Rheological properties of synthetic mucus for airway clearance

Effects of Kinematic Hardening of Mucus Polymers in an Airway Closure Model

The formation of a liquid plug inside a human airway, known as airway closure, is computationally studied by considering the elastoviscoplastic (EVP) properties of the pulmonary mucus covering the airway walls for a range of liquid film thicknesses and Laplace numbers. The airway is modeled as a rigid tube lined with a single layer of an EVP liquid. The Saramito-Herschel-Bulkley (Saramito-HB) model is coupled with an Isotropic Kinematic Hardening model (Saramito-HB-IKH) to allow energy dissipation at low strain rates. The rheological model is fitted to the experimental data under healthy and cystic fibrosis (CF) conditions. Yielded/unyielded regions and stresses on the airway wall are examined throughout the closure process. Yielding is found to begin near the closure in the Saramito-HB model, whereas it occurs noticeably earlier in the Saramito-HB-IKH model. The kinematic hardening is seen to have a notable effect on the closure time, especially for the CF case, with the effect being more pronounced at low Laplace numbers and initial film thicknesses. Finally, standalone effects of rheological properties on wall stresses are examined considering their physiological values as baseline.

Comprehensive Characterization of the Viscoelastic Properties of Bovine Submaxillary Mucin (BSM) Hydrogels and the Effect of Additives

This study presents a comprehensive characterization of the viscoelastic and structural properties of bovine submaxillary mucin (BSM), which is widely used as a commercial source to conduct mucus-related research. We conducted concentration studies of BSM and examined the effects of various additives, NaCl, CaCl2, MgCl2, lysozyme, and DNA, on its rheological behavior. A notable connection between BSM concentration and viscoelastic properties was observed, particularly under varying ionic conditions. The rheological spectra could be well described by a fractional Kelvin-Voigt model with a minimum of model parameters. A detailed proteomics analysis provided insight into the protein, especially mucin composition within BSM, showing MUC19 as the main component. Cryo-scanning electron microscopy enabled the visualization of the porous BSM network structure. These investigations give us a more profound comprehension of the BSM properties, especially those pertaining to viscoelasticity, and how they are influenced by concentration and environmental conditions, aspects relevant to the field of mucus research.

Mucus-Mimicking Mucin-Based Hydrogels by Tandem Chemical and Physical Crosslinking

Mucosal tissues represent a major interface between the body and the external environment and are covered by a highly hydrated mucins gel called mucus. Mucus lubricates, protects and modulates the moisture levels of the tissue and is capitalized in transmucosal drug delivery. Pharmaceutical researchers often use freshly excised animal mucosal membranes to assess mucoadhesion and muco-penetration of pharmaceutical formulations which may struggle with limited accessibility, reproducibility, and ethical questions. Aiming to develop a platform for the rationale study of the interaction of drugs and delivery systems with mucosal tissues, in this work mucus-mimicking mucin-based hydrogels are synthesized by the tandem chemical and physical crosslinking of mucin aqueous solutions. Chemical crosslinking is achieved with glutaraldehyde (0.3% and 0.75% w/v), while physical crosslinking by one or two freeze-thawing cycles. Hydrogels after one freeze-thawing cycle show water content of 97.6-98.1%, density of 0.0529-0.0648 g cm⁻3, and storage and loss moduli of ≈40-60 and ≈3-5 Pa, respectively, that resemble the properties of native gastrointestinal mucus. The mechanical stability of the hydrogels increases over the number of freeze-thawing cycles. Overall results highlight the potential of this simple, reproducible, and scalable method to produce artificial mucus-mimicking hydrogels for different applications in pharmaceutical research.

The yielding behaviour of human mucus

Mucus is a viscoelastic material with non-linear rheological properties such as a yield stress of the order of a few hundreds of millipascals to a few tens of pascals, due to a complex network of mucins in water along with non-mucin proteins, DNA and cell debris. In this review, we discuss the origin of the yield stress in human mucus, the changes in the rheology of mucus with the occurrence of diseases, and possible clinical applications in disease detection as well as cure. We delve into the domain of mucus rheology, examining both macro- and microrheology. Macrorheology involves investigations conducted at larger length scales (∼ a few hundreds of μm or higher) using traditional rheometers, which probe properties on a bulk scale. It is significant in elucidating various mucosal functions within the human body. This includes rejecting unwanted irritants out of lungs through mucociliary and cough clearance, protecting the stomach wall from the acidic environment as well as biological entities, safeguarding cervical canal from infections and providing a swimming medium for sperms. Additionally, we explore microrheology, which encompasses studies performed at length scales ranging from a few tens of nm to a μm. These microscale studies find various applications, including the context of drug delivery. Finally, we employ scaling analysis to elucidate a few examples in lung, cervical, and gastric mucus, including settling of irritants in lung mucus, yielding of lung mucus in cough clearance and cilial beating, spreading of exogenous surfactants over yielding mucus, swimming of Helicobacter pylori through gastric mucus, and lining of protective mucus in the stomach. The scaling analyses employed on the applications mentioned above provide us with a deeper understanding of the link between the rheology and the physiology of mucus.

The synthesis of Tween80/calcium phosphate nanocomposite bioactive gelatine-based gels as a proof of concept for tooth sensitivity home-treatment

Dentine hypersensitivity interferes with the lifestyle owing to pain arising from exposed dentine surfaces in response to stimuli. One common way to treat this problem is to occlude the exposed tubules. In this work, we have proposed a home-based treatment gel for tooth sensitivity. The gel was prepared using the emulsion method and contained Tween80/calcium phosphate nanocomposite that could occlude the tubules after 10 h of application. In preparation, Tween80 was used as a surfactant, and oleic acid was used as an oil phase to form a water-in-oil nanoreactor for calcium phosphate synthesis. Finally, different concentrations of gelatine were used to transform the emulsion into a stable gel. The nanoparticles had a uniform spherical shape and a diameter of approximately 300 nm. The nanocomposite gel containing the lowest amount of gelatine (Gel-T80-5%GE) exhibited the best liquid-like property and the highest occlusion rate of 95%.

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.

Mucociliary clearance affected by mucus-periciliary interface stimulations using analytical solution during cough and sneeze

Assessment of mucus velocity variations under different conditions including viscosity variation and boundary conditions is useful to develop mucosal-based medical treatments. This paper deals with the analytical investigation of mucus-periciliary velocities under mucus-periciliary interface movements and mucus viscosity variations. The results for mucus velocity show that there is no difference between the two cases under the free-slip condition. Therefore, power-law mucus can be substituted with a high viscosity Newtonian fluid since the upper boundary of the mucus layer is exposed to the free-slip condition. However, when the upper boundary of the mucus layer is under nonzero shear stress levels, including cough or sneeze, the assumption of a high viscosity Newtonian mucus layer is invalid. Moreover, mucus viscosity variations are investigated for both Newtonian and power-law mucus layers under sneeze and cough to propose a mucosal-based medical treatment. The results indicate by varying mucus viscosity up to a critical value, the direction of mucus movement changes. The critical values of viscosity in sneezing and coughing for Newtonian and power-law mucus layers are 10-4 and 5 × 10-5 and 0.0263 and 006.024 m2 s-1, respectively. Therefore, the pathogen entry into the respiratory system can be prevented by varying mucus viscosity during sneeze and cough.

Effectiveness of Autogenic Drainage in Improving Pulmonary Function in Patients with Cystic Fibrosis

The use of autogenic drainage (AD) in patients with cystic fibrosis (CF) has been officially approved; therefore, the purpose of this study was to compare the efficiency of the leading therapeutic techniques based on AD in patients with CF; Among patients with CF assessments were made of spirometric parameters, percent blood oxygen saturation, and the general feeling of the patients (Borg, VAS, and mMRC dyspnea scale) before and after therapy using AD, using AD in connection with a belt or a Simeox device and AD in combination with both a belt and Simeox device simultaneously. The best therapeutic effects were generated by the combination of AD with the belt and with the Simeox device. The greatest improvements were observed for FEV1, FVC, PEF, FET, saturation, and patient comfort. In patients <10.5 years of age, the increase in the level of FEV3 and FEV6 was significant in comparison to older patients. Due to their efficacy, therapies connected with AD should be applied not only in hospital departments but also during daily patient care. Given the particular benefits observed in patients <10.5 years of age, it is important to guarantee real accessibility to this form of physiotherapy, especially in this age group.

Experimental studies and mathematical modeling of the viscoelastic rheology of tracheobronchial mucus from respiratory healthy patients

Background

Tracheobronchial mucus plays a crucial role in pulmonary function by providing protection against inhaled pathogens. Due to its composition of water, mucins, and other biomolecules, it has a complex viscoelastic rheological behavior. This interplay of both viscous and elastic properties has not been fully described yet. In this study, we characterize the rheology of human mucus using oscillatory and transient tests. Based on the transient tests, we describe the material behavior of mucus under stress and strain loading by mathematical models.

Methods

Mucus samples were collected from clinically used endotracheal tubes. For rheological characterization, oscillatory amplitude-sweep and frequency-sweep tests, and transient creep-recovery and stress-relaxation tests were performed. The results of the transient test were approximated using the Burgers model, the Weibull distribution, and the six-element Maxwell model. The three-dimensional microstructure of the tracheobronchial mucus was visualized using scanning electron microscope imaging.

Results

Amplitude-sweep tests showed storage moduli ranging from 0.1 Pa to 10,000 Pa and a median critical strain of 4%. In frequency-sweep tests, storage and loss moduli increased with frequency, with the median of the storage modulus ranging from 10 Pa to 30 Pa, and the median of the loss modulus from 5 Pa to 14 Pa. The Burgers model approximates the viscoelastic behavior of tracheobronchial mucus during a constant load of stress appropriately (R2 of 0.99), and the Weibull distribution is suitable to predict the recovery of the sample after the removal of this stress (R2 of 0.99). The approximation of the stress-relaxation test data by a six-element Maxwell model shows a larger fit error (R2 of 0.91).

Conclusions

This study provides a detailed description of all process steps of characterizing the rheology of tracheobronchial mucus, including sample collection, microstructure visualization, and rheological investigation. Based on this characterization, we provide mathematical models of the rheological behavior of tracheobronchial mucus. These can now be used to simulate mucus flow in the respiratory system through numerical approaches.

Mucus from human bronchial epithelial cultures: rheology and adhesion across length scales

Mucus is a viscoelastic aqueous fluid that participates in the protective barrier of many mammals' epithelia. In the airways, together with cilia beating, mucus rheological properties are crucial for lung mucociliary function, and, when impaired, potentially participate in the onset and progression of chronic obstructive pulmonary disease (COPD). Samples of human mucus collected in vivo are inherently contaminated and are thus poorly characterized. Human bronchial epithelium (HBE) cultures, differentiated from primary cells at an air-liquid interface, are highly reliable models to assess non-contaminated mucus. In this paper, the viscoelastic properties of HBE mucus derived from healthy subjects, patients with COPD and from smokers are measured. Hallmarks of shear-thinning and elasticity are obtained at the macroscale, whereas at the microscale mucus appears as a heterogeneous medium showing an almost Newtonian behaviour in some extended regions and an elastic behaviour close to boundaries. In addition, we developed an original method to probe mucus adhesion at the microscopic scale using optical tweezers. The measured adhesion forces and the comparison with mucus-simulants rheology as well as mucus imaging collectively support a structure composed of a network of elastic adhesive filaments with a large mesh size, embedded in a very soft gel.

An optimized protocol for assessment of sputum macrorheology in health and muco-obstructive lung disease

Background: Airway mucus provides important protective functions in health and abnormal viscoelasticity is a hallmark of muco-obstructive lung diseases such as cystic fibrosis (CF). However, previous studies of sputum macrorheology from healthy individuals and patients with CF using different experimental protocols yielded in part discrepant results and data on a systematic assessment across measurement settings and conditions remain limited.

Objectives: The aim of this study was to develop an optimized and reliable protocol for standardized macrorheological measurements of airway mucus model systems and native human sputum from healthy individuals and patients with muco-obstructive lung disease.

Methods: Oscillatory rheological shear measurements were performed using bovine submaxillary mucin (BSM) at different concentrations (2% and 10% solids) and sputum samples from healthy controls (n = 10) and patients with CF (n = 10). Viscoelastic properties were determined by amplitude and frequency sweeps at 25°C and 37°C with or without solvent trap using a cone-plate geometry.

Results: Under saturated atmosphere, we did not observe any temperature-dependent differences in 2% and 10% BSM macrorheology, whereas in the absence of evaporation control 10% BSM demonstrated a significantly higher viscoelasticity at 37°C. Similarly, during the measurements without evaporation control at 37°C we observed a substantial increase in the storage modulus G′ and the loss modulus G″ of the highly viscoelastic CF sputum but not in the healthy sputum.

Conclusion: Our data show systematically higher viscoelasticity of CF compared to healthy sputum at 25°C and 37°C. For measurements at the higher temperature using a solvent trap to prevent evaporation is essential for macrorheological analysis of mucus model systems and native human sputum. Another interesting finding is that the viscoelastic properties are not much sensitive to the applied experimental deformation and yield robust results despite their delicate consistency. The optimized protocol resulting from this work will facilitate standardized quantitative assessment of abnormalities in viscoelastic properties of airway mucus and response to muco-active therapies in patients with CF and other muco-obstructive lung diseases.

The importance of pre-formulation studies and of 3D-printed nasal casts in the success of a pharmaceutical product intended for nose-to-brain delivery

This review aims to cement three hot topics in drug delivery: (a) the pre-formulation of new products intended for nose-to-brain delivery; (b) the development of nasal casts for studying the efficacy of potential new nose-to-brain delivery systems at the early of their development (pre-formulation); (c) the use of 3D printing based on a wide variety of materials (transparent, biocompatible, flexible) providing an unprecedented fabrication tool towards personalized medicine by printing nasal cast on-demand based on CT scans of patients. This review intends to show the links between these three subjects. Indeed, the pathway selected to administrate the drug to the brain not only influence the formulation strategies to implement but also the design of the cast, to get the most convincing measures from it. Moreover, the design of the cast himself influences the choice of the 3D-printing technology, which, in its turn, bring more constraints to the nasal replica design. Consequently, the formulation of the drug, the cast preparation and its realisation should be thought of as a whole and not separately.

A 3D "In Vitro" Model to Study Hyaluronan Effect in Nasal Epithelial Cell Line Exposed to Double-Stranded RNA Poly(I:C)

Environmental agents, including viral and bacterial infectious agents, are involved in the alteration of physicochemical and biological parameters in the nasal epithelium. Hyaluronan (HA) has an important role in the regulation of tissue healing properties. High molecular weight HA (HMW-HA) shows greater anti-inflammatory responses than medium molecular weight HA (MMW-HA) and low molecular weight HA (LMW-HA). We investigated the effect of HMW-HA, MMW-HA and LMW-HA on the regulation of physicochemical and biological parameters in an “in vitro” model that might mimic viral infections of the nasal epithelium. Human nasal epithelial cell line RPMI2650 was stimulated with double-stranded RNA (dsRNA) Poly(I:C) for 5 days in air-liquid-interface (ALI) culture (3D model of airway tissue). dsRNA Poly(I:C) treatment significantly decreased transepithelial electrical resistance (TEER) in the stratified nasal epithelium of RPMI2650 and increased pH values, rheological parameters (elastic G’ and viscous G’’), and Muc5AC and Muc5B production in the apical wash of ALI culture of RPMI2650 in comparison to untreated cells. RPMI2650 treated with dsRNA Poly(I:C) in the presence of HMW-HA showed lower pH values, Muc5AC and Muc5B production, and rheological parameters, as well as increased TEER values in ALI culture, compared to cells treated with Poly(I:C) alone or pretreated with LMW-HA and MMW-HA. Our 3D “in vitro” model of epithelium suggests that HMW-HA might be a coadjuvant in the pharmacological treatment of viral infections, allowing for the control of some physicochemical and biological properties affecting the epithelial barrier of the nose during infection.

Why antiplectic metachronal cilia waves are optimal to transport bronchial mucus

The coordinated beating of epithelial cilia in human lungs is a fascinating problem from the hydrodynamics perspective. The phase lag between neighboring cilia is able to generate collective cilia motions, known as metachronal waves. Different kinds of waves can occur, antiplectic or symplectic, depending on the direction of the wave with respect to the flow direction. It is shown here, using a coupled lattice Boltzmann-immersed boundary solver, that the key mechanism responsible for their transport efficiency is a blowing-suction effect that displaces the interface between the periciliary liquid and the mucus phase. The contribution of this mechanism on the average flow generated by the cilia is compared to the contribution of the lubrication effect. The results reveal that the interface displacement is the main mechanism responsible for the better efficiency of antiplectic metachronal waves over symplectic ones to transport bronchial mucus. The conclusions drawn here can be extended to any two-layer fluid configuration having different viscosities, and put into motion by cilia-shaped or comb-plate structures, having a back-and-forth motion with phase lags.

Influence of frequency and amplitude on the mucus viscoelasticity of the novel mechano-acoustic Frequencer™

BACKGROUND

Cystic fibrosis affects 1/3200 Caucasians. This genetic disease disturbs the ion and water homeostasis across epithelia, thus rendering mucus more viscous and harder to expel. Conventional treatments rely on the clapping method coupled with postural drainage. Despite the effectiveness of these procedures, they are invasive and enervating.

METHODS

Here we study a new mechano-acoustic treatment device to help patients expectorate excess mucus, the Frequencer™. We test both normal and pathological synthetic mucin solutions (1 % and 4 % by weight) in vitro. We varied the frequency applied (from 20 Hz to 60 Hz) as well as the amplitude (from 50 % to 100 % intensity). Moreover, we assessed the effect of NaCl on mucus rehydration.

RESULTS

A frequency of 40 Hz coupled with a 0.5 gL^-1NaCl solution provokes partial mucus rehydration, regardless of the amplitude selected, as the work of adhesion measurements evidenced.

CONCLUSIONS

Mechanical solicitation is fundamental to help patients affected by cystic fibrosis expectorate mucus. With an operating frequency of 20 Hz to 65 Hz, the Frequencer™ provides a gentler therapy than traditional methods (conventional chest physiotherapy). The Frequencer™ proved to be effective in the homogenization of synthetic mucin solutions in vitro in 20 min and elicited improved effectiveness in a mucin-rich environment.

Fractional Burgers Fluid Flow Due to Metachronal Ciliary Motion in an Inclined Tube

Cilia-induced flow of fractional Burgers fluid is studied in an inclined tube for both symplectic and antiplectic wave patterns. The solution of the problem is persued under the long wave length limitation. The fractional Adomian decomposition method is employed to evaluate the pressure gradient. Mathematical expressions for the axial velocity, frictional force, pressure gradient, and stream function are obtained and the influence of the main operating parameters is discussed in detail. It is noted that the velocity profile is more dominant in the case of antiplectic metachronal waves compared to symplectic ones, which confirms former results on the better capability of antiplectic waves to transport mucus, obtained with more complex numerical solvers.