Page 357 - Control Theory in Biomedical Engineering

Tunable stiffness using negative Poisson's ratio 323

Conventional Auxetic
(Positive Poisson's ratio) (Negative Poisson's ratio)
Pull Pull

Pull Pull
Fig. 3 Comparison of a positive Poisson’s ratio material (left) vs a negative Poisson’s
ratio material (right).

These types of materials display interesting properties, such as light
weight, indentation resistance, impact resistance, shear resistance, and better
energy absorption capabilities as compared to conventional materials. While
there has been previous work done in the field of purely auxetic materials,
they have focused more on theoretical modeling than exploring the practical
applications of such materials in the field of variable stiffness devices. In this
study, we develop a stiffness tuning framework using continuum tubular
auxetic materials.
For a proof-of-concept, we restricted ourselves to fabricating these
materials using commonly available and cheap processes, thereby improv-
ing the speed and economic feasibility of manufacture. We employed
structures that could be fabricated by simple 3D printing as well as kiri-
gami and origami folding structures as simple, fast, and low-cost methods
of fabrication to prove the concepts with the following design require-
ments in mind:
• Geometric constraint: 10–12mm maximum outer diameter
• Compatible with the tendon driven actuation method
• Shift from flexible to stiff and vice versa without using temperature
regulations

4 Concentric continuum metastructures

Our primary hypothesis for a conceptualized mechanism is to vary the stiff-
ness of our tubular surgical device by using the concept of jamming. How-
ever, unlike layer or space jamming, we use the inherent mechanism of
auxetic materials, which expands upon elongation. This is more advanta-
geous as compared to using a vacuum as there is no need for sealing, which
is often one of the biggest challenges associated with jamming. By restricting
the space available for the expansion of the auxetic material, it is possible to