## 6 Discussion  
This study provided important knowledge with regard to the manufacturing process of MRE transducers and the addition of nanoparticles. Experimental tests were performed to cure the MRE layers. Some of them did not work, and acceptable results were obtained when both layers were cured, and the nanoparticles were added in the correct proportion. Furthermore, the mold max 30 silicone rubber with its catalyst worked well for the 3D printing of MRE molds. It was crucial to keep in mind the goal of replacing the strain-gauge transducer with an MRE transducer. After several tests, the composition of 90% silicone rubber and 10% catalyst generated a satisfactory result; on that basis, the nanoparticles were included in the MRE. The initial tests were carried out with iron (II and III) oxide nanopowder, but the relationship between strain and electrical  
Figure 28: Measured output voltage (Vout) for a 7.5 cm length cantilever beam with three piezoelectric sensors, as shown in Figure 13. Three samples were applied for each test (length and frequency). Small black numbers, accumulated voltage measurements. Figure 29: Rectangular rosette \(0^{\circ}/45^{\circ}/90^{\circ}\) strain-gauge array; model: SGD-6/350-RYT21; 350 \(\Omega\) of nominal resistance, 16.3 mm per side with a gauge factor of 2.13.  
resistance was not proportional to allow strain measurement, so it was decided to use multiwall carbon nanotubes with iron, and the experiment delivered satisfactory results. Regarding the ratio of silicone rubber and multiwall carbon particles, the ratio of 97% silicone rubber (ratio 10:1 of silicone rubber and catalyst) and 3% MWCNT-Fe particles presented satisfactory electrical-resistance characteristics, as shown in Figures 21 and 22. On the basis of the MRE transducer results, from the tensile tests in Figure 21, for the four-line parallel lines pattern, the replicas showed similar behavior. At the beginning of the test, a linear tendency was observed, but the slopes of the graphs were different. The rate of resistance change was higher in Replica 2 than that in Replica 1. For the 90-degree array, similarity between the two replicas was observed. The behavior could not be linearly described because, in Replica 2, electrical resistance did not vary as much as that in Replica 1 did. For the 45-degree array, Replica 1 had an almost perfectly linear tendency throughout the whole test. However, Replica 2 showed an increase in resistance as the force went up, but some noise was present from 5 to 35 N.  
According to the results of the compression tests shown in Figure 22, a high increase in electrical resistance was instantly observed when the load was applied; then, it declined as the system remained at rest. This behavior was detected for all arrays and with both replicas. Moreover, Replica 1 of the 90-degree array showed the most straightforward relation between applied force and electrical resistance before crossing the 1000 N border. Electrical resistance technically remained the same when the load was static. For the horizontal resistance of the 45-degree array, Replicas 1 and 2 showed almost equal behavior, although there was an offset between the two responses. A possible justification for this is that the base resistance of each was different, but the increasing steps were similar. In addition, there was some noise in the output resistance when the forces were applied; however, variation was very low and output resistance could be considered as being at a constant value throughout the force-step increase. The observed results in Figures 21 and 22 were dependent on the hands-on manufacturing of the MRE transducers. The MREs were manually manufactured without following rigorous industrial standards, and this led to variations in the electrical resistance. For example, distribution of the multiwall carbon nanotubes with iron (MWCNT-Fe) following the guidelines in Figure 5 was not the same in all three lines, and this affected the relationship between electrical resistance and applied force.