IEEE Robotics & Automation Magazine - December 2020 - 46

the fact that a firm physical crosslink can be established when
only a fraction of the hydrogen bonds of the complex have
been formed. This is the case for SupraB, which instantaneously recovers part of its strength after the fracture surfaces
are placed together [18], [19]. Longer healing times further
increase the strength since a growing fraction of the hydrogen
bonds is formed in the crosslink complex.
It is important to mention that, aside from the DA polymer used in this article, other autonomous, intrinsic SH polymers that rely on various mechanisms have mechanical and
healing properties suitable to introduce healability with high

efficiency in soft robotics. Other intrinsic SH polymers with
interesting properties (Young's modulus E, fracture stress v,
fracture strain f, and healing efficiency h at 25 °C) are listed in
Table 1, and others are discussed in [20].

Healing in DA Networks
DA polymers are networks containing reversible covalent crosslinks formed by a DA bond between a furan and a maleimide
[Figure 1(a)]. The networks are formed using two monomers;
bismaleimide (DPBM) and a furan-functionalized Jeffamine
(FTx), obtained by irreversibly binding FGE on a Jeffamine
(JTx) using an epoxy-amine reaction
(further details appear in the " Re--
agents " section). The DA reaction
Table 1. Autonomous SH networks suitable for soft robotics.
between a furan and a maleimide is an
Reversible
E
v (Maximum) f (Maximum) h at 25 °C
equilibrium reaction; consequently, the
-Reaction
Institution
(MPa)
(MPa)
(%)
(%)
crosslink bonds are dynamic. This
Hydrogen
SupraPolix [18] 0.8
0.35
250
90 (24 h)
means that, for a specific temperature
(in equilibrium conditions), a crosslink
ESPCI Paris [17] 0.25
3
550
90 (3 h)
density can be defined. Although the
UC [21]
17
1.94
780
90 (24 h)
crosslink density remains constant if
Metal ligand SU [22]
0.5
0.25
4,500
90 (48 h)
the temperature continues unchanged,
Exchange
Cidetec [23]
0.05
0.5
3,000
97 (24 h)
bonds are constantly broken and reDA
VUB
0.12
0.1
245
80-97 (Seven
formed in the dynamic network.
to 14 days)
DA networks, in general, require
UC: University of California; VUB: Vrije Universiteit Brussel; SU: Stanford University.
heat to heal macroscopic damage
[Figure 1(a)]. When damage occurs,

Furan
o
R1

Maleimide
o
N

DA Adduct
o

R

o

o

N R2

R1

o

DPBM-FT3000, r 1

DPBM-FT5000, r 1
(c)

(a)

DPBM-FT5000, r 1

DPBM-FT5000, r 0.5
(d)

(b)

Figure 1. (a) Separated DA parts can be healed using a mild heating treatment. The result is a strong interface due to covalent
DA bonds formed between a reactive furan and a maleimide. (b) The self-sealing zipping effect in DA networks with sufficient
microscopic mobility enables cavities to slowly seal. (c) The molecular mobility in the network can be increased by decreasing the
crosslink density, by raising the molecular weight of the monomer units, in this case, the molecular weight of the furan-functionalized
compound (green). (d) Alternatively, the crosslink density can be decreased by lessening the maleimide-to-furan (r) ratio and by
employing a deficit of reactive maleimide components (red) in the synthesis.

46

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IEEE ROBOTICS & AUTOMATION MAGAZINE

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DECEMBER 2020
IEEE Robotics & Automation Magazine - December 2020

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