## 1 Introduction  
Rubber sensors are valuable elements in a wide variety of engineering applications that affect many aspects of our daily life. The elasticity and flexibility of rubber have resulted in significant progress in several applications. To advance this field, we have investigated the mechanical, electrical and photovoltaic characteristics of a proposed magnetically responsive fluid called MCF involving rubber latex, the so-called magnetic compound fluid (MCF) rubber. This material exhibits hybrid properties during electrolytic polymerization under an applied magnetic field [1; 2; 3; 4; 5; 6; 7]. MCF rubber is a soft rubber that can be easily made using the latex of natural rubber (NR), isoprene rubber (IR), chloroprene rubber (CR), and butadiene rubber (BR) based on our proposed electrolytic polymerization procedure [1]. These rubbers contain C=C bonds so that they can be electrolyzed for vulcanization [1; 4]. However, given that nitrile rubber (NBR) or styrene-butadiene rubber (SBR) potentially has a high viscosity, electrolytic polymerization is difficult. In addition, the MCF rubber can be used as a sensor by exploitingour proposed novel adhesion technique [8] whereby the MCF rubber is electrolytically polymerized in conjunction with a metal complex hydrate so that the metal electrodes strictly adhere to the MCF rubber. Based on the reaction of the MCF rubber with electromagnetic waves, a variety of engineering applications can be developed. In a previous report [9], the effects of \(\gamma\)-rays, infrared and microwaves on the MCF rubber sensor were evaluated, establishing the feasibility of using the MCF rubber sensor for sensing various solar and thermal sources, etc., related to energy harvesting. In particular, it is suitable for use as a sensor installed in a robot that operates in a nuclear reactor building. However, one issue is the stability of the MCF rubber sensor. The stability of the changes of electrical properties with time is a significant problem. In the present report, we investigate the causes of the instability and deterioration in MCF rubber. Next, we resolve these problems by introducing a combination of non-diene rubbers. The non-diene rubbers include butyl rubber (IIR), ethylene-propylene rubber (EPM or EPDM), urethane rubber (UR), silicone rubber (Q), fluoro rubber (FKM), etc., which do not or seldom contain C=C bonds. Non-diene rubbers show superior weather, ozone, chemical, and oxidation resistance, less deterioration after years of usage under a variety of environmental conditions, and they can be used at any temperature. In addition, in terms of the sensitivity of a robot operated in a nuclear building, we also investigate in the present report the feasibility of an underwater MCF rubber sensor. To date, few studies have attempted to develop rubber-type water sensors in contrast with typical sensors including acoustic ones, etc. [10; 11]. Regarding the combination of any material into a rubber, there have been many studies on the combination of fillers such as carbon, metal, etc. particles in NR [12], CR [13], NBR [14], SBR [15], Q [16; 17] rubbers and in the combinations of NR and SBR [18; 19; 20; 21], NR and BR [21], NR and EPDM [22]. However, in terms of the latter combination, vulcanization with sulfur has been used. As observed in the results for \(\gamma\)-irradiation effect on the MCF rubber sensor in the previous report, our electrolytic polymerization for rubber vulcanization without using sulfur results in some typical characteristics, for example, the enhancement of softness during the elongation. Therefore, the utilization of electrolytic polymerization in the combination of several rubbers is expected to be effective. In addition, given that non-diene rubber offers excellent resistance as previously mentioned, the combination of diene and non-diene rubbers is also suitable. However, fabrication of this combination is akin to mixing water and oil because each rubber is generally similar to water and oil, respectively.