Dispersion and adhesion of blood components in porous media

The transport and capture of particulate matter in suspension, while flowing through porous media, have received significant attention because of extensive involvement of filtration and colmatage in water and air purification, crude oil production from reservoirs, blood processing, functioning of biological organs such as the kidney and recently, in micro-fabricated arrays and nano-medicine. The focus is on blood processing in this thesis. White blood cells (WBC) can cause serious clinical complications to the recipients of transfused blood, hence the need for its removal. Current filters for selective removal of WBC among the other formed elements are inefficient. Platelets are often damaged, activated or captured in the process. This thesis is on the physics of transport and capture of white cells and platelets in fibrous porous media. Porous media are typically complex structures of fibres or particles randomly arranged on the micro-scale, but characterized by macroscopic properties such as porosity and permeability. When pore spaces are large relative to the particles in suspension, steric hindrance is unimportant and short-range forces between the particles and the matrix determine the effectiveness of particle capture in "deep bed filtration". In the present work, dispersion and capture of finite-sized and rigid particles such as white blood cells and platelets suspended in platelet-rich-plasma or red cell-free blood is modeled as the suspension percolates the interstitial spaces of a fibrous porous medium. At the micro-scale level, a unit cell comprises a fibre and the space around it. A single cell (in dilute suspension) enters the space and the force fields between the cell and fibre interact. Such local events are transformed into a macro-scale level by adopting periodic boundary conditions for contiguous unit-cells and applying Taylor-Aris convective-diffusive-adhesive theory known as macrotransport theory. Previous macrotransport models have dealt exclusively with point-sized particles as would be consistent with small particle/collector aspect ratios. This is modified to incorporate finite-sized particles.; The theory involves parameters which have to be determined experimentally. Atomic Force Microscopy (AFM) experiments were performed to determine short range forces between blood cells and fibres. Soft lithographic micro fabrication techniques were used to produce perfusion channels which allowed visualization of cell movements and interactions with the posts representing fibres. Short range forces and drag forces on a blood cell result in a velocity field for the blood cell in the interstitial spaces. The velocity field is an important variable in macrotransport theory. The developed theory is used to investigate the importance of parameters like fibre size, percolation velocity, bed porosity and bed thickness in filter design.; The model results for the commercial filters [3mum diameter fibre, 0.87 porosity, 1mm bed thickness at a percolation rate of 2.4(10)-4 m/s] are ∼71% white cell capture and ∼47% platelet recovery. The model predicts that an 8-mm thick filter bed with a void space of 0.93 that comprises fibres with 3mum diameter can remove more than 95% of the white cells while allowing through more than 55% of the platelets at a percolation velocity of 5(10)-5 m/s.