Auxetic lattice sensor for In-socket load evaluation

Abstract

Auxetic metamaterials present an uncommon dome shape when subjected to an out-of-plane bending moment, known as synclasticity. This property has them potential candidates in aerospace, biomedical and textiles. Currently, the use of wearable devices has increased. These sensors allow the tracking of physical activity of the human body, which provide useful information about health. They need to withstand repeated large deformations and conform to the complex curved geometries of the human body without loss in performace. Conformability has presented a challenge in materials science and engineering and one approach to overcome this, has been the implementation of auxetic topologies. Still, most applications remain in their infancy and require more research. Despite biomedical sensors being subjected to complex loading conditions, most of the literature has focused on auxetic metamaterials under simple tensile and compressive loadings.

The geometrical parameter-Poisson´s ratio was thoroughly characterized bia Finite element modeling (FEM). This brought up a thorough relation between their geometrical parameters and auxeticity. Their out-of-plane stiffness was also characterized via FEM and corroborated with additive manufactured samples subjected to the same boundary conditions. A conformability ratio was computed with digital image processing, and a generalized linear model of 95% confidence interval exhibited the relation between each parameter and this property. Topologies with similar conformability ratio were found, which allowed to establish a relation between geometrical parameters, conformability and stiffness.

Finally, the fabrication of pressure-sensing devices was achieved by the instrumentation of velostat with different auxetic porous arrangements. This exposed a general view of their electric response under different loading conditions. These devices were also tested as in-socket pressure sensors, establishing a link between their electric and mechanical response while being stretched to conform an artificial residual limb. This, in addition to in-plane, and out-of-plane characterization, lead to key properties when deciding the geometry specific for applications; deformation mechanism, relative density, auxetic behavior and stiffness.