Investigation on Microparticle Transport and Deposition Mechanics in Rhythmically Expanding Alveolar Chip

Design of a multilayer lung chip with multigenerational alveolar ducts to investigate the inhaled particle deposition

We present the development and application of a multilayer microfluidic lung chip designed to accurately replicate the human respiratory bronchi, providing an innovative platform for controlled particle deposition in the lung. By employing a quantitative control method of fluid velocity through the deformation of an elastic PDMS membrane, this platform mimics the passive breathing process in humans and allows for precise simulation of the respiration cycle. We utilized time-lapse photography of fluorescent particles in a water/glycerol solution to qualitatively observe fluid morphology in the channel, while a chip-aerosol exposure device combined with microscopy imaging was employed to visualise aerosol deposition. Both experimental and numerical simulation results showed that particle concentration decreased towards the distal generations of the lung, and that changes in breathing pattern significantly affected particle deposition trends. Furthermore, we found that increasing the residence time of particles in the channel facilitated deeper particle deposition, achievable by adjusting parameters such as breath-hold time, exhalation time, respiration cycle length, and tidal volume. The proposed microfluidic lung chip device has significant potential for future research in respiratory health and inhaled drug delivery, providing an efficient, cost-effective, and ethical alternative to traditional in vivo studies.

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.

Microflows in two-generation alveolar cells at an acinar bifurcation

The alveolus is a basic functional unit of the human respiratory system, and the airflow in the alveoli plays an important role in determining the transport and deposition of particulate matter, which is crucial for inhaled disease diagnosis and drug delivery. In the present study, taking advantage of the precise control ability of the microfluidic technique, a rhythmically expanding alveolar chip with multiple alveoli in two generations is designed and both the geometric and kinematic similarities are matched with the real human respiration system. With the help of a micro-PIV measurement system, the microflow patterns inside each alveolus can be studied. The observed vortex and radial flow patterns and the discovery of stagnant saddle points are similar to those captured in our previous platform with only one alveolus [Lv et al., Lab Chip 20, 2394-2402 (2020)]. However, the interactions between multiple alveoli also uncover new phenomena, such as the finding of stagnant saddle points in non-vortex flow patterns and significant differences in the flow pattern around the points between the time of T/4 and 3T/4. The obtained results could enrich the understanding of microflow in a whole alveolar tree with multiple generations.

Microparticle Transport and Sedimentation in a Rhythmically Expanding Alveolar Chip

Understanding the mechanism of particle transport and sedimentation in pulmonary alveolus is important for deciphering the causes of respiratory diseases and helping the development of drug delivery. In this study, taking advantage of the microfluidic technique, an experimental platform was developed to study particle behavior in a rhythmically expanding alveolar chip for a sufficient number of cycles. The alveolar flow patterns at different generations were measured for two cases with the gravity direction parallel or vertical to the alveolar duct. Affected by both the vortex flow inside the alveoli and the shear flow in the duct simultaneously, it was observed that particles inside the alveoli either escaped from the inlet of the alveolar duct or stayed in the alveoli, revealing the irreversibility of particle transport in the alveoli. At the earlier acinar generations, particles were inclined to deposit on the distal alveolar wall. The settling rates of particles of different sizes in the alveoli were also compared. This study provides valuable data for understanding particle transport and sedimentation in the alveoli.

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.

Revisiting Airflow and Aerosol Transport Phenomena in the Deep Lungs with Microfluidics

The dynamics of respiratory airflows and the associated transport mechanisms of inhaled aerosols characteristic of the deep regions of the lungs are of broad interest in assessing both respiratory health risks and inhalation therapy outcomes. In the present review, we present a comprehensive discussion of our current understanding of airflow and aerosol transport phenomena that take place within the unique and complex anatomical environment of the deep lungs, characterized by submillimeter 3D alveolated airspaces and nominally slow resident airflows, known as low-Reynolds-number flows. We exemplify the advances brought forward by experimental efforts, in conjunction with numerical simulations, to revisit past mechanistic theories of respiratory airflow and particle transport in the distal acinar regions. Most significantly, we highlight how microfluidic-based platforms spanning the past decade have accelerated opportunities to deliver anatomically inspired in vitro solutions that capture with sufficient realism and accuracy the leading mechanisms governing both respiratory airflow and aerosol transport at true scale. Despite ongoing challenges and limitations with microfabrication techniques, the efforts witnessed in recent years have provided previously unattainable in vitro quantifications on the local transport properties in the deep pulmonary acinar airways. These may ultimately provide new opportunities to explore improved strategies of inhaled drug delivery to the deep acinar regions by investigating further the mechanistic interactions between airborne particulate carriers and respiratory airflows at the pulmonary microscales.