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

双光子直接激光写入技术（DLW）是一种克服常规3D打印机分辨率限制的有前途的制造技术，也称为3D激光光刻技术。这种基于光的微纳米印刷方法是基于多光子聚合过程的，其中两个或多个光子同时被吸收。由于多光子过程的非线性，化学反应只发生在激光的焦点光斑上，并允许局部定义的交联。因此，DLW能够在亚微米尺度上创建复杂的3D结构。这种高分辨率——在某些情况下利用STED原理的亚衍射——对于需要高精度的非常复杂的结构的应用特别有吸引力，包括光子学、超材料、生物医学和微电子。用DLW获得的大多数三维微结构都是不可逆交联成永久形状的。然而，对于许多应用来说，如果3D结构由一种材料组成，这种材料在一定时间后是可拆卸或可更换的，这将是有益的。这种方法，即所谓的“后dlw降解”，对于3D结构作为支架或在短寿命内导致自然损坏并需要更换的应用特别有趣。由于其巨大的潜力，DLW在可降解三维微结构领域的进一步研究有很大的需求。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特性的DLW功能光阻剂。特别是，我对具有可切割键的交联剂的设计和制备感兴趣。这些不稳定的分子应该对微观结构将受到的条件是惰性的，并且需要在特定和温和的条件下进行裂解以降解交联材料。为了建立一个多功能的可切割光刻胶工具箱，我将研究不同的交联剂系统，可以完全降解或局部控制。准确地说，我将合成基于以下材料的交联剂：(I)可被氟离子（如四丁基氟化铵）切割的硅醚（ii）可被紫外光切割的邻硝基苄氧基（iii）可被磷酸二酯酶切割的磷酸酯。此外，我将研究交联剂结构对可切割激光直接书写结构材料性能的影响。为了确定最佳的交联剂结构，我将准备一系列不同类型、间隔和功能的交联剂。最理想的情况是，我可以将知识从模型系统转移到其他具有选择性裂解触发器的交联剂。最后，我将使用两种或更多的可切割光刻剂来制备纳米和微观结构，这些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.