## 3 Results and Discussion
### Control of the Resistivity Anisotropy of Printed Parts  
The internal structure of the printed parts inherently appears anisotropic, and when the printed parts serve as a conductive medium, the resistivity is also anisotropic [18]. In the vertical direction, the current must pass through adjacent fiber layers but, in the horizontal directions, the current is not distributed only along the individual fiber but also across parallel fibers. As the process parameters influence the structure of printed parts, it also impacts the resistivity anisotropy ratios of the parts. The resistivity anisotropy ratios for the different parameter combinations are shown in Figure 5a by calculating the ratio of the resistivity in the vertical direction to that in the horizontal direction. The maximum anisotropy ratio is 3.59 and the minimum ratio is 1.47. The effect of the main factors of the anisotropy ratio for the three parameters is indicated in Figure 5b. Similar to that of resistivity in the vertical direction, the most influential parameter was the raster width, and the anisotropy ratio decreased with increasing raster width.  
Figure 5: Resistivity anisotropy of 3D-printed ABS/CB parts. (**a**) Anisotropy ratio of the printed parts in different parameter combinations from factorial design; (**b**) the main effects of the anisotropy ratio, including average values and standard deviations; an (**c**) the relationship between the anisotropy ratio and raster width ranged from 0.30 to 0.70 mm at intervals of 0.05 mm.  
To further clarify the relationship between the anisotropy ratio and raster width, the anisotropy ratio for the raster width range of 0.30 to 0.70 mm at intervals of 0.05 mm was examined. It can be seen from Figure 5c that with increasing raster width, the resistivity anisotropy ratio decreased monotonically. Although the structure of the printed parts was anisotropic when the raster width was 0.70 mm, the resistivity anisotropy ratio was close to 1.0. This result suggested that the resistivity of the printed parts can be adjusted to be nearly isotropic. This indicated that although the internal structure of printed parts was inevitably anisotropic, isotropic resistivity for conductive composites can be achieved by adjusting the process parameters. The relationship between the anisotropy ratio and raster width was fitted by an exponential function as shown in Figure 5c. The correlation coefficient was 0.993. It can be concluded that the resistivity anisotropy ratio can be adjusted from 1.01 to 3.59 by directly altering the raster width. Therefore, this could be considered as a new approach to fabricate parts with complex shapes and desired resistivity anisotropy ratio. Further, although the resistivity anisotropy ratio manipulation can also be achieved by printing with two kinds of materials with different resistivities using a two-nozzle printer, the advantage of this method was that only one conductive composites and the most widely used one-nozzle printer were needed.  
Since controllable and well-defined anisotropy materials are required in the construction of phantoms to test methods that evaluate medical imaging equipment, such as diffusion tensor imaging [39] and electrical impedance tomography [40], the process we detailed to manipulate resistivity anisotropy may serve as a new approach to fabricate a model by 3D printing with a complex shape and the desired resistivity anisotropy for imaging electrical anisotropy.