Page 379 - Control Theory in Biomedical Engineering

Tunable stiffness using negative Poisson's ratio 345

7.1.3 High-density foam
As an alternative to silicon, we used another flexible material—high-density
foam (Fig. 25). We theorized that the amorphous nature of foam would slow
down the crack propagation and it could also be more easily stretched com-
pared to rigid cardboard paper. The same testing process was carried out, and
after the tests, we noticed a marked improvement in terms of the tearing of
hinges. However, this material failed to stiffen through our theorized means.
This is because even after closing the bulk gaps between each of the indi-
vidual sections, there were still minute natural gaps within each section
because of the nature of foam. Hence, the application of a load leads to huge
deformities, so a pure foam-like material is not suited for such applications.
From these tests, we concluded that the perfect mix for such applications
is a combination of rigid and flexible materials. The rotating squares must be
stiff to bear load and exhibit stiffness characteristics when required. How-
ever, the hinges connecting the different squares must be made from a flex-
ible material to ease the stretching of the structure into its auxetic form. To
meet these requirements, we could use a multimaterial design by using
Vero-White as the rigid material for the squares and a mixture of Tango+
with other materials to create an amorphous flexible material for the hinges.
Furthermore, we could make design improvements by altering the hinge
shapes. Instead of a straight cut hinge, which introduces a lot of stress
concentration, we could use a curved cut to reduce the chance of tearing
and improve repeatability. However, the use of multimaterial 3D printers
would significantly increase the time and cost of fabrication, which goes

Fig. 25 (A) Nonrigid structure when the spaces are closed; (B) High-density foam
auxetic behavior.