Page 388 - Control Theory in Biomedical Engineering

354 Control theory in biomedical engineering

crisscrossing of the tendons from the outside, yields bending. As the focus of
the chapter is on the means of achieving variable stiffness, we will not go into
detail about tendon-routing methods.
Twisting of the tube along its central axis (Fig. 38) yielded another inter-
esting result. This twist motion effectively changed the metamaterial struc-
ture in such a way to resist further loading. Hence, by having an on/off state
for the twist of the structure, we found that we could control and vary the
stiffness of the tubular structure. Furthermore, the twist did not need to be
constrained to a binary state. A graded change in stiffness was observed by
varying the extent of twist anywhere between no twist and the fully
squeezed version. The detailed kinematic analysis of the change in metama-
terial structure and the associated change in stiffness was quantified by Feng
(Feng et al., 2018).
We harnessed this interesting property by developing a method of con-
trolling the amount of twist from a distal point. A thin tensile spring was run
through the central channel of the waterbomb tube and fixated at the top
using a layer of silicone rubber. Tensile springs were used since their com-
pliant nature allows them to deform and follow the bending motion of the
waterbomb tube. Furthermore, these springs are also able to maintain their
curvature and provide a torsional force at their tip when required. This use-
ful property allowed us to deliver a method of twisting the waterbomb tube
to modulate the stiffness of the structure. The twist can also be applied when
the structure is bent, which would allow the device to maintain its shape
throughout manipulation tasks during surgery. Fig. 39 shows the stiffness
capability of the waterbomb structure upon the application of twist. Notice
the curvature of the structure before and after the application of twist. It is
evident that an untwisted structure has a lower bending stiffness. Thus, we

Fig. 38 (A) Waterbomb tube before twist. (B) Waterbomb tube after twist.