# Hybridizing Lithography-Based Ceramic Additive Manufacturing with Two-Photon-Polymerization
## 1 Introduction  
The additive manufacturing (AM) of ceramics in the micrometer scale opens new applications, associated with their inherent unique properties such as thermally stability, chemical inertness, abrasion resistance, biocompatibility and various functional properties [1]. Due to the large surface volume ratio of microstructures, corrosion and chemical resistance play an important role. Conventional ceramic processing of very small structures is expensive and often limited by geometry (e.g., wall thickness). In such cases, AM-technologies can help to create ceramic structures in unmatched shapes. One of those technologies is two-photon polymerization (2 PP), which enables the highest accuracy of all AM-technologies so far. Feature sizes below 100 nm have been reported [2]. This accuracy has so far been applicable to a few types of resins, such as homogeneous polymers, nano-composites, bio-polymers [3] and polymer derived ceramics (PDCs) [4]. All these systems meet one major requirement for 2 PP, which is transparency. This concept has been successfully transferred to a ceramic slurry of 70 wt% yttria stabilized zirconia (YSZ) nano-particles [5]. The zirconia slurry can provide high transmittance in the relevant wavelength range of around 800 nm together with a high-solid loading, thus enabling the manufacturing of technical ceramics utilizing the 2 PP process.  
Even though the opportunities opened by this new feedstock are thrilling, the achievement comes with a major drawback. Printing millimeter- or even centimeter-sized features with 2 PP-accuracy takes a tremendously long processing time, making this approach excessively expensive for the manufacture of macroscopic parts. In this regard, there are technologies in development to speed up 2 PP-printing, e.g., using parallel beams or increasing the scanning speed. However, they may not reach the printing speed of larger vat-photopolymerization (VPP) techniques, such as lithography-based ceramic manufacturing (LCM). Combining the accuracy of 2 PP with the printing speed of LCM appears, in this context, to be very appealing [6]. Chemistry-wise, the process is the same, as only different photo-initiators are needed. The same principle has been already tested with PDCs [7], which are suitable for both 2 PP and LCM.  
In order to apply the 2 PP process to ceramic slurries, a certain level of transparency is required. Hybridizing 2 PP with stereolithography, in particular LCM, is not naturally given, as feedstocks for LCM are generally not transparent for the light used to crosslink the photoactive slurry. The light might penetrate too deep into the slurry and definition of the part in z-direction may be lost. On the other hand, for crosslinking more massive structures in one UV-illumination event with a conventional UV lamp, e.g., for the generation of flat substrates to support 2 PP structures of the same material [5], an even higher transmissivity is required.  
The present study takes the advantage of the fact that the transparent 2 PP-resins are printable with stereolithography technologies. The first results in hybridizing these two technologies are presented on a zirconia material, and it is demonstrated that the YSZ feedstock can be used for 2 PP printing, crosslinking substrates with a UV lamp and the LCM process. Moreover, the specimens fabricated are generated from the same slurry-composition, and successively printed with 2 PP onto LCM-manufactured parts and UV-cured substrates. It is not only possible to print structures with 2 PP in high resolution onto macroscopic substrates, but also the UV-cured, or LCM-manufactured structures can be sintered together with 2 PP features, resulting in a uniform shrinkage with low distortion in the filigree 2 PP structures.