## 3 Results and Discussion
### Pump Demonstrator Manufacturing and Testing  
The aim to fabricate a prototype design of an RBP with magnetic bearing and magnetic drive coupling was achieved. The pump was successfully printed in one print (Figure 1c) with a print time of approximately 15 h and tested after support material removal and silicone coating.  
During multiple-material printing, the use of a single-nozzle printer can cause loss of printing precision and process stability. Occasionally, the MagFil filament is ground up in the extruder due to increased stress during filament changes. The necessity to co-print a wipe-tower to clean the nozzle after each filament change drastically increases the printing time. Contamination of the nozzle with the oily dispersing agent from the MagFil material diminishes proper layer adhesion of the concomitantly printed filaments, PLA and BVOH. Furthermore, the feature size for PLA- and BVOH-regions is limited due to the use of a large 0.6 mm diameter nozzle necessary to prevent clogging by NdFeB particles. A simple solution targeting all named problems with low investment costs could be the  
Figure 3: Comparison of remanences of NdFeB magnets in the form of commercially available sintered magnets [21], polymer-bonded magnets [22], magnets printed by selective laser melting (SLM) [6], big area additive manufacturing (BAAM) [2], and fused deposition modelling (FDM) [5] (Printed, Literature), theoretical remanences of the developed polymer-bonded magnetic compound as dense material and according for a porosity of 24% (Theoretical, MagFil) and the measured remanence of printed MagFil cylinders (Printed, MagFil; SD = 7%).  
use of a multi-nozzle printer that renders filament changes unnecessary and allows use of different nozzle diameters for each material. While this is expected to raise the printing quality to the standard FDM level, the FDM-inherent residual surface roughness will remain, and might limit the range of applicability for the present method.  
An operation of the pump prototype at a maximum rotational speed of 1000 rpm, with a flow rate of 3 L/min against a pressure head of 6 mmHg was achieved. At higher rotational speeds, the magnetic coupling broke off and the delivered flow rate decreased concomitantly. The pump prototype could therefore not deliver a sufficient flow rate at head pressures that are realistic for clinically used RBPs. This can be linked to the inferior printing quality caused by the above-described difficulties during multi-material printing. Due to insufficient layer adhesion of the PLA material and subsequent layer delamination, the pump surfaces were rougher and small features less precise compared to usual FDM parts. This caused additional friction and imbalances, for which the strength of the printed magnets was insufficient to compensate. Furthermore, the rough inner surfaces and material choice render hemocompatibility highly unlikely [23].  
The presented method allows only one polarization over all magnetic parts of one print, as a demagnetized powder is used during printing and the magnetic parts are magnetized when already embedded. A possible approach is to use already magnetized hard magnetic particles or to magnetize the particles during the printing process. These particles could then be aligned when exiting the nozzle by magnetic fields, as shown by [PERSON] et al., for direct ink writing of magnetic elastomer composites [24].