Technological strategies to estimate and control diffusive passage times through the mucus barrier in mucosal drug delivery

The power of weak, transient interactions across biology: A paradigm of emergent behavior

A growing list of diverse biological systems and their equally diverse functionalities provides realizations of a paradigm of emergent behavior. In each of these biological systems, pervasive ensembles of weak, short-lived, spatially local interactions act autonomously to convey functionalities at larger spatial and temporal scales. In this article, a range of diverse systems and functionalities are presented in a cursory manner with literature citations for further details. Then two systems and their properties are discussed in more detail: yeast chromosome biology and human respiratory mucus.

The role of mucosal barriers in disease progression and transmission

Mucus is a biological hydrogel that coats and protects all non-keratinized wet epithelial surfaces. Mucins, the primary structural components of mucus, are critical components of the gel layer that protect against invading pathogens. For communicable diseases, pathogen-mucin interactions contribute to the pathogen's fate and the potential for disease progression in-host, as well as the potential for onward transmission. We begin by reviewing in-host mucus filtering mechanisms, including size filtering and interaction filtering, which regulate the permeability of mucus barriers to all molecules including pathogens. Next, we discuss the role of mucins in communicable diseases at the point of transmission (i.e. how the encapsulation of pathogens in emitted mucosal droplets externally to hosts may modulate pathogen infectivity and viability). Overall, mucosal barriers modulate both host susceptibility as well as the dynamics of population-level disease transmission. The study of mucins and their use in models and experimental systems are therefore crucial for understanding the mechanistic biophysical principles underlying disease transmission and the early stages of host infection.

Nasal delivery as a strategy for the prevention and treatment of COVID-19

INTRODUCTION

The upper respiratory tract is a major route of infection for COVID-19 and other respiratory diseases. Thus, it appears logical to exploit the nose as administration site to prevent, fight, or minimize infectious spread and treat the disease. Numerous nasal products addressing these aspects have been considered and developed for COVID-19.

AREAS COVERED

This review gives a comprehensive overview of the different approaches involving nasal delivery, i.e., nasal vaccination, barrier products, and antiviral pharmacological treatments that have led to products on the market or under clinical evaluation, highlighting the peculiarities of the nose as application and absorption site and pointing at key aspects of nasal drug delivery.

EXPERT OPINION

From the analysis of nasal delivery strategies to prevent or fight COVID-19, it emerges that, especially for nasal immunization, formulations appear the same as originally designed for parenteral administration, leading to suboptimal results. On the other hand, mechanical barrier and antiviral products, designed to halt or treat the infection at early stage, have been proven effective but were rarely brought to the clinics. If supported by robust and targeted product development strategies, intranasal immunization and drug delivery can represent valid and sometimes superior alternatives to more conventional parenteral and oral medications.

Nanoparticle Diffusion in Respiratory Mucus Influenced by Mucociliary Clearance: A Review of Mathematical Modeling

Background: Inhalation and deposition of particles in human airways have attracted considerable attention due to importance of particulate pollutants, transmission of infectious diseases, and therapeutic delivery of drugs at targeted areas. We summarize current state-of-the art research in particle deposition on airway surface liquid (ASL) influenced by mucociliary clearance (MCC) by identifying areas that need further investigation. Methodology: We aim to review focus on governing and constitutive equations describing MCC geometry followed by description of mathematical modeling of ciliary forces, mucus rheology properties, and numerical approaches to solve modified time-dependent Navier-Stokes equations. We also review mathematical modeling of particle deposition in ASL influenced by MCC, particle transport in ASL in terms of Eulerian and Lagrangian approaches, and discuss the corresponding mass transport issues in this layer. Whenever required, numerical predictions are contrasted with the pertinent experimental data. Results: Results indicate that mean mucus and periciliary liquid velocities are strongly influenced by mucus rheological characteristics as well as ciliary abnormalities. However, most of the currently available literature on mucus fiber spacing, ciliary beat frequency, and particle surface chemistry is based on particle deposition on ASL by considering a fixed value of ASL velocity. The effects of real ASL flow regimes on particle deposition in this layer are limited. In addition, no other study is available on modeling nonhomogeneous and viscoelastic characteristics of mucus layer on ASL drug delivery. Conclusion: Simplification of assumptions on governing equations of drug delivery in ASL influenced by MCC leads to imposing some limitations on numerical results.

Impact of hydrogel biophysical properties on tumor spheroid growth and drug response

The extracellular matrix (ECM) plays a critical role in regulating cell-matrix interactions during tumor progression. These interactions are due in large part to the biophysical properties responding to cancer cell interactions. Within in vitro models, the ECM is mimicked by hydrogels, which possess adjustable biophysical properties that are integral to tumor development. This work presents a systematic and comparative study on the impact of the biophysical properties of two widely used natural hydrogels, Matrigel and collagen gel, on tumor growth and drug response. The biophysical properties of Matrigel and collagen including complex modulus, loss tangent, diffusive permeability, and pore size, were characterised. Then the spheroid growth rates in these two hydrogels were monitored for spheroids with two different sizes (140 μm and 500 μm in diameters). An increased migratory growth was observed in the lower concentration of both the gels. The effect of spheroid incorporation within the hydrogel had a minimal impact on the hydrogel's complex modulus. Finally, 3D tumor models using different concentrations of hydrogels were applied for drug treatment using paclitaxel. Spheroids cultured in hydrogels with different concentrations showed different drug response, demonstrating the significant effect of the choice of hydrogels and their concentrations on the drug response results despite using the same spheroids. This study provides useful insights into the effect of hydrogel biophysical properties on spheroid growth and drug response and highlights the importance of hydrogel selection and in vitro model design.

Altering the viscoelastic properties of mucus-grown Pseudomonas aeruginosa biofilms affects antibiotic susceptibility

The viscoelastic properties of biofilms are correlated with their susceptibility to mechanical and chemical stress, and the airway environment in muco-obstructive pulmonary diseases (MOPD) facilitates robust biofilm formation. Hyperconcentrated, viscoelastic mucus promotes chronic inflammation and infection, resulting in increased mucin and DNA concentrations. The viscoelastic properties of biofilms are regulated by biopolymers, including polysaccharides and DNA, and influence responses to antibiotics and phagocytosis. We hypothesize that targeted modulation of biofilm rheology will compromise structural integrity and increase antibiotic susceptibility and mucociliary transport. We evaluate biofilm rheology on the macro, micro, and nano scale as a function of treatment with a reducing agent, a biopolymer, and/or tobramycin to define the relationship between the viscoelastic properties of biofilms and susceptibility. Disruption of the biofilm architecture is associated with altered macroscopic and microscopic moduli, rapid vector permeability, increased antibiotic susceptibility, and improved mucociliary transport, suggesting that biofilm modulating therapeutics will improve the treatment of chronic respiratory infections in MOPD.

Antibody protection from SARS-CoV-2 respiratory tract exposure and infection

The COVID-19 pandemic has underscored the need to understand the dynamics of SARS-CoV-2 respiratory infection and protection provided by the immune response. SARS-CoV-2 infections are characterized by a particularly high viral load, and further by the small number of inhaled virions sufficient to generate a high viral titer in the nasal passage a few days after exposure. SARS-CoV-2 specific antibodies (Ab), induced from vaccines, previous infection, or inhaled monoclonal Ab, have proven effective against SARS-CoV-2 infection. Our goal in this work is to model the protective mechanisms that Ab can provide and to assess the degree of protection from individual and combined mechanisms at different locations in the respiratory tract. Neutralization, in which Ab bind to virion spikes and inhibit them from binding to and infecting target cells, is one widely reported protective mechanism. A second mechanism of Ab protection is muco-trapping, in which Ab crosslink virions to domains on mucin polymers, effectively immobilizing them in the mucus layer. When muco-trapped, the continuous clearance of the mucus barrier by coordinated ciliary propulsion entrains the trapped viral load toward the esophagus to be swallowed. We model and simulate the protection provided by either and both mechanisms at different locations in the respiratory tract, parametrized by the Ab titer and binding-unbinding rates of Ab to viral spikes and mucin domains. Our results illustrate limits in the degree of protection by neutralizing Ab alone, the powerful protection afforded by muco-trapping Ab, and the potential for dual protection by muco-trapping and neutralizing Ab to arrest a SARS-CoV-2 infection. This manuscript was submitted as part of a theme issue on "Modelling COVID-19 and Preparedness for Future Pandemics".

Screening confinement of entanglements: Role of a self-propelling end inducing ballistic chain reptation

Synthetic and natural nanomaterials with self-propelling mechanisms continue to be explored to boost chain mobility beyond normal reptation in the crowded environments of entangled chains. Here we employ scaling theory and numerical simulations to demonstrate that activating one chain end of a singular or isolated chain boosts entanglement-constrained chain reptation from the one-dimensional diffusive mobility as described by the de Gennes-Edwards-Doi model to ballistic motion along the entanglement tube contour. The active chain is effectively screened from the constraint of entanglements on length scales exceeding the tube size.

From Static to Dynamic: A Review on the Role of Mucus Heterogeneity in Particle and Microbial Transport

Mucus layers (McLs) are on the front line of the human defense system that protect us from foreign abiotic/biotic particles (e.g., airborne virus SARS-CoV-2) and lubricates our organs. Recently, the impact of McLs on human health (e.g., nutrient absorption and drug delivery) and diseases (e.g., infections and cancers) has been studied extensively, yet their mechanisms are still not fully understood due to their high variety among organs and individuals. We characterize these variances as the heterogeneity of McLs, which lies in the thickness, composition, and physiology, making the systematic research on the roles of McLs in human health and diseases very challenging. To advance mucosal organoids and develop effective drug delivery systems, a comprehensive understanding of McLs' heterogeneity and how it impacts mucus physiology is urgently needed. When the role of airway mucus in the penetration and transmission of coronavirus (CoV) is considered, this understanding may also enable a better explanation and prediction of the CoV's behavior. Hence, in this Review, we summarize the variances of McLs among organs, health conditions, and experimental settings as well as recent advances in experimental measurements, data analysis, and model development for simulations.

MPTHub: An Open-Source Software for Characterizing the Transport of Particles in Biorelevant Media

The study of particle transport in different environments plays an essential role in understanding interactions with humans and other living organisms. Importantly, obtained data can be directly used for multiple applications in fields such as fundamental biology, toxicology, or medicine. Particle movement in biorelevant media can be readily monitored using microscopy and converted into time-resolved trajectories using freely available tracking software. However, translation into tangible and meaningful parameters is time consuming and not always intuitive. We developed new software—MPTHub—as an open-access, standalone, user-friendly tool for the rapid and reliable analysis of particle trajectories extracted from video microscopy. The software was programmed using Python and allowed to import and analyze trajectory data, as well as to export relevant data such as individual and ensemble time-averaged mean square displacements and effective diffusivity, and anomalous transport exponent. Data processing was reliable, fast (total processing time of less than 10 s), and required minimal memory resources (up to a maximum of around 150 MB in random access memory). Demonstration of software applicability was conducted by studying the transport of different polystyrene nanoparticles (100–200 nm) in mucus surrogates. Overall, MPTHub represents a freely available software tool that can be used even by inexperienced users for studying the transport of particles in biorelevant media.

Machine learning-informed predictions of nanoparticle mobility and fate in the mucus barrier

Nanomaterial diffusion through mucus is important to basic and applied areas of research such as drug delivery. However, it is often challenging to interpret nanoparticle dynamics within the mucus gel due to its heterogeneous microstructure and biochemistry. In this study, we measured the diffusion of polyethylene glycolylated nanoparticles (NPs) in human airway mucus ex vivo using multiple particle tracking and utilized machine learning to classify diffusive vs sub-diffusive NP movement. Using mathematic models that account for the mode of NP diffusion, we calculate the percentage of NPs that would cross the mucus barrier over time in airway mucus with varied total solids concentration. From this analysis, we predict rapidly diffusing NPs will cross the mucus barrier in a physiological timespan. Although less efficient, sub-diffusive "hopping" motion, a characteristic of a continuous time random walk, may also enable NPs to cross the mucus barrier. However, NPs exhibiting fractional Brownian sub-diffusion would be rapidly removed from the airways via mucociliary clearance. In samples with increased solids concentration (>5% w/v), we predict up to threefold reductions in the number of nanoparticles capable of crossing the mucus barrier. We also apply this approach to explore diffusion and to predict the fate of influenza A virus within human mucus. We predict only a small fraction of influenza virions will cross the mucus barrier presumably due to physical obstruction and adhesive interactions with mucin-associated glycans. These results provide new tools to evaluate the extent of synthetic and viral nanoparticle penetration through mucus in the lung and other tissues.

Modeling insights into SARS-CoV-2 respiratory tract infections prior to immune protection

Mechanistic insights into human respiratory tract (RT) infections from SARS-CoV-2 can inform public awareness as well as guide medical prevention and treatment for COVID-19 disease. Yet the complexity of the RT and the inability to access diverse regions pose fundamental roadblocks to evaluation of potential mechanisms for the onset and progression of infection (and transmission). We present a model that incorporates detailed RT anatomy and physiology, including airway geometry, physical dimensions, thicknesses of airway surface liquids (ASLs), and mucus layer transport by cilia. The model further incorporates SARS-CoV-2 diffusivity in ASLs and best-known data for epithelial cell infection probabilities, and, once infected, duration of eclipse and replication phases, and replication rate of infectious virions. We apply this baseline model in the absence of immune protection to explore immediate, short-term outcomes from novel SARS-CoV-2 depositions onto the air-ASL interface. For each RT location, we compute probability to clear versus infect; per infected cell, we compute dynamics of viral load and cell infection. Results reveal that nasal infections are highly likely within 1-2 days from minimal exposure, and alveolar pneumonia occurs only if infectious virions are deposited directly into alveolar ducts and sacs, not via retrograde propagation to the deep lung. Furthermore, to infect just 1% of the 140 m2 of alveolar surface area within 1 week, either 103 boluses each with 106 infectious virions or 106 aerosols with one infectious virion, all physically separated, must be directly deposited. These results strongly suggest that COVID-19 disease occurs in stages: a nasal/upper RT infection, followed by self-transmission of infection to the deep lung. Two mechanisms of self-transmission are persistent aspiration of infected nasal boluses that drain to the deep lung and repeated rupture of nasal aerosols from infected mucosal membranes by speaking, singing, or cheering that are partially inhaled, exhaled, and re-inhaled, to the deep lung.

Biophysical properties of hydrogels for mimicking tumor extracellular matrix

The extracellular matrix (ECM) is an essential component of the tumor microenvironment. It plays a critical role in regulating cell-cell and cell-matrix interactions. However, there is lack of systematic and comparative studies on different widely-used ECM mimicking hydrogels and their properties, making the selection of suitable hydrogels for mimicking different in vivo conditions quite random. This study systematically evaluates the biophysical attributes of three widely used natural hydrogels (Matrigel, collagen gel and agarose gel) including complex modulus, loss tangent, diffusive permeability and pore size. A new and facile method was developed combining Critical Point Drying, Scanning Electron Microscopy imaging and a MATLAB image processing program (CSM method) for the characterization of hydrogel microstructures. This CSM method allows accurate measurement of the hydrogel pore size down to nanometer resolution. Furthermore, a microfluidic device was implemented to measure the hydrogel permeability (P_d) as a function of particle size and gel concentration. Among the three gels, collagen gel has the lowest complex modulus, medium pore size, and the highest loss tangent. Agarose gel exhibits the highest complex modulus, the lowest loss tangent and the smallest pore size. Collagen gel and Matrigel produced complex moduli close to that estimated for cancer ECM. The P_d of these hydrogels decreases significantly with the increase of particle size. By assessing different hydrogels' biophysical characteristics, this study provides valuable insights for tailoring their properties for various three-dimensional cancer models.

Influenza A virus diffusion through mucus gel networks

Mucus in the lung plays an essential role as a barrier to infection by viral pathogens such as influenza A virus (IAV). Previous work determined mucin-associated sialic acid acts as a decoy receptor for IAV hemagglutinin (HA) binding and the sialic-acid cleaving enzyme, neuraminidase (NA), facilitates virus passage through mucus. However, it has yet to be fully addressed how the physical structure of the mucus gel influences its barrier function and its ability to trap viruses via glycan mediated interactions to prevent infection. To address this, IAV and nanoparticle diffusion in human airway mucus and mucin-based hydrogels is quantified using fluorescence video microscopy. We find the mobility of IAV in mucus is significantly influenced by the mesh structure of the gel and in contrast to prior reports, these effects likely influence virus passage through mucus gels to a greater extent than HA and NA activity. In addition, an analytical approach is developed to estimate the binding affinity of IAV to the mucus meshwork, yielding dissociation constants in the mM range, indicative of weak IAV-mucus binding. Our results provide important insights on how the adhesive and physical barrier properties of mucus influence the dissemination of IAV within the lung microenvironment.

Influenza A virus movement in mucus is found to be affected by the mesh structure of the gel network and further analysis reveals weak IAV-mucus binding.

Principles of slowed hydrogen diffusion through a mucus layer

The control of transport through mucus layers is a ubiquitous phenomenon in physiological systems. Mucus is often tasked with the mediation of passive, diffusive transport of small ionic species. However, questions remain regarding how mucin gel characteristics (charge density of the polymeric network, binding affinity of ions with mucus) govern the rate at which ions diffuse through mucus layers. Experimental studies measuring hydrogen diffusion through gastric mucus have provided conflicting results, and it is not clear if the rate of ionic diffusion through mucus layers is appreciably different than in aqueous environments (depending on experimental preparation). Here, we present a mathematical analysis of electrodiffussion of two ionic species (hydrogen and chloride) through a mucus layer. In addition to accounting for the chemical binding of hydrogen to the mucus network, we enforce a zero net current condition (as mucus layers in physiological systems are not generally electrogenic) and calculate the Donnan potential that occurs at the edge of the mucus layer. The model predicts the steady-state fluxes of ionic species and the induced potential across the layer. We characterize the dependence of these quantities on the chemical properties of the mucus gel, the composition of the bath solution, and the molecular mobility of the dissolved anion, and we show that the model predictions are consistent with a large portion of the experimental literature. Our analysis predicts that mucus layers generically slow the diffusive transport of hydrogen, but that chemical binding with the network attenuates this effect.

Molecular Dynamics Simulations to Explore the Structure and Rheological Properties of Normal and Hyperconcentrated Airway Mucus

We develop the first molecular dynamics model of airway mucus based on the detailed physical properties and chemical structure of the predominant gel-forming mucin MUC5B. Our airway mucus model leverages the LAMMPS open-source code [https://lammps.sandia.gov], based on the statistical physics of polymers, from single molecules to networks. On top of the LAMMPS platform, the chemical structure of MUC5B is used to superimpose proximity-based, non-covalent, transient interactions within and between the specific domains of MUC5B polymers. We explore feasible ranges of hydrophobic and electrostatic interaction strengths between MUC5B domains with 9 nanometer spatial and 1 nanosecond temporal resolution. Our goal here is to propose and test a mechanistic hypothesis for a striking clinical observation with respect to airway mucus: a 10-fold increase in non-swellable, dense structures called flakes during progression of cystic fibrosis disease. Among the myriad possible effects that might promote self-organization of MUC5B networks into flake structures, we hypothesize and confirm that the clinically confirmed increase in mucin concentration, from 1.5 to 5 mg/mL, alone is sufficient to drive the structure changes observed with scanning electron microscopy images from experimental samples. We post-process the LAMMPS simulated datasets at 1.5 and 5 mg/mL, both to image the structure transition and compare with scanning electron micrographs and to show that the 3.33-fold increase in concentration induces closer proximity of interacting electrostatic and hydrophobic domains, thereby amplifying the proximity-based strength of the interactions.

Design of ophthalmic micelles loaded with diclofenac sodium: effect of chitosan and temperature on the block-copolymer micellization behaviour

Diclofenac sodium 0.1% is a commonly used NSAID with well-documented clinical efficacy in reducing postoperative inflammation; however, its corneal tolerability and ophthalmic tissue bioavailability require further improvement. Advanced micellar delivery systems composed of block-copolymers and chitosan showing fine balance between the mucoadhesion and mucus permeation, capable to slip through the mucus barrier and adhere to the epithelial ocular surface, may be used to tackle both challenges. The aggregation behaviour of the block-copolymers in the presence of different additives will dramatically influence the quality attributes like particle size, particle size distribution, drug-polymer interaction, zeta potential, drug incorporation, important for the delicate balance among mucoadhesion and permeation, as well as safety and efficacy of the ophthalmic micelles. Therefore, quality by design approach and D-optimal experimental design model were used to create a pool of useful data for the influence of chitosan and the formulation factors on the block copolymer's aggregation behaviour during the development and optimization of Diclofenac loaded Chitosan/Lutrol F127 or F68 micelles. Particle size, polydispersity index, dissolution rate, FTIR and DSC studies, NMR spectroscopy, cytotoxicity, mucoadhesivity, mucus permeation studies, and bioadhesivity were assessed as critical quality attributes. FTIR and DSC studies pointed to the chaotropic effect of chitosan during the micelle aggregation. Mainly, Pluronic F68 micellization behaviour was more dramatically affected by the presence of chitosan, and self-aggregation into larger micelles with high polydispersity index was favoured at higher chitosan concentration. The optimized formulation with highest potential for ophthalmic delivery of diclofenac sodium, good cytotoxicity profile, delicate balance of the mucoadhesivity, and mucus permeation was in the design space of Chitosan/Lutrol F127 micelles.

Functionalized Particles Designed for Targeted Delivery

Pure bioactive compounds alone can only be exceptionally administered in medical treatment. Usually, drugs are produced as various forms of active compounds and auxiliary substances, combinations assuring the desired healing functions. One of the important drug forms is represented by a combination of active substances and particle-shaped polymer in the nano- or micrometer size range. The review describes recent progress in this field balanced with basic information. After a brief introduction, the paper presents a concise overview of polymers used as components of nano- and microparticle drug carriers. Thereafter, progress in direct synthesis of polymer particles with functional groups is discussed. A section is devoted to formation of particles by self-assembly of homo- and copolymer-bearing functional groups. Special attention is focused on modification of the primary functional groups introduced during particle preparation, including introduction of ligands promoting anchorage of particles onto the chosen living cell types by interactions with specific receptors present in cell membranes. Particular attention is focused on progress in methods suitable for preparation of particles loaded with bioactive substances. The review ends with a brief discussion of the still not answered questions and unsolved problems.

Chain stiffness boosts active nanoparticle transport in polymer networks

Recent advances in technologies such as nanomanufacturing and nanorobotics have opened new pathways for the design of active nanoparticles (NPs) capable of penetrating biolayers for biomedical applications, e.g., for drug delivery. The coupling and feedback between active NP motility (with large stochastic increments relative to passive NPs) and the induced nonequilibrium deformation and relaxation responses of the polymer network, spanning scales from the NP to the local structure of the network, remain to be clarified. Using molecular dynamics simulations, combined with a Rouse mode analysis of network chains and position and velocity autocorrelation functions of the NPs, we demonstrate that the mobility of active NPs within cross-linked, concentrated polymer networks is a monotonically increasing function of chain stiffness, contrary to passive NPs, for which chain stiffness suppresses mobility. In flexible networks, active NPs exhibit a behavior similar to passive NPs, with a boost in mobility proportional to the self-propulsion force. These results are suggestive of design strategies for active NP penetration of stiff biopolymer matrices.

ENHANCED ELECTRODIFFUSIVE TRANSPORT ACROSS A MUCUS LAYER

Diffusive transport of small ionic species through mucus layers is a ubiquitous phenomenon in physiology. However, some debate remains regarding how the various characteristics of mucus (charge of the polymers themselves, binding affinity of ions with mucus) impact the rate at which small ions may diffuse through a hydrated mucus gel. Indeed it is not even clear if small ionic species diffuse through mucus gel at an appreciably different rate than they do in aqueous solution. Here, we present a mathematical description of the transport of two ionic species (hydrogen and chloride) through a mucus layer based on the Nernst-Planck equations of electrodiffusion. The model explicitly accounts for the binding affinity of hydrogen to the mucus material, as well as the Donnan potential that occurs at the interface between regions with and without mucus. Steady state fluxes of ionic species are quantified, as are their dependencies on the chemical properties of the mucus gel and the composition of the bath solution. We outline a mechanism for generating enhanced diffusive flux of hydrogen across the gel region, and hypothesize how this mechanism may be relevant to the apparently contradictory experimental data in the literature.

The Cervicovaginal Mucus Barrier

Preterm births are a global health priority that affects 15 million babies every year worldwide. There are no effective prognostic and therapeutic strategies relating to preterm delivery, but uterine infections appear to be a major cause. The vaginal epithelium is covered by the cervicovaginal mucus, which is essential to health because of its direct involvement in reproduction and functions as a selective barrier by sheltering the beneficial lactobacilli while helping to clear pathogens. During pregnancy, the cervical canal is sealed with a cervical mucus plug that prevents the vaginal flora from ascending toward the uterine compartment, which protects the fetus from pathogens. Abnormalities of the cervical mucus plug and bacterial vaginosis are associated with a higher risk of preterm delivery. This review addresses the current understanding of the cervicovaginal mucus and the cervical mucus plug and their interactions with the microbial communities in both the physiological state and bacterial vaginosis, with a focus on gel-forming mucins. We also review the current state of knowledge of gel-forming mucins contained in mouse cervicovaginal mucus and the mouse models used to study bacterial vaginosis.

Transmucosal drug administration as an alternative route in palliative and end-of-life care during the COVID-19 pandemic

The Coronavirus disease 2019 (COVID-19) pandemic has led to a surge in need for alternative routes of administration of drugs for end of life and palliative care, particularly in community settings. Transmucosal routes include intranasal, buccal, sublingual and rectal. They are non-invasive routes for systemic drug delivery with the possibility of self-administration, or administration by family caregivers. In addition, their ability to offer rapid onset of action with reduced first-pass metabolism make them suitable for use in palliative and end-of-life care to provide fast relief of symptoms. This is particularly important in COVID-19, as patients can deteriorate rapidly. Despite the advantages, these routes of administration face challenges including a relatively small surface area for effective drug absorption, small volume of fluid for drug dissolution and the presence of a mucus barrier, thereby limiting the number of drugs that are suitable to be delivered through the transmucosal route. In this review, the merits, challenges and limitations of each of these transmucosal routes are discussed. The goals are to provide insights into using transmucosal drug delivery to bring about the best possible symptom management for patients at the end of life, and to inspire scientists to develop new delivery systems to provide effective symptom management for this group of patients.

LPS-binding IgG arrests actively motile Salmonella Typhimurium in gastrointestinal mucus

The gastrointestinal (GI) mucosa is coated with a continuously secreted mucus layer that serves as the first line of defense against invading enteric bacteria. We have previously shown that antigen-specific immunoglobulin G (IgG) can immobilize viruses in both human airway and genital mucus secretions through multiple low-affinity bonds between the array of virion-bound IgG and mucins, thereby facilitating their rapid elimination from mucosal surfaces and preventing mucosal transmission. Nevertheless, it remains unclear whether weak IgG-mucin crosslinks could reinforce the mucus barrier against the permeation of bacteria driven by active flagella beating, or in predominantly MUC2 mucus gel. Here, we performed high-resolution multiple particle tracking to capture the real-time motion of hundreds of individual fluorescent Salmonella Typhimurium in fresh, undiluted GI mucus from Rag1^-/- mice, and analyzed the motion using a hidden Markov model framework. In contrast to control IgG, the addition of anti-lipopolysaccharide IgG to GI mucus markedly reduced the progressive motility of Salmonella by lowering the swim speed and retaining individual bacteria in an undirected motion state. Effective crosslinking of Salmonella to mucins was dependent on Fc N-glycans. Our findings implicate IgG-mucin crosslinking as a broadly conserved function that reduces mucous penetration of both bacterial and viral pathogens.

Novel O/W nanoemulsions for nasal administration: Structural hints in the selection of performing vehicles with enhanced mucopenetration

We propose novel oil-in-water nanoemulsions (O/W NEs) including PEGylated surfactants and chitosan, showing good biocompatibility and optimization for nasal administration of drugs or vaccines. The transmucosal route has been shown to be ideal for a fast and efficient absorption and represents a viable alternative when the oral administration is problematic. The critical structural features in view of optimal encapsulation and transmucosal delivery were assessed by characterizing the NEs with complementary scattering techniques, i.e. dynamic light scattering (DLS), small angle X-ray (SAXS) and neutron scattering (SANS). Combined results allowed for selecting the formulations with the best suited structural properties and in addition establishing their propensity to enter the mucus barrier. To this scope, mucin was used as a model system and the effect of adding chitosan to the NEs, as adjuvant, was investigated. Remarkably, the presence of chitosan had a positive impact on the diffusion of the NE particles through the mucin matrix. We can infer that chitosan-mucin interaction induces density inhomogeneity and an increase in the pore size within the gel matrix that enhances the PEGylated NEs mobility. The coupling of mucoadhesive and mucopenetrating agents is shown to be a promising strategy for innovative transmucosal delivery systems.

Engineering Polymer-Binding Bispecific Antibodies for Enhanced Pretargeted Delivery of Nanoparticles to Mucus-Covered Epithelium

Mucus represents a major barrier to sustained and targeted drug delivery to mucosal epithelium. Ideal drug carriers should not only rapidly diffuse across mucus, but also bind the epithelium. Unfortunately, ligand-conjugated particles often exhibit poor penetration across mucus. In this work, we explored a two-step "pretargeting" approach through engineering a bispecific antibody that binds both cell-surface ICAM-1 and polyethylene glycol (PEG) on the surface of nanoparticles, thereby effectively decoupling cell targeting from particle design and formulation. When tested in a mucus-coated Caco-2 culture model that mimics the physiological process of mucus clearance, pretargeting increased the amount of PEGylated particles binding to cells by around 2-fold or more compared to either non-targeted or actively targeted PEGylated particles. Pretargeting also markedly enhanced particle retention in mouse intestinal tissues. Our work underscores pretargeting as a promising strategy to improve the delivery of therapeutics to mucosal surfaces.

Optical catapulting of microspheres in mucus models-toward overcoming the mucus biobarrier

The generalized phase contrast method is employed as an efficient "phase-only" laser beam-shaping technique in an optical setup built for catapulting microspheres through simple mucus models. The influence of the laser power and mucin concentration on the motion of the microspheres is investigated in terms of instant and average velocities on a 250-μm trajectory, corresponding to the mucus thickness in the human gastrointestinal tract. Increasing the laser power leads to higher velocities in all the tested samples, while increasing the mucin concentration leads to significant velocity decrease for similar laser input power. However, velocities of up to 95  μm  ·  s  -  1 are demonstrated in a 5% mucin simple mucus model using our catapulting system. This study contributes to understanding and overcoming the challenges of optical manipulation in mucus models. This paves the way for efficient optical manipulation of three-dimensional-printed light-controlled microtools with the ability to penetrate the mucus biobarrier for in vitro drug-delivery studies.

Pathological mucus and impaired mucus clearance in cystic fibrosis patients result from increased concentration, not altered pH

Cystic fibrosis (CF) is a recessive genetic disease that is characterised by airway mucus plugging and reduced mucus clearance. There are currently alternative hypotheses that attempt to describe the abnormally viscous and elastic mucus that is a hallmark of CF airways disease, including: 1) loss of CF transmembrane regulator (CFTR)-dependent airway surface volume (water) secretion, producing mucus hyperconcentration-dependent increased viscosity, and 2) impaired bicarbonate secretion by CFTR, producing acidification of airway surfaces and increased mucus viscosity.A series of experiments was conducted to determine the contributions of mucus concentration versus pH to the rheological properties of airway mucus across length scales from the nanoscopic to macroscopic.For length scales greater than the nanoscopic, i.e. those relevant to mucociliary clearance, the effect of mucus concentration dominated over the effect of airway acidification.Mucus hydration and chemical reduction of disulfide bonds that connect mucin monomers are more promising therapeutic approaches than alkalisation.

A New Paradigm of Pharmaceutical Drug Delivery Systems (DDS): Challenges for Space, Time, and Shapes

Using 3D food printing with the Internet of Things (IoT) technology, patients can receive diagnoses and prescriptions from their doctors while in the comfort of their homes. The patient-specific prescription has been innovated by converging 3D food printing technology with drug delivery systems (DDSs). Quantitative drug dosages can be incorporated into the composition of food and produced in any shape within a short time. Automating food and DDSs makes promising implications for healing patients remotely, as well. Each of these aspects, along with IoT technology, have contributed to increased health care for patients, no matter their location. The quantitative discharge of vitamin C melted in water, mayonnaise, ketchup, and peanut butter has been verified using the Piston Typed Extrusion (PTE) method. Designs with different curves and shapes were repeatedly printed with a head speed of 1.6×10^-2 m/s, and it was confirmed that effective control while printing the shapes was possible. The Hagen-Poiseuille (HP) formula was utilized to simulate the overall printing time. This simulation affirmed that increasing the head speed from 1.6×10^-2 m/s to 4.0×10^-2 m/s had reduced the printing time consistently, but the time was not reduced continuously after 4.0×10^-2 m/s, depending on the materials' viscosities and how much curvature exists in the designs. The precision of printing was adjusted within 5% of the theoretical value during printing, and the IoT technology allowed printing of the materials within five minutes, regardless of the patient's location.

Mucins and Their Role in Shaping the Functions of Mucus Barriers

We review what is currently understood about how the structure of the primary solid component of mucus, the glycoprotein mucin, gives rise to the mechanical and biochemical properties of mucus that are required for it to perform its diverse physiological roles. Macroscale processes such as lubrication require mucus of a certain stiffness and spinnability, which are set by structural features of the mucin network, including the identity and density of cross-links and the degree of glycosylation. At the microscale, these same features affect the mechanical environment experienced by small particles and play a crucial role in establishing an interaction-based filter. Finally, mucin glycans are critical for regulating microbial interactions, serving as receptor binding sites for adhesion, as nutrient sources, and as environmental signals. We conclude by discussing how these structural principles can be used in the design of synthetic mucin-mimetic materials and provide suggestions for directions of future work in this field.