Leaf-Inspired Self-Healing Polymers

Article abstract of DOI:10.1016/j.chempr.2018.06.001

Hierarchical multiphase fibrous morphologies provide strength and elasticity for biological species, facilitating responses to environmental changes. Wound closure of leaves is one example. If polymers can be formed in a similar manner by introducing mult

Full text of DOI:10.1016/j.chempr.2018.06.001

Article
Leaf-Inspired Self-Healing Polymers
Ying Yang, Dmitriy Davydovich,
Chris C. Hornat, Xiaolin Liu,
Marek W. Urban
mareku@clemson.edu
HIGHLIGHTS
Inspired by plants leaves, self-
healing fibers are developed
Self-healing occurs as a result of
favorable interphase
morphologies
Molecular events leading to self-
healing are examined by
spectroscopic analysis
Correlations between
morphology, shape memory, and
self-healing are established
This study demonstrates the role of phase morphology on inducing shape memory
effects leading to the self-healing of polymers. Molecular events that occur during
this process are established.
Yang et al., Chem 4, 1–9
August 9, 2018 ª 2018 Elsevier Inc.
https://doi.org/10.1016/j.chempr.2018.06.001

Please cite this article in press as: Yang et al., Leaf-Inspired Self-Healing Polymers, Chem (2018), https://doi.org/10.1016/j.chempr.2018.06.001
Article
Leaf-Inspired Self-Healing Polymers
Ying Yang,1 Dmitriy Davydovich,1 Chris C. Hornat,1 Xiaolin Liu,1 and Marek W. Urban1,2,3,
*
SUMMARY
The Bigger Picture
Developments of self-healing
polymers are primarily driven by
the desire to prolong materials’
lifetime while maintaining their
functions. Significant synthetic
efforts have been made over the
last two decades via the
Hierarchical multiphase fibrous morphologies provide strength and elasticity
for biological species, facilitating responses to environmental changes. Wound
closure of leaves is one example. If polymers can be formed in a similar manner
by introducing multiphase-separated morphologies, self-healing in a variety of
commodity materials can be achieved. In these studies, we demonstrate the role
of phase morphologies, interphases, and viscoelasticity-driven shape memory
effects on self-healing. We synthesized phase-separated polycaprolactone-
polyurethane fibrous thermoplastic polymers in which microphase separation
facilitates the formation of stable interfacial regions between hard and soft seg-
ments. Self-healing can be repeated many times. This behavior is attributed to
the shape memory effect, given that micron-scale interphase reduces chain slip-
page, enabling entropic energy storage during damage. Chemically identical
but nanophase-separated copolymers do not exhibit this behavior. These
studies show that self-healing can be achieved by morphology control and facil-
itated by thermal or other volume-induced transitions.
incorporation of dynamic bonds
capable of reversible breaking
and reforming. However, the role
of physical network design in
achieving self-healing properties
in commodity polymers remains
unclear.
Inspired by the self-healing
behavior of leaves, we built self-
healing into polycaprolactone-
polyurethane fibers by controlling
morphological features.
INTRODUCTION
Biological species are able to self-repair repeatedly and autonomously on multiple
scale lengths, from DNA macromolecules to larger organs, such as veins, soft
or hard tissues, and muscles. Over the last couple of decades, numerous studies
have focused on mimicking biological systems to develop self-healing materials.
The main motivation behind these efforts is to prolong the lifespan while retaining
desired functions of man-made materials. Recent advances in materials capable of
self-healing can be classified by a handful of approaches: (1) embedding reactive
encapsulated fluids that burst open upon crack propagation to fill and repair
damaged areas,1,2 (2) incorporating reversible covalent and non-covalent bonds
into existing structures capable of rebonding after damage,3–16 (3) physically
dispersing superparamagnetic or other nanomaterials that remotely respond to
magnetic, electromagnetic, or other energy sources,17,18 and (4) embedding living
organisms capable of re-mending damaged structural features.19,20 Although
these inherently different chemical and physical approaches significantly advanced
our understanding of synthetic self-healing materials, biological systems are
capable of on-demand self-healing with an incredible degree of efficiency. One
example is healing upon mechanical damage of the Delosperma cooperi leaves
shown in Figure 1A, where wounds can be closed within 60 min by tissue bending
or contraction. This response is believed to be due to stored elastic stress built-
in within the heterogeneous structure during growth.21 When the equilibrium
is disturbed by mechanical damage the energy will be released, causing
viscoelastic shape transformation, bringing the wound edges into contact to
heal. Similarly, in mammals, microstructural heterogeneity in cancellous bones is
responsible for enhanced shape recovery upon mechanical deformations.22
Achieving similar functions in real-world polymer-based materials is challenging
to say the least.23
Favorable viscoelastic properties
originating from interphase
features facilitate shape memory
effects and lead to autonomous
damage closure and subsequent
self-healing. The creation of
morphology-controlled dynamic
polymers can be utilized in
numerous applications ranging
from soft robotics to molecular
actuators or morphology-induced
information storage to
thermomechanical sensing and
other devices.
Chem 4, 1–9, August 9, 2018 ª 2018 Elsevier Inc.
1

Please cite this article in press as: Yang et al., Leaf-Inspired Self-Healing Polymers, Chem (2018), https://doi.org/10.1016/j.chempr.2018.06.001
Figure 1. Self-Healing Polycaprolactone-Polyurethane Fibers Inspired by Leaves of Plants
For a Figure360 author presentation of Figure 1, see http//dx.doi:10.1016/j.chempr.2018.06.
001#mmc2.
(A) Succulent plant Delosperma cooperi (left) and a leaf showing a healed injury (right, arrow in
circled area). Image reprinted from Speck et al.21 ª 2018 Speck et al.; licensee Beilstein-Institut
under CC BY 4.0.
(B) Chemical composition of thermoplastic polycaprolactone-polyurethane (PURP) fibers.
(C) Self-healing of PURP fibers upon making a 100-mm deep cut.
Inspired by the role of heterogeneous morphologies in biological systems with built-
in stresses, this study explores the role of deliberately introduced phase-separated
morphologies that enable energy storage during mechanical damage and visco-
elastic shape recovery attributed to built-in shape memory effect (SME) leading to
self-healing. Although intuitively SME may potentially contribute to the self-healing
of polymers, molecular-level relationships between these events are not obvious
and have not been addressed. The early approaches focused on practical applica-
tions to improve the self-healing performance of composites by embedding pre-
stretched shape memory alloy wires.24 As the fields of self-healing25 and shape
memory polymers26–29 evolved independently, shape-memory-assisted self-healing
emerged.30–36 Although practical applications are promising, limited molecular-
level mechanisms that would correlate SMEs with damage closure were identified.
This study aims to explore how SMEs can facilitate microscale damage closure
and identify the role of polymer heterogeneities, if any, in self-healing.
RESULTS AND DISCUSSION
To develop heterogeneities of desirable sizes, we prepared a polymer consisting of
well-defined thermoplastic polyurethane copolymerized from polycaprolactone diol
(PCL-diol), 1,4-butanediol (BDO), hydroxyl modified spiropyran (SP), and hexamethy-
lene diisocyanate (HDI) drawn into fibers from solution during polymerization (PURP,
1Department of Materials Science and
where PUR stands for polyurethane and P represents that the fibers are produced
by cold drawing during polymerization). Whereas polymerization methods are pro-
vided in Supplemental Experimental Procedures, Scheme S1, and Figure S1, Figure 1B
illustrates PURP copolymer structure. The choice of this copolymer was dictated by the
ability of PCL to recrystallize after melting at 65ꢀC and achieve controlled phase-sepa-
rated morphologies by copolymerizing with BDO and HDI components. As a visual co-
lor change indicator of mechanical damage, well-known SP mechanophores were co-
polymerized into the polymer backbone.37 Optical images shown in Figure 1C
Engineering, Center for Optical Materials Science
Engineering Technologies (COMSET), Clemson
University, Clemson, SC 29634, USA
2Department of Chemistry, Clemson University,
Clemson, SC 29634, USA
3Lead Contact
*Correspondence: mareku@clemson.edu
https://doi.org/10.1016/j.chempr.2018.06.001
2
Chem 4, 1–9, August 9, 2018

Please cite this article in press as: Yang et al., Leaf-Inspired Self-Healing Polymers, Chem (2018), https://doi.org/10.1016/j.chempr.2018.06.001
Figure 2. Phase Morphology of PURP Polymers Showing Micron-Scale Phase Separation along with Gradient Interphase
For a Figure360 author presentation of Figure 2, see http//dx.doi:10.1016/j.chempr.2018.06.001#mmc2.
(A-1) Schematic representation of PURP cross-section of a 200-mm thick fiber.
(A-2) IR images of the 1,685 cmꢁ1 (a–d) and 1,724 cmꢁ1 (a0–d0) bands due to PUR and PCL C=O vibrations, respectively, collected from the cross-sections
1–4 marked as the squares in (A-1) for PURP fiber.
(A-3) Corresponding IR spectra recorded from PUR-rich (blue in a0–d0) and PCL-rich (red in a0–d0) domains indicated by the black arrows. The red arrow
indicates that the amide II band shifted from 1,535 cmꢁ1 for the PUR-rich doamin to 1,530 cmꢁ1 for the PCL-rich domain.
(B-1) IR image of the 1,685 cmꢁ1 band enlarged on a PUR-rich domain (marked by the square in A-2a).
(B-2) IR spectra of carbonyl regions, where traces 0–5 were collected from the corresponding areas shown in (B-1) marked by 0–5, respectively.
(B-3) Percentage of the 1,724 cmꢁ1 (ester C=O) and 1,685 cmꢁ1 (urethane C=O) band areas (A1724 and A1685) within 1,780–1,640 cmꢁ1 region plotted as a
function of the distance from the center of PUR-rich domain shown in (B-1) (0–9 mm). The percentage is calculated on the basis of integrated areas of the
deconvoluted bands.
illustrate that when a PURP fiber is damaged, a 100-mm-deep cut vanishes within
10 min upon exposure slightly above melting transition of PCL (Tm = 65ꢀC).
Figure 2A-1 shows four 100-mm-thick cross-sections of PURP fibers. As seen, each
sliced cross-section exhibits micron-size phase separation demonstrated by infrared
(IR) images of urethane groups shown in Figure 2A-2, a–d. The micron-size PUR-rich
domains (red) are formed within a PUR-deficient matrix (blue). The corresponding
PCL distribution is detected upon tuning to the PCL ester linkages (Figure 2A-2,
a0–d0), where the PCL segments are mainly distributed within the matrix (red and yel-
low regions). Similar morphologies are detected along the longitudinal direction of
individual fibers. Chemical analysis of PUR- and PCL-rich domains shown in Fig-
ure 2A-3 illustrates the carbonyl and amide II band intensity differences of the two
Chem 4, 1–9, August 9, 2018
3

Please cite this article in press as: Yang et al., Leaf-Inspired Self-Healing Polymers, Chem (2018), https://doi.org/10.1016/j.chempr.2018.06.001
domains. Another feature shown in Figures 2B-1 and 2B-2 is a gradual decrease of
the urethane content (1,685 cmꢁ1) when going from the center of PUR-rich domain
outward toward the PCL-rich domain in 1-mm increments (0 0 5). Figure 2B-3 plots
relative intensities of urethane and ester C=O groups as a function of the distance
from the PUR-rich center. These results show the presence of micron-scale inter-
phase between the two domains. The phase separation is also confirmed by differ-
ential scanning calorimetry measurements with two melting transitions (Tm) at 51ꢀC
and 168ꢀC due to PCL- and PUR-rich domains, respectively (Figure S2).
To achieve nanoscale phase separation, we applied a melt-drawn process to chemically
identical polymers (Scheme S1). Designated as PURM (where PUR stands for polyure-
thane and M stands for the melt-drawing method), nanoscale phase separation
becomes indistinguishable in IR imaging (Figure S3; IR imaging spatial resolution
ꢂ1 mm), but the presence of two melting points (Tm) at 51ꢀC and 162ꢀC (Figure S2) along
with atomic force microscopy phase images (Figure S3) indicate nanoscale phase sepa-
ration. Although a 6ꢀC lower Tm signifies smaller-size crystals of the hard segments, in
contrast to microphase-separated PURPs, nanoscale phase-separated PURM fibers
virtually do not self-heal within the same time frame. As seen in Figure 3A-1, a cut in
the microphase-separated PURP (t = 0 min) closes within 10 min. After damage, tensile
strain and stress at break plotted as a function of healing time shown in Figure 3A-2 illus-
trate that compared with the undamaged state with the strain of 1,015% and stress at
29.2 MPa, upon self-healing, 35.8% and 52.5% of the original strain and stress is recov-
ered within first 10 min, to reach 82.6% and 87.0% after 120 min. Parallel experiments
conducted on nanophase-separated PURM fibers are shown in Figures 3B-1 and 3B-2.
The ultimate strain and stress of undamaged PURM are 1,090% and 34.3 MPa, respec-
tively, and after 120 min the recovery reaches only ꢂ48% of the original strain and ꢂ51%
of the original tenacity (note that these values for tensile stress and strain at break repre-
sent an average of six measurements [for standard deviation, see Table S1]; the plots
shown in Figures 3A-2 and 3B-2 represent an example of one measurement). Notably,
at the healing temperature, PURP and PURM maintain elasticity within the rubbery
plateau regions (dynamic mechanical analysis [DMA]; Figure S4).
Since the microphase-separated PURPs exhibit ꢂ35% higher self-healing efficiency,
the question is why chemically identical polymers with the same molecular weight,
but different domain sizes and degrees of heterogeneity, exhibit entirely different
self-healing characteristics. Considering viscoelastic and recently quantified entropic
contributions to SME in synthetic polymers,38 thermomechanical shape memory cy-
cles (SMCs) were conducted in which a ‘‘cold’’ programming cycle was employed to
determine the SME on self-healing under the same conditions. As shown inFigure 3C,
microphase-separated (PURP) and nanophase-separated (PURM) fibers were uniax-
ially deformed to 200% strain ( 3 max) at room temperature (23ꢀC), which is below their
melting transitions, and at a strain rate of 100%/min. Upon load removal, microphase-
separated (PURP) and nanophase-separated (PURM) fibers contracted to 126% and
147% strains ( 3 ), respectively, with fixity ratios ðR 3 = 3 maxÞ of 63.0% and 73.5%,
f
f =
f
respectively. When the temperature was increased to 65ꢀC, shape recovery was initi-
ated, and the residual strains detected after recovery ( 3 ) were 43% and 85%, respec-
r
tively. At the same time, the shape recovery ratios ðRr = ð 3 max ꢁ 3 Þ= 3 maxÞ were 78.5%
r
and 57.5%, respectively. These bulk measurements clearly suggest a relationship
between the degree of phase separation and shape recovery. For comparison, stan-
dard thermomechanical SMCs of PURP and PURM are provided in Figure S5.
To identify molecular processes responsible for SME, we conducted X-ray diffraction
(XRD) and Fourier transform IR analysis. Figure 3Di plots the percentage of
4
Chem 4, 1–9, August 9, 2018

Please cite this article in press as: Yang et al., Leaf-Inspired Self-Healing Polymers, Chem (2018), https://doi.org/10.1016/j.chempr.2018.06.001
Figure 3. Self-Healing and Shape Memory Properties of PURP and PURM Fibers
(A-1) Optical images of self-healable microphase-separated PURP fibers.
(A-2) Tensile stress and strain at break are plotted as a function of self-healing time for PURP fibers. The dimensions of the scratch are 8 3 50 mm (width 3
depth), and self-healing was conducted at 65ꢀC.
(B-1) Optical images of self-healing for nano-phase-separated (PURM) fibers.
(B-2) tensile stress and strain at break are plotted as a function of self-healing time for PURM fibers. The dimensions of the scratch are 8 3 50 mm (width 3
depth), and self-healing was conducted at 65ꢀC.
(C) Thermomechanical SMCs of PURP and PURM polymers.
(D) The degree of crystallinity in deformed (i) and shape-recovered (ii) states for six consecutive SMCs; ccꢁf is the percentage of crystallinity at unload
strain, and ccꢁr is the same after shape recovery.
crystallinity (cc) for microphase- and nanophase-separated fibers at unload strain ( 3 )
f
for six consecutive SMCs (% ccꢁf) (XRD reference data are provided in Figure S6). The
strain-induced crystallinity increases by ꢂ30% after the first SMC, but for the remain-
ing cycles only a slight %ccꢁf increase for microphase-separated fiber is detected. In
contrast, the %ccꢁf in nanophase-separated fiber initially increases and then de-
creases after first four cycles and levels off. The percentage of crystallinity after
shape recovery (% ccꢁr) plotted in Figure 3Dii shows ꢂ8% increase after the first
SMC for microphase-separated (PURP) and nanophase-separated (PURM) fibers,
which is attributed to the residual strain (Figure 3C). After six SMCs, % ccꢁf and
% ccꢁr values are lower in PURM than in PURP. Our hypothesis for this behavior is
that chain slippage of urethane segments from PUR-rich to PCL-rich domains occurs
during deformation of PURM, thus inhibiting PCL chain packing. On the contrary, the
Chem 4, 1–9, August 9, 2018
5

Please cite this article in press as: Yang et al., Leaf-Inspired Self-Healing Polymers, Chem (2018), https://doi.org/10.1016/j.chempr.2018.06.001
Figure 4. 2D-FTIR Analysis Identifying Molecular Events during Self-Healing and Shape Memory Cycles
Asynchronous 2D-FTIR correlation spectra of (A-1) PURP before and after self-healing, (A-2) PURP before and after the first SMC, (B-1) PURM before and
after self-healing, and (B-2) PURM before and after the first SMC zoomed in to the 1,555–1,510 cmꢁ1 range. The 2D-FTIR spectra were calculated with no
reference and provide information on the bands’ relative intensity changes. n1 corresponds to the IR spectra of the original material before damage or
deformation, and n2 corresponds to spectra collected after perturbation such as self-healing or SMC. Schematic illustrations of molecular events in
micro- and nanophase-separated PURP and PURM copolymers upon self-healing are shown in (A-3) and (B-3), respectively.
distribution of PUR-rich and PCL-rich domains in microphase-separated PURP re-
mains less disturbed. These conclusions are also supported by IR imaging analysis
of microphase-separated PURP cross-sections after deformation (at unloading
strain) as well as after the SMCs (Figure S7). The microscale phase-separated inter-
facial regions may act as stable junction points inhibiting chain slippage during me-
chanical deformation.
To examine this hypothesis and further substantiate whether the same phenomenon
governs SME and self-healing, we utilized 2D-FTIR correlation spectroscopy. Figures
4A-1 and 4B-1 show the results of asynchronous 2D-FTIR correlation of molecular vi-
brations that are affected by the damage-repair cycle in microphase- and nano-
phase-separated copolymers, respectively. For microphase separate PURPs, two
antisymmetric crosspeaks along the diagonal lines are detected, indicating two
types of H bonding: urethane-urethane (1,536 cmꢁ1) and urethane-ester (PCL)
(1,521 cmꢁ1). Detailed 2D-FTIR analysis is provided in the Supplemental Experi-
mental Procedures and Figures S8–S12. The presence of the positive crosspeak (Fig-
ure 4A-1) after self-healing is attributed to the increased urethane-urethane
H bonding with respect to urethane-ester counterpart. In contrast, an opposite
trend is observed for PURM. The negative intensity shown in Figure 4B-1 after the
6
Chem 4, 1–9, August 9, 2018

Please cite this article in press as: Yang et al., Leaf-Inspired Self-Healing Polymers, Chem (2018), https://doi.org/10.1016/j.chempr.2018.06.001
damage-repair cycle indicates the decrease of urethane-urethane H bonding with
respect to the urethane-ester. If the damage-repair cycle is attributed to the SME,
the same spectroscopic responses are expected after the first SMC. Indeed, Figures
4A-2 and 4B-2 illustrate asynchronous 2D-FTIR correlation before and after one SMC
and clearly show that the same responses are detected for PURP and PURM. Thus, a
micron-scale damage-repair cycle and macroscopic shape memory recovery are
governed by the same molecular events: the increase of urethane-urethane
H bonding in microphase-separated PURP copolymers and further phase separation
at the healing temperatures as the two phases are immiscible. On the other hand,
the increase of urethane-ester interactions in nanophase-separated PURM indicates
that PUR blocks are partially dispersed in PCL-rich domains after SMC and self-heal-
ing, thus indicating that nanosize hard-segment crystals resulting from urethane-ure-
thane H bonding dissociate during deformation. Further support for the dissociation
of H bonding as well as the % cc decrease after mechanical damage for microphase-
and nanophase-separated fibers is provided in Figures S12–S14. The analysis of
junction densities due to hard segments and chain entanglements after the first
shape memory cycle shows that for microphase-separated PURP, there is virtually
no change (0.68–0.65 kmol/m3, corresponding to the molecular weight between
junction points from 1.68 to 1.75 kDa). In contrast, the junction density in nano-
phase-separated PURM is reduced over 3-fold from 1.8 to 0.46 kmol/m3, also re-
flected in the increase of the molecular weight between the junction points from
0.63 to 2.5 kDa (determined from the rubbery plateau moduli of DMA curves; Fig-
ure S4). These results along with 2D FTIR and XRD provide further evidence that mi-
crophase-separated PUR-rich domains along with gradually changing PUR-to-PCL
microscale interphase have a stabilizing effect by reducing the degree of chain slip-
page during thermomechanical SMC, thus allowing entropic energy storage during
damage, which is responsible for shape recovery. On the other hand, plastic defor-
mations associated with the dissociations of hard-segment crystalline domains lead
to lower shape recovery ratios in nanoscale phase-separated PURMs.
On the basis of these results, the following mechanism of wound closure is pro-
posed. For a microphase-separated system (Figure 4A-3), a PUR-rich domain and
the microscale interphase inhibits PUR blocks from disengagement under mechan-
ical forces. As a result, PCL-rich domains are able to store entropic energy, whereby
the crystalline blocks unfold and low Tg amorphous segments elongate, thus
reducing entropy. However, when temperature is raised and the chains in the
PCL-rich domain gain sufficient mobility, they return to the randomized state to
maximize entropy. The stored energy is released and shape recovery occurs, which
is responsible for damage closure in microphase-separated polymers. This is sche-
matically depicted in Figure 4A-3. During this process, localized chain diffusion,
recrystallization of PCL chains, and reformation of H bonds localized within the
damaged interface occur, resulting in self-healing. On the contrary, for nano-
phase-separated copolymers depicted in Figure 4B-3, the disengagement of PUR
blocks from PUR-rich domains and slippage into a PCL-rich phase upon mechanical
deformation leads to energy dissipation during deformation. As a result, shape re-
covery and wound closure do not occur to the same extent. Thus, built-in control-
lable heterogeneities at different physical length scales of chemically identical com-
ponents may facilitate entropy storage that leads to self-healing via SME. Additional
evidence that the role of heterogeneities in self-healing is demonstrated by varying
the PUR-rich domain sizes in microphase PURPs by changing the content of PUR hard
segments (Table S1 and Figure S15), which compromises self-healing efficiency.
Reversible color changes during the damage-repair cycle can be induced as a result
of the presence of spiropyran incorporated into the polymer backbone (Figure S16).
Chem 4, 1–9, August 9, 2018
7

Please cite this article in press as: Yang et al., Leaf-Inspired Self-Healing Polymers, Chem (2018), https://doi.org/10.1016/j.chempr.2018.06.001
Conclusion
In summary, felicitous heterogeneities intentionally induced in polymers enable
elastic energy storage during damage, which can be recovered during self-heal-
ing. These studies identified the relationship between the SMEs and self-healing
in microphase-separated polymers whereby the ability of polymer networks to
minimize plastic deformation and store entropic energy during deformation is
responsible for shape recovery, thus facilitating damage closure. The presence
of phase-separated microdomains with stabilizing interfacial regions serve as an
energy reservoir, facilitating polymer recovery upon mechanical damage without
elaborate chemical modifications. The damage-repair cycle can be repeated
many times while retaining mechanical properties, thus enhancing the material’s
sustainability. Although it is well known that embedding rigid nano-objects usu-
ally enhances physical properties, introducing stabile interfacial regions facili-
tating energy storage during mechanical damage offers alternative approaches
for the development of self-healable commodity polymers without chemical
modification.
EXPERIMENTAL PROCEDURES
Detailed synthesis and characterization procedures are provided in the Supple-
mental Information.
SUPPLEMENTAL INFORMATION
Supplemental Information includes Supplemental Experimental Procedures, 16 fig-
ures, 1 table, and 1 scheme and can be found with this article online at https://doi.
org/10.1016/j.chempr.2018.06.001.
ACKNOWLEDGMENTS
This work was supported in part by the Established Program to Stimulate Compet-
itive Research under National Science Foundation award OIA-1655740, the Division
of Materials Research program under award DMR1744306, and the J.E. Sirrine Foun-
dation Endowment at Clemson University.
AUTHOR CONTRIBUTIONS
Conceptualization, Y.Y. and M.W.U.; Methodology, Y.Y. and M.W.U.; Investigation,
Y.Y., D.D., C.C.H., and X.L.; Writing – Original Draft, Y.Y. and M.W.U; Writing – Re-
view & Editing, Y.Y. and M.W.U.; Supervision. M.W.U.; Funding Acquisition, M.W.U.
DECLARATION OF INTERESTS
The authors declare no competing financial interests.
Received: January 11, 2018
Revised: March 28, 2018
Accepted: May 31, 2018
Published: June 28, 2018
REFERENCES AND NOTES
1. White, S.R., Sottos, N.R., Geubelle, P.H.,
Moore, J.S., Kessler, M.R., Sriram, S.R., Brown,
E.N., and Viswanathan, S. (2001). Autonomic
healing of polymer composites. Nature 409,
794–797.
materials with microvascular networks. Nat.
Mater. 6, 581–585.
4. Ghosh, B., and Urban, M.W. (2009). Self-
repairing oxetane-substituted chitosan
polyurethane networks. Science 323, 1458–
1460.
3. Chen, X., Dam, M.A., Ono, K., Mal, A.,
Shen, H., Nutt, S.R., Sheran, K., and Wudl,
F. (2002). A thermally re-mendable cross-
linked polymeric material. Science 295,
1698–1702.
5. Imato, K., Nishihara, M., Kanehara, T.,
Amamoto, Y., Takahara, A., and Otsuka, H.
(2012). Self-healing of chemical gels
2. Toohey, K.S., Sottos, N.R., Lewis, J.A., Moore,
J.S., and White, S.R. (2007). Self-healing
8
Chem 4, 1–9, August 9, 2018

Please cite this article in press as: Yang et al., Leaf-Inspired Self-Healing Polymers, Chem (2018), https://doi.org/10.1016/j.chempr.2018.06.001
cross-linked by diarylbibenzofuranone-based
trigger-free dynamic covalent bonds at room
temperature. Angew. Chem. Int. Ed. 51, 1138–
1142.
16. Yang, Y., and Urban, M.W. (2017). Self-healing
of glucose-modified polyurethane networks
facilitated by damage-induced primary
amines. Polym. Chem. 8, 303–309.
27. Xie, T. (2010). Tunable polymer multi-shape
memory effect. Nature 464, 267.
28. Liu, C., Qin, H., and Mather, P. (2007). Review of
progress in shape-memory polymers. J. Mater.
Chem. 17, 1543–1558.
6. Ying, H., Zhang, Y., and Cheng, J. (2014).
Dynamic urea bond for the design of reversible
and self-healing polymers. Nat. Commun. 5,
3218.
17. Corten, C.C., and Urban, M.W. (2009).
Repairing polymers using oscillating magnetic
field. Adv. Mater. 21, 5011–5015.
29. Zhao, Q., Qi, H.J., and Xie, T. (2015). Recent
progress in shape memory polymer: new
behavior, enabling materials, and mechanistic
understanding. Prog. Polym. Sci. 49, 79–120.
18. Huang, L., Yi, N., Wu, Y., Zhang, Y., Zhang, Q.,
Huang, Y., Ma, Y., and Chen, Y. (2013).
Multichannel and repeatable self-healing of
mechanical enhanced graphene-thermoplastic
polyurethane composites. Adv. Mater. 25,
2224–2228.
7. Sijbesma, R.P., Beijer, F.H., Brunsveld, L.,
Folmer, B.J., Hirschberg, J.K., Lange, R.F.,
Lowe, J.K., and Meijer, E. (1997). Reversible
polymers formed from self-complementary
monomers using quadruple hydrogen
bonding. Science 278, 1601–1604.
30. Nji, J., and Li, G. (2010). A biomimic shape
memory polymer based self-healing
particulate composite. Polymer 51, 6021–6029.
19. Jonkers, H.M., Thijssen, A., Muyzer, G.,
Copuroglu, O., and Schlangen, E. (2010).
Application of bacteria as self-healing agent
for the development of sustainable concrete.
Ecol. Eng. 36, 230–235.
31. Rodriguez, E.D., Luo, X., and Mather, P.T.
(2011). Linear/network poly ( 3 -caprolactone)
blends exhibiting shape memory assisted self-
healing (SMASH). ACS Appl. Mater. Interfaces
3, 152–161.
8. Kalista, S.J., Jr., Ward, T.C., and Oyetunji, Z.
(2007). Self-healing of poly (ethylene-co-
methacrylic acid) copolymers following
projectile puncture. Mech. Adv. Mater. Struct.
14, 391–397.
20. Wang, J., Soens, H., Verstraete, W., and De
Belie, N. (2014). Self-healing concrete by use of
microencapsulated bacterial spores. Cem.
Concr. Res. 56, 139–152.
32. Heo, Y., and Sodano, H.A. (2014). Self-healing
polyurethanes with shape recovery. Adv. Funct.
Mater. 24, 5261–5268.
9. Cordier, P., Tournilhac, F., Soulie´ -Ziakovic, C.,
and Leibler, L. (2008). Self-healing and
thermoreversible rubber from supramolecular
assembly. Nature 451, 977–980.
33. Rivero, G., Nguyen, L.-T.T., Hillewaere, X.K.,
and Du Prez, F.E. (2014). One-pot thermo-
remendable shape memory polyurethanes.
Macromolecules 47, 2010–2018.
10. Burnworth, M., Tang, L., Kumpfer, J.R., Duncan,
A.J., Beyer, F.L., Fiore, G.L., Rowan, S.J., and
Weder, C. (2011). Optically healable
supramolecular polymers. Nature 472,
334–337.
21. Speck, O., Schlechtecdahl, M., Borm, F.,
Kampowski, T., and Speck, T. (2018). Humidity-
dependent wound sealing in succulent leaves
of Delosperma cooperi—an adaptation to
seasonal drought stress. Beilstein J.
34. Ur Rehman, H., Chen, Y., Hedenqvist, M.S., Li,
H., Xue, W., Guo, Y., Guo, Y., Duan, H., and Liu,
H. (2018). Self-healing shape memory PUPCL
copolymer with high cycle life. Adv. Funct.
Mater. 28, 1704109.
Nanotechnol. 9, 175–186.
11. Holten-Andersen, N., Harrington, M.J.,
Birkedal, H., Lee, B.P., Messersmith, P.B., Lee,
K.Y.C., and Waite, J.H. (2011). pH-induced
metal-ligand cross-links inspired by mussel
yield self-healing polymer networks with near-
covalent elastic moduli. Proc. Natl. Acad. Sci.
USA 108, 2651–2655.
22. Torres, A.M., Matheny, J.B., Keaveny, T.M.,
Taylor, D., Rimnac, C.M., and Hernandez, C.J.
(2016). Material heterogeneity in cancellous
bone promotes deformation recovery after
mechanical failure. Proc. Natl. Acad. Sci. USA
113, 2892–2897.
35. Miyamae, K., Nakahata, M., Takashima, Y., and
Harada, A. (2015). Self-healing, expansion-
contraction, and shape-memory properties of a
preorganized supramolecular hydrogel
through host-guest interactions. Angew.
Chem. Int. Ed. 54, 8984–8987.
12. Nakahata, M., Takashima, Y., Yamaguchi, H.,
and Harada, A. (2011). Redox-responsive self-
healing materials formed from host–guest
polymers. Nat. Commun. 2, 511.
23. Patrick, J.F., Robb, M.J., Sottos, N.R., Moore,
J.S., and White, S.R. (2016). Polymers with
autonomous life-cycle control. Nature 540,
363–370.
36. Fang, Z., Zheng, N., Zhao, Q., and Xie, T. (2017).
Healable, reconfigurable, reprocessable
13. Chen, Y., Kushner, A.M., Williams, G.A., and
Guan, Z. (2012). Multiphase design of
autonomic self-healing thermoplastic
elastomers. Nat. Chem. 4, 467–472.
24. Kirkby, E.L., Rule, J.D., Michaud, V.J., Sottos,
N.R., White, S.R., and Ma˚nson, J.A.E. (2008).
Embedded shape-memory alloy wires for
improved performance of self-healing
thermoset shape memory polymer with highly
tunable topological rearrangement kinetics.
ACS Appl. Mater. Interfaces 9, 22077–22082.
37. Davis, D.A., Hamilton, A., Yang, J., Cremar,
L.D., Van Gough, D., Potisek, S.L., Ong, M.T.,
Braun, P.V., Martı´nez, T.J., and White, S.R.
(2009). Force-induced activation of covalent
bonds in mechanoresponsive polymeric
materials. Nature 459, 68–72.
polymers. Adv. Funct. Mater. 18, 2253–2260.
14. Yang, Y., and Urban, M.W. (2014). Self-
repairable polyurethane networks by
atmospheric carbon dioxide and water.
Angew. Chem. Int. Ed. 53, 12142–12147.
25. Yang, Y., and Urban, M.W. (2013). Self-healing
polymeric materials. Chem. Soc. Rev. 42, 7446–
7467.
15. Li, C.-H., Wang, C., Keplinger, C., Zuo, J.-L., Jin,
L., Sun, Y., Zheng, P., Cao, Y., Lissel, F., Linder,
C., et al. (2016). A highly stretchable
autonomous self-healing elastomer. Nat.
Chem. 8, 618–624.
26. Lendlein, A., and Langer, R. (2002).
Biodegradable, elastic shape-memory
polymers for potential biomedical
applications. Science 296, 1673–1676.
38. Hornat, C.C., Yang, Y., and Urban, M.W. (2016).
Quantitative predictions of shape-memory
effects in polymers. Adv. Mater. 29, 1603334.
Chem 4, 1–9, August 9, 2018
9