Page 362 - Control Theory in Biomedical Engineering

328 Control theory in biomedical engineering

Fig. 7 Re-entrant structure CAD.

the ligaments are pulled (i.e., a longitudinal strain is applied), the rigid ring
rotates, causing unwinding of the ligaments, which leads to its auxetic
behavior.
As shown in Fig. 8, we designed and fabricated a tetra-chiral structure
with each central node connected to four other nodes. While we wanted
to construct a hexachiral structure, the geometric constraints set by our
design specifications did not allow us to construct such designs as the tube
diameter would have had to be increased to 20mm.

5.3.3 Star honeycomb structure
A star honeycomb structure is a subset of re-entrant structures and a variant
of the traditional hexagonal re-entrant design. As is common with
re-entrancy, the deformation of the star structure is through the hinging
of the walls of the cells, as shown in Fig. 9. These star-shaped building blocks
have a rotational symmetry on the order n¼3, 4, and 6. We have designed a
Star 4 structure, which is said to display a Poisson’s ratio of 0.845 (Kolken
and Zadpoor, 2017).