IEEE Robotics & Automation Magazine - September 2016 - 102

research works already rely on the use of these materials for
fast manufacturing, self-healing, or reconfigurability [58],
but not for the ability to change its mechanical properties.
Temperature-sensitive plastics and wax have demonstrated
their potential, but higher stiffness variation can be obtained
if metals are used instead. Moreover, in principle the metal
itself can be used as a source of heat from electric energy
(resolving the issue of providing an external heat source). But

to fully exploit this advantage, encapsulation is needed to contain possible liquid pouring. A smart solution is the use of
microfabricated elastomeric structures [59] [Figure 5(c)].
This is based on Cerrolow 117, a low melting point alloy
(LMPA), embedded in soft polydimethylsiloxane (PDMS).
The devices demonstrate a relative stiffness change higher
than 25 times (theoretically > 1,000 times) and a fast transition from rigid to soft states (< 1 s) at low power (< 500 mW).

Table 2. A direct comparison of stiffening mechanisms (qualitative evaluation scale: ++, +, o, -, - from
the highest to the lowest values).
Actuation
Technology

ACTIVE
(antagonistic arrangement)

Active-Active

EAP + EAP

Depending on

Speed of
Destiffening

++

Voltage amplifier

++

Depending on
Discharge

Fluidics + fluidics

+

Fluidic inflow

o

Fluidic outflow

Tendons + fluidics

o

Motor's
velocity and
fluidic inflow

o

Motor's velocity
and fluidic outflow

Fluidics + braided sleeves

+

Fluidic inflow

o

Fluidic outflow

SMA + braided sleeve

+

Electric power

-

Thermal conditions

SMA + flexible layers

-

Electric power

-

Thermal conditions

EAP + flexible layers

+

Voltage amplifier

Granular jamming

+

Vacuum pump
outflow

o

Vacuum pump
inflow

Layer Jamming

+

Vacuum pump
outflow

o

Vacuum pump
inflow

++

Magnetic field

++

Magnetic field

MREs

++

Magnetic field

++

Magnetic field

EREs

++

Electric field

++

Discharge

Wax

-

Thermal
conditions

-

Heat source

Polymers

-

Thermal
conditions

o

Heat source

Alloys

-

Electric power

-

Thermal conditions

Glass transition-based

-

Thermal
conditions

o

Heat source

SMMs

Polymers

-

Heat source

-

Thermal conditions

Alloys

-

Electric power

-

Thermal conditions

Conductive polymers

-

Thermal
conditions

-

Electric popup

Chemical-based

-

Drying
conditions

-

Hydration speed

Active-
Passive
Structure

Jamming-based

ER and MR materials MR fluids

SEMI-ACTIVE
(intrinsic rigidity tuning)

Speed of
Stiffening

Low melting point
materials

++

Discharge

Independent control stiffness and equilibrium position: the ability to vary the stiffness independently from the equilibrium position
Passive deformation: the maximum elastic deformation of the structure
Number of modes of stiffening: the modes considered are 1) bending, 2) elongation/compression, and 3) torsion.
Allowed workspace: the workspace of the robotic device in which the change of stiffness is allowed

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