Isotropic auxetic structure with fixed Poisson's ratio for piezo-resistive strain sensor to detect human body motion

Auxetic materials have different types whose mechanical behaviors are different from each other, and these differences affect their sensitivity. However, in the piezo-resistive strain sensor field, less attention has been paid to this variety of structures. In this article, the isotropic auxetic structure with three axes of symmetry was used to construct the isotropic linear piezo-resistive strain sensor. Structures that have only three or six symmetry axes have isotropic mechanical behavior, and structures with many symmetry axes other than these two cases are not isotropic. Also, to obtain a sensor with a linear sensing performance, the Poisson's ratio of the sensor must be constant during the strain. Most of the structures presented in the previous works have a variable Poisson's ratio along the strain. When the Poisson’s ratio of the structure is constant during the strain, it causes the sensing elements to have the same increased space to separate from each other during the strain, and as a result, the sensor will have a linear performance. A computer numerical control device was utilized to manufacture the sensor mold. Silicon RTV2 as a substrate material, and graphite powder as a sensing element were mixed to provide the sensor constitutive material, and molded into the manufactured die. To evaluate the response of the sensor in different directions, the sensitivity tests were performed in the directions of 0°, 60°, and 120°, which are compatible with each other, and also the suggested sensor has linear sensitivity during the strain.