Page 378 - Control Theory in Biomedical Engineering

344 Control theory in biomedical engineering

b1

b2

t d2 d1

Fig. 24 Maximum load-bearing capacity of a hollow triangle (b 1 , b 2 defines the edge
length of the outer and inner triangle, respectively; t is the edge thickness and d 1 , d 2
defines the height for the outer and inner triangle, respectively).

where
3 3
ð
b 1 d 1 b 2 d 2 2tÞ 2
I Hollow ¼ ð tb 2 d 2 Þ: (7)
36 36 3
Since this structure is segmented into different sections, it is possible to
selectively stiffen certain sections of the tube while keeping the rest of the
tube flexible. This translates to a dynamic means of load bearing as it can
enable us to achieve complex shapes and maneuvering behavior for our
applications.

7.1.2 Silicone rubber
As silicon is a flexible material, we theorized that it will display better char-
acteristics compared to the rigid cardboard material as it can be easily
stretched into its auxetic nature. We used 1.5mm silicon rubber sheets using
a similar cutting technique.
Firstly, we observed that even the slightest loads at the ends caused buck-
ling of the tube even under closed conditions. This is because the slits intro-
duce structural problems given that silicon is a very soft and flexible material.
We also observed that the rotation of the squares was not as smooth as the
friction between the silicon layers, causing many out-of-plane rotations. As a
result, the cell-opening mechanism was not satisfactory, and the auxetic
behavior was minimal. Additionally, the load-bearing capacity was not as
good as that of rigid cardboard paper as there were already initial deforma-
tions when a similar load was applied. Furthermore, we noticed that cracks
propagated very easily in the silicon layers and the hinges kept tearing away
frequently. The hinges of the tube were torn away when a load was applied.
Thus, we did not choose silicon sheets as the base material.