Integration of Two-Photon Polymerized Structures in Microfluidic Channels for Cell Handling Applications

In this thesis, the investigations to establish a processing sequence for the realization of 3D microstructures and -electrodes fabricated by two-photon polymerization (2PP), that can be utilized for precise and efficient manipulation of individual cells in microfluidic environments, are presented. 2PP is a 3D printing approach, based on direct laser writing, which enables the realization of structures with sub-micrometer features. By integrating both passive and active manipulation methods, new design strategies for
microscale cell handling are explored and demonstrated.
As a passive cell handling method, a self-closing single cell trap is realized, which is based on a rotating element to close the trap. To realize the rotating element, newly designed thin support beams are implemented during printing to connect the rotating flap and a stator. These support beams improve the printing outcome, as they give stability and prevent deformation during printing. Rotor and stator can later be easily disconnected by breaking the support beams; for example via a directed nitrogen stream. The flaps rotate freely around the rotor, both in nitrogen and in a liquid. With the flap element placed in front of the trap, the closing mechanism is initiated by the drag force of the liquid. With that, single cells can be captured successfully.
As an active cell handling method, 3D ring-electrodes are realized by 2PP to generate a non-uniform electric field to manipulate cells via dielectrophoresis (DEP). When exposed to such a field, cells experience a force due to induced polarization, allowing them to be manipulated in the direction of the higher or lower field gradient. To realize 3D electrodes via 2PP, a unique fabrication process is designed to implement them in high microfluidic channels. With a ring-shape, compared to bottom or ceiling electrodes, it is possible to span the electric field over the entire channel, thus influencing cells more effectively. When applying a flow, cells can be successfully focused towards the middle of the channel in a straight line.
These two cell handling approaches illustrate the potential of integrating two-photon polymerized structures into microfluidic systems for precise cell manipulation. The
unique design flexibility offered by applying 2PP in a microchannel enables enhanced control over cell behavior, which is an essential factor for improving the functionality of microfluidic applications. Overall, this thesis highlights the promise of 2PP in microfluidics and lays a groundwork for future technological advancements in the field.