Designing Advanced Photoresists for Degradable Nano- and Microstructures by 3D Laser Lithography

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A promising fabrication technique that overcomes the resolution limitation of regular 3D printers is the two-photon direct laser writing (DLW), also known as 3D laser lithography. This light-based micro- and nano-sized printing method is based on a multi-photon polymerization process, in which two or more photons are absorbed simultaneously. Due to the non-linearity of the multi-photon process, the chemical reaction occurs exclusively in the focal spot of the laser and allows for locally defined crosslinking. For this reason, DLW is capable of creating complex 3D structures in the submicron length scale. This high resolution – in some instances sub-diffraction by exploiting STED principles – is particularly attractive for applications where very sophisticated structures with high precision are needed including photonics metamaterials, biomedicine, and microelectronic. Most 3D microstructures obtained with DLW are irreversibly crosslinked into a permanent shape. For many applications, however, it would be beneficial if the 3D structure consists of a material, which is removable or replaceable after a certain amount of time. This approach, so-called Post-DLW degradation on demand, is particularly interesting for applications where the 3D structure acts as a scaffold or natural damage results in a short lifetime and requires replacement. Due to its enormous potential, there is a high demand for further research in the area of degradable 3D microstructures via DLW. In my postdoctoral project, I will address this demand and investigate functional photoresists for DLW with tuneable degradation properties. In particular, I am interested in the design and preparation of crosslinkers that exhibit cleavable bonds. These labile molecules should be inert towards conditions to which the microstructure will be subjected and need to be cleaved under specific and mild conditions to degrade the crosslinked material. To establish a versatile toolbox of cleavable photoresists, I will investigate different crosslinker systems that can be degraded either completely or locally controlled. Precisely, I will synthesize crosslinker based on: (i) silyl ether that can be cleaved with fluoride-ions (e.g. tetrabutylammonium fluoride)(ii) o-nitrobenzyloxy that can be cleaved with UV light(iii) phosphoester that can be cleaved with the enzyme phosphodiesteraseIn addition, I will investigate the influence of the crosslinker structure on the material properties of the cleavable direct-laser-written structure. In order to identify the optimal crosslinker structure, I will prepare a series of different crosslinker varying in type, spacer, and functionality. Optimally, I can transfer the knowledge from the model system to other crosslinkers with alternative cleavage trigger. Finally, I will use two or more cleavable photoresists to prepare nano- and microstructures that can be sequentially degraded in a controlled manner.