Measuring barrier function in organ-on-chips with cleanroom-free integration of multiplexable electrodes

Standardizing designed and emergent quantitative features in microphysiological systems

Microphysiological systems (MPSs) are cellular models that replicate aspects of organ and tissue functions in vitro. In contrast with conventional cell cultures, MPSs often provide physiological mechanical cues to cells, include fluid flow and can be interlinked (hence, they are often referred to as microfluidic tissue chips or organs-on-chips). Here, by means of examples of MPSs of the vascular system, intestine, brain and heart, we advocate for the development of standards that allow for comparisons of quantitative physiological features in MPSs and humans. Such standards should ensure that the in vivo relevance and predictive value of MPSs can be properly assessed as fit-for-purpose in specific applications, such as the assessment of drug toxicity, the identification of therapeutics or the understanding of human physiology or disease. Specifically, we distinguish designed features, which can be controlled via the design of the MPS, from emergent features, which describe cellular function, and propose methods for improving MPSs with readouts and sensors for the quantitative monitoring of complex physiology towards enabling wider end-user adoption and regulatory acceptance.

Skin-on-a-chip technologies towards clinical translation and commercialization

Skin is the largest organ of the human body which plays a critical role in thermoregulation, metabolism (e.g. synthesis of vitamin D), and protection of other organs from environmental threats, such as infections, microorganisms, ultraviolet radiation, and physical damage. Even though skin diseases are considered to be less fatal, the ubiquity of skin diseases and irritation caused by them highlights the importance of skin studies. Furthermore, skin is a promising means for transdermal drug delivery, which requires a thorough understanding of human skin structure. Current animal andin vitrotwo/three-dimensional skin models provide a platform for disease studies and drug testing, whereas they face challenges in the complete recapitulation of the dynamic and complex structure of actual skin tissue. One of the most effective methods for testing pharmaceuticals and modeling skin diseases are skin-on-a-chip (SoC) platforms. SoC technologies provide a non-invasive approach for examining 3D skin layers and artificially creating disease models in order to develop diagnostic or therapeutic methods. In addition, SoC models enable dynamic perfusion of culture medium with nutrients and facilitate the continuous removal of cellular waste to further mimic thein vivocondition. Here, the article reviews the most recent advances in the design and applications of SoC platforms for disease modeling as well as the analysis of drugs and cosmetics. By examining the contributions of different patents to the physiological relevance of skin models, the review underscores the significant shift towards more ethical and efficient alternatives to animal testing. Furthermore, it explores the market dynamics ofin vitroskin models and organ-on-a-chip platforms, discussing the impact of legislative changes and market demand on the development and adoption of these advanced research tools. This article also identifies the existing obstacles that hinder the advancement of SoC platforms, proposing directions for future improvements, particularly focusing on the journey towards clinical adoption.

Establishment and evaluation of on-chip intestinal barrier biosystems based on microfluidic techniques

As a booming engineering technology, the microfluidic chip has been widely applied for replicating the complexity of human intestinal micro-physiological ecosystems in vitro. Biosensors, 3D imaging, and multi-omics have been applied to engineer more sophisticated intestinal barrier-on-chip platforms, allowing the improved monitoring of physiological processes and enhancing chip performance. In this review, we report cutting-edge advances in the microfluidic techniques applied for the establishment and evaluation of intestinal barrier platforms. We discuss different design principles and microfabrication strategies for the establishment of microfluidic gut barrier models in vitro. Further, we comprehensively cover the complex cell types (e.g., epithelium, intestinal organoids, endothelium, microbes, and immune cells) and controllable extracellular microenvironment parameters (e.g., oxygen gradient, peristalsis, bioflow, and gut-organ axis) used to recapitulate the main structural and functional complexity of gut barriers. We also present the current multidisciplinary technologies and indicators used for evaluating the morphological structure and barrier integrity of established gut barrier models in vitro. Finally, we highlight the challenges and future perspectives for accelerating the broader applications of these platforms in disease simulation, drug development, and personalized medicine. Hence, this review provides a comprehensive guide for the development and evaluation of microfluidic-based gut barrier platforms.

Innovative electrode and chip designs for transendothelial electrical resistance measurements in organs-on-chips

Many different epithelial and endothelial barriers in the human body ensure the proper functioning of our organs by controlling which substances can pass from one side to another. In recent years, organs-on-chips (OoC) have become a popular tool to study such barriers in vitro. To assess the proper functioning of these barriers, we can measure the transendothelial electrical resistance (TEER) which indicates how easily ions can cross the cell layer when a current is applied between electrodes on either side. TEER measurements are a convenient method to quantify the barrier properties since it is a non-invasive and label-free technique. Direct integration of electrodes for TEER measurements into OoC allows for continuous monitoring of the barrier, and fixed integration of the electrodes improves the reproducibility of the measurements. In this review, we will give an overview of different electrode and channel designs that have been used to measure the TEER in OoC. After giving some insight into why biological barriers are an important field of study, we will explain the theory and practice behind measuring the TEER in in vitro systems. Next, this review gives an overview of the state of the art in the field of integrated electrodes for TEER measurements in OoC, with a special focus on alternative chip and electrode designs. Finally, we outline some of the remaining challenges and provide some suggestions on how to overcome these challenges.

Humanized brain organoids-on-chip integrated with sensors for screening neuronal activity and neurotoxicity

The complex structure and function of the human central nervous system that develops from the neural tube made in vitro modeling quite challenging until the discovery of brain organoids. Human-induced pluripotent stem cells-derived brain organoids offer recapitulation of the features of early human neurodevelopment in vitro, including the generation, proliferation, and differentiation into mature neurons and micro-macroglial cells, as well as the complex interactions among these diverse cell types of the developing brain. Recent advancements in brain organoids, microfluidic systems, real-time sensing technologies, and their cutting-edge integrated use provide excellent models and tools for emulation of fundamental neurodevelopmental processes, the pathology of neurological disorders, personalized transplantation therapy, and high-throughput neurotoxicity testing by bridging the gap between two-dimensional models and the complex three-dimensional environment in vivo. In this review, we summarize how bioengineering approaches are applied to mitigate the limitations of brain organoids for biomedical and clinical research. We further provide an extensive overview and future perspectives of the humanized brain organoids-on-chip platforms with integrated sensors toward brain organoid intelligence and biocomputing studies. Such approaches might pave the way for increasing approvable clinical applications by solving their current limitations.

Vasculature-on-a-Chip with a Porous Membrane Electrode for In Situ Electrochemical Detection of Nitric Oxide Released from Endothelial Cells

Vasculature-on-a-chip is a microfluidic cell culture device used for modeling vascular functions by culturing endothelial cells. Porous membranes are widely used to create cell culture environments. However, in situ real-time measurements of cellular metabolites in microchannels are challenging. In this study, a novel microfluidic device with a porous membrane electrode was developed for the in situ monitoring of nitric oxide (NO) released by endothelial cells in real time. In this system, a porous Au membrane electrode was placed directly beneath the cells for in situ and real-time measurements of NO, a biomarker of endothelial cells. First, the device was electrochemically characterized to construct a calibration plot for NO. Next, NO released by human umbilical vein endothelial cells under l-arginine stimulation was successfully quantified. Furthermore, the changes in NO release with culture time (in days) using the same sample were successfully recorded by exploiting minimally invasive measurements. This is the first report on the combination of a microfluidic device and porous membrane electrode for the electrochemical analysis of endothelial cells. This device will contribute to the development of organ-on-a-chip technology for real-time in situ cell analyses.

Rapid integration of screen-printed electrodes into thermoplastic organ-on-a-chip devices for real-time monitoring of trans-endothelial electrical resistance

Trans-endothelial electrical resistance (TEER) is one of the most widely used indicators to quantify the barrier integrity of endothelial layers. Over the last decade, the integration of TEER sensors into organ-on-a-chip (OOC) platforms has gained increasing interest for its efficient and effective measurement of TEER in OOCs. To date, microfabricated electrodes or direct insertion of wires has been used to integrate TEER sensors into OOCs, with each method having advantages and disadvantages. In this study, we developed a TEER-SPE chip consisting of carbon-based screen-printed electrodes (SPEs) embedded in a poly(methyl methacrylate) (PMMA)-based multi-layered microfluidic device with a porous poly(ethylene terephthalate) membrane in-between. As proof of concept, we demonstrated the successful cultures of hCMEC/D3 cells and the formation of confluent monolayers in the TEER-SPE chip and obtained TEER measurements for 4 days. Additionally, the TEER-SPE chip could detect changes in the barrier integrity due to shear stress or an inflammatory cytokine (i.e., tumor necrosis factor-α). The novel approach enables a low-cost and facile fabrication of carbon-based SPEs on PMMA substrates and the subsequent assembly of PMMA layers for rapid prototyping. Being cost-effective and cleanroom-free, our method lowers the existing logistical and technical barriers presenting itself as another step forward to the broader adoption of OOCs with TEER measurement capability.

Bio-Microfabrication of 2D and 3D Biomimetic Gut-on-a-Chip

Traditional goal of microfabrication was to limitedly construct nano- and micro-geometries on silicon or quartz wafers using various semiconductor manufacturing technologies, such as photolithography, soft lithography, etching, deposition, and so on. However, recent integration with biotechnologies has led to a wide expansion of microfabrication. In particular, many researchers studying pharmacology and pathology are very interested in producing in vitro models that mimic the actual intestine to study the effectiveness of new drug testing and interactions between organs. Various bio-microfabrication techniques have been developed while solving inherent problems when developing in vitro micromodels that mimic the real large intestine. This intensive review introduces various bio-microfabrication techniques that have been used, until recently, to realize two-dimensional and three-dimensional biomimetic experimental models. Regarding the topic of gut chips, two major review subtopics and two-dimensional and three-dimensional gut chips were employed, focusing on the membrane-based manufacturing process for two-dimensional gut chips and the scaffold-based manufacturing process for three-dimensional gut chips, respectively.

Recent advances in sensor-integrated brain-on-a-chip devices for real-time brain monitoring

Brain science has remained in the global spotlight as an important field of scientific and technological discovery. Numerous in vitro and in vivo animal studies have been performed to understand the pathological processes involved in brain diseases and develop strategies for their diagnosis and treatment. However, owing to species differences between animals and humans, several drugs have shown high rates of treatment failure in clinical settings, hindering the development of diagnostic and treatment modalities for brain diseases. In this scenario, microfluidic brain-on-a-chip (BOC) devices, which allow the direct use of human tissues for experiments, have emerged as novel tools for effectively avoiding species differences and performing screening for new drugs. Although microfluidic BOC technology has achieved significant progress in recent years, monitoring slight changes in neurochemicals, neurotransmitters, and environmental states in the brain has remained challenging owing to the brain's complex environment. Hence, the integration of BOC with new sensors that have high sensitivity and high selectivity is urgently required for the real-time dynamic monitoring of BOC parameters. As sensor-based technologies for BOC have not been summarized, here, we review the principle, fabrication process, and application-based classification of sensor-integrated BOC, and then summarize the opportunities and challenges for their development. Generally, sensor-integrated BOC enables real-time monitoring and dynamic analysis, accurately measuring minute changes in the brain and thus enabling the realization of in vivo brain analysis and drug development.

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.

Primary Ciliary Dyskinesia Patient-Specific hiPSC-Derived Airway Epithelium in Air-Liquid Interface Culture Recapitulates Disease Specific Phenotypes In Vitro

Primary ciliary dyskinesia (PCD) is a rare heterogenic genetic disorder associated with perturbed biogenesis or function of motile cilia. Motile cilia dysfunction results in diminished mucociliary clearance (MCC) of pathogens in the respiratory tract and chronic airway inflammation and infections successively causing progressive lung damage. Current approaches to treat PCD are symptomatic, only, indicating an urgent need for curative therapeutic options. Here, we developed an in vitro model for PCD based on human induced pluripotent stem cell (hiPSC)-derived airway epithelium in Air-Liquid-Interface cultures. Applying transmission electron microscopy, immunofluorescence staining, ciliary beat frequency, and mucociliary transport measurements, we could demonstrate that ciliated respiratory epithelia cells derived from two PCD patient-specific hiPSC lines carrying mutations in DNAH5 and NME5, respectively, recapitulate the respective diseased phenotype on a molecular, structural and functional level.

Advances in TEER measurements of biological barriers in microphysiological systems

Biological barriers are multicellular structures that precisely regulate the transport of ions, biomolecules, drugs, cells, and other organisms. Transendothelial/epithelial electrical resistance (TEER) is a label-free method for predicting the properties of biological barriers. Understanding the mechanisms that control TEER significantly enhances our knowledge of the physiopathology of different diseases and aids in the development of new drugs. Measuring TEER values within microphysiological systems called organ-on-a-chip devices that simulate the microenvironment, architecture, and physiology of biological barriers in the body provides valuable insight into the behavior of barriers in response to different drugs and pathogens. These integrated systems should increase the accuracy, reproducibility, sensitivity, resolution, high throughput, speed, cost-effectiveness, and reliable predictability of TEER measurements. Implementing advanced micro and nanoscale manufacturing techniques, surface modification methods, biomaterials, biosensors, electronics, and stem cell biology is necessary for integrating TEER measuring systems with organ-on-chip technology. This review focuses on the applications, advantages, and future perspectives of integrating organ-on-a-chip technology with TEER measurement methods for studying biological barriers. After briefly reviewing the role of TEER in the physiology and pathology of barriers, standard techniques for measuring TEER, including Ohm's law and impedance spectroscopy, and commercially available devices are described. Furthermore, advances in TEER measurement are discussed in multiple barrier-on-a-chip system models representing different organs. Finally, we outline future trends in implementing advanced technologies to design and fabricate nanostructured electrodes, complicated microfluidic chips, and membranes for more advanced and accurate TEER measurements.

Building Blood Vessel Chips with Enhanced Physiological Relevance

Blood vessel chips are bioengineered microdevices, consisting of biomaterials, human cells, and microstructures, which recapitulate essential vascular structure and physiology and allow a well-controlled microenvironment and spatial-temporal readouts. Blood vessel chips afford promising opportunities to understand molecular and cellular mechanisms underlying a range of vascular diseases. The physiological relevance is key to these blood vessel chips that rely on bioinspired strategies and bioengineering approaches to translate vascular physiology into artificial units. Here, we discuss several critical aspects of vascular physiology, including morphology, material composition, mechanical properties, flow dynamics, and mass transport, which provide essential guidelines and a valuable source of bioinspiration for the rational design of blood vessel chips. We also review state-of-art blood vessel chips that exhibit important physiological features of the vessel and reveal crucial insights into the biological processes and disease pathogenesis, including rare diseases, with notable implications for drug screening and clinical trials. We envision that the advances in biomaterials, biofabrication, and stem cells improve the physiological relevance of blood vessel chips, which, along with the close collaborations between clinicians and bioengineers, enable their widespread utility.

Integrated technologies for continuous monitoring of organs-on-chips: Current challenges and potential solutions

Organs-on-chips (OoCs) are biomimetic in vitro systems based on microfluidic cell cultures that recapitulate the in vivo physicochemical microenvironments and the physiologies and key functional units of specific human organs. These systems are versatile and can be customized to investigate organ-specific physiology, pathology, or pharmacology. They are more physiologically relevant than traditional two-dimensional cultures, can potentially replace the animal models or reduce the use of these models, and represent a unique opportunity for the development of personalized medicine when combined with human induced pluripotent stem cells. Continuous monitoring of important quality parameters of OoCs via a label-free, non-destructive, reliable, high-throughput, and multiplex method is critical for assessing the conditions of these systems and generating relevant analytical data; moreover, elaboration of quality predictive models is required for clinical trials of OoCs. Presently, these analytical data are obtained by manual or automatic sampling and analyzed using single-point, off-chip traditional methods. In this review, we describe recent efforts to integrate biosensing technologies into OoCs for monitoring the physiologies, functions, and physicochemical microenvironments of OoCs. Furthermore, we present potential alternative solutions to current challenges and future directions for the application of artificial intelligence in the development of OoCs and cyber-physical systems. These "smart" OoCs can learn and make autonomous decisions for process optimization, self-regulation, and data analysis.

The Use of Sensors in Blood-Brain Barrier-on-a-Chip Devices: Current Practice and Future Directions

The application of lab-on-a-chip technologies in in vitro cell culturing swiftly resulted in improved models of human organs compared to static culture insert-based ones. These chip devices provide controlled cell culture environments to mimic physiological functions and properties. Models of the blood-brain barrier (BBB) especially profited from this advanced technological approach. The BBB represents the tightest endothelial barrier within the vasculature with high electric resistance and low passive permeability, providing a controlled interface between the circulation and the brain. The multi-cell type dynamic BBB-on-chip models are in demand in several fields as alternatives to expensive animal studies or static culture inserts methods. Their combination with integrated biosensors provides real-time and noninvasive monitoring of the integrity of the BBB and of the presence and concentration of agents contributing to the physiological and metabolic functions and pathologies. In this review, we describe built-in sensors to characterize BBB models via quasi-direct current and electrical impedance measurements, as well as the different types of biosensors for the detection of metabolites, drugs, or toxic agents. We also give an outlook on the future of the field, with potential combinations of existing methods and possible improvements of current techniques.

Gut-on-a-chip models for dissecting the gut microbiology and physiology

Microfluidic technologies have been extensively investigated in recent years for developing organ-on-a-chip-devices as robust in vitro models aiming to recapitulate organ 3D topography and its physicochemical cues. Among these attempts, an important research front has focused on simulating the physiology of the gut, an organ with a distinct cellular composition featuring a plethora of microbial and human cells that mutually mediate critical body functions. This research has led to innovative approaches to model fluid flow, mechanical forces, and oxygen gradients, which are all important developmental cues of the gut physiological system. A myriad of studies has demonstrated that gut-on-a-chip models reinforce a prolonged coculture of microbiota and human cells with genotypic and phenotypic responses that closely mimic the in vivo data. Accordingly, the excellent organ mimicry offered by gut-on-a-chips has fueled numerous investigations on the clinical and industrial applications of these devices in recent years. In this review, we outline various gut-on-a-chip designs, particularly focusing on different configurations used to coculture the microbiome and various human intestinal cells. We then elaborate on different approaches that have been adopted to model key physiochemical stimuli and explore how these models have been beneficial to understanding gut pathophysiology and testing therapeutic interventions.

Integrating mechanical sensor readouts into organ-on-a-chip platforms

Organs-on-a-chip have emerged as next-generation tissue engineered models to accurately capture realistic human tissue behaviour, thereby addressing many of the challenges associated with using animal models in research. Mechanical features of the culture environment have emerged as being critically important in designing organs-on-a-chip, as they play important roles in both stimulating realistic tissue formation and function, as well as capturing integrative elements of homeostasis, tissue function, and tissue degeneration in response to external insult and injury. Despite the demonstrated impact of incorporating mechanical cues in these models, strategies to measure these mechanical tissue features in microfluidically-compatible formats directly on-chip are relatively limited. In this review, we first describe general microfluidically-compatible Organs-on-a-chip sensing strategies, and categorize these advances based on the specific advantages of incorporating them on-chip. We then consider foundational and recent advances in mechanical analysis techniques spanning cellular to tissue length scales; and discuss their integration into Organs-on-a-chips for more effective drug screening, disease modeling, and characterization of biological dynamics.

Organ-On-A-Chip Models of the Blood-Brain Barrier: Recent Advances and Future Prospects

The human brain and central nervous system (CNS) present unique challenges in drug development for neurological diseases. One major obstacle is the blood-brain barrier (BBB), which hampers the effective delivery of therapeutic molecules into the brain while protecting it from blood-born neurotoxic substances and maintaining CNS homeostasis. For BBB research, traditional in vitro models rely upon Petri dishes or Transwell systems. However, these static models lack essential microenvironmental factors such as shear stress and proper cell-cell interactions. To this end, organ-on-a-chip (OoC) technology has emerged as a new in vitro modeling approach to better recapitulate the highly dynamic in vivo human brain microenvironment so-called the neural vascular unit (NVU). Such BBB-on-a-chip models have made substantial progress over the last decade, and concurrently there has been increasing interest in modeling various neurological diseases such as Alzheimer's disease and Parkinson's disease using OoC technology. In addition, with recent advances in other scientific technologies, several new opportunities to improve the BBB-on-a-chip platform via multidisciplinary approaches are available. In this review, an overview of the NVU and OoC technology is provided, recent progress and applications of BBB-on-a-chip for personalized medicine and drug discovery are discussed, and current challenges and future directions are delineated.

Contributions of the microbiome to intestinal inflammation in a gut-on-a-chip

The intestinal microbiome affects a number of biological functions of the organism. Although the animal model is a powerful tool to study the relationship between the host and microbe, a physiologically relevant in vitro human intestinal system has still unmet needs. Thus, the establishment of an in vitro living cell-based system of the intestine that can mimic the mechanical, structural, absorptive, transport and pathophysiological properties of the human intestinal environment along with its commensal bacterial strains can promote pharmaceutical development and potentially replace animal testing. In this paper, we present a microfluidic-based gut model which allows co-culture of human and microbial cells to mimic the gastrointestinal structure. The gut microenvironment is recreated by flowing fluid at a low rate (21 μL/h) over the microchannels. Under these conditions, we demonstrated the capability of gut-on-a-chip to recapitulate in vivo relevance epithelial cell differentiation including highly polarized epithelium, mucus secretion, and tight membrane integrity. Additionally, we observed that the co-culture of damaged epithelial layer with the probiotics resulted in a substantial responded recovery of barrier function without bacterial overgrowth in a gut-on-a-chip. Therefore, this gut-on-a-chip could promote explorations interaction with host between microbe and provide the insights into questions of fundamental research linking the intestinal microbiome to human health and disease.