was used to assess the cellular association and transport of individual dioxin congeners across the Caco-2 cell monolayers. After 24 h, the amount of transported dioxin mixture was similar in both the dynamic gut-on-a-chip model and the static Transwell model. The transport of individual congeners corresponded with their number of chlorine atoms and substitution patterns as revealed by quantitative structure-property relationship modelling. These results show that the gut-on-a-chip model can be used, as well as the traditional static Transwell system, to study the cellular association and transport of lipophilic compounds like dioxins.

Static and Dynamic Permeability Assay for Hydrophilic Small Molecules Using a Planar Droplet Interface Bilayer

Because numerous drugs are administered through an oral route and primarily absorbed at the intestine, the prediction of drug permeability across an intestinal epithelial cell membrane has been a crucial issue in drug discovery. Thus, various in vitro permeability assays have been developed such as the Caco-2 assay, the parallel artificial membrane permeability assay (PAMPA), the phospholipid vesicle-based permeation assays (PVPA) and Permeapad. However, because of the time-consuming and quite expensive process for culturing cells in the Caco-2 assay and the unknown microscopic membrane structures of the other assays, a simpler yet more accurate and versatile technique is still required. Accordingly, we developed a new platform to measure the permeability of small molecules across a planar freestanding lipid bilayer with a well-defined area and structure. The lipid bilayer was constructed within a conventional UV spectrometer cell, and the transport of drug molecules across the bilayer was recorded by UV absorbance over time. We then computed the permeability from the time-dependent diffusion equation. We tested this assay for five exemplary hydrophilic drugs and compared their values with previously reported ones. We found that our assay has a much higher permeability compared to the other techniques, and this higher permeability is related to the thickness of the lipid bilayer. Also we were able to measure the dynamic permeability upon the addition of a membrane-disrupting surfactant demonstrating that our assay has the capability to detect real-time changes in permeability across the lipid bilayer.

Organ-on-a-Chip Technology for Reproducing Multiorgan Physiology

In the drug development process, the accurate prediction of drug efficacy and toxicity is important in order to reduce the cost, labor, and effort involved. For this purpose, conventional 2D cell culture models are used in the early phase of drug development. However, the differences between the in vitro and the in vivo systems have caused the failure of drugs in the later phase of the drug-development process. Therefore, there is a need for a novel in vitro model system that can provide accurate information for evaluating the drug efficacy and toxicity through a closer recapitulation of the in vivo system. Recently, the idea of using microtechnology for mimicking the microscale tissue environment has become widespread, leading to the development of "organ-on-a-chip." Furthermore, the system is further developed for realizing a multiorgan model for mimicking interactions between multiple organs. These advancements are still ongoing and are aimed at ultimately developing "body-on-a-chip" or "human-on-a-chip" devices for predicting the response of the whole body. This review summarizes recently developed organ-on-a-chip technologies, and their applications for reproducing multiorgan functions.

Direct Optofluidic Measurement of the Lipid Permeability of Fluoroquinolones

Quantifying drug permeability across lipid membranes is crucial for drug development. In addition, reduced membrane permeability is a leading cause of antibiotic resistance in bacteria, and hence there is a need for new technologies that can quantify antibiotic transport across biological membranes. We recently developed an optofluidic assay that directly determines the permeability coefficient of autofluorescent drug molecules across lipid membranes. Using ultraviolet fluorescence microscopy, we directly track drug accumulation in giant lipid vesicles as they traverse a microfluidic device while exposed to the drug. Importantly, our measurement does not require the knowledge of the octanol partition coefficient of the drug – we directly determine the permeability coefficient for the specific drug-lipid system. In this work, we report measurements on a range of fluoroquinolone antibiotics and find that their pH dependent lipid permeability can span over two orders of magnitude. We describe various technical improvements for our assay, and provide a new graphical user interface for data analysis to make the technology easier to use for the wider community.

The motility of Entamoeba histolytica: finding ways to understand intestinal amoebiasis

The pathogenic amoeba Entamoeba histolytica is able to migrate within various compartments of the human body. The present article reviews progress in understanding the mechanisms of cell motility in E. histolytica during human intestinal invasion and, in particular, how the three-dimensional characteristics of the environment regulate the parasite's behaviour. The amoeboid mode of migration that applies to E. histolytica's displacements on two-dimensional surfaces is also expected to apply to the three-dimensional environment in the human intestine although several unknown, distinct modalities may be involved. Recent advances in the field of tissue engineering have provided clues on how the construction of a human colon model could help us to understand the host's intestinal physiology and its changes following amoebic infection.

Biomedical microelectromechanical systems (BioMEMS): Revolution in drug delivery and analytical techniques

Advancement in microelectromechanical system has facilitated the microfabrication of polymeric substrates and the development of the novel class of controlled drug delivery devices. These vehicles have specifically tailored three dimensional physical and chemical features which together, provide the capacity to target cell, stimulate unidirectional controlled release of therapeutics and augment permeation across the barriers. Apart from drug delivery devices microfabrication technology’s offer exciting prospects to generate biomimetic gastrointestinal tract models. BioMEMS are capable of analysing biochemical liquid sample like solution of metabolites, macromolecules, proteins, nucleic acid, cells and viruses. This review summarized multidisciplinary application of biomedical microelectromechanical systems in drug delivery and its potential in analytical procedures.

Microfluidic Organ-on-a-Chip Technology for Advancement of Drug Development and Toxicology

In recent years, the exploitation of phenomena surrounding microfluidics has seen an increase in popularity, as researchers have found a way to use their unique properties to create superior design alternatives. One such application is representing the properties and functions of different organs on a microscale chip for the purpose of drug testing or tissue engineering. With the introduction of "organ-on-a-chip" systems, researchers have proposed various methods on various organ-on-a-chip systems to mimic their in vivo counterparts. In this article, a systematic approach is taken to review current technologies pertaining to organ-on-a-chip systems. Design processes with attention to the particular instruments, cells, and materials used are presented.

TEER measurement techniques for in vitro barrier model systems

Transepithelial/transendothelial electrical resistance (TEER) is a widely accepted quantitative technique to measure the integrity of tight junction dynamics in cell culture models of endothelial and epithelial monolayers. TEER values are strong indicators of the integrity of the cellular barriers before they are evaluated for transport of drugs or chemicals. TEER measurements can be performed in real time without cell damage and generally are based on measuring ohmic resistance or measuring impedance across a wide spectrum of frequencies. The measurements for various cell types have been reported with commercially available measurement systems and also with custom-built microfluidic implementations. Some of the barrier models that have been widely characterized using TEER include the blood-brain barrier (BBB), gastrointestinal (GI) tract, and pulmonary models. Variations in these values can arise due to factors such as temperature, medium formulation, and passage number of cells. The aim of this article is to review the different TEER measurement techniques and analyze their strengths and weaknesses, determine the significance of TEER in drug toxicity studies, examine the various in vitro models and microfluidic organs-on-chips implementations using TEER measurements in some widely studied barrier models (BBB, GI tract, and pulmonary), and discuss the various factors that can affect TEER measurements.

Optimizing Wnt-3a and R-spondin1 concentrations for stem cell renewal and differentiation in intestinal organoids using a gradient-forming microdevice

A gradient-generating device assayed the impact of Wnt-3a and R-spondin1 on colonoids identifying concentrations required to yield a physiologically-relevant epithelium.

The promise of organotypic hepatic and gastrointestinal models

Advances in the design and assembly of in vitro organotypic liver and gastrointestinal (GI) models can accelerate our understanding of metabolism, nutrient absorption, and the effect of microbial flora. Such models can provide comprehensive information on how of environmental toxins, drugs, and pharmaceuticals interact with and within these organs. Information obtained from such models could elucidate the complicated cascades of signaling mechanisms that occur in vivo. Because experiments on large-scale animal models are expensive and resource intensive, the design of organotypic models has renewed significance. The challenges and approaches to designing liver and GI models are similar. Because these organs are in close proximity and interact continually, we have described recent design considerations to guide future tissue models.

Biomimetic microfluidic device for in vitro antihypertensive drug evaluation

Microfluidic devices have emerged as revolutionary, novel platforms for in vitro drug evaluation. In this work, we developed a facile method for evaluating antihypertensive drugs using a microfluidic chip. This microfluidic chip was generated using the elastic material poly(dimethylsiloxane) (PDMS) and a microchannel structure that simulated a blood vessel as fabricated on the chip. We then cultured human umbilical vein endothelial cells (HUVECs) inside the channel. Different pressures and shear stresses could be applied on the cells. The generated vessel mimics can be used for evaluating the safety and effects of antihypertensive drugs. Here, we used hydralazine hydrochloride as a model drug. The results indicated that hydralazine hydrochloride effectively decreased the pressure-induced dysfunction of endothelial cells. This work demonstrates that our microfluidic system provides a convenient and cost-effective platform for studying cellular responses to drugs under mechanical pressure.

Reliable permeability assay system in a microfluidic device mimicking cerebral vasculatures

Since most of the bioavailable drugs are impermeable through the blood-brain barrier (BBB), development of a rapid and reliable permeability assay system has been a challenge in drug discovery targeting central nervous system (CNS). Here, we designed a microfluidic device to monitor the drug permeability into the CNS. Human umbilical vein endothelial cells (HUVECs) were shortly (2 ~ 3 h) incubated with astrocyte-conditioned medium after being trapped on microholes in the microfluidic device and tested for chip-based permeability measurement of drugs. The measured permeability values were highly correlated with those measured by conventional in vitro methods and the brain uptake index representing the quantity of transported substances across the in vivo BBB of rats. Using the microfluidic device, we could easily monitor the effect of hydrogen peroxide on the trans-endothelial permeability, which are consistent with the finding that the same treatment disrupted the formation of tight junctions between endothelial cells. Considering relatively short period of time needed for endothelial cell culture and ability to monitor the BBB physiology continuously, we propose that this novel system can be used as an invaluable first-line tool for CNS-related drug development.

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

In this study, an integrated microfluidic device was developed for drug permeability assays with real-time online detection by a directly coupled mass spectrometer. The integrated microfluidic device contained two independent channels sandwiched by a semipermeable polycarbonate membrane for cell culture, and micro solid-phase extraction (SPE) columns for sample clean-up and concentration prior to mass spectrometry detection. Curcumin, a model drug, was delivered to an upper or bottom channel by a pressure-driven flow to mimic dynamic in vivo conditions, and it was forced to permeate into the other side channel. The concentration of curcumin permeated with time was directly detected by an electrospray ionization quadrupole time-of-flight mass spectrometer (ESI-Q-TOF MS) with high sensitivity after micro-SPE pretreatment. The total analysis time only needed about 30 min, and only 6 μL of the drug solution was required for each permeation experiment. The measured permeability of curcumin was consistent with the literature reported value. In addition, this technique offers the potential for parallelization and increasing throughput compared to conventional methods. Thus, the established platform provides a useful tool for drug permeability studies, which is crucial for drug discovery and development.

Microfabrication technologies for oral drug delivery

Micro-/nanoscale technologies such as lithographic techniques and microfluidics offer promising avenues to revolutionalize the fields of tissue engineering, drug discovery, diagnostics and personalized medicine. Microfabrication techniques are being explored for drug delivery applications due to their ability to combine several features such as precise shape and size into a single drug delivery vehicle. They also offer to create unique asymmetrical features incorporated into single or multiple reservoir systems maximizing contact area with the intestinal lining. Combined with intelligent materials, such microfabricated platforms can be designed to be bioadhesive and stimuli-responsive. Apart from drug delivery devices, microfabrication technologies offer exciting opportunities to create biomimetic gastrointestinal tract models incorporating physiological cell types, flow patterns and brush-border like structures. Here we review the recent developments in this field with a focus on the applications of microfabrication in the development of oral drug delivery devices and biomimetic gastrointestinal tract models that can be used to evaluate the drug delivery efficacy.

Technique for real-time measurements of endothelial permeability in a microfluidic membrane chip using laser-induced fluorescence detection

Characterizing permeability of the endothelium that lines blood vessels and heart valves provides fundamental physiological information and is required to evaluate uptake of drugs and other biomolecules. However, current techniques used to measure permeability, such as Transwell insert assays, do not account for the recognized effects of fluid flow-induced shear stress on endothelial permeability or are inherently low-throughput. Here we report a novel on-chip technique in a two-layer membrane-based microfluidic platform to measure real-time permeability of endothelial cell monolayers on porous membranes. Bovine serum albumin (a model protein) conjugated with fluorescein isothiocyanate was delivered to an upper microchannel by pressure-driven flow and was forced to permeate a poly(ethylene terephthalate) membrane into a lower microchannel, where it was detected by laser-induced fluorescence. The concentration of the permeate at the point of detection varied with channel flow rates in agreement to less than 1% with theoretical analyses using a pore flow model. On the basis of the model, a sequential flow rate stepping scheme was developed and applied to obtain the permeability of cell-free and cell-bound membrane layers. This technique is a highly sensitive, novel microfluidic approach for measuring endothelial permeability in vitro, and the use of micrometer-sized channels offers the potential for parallelization and increased throughput compared to conventional shear-based permeability measurement methods.

Breast cancer diagnosis using a microfluidic multiplexed immunohistochemistry platform

Background

Biomarkers play a key role in risk assessment, assessing treatment response, and detecting recurrence and the investigation of multiple biomarkers may also prove useful in accurate prediction and prognosis of cancers. Immunohistochemistry (IHC) has been a major diagnostic tool to identify therapeutic biomarkers and to subclassify breast cancer patients. However, there is no suitable IHC platform for multiplex assay toward personalized cancer therapy. Here, we report a microfluidics-based multiplexed IHC (MMIHC) platform that significantly improves IHC performance in reduction of time and tissue consumption, quantification, consistency, sensitivity, specificity and cost-effectiveness.

Methodology/Principal Findings

By creating a simple and robust interface between the device and human breast tissue samples, we not only applied conventional thin-section tissues into on-chip without any additional modification process, but also attained perfect fluid control for various solutions, without any leakage, bubble formation, or cross-contamination. Four biomarkers, estrogen receptor (ER), human epidermal growth factor receptor 2 (HER2), progesterone receptor (PR) and Ki-67, were examined simultaneously on breast cancer cells and human breast cancer tissues. The MMIHC method improved immunoreaction, reducing time and reagent consumption. Moreover, it showed the availability of semi-quantitative analysis by comparing Western blot. Concordance study proved strong consensus between conventional whole-section analysis and MMIHC (n = 105, lowest Kendall's coefficient of concordance, 0.90). To demonstrate the suitability of MMIHC for scarce samples, it was also applied successfully to tissues from needle biopsies.

Conclusions/Significance

The microfluidic system, for the first time, was successfully applied to human clinical tissue samples and histopathological diagnosis was realized for breast cancers. Our results showing substantial agreement indicate that several cancer-related proteins can be simultaneously investigated on a single tumor section, giving clear advantages and technical advances over standard immunohistochemical method. This novel concept will enable histopathological diagnosis using numerous specific biomarkers at a time even for small-sized specimens, thus facilitating the individualization of cancer therapy.