## 3 Materials and Methods
### Design and Manufacturing
#### 3.1.1 Transducer: Magnetorheological Elastomer (MRE)
The first elastomer was fabricated with iron (II and III) oxide nanopowder with a particle size of 50-100 nm, and 97% trace metal basis. For the preparation of the sample, the first layer had a mass equal to 3 g consisting of 70% silicone rubber (ratio 10:1 of silicone rubber to catalyst) and 30% iron oxide nanoparticles. For the second layer,the mass was equal to 10 g and it was composed of 90% silicone rubber (ratio 10:1 of silicone rubber to catalyst), and 10% iron oxide nanoparticles. Figure 3: CAD images of three implemented MRE patterns. (**a**) Horizontal array with 4 lines; (**b**) 3-line array with 90\({}^{\circ}\) and 45\({}^{\circ}\) between them; (**c**) 4-line array with 90\({}^{\circ}\) between them. Figures taken from [39]. Figure 2: Computer-aided-design (CAD) magnetorheological-elastomer (MRE) mold with 3 layers: floor, middle, and top. Electrical-resistance measurements were carried out on the cured sample, but the relation between the applied force and electrical resistance was not evident. The reason for this was that the electrical conductivity of the MRE with iron oxide nanoparticles was too small to exhibit the expected behavior of a potential transducer. To mitigate this, and on the basis of laboratory availability, we used multi-wall carbon nanotubes with iron (MWCNT-Fe). [43]. To overcome the aforementioned problem, rubber and catalyst percentages were changed. The first layer consisted of 97% silicone rubber (ratio 10:1 of silicone rubber to catalyst), and 3% MWCNT-Fe particles (some trials with a different percentage of particles were carried out before selecting this rubber-nanoparticle percentage ratio). This ratio of silicone rubber to MWCNT-Fe particles was chosen because the second test had satisfactory results--the observed electrical-resistance characteristics were suitable for sensing (i.e., the elastomer exhibited the required elastic behavior). The manufactured 2-layer MR elastomer with nanoparticles is shown in Figure 5.  
A graphical way of presenting the set of methods and carried out experiments (planned and executed in sequence) is presented in Figure 6. The image depicts the ordered sequence of laboratory tests. It started with the CAD designs of the molds and continued with the implemented tests for different concentrations of silicone rubber and catalyst. Later, the nanoparticles were added to the correct mixture among layers. In the first trial, the tests included different concentrations of iron (II and III) oxide nano powder but results were not satisfactory, i.e., the relation force vs. electrical resistance  
Figure 4: Examples of two generated MR elastomers with 90% silicone rubber and 10% catalyst. Depicted patterns are (**a**) 3-line array with 90\({}^{\circ}\) and 45\({}^{\circ}\) between them, and (**b**) 4-line array with 90\({}^{\circ}\) between them. Figure 3.0 Manufactured 2-layer MR elastomer with nanoparticles. was poor. Subsequently, the experiments were carried out with MWCNT-Fe and the relation cause (force)- consequence (electrical resistance) was obtained. Ohmmeter tests were applied to the generated samples and based on these measurements, it was validated that the sample with MWCNT-Fe had a better force vs. electrical resistance relation than the iron (II and III) oxide nano powder. The outcomes supported the idea of applying the manufactured MRE as a transducer. With this lately result, three different MRE instances were manufactured (4-lines in parallel, 4-lines in 90\({}^{\circ}\), and 3-lines in 90\({}^{\circ}\)/45\({}^{\circ}\)). The MRE output is the electrical resistance, which must go through a signal-conditioning stage prior to being processed by the CPU. For this purpose, two required circuits were implemented, a Wheatstone bridge and a signal amplifier with operational amplifiers [1].