Page 383 - Control Theory in Biomedical Engineering

Tunable stiffness using negative Poisson's ratio 349

actuation methods like bending. The structure yielded pure compressions
and extensions without any bending. Furthermore, even while compressing
and extending, the device undergoes an involuntary twist that cannot be
overcome. This is because the complete structure had a certain twist to it
as all the mountain and valley folds pointed in one direction. Hence, it
was deemed important to eliminate this directional twist and achieve a
net zero twist structure while preserving its collapsible nature to add on
bending capabilities.

7.2.2 Miura origami structure
Miura origami, or as it is called in English, the herringbone tessellation, is a
form of an auxetic structure made from parallelograms. As shown in Fig. 28,
this tessellating structure displays excellent auxetic behavior.
For our design, the two-dimensional herringbone tessellation was com-
bined with the collapsible fold from the previous design. Moreover, the par-
allelograms were alternated in each layer such that the twist from one layer
canceled out the opposite twist from its adjacent layer, resulting in a net zero
twist. This yielded a hybrid composite structure that was compatible with
the current method of actuation and displayed variable stiffness. The folding
pattern and the complete folded structure are shown in Figs. 29 and 30.
A similar process of precreasing, gluing, and folding was followed to get
the complete structure.
After obtaining our complete structure, we then tested its ability for actu-
ation compatibility using a tendon-driven mechanism as well as variable
stiffness using the collapsible method. The structure was fixated with ten-
dons routing from the inner channel. Our tests showed that this structure

Fig. 28 (A) Herringbone fold crease pattern. (B) Herringbone folded. (C) Herringbone
unfolded.