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

克服常规3D打印机分辨率限制的一种有前途的制造技术是双光子直接激光写入（DLW），也称为3D激光光刻。这种基于光的微米和纳米尺寸的印刷方法基于多光子聚合过程，其中两个或更多个光子同时被吸收。由于多光子过程的非线性，化学反应仅发生在激光的焦点中，并允许局部限定的交联。因此，DLW能够在亚微米长度尺度上创建复杂的3D结构。这种高分辨率-在某些情况下，通过利用STED原理的子衍射-对于需要具有高精度的非常复杂的结构的应用特别有吸引力，包括光子超材料，生物医学和微电子。用DLW获得的大多数3D微结构不可逆地交联成永久形状。然而，对于许多应用，如果3D结构由在一定量的时间之后可移除或可替换的材料组成，则将是有益的。这种方法，即所谓的按需后DLW降解，对于3D结构充当支架或自然损坏导致寿命短并需要更换的应用特别有趣。由于其巨大的潜力，有一个高的需求，在该领域的可降解三维微结构通过DLW的进一步研究。在我的博士后项目中，我将解决这一需求，并研究具有可调降解性能的DLW功能性光刻胶。特别是，我感兴趣的设计和制备的交联剂，表现出可裂解的债券。这些不稳定分子应该对微结构将经受的条件是惰性的，并且需要在特定和温和的条件下裂解以降解交联材料。为了建立一个多功能的工具箱可裂解的光致抗蚀剂，我将研究不同的交联剂系统，可以完全或局部控制降解。准确地说，我将合成交联剂的基础上：（i） 可被氟离子裂解的甲硅烷基醚（例如氟化四丁基铵）（ii） 可以用UV光裂解的邻硝基苄氧基（iii） 此外，我将研究交联剂结构对可裂解的直接激光写入结构的材料性质的影响。为了确定最佳的交联剂结构，我将制备一系列不同的交联剂，这些交联剂在类型、间隔基和官能度上各不相同。最佳地，我可以将知识从模型系统转移到具有替代切割触发剂的其他交联剂。最后，我将使用两种或更多种可裂解的光致抗蚀剂来制备纳米和微观结构，这些纳米和微观结构可以以可控的方式依次降解。

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.