Modeling nanofiber transport and deposition in human airways

Sammanfattning: An understanding of the complex motion of fibrous particles is important in a wide range of areas, including inhalation toxicology, targeted drug delivery, composites manufacturing, paper making, and food processing. While the applications are diverse, the governing equations for the fiber motion may be shared.

Fiber transport and deposition properties are determined by particle characteristics, flow pattern and bounding geometry. One of the most important mechanical parameters is particle size, and while there are quite many publications on the behavior of micron particles, there are few covering nanoparticles. Further, most studies are limited to particles of spherical shape.

This project was originally initiated to gain information regarding health risks associated with inhalation of carbon nanotubes. Thus, the core of the thesis is dealing with particle inhalation and deposition in lung flows, but the theory developed and the knowledge acquired is also applied to fiber flows related to composites manufacturing. The problem was addressed by the use of theoretical models accompanied by numerical simulations, and a considerable part of the total work was devoted to the development of a  model for the combined translational and rotational motion of a fibrous particle in an arbitrary flow and geometry.

The results suggest that the deposition of spherical nanoparticles is close to negligible in the large airways; the inclusion of cartilaginous rings leads to a somewhat increased and more inhomogeneous deposition, but the extent of deposition is still very small.

Decrease in particle size means reduced efficiency of sedimentation but increased intensity of Brownian diffusion; and if regarding the influence of particle shape, results suggest that respiratory deposition both due to gravitational settling and Brownian diffusion decreases with increased fiber aspect ratio. For fibers with small aspect ratios, minimum deposition is achieved for fiber diameters around 0.5 micrometer, as expected from published work on spherical particles, whereas the minimum deposition for fibers with large aspect ratios is shifted towards smaller diameters. In regards to the combined effect of fiber size and shape, results indicate that the probability of reaching the vulnerable gas-exchange region in the deep lung is highest for particles with diameters in the size-range range 10-100 nm and lengths of several micrometers. Note that the popular multi-walled carbon nanotubes fall into this size-range.

The respiratory deposition of nanofibers was independently predicted with two models: the fiber model mentioned above and an asymptotic approach based on the concentration of fibers. Considering the rather large differences in the two approaches the agreement was overall quite good, which further supports the soundness of the results.

The motion of carbon nanotubes during the impregnation of dual-scale fabrics in composites manufacturing was examined for the flow in two types of channels: one representative for the flow within the fiber bundles (micro-scale) and one for the flow between the bundles (meso-scale). Simulation results suggest that the nanofibers basically go straight through the meso-scale channels, whereas in the micro-scale channel they experience Brownian fluctuations that are large compared to the channel width, and hence efficient deposition may occur which ultimately may lead to a total blockage of the flow. In most cases, however, this will not effect the overall filling time since most of the flow takes place in the inter-bundle channels, and these only experience modest deposition.