Turn-On Fluorescence Sensing of Oxygen with Dendrimer-Encapsulated Platinum Nanoparticles as Tunable Oxidase Mimics for Spatially Resolved Measurement of Oxygen Gradient in a Human Gut-on-a-Chip

An Array of Carbon Nanofiber Bundle_Based 3D In Vitro Intestinal Microvilli for Mimicking Functional and Physical Activities of the Small Intestine

Researchers have developed in vitro small intestine models of biomimicking microvilli, such as gut-on-a-chip devices. However, fabrication methods developed to date for 2D and 3D in vitro gut still have unsolved limitations. In this study, an innovative fabrication method of a 3D in vitro gut model is introduced for effective drug screening. The villus is formed on a patterned carbon nanofiber (CNF) bundle as a flexible and biocompatible scaffold. Mechanical properties of the fabricated villi structure are investigates. A microfluidic system is applied to induce the movement of CNFs villi. F-actin and Occludin staining of Caco-2 cells on a 2D flat-chip as a control and a 3D gut-chip with or without fluidic stress is observed. A permeability test of FD20 is performed. The proposed 3D gut-chip with fluidic stress achieve the highest value of Papp. Mechano-active stimuli caused by distinct structural and movement effects of CNFs villi as well as stiffness of the suggested CNFs villi not only can help accelerate cell differentiation but also can improve permeability. The proposed 3D gut-chip system further strengthens the potential of the platform to increase the accuracy of various drug tests.

Bioengineered human gut-on-a-chip for advancing non-clinical pharmaco-toxicology

INTRODUCTION

There is a growing need for alternative models to advance current non-clinical experimental models because they often fail to accurately predict drug responses in human clinical trials. Human organ-on-a-chip models have emerged as promising approaches for advancing the predictability of drug behaviors and responses.

AREAS COVERED

We summarize up-to-date human gut-on-a-chip models designed to demonstrate intricate interactions involving the host, microbiome, and pharmaceutical compounds since these models have been reported a decade ago. This overview covers recent advances in gut-on-a-chip models as a bridge technology between non-clinical and clinical assessments of drug toxicity and metabolism. We highlight the promising potential of gut-on-a-chip platforms, offering a reliable and valid framework for investigating reciprocal crosstalk between the host, gut microbiome, and drug compounds.

EXPERT OPINION

Gut-on-a-chip platforms can attract multiple end users as predictive, human-relevant, and non-clinical model. Notably, gut-on-a-chip platforms provide a unique opportunity to recreate a human intestinal microenvironment, including dynamic bowel movement, luminal flow, oxygen gradient, host-microbiome interactions, and disease-specific manipulations restricted in animal and in vitro cell culture models. Additionally, given the profound impact of the gut microbiome on pharmacological bioprocess, it is critical to leverage breakthroughs of gut-on-a-chip technology to address knowledge gaps and drive innovations in predictive drug toxicology and metabolism.

Scanning electrochemical microscopy for determining oxygen consumption rates of cells in hydrogel fibers fabricated using an extrusion 3D bioprinter

Three-dimensional (3D)-cultured cells have attracted the attention of researchers in tissue engineering- and drug screening-related fields. Among them, 3D cellular fibers have attracted significant attention because they can be stacked to prepare more complex tissues and organs. Cellular fibers are widely fabricated using extrusion 3D bioprinters. For these applications, it is necessary to evaluate cellular activities, such as the oxygen consumption rate (OCR), which is one of the major metabolic activities. We previously reported the use of scanning electrochemical microscopy (SECM) to evaluate the OCRs of cell spheroids. However, the SECM approach has not yet been applied to hydrogel fibers prepared using the bioprinters. To the best of our knowledge, this is the first study to evaluate the OCR of cellular fibers printed by extrusion 3D bioprinters. First, the diffusion theory was discussed to address this issue. Next, diffusion models were simulated to compare realistic models with this theory. Finally, the OCRs of MCF-7 cells in the printed hydrogel fibers were evaluated as a proof of concept. Our proposed approach could potentially be used to evaluate the OCRs of tissue-engineered fibers for organ transplantation and drug screening using in-vitro models.

Chiral MoSe2 Nanoparticles for Ultrasensitive Monitoring of Reactive Oxygen Species In Vivo

Reactive oxygen species (ROS) are involved in neurodegenerative diseases, cancer, and acute hepatitis, and quantification of ROS is critical for the early diagnosis of these diseases. In this work, a novel probe is developed, based on chiral molybdenum diselenide (MoSe2 ) nanoparticles (NPs) modified by the fluorescent molecule, cyanine 3 (Cy3). Chiral MoSe2 NPs show intensive circular dichroism (CD) signals at 390 and 550 nm, whereas the fluorescence of Cy3 at 560 nm is quenched by MoSe2 NPs. In the presence of ROS, the probe reacts with the ROS and then oxidates rapidly, resulting in decreased CD signals and the recovery of the fluorescence. Using this strategy, the limit of detection values of CD and fluorescent signals in living cells are 0.0093 nmol/106 cells and 0.024 nmol/106 cells, respectively. The high selectivity and sensitivity to ROS in complex biological environments is attributed to the Mo4+ and Se2- oxidation reactions on the surface of the NPs. Furthermore, chiral MoSe2 NPs are able to monitor the levels of ROS in vivo by the fluorescence. Collectively, this strategy offers a new approach for ROS detection and has the potential to inspire others to explore chiral nanomaterials as biosensors to investigate biological events.

Galvanic Bipolar Electrode Arrays with Self-Driven Optical Readouts

In this work, we report a bipolar electrode (BPE) array system with self-driven optical readouts of the faradic current flowing through the BPEs. The BPE array system is based on the spontaneous redox reactions that are respectively occurring at opposite poles of the BPEs with appropriate electrocatalysts on the poles; this system is analogous to one consisting of galvanic electrochemical cells. The galvanic BPE array system operates in a self-powered mode that requires there to be neither a direct electrical connection nor external electrical polarization to each BPE. Importantly, the appropriate electrocatalysts on the poles play a critical role in the galvanic BPE array system to induce the spontaneous redox reactions occurring at the poles of BPEs. Moreover, the galvanic BPE array system provides self-driven optical readouts, including fluorometric and colorimetric ones, to report the faradaic current resulting from the spontaneous redox reactions on the BPE poles. Based on the unique benefits that the galvanic BPE array system has over conventional BPEs, we demonstrated the promising potential of galvanic BPE arrays for the simple yet rapid and quantitative screening of electrocatalysts for the oxygen reduction reaction as well as sensitive sensing of H2O2 in parallel.

Colloidal Polymer-Templated Formation of Inorganic Nanocrystals and their Emerging Applications

Inorganic nanocrystals possess unique physicochemical properties compared to their bulk counterparts. Stabilizing agents are commonly used for the preparation of inorganic nanocrystals with controllable properties. Particularly, colloidal polymers have emerged as general and robust templates for in situ formation and confinement of inorganic nanocrystals. In addition to templating and stabilizing inorganic nanocrystals, colloidal polymers can tailor their physicochemical properties such as size, shape, structure, composition, surface chemistry, and so on. By incorporating functional groups into colloidal polymers, desired functions can be integrated with inorganic nanocrystals, advancing their potential applications. Here, recent advances in the colloidal polymer-templated formation of inorganic nanocrystals are reviewed. Seven types of colloidal polymers, including dendrimer, polymer micelle, stare-like block polymer, bottlebrush polymer, spherical polyelectrolyte brush, microgel, and single-chain nanoparticle, have been extensively applied for the synthesis of inorganic nanocrystals. Different strategies for the development of these colloidal polymer-templated inorganic nanocrystals are summarized. Then, their emerging applications in the fields of catalysis, biomedicine, solar cells, sensing, light-emitting diodes, and lithium-ion batteries are highlighted. Last, the remaining issues and future directions are discussed. This review will stimulate the development and application of colloidal polymer-templated inorganic nanocrystals.

Mechanisms of Nanoparticle Transport across Intestinal Tissue: An Oral Delivery Perspective

Oral drug administration has been a popular choice due to patient compliance and limited clinical resources. Orally delivered drugs must circumvent the harsh gastrointestinal (GI) environment to effectively enter the systemic circulation. The GI tract has a number of structural and physiological barriers that limit drug bioavailability including mucus, the tightly regulated epithelial layer, immune cells, and associated vasculature. Nanoparticles have been used to enhance oral bioavailability of drugs, as they can act as a shield to the harsh GI environment and prevent early degradation while also increasing uptake and transport of drugs across the intestinal epithelium. Evidence suggests that different nanoparticle formulations may be transported via different intracellular mechanisms to cross the intestinal epithelium. Despite the existence of a significant body of work on intestinal transport of nanoparticles, many key questions remain: What causes the poor bioavailability of the oral drugs? What factors contribute to the ability of a nanoparticle to cross different intestinal barriers? Do nanoparticle properties such as size and charge influence the type of endocytic pathways taken? In this Review, we summarize the different components of intestinal barriers and the types of nanoparticles developed for oral delivery. In particular, we focus on the various intracellular pathways used in nanoparticle internalization and nanoparticle or cargo translocation across the epithelium. Understanding the gut barrier, nanoparticle characteristics, and transport pathways may lead to the development of more therapeutically useful nanoparticles as drug carriers.

Platinum-Nickel Nanoparticles with Enhanced Oxidase-like Activity for Total Antioxidant Capacity Bioassay

While great progress in nanozyme-enabled analytical chemistry has been made, most current nanozyme-based biosensing platforms are based on peroxidase-like nanozymes. However, peroxidase-like nanozymes with multienzymatic activities can influence the detection sensitivity and accuracy, while the use of unstable hydrogen peroxide (H₂O₂) in a peroxidase-like catalytic reaction may result in the reproducibility challenge of sensing signals. We envision that constructing biosensing systems by using oxidase-like nanozymes can address these limitations. Herein, we reported that platinum-nickel nanoparticles (Pt-Ni NPs) with Pt-rich shells and Ni-rich cores possessed high oxidase-like catalytic efficiency, exhibiting a 2.18-fold higher maximal reaction velocity (v_max) than initial pure Pt NPs. The oxidase-like Pt-Ni NPs were applied to develop a colorimetric assay for the determination of total antioxidant capacity (TAC). The antioxidant levels of four bioactive small molecules, two antioxidant nanomaterials, and three cells were successfully measured. Our work not only provides new insights for preparing highly active oxidase-like nanozymes but also manifests their applications for TAC analysis.

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

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

Gut-on-a-Chip Models: Current and Future Perspectives for Host-Microbial Interactions Research

The intestine contains the largest microbial community in the human body, the gut microbiome. Increasing evidence suggests that it plays a crucial role in maintaining overall health. However, while many studies have found a correlation between certain diseases and changes in the microbiome, the impact of different microbial compositions on the gut and the mechanisms by which they contribute to disease are not well understood. Traditional pre-clinical models, such as cell culture or animal models, are limited in their ability to mimic the complexity of human physiology. New mechanistic models, such as organ-on-a-chip, are being developed to address this issue. These models provide a more accurate representation of human physiology and could help bridge the gap between clinical and pre-clinical studies. Gut-on-chip models allow researchers to better understand the underlying mechanisms of disease and the effect of different microbial compositions on the gut. They can help to move the field from correlation to causation and accelerate the development of new treatments for diseases associated with changes in the gut microbiome. This review will discuss current and future perspectives of gut-on-chip models to study host-microbial interactions.

In Situ Confocal Fluorescence Lifetime Imaging of Nanopore Electrode Arrays with Redox Active Fluorogenic Amplex Red

Direct imaging of electrochemical processes on electrode surfaces is a central part of understanding spatially heterogeneous electrochemical processes on the surfaces. Herein, we report a strategy for the spatially resolved imaging of local faradaic processes on nanoscale electrochemical interfaces. This strategy is based on fluorescence lifetime imaging microscopy (FLIM) with the use of Amplex Red as a fluorogenic redox probe. After verifying the capability of Amplex Red for fluorescence lifetime imaging, we demonstrated the turn-on FLIM-based imaging of faradaic processes on the electrochemical interfaces of different dimensions. In particular, we achieved spatially resolved visualization of the local electrochemical processes occurring on even nanopore electrode arrays as well as conventional microelectrodes, including disk-shaped ultramicroelectrodes and interdigitated array microelectrodes.

Integrated biosensors for monitoring microphysiological systems

Microphysiological systems (MPSs), also known as organ-on-a-chip models, aim to recapitulate the functional components of human tissues or organs in vitro. Over the last decade, with the advances in biomaterials, 3D bioprinting, and microfluidics, numerous MPSs have emerged with applications to study diseased and healthy tissue models. Various organs have been modeled using MPS technology, such as the heart, liver, lung, and blood-brain barrier. An important aspect of in vitro modeling is the accurate phenotypical and functional characterization of the modeled organ. However, most conventional characterization methods are invasive and destructive and do not allow continuous monitoring of the cells in culture. On the other hand, microfluidic biosensors enable in-line, real-time sensing of target molecules with an excellent limit of detection and in a non-invasive manner, thereby effectively overcoming the limitation of the traditional techniques. Consequently, microfluidic biosensors have been increasingly integrated into MPSs and used for in-line target detection. This review discusses the state-of-the-art microfluidic biosensors by providing specific examples, detailing their main advantages in monitoring MPSs, and highlighting current developments in this field. Finally, we describe the remaining challenges and potential future developments to advance the current state-of-the-art in integrated microfluidic biosensors.

Stimuli-Responsive Dendritic Macromolecules for Optical Detection of Metal Ions and Acidic Vapors by the Photoinduced Electron Transfer Mechanism: Paper-Based Indicator for Food Spoilage Sensing

Visual detection of analytes has been a significant challenge in the design and development of optical chemosensors. Sensing of analytes in aqueous solution by organic molecules has encountered some issues, such as poor water solubility and quenching of optical properties. In this study, a new category of smart dendritic macromolecules was designed and synthesized by functionalization of the poly(amidoamine) (PAMAM) dendrimer with spiropyran molecules to afford a photoluminescent dendritic structure (SP-PAMAM). Smart optical sensors were prepared by physical incorporation of four different oxazolidine derivatives containing hydroxyl and nitro substituted groups into the SP-PAMAM structure. Investigation of optical properties demonstrated photoinduced electron transfer (PET) between the spiropyran end group of SP-PAMAM and oxazolidine derivatives (in a concentration of about 0.0002 M), which can result in quenching of fluorescence emission of spiropyran photoswitch in the form of merocyanine (MC). Treatment of the oxazolidine-doped SP-PAMAM samples with metal ions resulted in changes in the PET mechanism (switching on or off), as observed in the case of Fe³⁺, Pb²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Co²⁺, and Ni²⁺ by different oxazolidine derivatives through various mechanisms (increase or decrease of fluorescence emission). These smart photoluminescent dendritic macromolecules have potential applications for photodetection of metal ions in aqueous media as optical chemosensors. In addition, the smart macromolecules displayed disconnection of PET between MC and oxazolidine and also showed red fluorescence emission under acidic conditions (pH 1-5). It is due to the protonation of the MC to MCH form and demonstrates a remarkable red shift in fluorescence spectra. The pH-responsivity of smart macromolecules was used for designing a paper-based pH indicator for visual detection of spoilage in the food industry, especially in the case of milk. The prepared papers applied on cap of the milk bottles did not show any fluorescence emission in the case of fresh milk; however, a red fluorescence emission was observed after milk spoilage as a result of adsorption of acidic volatile components generated by bacterial degradation and oxidation process on the paper surface. The reported smart papers can serve as optical portable pH indicators for timely detection of spoilage in food materials, which are usable in food packaging as smart indicator tags.

In vitro triple coculture with gut microbiota from spondyloarthritis patients is characterized by inter-individual differences in inflammatory responses

Spondyloarthritis is a group of chronic inflammatory diseases that primarily affects axial or peripheral joints and is frequently associated with inflammation at non-articular sites. The disease is multifactorial, involving genetics, immunity and environmental factors, including the gut microbiota. In vivo, microbiome contributions are difficult to assess due to the multifactorial disease complexity. In a proof-of-concept approach, we therefore used a triple coculture model of immune-like, goblet and epithelial cells to investigate whether we could detect a differential impact from spondyloarthritis- vs. healthy-derived gut microbiota on host cell response. Despite their phylogenetic resemblance, flow cytometry-based phenotypic clustering revealed human-derived gut microbiota from healthy origin to cluster together and apart from spondyloarthritis donors. At host level, mucus production was higher upon exposure to healthy microbiota. Pro-inflammatory cytokine responses displayed more inter-individual variability in spondyloarthritis than in healthy donors. Interestingly, the high dominance in the initial sample of one patient of Prevotella, a genus previously linked to spondyloarthritis, resulted in the most differential host response upon 16 h host-microbe coincubation. While future research should further focus on inter-individual variability by using gut microbiota from a large cohort of patients, this study underscores the importance of the gut microbiota during the SpA disease course.

Back to the Drawing Board: A Unifying First-Principle Model for Correlating Sample UV-Vis Absorption and Fluorescence Emission

The popular textbook and literature model I(λ_x,λ_m) = K(λx,λm)(1-10-Ax) or its variants for correlating the sample absorption and fluorescence often fails even for the simplest samples where the fluorophore is the only light absorber. Reported is a first-principle model I(λ_x,λ_m) = K(λx,λm)Ax,f10-(Ax,sdx+Am,sdm) for correlating the sample fluorescence measured with a conventional spectrofluorometer and its UV-vis absorbance quantified with a conventional UV-vis spectrophotometer. This model can be simplified or expanded for a variety of fluorescence analyses. First, it enables curve-fitting fluorescence intensity as a function of the fluorophore or sample absorbance over a sample concentration range impossible with existing models. Second, it provides the theoretical foundation for an inner-filter-effect (IFE)-correction method developed earlier and explains mathematically the linearity between the IFE-corrected fluorescence and the fluorophore concentration or absorbance. Third, this model can be expanded for quantitative mechanistic studies of fluorescence intensity variations triggered by stimuli treatments. One demonstrated example is to quantify temperature effects on the emission-wavelength-specific and total fluorescence quantum yield of anthracene. We expect that this first-principle model will be broadly adopted for both student education that promotes evidence-based learning and a variety of fluorescence applications where disentangling sample absorption and emission are critical for reliable data analysis.