## 2 Stability  
### Factor That Influence Instability  
Firstly, we will begin by discussing the factors that influence the instability of an MCF rubber sensor. In the previous study, we evaluated the secular stability of electrolytically polymerized MCF rubber sheets fabricated by sandwiching between opposing magnets [1]. However, it is typically necessary to measure the electric properties during sandwiching of the MCF rubber between the opposing electrodes. This electric property has secular stability. However, when the electrodes are separated, the stability deteriorates with time. Instability can be divided into two phases: (a) temporary instability during sensing that fluctuates with time; (b) instability over a long span of time leading to aging degradation. However, MCF rubber sheets must be considered because they are fundamental components of MCF rubber sensors. From this perspective, examples of factors that influence instability include: (1) the evaporation of water from the MCF rubber because water is substantially involved in the fabrication process; (2) the vulcanization of the MCF rubber occurs during the electrolytic polymerization process with surpassing this point, in contrast to electrolytic polymerization in our previous studies; (3) the deformation of the MCF rubber by the Mullins effect. Factors (1-3) are related to (a) and (b). In the case of (1), water is involved and is trapped in the inner MCF rubber, even if electrolytic polarization is performed, because the NR and CR latexes contain water. Therefore, due to the evaporation of water from the electrolytically polymerized MCF rubber, the MCF rubber sensor can exhibit a secular change of its electrical and mechanical properties. This will be discussed in the following section. In the case of (2), the MCF rubber liquid should be taken into account because it is fundamental in the production of MCF rubber sensors. In the previous study, we experimentally investigated the influence of the aggregation of magnetic particles and rubber molecules of MCF rubber liquid on their electrical characteristics when a magnetic field was applied [24]. The application of a magnetic field to a container immersed in the MCF rubber liquid resulted in the aggregation of particles and molecules to sediment. In the present report, we will introduce the result that the MCF rubber in the middle of vulcanization induces temporal instability by using an experimental apparatus with large electrodes gap as shown in Figure 11 in the Appendix A. The MCF rubber liquid consisted of 12 g Ni powder with particles on the order of microns and bumps on the surface (No. 123 by Yamaishi Co. Ltd., Noda, Japan), 3 g water-based MF with 50 wt% Fe\({}_{3}\)O\({}_{4}\) (M-300, Sigma Hi-Chemical Co. Ltd., Tsutsujigasaki, Japan), 16 g NR-latex (Ulacol, Rejitex Co. Ltd., Atsugi, Japan) and 31 g of water. Using electrolytic polymerization, the MCF rubber is vulcanized as shown in Figure 12 in the Appendix A. The vulcanized MCF rubber grows from the anode surface towards the direction of the cathode as a crystal. The thickness of the vulcanized MCF rubber increases with time as shown in Figure 13 in the Appendix A. Isoprene molecules and magnetic clusters are aligned along the same direction as the electric and magnetic field lines, as explained in previous studies [1,4]. The structure can be considered to be approximately quasi-regular crystalline, but not a crystalline lattice such as in hexagonal close-packed, body-centered cubic, and face-centered cubic structures. The density of the aggregation at 0 mm was larger than that at 15 mm such that the thickness of the vulcanize MCF rubber at 0 mm was larger than that at 15 mm. As shown in Figure 14 in the Appendix A, as the thickness of the vulcanized MCF rubber increases, the electrical conductivity decreases.