## 1 Introduction  
Sensors are currently used in many fields to detect force, temperature, pressure, etc. However, in robotics, sensors must have the mechanical durability to withstand immense or impulsive pressure, as well as large elasticity or extensibility. Certain sensors should be suitable for installation as an outer layer on robots, or for assimilation into an artificial skin. These characteristics are not, of course, limited to the sensors in robotics; sensors with mechanical durability are also required in other engineering fields. Regarding the concept of a skin sensor, there has been only a few general investigations [1]. The paradigm is that the sensor should be based on the effective control or management of human-machine systems. One new natural-contact biosensor has been designed as an interface between humans and machines for nonintrusive and real-time sensing. Although this sensor utilizes microelectromechanical systems (MEMS) or nanoelectromechanical systems (NEMS) to enable miniature sensors, flexibility remains a fundamental problem. Therefore, as described above, there is a need for a suitable rubber material with the durability to withstand impulsive, immense or repeated force while maintaining large elasticity and extensibility [2]. Researchers have examined many materials in the search for a sensor made of rubber: silicon oil rubber [3; 4; 5; 6; 7; 8], chloroprene [9], styrene-butadiene [10], acrylonitrile butadiene [11], nitrile butadiene rubber [12], and natural rubber (NR-latex) [13; 14; 15; 16]. [PERSON] proposed a new sensor made of silicon-oil rubber that uses magnetic compound fluid (MCF) as filler (called MCF rubber) [17]. MCF was devised in 2001 as a colloidal fluid containing Fe\({}_{3}\)O\({}_{4}\) sphere particles on the order of 10 nm, coated by oleic acid and other metal particles such as Ni, Fe, and Cu, on the order of 1 \(\upmu\)m. These particles are dispersed in a solvent such as water, kerosene, or silicone oil to compound a magnetic fluid (MF), which is an ordinary intelligent fluid responsive to the magnetic field with Fe\({}_{3}\)O\({}_{4}\) particles. When a magnetic field is applied, the Fe\({}_{3}\)O\({}_{4}\) particles play a bonding role among the metal particles, causing numerous metal and Fe\({}_{3}\)O\({}_{4}\) particles to aggregate [18]. Due to these magnetic clusters, MCF is useful for many engineering applications [19]. The mechanism of electric conductivity of silicon-oil type MCF rubber is explained mainly by the percolation theory [20] and tunnel theory [21]. Percolation threshold is significant factor in the former. Therefore, in the case of a small mass concentration of MCF rubber, it is possible that electric conductivity is demonstrated by this theory. On the other hand, for a large aggregation of particles with large aspect ratio, the latter theory can better explain the electric conductivity of MCF rubber. Electrons jump between metal particles such as Fe\({}_{3}\)O\({}_{4}\) and Ni through the barrier of non-conductive materials such as silicon-oil rubber and oleic acid coated around Fe\({}_{3}\)O\({}_{4}\). Silicon-oil type MCF rubber has a large electrical resistivity, however, which does not fulfill the needs of a skin sensor. In order to obtain sensitivity at a shearing motion, such as touching a body and scrubbing, a skin sensor requires high sensitivity, as shown by a previous investigation. When two electrodes are set between skin rubber and supplied with voltage, the electric current flowing inside the rubber can be measured. The electrical conductivity changed by the force applied on the skin rubber is related to the degree of sensitivity. In order to achieve the requirement of high sensitivity, we can enhance the sensitivity of MCF rubber using NR-latex. NR-latex type MCF rubber is made by solidification under a constant temperature (the drying method (DM)) and the application of a magnetic field. The DM has also been used to produce ordinary, commercial, pressure-sensitive electrically conductive rubber (PSECR).