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

Hydrogel-Integrated Millifluidic Systems: Advancing the Fabrication of Mucus-Producing Human Intestinal Models

The luminal surface of the intestinal epithelium is protected by a vital mucus layer, which is essential for lubrication, hydration, and fostering symbiotic bacterial relationships. Replicating and studying this complex mucus structure in vitro presents considerable challenges. To address this, we developed a hydrogel-integrated millifluidic tissue chamber capable of applying precise apical shear stress to intestinal models cultured on flat or 3D structured hydrogel scaffolds with adjustable stiffness. The chamber is designed to accommodate nine hydrogel scaffolds, 3D-printed as flat disks with a storage modulus matching the physiological range of intestinal tissue stiffness (~3.7 kPa) from bioactive decellularized and methacrylated small intestinal submucosa (dSIS-MA). Computational fluid dynamics simulations were conducted to confirm a laminar flow profile for both flat and 3D villi-comprising scaffolds in the physiologically relevant regime. The system was initially validated with HT29-MTX seeded hydrogel scaffolds, demonstrating accelerated differentiation, increased mucus production, and enhanced 3D organization under shear stress. These characteristic intestinal tissue features are essential for advanced in vitro models as they critically contribute to a functional barrier. Subsequently, the chamber was challenged with human intestinal stem cells (ISCs) from the terminal ileum. Our findings indicate that biomimicking hydrogel scaffolds, in combination with physiological shear stress, promote multi-lineage differentiation, as evidenced by a gene and protein expression analysis of basic markers and the 3D structural organization of ISCs in the absence of chemical differentiation triggers. The quantitative analysis of the alkaline phosphatase (ALP) activity and secreted mucus demonstrates the functional differentiation of the cells into enterocyte and goblet cell lineages. The millifluidic system, which has been developed and optimized for performance and cost efficiency, enables the creation and modulation of advanced intestinal models under biomimicking conditions, including tunable matrix stiffness and varying fluid shear stresses. Moreover, the readily accessible and scalable mucus-producing cellular tissue models permit comprehensive mucus analysis and the investigation of pathogen interactions and penetration, thereby offering the potential to advance our understanding of intestinal mucus in health and disease.

In Vitro Models for Investigating Intestinal Host-Pathogen Interactions

Infectious diseases are increasingly recognized as a major threat worldwide due to the rise of antimicrobial resistance and the emergence of novel pathogens. In vitro models that can adequately mimic in vivo gastrointestinal physiology are in high demand to elucidate mechanisms behind pathogen infectivity, and to aid the design of effective preventive and therapeutic interventions. There exists a trade-off between simple and high throughput models and those that are more complex and physiologically relevant. The complexity of the model used shall be guided by the biological question to be addressed. This review provides an overview of the structure and function of the intestine and the models that are developed to emulate this. Conventional models are discussed in addition to emerging models which employ engineering principles to equip them with necessary advanced monitoring capabilities for intestinal host-pathogen interrogation. Limitations of current models and future perspectives on the field are presented.

Protein Nanotubes as Advanced Material Platforms and Delivery Systems

Protein nanotubes (PNTs) as state-of-the-art nanocarriers are promising for various potential applications both in the food and pharmaceutical industries. Derived from edible starting sources like α-lactalbumin, lysozyme, and ovalbumin, PNTs bear properties of biocompatibility and biodegradability. Their large specific surface area and hydrophobic core facilitate chemical modification and loading of bioactive substances, respectively. Moreover, their enhanced permeability and penetration ability across biological barriers such as intestinal mucus, extracellular matrix, and thrombus clot, make it promising platforms for health-related applications. Most importantly, their simple preparation processes enable large-scale production, supporting applications in the biomedical and nanotechnological fields. Understanding the self-assembly principles is crucial for controlling their morphology, size, and shape, and thus provides the ground to a multitude of applications. Here, the current state-of-the-art of PNTs including their building materials, physicochemical properties, and self-assembly mechanisms are comprehensively reviewed. The advantages and limitations, as well as challenges and prospects for their successful applications in biomaterial and pharmaceutical sectors are then discussed and highlighted. Potential cytotoxicity of PNTs and the need of regulations as critical factors for enabling in vivo applications are also highlighted. In the end, a brief summary and future prospects for PNTs as advanced platforms and delivery systems are included.

Gels That Serve as Mucus Simulants: A Review

Mucus is a critical part of the human body's immune system that traps and carries away various particulates such as anthropogenic pollutants, pollen, viruses, etc. Various synthetic hydrogels have been developed to mimic mucus, using different polymers as their backbones. Common to these simulants is a three-dimensional gel network that is physically crosslinked and is capable of loosely entrapping water within. Two of the challenges in mimicking mucus using synthetic hydrogels include the need to mimic the rheological properties of the mucus and its ability to capture particulates (its adhesion mechanism). In this paper, we review the existing mucus simulants and discuss their rheological, adhesive, and tribological properties. We show that most, but not all, simulants indeed mimic the rheological properties of the mucus; like mucus, most hydrogel mucus simulants reviewed here demonstrated a higher storage modulus than its loss modulus, and their values are in the range of that found in mucus. However, only one mimics the adhesive properties of the mucus (which are critical for the ability of mucus to capture particulates), Polyvinyl alcohol-Borax hydrogel.

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.

Development and validation of a porcine artificial colonic mucus model reflecting the properties of native colonic mucus in pigs

Colonic mucus plays a key role in colonic drug absorption. Mucus permeation assays could therefore provide useful insights and support rational formulation development in the early stages of drug development. However, the collection of native colonic mucus from animal sources is labor-intensive, does not yield amounts that allow for routine experimentation, and raises ethical concerns. In the present study, we developed an in vitro porcine artificial colonic mucus model based on the characterization of native colonic mucus. The structural properties of the artificial colonic mucus were validated against the native secretion for their ability to capture key diffusion patterns of macromolecules in native mucus. Moreover, the artificial colonic mucus could be stored under common laboratory conditions, without compromising its barrier properties. In conclusion, the porcine artificial colonic mucus model can be considered a biorelevant way to study the diffusion behavior of drug candidates in colonic mucus. It is a cost-efficient screening tool easily incorporated into the early stages of drug development and it contributes to the implementation of the 3Rs (refinement, reduction, and replacement of animals) in the drug development process.

Optimisation of a High-Throughput Model for Mucus Permeation and Nanoparticle Discrimination Using Biosimilar Mucus

High-throughput permeation models are essential in drug development for timely screening of new drug and formulation candidates. Nevertheless, many current permeability assays fail to account for the presence of the gastrointestinal mucus layer. In this study, an optimised high-throughput mucus permeation model was developed employing a highly biorelevant mucus mimic. While mucus permeation is primarily conducted in a simple mucin solution, the complex chemistry, nanostructure and rheology of mucus is more accurately modelled by a synthetic biosimilar mucus (BSM) employing additional protein, lipid and rheology-modifying polymer components. Utilising BSM, equivalent permeation of various molecular weight fluorescein isothiocyanate-dextrans were observed, compared with native porcine jejunal mucus, confirming replication of the natural mucus permeation barrier. Furthermore, utilising synthetic BSM facilitated the analysis of free protein permeation which could not be quantified in native mucus due to concurrent proteolytic degradation. Additionally, BSM could differentiate between the permeation of poly (lactic-co-glycolic) acid nanoparticles (PLGA-NP) with varying surface chemistries (cationic, anionic and PEGylated), PEG coating density and size, which could not be achieved by a 5% mucin solution. This work confirms the importance of utilising highly biorelevant mucus mimics in permeation studies, and further development will provide an optimal method for high-throughput mucus permeation analysis.

Bioinspired in vitro intestinal mucus model for 3D-dynamic culture of bacteria

The intestinal mucus is a biological barrier that supports the intestinal microbiota growth and filters molecules. To perform these functions, mucus possesses optimized microstructure and viscoelastic properties and it is steadily replenished thus flowing along the gut. The available in vitro intestinal mucus models are useful tools in investigating the microbiota-human cells interaction, and are used as matrices for bacterial culture or as static component of microfluidic devices like gut-on-chips. The aim of this work is to engineer an in vitro mucus models (I-Bac3Gel) addressing in a single system physiological viscoelastic properties (i.e., 2-200 Pa), 3D structure and suitability for dynamic bacterial culture. Homogeneously crosslinked alginate hydrogels are optimized in composition to obtain target viscoelastic and microstructural properties. Then, rheological tests are exploited to assess a priori the hydrogels capability to withstand the flow dynamic condition. We experimentally assess the suitability of I-Bac3Gels in the evolving field of microfluidics by applying a dynamic flow to a bacterial-loaded mucus model and by monitoring E. coli growth and survival. The engineered models represent a step forward in the modelling of the mucus, since they can answer to different urgent needs such as a 3D structure, bioinspired properties and compatibility with dynamic system.