Particle transport in human lung : effects of particle size and shape

Sammanfattning: Recent studies have shown that nanoparticles may be more toxic than larger particles of the same material, but the health risks associated with widespread use largely depend on the extent of exposure. When dealing with potentially toxic particles, precautionary measures have to be taken in order to minimize contact. For larger particles, mechanical filtering is commonly used. Nanoparticles, however, are too small to be effectively impeded by these filters and thus alternative methods need to be developed. Experiments are performed where clusters of carbon nanotubes are dropped vertically into a region with an electric field, generated between two parallel plates. The clusters are strongly affected by the field and move swiftly towards the electrodes. In this setup, most clusters simply bounce between the electrodes. By adding an electrically insulating layer to one of the plates, however, the particles get stuck. This implies that electrostatic filtration is an effective means of collecting airborne carbon nanotubes. Nanoparticles may enter human lung regardless if filtration is used or not. To examine the health risks, therefore, knowledge of transport and deposition properties of aerosol particles in lung flows is necessary. This information is also essential in the optimization of targeted drug delivery with pharmaceutical aerosols. In vivo and in vitro studies are cost-intensive and difficult to perform for studying particle deposition in the airways. Hence, numerical simulations constitute a valuable complement. The extent and location of particle deposition depend on particle properties, airway geometry and breathing pattern. To start with, Computational Fluid Dynamics simulations are performed for spherical particles, 15 nanometer to 50 micrometer in diameter, in a multiply bifurcated asymmetric 3D model, representing trachea to the segmental bronchi. Steady, laminar flow is considered for inhalation flow rates of 0.1 and 0.5 l/s. The largest particles are captured near the first bifurcation, whereas smaller microparticles are less efficiently, but more uniformly, deposited. The site of deposition is also affected by geometric asymmetry. The nanoparticles essentially follow the streamlines and travel unaffected through the region modeled. Thus, transport to the distal airways can be assumed extensive. Because of their specific shape, fibers may cause additional harm compared to spherical particles. Asbestos is a well-known example of hazardous fibrous materials. More recently, this has also called for concern on the extended use of nanotubes. A numerical model is developed for fiber transport in the respiratory airways. The coupled equations for fiber rotation and translation are solved using MATLAB. The model is valid for arbitrary Stokes flows at low particle concentrations and for particle sizes from nano- to the micro range. The results suggest that the potential of a fiber to reach the distal airways increases with increased fiber aspect ratio, regardless of particle size.