In-vitro visualization of aerosol deposition by laser-induced fluorescence in a human airway model

Abstract

This study presents the first in-vitro visualization of aerosol deposition from the trachea up to the 10th bifurcation in a transparent airway model. The airway model simulates a representative respiratory tract of the left lower lobe and consists of all 23 generations of a human lung. Laser- induced fluorescence (LIF) was used to investigate the temporal and spatial deposition behavior of aerosols generated by a jet nebulizer along the representative airway. The in-situ measured LIF signal correlates to the amount of deposited aerosol during spontaneous breathing. By the means of LIF-image post-processing the temporal course of the LIF signal in each of the first eight bifurcations is correlated to the main aerosol deposition mechanisms, i.e. inertial impaction and gravitational settling, in the conductive airways. Depending on the time point in the respiratory cycle either one of both deposition mechanisms dominates the current aerosol deposition. The spatial analysis over eight subsequent bifurcations shows the diminishing influence of the inertial deposition mechanism over deeper bifurcations. Further, the duration of gravitational settling decreases over the bifurcations depending on the accompanying airway diameters of each bifurcation. The introduction of the dimensionless Froude number allows the comparison of the measured aerosol deposition to existing research and demonstrates that the proposed threshold limit in literature of Froude <5 matches well to the gravitational settling regime observed in the transparent airway. For Froude >5 mainly the inertial impaction is observed as mechanism for aerosol deposition in this set-up. An error analysis is performed for evaluating the influence of the low relative humidity of the inhaled air on the aerosol evaporation and deposition. This feasibility study shows the capability of the measurement method in combination with the airway model to resolve the aerosol deposition up to the eighth bifurcation. In future, this analysis should be extended to higher airway generations by microscopic LIF imaging to evaluate the deposition mechanisms in all 23 generations of the transparent airway model.