## 3 Materials and Methods
### Design and Manufacturing
#### 3.1.1 Transducer: Magnetorheological Elastomer (MRE)  
A magnetorheological elastomer (MRE) was designed and manufactured to convert strain and vibration measurements into electrical resistance. A general mold was designed using computer-aided-design (CAD) software, and different patterns to measure the strain were tested. Furthermore, this subsection explains, in detail, the manufacturing process used to create three MRE prototypes. The MRE transducer mold was designed as a 3-layer device. The floor layer contained cavities with a specific pattern based on a strain-gauge rosette. The middle layer was screwed to the floor, and it allowed the samples to be extracted. The top layer was screwed to the middle layer to close the mold. The complete CAD mold is shown in Figure 2.  
As part of the research activities, three different designs for the MRE transducer were implemented--a horizontal array with 4 lines, 4 lines in a rosette array with 90\({}^{\circ}\) among lines, and another rosette arrangement with 3 lines and 45\({}^{\circ}\) among them. These designs were modeled in Autodesk Inventor 2018, and printed in 3D with polylactic acid (PLA) material. The array of 4 lines had 4 cavities in the floor layer, describing a pattern of four parallel lines to measure electrical-resistance variations just in one axis. The rosette arrangement with 3 lines of 45\({}^{\circ}\) had a different pattern in the floor layer; it had 3 cavities, one at 0\({}^{\circ}\), another line at 45\({}^{\circ}\), and the third at 90\({}^{\circ}\) to measure electrical-resistance variations in two axes and their components at 45\({}^{\circ}\). The third pattern proposal had two pairs of lines on its floor layer that were placed oppositely and a 90\({}^{\circ}\) rosette strain gauge. The CAD drawings of all the developed patterns are shown in Figure 3.  
After the molds were manufactured, to continue the MRE preparation process, preliminary tests of silicon rubber adhesion were performed. Experiments were carried out to verify that a silicone layer could be adhered to another one without mixing. The goal of this experiment was to confirm that both layers were able to merge once they were fully cured. This procedure was applied to the MRE preparation process to combine a layer with a satisfactory percentage of nanoparticles to guarantee an accurate measurement of electrical resistance, with another of lower concentration that served as the  
Figure 1: Schematic of the designed and implemented smart sensor. Red text—separately tested subsystems with preliminary results. base material for the transducer. The employed materials for this task were a mold max 30 silicone rubber with its catalyst and the 3D printed molds fabricated from the designs in Figure 3.  
Silicone rubber and catalyst were applied to generate the MRE. The first experiment used a ratio of 50:1 (98% silicone rubber and 2% catalyst). The results were not satisfactory because the low-concentration of catalyst caused the sample to begin the curing process after 20 h. The second and third experiments used a ratio of 10:1 (90% silicone rubber and 10% catalyst). The new results from the second and third experiments were acceptable because the mixture started to cure after 2 h, which complied with the reported reaction times during the MRE fabrication process [42]. After a series of trials, the tests were carried out with 90% silicone rubber and 10% catalyst, and the results were satisfactory. The layers were successfully merged, and this ratio of rubber to catalyst was used in the subsequent experiments. The manufactured transducers are shown in Figure 4.  
The next step was to include nanoparticles in the MRE. An MR elastomer sample composed of two layers was made; the first had a higher concentration of nanoparticles to obtain better electrical conductivity and conduct electrical-resistance measurements in the material, and the second layer had a lower concentration of nanoparticles, with the aim of using it as the base layer.