Highly sensitive, stretchable, piezoresistive auxetic sensor based on graphite powders sandwiched between silicon rubber layers

As a measuring device, an auxetic piezo-resistant sensor must have the characteristics of the measuring equipment. One of these features is to have the correct and same answer in different strains and directions (same answer in different conditions). This issue should be considered in the case of auxetic piezo-resistant sensors because of having different Poisson's ratios at different strains and also being generally anisotropic. In this paper, the problem of instability of Poisson's ratio during the strain for auxetic structures was investigated and showed that the sensing performance of re-entrant auxetic piezo-resistant sensors is nonlinear due to the nonlinearity of Poisson's ratio over a broad range of stain values. To investigate this issue, highly sensitive, stretchable, piezoresistive sensors based on graphite powders sandwiched between silicon rubber layers were prepared using a re-entrant auxetic structure. The sensing performance of the auxetic sensors was identified and compared with previous sensors. The presented sensor showed piezoresistive enhancement under tension loading, with improvement up to 176 and 380% (in terms of gauge factor) in comparison with previous mix auxetic and conventional composite sandwich sensors, respectively. Furthermore, we proposed an auxetic structure that has a constant Poisson's ratio over a broad range of strains. Using this auxetic structure made it possible to design an auxetic sensor with linear sensitivity over a broad spectrum of strain values. The auxetic sensor with a constant value of Poisson's ratio has stable sensitivity changes at different strains. It means that the sensor sensitivity has a fixed slope. In the next works, we intend to use isotropic auxetic structures to make piezo-resistant auxetic sensors.