Piezoelectric materials have a property that they can convert the mechanical energy to the electric energy and vice versa, and making use of such property, they are used as sensors and actuators. The conventional piezoelectric materials which are piezoelectric ceramics are very hard and brittle, accordingly their application places are restricted. Instead of brittle piezoelectric ceramics, we have focused on flexible piezoelectric rubber for engineering use. At first, the “mixed-type” piezoelectric rubber was fabricated by mixing the rubber and piezoelectric ceramics particles with roll machine; however, its piezoelectricity was very low. Therefore, the “aligned-type” piezoelectric rubber which was aligned piezoelectric ceramics particles in rubber by applying voltage with rubber vulcanization was fabricated. The piezoelectricity of the aligned-type was higher than that of the mixed-type even though it has smaller amount of particles. In addition, it was confirmed that the increase of the force applied to aligning particles enhanced the piezoelectricity of the aligned-type. In this paper, we report the fabrication method and piezoelectricity of piezoelectric rubber, and also reported the result of applications of piezoelectric rubbers to a pinching sensor of foreign matter at the slide door of the rolling stock, and a damage detection sensor at the axle bearing of the rolling stock.

Conventionally, in order to measure the strain of the rigid, such as metal, the strain gauges are widely used. The strain measurement of the flexible object, and a wide dynamic range (for example 100% or more) strain sensor are required. Flexible strain sensor is expected to realize a variety of technologies. For example, measurement applications, human interface, smart wear, skin-motion monitoring, and robotic skin.

We have developed a stretchable strain sensor C-STRETCH^®, used the compounding techniques of elastomer and elastic conductive materials. This sensor is wide dynamic range (up to 200% elongation), very soft, very thin film, high responsiveness, and excellent measurement accuracy.

This review article highlights an overview of cellulose derivatives for novel mechanical stress sensing. Natural polymers such as cellulose are known to form cholesteric liquid crystal (CLC) phase typically over 100 °C after chemical modification of their side chains. Bragg reflection is one of the most unique and important optical properties of the CLC helical structures. When hydroxypropyl cellulose (HPC) was esterified with both propionyl and another groups, we serendipitously found the continuous changes of Bragg reflection peak throughout the full visible-wavelength range between 400 nm and 800 nm at relatively low CLC mesophase temperatures below 100 °C. Furthermore, full-color imaging could be realized by crosslinkable HPC derivatives tethering acryloyl side chains. More interestingly, we succeeded in the demonstration of novel mechanical stress sensing by the elastic CLC films of crosslinked HPC derivatives, leading to the visualization of external mechanical stress as the changes in Bragg reflection color. In this way, the elastic cellulose CLC films are available as the social infrastructure sensors for deterioration or damage of concretes, wearable sensors for human medical care, and so forth.

To achieve the sensing shear stress as well as normal force, and temperature, the technique of their sensitivity of flexible sensors is introduced by utilizing natural rubber (NR)-latex through the application of electrolytic polymerization focused on the isoprene C=C bonds on natural rubber and by aiding a magnetic field and magnetic compound fluid (MCF) as magnetically responsive fluid. By the application of the magnetic, electric fields and the MCF, we can align the electrolytically polymerized C=C along the magnetic field line with the magnetic clusters formed by the aggregation of magnetite and metal particles of MCF so as to enhance the sensing effect. The flexible sensor has many intelligent properties of piezoresistivity, piezoelectricity, photovoltaics, et al. simultaneously.