## 4 Discussion  
From the above results, we can safely assume that thermal sintering plays an important role not only in the obtained resistance of the conductive pattern that is fabricated using FMD 3D technology, as in the cases of inkjet printing or screen printing [28; 31], but also in the main characteristics of the resulting sensor, that is, gauge factor, hysteresis, and linearity (\(\sigma_{\text{AR}}\)). In more detail, as shown in Figure 9a,b, in the devices that did not undergo sintering, a notable hysteresis is observed since the final value of the recorded resistance is decreased by approximately 0.6% compared to the initial value. Note that the level of hysteresis is significantly reduced (that is, 0.4%) with the application of sintering for even one hour, while a twenty-hour thermal treatment completely eliminated hysteresis. Moreover, as shown in Table 2, sintering clearly affects the resulting resistance change \(\Delta\)R for a given displacement (therefore, the gauge factor), while its duration notably influences the resulting linearity (\(\sigma_{\text{R}}\)) of the sensor. Figure 12: (**a**) Alternate bending response for 20 h annealed sample; (**b**) alternate bending response for 1 h annealed sample; (**c**) bending geometry corresponding to minimum and maximum resistance for 20 h annealed sample. In general, the mechanisms by which conductivity is achieved in polymer composites enriched with conductive fillers (such as CNTs) are closely related to the conductive filler loading and can be classified into two main types: tunnelling effect and ohmic conductance [34]. The tunnelling effect primarily occurs in composites with lower conductive filler loading, where most filler particles are covered by a polymer layer during mixing. According to [PERSON] et al. [35], in CNT-filled polymers, such as the conductive filament employed, the CNTs are entangled and folded in complex networks inside the polymer matrix. When an external strain is applied, the unfolding of entangled elastic nanomaterials is more probable than sliding in their axial directions, changing the tunnelling resistance; as a result, the conductivity of the polymer also changes. When the concentration of the conductive filler is increased beyond a specific point, referred to as the \"Percolation Threshold\", conductive networks are formed within the polymer matrix, providing relatively low-resistance paths for the movement of free electrons. As a result, ohmic conductance is the primary conductive mechanism. According to the results presented in Section 3.1 (SEM and elemental chemical analysis), the conductive layer of the sensor primarily consists of C (81.4 \(\pm\) 0.4% wt), O (7.2 \(\pm\) 0.4% wt), and N (11.3 \(\pm\) 0.1% wt). Taking into consideration that a typical C concentration for pure PLA material is under 60% wt [36], we can safely assume that Ohmic conductance is the main conductive mechanism for the particular filament and not the tunnelling effect since the filler concentration (CNTs) is over the typical percolation threshold; note that a typical percolation threshold of 0.1 wt% might be sufficient for nearly any CNT/polymer system with optimized dispersion of the conductive nanofillers [37]. In addition, according to [34], polymers enriched with CNT usually possess a very low percolation threshold because of their nanosized diameter and large aspect ratio. The piezoresistor's conductive filament is made using PLA enriched with CNTs, according to the manufacturer's specifications [29]. [PERSON] et al. [38] reported that annealing of PLA at a temperature of 95 C for 4 h resulted in the modification of its microstructure, which changed from an amorphous state to a semi-crystalline state, exhibiting two different crystal morphologies.