## 5 Conclusions  
A fully alveolated fluid-structure interaction model has been developed for alveolar breathing mechanics and underlying mechanisms of aerosol deposition in idealised alveolar sacs. We utilized four treatment group breathing periods and twelve visco-elastic properties of lung parenchyma tissue models for investigating the relative importance of the variables affecting deposition such as particle size, gravity orientation, airflow rate or generation number, tissue properties, breathing patterns and particle-fluid interactions. Airflow streamlines, as well as one- and two-way coupling particles deposition efficiencies, have been predicted by the model rather than particle trajectories in the duct model explicitly. The results indicate that modelling the two-way coupling particle system is important because the two-way DPM approach despite its complexity provides more extensive particle interactions and is more reliable than transport results from the one-way DPM approach. The substantial difference between the results of the two approaches is likely due to particle-fluid interactions, which re-suspend the sediment particles in the airway stream and hence pass from the current generation. Some results of the simulations were carried out for different orientations of the model with respect to gravity forces in 18-22 generations of the human lung, and demonstrated the importance of gravity orientation on particle efficiency. Model orientations have important effects on the overall deposition which changed by a factor of approximately 8 between the \(x\)-axis \([-1,\,0,\,0]\) and \(y\)-axis \([0,\,-1,\,0]\) orientations for all particle sizes in the 22 nd generation simulated. The \(x\)-axis \([-1,\,0,\,0]\) and \(z\)-axis \([0,\,0,\,-1]\) orientations have approximately similar results because of the radial walls used in the alveoli model in this study. The results clearly show that deposition efficiency of one- and two-way DPM particles is increased by increasing the particle diameter and generation number. Such a phenomenon is due to a reduction of the Reynolds number and dominant viscous airflow effect for larger particles as well as lower generations. In addition, the results describe the effects of tissue properties and breathing patterns on particle dynamic transport and deposition efficiency. Tissue Properties: It is clear that increasing the tissue elasticity (E) causes less wall motion, which lowers flow velocities. Therefore, the value of convention forces that pass the particles out of generations is reduced by decreasing the flow velocities. Thus, the particles are pushed into the radial alveoli by the lower-velocity airflow. As a consequence, the effect of tissue elasticity has been observed explicitly in higher generations because particle inertia and Brownian diffusion is magnified during these generations and causes more particles to pass out of streamlines and become deposited on the walls. It can be inferred that under microgravity (a special condition outside of the human body) the effect of tissue properties can be seen more and more clearly. Therefore, under normal gravity conditions, the propagation depth of the particles is not significantly influenced by tissue mechanical properties because the particles settled to the bottom of the alveolar structure. In addition, during actual conditions, on which the current study is based, gravity effects dominate the particle dynamics with increasing particle size and generation number, and cause rapid particle deposition, which decreases the diffusion forces. Therefore, tissue viscosity has no significant effect on the particle deposition efficiency. Breathing Patterns: The same results have been obtained for the effect of breathing pattern (different breathing periods) on particle deposition efficiency. The results show that larger particles exhibit little dependence on breathing periods because they experience more sedimentation transport than small particles.