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Acknowledgements
We thank A. Ben-Kish, J. Bollinger, J. Britton, N. Gisin, P. Knight, P. Kwiat and I. Percival
for useful discussions and comments on the manuscript. This work was supported by the
US National Security Agency (NSA) and the Advanced Research and Development
Activity (ARDA), the US Of®ce of Naval Research, and the US Army Research Of®ce. This
paper is a contribution of the National Institute of Standards and Technology and is not
subject to US copyright.
Correspondence and requests for materials should be addressed to D.J.W.
(e-mail: david.wineland@boulder.nist.gov).
letters to nature
794 NATURE
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Autonomic healing of
polymer composites
S. R. White*, N. R. Sottos
²
, P. H. Geubelle*, J. S. Moore
³
, M. R. Kessler
²
,
S. R. Sriram
³
, E. N. Brown
²
& S. Viswanathan*
* Department of Aeronautical and Astronautical Engineering,
²
Department
of Theoretical and Applied Mechanics,
³
Department of Chemistry, University of
Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
..............................................................................................................................................
Structural polymers are susceptible to damage in the form of
cracks, which form deep within the structure where detection is
dif®cult and repair is almost impossible. Cracking leads to
mechanical degradation
1±3
of ®bre-reinforced polymer com-
posites; in microelectronic polymeric components it can also
lead to electrical failure
4
. Microcracking induced by thermal
and mechanical fatigue is also a long-standing problem in poly-
mer adhesives
5
. Regardless of the application, once cracks have
formed within polymeric materials, the integrity of the structure
is signi®cantly compromised. Experiments exploring the concept
of self-repair have been previously reported
6±8
, but the only
successful crack-healing methods that have been reported so far
require some form of manual intervention
10±18
. Here we report a
structural polymeric material with the ability to autonomically
heal cracks. The material incorporates a microencapsulated heal-
ing agent that is released upon crack intrusion. Polymerization of
the healing agent is then triggered by contact with an embedded
catalyst, bonding the crack faces. Our fracture experiments yield
as much as 75% recovery in toughness, and we expect that our
approach will be applicable to other brittle materials systems
(including ceramics and glasses).
Figure 1 illustrates our autonomic healing concept. Healing is
accomplished by incorporating a microencapsulated healing agent
and a catalytic chemical trigger within an epoxy matrix. An
approaching crack ruptures embedded microcapsules, releasing
healing agent into the crack plane through capillary action. Poly-
merization of the healing agent is triggered by contact with the
embedded catalyst, bonding the crack faces. The damage-induced
triggering mechanism provides site-speci®c autonomic control of
repair. An additional unique feature of our healing concept is the use
of living (that is, having unterminated chain-ends) polymerization
catalysts, thus enabling multiple healing events. Engineering this
self-healing composite involves the challenge of combining polymer
science, experimental and analytical mechanics, and composites
processing principles.
We began by analysing the effects of microcapsule geometry
and properties on the mechanical triggering process. For example,
capsule walls that are too thick will not rupture when the crack
approaches, whereas capsules with very thin walls will break during
processing. Other relevant design parameters are the toughness and
the relative stiffness of the microcapsules, and the strength of
the interface between the microcapsule and the matrix. Micro-
mechanical modelling with the aid of the Eshelby±Mura equivalent
inclusion method
19
has been used to study various aspects of the
complex three-dimensional interaction between a crack and a
microcapsule. An illustrative result from these studies is presented
in Fig. 2a, which shows the effect of the relative stiffness of the
microcapsule on the propagation path of an approaching crack.
The crack, the sphere and the surrounding matrix are subjected to
a far-®eld tensile loading, j
`
, perpendicular to the crack plane.
Catalyst
Polymerized
healing agent
Healing agent
Microcapsule
Crack
a
b
c
Figure 1 The autonomic healing concept. A microencapsulated healing agent is
embedded in a structural composite matrix containing a catalyst capable of polymerizing
the healing agent. a, Cracks form in the matrix wherever damage occurs; b, the crack
ruptures the microcapsules, releasing the healing agent into the crack plane through
capillary action; c, the healing agent contacts the catalyst, triggering polymerization that
bonds the crack faces closed.
© 2001 Macmillan Magazines Ltd
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As apparent from the j
22
stress distribution in the equatorial plane
of the sphere in Fig. 2a, the stiffness of the sphere relative to the
matrix strongly affects the stress state in the proximity of the crack
tip and around the sphere itself. In the case of a stiffer inclusion, the
stress ®eld in the immediate vicinity of the crack tip shows an
asymmetry that indicates an undesirable tendency of the crack to be
de¯ected away from the inclusion. The situation is reversed in the
case of a more compliant spherical inclusion, and the crack is
attracted toward the microcapsule, a necessary condition for its
rupture and the triggering of the healing process.
Observations by optical and scanning electron microscopy con-
®rmed the healing concept in Fig. 1 and substantiated the main
®ndings from analytical studies in Fig. 2a. The time sequence of
optical images in Fig. 2b shows the rupture of an embedded
microcapsule ®lled with a red dye and the subsequent release of
the healing agent into the crack plane. The scanning electron
a
Crack
σ
22
/σ
∞
σ
∞
1.8
0.5
–2
–2
–1
0
1
–1 0
x
1
/R
x
2
/R
12–2
–2
–1
0
1
–1 0
x
1
/R
x
2
/R
12
Sphere
R
E*= 3
E*=1/3
Figure 2 Rupture and release of the microencapsulated healing agent. a, Stress state in
the vicinity of a planar crack as it approaches a spherical inclusion embedded in a linearly
elastic matrix and subjected to a remote tensile loading perpendicular to the fracture
plane. The left and right ®gures correspond to an inclusion three times stiffer
(E * E
sphere
=E
matrix
3) and three times more compliant (E * 1=3) than the
surrounding matrix, respectively. The Poisson's ratios of the sphere and matrix are equal
(0.30). b, A time sequence of video images shows the rupture of a microcapsule and the
release of the healing agent. A red dye was added for visualization. The elapsed time from
the left to right image is 1/15 s. Scale bar, 0.25 mm. c, A scanning electron microscope
image shows the fracture plane of a self-healing material with a ruptured urea-
formaldehyde microcapsule in a thermosetting matrix.
DCPD
monomer
Crosslinked polymer
network
Grubbs' catalyst
1,400 1,200 1,000 800
Wavenumber (cm
–1
)
600
100 µm
a
b
Figure 3 Chemistry of self-healing. a, Ruthenium-based Grubbs' catalyst initiates
ring-opening metathesis polymerization (ROMP) of dicyclopentadiene (DCPD).
b, Environmental ESEM micrograph and infrared analyses. IR spectra correspond to neat
DCPD (top), an authentic sample of poly(DCPD) prepared with Grubbs' catalyst and DCPD
monomer (middle), and poly(DCPD) ®lm formed at the healed interface (bottom). The
highlighted peak at 965 cm
-1
is characteristic of trans double bonds of ring-opened
poly(DCPD).
© 2001 Macmillan Magazines Ltd
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microscope image of the fracture plane in Fig. 2c further illustrates
the rupture process of an embedded microcapsule.
Completion of the self-healing process requires a suitable chem-
istry to polymerize the healing agent in the fracture plane. We
identi®ed the living ring-opening metathesis polymerization
(ROMP) as meeting the diverse set of requirements of the self-
healing system, which includes long shelf life, low monomer
viscosity and volatility, rapid polymerization at ambient conditions,
and low shrinkage upon polymerization. The ROMP reaction
invokes the use of a transition metal catalyst (Grubbs' catalyst)
that shows high metathesis activity while being tolerant of a wide
range of functional groups as well as oxygen and water
20±22
. The
reaction polymerizes dicyclopentadiene (DCPD) at room tem-
perature in several minutes to yield a tough and highly cross-
linked polymer network (Fig. 3a). DCPD-®lled microcapsules (50±
200 mm) with a urea-formaldehyde shell were prepared using
standard microencapsulation techniques. The microcapsule shell
provides a protective barrier between the catalyst and DCPD to
prevent polymerization during the preparation of the composite.
In order to test the stability and activity of the catalyst in an epoxy
matrix, solution-state
1
H-NMR and solid-state
31
P-NMR spectro-
scopies of the self-healing polymer composite have been performed.
Solutions of Grubbs' catalyst with the epoxy prepolymer and
diethylenetriamine are chemically compatible as revealed by the
characteristic benzylidene resonance at 23.3 p.p.m. Solid state
31
P-
NMR measurements of a cured epoxy material with Grubbs' catalyst
shows a characteristic signal corresponding to the tricyclohexyl-
phosphine (PCy
3
) coordinated to the ruthenium metal present in
the catalyst. Solid state
1
H-NMR spectroscopy of the self-healing
polymer composite also indicates the presence of a liquid DCPD
monomer phase within the cured epoxy matrix.
Evidence of polymerization of the healing agent induced by
damage is provided by environmental scanning electron microscopy
(ESEM) and infrared spectroscopy (Fig. 3b). ESEM micrographs
reveal the presence of a thin polymer ®lm on the fracture surface.
Infrared spectroscopy of this ®lm indicates an absorption at 965 cm
-1
characteristic of the ring-opened product, poly(DCPD). Control
samples in which the catalyst was excluded showed no ability to
polymerize the DCPD monomer, providing evidence that the
embedded catalyst initiated the polymerization of the healing agent.
To assess the crack-healing ef®ciency of these composite
materials, fracture tests were performed using a tapered double-
cantilever beam (TDCB) specimen (Fig. 4). Self-healing composite
and control samples were fabricated. Control samples consisted of:
(1) neat epoxy containing no Grubbs' catalyst or microspheres; (2)
epoxy with Grubbs' catalyst but no microspheres; and (3) epoxy
with microspheres but no catalyst. A sharp pre-crack was created in
the tapered samples by gently tapping a razor blade into a moulded
starter notch. Load was applied in a direction perpendicular to the
precrack (Mode I) with pin loading grips as shown in Fig. 4a. The
virgin fracture toughness was determined from the critical load to
propagate the crack and fail the specimen. After failure, the load was
removed and the crack allowed to heal at room temperature with no
manual intervention. Fracture tests were repeated after 48 hours to
quantify the amount of healing and in all of the healed samples, the
crack propagated along the original (virgin) crack plane. The
intrinsic ability of the healing agent to rebond epoxy is shown by
the upper horizontal dotted line in Fig. 4a which represents the
average fracture load achieved for control samples (1) manually
healed by injecting a mixture of DCPD and Grubbs' catalyst into the
crack plane.
A representative load±displacement curve for a self-healing
composite sample is plotted in Fig. 4a demonstrating recovery of
about 75% of the virgin fracture load. In great contrast, all three
types of control samples showed no healing and were unable to
carry any load upon reloading. A set of four independently prepared
self-healing composite samples showed an average healing ef®ciency
of 60%. When the healing ef®ciency is calculated relative to the
critical load for the virgin, neat resin control (lower horizontal
dotted line in Fig. 4a), a value slightly greater than 100% is achieved.
The average critical load for virgin self-healing samples containing
microspheres and Grubbs' catalyst was 20% larger than the average
value for the neat epoxy control samples, indicating that the
addition of microspheres and catalyst increases the inherent tough-
ness of the epoxy.
Self-healing composites possess great potential for solving some
of the most limiting problems of polymeric structural materials:
microcracking and hidden damage. Microcracks are the precursors
to structural failure and the ability to heal them will enable
structures with longer lifetimes and less maintenance. Filling
microcracks will also mitigate the deleterious effects of environ-
mentally assisted degradation such as moisture swelling and stress
corrosion cracking. Although the potential bene®ts are quite high,
the speci®c composite described here has some practical limitations
on crack-healing kinetics and the stability of the catalyst to environ-
mental conditions.
We have thus developed a new structural polymeric material that
possesses the ability to heal cracks autonomically and recover
0
50
100
150
200
250
0 200 400 600 800
Load (N)
Displacement (µm)
Self-
healed
Virgin
Neat epoxy
a
Injected DCPD + catalyst
Figure 4 Self-healing ef®ciency in an epoxy polymer. a, Healing ef®ciency is obtained by
fracture toughness testing of tapered double-cantilever beam (TDCB) specimens. The
virgin fracture toughness is determined by propagating the starter crack along the mid-
plane of the specimen. Subsequently, the load is removed and the crack allowed to heal at
room temperature with no manual intervention. The healed fracture toughness is then
measured by retesting the specimen. b, Post-fracture analysis of the specimens revealed
that the healed crack failed in an interfacial manner from one side of the epoxy/poly(DCPD)
interface to the other. The ESEM image in b shows one area of the fracture plane of a
healed specimen in which the poly(DCPD) ®lm is still attached to the interface on the right
side of the image. The ®lm that originally covered the interface on the left side of the image
is found on the opposite mating surface of the specimen.
© 2001 Macmillan Magazines Ltd
letters to nature
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structural function. Such materials will increase the reliability and
service life of thermosetting polymers used in a wide variety of
applications ranging from microelectronics to aerospace. These
concepts may also be applicable to a broad class of brittle materials
including ceramics and glasses. We expect that the ®eld of self-
healing, although still in its infancy, will evolve beyond the method
presented here until true biomimetic healing is achieved by incorpor-
ating a circulatory system that continuously transports the necessary
chemicals and building blocks of healing to the site of damage.
M
Methods
Preparation of microcapsules by in situ polymerization
In a 600 ml beaker we dissolved urea (0.11 mol, 7.0 g) followed by resorcinol (0.5 g) and
ammonium chloride (0.5 g) in water (150 ml). A 5 wt.% solution of ethylene maleic
anhydride copolymer (100 ml) was added to the reaction mixture and the pH of the
reaction mixture was adjusted to 3.5 using 10% NaOH solution. The reaction mixture was
agitated at 454 r.p.m. and to the stirred solution we added 60 ml of dicyclopentadiene to
achieve an average droplet size of 200 mm. To the agitated emulsion was added 37%
formaldehyde (0.23 ml, 18.91 g) solution and then the temperature of the reaction mixture
was raised to 50 8C and maintained for 2 h. After 2 h, 200 ml of water was added to the
reaction mixture. After 4 h, the reaction mixture was cooled to room temperature and the
microcapsules were separated. The microcapsule slurry was diluted with an additional
200 ml of water and washed with water (3 3 500 ml). The capsules were isolated by
vacuum ®ltration, and air dried. The yield was 80%. Their average size was 220 mm.]
Self-healing epoxy specimen manufacture
The epoxy matrix composite was prepared by mixing 100 parts EPON 828 (Shell
Chemicals Inc.) epoxide with 12 parts DETA (diethylenetriamine) curing agent (Shell
Chemicals Inc.). Self-healing epoxy specimens were prepared by mixing 2.5% (by weight)
Grubbs' catalyst and 10% (by weight) microcapsules with the resin mixture described
above. The resin was then poured into silicone rubber moulds and cured for 24 h at room
temperature, followed by postcuring at 40 8C for 24 h.
TDCB specimens
The TDCB sample was introduced by Mostovoy and co-workers
23
and is designed so that
the compliance of the specimen changes linearly with crack length during the test. This
tapered geometry enables controlled crack growth across the centre of a brittle sample such
as epoxy. The fracture toughness for TDCB specimens depends only on the applied load
and is independent of the crack length so that K
IC
aP
c
where a is a function of geometry
and material properties and P
c
is the critical load at fracture. K
IC
is the experimentally
determined mode-I critical stress intensity factor. A taper angle of 408 was used and a was
measured to be 7,700 m
-3/2
.
Quantifying healing ef®ciency
Previous studies of crack healing in thermoplastic polymers quanti®ed healing effects by
comparing the fracture toughness of the virgin material to the fracture toughness
measured after crack closure and healing.
13±16
. An ef®ciency of healing was de®ned as the
ratio of the fracture toughness of healed and virgin materials such that h K
healed
IC
=K
virgin
IC
where h is the healing ef®ciency.
Received 15 August; accepted 12 December 2000.
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glass. J. Am. Ceram. Soc. 68, 704±706 (1985).
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Polym. Bull. 1, 697±707 (1979).
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polymers. J. Mater. Sci. 16, 204±210 (1981).
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(1982).
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907±915 (1989).
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Acknowledgements
This work has been sponsored by the University of Illinois Critical Research Initiative
Program and the AFOSR Aerospace and Materials Science Directorate. Electron
microscopy was carried out in the Center for Microanalysis of Materials, University of
Illinois, which is supported by the US Department of Energy.
Correspondence and requests for materials should be addressed to S.W.
(e-mail: swhite@uiuc.edu).
.................................................................
A chiroselective peptide replicator
Alan Saghatelian, Yohei Yokobayashi, Kathy Soltani & M. Reza Ghadiri
Departments of Chemistry and Molecular Biology and the Skaggs Institute for
Chemical Biology, The Scripps Research Institute, La Jolla, California 92037, USA
..............................................................................................................................................
The origin of homochirality in living systems is often attributed to
the generation of enantiomeric differences in a pool of chiral
prebiotic molecules
1,2
, but none of the possible physiochemical
processes considered
1±7
can produce the signi®cant imbalance
required if homochiral biopolymers are to result from simple
coupling of suitable precursor molecules. This implies a central
role either for additional processes that can selectively amplify an
initially minute enantiomeric difference in the starting
material
1,8±12
, or for a nonenzymatic process by which biopoly-
mers undergo chiroselective molecular replication
13±16
. Given that
molecular self-replication and the capacity for selection are
necessary conditions for the emergence of life, chiroselective
replication of biopolymers seems a particularly attractive process
for explaining homochirality in nature
13±16
. Here we report that a
32-residue peptide replicator, designed according to our earlier
principles
17±20
, is capable of ef®ciently amplifying homochiral
products from a racemic mixture of peptide fragments through
a chiroselective autocatalytic cycle. The chiroselective ampli®ca-
tion process discriminates between structures possessing even
single stereochemical mutations within otherwise homochiral
sequences. Moreover, the system exhibits a dynamic stereochemi-
cal `editing' function; in contrast to the previously observed error
correction
20
, it makes use of heterochiral sequences that arise
through uncatalysed background reactions to catalyse the pro-
duction of the homochiral product. These results support the idea
that self-replicating polypeptides could have played a key role in
the origin of homochirality on Earth.
Chiroselective ampli®cation refers to an autocatalytic process in
which a homochiral template instructs the synthesis of a homo-
chiral polymer of the same handedness
13
. Past efforts aimed at
establishing the feasibility of nonenzymatic chiroselective ampli®-
cation, using nucleic acids and their analogues, have been hampered
by lack of template turnover
13
and/or enantiomeric cross-inhibition
processes in template-directed oligomerization of activated mono-
meric building blocks
14±16
.
The peptide used in the present study was identi®ed using
design and mechanistic principles that are similar to previously
© 2001 Macmillan Magazines Ltd
PPARa were determined by chemical-mediated fluorescence energy transfer assays using
the AlphaScreen Technology from Packard BioScience
30
. The experiments were conducted
with 5 nM PPARa LBD of biotinylated peptide containing individual motifs (Fig. 3a),
following the manufacturer’s instructions for the hexahistidine detection kit in a buffer
containing 50 mM MOPS, pH 7.4, 50 mM NaF, 0.05 mM CHAPS, 0.1 mg ml
-1
bovine
serum albumin, and 10 mM dithiothreitol (DTT). The binding signals were detected with
the increasing concentrations of GW6471, and the results from four repeated experiments
were normalized as a percentage of the binding in the absence of GW6471.
The effects of GW6471 on the affinity of the SMRT or N-CoR peptides with purified
PPARa LBD were determined by fluorescence polarization in a buffer containing 10 mM
HEPES, pH 7.4, 0.15 M NaCl, 3 mM EDTA, 0.005% polysorbate-20, 5 mM DTT and 2.5%
DMSO. Varied concentration of PPARa LBD in the presence or absence of 40
m
M GW6471
were incubated at room temperature with 10 nM of a fluorescein-labelled peptide of N-
CoR2 or SMRT2 (Fig. 3a). The fluorescence polarization values for each concentration of
receptor were determined using a BMG PolarStar Galaxy fluorescence reader with 485 nm
excitation and 520 nm emission filters. The apparent dissociation constant (K
d
) values
were determined by the binding curves derived from a nonlinear least-squares-fit of the
data for a simple 1:1 interaction.
Mutational analysis of the SMRT co-repressor motif interaction with the PPARa and
TR
b
LBDs was also performed by fluorescence polarization. To determine the importance
of each amino acid in the SMRT motif for binding to nuclear receptors, SMRT peptides
with alanine substitution at each position were added to inhibit the binding of 1
m
MTR
b
LBD or 2
m
MPPARa to the fluorescent N-CoR2 peptide. For the PPARa experiments we
added 10
m
M GW6471. The inhibition curves were constructed and IC
50
values were
determined by nonlinear least-squares-fit of the data to a simple 1:1 interaction.
Received 8 November; accepted 12 December 2001.
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Acknowledgements
We thank B. Wisely and R. Bledsoe for making co-repressor expression constructs in early
crystallization studies; W. Burkart and M. Moyer for protein sequencing; M. Iannone for
compound characterizations; G. Waitt and C. Wagner for mass spectroscopy and amino-
acid content analysis; and J. Chrzas and A. Howard for assistance with data collections at
17-ID. Use of the Advanced Photon Source was supported by the US Department of
Energy, Basic Energy Sciences, and Office of Science. We also thank L. Kuyper and
D. Eggleston for support and critical reading of the manuscript.
Competing interests statement
The authors declare that they have no competing financial interests.
Correspondence and requests for materials should be addressed to H.E.X.
(e-mail: ex11957@gsk.com). The Protein Data Bank code for the PPARa/GW6471/SMRT
complex and the PPARa/GW409544/SRC-1 complex is 1KQQ and 1K7L, respectively.
..............................................................
correction
Autonomic healing of polymer
composites
S. R. White, N. R. Sottos, P. H. Geubelle, J. S. Moore, M. R. Kessler,
S. R. Sriram, E. N. Brown & S. Viswanathan
Nature 409, 794–797 (2001).
.............................................................................................................................................................................
In this Letter, the middle infrared spectrum in Fig. 3b, correspond-
ing to an authentic sample of poly(DCPD) prepared with Grubbs’
catalyst and DCPD monomer, was a duplicate of the top spectrum
owing to a formatting error. The corrected spectra are shown
below. A
1,400 1,200 1,000 800
Wavenumber (cm
–1
)
600
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