IEEE Robotics & Automation Magazine - September 2016 - 97

(namely DEAs) instead of SMAs to enable the stiffness
change. The mechanism is based on the same principle, and
the results show the possibility of increasing the stiffness by
up to two orders of magnitude.
Semiactive (Intrinsic Rigidity Tuning)
Other than antagonistic arrangement, variable-stiffness
mechanisms can be obtained by using semiactive actuators
whose working principle relies on the modulation of the
intrinsic passive mechanical properties of the material itself.
This translates into the capability of varying the energy dissipation of the system.
Jamming-Based Systems
Among the suitable strategies for changing the stiffness of a
soft robot, the jamming-based systems are emerging with a
new set of possibilities. Despite already being investigated for
a long time (granular jamming is a well-known issue in agriculture and the food industry), it recently gained the attention
of roboticists because of its capability of enabling a reversible
transition between a fluid-like and a solid-like material without (or with very limited) volume variation [23], [24].
There exist two systematic approaches that exploit the jamming phenomenon: membranes filled with granular matter or
with thin sheets. Both refer to the same activation principle:
vacuum triggers the "phase change," increasing the relative
shear stress experienced by the particles or layers in the elastic
membrane. Without attempting to catch the phenomenon
from a physical point of view, some research works have tried
to identify the key factors affecting the jamming mechanism at
the macroscale. Among the most accredited are analyses of the
number, shape, and dimension of particles or of overlapping
surfaces (contact surfaces); the mechanical properties of the
elastic membrane; and the vacuum level and the shear stress
experienced by the system [25]-[28].
One of the first and most effective efforts to exploit such a
phenomenon is represented by the universal gripper developed by Brown and colleagues [29]-a robotic end effector
able to pick up unfamiliar objects of widely varying shape and
surface properties [Figure 3(a)]. When pressed onto a target
object, the gripper flows around it and conforms to its shape.
Upon application of a vacuum, the granular material contracts and hardens quickly to pinch and hold the object. This
basic yet effective functionality has been increased in a more
recent work by Amend et al. [30], where they introduced the
possibility of adding a positive pressure in the elastomeric
membrane to make the recovery phase faster and to increase
the reliability and the error tolerance of the gripper, while
decreasing the force needed on target objects.
Used in an anthropomorphic gripper in combination with
PneuFlex actuators (a particular kind of FFA), a granular jamming-based system can allow a selective stiffening or a shape
locking of any bending state by retaining the motion of the
elastic top side of the fingers. This effect, in combination with
the flexible but nonstretchable material placed in the bottom
side, increases the flexural stiffness [31].

In a similar approach, Cheng et al. [32] developed a robust,
modular, and highly articulated manipulator that utilizes jamming of granular media for achieving local stiffness control,
while actuation cables along the robotic manipulator allow the
control of its shape and position [Figure 3(b)]. Jamming mechanisms provided the ground for the development of a new paradigm for soft robots, as referenced in [33]. The novelty is also
related to the capability of introducing selective anisotropies in
the behavior of the material. A soft mobile robot [Figure 3(d)]
has been developed by using jamming-based cells arranged on
the external surface of a sphere-shaped robot. By jamming specific cells with respect to others that remain in a fluid-like state,
it is possible to steer the deformation along preferred directions
that can be used to initiate a rolling locomotion pattern. With a
similar strategy, Kaufhold et al. [34] implemented a variablestiffness mechanism based on granular jamming to locally
interrupt the symmetrical behavior of their amoeba-like robot
[Figure 3(e)]. The rotating magnetic system used to induce a
vibration in the robot implements a random circular locomotion until the stiffness change brakes the symmetry, causing a
modification in the locomotion direction.
The simplicity, feasibility, and reliability of the jamming-based
technologies are enabling their use in completely new and innovative scenarios. In recent years, new application areas have
emerged, such as the stiffness modulation used as feedback for haptic or tactile interfaces [35]-[37] [Figure 3(f)].
But the most straightforward and promising exploitation scenario for this technology is undoubtedly the medical field. In
minimally invasive surgery (MIS) procedures, clinicians need
instruments that are flexible enough to enable insertion
through body cavities without damaging tissues but that are
also able to stiffen enough for applying forces on the target
site. Up to now, different endoscopes with these requirements
have been developed by exploiting the jamming mechanism
for actively varying stiffness [38]. The STIFF-FLOP surgical
manipulator is one of these. Three FFAs arranged at 120° are
used to generate omnidirectional bending and elongation and
combined with a central hollow channel that hosts the granular jamming mechanism to enable a selective stiffening of up
to 46% [39]-[41] [Figure 3(c)].
Particle jamming has interesting features, such as high
deformability in the ﬂuid state and a drastic stiffness increase
in the solid state without a signiﬁcant change in volume.
However, it requires a substantial volume of granular material
to achieve a signiﬁcant stiffness variation. From this point of
view, layer jamming technology could represent an even better alternative. Here, overlapping surfaces present a large contact area that translates into an increased friction force that
can be generated through vacuum application. A snake-like
manipulator based on this principle has been developed by
Kim and colleagues [42], [43]. The cylindrical shape of the
manipulator develops by overlapping layers into a helical
shape for maximizing the friction effects between layers
[Figure 3(h)]. The scalability of the technology is not particularly remarkable, but it allows obtaining a manipulator with
features compatible with various MIS applications.
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