Page 359 - Control Theory in Biomedical Engineering

Tunable stiffness using negative Poisson's ratio 325

We reasoned that we could further improve this design to increase the
amount of jamming at the surface by having a counter force at the boundary
pushing in the other direction. This would effectively double the normal
force and, hence, double the friction. To implement this process, we used
a combination of an auxetic tube material surrounded by a normal PPR
material. The inner auxetic tube had a diameter of 11mm while the outer
tube had a diameter of 12mm such that they fit snugly into each other with a
space of 1mm as shown in Fig. 6. We theorized that when the same tensile
force is applied to both the PPR and NPR materials, the NPR material will
expand while the PPR material will contract. Thus, the radius of the inner
tube will increase, and the radius of the outer tube will decrease until the free
space is occupied. As we continue to apply the tensile load, jamming will
occur, and the resulting friction between the tubes will resist any bending
forces, leading to a stiff structure. The load-bearing capacity can be increased
by exerting a higher tensile force, which leads to a larger frictional force
between the tubes.

Fig. 6 CAD of the improved concept.