## 1 Introduction
From these causes, the conception of a skin sensor is very significant, and there are many works on skin sensors, for example: sensing in a human-machine system [12]; piezoresistive sensors utilizing carbon nanotubes [13]; electronic skins (e-skins) [14]. On the other hand, [PERSON] attempted to develop a sensor that fulfills these demands using natural rubber (NR) mixed with metal particles [15]. Large extension and softness were achieved with NR-latex as the natural rubber. By enhancing the electric conductivity of the natural rubber with metal particles mixed into the NR-latex, haptic sensibility was attained. The metal particles consisted of particles involved in a magnetic compound fluid (MCF) as follows: a magnetic fluid (MF), which is an ordinary intelligent fluid responsive to the magnetic field used in rotating shaft sealing, contains magnetite Fe\({}_{3}\)O\({}_{4}\) sphere particles on the order of 10 nm. In the initial stage, [PERSON] devised in 2001 a new intelligent fluid responsive to a magnetic field by compounding MF and metal particles, such as Ni, Fe, Cu, etc., on the order of 1 \(\upmu\)m. He named the new fluid magnetic compound fluid (MCF) [16]. Today MCF is defined as a colloidal fluid containing Fe\({}_{3}\)O\({}_{4}\) and other metal particles on the order of 10 nm dispersed in a solvent. Fe\({}_{3}\)O\({}_{4}\) particles play a bonding role among the metal particles, allowing numerous metal and Fe\({}_{3}\)O\({}_{4}\) particles to aggregate. As a result, magnetic clusters formed by the aggregation take shapes such as prolate spheroid, needle, or network, and those shapes can be verified by the new method of extracting magnetic clusters from colloidal fluid devised by [PERSON] [17]. These magnetic clusters range in size from the order of nm to mm, and are aligned along the magnetic field lines by the application of a magnetic field. MCF is useful for engineering applications owing to the magnetic clusters. It is effective in polishing [18], and as a material for use as a damper [19] and composite material due to these magnetic clusters [20]. As for MCF damper, the damping effect is enhanced by the increasing viscosity of the MCF owing to the magnetic clusters. With regard to MCF polishing, the magnetic clusters act as magnetic brushes, like toothbrushes, and produce an enhanced mirror-finishing effect [21]. [PERSON] showed that the properties of the fluid in these engineering applications could be enhanced by varying the behavior of the magnetic clusters [22]. The rubber mixed with MCF is called MCF rubber. The magnetic clusters in MCF rubber produce the electric conductivity that leads to sensitivity. At the beginning of MCF rubber research, the rubber was mixed using silicon oil rubber, and vulcanized under a constant temperature and magnetic field application. MCF rubber has a switching effect, which means the electric resistance is large, more than M\(\Omega\) when touched with almost negligibly small pressure, and this resistance is reduced abruptly up to several \(\Omega\) by pressure [23]. The electric resistance can be as large as 335 \(\Omega\) at the split second that an electric current flows, whether the pressure touches the rubber or not; at that time its pressing force F\({}_{\rm s}\) is 0.294 N. The F\({}_{\rm s}\) values of the commercial pressure-sensitive electrically conductive rubbers (PSERs) are larger than this, for example, 6.53 N, 38.1 N and 75.5 N in PSERs made by the Japanese Co. Ltd. It is generally said that the F\({}_{\rm s}\) must be on an order less than 0.01 N in order to reflect the sensitivity of human skin. If natural rubber (NR-latex) is used in place of silicon oil rubber as a constituent of the MCF rubber, the F\({}_{\rm s}\) becomes less than 0.01 N. In fact, [PERSON] obtained the result that F\({}_{\rm s}\) is 0.0149 N [15]. NR-latex constitutes a hydrophilic system, while silicon oil rubber is a lipophilic one. It is easier for an electric current to flow in the former than in the latter.