Heterogeneous structure and surface tension effects on mechanical response in pulmonary acinus: A finite element analysis

Numerical simulation of voluntary respiration in a model of the whole human lower airway

The lung model construction is limited to the local scale, and the numerical simulation of autonomous breathing is mostly computed from top to bottom in recent research. In this study, models of the entire lower airway from G0-G23 were constructed, and computational simulations were performed for the alveolar model using coupled fluid-solid analysis with pressure changes on the wall and for the rigid bronchial model using computational fluid dynamics by transmitting the boundary conditions step from bottom to top. This paper provides the results under spontaneous respiration, including the ventilation volume of the tracheobronchial tree, the situation of the internal flow field, and the mechanical characteristics of the lung tissues. The mechanical characteristics and the lung functions computed by the models were consistent with clinical or experimental data. This model could provide quantitative analysis results of respiratory mechanics in the lower respiratory tract of the human, which offers a reference for mechanical studies, such as the morphological changes and differentiation of cell types induced by force stimulation and tumor induction. Furthermore, various pathological models can be developed based on this model.

Alveolar wall hyperelastic material properties determined using alveolar cluster model with experimental stress-stretch and pressure-volume data

Micro-scale models of lung tissue have been employed by researchers to investigate alveolar mechanics; however, they have been limited by the lack of biofidelic material properties for the alveolar wall. To address this challenge, a finite element model of an alveolar cluster was developed comprising a tetrakaidecahedron array with the nominal characteristics of human alveolar structure. Lung expansion was simulated in the model by prescribing a pressure and monitoring the volume, to produce a pressure-volume (PV) response that could be compared to experimental PV data. The alveolar wall properties in the model were optimized to match experimental PV response of lungs filled with saline, to eliminate surface tension effects and isolate the alveolar wall tissue response. When simulated in uniaxial tension, the model was in agreement with reported experimental properties of uniaxial tension on excised lung tissue. The work presented herein was able to link micro-scale alveolar response to two disparate macroscopic experimental datasets (stress-stretch and PV response of lung) and presents hyperelastic properties of the alveolar wall for use in alveolar scale finite element models and multi-scale models. Future research will incorporate surface tension effects, and investigate alveolar injury mechanisms.

Digital twins for chronic lung diseases

Digital twins have recently emerged in healthcare. They combine advances in cyber-physical systems, modelling and computation techniques, and enable a bidirectional flow of information between the physical and virtual entities. In respiratory medicine, progress in connected devices and artificial intelligence make it technically possible to obtain digital twins that allow real-time visualisation of a patient's respiratory health. Advances in respiratory system modelling also enable the development of digital twins that could be used to predict the effectiveness of different therapeutic approaches for a patient. For researchers, digital twins could lead to a better understanding of the gene-environment-time interactions involved in the development of chronic respiratory diseases. For clinicians and patients, they could facilitate personalised and timely medicine, by enabling therapeutic adaptations specific to each patient and early detection of disease progression. The objective of this review is to allow the reader to explore the concept of digital twins, their feasibility in respiratory medicine, their potential benefits and the challenges to their implementation.

Surface tension effects on flow dynamics and alveolar mechanics in the acinar region of human lung

Background

Computational fluid dynamics (CFD) simulations, in-vitro setups, and experimental ex-vivo approaches have been applied to numerous alveolar geometries over the past years. They aimed to study and examine airflow patterns, particle transport, particle propagation depth, particle residence times, and particle-alveolar wall deposition fractions. These studies are imperative to both pharmaceutical and toxicological studies, especially nowadays with the escalation of the menacing COVID-19 virus. However, most of these studies ignored the surfactant layer that covers the alveoli and the effect of the air-surfactant surface tension on flow dynamics and air-alveolar surface mechanics.

Methods

The present study employs a realistic human breathing profile of 4.75s for one complete breathing cycle to emphasize the importance of the surfactant layer by numerically comparing airflow phenomena between a surfactant-enriched and surfactant-deficient model. The acinar model exhibits physiologically accurate alveolar and duct dimensions extending from lung generations 18 to 23. Airflow patterns in the surfactant-enriched model support previous findings that the recirculation of the flow is affected by its propagation depth. Proximal lung generations experience dominant recirculating flow while farther generations in the distal alveolar region exhibit dominant radial flows. In the surfactant-enriched model, surface tension values alternate during inhalation and exhalation, with values increasing to 25 mN/m at the inhalation and decreasing to 1 mN/m at the end of the exhalation. In the surfactant-deficient model, only water coats the alveolar walls with a high surface tension value of 70 mN/m.

Results

Results showed that surfactant deficiency in the alveoli adversely alters airflow behavior and generates unsteady chaotic breathing through the production of vorticities, accompanied by higher vorticity magnitudes (100% increase at the end of exhalation) and higher velocity magnitudes (8.69% increase during inhalation and 11.9% increase during exhalation). In addition, high air-water surface tension in the surfactant-deficient case was found to induce higher shear stress values (by around a factor of 10) on the alveolar walls than that of the surfactant-enriched case.

Conclusion

Overall, it was concluded that the presence of the surfactant improves respiratory mechanics and allows for smooth breathing and normal respiration.

Mathematical modeling of pulmonary acinus structure: Verification of acinar shape effects on pathway structure using rat lungs

The pulmonary acinus is the gas exchange unit in the lung and has a very complex microstructure. The structure model is essential to understand the relationship between structural heterogeneity and mechanical phenomena at the acinus level with computational approaches. We propose an acinus structure model represented by a cluster of truncated octahedra in conical, double-conical, inverted conical, or chestnut-like conical confinement to accommodate recent experimental information of rodent acinar shapes. The basis of the model is the combined use of Voronoi and Delaunay tessellations and the optimization of the ductal tree assuming the number of alveoli and the mean path length as quantities related to gas exchange. Before applying the Voronoi tessellation, controlling the seed coordinates enables us to model acinus with arbitrary shapes. Depending on the acinar shape, the distribution of path length varies. The lengths are more widely spread for the cone acinus, with a bias toward higher values, while most of the lengths for the inverted cone acinus primarily take a similar value. Longer pathways have smaller tortuosity and more generations, and duct length per generation is almost constant irrespective of generation, which agrees well with available experimental data. The pathway structure of cone and chestnut-like cone acini is similar to the surface acini's features reported in experiments. According to space-filling requirements in the lung, other conical acini may also be acceptable. The mathematical acinus structure model with various conical shapes can be a platform for computational studies on regional differences in lung functions along the lung surface, underlying respiratory physiology and pathophysiology.

Recent advances in the understanding of alveolar flow

Understanding the dynamics of airflow in alveoli and its effect on the behavior of particle transport and deposition is important for understanding lung functions and the cause of many lung diseases. The studies on these areas have drawn substantial attention over the last few decades. This Review discusses the recent progress in the investigation of behavior of airflow in alveoli. The information obtained from studies on the structure of the lung airway tree and alveolar topology is provided first. The current research progress on the modeling of alveoli is then reviewed. The alveolar cell parameters at different generation of branches, issues to model real alveolar flow, and the current numerical and experimental approaches are discussed. The findings on flow behavior, in particular, flow patterns and the mechanism of chaotic flow generation in the alveoli are reviewed next. The different flow patterns under different geometrical and flow conditions are discussed. Finally, developments on microfluidic devices such as lung-on-a-chip devices are reviewed. The issues of current devices are discussed.

Pulmonary Stretch and Lung Mechanotransduction: Implications for Progression in the Fibrotic Lung

Lung fibrosis results from the synergic interplay between regenerative deficits of the alveolar epithelium and dysregulated mechanisms of repair in response to alveolar and vascular damage, which is followed by progressive fibroblast and myofibroblast proliferation and excessive deposition of the extracellular matrix. The increased parenchymal stiffness of fibrotic lungs significantly affects respiratory mechanics, making the lung more fragile and prone to non-physiological stress during spontaneous breathing and mechanical ventilation. Given their parenchymal inhomogeneity, fibrotic lungs may display an anisotropic response to mechanical stresses with different regional deformations (micro-strain). This behavior is not described by the standard stress–strain curve but follows the mechano-elastic models of “squishy balls”, where the elastic limit can be reached due to the excessive deformation of parenchymal areas with normal elasticity that are surrounded by inelastic fibrous tissue or collapsed induration areas, which tend to protrude outside the fibrous ring. Increasing evidence has shown that non-physiological mechanical forces applied to fibrotic lungs with associated abnormal mechanotransduction could favor the progression of pulmonary fibrosis. With this review, we aim to summarize the state of the art on the relation between mechanical forces acting on the lung and biological response in pulmonary fibrosis, with a focus on the progression of damage in the fibrotic lung during spontaneous breathing and assisted ventilatory support.

Correlating Local Volumetric Tissue Strains with Global Lung Mechanics Measurements

The mechanics of breathing is a fascinating and vital process. The lung has complexities and subtle heterogeneities in structure across length scales that influence mechanics and function. This study establishes an experimental pipeline for capturing alveolar deformations during a respiratory cycle using synchrotron radiation micro-computed tomography (SR-micro-CT). Rodent lungs were mechanically ventilated and imaged at various time points during the respiratory cycle. Pressure-Volume (P-V) characteristics were recorded to capture any changes in overall lung mechanical behaviour during the experiment. A sequence of tomograms was collected from the lungs within the intact thoracic cavity. Digital volume correlation (DVC) was used to compute the three-dimensional strain field at the alveolar level from the time sequence of reconstructed tomograms. Regional differences in ventilation were highlighted during the respiratory cycle, relating the local strains within the lung tissue to the global ventilation measurements. Strains locally reached approximately 150% compared to the averaged regional deformations of approximately 80-100%. Redistribution of air within the lungs was observed during cycling. Regions which were relatively poorly ventilated (low deformations compared to its neighbouring region) were deforming more uniformly at later stages of the experiment (consistent with its neighbouring region). Such heterogenous phenomena are common in everyday breathing. In pathological lungs, some of these non-uniformities in deformation behaviour can become exaggerated, leading to poor function or further damage. The technique presented can help characterize the multiscale biomechanical nature of a given pathology to improve patient management strategies, considering both the local and global lung mechanics.

Characterization of air flow and lung function in the pulmonary acinus by fluid-structure interaction in idiopathic interstitial pneumonias

Background and objective

The idiopathic interstitial pneumonias (IIPs) are diffuse parenchymal lung disorders that are associated with substantial morbidity and mortality. Early diagnosis and disease stratification of IIP patients are important because these are related with the treatment and prognosis. Idiopathic pulmonary fibrosis (IPF) and nonspecific interstitial pneumonia (NSIP) are two major distinctive pathologic patterns of pulmonary fibrosis. We researched the application of the fluid-structure interaction (FSI) to the respiratory system and compared the pulmonary acinus mechanics and functions in healthy and IIP models.

Methods

The human pulmonary alveolus is idealized by a three-dimensional honeycomb-like geometry, and a fluid-structure interaction analysis is performed to study the normal and diseased breathing mechanics. The computational domain consists of two generations of alveolar ducts within the pulmonary acinus, with alveolar geometries approximated as closely packed 14-sided polygons.

Findings

In a normal breathing cycle, the flow rate of the healthy model is significantly larger than that of the NSIP and IPF models. Similar trends are observed for the volume change and the maximum pressure drop. The flow rate and the volume change of the NSIP are almost the same as those of IPF. The maximum pressure drop of NSIP is 5.5% larger than that of IPF. There is a 47% decrease in the pulmonary acinus compliance for the NSIP and IPF compared with that of the healthy model. The acinus resistances of NSIP and IPF are higher than those of the healthy lung by 6.4~11.2%. In particular, the pulmonary acinus resistance of the NSIP lung is higher than that of the IPF lung by 4.5%.

Conclusions

Our study demonstrates the differences of air flow and lung function in the pulmonary acinus between the healthy and the IIP models. These changes in the lung are important considerations for early diagnosis and disease stratification in patients. Patient-based geometry can to be included in the computational models in future studies.

Fluid dynamic assessment of tracheal flow in infants with congenital tracheal stenosis before and after surgery

Tracheal flow in infants with congenital tracheal stenosis (CTS) was numerically investigated using subject-specific airway models before and after reconstructive surgery. We quantified tracheal flow based on airway resistance during inhalation, and compared it between controls and patients before and after surgery. The airway resistance in each subject was assessed using geometrical parameters of the trachea: the minimum cross-sectional area A_min, the minimum cross-sectional area normalized by the standard deviation of the cross-sectional area A_min/σ_A, the area ratio of the minimum and maximum cross-sectional area A_min/A_max, and ratio of the normalized standard deviation of cross-sectional area to the mean cross-sectional area σ_A/A_mean. Our numerical results demonstrated that such geometrical parameters could be used to assess the severity of CTS. Since subjects can be more clearly categorized as controls and most preoperative patients in terms of the airway resistance, a simulation using subject-specific airway models can lead us to a precise understanding of tracheal flow, and also provide knowledge about therapeutic decision. Our numerical results also demonstrated that significant surgical expansion of cross-sectional area did not help recover tracheal flow because of expansion loss. These results will be helpful not only when making therapeutic decisions about surgery but also when assessing quality of life in postoperative patients. Graphical abstract.