## 3 Results  
### SEM Characterization and Elemental Chemical Analysis  
As already mentioned, the current investigation included a SEM characterization of appropriate printed samples to investigate the morphology of the structure, homogeneity, and chemical composition of the resulting piezoresistor. Therefore, SEM images of 3D printed samples employing Fiber Force Nylforce CNT Conductive filament were recorded on a JEOL JSM-6510 LV-EDAX Scanning Electron Microscope (Oxford Instruments (Abingdon, UK), 10 mm\({}^{2}\) Silicon Drift Detector-x-act). The dimensions of the samples were 1 cm \(\times\) 1 cm \(\times\) 1 cm.  
The views of the specimens along the different cross-sectional planes are presented in Figure 4. A fairly uniform picture arising from the carbon particles embedded in the plastic matrix was observed along every cross section (Figure 4a-c). This uniformity was enhanced upon annealing (Figures 4d-f and 5). The morphology of the microstructure was the same and independent of the orientation of the cross-sectional plane. We should note that, as anticipated, no individual CNTs were detected at this observation level (nor at higher magnifications employing appropriately polished samples, reaching up to \(\times\)13,000) since the dimensions of individual CNTs are expected to be in the nanoscale. The purpose of SEM characterization, however, was not to observe individual CNTs but to detect (in combination with EDAX analysis results) potential non-uniform dispersion of CNTs (i.e., aggregates of CNTs) in the polymer matrix. From the obtained results, no distinctive particle formations were observed (both from SEM and corresponding EDAX carbon mapping), suggesting a uniform distribution of the CNTs within the polymer matrix. Therefore, a continuous and homogeneous solid material is built.  
Elemental chemical analysis of the specimen was also performed through Energy Dispersive X-ray Spectroscopy (EDS)/EDAX. According to the obtained results, the specimen consisted mainly of C (81.4 \(\pm\) 0.4% wt), O (7.2 \(\pm\) 0.4% wt),  
Figure 4: Cross–sectional SEM images of the printed specimen along different planes before (**a–c**) and after (**d–f**) annealing.  
Small amounts of Al (approximately 0.1% wt) were also detected which can probably be attributed to impurities present in the specific filament. We should note that the recorded value for C is significantly larger than the corresponding one for a typical PLA filament which is not enriched with CNTs (see Section 4). A cross-sectional image of the specimen recorded by SEM is presented in Figure 6a, together with the EDAX carbon and oxygen elemental analysis (Figure 6b) and element distribution of C and O (Figure 6c,d). The presence of C is attributed to the conductive nanoparticles as well as the polymer matrix, whereas O and N originate from the matrix material. An isotropic behavior of the material is anticipated, based on the uniform distribution of the elements observed in the images.  
Figure 5: Cross-sectional SEM images of the printed specimen along the x–y plane before (**a**) and after (**b**) annealing.  
Figure 6: View of an x–y cross-section of the printed specimen before annealing: (**a**) SEM image, (**b**) EDAX spectrum, (**c**) carbon mapping, and (**d**) oxygen mapping.