Polydimethylsiloxane-based Microfluidic Devices for Separating Particle Systems by Deterministic Lateral Displacement

The separation of particles in microfluidic devices is a fast-growing area of research as it offers the potential for more precise, faster, and reliable separation than conventional techniques while also enabling point-of-care applications. Deterministic Lateral Displacement (DLD) devices are fascinating due to their excellent particle size resolution and numerous potential parameters to manipulate and tune device performance. For many applications, the throughput of microfluidic devices is a relevant factor. DLD devices especially exhibit inherent high hydraulic resistance, which can result in high operational pressures. This trend is even further intensified if structure dimensions in the device are decreased to separate even smaller particles in the sub-micrometer or nanometer regime. Furthermore, polydimethylsiloxane (PDMS) is a popular material for rapid microfluidic device manufacturing, as it offers fast, easy, and cheap prototyping. However, PDMS's soft and flexible nature can pose manufacturing and operational challenges. In this thesis, the manufacturing of PDMS-based DLD devices is investigated and optimized to achieve reliable manufacturing. While different approaches were evaluated, the best manufacturing process was based on PDMS molding from master structures processed with SU-8 lithography. With this approach, precise pillar structures with spacing down to 2.7 µm and heights of 32 µm were manufactured, enabling higher device throughputs than shallower devices with just 3 to 4 µm height reported in the literature. The separation characteristics of different DLD devices were evaluated in-depth. The DLD device design with the smallest critical diameter could reliably separate particles in the sub-micrometer regime, which was only scarcely shown before for PDMS-based devices. Furthermore, the operation of DLD devices at increased flow rates to achieve higher throughput was investigated. DLD devices capable of separating particles in the sub-micrometer regime operating at up to 80 times increased throughput compared to the literature were realized. However, pronounced PDMS structure deformations at increased flow rates were encountered and had to be reduced. A strategy to minimize deformations by thermal stiffness tuning of PDMS was developed. An appropriate thermal treatment increased PDMS stiffness by roughly 69 %. However, the reduced structural deformation also affected throughput limits as the pressure in the DLD devices was higher compared to DLD devices without the thermal treatment. In the end, a model was derived to describe and explain the structural deformation and throughput limitations encountered in PDMS-based DLD devices with different PDMS stiffnesses. Furthermore, the model can also be applied to predict the separation performance of DLD devices operating at different flow rates. Thus, it is an excellent tool for DLD device design.