In vitro particle size characterization via inertial impaction

Among the particle size characteristics determined by these models are the mass median aerodynamic diameter (MMAD) and particle size distribution. where particles less than 5 µm are thought to reach deep pulmonary regions but those of 5 µm or more are preferably deposited in the oropharyngeal region, not the lungs. These in vitro models depend on the presence of a distinct device called cascade impactor to determine the particle characteristics and the aerosol particle size distribution like Next-generation impactor (NGI) and Anderson cascade impactor (ACI). Precisely, these devices can assess the particle size and define the range of aerosolized particles within the inhaled dosage form. The in vitro particle size characterization model simply contains an electrostatic filter in a holder, breathing simulator to simulate the patient condition, two T-pieces connected in addition to the cascade impactor. The cascade impactor should be first placed in the refrigerator before use for one hour in case of the ACI while 1.5 hours is needed for the NGI (16,17). The exact arrangement of these parts in a single limb NIV circuit is shown in Figure 2A where Figure 2B shows that of a dual limb. As noted, the cascade impactor is connected to a vacuum pump to withdraw air at an estimated flow rate of 15 L/min which in turn stimulates the patient inspiratory flow rate (quiet breathing pattern) and at the same time allows the entry of an aerosol sample to the cascade impactor device to be analyzed. Particle characterization is done only for the entered sample, not all the aerosol emitted which is considered the major drawback of the previously described setup. Hassan et al. determined the aerodynamic characteristics of aerosol droplets released from 4 different nebulizers during NIV using Anderson cascade impactor to measure MMAD, and fine particle fraction (12). The model consisted of a ventilator, breathing simulator, Anderson cascade impactor, and NIV breathing circuit which consisted of a 180 cm length of corrugated tubing (diameter of 22 mm) and a fixed leak expiration port. Although jet nebulizer exhibited a higher fine particle fraction the VMNs delivered the highest fine particle doses (12).

Schematic of breathing simulator and bi-level ventilator circuit for aerodynamic particle size characterization of the respirable dose, (A) single limb, (B) dual limb and (C) for determination of the whole aerosol emitted.

One adjustment can be performed within the NIV circuit to allow the characterization of all delivered aerosol through the use of mixing inlet as shown in Figure 2C where a Y-tube is used to connect a supplementary air delivered at a flow rate of 15 L/min, breathing simulator and the mixing inlet to the cascade impactor (18). This adjustment was only tested with a dual limb NIV circuit with the possibility of applying it in a single limb. The presence of this mixing inlet allows additional air to be delivered at an equal flow rate to that withdrawn by the vacuum pump connected to the cascade impactor which in turn results in zero flow rate withdrawn from the ventilation circuit. Using this setup, when the inspiratory phase of the breathing simulator was withdrawn from the supplementary air, the inhalation profile is replayed within the ventilation circuit; and because of the constant flow (15 L/min) of air being withdrawn through the cascade impactor. Hence, the aerosolized drug can be withdrawn from the circuit which is then withdrawn into the cascade impactor. When the expiratory phase started, it added more airflow to the supplementary air. The inhalation profile again is replayed within the ventilation circuit, because of the constant flow of air being withdrawn through the cascade impactor. So, the aerosolized drug is kept within the circuit till the next inspiratory phase resampling what truly happens when delivering aerosol to the patient (18). This model has been used to determine the emitted dose in a study by Elhansy et al. They conducted an in vitro study comparing VMN and Jet nebulizer with a spacer (18). Similar to what was found in the previous study; Jet nebulizer showed the highest fine particle fraction compared with VMNs, while VMNs showed the highest fine particle doses.

To simulate the human lung, the cascade impactor consists of several stages arranged in a descending manner according to the pore diameter of each stage where the stage with the largest pore diameter is placed first near the device mouth inlet followed by that of lower diameter and so on until reaching the last stage (19). The known cut-off diameters for both ACI and NGI stages are displayed at 28.3, 60, and 90 L/min. However, other flow rates can be used with ACI but adjustment of the stages cut-off diameters is needed at the newer flow rate which can be done by applying this equation: ECDF₂ = ECD_28.3 (28.3/F2)^0.5. ECDF_2, ECD_28.3, and F₂ symbols stand for the effective cut off diameters at the newly applied flow, the effective cut off diameters at the standard flow (28.3 L/min), the newer flow in L/min, respectively (20). As particles pass through cascade impactor, particles with sufficient inertia will impact within a certain stage while the rest of particles pass through the stage pores to the following stages until settling of all emitted particles. The amount of drug deposited on each stage can be eluted by washing the stage with a specific solvent carefully chosen to match the solubility of the studied drug and then mass determination is performed by either high-performance liquid chromatography (HPLC) or ultra-violet (UV) spectrophotometer. Then, the mass values of the measured drug in each stage are entered into certain software supplied by the cascade impactor manufacturer to translate them into the aerodynamic characteristics of the administered aerosol; MMAD, fine particle dose (FPD), fine particle fraction (FPF), and geometric standard deviation (GSD). MMAD is a measure of central tendency and refers to the cutoff size where half particles are smaller than that referred particle size and the other half is higher. FPD refers to the fraction of the dose with particle size lower than 5 µm where FDF is simply the percentage of FPD. GSD is used to describe the range of distribution of inhaled particles where the highly destructed or distributed aerosol is indicated by a higher GSD number. As mentioned before, particles of 5 µm or more will impact on the oropharyngeal region and can participate in the systemic drug effects if orally absorbed besides increasing its adverse effects. However, particles smaller than 2 µm in diameter are mainly deposited in the alveolar region by sedimentation so the inhaled aerosol with smaller MMAD (<2 µm) is still preferred for many respiratory diseases.