Page 380 - Control Theory in Biomedical Engineering

346 Control theory in biomedical engineering

against the spirit of our design requirements. Hence, we explored alternative
methods for achieving variable stiffness using origami structures.
7.2 Origami methods

While the method of kirigami was promising, it had many issues with respect
to reliability and repeatability as the delicate cuts can give way at any time. It
also had a low cycle count before the cuts led to tearing. Other designs for
auxetic kirigami have similar problems, so we decided our design should
minimize or, if possible, eliminate any cutting required.
As an alternative to kirigami, we used origami, which has more robust
material characteristics and, as such, is able to withstand higher cycle counts
and loads. Through intricate folding patterns using auxetic design, we can
integrate multiple units into a single system capable of providing both flex-
ible actuation as well as variable stiffness. The following sections will detail
and test such designs. Note that throughout this section, there is not much
importance placed on the overall dimensions as origami folding can be
highly miniaturized depending on the dexterity of the operator. Instead, care
is taken to make the crease pattern clear and visible for first-time users, so the
dimensions are generally larger than our design specifications.

7.2.1 Collapsible origami structure
Our starting point of inspiration comes from an on-demand collapsible
origami structure that also has load-bearing ability (Zhai et al., 2018). A col-
lapsible structure is a structure that can reconfigure and change shape/size
mainly from folding and unfolding. Stents are an excellent example of
collapsible structures used in the biomedical industry. The collapsible structure
has a stiffness tunable capability, as the method of extension and the method of
collapsing follow two different paths. More specifically, extension is easy,
whereas after complete extension, collapsing it back is harder. This metama-
terial design is inspired by the triangular cylindrical pattern adopted by the
authors and it is a relatively common fold in collapsible origami structures
(Zhai et al., 2018).
The structure was created using the base fold design as shown in Fig. 26.
A simple A4 printing paper was used as the material, and precreasing was
done using the Silhouette Curio machine to help in the folding process.
Creasing was done by using the score option to make tiny cuts that did
not fully penetrate through the paper. The edges were formed into a cylin-
der and glued using a thin layer of nonwater-based glue. The precrease was
then collapsed into the final shape before testing.