Adapting the Aerogen Mesh Nebulizer for Dried Aerosol Exposures Using the PreciseInhale Platform

Abstract

Background: Many substances used in inhalation research are water soluble and can be administered as nebulized solutions. Typical examples are therapeutic, small-molecular agents, or macromolecules. Another category is a number of water-soluble agents used for airway diagnostics or disease modeling. Mesh nebulizers have facilitated well-controlled liquid aerosol exposures. Meanwhile, a benchtop inhalation platform, PreciseInhale, was developed for providing small-scale, well-controlled aerosol exposures in preclinical configurations. The purpose of the current research was to adapt the Aerogen mesh nebulizer to work within the PreciseInhale system for both cell culture and rodent exposures. Methods: The wet aerosols produced with the Aerogen Pro nebulizer were dried out in an aerosol holding chamber by supplying dry carrier air, which was provided by passing the incoming ambient air through a column with silica gel. The nebulizer was installed in an aerosol holding chamber between an upstream flow-rate pneumotach and a downstream aerosol monitor. By pulsing, the nebulizer output was reduced to 1%-10% of continuous operation to better match the exposure ventilation requirements. Additional drying was obtained by mantling the holding chamber with dried paper. Results and Conclusions: The nebulizer output was reduced to 3-30 μL/min and dried out before reaching the in vitro or in vivo exposure modules. Using solute concentrations in the range of 0.5%-2% (w/w), dried aerosols were produced with a mass median aerodynamic diameter of 1.5-2.0 μm, compared to the 4-5 μm droplets emitted by the nebulizer. Controlling the Aerogen nebulizer under a reduced output scheme within the PreciseInhale platform gave two major advantages: (i) by reducing aerosol output to better match exposure flow rates of single rodents, increased airway deposition yields were obtained in a range of 1%-10% relative to the nebulized amount of test substance and (ii) shrinking aerosol particle sizes through drying improved the peripheral lung deposition of test aerosols.

Keywords: Aerogen; PreciseInhale; aerosol drying; mesh nebulizer.

Conflict of interest statement

Dr. Per Gerde, Mr. Mattias Nowenwik, Dr. Ewa Selg, and Mr. Carl-Olof Sjöberg are minority shareholders in Inhalation Sciences Sweden AB.

Figures

FIG. 1.

(A) The principle of operation of the nebulizer unit. The Aerogen Pro mesh nebulizer was connected to the PreciseInhale platform in an aerosol holding chamber between an upstream flow rate pneumotach and a downstream aerosol monitor and vacuum source. The liquid reservoir of the nebulizer was sealed with a vacuum lid. For the humidity measurements, the humidity probe was installed at a branch tube immediately before the aerosol light dispersion instrument. The incoming air was dried and filtered in a drying column with silica gel. The exposure module was located immediately downstream of the aerosol monitor. (B) A picture of the PreciseInhale platform with the liquid aerosol generator properly installed. Vacuum is provided by a vacuum eductor contained in the nebulizer holding assembly and regulated by the needle valve in the control box next to the nebulizer head. (C) The aerosol holding chamber with a scrolled drying paper to be inserted. This drying mantle consists of a filter paper (Munktell Filter, Quality 1600, Sweden).

FIG. 2.

Examples of the wet aerosol production rate from the nebulizer head at different output settings in the lower output range with a low-salt solution and a 2% terbutaline solution. The nebulizer operation was set to one spray pulse of adjustable duration per second. The data are mean ± standard deviation (n = 3).

FIG. 3.

Relative humidity of air exiting the aerosol collection chamber following aerosol generation with different operative settings. These sets of experiments were all performed with dry inlet air (RH

FIG. 4.

Flow rate monitoring during a 4 seconds period of pulsed aerosol generation with dry inlet air at a flow rate of 400 mL/min. The duration between the flow rate spikes was 1 second, and the most rapid evaporation on each aerosol pulse affecting flow rate lasted less than 0.4 seconds. The low-salt solution was nebulized at 5.5 μL/min, and the 2% terbutaline solution was nebulized at 5.7 μL/min.

FIG. 5.

The concentration of dry terbutaline aerosol measured with the light dispersion instrument during the two types of drying strategies: with dry inlet air or with dry inlet air combined with a drying paper mantle in the holding chamber. After each test run, the instrument signal was corrected for the amount of terbutaline collected on the downstream exposure filters and the carrier air flow rate to calculate apparent aerosol concentration. The thin lines indicate ± one standard deviation from three experiments. The wet aerosol output from the nebulizer during the experiments was 5.7 ± 0.3 μL/min, and the carrier air flow rate was 400 mL/min.

FIG. 6.

Scanning electron microscopy pictures of generated dried aerosols. (A) Terbutaline 2%; (B) terbutaline 0.5%; (C) methacholine 2%.