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336 Control theory in biomedical engineering

6.2.3 Double arrow honeycomb structure tests
Like the previous two designs, the TPU material exhibited an increase in
stiffness compared to the case with no elongation (Fig. 17). Like the missing
rib design, this increase was consistent across different loads. Tests with
Pythonflex again showed no significant change in stiffness. We observed that
the variation in stiffness when using PVA was very high. It is unclear at this
point if such a difference was actually present or if tiny human errors in load-
ing geometry might have led to such a difference. Repeated tests are
required to confirm this. Furthermore, using a 6mm elongation in PVA
material is not recommended, as after a certain load the material starts to rup-
ture or develop a crack as shown by the sharp drop in the load in the graph.

6.2.4 Chiral structure tests
The chiral design was the only design that showed an increase in stiffness
upon elongation with more than one material (Fig. 18). Both TPU and
PVA data showed that the chiral design had higher stiffness, especially at
higher loads. Like the previous two cases, Pythonflex did not show much
variation among the three cases. The increase in stiffness was consistent
across the loads when using PVA. However, it was inconsistent when
using TPU.

6.2.5 Star structure tests
This star design showed least capability of stiffness tuning out of all the five
designs across the three materials (Fig. 19). We saw a relatively wider var-
iance among the base case and elongation cases, and in all three materials,
the base case had higher stiffness. This means that upon elongation, the sam-
ples displayed a lower stiffness, which opposed our hypothesized concept of
increasing stiffness through jamming.

6.3 Results discussions
In the previous section, we tested the possibility of using the mechanism of
jamming created by the expansions of auxetic materials to achieve variable
stiffness. We tested five designs and three flexible materials with 0, 3, and
6mm horizontal elongations. Our results showed that across the designs
and materials, there was little evidence to show that there was a significant
difference in the change in stiffness. In some cases, the change in stiffness was
observable as the load vs vertical extension curve shifted upwards under hor-
izontal elongation. However, this change was not consistent and not observ-
able in the test cases. Among all the different auxetic designs, the change in