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Tunable stiffness using negative Poisson's ratio 327

5.2.1 Material filaments
As it is required for the part to be flexible, we used thermoplastic polyure-
thane (TPU), polyvinyl alcohol (PVA), and Pythonflex as three different
materials to test our designs. All these materials are easily available and com-
patible with a standard FDM 3D printer.
Thermoplastic polyurethane (TPU): We use a 1.75mm filament of TPU, with
high elasticity and resistance to grease, oil, and abrasion.
Polyvinyl alcohol (PVA): PVA is a water-soluble material and is commonly used
as a sacrificial support material for complex geometrical parts as they can be dis-
solved in water. Although their primary use is in making molds and supports, the
material displays excellent elasticity when extruded at a slightly lower temperature
( 10–20° lower than the rated temperature), the material has excellent elasticity
and springback and hence can be used as a flexible spring-like material. However,
care should be taken, as slight deviations from this range of temperature can quickly
make the material brittle and hard.
Pythonflex: Pythonflex is a high-performance variant made from specially formu-
lated TPU material.

5.3 Auxetic material designs
As mentioned earlier, we used five different auxetic designs to cover a
wide range of concepts of auxetic behavior and to explore the robustness
of our results. Each of our five designs were printed with the three mate-
rials mentioned in the previous section and subjected to mechanical tests.
The conceptual theory behind the auxetic behavior of each design is
elaborated below.

5.3.1 Hexagonal re-entrant honeycomb structure
Fig. 7 shows a traditional hexagonal re-entrant structure. When a force is
applied in either direction, the diagonal ribs move and rotate in a way to
produce an auxetic effect in the other direction. The auxetic effect is
observed as the diagonal ribs aligned along the horizontal direction move
apart in the vertical direction under tension. Tests have shown that most
of the structures involving re-entrant honeycombs undergo deformation.

5.3.2 Chiral structure
A chiral formation is defined as a nonsuperimposable mirror image. Such
structures exhibit a Poisson’s ratio of close to 1(Saxena et al., 2016). When