IEEE Robotics & Automation Magazine - December 2020 - 45

low-efficiency, damaged state for a limited time, as the cut or
perforation will tear further, leading to complete failure.
Many soft actuators are constructed out of siliconebased elastomeric networks, such as the Ecoflex series
from Smooth-On [5], [6]. These elastomers are not expensive, and therefore it is relatively cheap to produce soft
actuators. One way to solve the problem of a failed soft
actuator is by replacing it completely with a new one.
Although the production is cheap, the replacement of a
soft actuator can be costly since it is usually done through
human intervention. Most soft actuators are manufactured
through casting silicone monomers in a (3D-printed)
mold, followed by a curing step. During curing, the silicone-based network is formed by irreversible crosslinking.
The permanently formed crosslinks do not enable the
material to be reprocessed. Consequently, like most elastomeric polymers, these silicone-based networks are not
recyclable. Hence, replacing failed soft actuators with new,
cheaply produced parts is not the most ecological solution.
Alternatively, the vulnerability of soft robots can be
addressed by overdimensioning the components through
substantial safety factors, avoiding the possibility of damage in dangerous situations. However, this leads to larger
designs and less-energy-efficient systems, while potentially
compromising the soft and safe characteristic.
Recently, by using smart materials, soft robotic designs
have been improved, and new actuation principles are being
generated [7]. This includes the use of shape memory, piezoelectric and self-healing (SH) materials, and integrating
embodied intelligence [8]. In previous work [9]-[12], soft
robotic actuators were developed out of SH polymers-more
specifically, out of reversible elastomeric networks. The
crosslinks in these elastomers are reversible DA bonds and
provide the material with a healing ability [13]. Macroscopic
damages, such as perforations and cuts, as well as microscopic fatigue cracks can be healed by heating these flexible materials. Out of these, SH soft grippers and hands were
constructed [9], [14]. All of them could entirely recover from
realistic damages.
Incorporating a healing function is an eco-friendlier solution to the vulnerability of soft robotics. In addition, a healing ability facilitates reducing the overdimensioning of
systems and supports optimizing a design based on the function to be performed instead on potential damaging conditions. If these healable actuators are damaged very badly,
they can still be recycled because the reversible characteristic
of the crosslinks in the network enables reprocessing [9].
This can further decrease the ecological footprint. For successful healing, typically, a temperature increase to 80 °C is
required. This can be done by heating the entire soft robotic
part (the soft gripper [9]) in a healing station (e.g., an oven)
or by integrating a heating device in the actuator design [15].
The need for a heat stimulus provides a certain control over
the healing process. However, additional controlled heating
systems are required, making the overall robotic system larger and more complex.

This research focuses on lowering the healing temperature
of DA polymers toward room temperature, avoiding the need
for an additional heating system. Such polymers that do not
need an external stimulus to heal, other than the mechanical
force of the formation of the damage itself, are called autonomous SH polymers. This article first introduces different
mechanisms for autonomous healing that hold potential for
soft robotics. Next, it gives a detailed explanation of how an
autonomous SH DA polymer was synthesized. Healing at
25 °C is experimentally validated through extensive tensile
testing. The applicability of this DA network for soft robotics
is proven through the development of a soft hand prototype
that is able to heal autonomously from centimeter-scale damage. Last, we address the recovery of the component's properties after healing.
Autonomous SH Polymers
There exist in the literature many autonomous SH polymers
that rely on divergent healing mechanisms. Extrinsic autonomous SH polymers depend on healing agents embedded in
micro/nanocapsules [16]. The downside of these extrinsic
mechanisms is that the healing action can take place only a
limited number of times at the same location. In addition, the
healing mechanism usually allows recovery from only relatively small injuries. For the capsules to crack open, their shell
must have a brittle characteristic, and the material of the
matrix must be stiffer than that of the shell [16]. Consequently, extrinsic healing mechanisms work well for thermoset
matrices, but they are not adequate for elastomers. Although
they have potential for hard polymer components in robotics,
extrinsic SH polymers are not interesting for constructing
flexible soft robotic components.
There are intrinsic autonomous SH polymers having a
healing mechanism that works at room temperature. These
rely on reversible bonds and complexes that have fast
dynamics at room temperature. A particularly impressive
example is the hydrogel presented by Leibler et al. [17], a
supramolecular network formed through physical hydrogen
bonds. When slicing these materials, the hydrogen bonds
are locally mechanically broken, producing many nonassociated groups near the fracture surface that are " eager " to
link again. When pressing the fracture surfaces back together immediately after damage, these nonassociated groups
will re-form hydrogen bonds, and the cut can be healed at
room temperature. Using tensile tests, Leibler et al. proved
that, after three hours of mending, up to 90% of the original
strength was regained.
More recently, SupraPolix reported another autonomous
SH elastomer, SupraB [18], with great potential for soft robotics. This supramolecular material is formed by hydrogen
bond complexes that act as physical crosslinks. Again, when
cut, the hydrogen complexes are de-bonded, and free groups
created at the fracture surface can reconnect when its fractured areas are pressed back together. Using hydrogen bond
complexes as crosslinks not only enhances the strength of
polymer but also allows faster healing. This is explained by
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