Uptake of silica particulate drug carriers in an intestine-on-a-chip: towards a better in vitro model of nanoparticulate carrier and mucus interactions

Intestinal organoid technology and applications in probiotics

The impacts of probiotics on maintaining the host's intestinal health have been extensively confirmed. Organoid technology revolutionizes intestinal health research by providing a unique platform to study the effects of probiotics. It overcomes challenges posed by animal models and 2D cell models in accurately simulating the in vivo environment. This review summarizes the development of intestinal organoid technology and its potential applications in intestinal health research as well as highlights the regulatory mechanisms of probiotics on intestinal health, which have been revealed using intestinal organoid technology. Furthermore, an overview of its potential applications in probiotic research has also been provided. This review aims to improve the understanding of intestinal organoid technology's applications in this field as well as to contribute to its further development.

A membrane-free microfluidic approach to mucus permeation for efficient differentiation of mucoadhesive and mucopermeating nanoparticulate systems

The gastrointestinal mucus barrier is a widely overlooked yet essential component of the intestinal epithelium, responsible for the body's protection against harmful pathogens and particulates. This, coupled with the increasing utilisation of biological molecules as therapeutics (e.g. monoclonal antibodies, RNA vaccines and synthetic proteins) and nanoparticle formulations for drug delivery, necessitates that we consider the additional absorption barrier that the mucus layer may pose. It is imperative that in vitro permeability methods can accurately model this barrier in addition to standardised cellular testing. In this study, a mucus-on-a-chip (MOAC) microfluidic device was engineered and developed to quantify the permeation kinetics of nanoparticles through a biorelevant synthetic mucus layer. Three equivalently sized nanoparticle systems, formulated from chitosan (CSNP), mesoporous silica (MSNP) and poly (lactic-co-glycolic) acid (PLGA-NP) were prepared to encompass various surface chemistries and nanostructures and were assessed for their mucopermeation within the MOAC. Utilising this device, the mucoadhesive behaviour of chitosan nanoparticles was clearly visualised, a phenomenon not often observed via standard permeation models. In contrast, MSNP and PLGA-NP displayed mucopermeation, with significant differences in permeation pattern due to specific mucus-nanoparticle binding. Further optimisation of the MOAC to include a more biorelevant mucus mimic resulted in 5.5-fold hindered PLGA-NP permeation compared to a mucin solution. Furthermore, tracking of PLGA-NP at a single nanoparticle resolution revealed rank-order correlations between particle diffusivity and MOAC permeation. This device, including utilisation of biosimilar mucus, provides a unique ability to quantify both mucoadhesion and mucopenetration of nano-formulations and elucidate mucus binding interactions on a microscopic scale.

Capturing and Quantifying Particle Transcytosis with Microphysiological Intestine-on-Chip Models

Understanding the intestinal transport of particles is critical in several fields ranging from optimizing drug delivery systems to capturing health risks from the increased presence of nano- and micro-sized particles in human environment. While Caco-2 cell monolayers grown on permeable supports are the traditional in vitro model used to probe intestinal absorption of dissolved molecules, they fail to recapitulate the transcytotic activity of polarized enterocytes. Here, an intestine-on-chip model is combined with in silico modeling to demonstrate that the rate of particle transcytosis is ≈350× higher across Caco-2 cell monolayers exposed to fluid shear stress compared to Caco-2 cells in standard "static" configuration. This relates to profound phenotypical alterations and highly polarized state of cells grown under mechanical stimulation and it is shown that transcytosis in the microphysiological model is energy-dependent and involves both clathrin and macropinocytosis mediated endocytic pathways. Finally, it is demonstrated that the increased rate of transcytosis through cells exposed to flow is explained by a higher rate of internal particle transport (i.e., vesicular cellular trafficking and basolateral exocytosis), rather than a change in apical uptake (i.e., binding and endocytosis). Taken together, the findings have important implications for addressing research questions concerning intestinal transport of engineered and environmental particles.

Mechanisms of uptake and transport of particulate formulations in the small intestine

Micro- and nano-scale particulate formulations are widely investigated towards improving the oral bioavailability of both biologics and drugs with low solubility and/or low intestinal permeability. Particulate formulations harnessing physiological intestinal transport pathways have recently yielded remarkably high oral bioavailabilities, illustrating the need for better understanding the specific pathways underpinning particle small intestinal absorption and the relative role of intestinal cells. Mechanistic knowledge has been hampered by the well acknowledged limitations of current in vitro, in vivo and ex vivo models relevant to the human intestinal physiology and the lack of standardization in studies reporting absorption data. Here we review the relevant literature and critically discusses absorption pathways with a focus on the role of specific intestinal epithelial and immune cells. We conclude that while Microfold (M) cells are a valid target for oral vaccines, enterocytes play a greater role in the systemic bioavailability of orally administrated particulate formulations, particularly within the sub-micron size range. We also comment on less-reported mechanisms such as paracellular permeability of particles, persorption due to cell damage and uptake by migratory immune cells.

Emerging Roles of Microfluidics in Brain Research: From Cerebral Fluids Manipulation to Brain-on-a-Chip and Neuroelectronic Devices Engineering

Remarkable progress made in the past few decades in brain research enables the manipulation of neuronal activity in single neurons and neural circuits and thus allows the decipherment of relations between nervous systems and behavior. The discovery of glymphatic and lymphatic systems in the brain and the recently unveiled tight relations between the gastrointestinal (GI) tract and the central nervous system (CNS) further revolutionize our understanding of brain structures and functions. Fundamental questions about how neurons conduct two-way communications with the gut to establish the gut-brain axis (GBA) and interact with essential brain components such as glial cells and blood vessels to regulate cerebral blood flow (CBF) and cerebrospinal fluid (CSF) in health and disease, however, remain. Microfluidics with unparalleled advantages in the control of fluids at microscale has emerged recently as an effective approach to address these critical questions in brain research. The dynamics of cerebral fluids (i.e., blood and CSF) and novel in vitro brain-on-a-chip models and microfluidic-integrated multifunctional neuroelectronic devices, for example, have been investigated. This review starts with a critical discussion of the current understanding of several key topics in brain research such as neurovascular coupling (NVC), glymphatic pathway, and GBA and then interrogates a wide range of microfluidic-based approaches that have been developed or can be improved to advance our fundamental understanding of brain functions. Last, emerging technologies for structuring microfluidic devices and their implications and future directions in brain research are discussed.

Microfluidic Fabrication of pH-Responsive Nanoparticles for Encapsulation and Colon-Target Release of Fucoxanthin

Improving the stability of fucoxanthin in the gastrointestinal tract is an important approach to enhance its oral bioavailability. The study proposed a new microfluidic device allowing for the synthesis of a structurally well-defined nanoscale delivery system with a uniform size for encapsulation and colon-target release of fucoxanthin. The rapid mixing in the microfluidic channel ensured that the mixing time was shorter than the aggregation time, thus realizing the controllable control of the coprecipitation of fucoxanthin and shellac polymer. In vitro digestion tests showed that a pH stimulus-responsive release of fucoxanthin from FX/SH NPs was observed under alkaline pH conditions. The fluorescence colocalization imaging indicated that FX/SH NPs did not affect the intestine function and had a protective effect on Caco-2 cells damaged by H₂O₂ by enhancing their antioxidant capacity. Overall, this work illustrated the promise of using a microfluidic approach to fabricate the biomimetic nanodelivery system for better biocompatibility and targeting efficacy.

Intestinal Models for Personalized Medicine: from Conventional Models to Microfluidic Primary Intestine-on-a-chip

Intestinal dysfunction is frequently driven by abnormalities of specific genes, microbiota, or microenvironmental factors, which usually differ across individuals, as do intestinal physiology and pathology. Therefore, it's necessary to develop personalized therapeutic strategies, which are currently limited by the lack of a simulated intestine model. The mature human intestinal mucosa is covered by a single layer of columnar epithelial cells that are derived from intestinal stem cells (ISCs). The complexity of the organ dramatically increases the difficulty of faithfully mimicking in vivo microenvironments. However, a simulated intestine model will serve as an indispensable foundation for personalized drug screening. In this article, we review the advantages and disadvantages of conventional 2-dimensional models, intestinal organoid models, and current microfluidic intestine-on-a-chip (IOAC) models. The main technological strategies are summarized, and an advanced microfluidic primary IOAC model is proposed for personalized intestinal medicine. In this model, primary ISCs and the microbiome are isolated from individuals and co-cultured in a multi-channel microfluidic chip to establish a microengineered intestine device. The device can faithfully simulate in vivo fluidic flow, peristalsis-like motions, host-microbe crosstalk, and multi-cell type interactions. Moreover, the ISCs can be genetically edited before seeding, and monitoring sensors and post-analysis abilities can also be incorporated into the device to achieve high-throughput and rapid pharmaceutical studies. We also discuss the potential future applications and challenges of the microfluidic platform. The development of cell biology, biomaterials, and tissue engineering will drive the advancement of the simulated intestine, making a significant contribution to personalized medicine in the future. Graphical abstract The intestine is a primary organ for digestion, absorption, and metabolism, as well as a major site for the host-commensal microbiota interaction and mucosal immunity. The complexity of the organ dramatically increases the difficulty of faithfully mimicking in vivo microenvironments, though physiological 3-dimensional of the native small intestinal epithelial tissue has been well documented. An intestinal stem cells-based microfluidic intestine-on-a-chip model that faithfully simulate in vivo fluidic flow, peristalsis-like motions, host-microbe crosstalk, and multi-cell type interactions will make a significant contribution.

Mimicking the Gastrointestinal Mucus Barrier: Laboratory-Based Approaches to Facilitate an Enhanced Understanding of Mucus Permeation

The gastrointestinal mucus layer plays a significant role in maintaining gut homeostasis and health, offering protective capacities against the absorption of harmful pathogens as well as commensal gut bacteria and buffering stomach acid to protect the underlying epithelium. Despite this, the mucus barrier is often overlooked during preclinical pharmaceutical development and may pose a significant absorption barrier to high molecular weight or lipophilic drug species. The complex chemical and physical nature of the dynamic mucus layer has proven problematic to reliably replicate in a laboratory setting, leading to the development of multiple mucus models with varying complexity and predictive capacity. This, coupled with the wide range of analysis methods available, has led to a plethora of possible approaches to quantifying mucus permeation; however, the field remains significantly under-represented in biomedical research. For this reason, the development of a concise collation of the available approaches to mucus permeation is essential. In this review, we explore widely utilized mucus mimics ranging in complexity from simple mucin solutions to native mucus preparations for their predictive capacity in mucus permeation analysis. Furthermore, we highlight the diverse range of laboratory-based models available for the analysis of mucus interaction and permeability with a specific focus on in vitro, ex vivo, and in situ models. Finally, we highlight the predictive capacity of these models in correlation with in vivo pharmacokinetic data. This review provides a comprehensive and critical overview of the available technologies to analyze mucus permeation, facilitating the efficient selection of appropriate tools for further advancement in oral drug delivery.

An ex Vivo Model Enables Systematic Investigation of the Intestinal Absorption and Transcytosis of Oral Particulate Nanocarriers

Nanoparticulate formulations are being developed toward enhancing the bioavailability of orally administrated biologics. However, the processes mediating particulate carriers' intestinal uptake and transport remains to be fully elucidated. Herein, an optical clearing-based whole tissue mount/imaging strategy was developed to enable high quality microscopic imaging of intestinal specimens. It enabled the distribution of nanoparticles within intestinal villi to be quantitatively analyzed at a cellular level. Two-hundred and fifty nm fluorescent polystyrene nanoparticles were modified with polyethylene glycol (PEG), Concanavalin A (ConA), and pectin to yield mucopenetrating, enterocyte targeting, and mucoadhesive model nanocarriers, respectively. Introducing ConA on the PEGylated nanoparticles significantly increased their uptake in the intestinal epithelium (∼4.16 fold for 200 nm nanoparticle and ∼2.88 fold for 50 nm nanoparticles at 2 h). Moreover, enterocyte targeting mediated the trans-epithelial translocation of 50 nm nanoparticles more efficiently than that of the 200 nm nanoparticles. This new approach provides an efficient methodology to obtain detailed insight into the transcytotic activity of enterocytes as well as the barrier function of the constitutive intestinal mucus. It can be applied to guide the rational design of particulate formulations for more efficient oral biologics delivery.

"Mucus-on-Chip": A new tool to study the dynamic penetration of nanoparticulate drug carriers into mucus

The mucus covering of epithelial tissues presents one significant biological barrier to the uptake and absorption of particulate carriers. Improved understanding of the mechanisms mediating the transport of nanoparticles across such mucus layers would accelerate their development as optimised mucosal drug delivery formulations (e.g. via oral and rectal routes). Herein, an in vitro mucus model ("Mucus-on-Chip") was developed to enable the interaction and transport of functionalised nanoparticles and reconstituted mucus to be quantitatively investigated in real-time. We verified that the diffusion of nanoparticles into mucus is highly dependent on their biointerfacial properties. Muco-inert modification (PEGylation) significantly enhanced the mucopenetration of 50 nm and 200 nm nanoparticles, whereas limited mucopenetration was observed for pectin coated mucoadhesive nanoparticles. Furthermore, this model can be easily adapted to mimic specific physiological mucus environments. Mucus pre-treated with a mucolytic agent displayed reduced barrier function and therefore significantly accelerated mucopenetration of nanoparticles, which was independent of their size and biointerfacial properties. This new "Mucus-on-Chip" methodology provides detailed insight into the dynamics of nanoparticle-mucus interaction, which can be applied to refine the design of particulate formulations for more efficient mucosal drug delivery.

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

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

Design of PEGylated Three Ligands Silica Nanoparticles for Multi-Receptor Targeting

The synthesis of silica nanoparticles (SiNPs) decorated on their surface with a range of various elements (e.g., ligands, drugs, fluorophores, vectors, etc.) in a controlled ratio remains a big challenge. We have previously developed an efficient strategy to obtain in one-step, well-defined multifunctional fluorescent SiNPs displaying fluorophores and two peptides ligands as targeting elements, allowing selective detection of cancer cells. In this paper, we demonstrate that additional level of controlled multifunctionality can be achieved, getting even closer to the original concept of “magic bullet”, using solely sol–gel chemistry to achieve conjugation of PEG chains for stealth, along with three different ligands. In addition, we have answered the recurrent question of the surface ungrafting by investigating the stability of different siloxane linkages with the ERETIC Method (Electronic Reference to Access In Vivo Concentrations) by ¹⁹F NMR quantification. We also compared the efficiency of the hybrid silylated fluorophore covalent linkage in the core of the SiNP to conventional methods. Finally, the tumor-cell-targeting efficiency of these multi-ligand NPs on human endothelial cells (HUVEC or HDMEC) and mixed spheroids of human melanoma cells and HUVEC displaying different types of receptors were evaluated in vitro.

In Vitro and In Vivo Models for Evaluating the Oral Toxicity of Nanomedicines

Toxicity studies for conventional oral drug formulations are standardized and well documented, as required by the guidelines of administrative agencies such as the US Food & Drug Administration (FDA), the European Medicines Agency (EMA) or European Medicines Evaluation Agency (EMEA), and the Japanese Pharmaceuticals and Medical Devices Agency (PMDA). Researchers tend to extrapolate these standardized protocols to evaluate nanoformulations (NFs) because standard nanotoxicity protocols are still lacking in nonclinical studies for testing orally delivered NFs. However, such strategies have generated many inconsistent results because they do not account for the specific physicochemical properties of nanomedicines. Due to their tiny size, accumulated surface charge and tension, sizeable surface-area-to-volume ratio, and high chemical/structural complexity, orally delivered NFs may generate severe topical toxicities to the gastrointestinal tract and metabolic organs, including the liver and kidney. Such toxicities involve immune responses that reflect different mechanisms than those triggered by conventional formulations. Herein, we briefly analyze the potential oral toxicity mechanisms of NFs and describe recently reported in vitro and in vivo models that attempt to address the specific oral toxicity of nanomedicines. We also discuss approaches that may be used to develop nontoxic NFs for oral drug delivery.

Hele Shaw microfluidic device: A new tool for systematic investigation into the effect of the fluid shear stress for organs-on-chips

This method describes a novel approach to systematically investigate the effect of the fluid shear stress (FSS) on epithelial cells thanks to a single microfluidic device based on Hele-Shaw geometry. The method was validated with intestinal Caco-2 cell monolayers and lung A549 cells. We provide guidelines to adjust the experimental parameters to apply specific ranges of FSS and to specify more accurately the area where to image the cells within the device by the performance of a computational simulation of the fluid flow. Most importantly, this simulation enables to validate the equation. This approach was successfully applied to systematically investigate Caco-2 cell monolayers-based intestine-on-chip models as reported in a companion article published in Biomaterials. This study showed that exposure to microfluidic FSS induces significant phenotypical and functional changes. A detailed understanding of the effects of the FSS will enable the realization of in vitro organs-on-chip models with well-defined characteristics tailored to a specific purpose. The Hele-Shaw approach used in this study could be readily applied to other cell types and adapted for a wide range of physiologically relevant FSS.•Fluid shear stress is a key parameter in the differentiation of epithelial cells cultured in organ-on-chip models.•A simple approach can be used to assess the effect of fluid shear on cellular monolayer cultured in microfluidic devices.•Careful optimization of fluid shear stress environment is necessary for the development of better-defined organ-on-chip models.•Computational simulation of the fluid flow gives an accurate definition of the FSS in a microfluidic channel necessary to interpret the results.

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.

Unlocking the Potential of Organ-on-Chip Models through Pumpless and Tubeless Microfluidics

Microfluidic organs-on-chips are rapidly being developed toward eliminating the shortcomings of static in vitro models and better addressing basic and translational research questions. A critical aspect is the dynamic culture environment they provide. However, the associated inherent requirement for controlled fluid shear stress (FSS) and therefore the need for precise pumps limits their implementation. To address this issue, here a novel approach to manufacture pumpless and tubeless organs-on-chips is reported. It relies on the use of a hydrophilic thread to provide a driving force for the perfusion of the cell culture medium through constant evaporation in the controlled conditions of a cell incubator. Well-defined and tuneable flow rates can be applied by adjusting the length and/or diameter of the thread. This approach for the preparation of an intestine-on-chip model based on the Caco-2 cell line is validated. Five days culture under 0.02 dyn·cm^-2 shear conditions yield monolayers similar to those prepared using a high-precision peristaltic pump. A pumpless device can also be used to delineate the effect of FSS on the phenotype of adenocarcinomic human alveolar basal epithelial A549 cells. It is anticipated that the pumpless approach will facilitate and herefore increase the use of organs-on-chips models in the future.

Tools for probing host-bacteria interactions in the gut microenvironment: From molecular to cellular levels

Healthy function of the gut microenvironment is dependent on complex interactions between the bacteria of the microbiome, epithelial and immune (host) cells, and the surrounding tissue. Misregulation of these interactions is implicated in disease. A range of tools have been developed to study these interactions, from mechanistic studies to therapeutic evaluation. In this Digest, we highlight select tools at the cellular and molecular level for probing specific cell-microenvironment interactions. Approaches are overviewed for controlling and probing cell-cell interactions, from transwell and microfluidic devices to engineered bacterial peptidoglycan fragments, and cell-matrix interactions, from three-dimensional scaffolds to chemical handles for in situ modifications.

Enhancing the Cellular Uptake and Antibacterial Activity of Rifampicin through Encapsulation in Mesoporous Silica Nanoparticles

An urgent demand exists for the development of novel delivery systems that efficiently transport antibacterial agents across cellular membranes for the eradication of intracellular pathogens. In this study, the clinically relevant poorly water-soluble antibiotic, rifampicin, was confined within mesoporous silica nanoparticles (MSN) to investigate their ability to serve as an efficacious nanocarrier system against small colony variants of Staphylococcus aureus (SCV S. aureus) hosted within Caco-2 cells. The surface chemistry and particle size of MSN were varied through modifications during synthesis, where 40 nm particles with high silanol group densities promoted enhanced cellular uptake. Extensive biophysical analysis was performed, using quartz crystal microbalance with dissipation (QCM-D) and total internal reflection fluorescence (TIRF) microscopy, to elucidate the mechanism of MSN adsorption onto semi-native supported lipid bilayers (snSLB) and, thus, uncover potential cellular uptake mechanisms of MSN into Caco-2 cells. Such studies revealed that MSN with reduced silanol group densities were prone to greater particle aggregation on snSLB, which was expected to restrict endocytosis. MSN adsorption and uptake into Caco-2 cells correlated well with antibacterial efficacy against SCV S. aureus, with 40 nm hydrophilic particles triggering a ~2.5-log greater reduction in colony forming units, compared to the pure rifampicin. Thus, this study provides evidence for the potential to design silica nanocarrier systems with controlled surface chemistries that can be used to re-sensitise intracellular bacteria to antibiotics by delivering them to the site of infection.

Biofabrication Strategies and Engineered In Vitro Systems for Vascular Mechanobiology

The vascular system is integral for maintaining organ-specific functions and homeostasis. Dysregulation in vascular architecture and function can lead to various chronic or acute disorders. Investigation of the role of the vascular system in health and disease has been accelerated through the development of tissue-engineered constructs and microphysiological on-chip platforms. These in vitro systems permit studies of biochemical regulation of vascular networks and parenchymal tissue and provide mechanistic insights into the biophysical and hemodynamic forces acting in organ-specific niches. Detailed understanding of these forces and the mechanotransductory pathways involved is necessary to develop preventative and therapeutic strategies targeting the vascular system. This review describes vascular structure and function, the role of hemodynamic forces in maintaining vascular homeostasis, and measurement approaches for cell and tissue level mechanical properties influencing vascular phenomena. State-of-the-art techniques for fabricating in vitro microvascular systems, with varying degrees of biological and engineering complexity, are summarized. Finally, the role of vascular mechanobiology in organ-specific niches and pathophysiological states, and efforts to recapitulate these events using in vitro microphysiological systems, are explored. It is hoped that this review will help readers appreciate the important, but understudied, role of vascular-parenchymal mechanotransduction in health and disease toward developing mechanotherapeutics for treatment strategies.

Striving Towards the Perfect In Vitro Oral Drug Absorption Model

Oral drug delivery systems have multiple goals, assessing and enabling intestinal absorption at efficacious doses being one of them. Here we highlight the in vitro advances in modeling drug absorption, which more faithfully reflect human intestinal physiology and reduce the reliance on animal models.