## 1 Introduction  
In iterative product development, conventional manufacturing methods with long lead times are a significant bottleneck for fast testing and validation. This holds true especially for the development of products that use magnets, where the fabrication of custom magnet shapes by e.g., sintering, hot-pressing, or injection molding is time- and cost-intensive and requires special tooling [1].  
Furthermore, conventional manufacturing techniques restrict the design of new and complex structures, not only for magnets but also in product development, because the assembly steps necessary to integrate the magnets into parts impose restrictions on design and prolong iteration cycles. The use of rapid manufacturing methods for magnets during prototyping phases could, therefore, be of great benefit.  
Recently, studies have applied 3D printing techniques to magnetic materials to produce magnets with complex geometries and tailored magnetic fields. [PERSON] et al. used pellets of a NdFeB/Nylon compound to print large magnets with a remanence of 510 mT by big area additive manufacturing (BAM) [2]. [PERSON] et al. used a similar compound to produce filaments with high NdFeB content for printing magnets with a remanence of 314 mT via fused deposition modeling (FDM) [3; 4; 5]. While these approaches are based on polymer-bonded magnetic compounds, [PERSON] et al. printed purely metallic magnets by means of selective laser melting of NdFeB powder, reaching the maximum remanence measured in any printed permanent magnet of 590 mT [6]. Further approaches for the additive manufacturing of magnets include binder jetting [7] and direct ink writing [8; 9].  
With the exception of [PERSON] et al. who demonstrated the use of a printed magnet in a conventional DC motor, the translation of 3D printed magnets to actual use cases has not been made [10]. For printing by FDM [3; 4; 5], an inhibiting factor might be the reported use of non-optimized injection molding compounds that result in brittle filaments poorly suited for unsupervised handling on conventional FDM printers.  
Extensive iterative testing is of critical importance, especially during the development of biomedical devices, to ensure reliable operation and successful approval procedures. Rotary blood pumps (RBPs) are, next to heart transplantation, the only option for patients suffering from end-stage heart failure. Such RBPs have magnets as critical components in the driving and bearing systems of the impeller. RBPs which are currently available have the undesired side effects of hemolysis and thrombus formation that have to be addressed in the design and development phase of next-generation devices [11; 12]. Although computational fluid dynamics are often used in the early design stage, validating computational data experimentally is crucial [13]. While standard 3D printing is already partly being applied in RBP development [14; 15; 16], to the best of the authors' knowledge, 3D printing of magnets is not yet being used for testing new designs of medical devices.  
In this work, a functional pump with a magnetic driving and bearing system is printed in one print to show the feasibility of combining multiple material printing and magnet printing for the rapid development of RBPs. For this purpose, a flexible, highly-loaded magnetic compound filament (MagFil) is developed. This filament can be processed on a low-cost, end-user multi-material FDM printer. The chosen design exploits the designing freedom of additive manufacturing and cannot be manufactured conventionally.