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Tunable stiffness using negative Poisson's ratio 321

applications make use of jamming for devices that are intended for direct
human manipulation, and, as such, they are typically on the larger scale
of a few centimeters in diameter ( Jiang et al., 2014). Our robotic manipu-
lator for MIS has a dimensional constraint of about 5mm, so granular jam-
ming is challenging given the complexities required for miniaturization of
the technique. Additionally, there is a limit to the amount of achievable
rigidity, as the internal pressure cannot be lower than absolute vacuum
( Jiang et al., 2012). Furthermore, the bulk material properties of such mate-
rials are not well understood, making mathematical modeling of the process
a challenge.

2.4 Negative pressure jamming

For a preliminary study, we explored the method of using negative pressure
to create a tunable stiffness module. In this method, a thin film of PET was
encapsulated around a flexible continuum robot with 3degrees of rotational
freedom. Fig. 2 shows the continuum robot with the encapsulation made by
3D-printed vertebrates supported by Ni-Ti wires. Pulling on one or two
wires simultaneously causes an unbalanced load and a bending moment in
the desired direction.
In order to change the configuration from a flexible to a rigid robot, a
negative pressure was created through the means of a vacuum pump. When
the pressure inside the membrane was lower than the atmospheric pressure,
there was a net inward force, which caused the encapsulation membrane to
clamp onto the. This normal contact between the membrane and the ver-
tebrates of our robot created a frictional force that resisted bending motion.

Fig. 2 (A) Flexible motion of the continuum robot through the means of Ni-Ti wires;
(B) Continuum robot with bag without negative pressure. (C) Continuum robot with
bag with negative pressure.