IEEE Robotics & Automation Magazine - December 2020 - 54

merging takes longer than the healing between two freshly
made fracture surfaces because few reactive maleimide and
furan components are available on the surface of the actuator,
in comparison with newly formed fracture surfaces. On fracture surfaces, upon damage, a large additional number of
maleimide and furan components is created by mechanically
breaking the DA bonds.
In the future, the formation of bonds between two surfaces
with equilibrium conversion and in contact should be experimentally validated. Luckily, soft robots, like soft grippers and
hands, are generally used in dynamic applications, where selfcontact occurs for short durations. If the healing time at application temperatures in next-generation SH polymers in soft
robotics can be further reduced, the undesired merging of
planes will be able to take place faster, a condition that will
definitely have to be considered.
In this research, the macroscopic damages were applied in
a clean lab environment and with clean blades. Of course,
when evolving toward industrial and commercial applications, the influence of contamination on the fracture surfaces
on the healing performance has to be investigated. In future

(a)
80

Bending Angle (°)

70
60
50
40
30
20

Healed
Initial

10
0

4
Pressure (kPa)
(b)

Figure 9. (a) After being cut through completely [Figure 8(d)]
and healed, the actuator is again completely airtight and can be
pressurized without leaking. (b) The bending characteristic of the
healed actuator is compared to the initial characterization prior
to the damage.

IEEE ROBOTICS & AUTOMATION MAGAZINE

DECEMBER 2020

applications, to enhance the cleanliness of the fracture surfaces prior to healing, dirt and dust can be blown off using compressed air, which is available in pneumatic systems, or
cleaned away by submerging a part in water, which can be
done because the DA materials are insoluble.
Tradeoff Between Mechanical Properties and
Healing at Room Temperature
According to our knowledge, for all intrinsic SH polymer
technologies (excluding healing agent-based materials), a
general tradeoff between mechanical properties (the Young's
modulus and the tensile strength) of the SH polymer network and its healing temperature has to be considered. As a
result, all autonomous intrinsic SH polymers have a limited
mechanical strength and a high flexibility, as illustrated in the
examples in Table 1. For the DA network described in this
article, the same tradeoff applies. Autonomously healable DA
networks must have a lot of molecular network mobility to
heal damage at room temperatures. Consequently, they need
to be very soft. The high flexibility of these networks limits
the force output of the actuators that are built from them.
Soft robotic applications where higher force outputs are
needed can be developed from DA material with a higher
Young's modulus and greater strength, such as the DPBMFT5000-r1 material [12], [14]. However, these networks have
less molecular mobility. The temperature must be substantially increased (to nearly 90 °C) before sufficient molecular
mobility enables the closing and healing of relevant macroscopic damages. Currently, a DA network with high mechanical properties and a healing mechanism that performs at
room temperature cannot be synthesized.
Conclusion
For the first time, soft robotic actuators have been developed
that are able to recover their performance at room temperature after severe damage, without the need for an externally
applied stimulus. These were constructed from a newly created autonomous SH polymer network that obviates the
need for additional heating devices that would increase the
complexity of the overall robotic system. The SH polymer
network based on the reversible DA reaction was designed to
increase the molecular mobility by means of working at a low
maleimide-to-furan ratio. This reduces the crosslink density
and results in an excess of furan groups, compensating for
the lower maleimide concentration. A DA network was synthesized that can autonomously heal catastrophic macroscopic damage at room temperature. The healing efficiency
of a fractured part, evaluated through the recovery of the
stress at fracture, is 62, 91, and 97% after three, seven, and 14
days, respectively.
This material was used to develop a healable soft pneumatic hand. Relevant large cuts could be entirely healed without the need of a heat stimulus. Depending on the size of the
damage and, even more, on the location of the damage, the
healing takes only seconds or up to one week. For this evaluation, the actuator was considered to be healed whenever it

IEEE Robotics & Automation Magazine - December 2020

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