Mussel-inspired healing of a strong and stiff polymer

Article abstract of DOI:10.1039/c8ta00760h

Self-healability will greatly improve the reliability, service life and maintenance of synthetic materials. However, it remains a big challenge to realize the self-healing of a strong and stiff polymer due to its poor molecular mobility. Inspired by the h

Full text of DOI:10.1039/c8ta00760h

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Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
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A
mussel-inspired strong and stiff polymer exhibits healability, renewability,
anti-flammability and solvent-tolerance.

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Mussel-inspired healing of strong and stiff polymer
Ning Chen, Liming Qin, Qinmin Pan*
(State Key Laboratory of Robotics and Systems, School of Chemistry and Chemical Engineering, Harbin Institute
of Technology, Harbin 150001, P. R. China)
1

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Abstract
Selfꢀhealability will greatly improve the reliability, service life and maintenance of synthetic materials.
However, it remains a big challenge to realize selfꢀhealing of a strong and stiff polymer due to its poor molecular
mobility. Inspired by hardness and selfꢀhealability of the cuticle of mussel byssal threads, here we demonstrate
that a stiff and strong polymer can heal itself through dynamic metalꢀligand interaction. The polymer is comprised
of nitrocatecholꢀsubstituted chitosan crossꢀlinked by catecholꢀFe complexation, which exhibits a tensile strength
(63.6 ± 2.2 MPa) and an elastic modulus (862.6 ± 31 MPa) comparable to or even better than those of common
engineering plastics. Once suffering from multiple physical damages, the stiff polymer is able to recover its
configuration integrity and mechanical properties autonomously via pHꢀinduced coordination between Fe3+ and
catecholic moiety. Moreover, the polymer also exhibits interesting renewability, flame retardance and
solventꢀtolerance. Application of the polymer is demonstrated by a weightꢀbearing test. Our investigation offers a
bioꢀinspired strategy to design healable rigid polymers that have potential applications in various technologic
fields.
Keywords: stiff polymer, healable, musselꢀinspired, dynamic catecholꢀFe coordination, pHꢀinduced
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Introduction
Selfꢀhealing is an impressive ability that can spontaneously repair mechanical damage, which is common in
living organisms. Inꢀinspired by this phenomenon, synthesizing selfꢀhealable materials had attracted extensive
interest over the past decades.1ꢀ3 Compared to its conventional equivalent, a synthetic selfꢀhealable material
possesses excellent reliability, easy maintenance, long service life, and reduced waste. In term of selfꢀhealing
mechanism, there are extrinsic selfꢀhealability and intrinsic selfꢀhealability. The extrinsic selfꢀhealability is
realized by preꢀstoring healing agents such as monomer or catalyst inside microcapsules or microvascular
networks.4,5 In contrast, intrinsically selfꢀhealing is mainly based on dynamic covalent bonding (Diels–Alder
reaction,6ꢀ8 disulfide,9ꢀ11 cycloaddition reaction,12,13 radical dimerization reaction,14 borate ester bonding15ꢀ17 and
metalꢀligand coordination,18ꢀ21 etc) or reversible physical interactions like hydrogen bonding,22ꢀ24 hostꢀguest
interaction25ꢀ27 or stacking interaction.28,29 For either extrinsic or intrinsic selfꢀhealable material, selfꢀhealing
π-π
behavior takes place only when its polymer chains can diffuse across the broken interface, which means that the
mobility of polymer chains is crucial for the selfꢀhealability. Actually most of existing selfꢀhealable polymers are
soft and flexible in order to ensure good chain mobility. However, polymers used in daily life and technological
fields are usually strong and rigid, their selfꢀhealing remains a big challenge due to poor chain mobility.30 It is
highly desired to achieve stiffness and selfꢀhealability simultaneously without conflict through a facile and
versatile strategy.
Nature often provides inspiration for designing novel functional materials. Mussel is a kind of marine organism
that can fasten itself to almost all kind solids in aqueous environments by byssal threads coated with a
proteinaceous cuticle.31,32 The cuticle exhibits not only a mechanical hardness up to ~0.1 GPa but also a breaking
strain of 100%, which endows mussel with strong adhesion even in the turbulent intertidal zone.33 It was recently
revealed that the cuticle is rich in the catecholꢀlike amino acid comprising 3,4ꢀdihydroxyphenylalanine (dopa),34ꢀ36
and the coordination of the dopa with a small amount of metal ions, mainly Fe3+, accounts for the hardness and
3

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high extensibility.37ꢀ38 More importantly, dopaꢀmetal bonds are apt to spontaneously reform after breaking,
enabling the damaged cuticle to heal itself through the regeneration of catecholꢀFe complexes.39ꢀ41 Therefore, it is
the coordination between catecholic moiety and ferric ion responsible for both hardness and selfꢀhealing
capability of the cuticle.42 All these motivate us to realize the healing of a rigid polymer by introducing
catecholꢀFe coordination into its backbone.
Here we report healing of a stiff and strong polymer via a musselꢀinspired strategy. The polymer is composed of
nitrocatecholꢀsubstituted chitosan crossꢀlinked by dynamic catecholꢀFe complexation, which exhibits a tensile
strength of 63.6 ± 2.2 MPa, elongation of 53.33 ± 6 % and an elastic modulus up to 862.6 ± 31 MPa. Once the
polymer is thoroughly cut off, it can spontaneously restore its configuration and mechanical properties via
pHꢀinduced coordination between Fe3+ and catecholic moiety. Although the restoration involves slight pressure for
the alignment of the cut parts, it is realized autonomously through a microscopic process as that occurs in a living
mussel. Therefore, the restoration can be considered as a selfꢀhealing process. Given that similar metalꢀligand
coordination can be introduced to a wide range of polymeric chains, various healable rigid polymers might be
synthesized through the bioꢀinspired strategy reported here.
4

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Experimental section
Preparation of the stiff CNC polymer
At first chitosan substituted with nitrocatechol (chitosanꢀnitrocatechol, CNC) was synthesized through a
reductive amination reported in the literatures (Fig. S1, ESI†).43,44 Then 2 g of CNC was dissolved in 10 mL water
and later 10ꢀ70 mg of FeCl3 was slowly added to the resulting solution under stirring. The acidity of the
CNC/FeCl3 solution was adjusted to pH 3 by 0.1 mol L–1 NaOH solution, giving rise to brown CNC hydrogel. The
hydrogel was then shaped in a mold and subsequently dipped in a NaAc buffer (0.5 mol L–1, pH = 5.5) for 1 h.
After drying at 50 oC, a strong and rigid CNC polymer was obtained.
For the preparation of the CNC filaments, the CNC/FeCl3 solution was first loaded into a 5 mL plastic syringe
with a nozzle of 1.2ꢀ2.0 mm in diameter. The hydrogel was then injected into a coagulation bath containing the
NaAc buffer via a syringe pump at a rate of 0.05ꢀ3 mL min–1. After coagulation for 60 min, the filaments were
taken out from the bath and then dried at 50 oC for 2 h.
Healing of the CNC polymer
Typically, a CNC bar with the size of 50 mm × 10 mm × 5 mm was first cut into halves by a razor blade. The
cut interfaces were wetted by 0.25 mol L–1 HCl solution for 10 min. The wetted halves were carefully placed in
contact for geometric matching. Here only very little pressure was needed for the contact and the matching was
evaluated by visual inspection. Later the contact region was wetted by the NaAc buffer for 1 h. The halves
autonomously healed into a single unit that could support its own weight. The healed bar was dried at 50 oC for 2
h and then subjected to tensile experiments and microscopic observations.
Tensile experiments
The original (or healed) CNC filaments with 0.6 mm diameter and 40 mm length were used as testing samples.
Two ends of each filament were fixed to the clips of a universal electromechanical testing machine (CREE 8007B).
Then the filaments were stretched at a rate of 15 mm s–1 until breaking. The stressꢀstrain curves of the samples
5

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were recorded and their mechanical healing efficiency ( , %) was determined by the formula η = Th/To × 100,
η
where Th and To are the tensile strength of the healed and the original samples, respectively.
Regeneration of the CNC polymer
A CNC filament was first cut into small pieces and then dissolved in 10 mL dilute HCl solution (0.25 mol L–1).
The obtained viscous solution was injected into the NaAc buffer to produce a hydrogel filament. After drying at
50 oC for 2 h, the resulting filament was subjected to tensile experiments to assess its mechanical properties.
Characterizations
Scanning electron microscopic (SEM) images were recorded by a Zeiss supra 55. Tensile experiments were
performed on a universal electromechanical testing machine (Cree 8007B). Optical microscopy observations were
conducted on a Phenix PH50ꢀ1B43LꢀA/PL (China). Proton nuclear magnetic resonance (1H NMR) was conducted
on a Bruker 400. Ultraviolet visible (UVꢀvis) spectroscopy was performed on a Mapada UVꢀ6100 spectrometer.
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Results and discussion
a)
c)
b)
Fig. 1 (a) Synthesis of the stiff CNC polymer. (b) Optical and (c) SEM images of the asꢀprepared CNC polymer.
The inset is the crossꢀsectional image.
Fig. 1a illustrates the synthesis of the reported stiff polymer. At first, nitrocatechol was synthesized through a
previous process and then it was substituted to chitosan via reductive amination (Fig. S1ꢀ2, ESI†). The obtained
polymer (chitosanꢀnitrocatechol, CNC) was dissolved in a 0.1 mol L–1 HCl solution containing a small amount of
FeCl3. As the acidity of the resulting solution was adjusted to pH 5.5, the CNC chains were crossꢀlinked by
o
catecholꢀFe coordination to form a brown hydrogel. Finally the hydrogel was dried at 50 C to produce a strong
and stiff CNC polymer (Fig. 1a). The resulting polymer showed a smooth and compact morphology (Fig. 1bꢀc).
7

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The water content in the CNC polymer was measured to be 3.3 wt% by thermogravimetric analysis (Fig. S3,
ESI†).
c)70
b) 9
a)
70
8
60
60
7
50
6
5
4
3
2
1
0
50
40
30
20
10
0
40
30
20
10
0
1000
800
600
400
200
0
50
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10
0
CNC polymer
15
CNC polymer
0
10
20
30
40
50
0
10
20
30
40
50
60
70
0
5
10
Strain (%)
20
Time (min)
Strain (%)
Fig. 2 (a) Tensile stress–strain, (b) compressive stress–strain and (c) stressꢀrelaxation curves of the CNC polymer
at 25 °C. The insets are the average Young’s modulus of the four equal samples. (d) Mechanism for the excellent
mechanical properties of the polymer. The catecholꢀFe coordination serves as sacrificial bonds in the case of
mechanical damage.
The mechanical properties of the CNC polymer were evaluated by uniaxial tensile and compressive
measurements in ambient environment (25 oC and RH = 25%). For reproducibility, four equal samples were tested
at a rate of 15 mm s–1 for each kind measurement. The results of the tensile experiments are presented in Fig. 2a.
The polymer exhibits a tensile strength of 65 MPa and an elastic modulus up to 862.6 ± 31 MPa. Notably, the
elongation at break reaches 53.3 ± 6% by applying a tensile strength of 65 MPa, indicating good extensibility.
Compressive measurements revealed that the polymer has a compressive modulus of 43.6 ± 2.3 MPa (Fig. 2b).
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The values are closed to or even better than those of common engineering plastics like polyoxymethylene (POM),
polyphenylene Oxide (PPO) and polycarbonate (PC) (Table 1). Stressꢀrelaxation experiments showed that the
polymer underwent substantial stress relaxation with time at 25 °C (Fig. 2c). However, the residual stress was still
above 20.7 MPa after 50 min relaxation. The outstanding mechanical properties are attributed to the crossꢀlinking
of CNC chains by nitrocatecholꢀFe complexation. In the presence of trace water, the catecholꢀFe coordination
exhibits dynamic nature.45 Once suffering from external force, the coordination ruptures preferentially as a
sacrificial bond to dissipate energy and thus prevents the crack to propagate,18,42,46ꢀ48 ensuring the robustness and
good extensibility of the polymer. The factors affecting the mechanical properties of the CNC polymer were
summarized in Fig. S4ꢀ8 of ESI†. It was found that the acidity of the coagulation solutions, the molar ratio of
Fe/nitrocatechol, drying temperature and time played an important role in the tensile strength and breaking strain
of the resulting polymer. All the results clearly demonstrate that our CNC is a hard and strong polymer with poor
chain mobility.
Table 1. Comparison on the mechanical properties of typical engineering plastics.
Engineering plastics
Nylonꢀ6
Tensile strength (MPa)
Breaking strain (%)
51ꢀ78
58ꢀ70
48ꢀ66
60ꢀ88
85ꢀ142
~110
150ꢀ250
15ꢀ75
35ꢀ60
80ꢀ95
2.1ꢀ8
20
Polyoxymethylene (POM)
Polyphenylene Oxide (PPO)
Polycarbonate (PC)
Polyphenelene sulfide (PPS)
polyetheretherketone (PEEK)
This work
52ꢀ82
8.7ꢀ54
Interestingly, the CNC polymer exhibited a pHꢀinduced healing behavior. Here a CNC bar with the size of 50
mm × 10 mm × 5 mm was first severed into halves by a razor blade. The cut interfaces were wetted with a dilute
HCl solution (0.25 mol L–1, pH = 0.6) and then placed in contact for geometric matching. The contacted pieces
were further treated with a NaAc buffer (0.5 mol L–1, pH = 5.5) and then dried at 50 oC. After drying for 2 h, the
halves merged into a single unit that could withstand its own weight (Fig. 3a). Both visual inspection and optical
microscopy observation confirmed that there was almost no gap at the contact region (Fig. 3b), indicating the
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healing of the broken sample. We further investigated the mechanical properties of the healed sample by tensile
experiments. Fig. 3c compares the stressꢀstrain curves of the sample before and after healing. The healed sample
exhibits a tensile strength of 62 MPa. Its mechanical healing efficiency, the ratio of the tensile strength between
the healed and original samples, is calculated to be 90%. Moreover, the healed sample also shows a breaking
strain (55%) closed to that of its original counterpart. The results confirm efficient recovery of the mechanical
properties.
a)
b)
c)
e)80
90
80
70
60
50
40
30
20
10
0
d) 70
100
80
60
40
20
0
f)
100
80
60
40
20
0
70
60
60
50
40
30
20
10
0
50
40
30
20
10
0
Original
Healed
1
2
5
pH 4
pH 5
Acidity of buffer
3
4
pH 5.5
pH 6
6
pH 1
pH 7
0
10
20
30
40
50
60
Cut/healing cycle
Strain (%)
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g)
Fig. 3 (a) Optical images show the healing behavior of the CNC polymer. (b) Optical microscopic and (c) SEM
images of the healed region. (d) Stressꢀstrain curves of the original and healed CNC samples. (e) Effect of the
acidity of the buffers on the mechanical healing efficiency of the CNC samples. (f) Variation of healing efficiency
after multiple breaking/healing cycles. (g) Illustration of the healing mechanism of the CNC polymer.
We also found that the pH variation was crucial for the healing behavior. For comparison, CNC samples were
first severed and their cut interfaces were wetted by a dilute HCl solution (pH 0.6), NaAc (pH 4ꢀ5.5) and KH2PO4
(pH 6ꢀ7) buffers, respectively. After the same contact and heat treatment, the resulting samples were investigated
by tensile experiments to evaluate their mechanical healing efficiency. Fig. 3d compares the healing efficiency of
the CNC samples healed under different conditions. For the sample only treated with the pH 0.6 HCl, it shows a
tensile strength of 32.1 ± 2.3 MPa and a healing efficiency of 50.4%. The values are much lower than those of the
sample treated with the dilute HCl and then the NaAc buffer. The maximum tensile strength and healing
efficiency can be achieved when the polymer is treated by the buffer with pH 5.5. In this case, the healing can take
place for at least 6 cycles with an efficiency over 90% (Fig. 3e). The polymer also exhibits a relatively low
healing efficiency as the acidity of the buffer is higher than pH 5.5.
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The unique healing behavior of the CNC polymer is attributed to pHꢀinduced coordination between
nitrocatecholic moiety and Fe3+.41,49ꢀ51 When the CNC filament was cut into halves, a large amount of catecholꢀFe
coordination was cleaved. After the cut interfaces were wetted by the dilute HCl solution, the equilibrium of
catecholꢀFe coordination shifted to disassociated state, resulting in decomplexation of trisꢀcatecholꢀFe chelates.
The decomplexation produced abundant free CNC chains that could interpenetrate or entangle at the contact
interface. At the pH higher than 4, these free CNC chains were crossꢀlinked again via bisꢀ or trisꢀFeꢀcatechol
coordination between free nitrocatecholic moieties and Fe3+ (Fig. S5, ESI†),41,44 leading to the reꢀconstruction of
the broken CNC network (Fig. 3f). In contrast, the similar reꢀcoordination did not take place for the sample healed
only in a strong acidic medium. Notably, the CNC network healed in the KH2PO4 buffer became fragile because
its chains were highly crossꢀlinked in this case.
Nitro substitution significantly affected the healing recyclability of the CNC polymer. To prove the importance,
we synthesized chitosan substituted with catechol (chitosanꢀcatechol, CC) as a control sample (Fig. S1, ESI†). The
control polymer failed to fully recover its tensile strength and elongation after the same cut/healing operations,
exhibiting a mechanical healing efficiency of only 22.6 % (Fig. S9, ESI†). The failure is due to the oxidation of
the catecholic moiety into semiquinone or quinone by Fe3+.52 53 The oxidized products are highly reactive,32
resulting in covalent crossꢀlinking of CC chains. The covalent crossꢀlinking significantly impedes the release of
CC chains in the subsequent decomplexation process. On the contrary, the similar oxidation is effectively hindered
by the electronꢀwithdrawn nitro group for the CNC polymer.
The strong and stiff CNC polymer was also renewable through the pH variation process. To confirm the
renewability, we first crushed the polymer into small pieces and subsequent dissolved them in a dilute HCl
solution (Fig. 4a). The resulting solution was extruded into a pH 5.5 NaAc buffer to produce a CNC filament. The
regenerated filament also showed a tensile strength of 64.1 MPa and breaking strain of 55.9 % after drying at 50
oC for 2 h (Fig. 4b), which is closed to those of its original counterpart (i.e., 64.9 MPa and 58.5%). The
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regeneration greatly reduces the waste and environmental impact. Moreover, the CNC polymer exhibited a
flameꢀretardant property. As shown in Fig. 4c, a CNC filament cannot be ignited even after being treated with a
flame for 1 min. In contrast, a polypropylene plate was ignited immediately and the flame propagated quickly
with a large amount of drippings (Fig. S10, ESI†). The flame retardance of the CNC polymer mainly lies in high
limit oxygen index and charring ability of its chitosan backbone. The former endows the polymer with
selfꢀextinguishing property and the later prevents it from melting dripping at high temperature.54ꢀ56 Moreover, the
grafted nitrocatecholic moiety acts as a radical scavenger that retards flame propagation.57,58 The
antiꢀflammability greatly increases the safety of the polymer in the fire.
More interestingly, the CNC could tolerate various organic solvents, although it is a kind of polymeric material.
To investigate the solvent tolerance, we treated the CNC filaments with different solvents such as tetrahydrofuran
(THF), toluene and dichloromethane for 24 h, and then their stressꢀstress curves were recorded by tensile
experiments. Fig. 4d summarizes the stress strength and elongation of the resulting samples. By comparing the
data with those of the original sample, we found that the solvent treatments did not significantly deteriorate the
mechanical properties. Moreover, the solventꢀtreated samples also kept their healing capability (Fig. S11, ESI†).
For example, a toluene treated filament recovered both tensile strength and elongation through the same
pHꢀinduced coordination processes. The excellent solvent tolerance enables the polymer to be used under harsh
conditions.
Application of the healable stiff polymer was demonstrated by a weightꢀbearing test (Fig. 4e). A CNC filament
with the diameter of 0.5 mm was first loaded with 1.0 kg weight. Then additional loading was added slowly until
the filament was fractured. The broken filament was treated by pH variation and dried at 50 oC for 2 h allowing
for healing. The healed filament could still withstand 1.0 kg loading without breaking. In this case, both original
and healed filaments could bear at least 10204 times of their own weight (i.e., 98 mg) and the weightꢀbearing test
was repeatable for multiple times. The excellent mechanical durability together with unique healability allows our
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CNC polymer to be a promising material in some special technologic fields including spatial and oceanic science.
a)
70
b)
e)
60
50
Original
Regenerated
40
30
20
10
0
0
10
20
30
40
50
60
Strain (%)
c)
d) 70
60
50
40
30
20
10
0
Fig. 4 (a) Optical images show the regeneration of the CNC polymer, and (b) stressꢀstrain curves of the polymer
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before and after regeneration. (c) Fireꢀretardant property of the CNC filament. (d) Tensile strength of the CNC
filaments after treating with different solvents for 24 h. (e) Weightꢀbearing test of the CNC filament with diameter
of 500 ꢁm before (left) and after (right) healing.
Conclusions
In summary, this study demonstrated the healing of a stiff and strong polymer through a musselꢀinspired
strategy. Although the polymer had hardness comparable to comment engineering plastics, it restored its
configuration and mechanical properties even after 6 cycles of cutoffs. The restoration is attributed to the
increased mobility of its chains via decomplexation of Feꢀcatechol coordination in the acidic media. It was
revealed that pHꢀinduced interpolymer crossꢀlinking played an important impact on the healing efficiency and
mechanical properties. Moreover, the polymer also exhibited interesting renewability, fireꢀretardant and
solventꢀtolerance. This bioꢀinspired strategy offers a new healing mechanism for rigid polymer through dynamic
metalꢀligand coordination, which is of scientific and technologic significance.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This study is financially supported by National Natural Science Foundation of China (51473041) and a
selfꢀplanned task of the State Key Laboratory of Robotics and System of Harbin Institute of Technology
(SKLRS201604C) and Foundation for Innovative Research Groups of the National Natural Science Foundation of
China (51521003).
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