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numbers as the particles acquire random motion due to very low bulk airflow. Deposition of nanoparticles in the respiratory tract depends on the aerodynamic prop- erties of the particles. One of the properties of particles is aerodynamic size. For airborne particles, including dry nanopowders, the aerodynamic size depends on a number of factors, including the shape, physical size and density of the par- ticles. For nonspherical particles of graphene derivatives, t h e a e ro d y n a m i c d i a m e t e r h a s b e e n g i ve n a s d a =d e ( ) 1/2 ρ p ρ 0 χ where d a is the aerodynamic diameter, d e is the volume equivalent diameter, ρ p is the density of the material, ρ 0 is the standard particle density and χ is the shape factor. 2 Mucus: A primary defense mechanism Mucus acts as a primary line of defense, as it lines dif- ferent organs in the body that are in direct contact with the environment. It is a thick, viscous secretion produced by goblet cells, which are present in the epi- thelial lining of the airways. Despite the fact that it removes harmful substances from the respiratory pathways, it also lubricates the passage of different objects. us, mucus helps prevent the entry of for- eign particles into the cells of the trachea. On one side, mucus acts as a barrier to different pathogens, yet on the other side, it also acts as a hydrating layer over the epithelium, which helps in gaseous exchange. Mucins are high molecular weight glycoproteins that have the characteristic feature of making gel forms. 11 Some mucins can be membrane-bound, due to the presence of hydrophobic domains that interact with the membrane. Yet generally, mucins are secretory products and act as key components of mucus or form a part of saliva. ese are highly glycosylated secretions and may contain 80% carbohydrates. As shown in Figure 1 (redrawn with permission from Yang, et al. 11 ), different interactions between mucins and foreign particles may be hydrophobic, electrostatic or involve hydrogen bonding. The ability of the substances to adhere to mucins is known as mucoadhesivity. Nanoparticles entering the airway pathways may inter- act with mucus in different ways. The interactions between the two are yet to be characterized definitively. As shown in Figure 1, the mucus meshwork can trap the nanoparticles and hinder their direct interaction with cells. e nanoparticles are removed from that surface by regular mucus clearing mechanisms through the action of ciliated epithelium. When nanoparticles are inhaled in large amounts, they may cause the mucin fibers to intermingle and increase the pore sizes of the meshwork, which may further increase the chance of other foreign particles reaching the cells. us, mucus acts a potential defender in blocking the interaction of nanoparticles with the epithelial cells. Inhalation August 2017 23