In these review we investigate the functional materials and the tactile sensor devices, presented in literature, exploiting flexible composites with piezoresistive properties. These materials are one of the best candidates to fabricate a sensing ��skin��, able to reproduce the tactile sense and to fit the shape of the robot structure. Beyond the high conformability required to mimic the human skin, these composite sensing materials can be employed to generate devices with a wide range of sensitivity, a low power consumption and an elevate mechanical resistance guaranteeing protection from external physical agents that could damage the sensor. The major drawbacks of these types of devices are represented by the temperature sensitivity and hysteresis phenomena of the sensing response, which could influence the repeatability of the measurements [2,3].

We perform a classification on the basis of the piezoresistive conduction mechanism dividing the tactile sensors into piezoresistors, strain gauges, percolative and quantum tunnelling devices. For each flexible tactile device family we analyze the physics behind the conduction mechanism and we describe the state-of-the-art from the point of view of the material employed and the adopted architecture. The design and the performance are also reviewed, with a perspective outlook on the main promising applications. To introduce the following detailed analysis a general qualitative comparison of the four different tactile sensor types is presented in Table 2.

Furthermore a table (Tables 3, ,4,4, ,55 and and6)6) with a quantitative comparison of each analyzed device is reported at the end of each section.Table 2.Comparison of the different flexible composite tactile sensors.Table 3.Comparison of tactile sensor solutions based on flexible piezoresistor.Table 4.Comparison of tactile sensor solutions based on flexible strain gauges.Table 5.Comparison of tactile sensor solutions based on percolation mechanism.Table 6.Comparison of tactile sensor solutions based on quantum tunnelling mechanism.2.?PiezoresistorsThe work principle in piezoresistors consists in a variation of the resistivity of the material itself due to an applied stress. In general piezoresistors are made of silicon or other semiconductors, like germanium.

Here the stress modifies the width of the band-g
The bell-shaped vibratory angular rate gyro (BVG) is a kind of solid wave gyro that detects the input angular rate using the standing wave’s precession on a bell-shaped resonator. Its core component is a bell-shaped resonator-like millimeter-scale Chinese traditional bell. The resonator uses the exciting AV-951 and detecting electrodes on its wall to control the resonator’s mode to generate a special standing wave and extracts the precession to detect the angular rate.