Session E21: Biofluids: Physiological IV - Experimental Studies in Respiratory Flows

Liquid Therapy Delivery Models Using Microfluidic Airways

The propagation and break-up of viscous and surfactant-laden liquid plugs in the lungs is an active area of research in view of liquid plug installation in the lungs to treat a host of different pulmonary conditions. This includes Infant Respiratory Distress Syndrome (IRDS) the primary cause of neonatal death and disability. Until present, experimental studies of liquid plugs have generally been restricted to low-viscosity Newtonian fluids along a single bifurcation. However, these fluids reflect poorly the actual liquid medication therapies used to treat pulmonary conditions. The present work attempts to uncover the propagation, rupture and break-up of liquid plugs in the airway tree using microfluidic models spanning three or more generations of the bronchiole tree. Our approach allows the dynamics of plug propagation and break-up to be studied in real-time, in a one-to-one scale \textit{in vitro} model, as a function of fluid rheology, trailing film dynamics and bronchial tree geometry. \textit{Understanding these dynamics are a first and necessary step to deliver more effectively boluses of liquid medication to the lungs} while minimizing the injury caused to epithelial cells lining the lungs from the rupture of such liquid plugs.   

Acinus-on-a-chip: a microfluidic platform for pulmonary acinar flows

Convective respiratory flows in the pulmonary acinus and their influence on the fate of inhaled particles are typically studied using computational fluid dynamics (CFD) or scaled-up experimental models. However, current experiments generally capture only flow dynamics, without inhaled particle dynamics, due to difficulties in simultaneously matching flow and particle dynamics. In an effort to overcome these limitations, we have designed a novel microfluidic device mimicking acinar flow conditions directly at the physiological scale. The model features an anatomically-inspired acinar geometry with five dichotomously branching airway generations lined with periodically expanding and contracting alveoli. Using micro-particle image velocimetry (PIV), we reveal experimentally a gradual transition of alveolar flow patterns along the acinar tree from recirculating to radial streamlines, in support of previous predictions from CFD simulations. We demonstrate the applicability of the device for studying the mechanisms of particle deposition in the pulmonary acinus by mapping deposition sites of airborne fluorescent micro-particles (0.1-1$\mu $m) and visualizing trajectories of airborne incense particles inside the system.   

Steady Flow in Subject-Specific Human Airways from Mouth to Sixth Bronchial Generation

Understanding the complex flow topology within the human lung is critical to assess gas exchange and particle transport as they relate to the development and treatment of respiratory diseases. While idealized airway models have been investigated extensively, only limited information is available for anatomically accurate geometries. We have measured the full three-dimensional, mean velocity field from the mouth to the sixth bronchial generation in a patient-specific geometry at steady inspiration. Magnetic resonance velocimetry is used to measure the flow of water at realistic Reynolds number in a 3D-printed model derived from the CT scan of a healthy subject. The canonical laryngeal jet is observed; however, its structure is altered by an upstream jet behind the tongue, which is not discussed in the literature. Regions of separation in the supraglottic space are found to generate streamwise vortices. The resulting swirl persists to the first bifurcation and modifies the vorticity distribution in the main bronchi relative to that of a symmetric bifurcation with uniform inlet conditions. An integral momentum distortion parameter is calculated along several complete bronchial paths to assess the impact of branching angle and generation length on the flow field.   

Characterization of Ventilatory Modes in Dragonfly Nymph

A dragonfly nymph's highly modified hindgut has multiple ventilatory modes: hyperventilation (i.e. jet propulsion), gulping ventilation (extended expiratory phase) and normal ventilation. Each mode involves dynamic manipulation of the exit diameter and pressure. To study the different fluid dynamics associated with the three modes, Anisopteran larvae of the family Aeshnidae were tethered onto a rod for flow visualization. The result showed distinct flow structures. The hyperventilation showed a highly turbulent and powerful jet that occurred at high frequency. The gulping ventilation produced a single vortex at a moderate frequency. The normal ventilation showed two distinct vortices, a low-Reynolds number vortex, followed by a high-Reynolds number vortex. Furthermore, a correlation of the formation of the vortices with the movement of the sternum showed that the dragonfly is actively controlling the timing and the speed of the vortices to have them at equal distance from the jet exit at the onset of inspiration. This behavior prevents inspiration of the oxygen deficient expirated water, resulting in the maximization of the oxygen intake.   

Particle Image Velocimetry Measurements in an Anatomically-Accurate Scaled Model of the Mammalian Nasal Cavity

The mammalian nose is a multi-purpose organ that houses a convoluted airway labyrinth responsible for respiratory air conditioning, filtering of environmental contaminants, and chemical sensing. Because of the complexity of the nasal cavity, the anatomy and function of these upper airways remain poorly understood in most mammals. However, recent advances in high-resolution medical imaging, computational modeling, and experimental flow measurement techniques are now permitting the study of respiratory airflow and olfactory transport phenomena in anatomically-accurate reconstructions of the nasal cavity. Here, we focus on efforts to manufacture an anatomically-accurate transparent model for stereoscopic particle image velocimetry (SPIV) measurements. Challenges in the design and manufacture of an index-matched anatomical model are addressed. PIV measurements are presented, which are used to validate concurrent computational fluid dynamics (CFD) simulations of mammalian nasal airflow. Supported by the National Science Foundation.