Cell Sorting in Deterministic Lateral Displacement Devices

The ability to sort cells is extremely desirable in many fields such as diagnostics, chemical processing, and biological analyses. Label-free sorting schemes in microfluidic devices represent a promising alternative to the fluorescence and immunomagnetic activated cell sorting methods which are current industry standards; permitting reduced costs, simpler operation, and increased portability. Furthermore, passive label-free sorting schemes such as deterministic lateral displacement (DLD) devices pose further advantages; for instance, sorting cells in accordance with their physical properties with no need for the application of external fields, magnetic beads, or fluorescent labels. Many of the cells and particles which require sorting are non-spherical and deformable, however the majority of theory for predicting particle behaviour in Deterministic Lateral Displacement (DLD) devices is only applicable to the limited case of rigid spheres in circular post arrays. The concept behind the separation mechanism for rigid spheres relies upon the assumption that DLD device geometry uniquely determines the fluid flow field, which subsequently defines a critical particle size and the lateral separation of particles of different sizes. Whilst the developed theories are exceedingly successful for designing devices for separating rigid beads, they are restricted to sorting particles by size, a characteristic which is ill-defined for non-spherical deformable particles in flow. Furthermore, this design approach squanders the full potential of DLD, which could utilise alternative particle characteristics such as elasticity and dynamic behaviour as the bases for separation of non-spherical deformable particles. This work uses a combination of mesoscale hydrodynamic simulations and microfluidics experiments to demonstrate how cells’ mechanical and dynamic properties can be used as separation criteria within different DLD device geometries. Two mesoscopic particle-based methods are employed in simulations to represent fluid; the dissipative particle dynamic (DPD) technique and the smoothed dissipative particle dynamic method (SDPD). Additionally, two-dimensional(2D) mesoscopic models of rigid beads and red blood cells (RBC) are employed to investigate particle motion in DLD devices in a qualitative manner, and a three-dimensional (3D) model of Red Blood Cell (RBC)s is used to obtain a more precise quantitative picture. RBCs are chosen because they represent a ubiquitous non-spherical deformable particle, and sorting based on their mechanical properties in DLD devices would aid in diagnosis of lethal diseases such as malaria. The 2D simulation results show that a critical particle size can be well defined for rigid spherical particles in circular post arrays depending on the inter-post spacing, the row-shift fraction, and the corresponding flow field; in quantitative agreement with current empirical findings and theory. Additional 2D simulations allow the empirical equation for the critical size of rigid spherical particles in circular post arrays to be generalized and extended for diamond, square and triangular post arrays. In contrast, 2D simulation results demonstrate that RBCs exhibit far more complex dynamics within DLD post arrays than rigid spheres, and the particle motion cannot be predicted by a single parameter such as the critical size. Instead, the dynamic behaviour and deformation of the RBC are found to strongly influence transit behaviour through DLD devices, providing the potential for novel sorting schemes. The 2D work motivates the further study of RBC behaviour in DLD devices using 3D simulations and supplementary experiments to gain a more quantitative understanding of the underlying physical mechanisms at play. 3D simulations and microfluidic experiments achieve excellent quantitative agreement and reveal that RBCs can travel in transit modes which are inaccessible to rigid spheres due to the complex interplay between hydrodynamic interactions with posts, RBC deformations, and RBC dynamic behaviour. The dynamic behaviour of RBCs is investigated by confining cells within flow channels of different heights and by altering the ratio between the intra-cellular fluid and the suspending medium. Thin devices inhibit the dynamic behaviour of RBCs, resulting in trajectories which are more like those of rigid spheres, however deformation still plays a key distinctive role. Thick devices which allow full re-orientation of RBCs and different dynamic behaviours are observed at different viscosity contrasts: At physiological viscosity contrasts RBC s are found to move in a tumbling motion and, under conditions where the intra- and extra- cellular fluid are equal, RBCs are found to favour tank-treading behaviour. Each of these distinct dynamic behaviours result in dramatically different RBC trajectories through DLD post arrays, demonstrating how non-spherical deformable particles can be sorted according to characteristics other than size, such as membrane shear modulus, membrane bending rigidity, or internal viscosity. Finally, 3D simulations are used to demonstrate a potential DLD device design for deformability-based sorting of RBCs; achieving good lateral separation between healthy RBCs and RBCs with pathological values for their shear modulus