Research and development item: ②(c) Development of a method for stable aerosol generation

Implemented by Hiroshima University

Final objective: To facilitate comparisons of the results obtained from inhalation toxicity test and intratracheal administration test, we improve techniques for aerosol generation that can control the concentration, size, shape, and stability of the aerosol fed during inhalation toxicity test. Finally, we compile and release a guide for the technique of producing aerosol particles for inhalation toxicity test.

Main results:
We developed techniques of preparing aerosol samples suitable for inhalation toxicity test and conducting inhalation toxicity test using the resulting aerosol samples. To this end, we used a spray-drying technique because of its various advantages with respect to aerosolization of the existing nanomaterials. This spray-drying technique is used to produce solid-particle aerosols by spraying a suspension containing nanomaterials into droplets containing the nanomaterials, followed by evaporation of water from the droplets.

Figure ②(c)-1 shows the experimental system used for aerosol generation and characterization of the aerosols. We fed a nanomaterial suspension through a two-fluid nozzle and sprayed the suspension with pressurized air to produce microdroplets. The microdroplets were passed airborne into a heating tube. Using various analyzers, we analyzed the generated aerosols thus produced.

Figure②(c)-1 Experimental system (aerosol production apparatus, exposure chamber for inhalation test, and various analyzers)

We conducted experiments on aerosol generation using NiO, TiO₂, and CeO₂ nanoparticle suspensions. We evaluated the effects of the properties of nanomaterial suspensions and aerosolization conditions on the size, shape, concentration, and other properties of aerosol particles. As an improvement to the spray-drying technique, we demonstrated that droplet breakup by a unipolar charge and rapid evaporation of the droplets were particularly effective for producing aerosol particles with a small particle size and high mass concentration.

We produced a smaller droplet particle from nanomaterial suspension using a phenomenon called Coulomb explosion of charged droplets. When the electrostatic repulsion of multivalent charged droplets formed by mixing with airborne ions of high concentration exceeds their surface tension, the droplet is broken up into numerous progeny droplets; therefore, the production of smaller aerosol particles was possible. We heated the droplets immediately after the spraying process to reduce inertial impaction along the flow path due to accelerated shrinkage of the droplet, which increased the mass concentration of the aerosol particles.

Figure②(c)-2 SEM images of aerosolized NiO nanoparticles:
(a) before and (b) after improvement through the combined use of droplet breakup and rapid evaporation

We used the improved aerosol production system in conjunction with the exposure chamber and fed aerosols for 6 h per day for 20 days to conduct inhalation toxicity test. We demonstrated that aerosol could be supplied steadily during the test period. The mean particle size of the aerosol particles in the exposure chamber was approximately 100 nm, and the mass concentration in air was approximately 2 or 10 mg/m³.