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Introduction: self-healing polymersand composites

Nancy Sottos1,2, Scott White1,3 and Ian Bond4,*

1Beckman Institute, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA2Department of Materials Science and Engineering, and 3Department of AerospaceEngineering,University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA

4Department of Aerospace Engineering, University of Bristol, Bristol BS8 1TR, UK

Self-healing materials are inspired by biological systemsin which damage triggers an autonomic healingresponse. This new paradigm in materials designhas led several exciting interdisciplinary approaches

involving chemistry, mechanics and materials proces-sing to successfully incorporate self-healing function-ality in polymers and composites. The first cleardemonstration of self-healing in an engineered materialssystem occurred in early 2001 with the introduction ofa microencapsulated healing agent and suspendedcatalyst phase in a polymer matrix. Since that time,rapid advancement has been made in the field followingthis conceptual approach, while alternative conceptshave emerged in the scientific literature. From molecu-lar-based reversible bonding schemes to macroscalestructural approaches using hollow glass fibres, therecurrently exists a number of concepts at various stagesof development for self-healing in polymers andcomposites.

Owing to the increasing interest and activity in thefield of self-healing, we believe that it is an opportunetime for a special issue of the Journal of the RoyalSociety Interface. This themed supplement drawstogether current research from eight leading researchgroups. The papers encompass a range of topics with thecommon aim of developing self-healing functionality.

Verberg et al. (2007) are seeking the implementationof synthetic leucocytes via microchannel deliverynetworks and microcapsules capable of releasing nano-

material repair agents. These active agents will not onlysense the state of the walls of the vasculature, but alsodirect the vital components to the damaged sites. Oncethere, the diffusion of the nanoparticles out of themicrocapsules and deposition at the damage site due tothe enthalpic environment leads to an autonomic repairand go function.

A means to ascertain instantaneous knowledge of amaterials structural integrity and, more importantly,when it has been compromised, remains a considerablechallenge in current systems and materials. In order forthe full potential of self-healing materials to be realized,

a sensory component is needed to determine in real timethe formation (and healing) of internal microcracks. Toaddress this challenge, Williams et al. (2007) reportrecent efforts towards the development of an electro-

nically conductive material that is structurally dynamicand responds to various types of external stimuli.

Functional repair components stored inside hollowglass fibres (HGFs) impart self-repair (and visualenhancement) of impact damage by a bleeding action.Work by Trask et al. (2007) considers the placement ofself-healing HGF layers within both glass fibre/epoxyand carbon fibre/epoxy composite laminates to mitigatedamage and restore mechanical strength. Their studyinvestigates the effect of embedded HGF on the hostlaminates mechanical properties and the resultinghealing efficiency after quasi-static impact damage.

The paper by Kersey et al. (2007) addresses self-

healing on a molecular level by incorporating load-bearing, reversible interactions onto surfaces (i.e. modelskeletons) or into covalent polymer networks. Self-healing is governed either kinetically as in the generationof new material (typically by the failure-inducedinitiation of chemical reactions) that fills and adheresto cracks/failure sites or thermodynamically as in theregeneration of the initial state of the original molecularcomponents (typically by annealing reversibleinteractions). To preserve a desired structural state,reversible interactions can be incorporated in a compo-sitematerial with components that preserve a memory ofthe desired state, for example by encasement within anexoskeleton or tethering to an endoskeleton.

Jones et al. (2007b) discuss the healing response ofdamaged fibre-reinforced composites produced with ahealing matrix resin. The system in question consists of athermoplastic healing agent dissolved in a conventionalthermosetting epoxy. Healing is accomplished by heat-ing the fractured components. Upon repeated impact inthe same location, the subsequent delamination area isreduced in comparison with the non-healing specimens.

Work by Mauldin et al. (2007) reports on recentadvancements in a composite material that autonomic-ally repairs crack damage by a healing process based onring-opening metathesis chemistry to rebond the crackfaces. Self-healing is initiated by the release of liquiddicyclopentadiene (DCPD) from fractured microcap-sules that are embedded in a polymer matrix along with

J. R. Soc. Interface (2007) 4, 347348

doi:10.1098/rsif.2006.0205

Published online 20 February 2007

One contribution of 9 to a themed supplement Self-healing polymersand composites.

*Author for correspondence (i.p.bond@bristol.ac.uk).

347 This journal is q 2007 The Royal Society

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Grubbs metathesis catalyst. While their original systemused the commercially available endo-isomer, theexo-stereoisomer of DCPD is known to have much fastermetathesis reaction kinetics. The group now reports onfracture toughness recovery as a function of healing timeand compares the kinetics of damage repair for endo-andexo-DCPD and mixtures of the two isomers.

Fatigue and the associated slow accumulation ofdamage and deterioration in performance plague allmaterials. Jones et al. (2007a) demonstrate a materialsystem which responds to fatigue loading by autonomicself-healing that leads to higher endurance limits andlife extension, or even complete arrest of crack growth(infinite life). The system consists of a self-healingpolymer that incorporates a microencapsulated healingagent and a solid chemical catalyst. Embedded micro-capsules are ruptured by the crack growth and theyrelease the healing agent into the crack plane throughcapillary action leading to the retardation or permanentarrest of further fatigue crack growth. It is shown that

endurance training by periodic exposure to stress canlead to significant life extension and is governed by theinterplay between the mechanical kinetics of crackpropagation and the chemical kinetics of healing inthe polymer.

A class of poly(ethylene-co-methacrylic acid) copo-lymers and ionomers has shown a unique ability to self-heal following projectile puncture. The healing processactive in these materials appears to be significantlydifferent in capability and mechanism than any of theother systems being studied elsewhere. Kalista & Ward(2007) examine thermal effects during self-healing andinvestigate the link to some of the behaviour observed in

other self-healing systems. To effectively designmaterials that can take advantage of this capability,the mechanism by which these materials heal muststill be determined. To provide further understanding ofthe self-healing capabilities, several new damagemethods are considered, including sawing, cutting andpuncture techniques.

The papers collected in this issue represent areasonable survey of the current efforts across the

world in the development of self-healing materials.We know of several other groups currently makingprogress in the field and look forward to theircontributions as they appear in the scientific literature.While the field is still quite young, we believe that acritical mass of scientific activities has been achievedthroughout the world. The future of self-healing is quite

strong and it holds great promise across a broadspectrum of technologies from transportation to man-ufacturing to biomedicine.

REFERENCES

Jones, A. S., Rule, J. D., Moore, J. S., Sottos, N. R. & White,S. R. 2007a Life extension of self-healing polymers withrapidly growing fatigue cracks. J. R. Soc. Interface 4,395403. (doi:10.1098/rsif.2006.0199)

Jones, F. R., Hayes, S. A. & Zhang, W. 2007b Self-healing ofdamage in fibre-reinforced polymermatrix composites.J. R. Soc. Interface 4, 381387. (doi:10.1098/rsif.2006.

0209)Kalista, S. J. & Ward, T. C. 2007 Thermal characteristics ofthe self-healing response in poly(ethylene-co/i-methacrylicacid) copolymers. J. R. Soc. Interface 4, 405411. (doi:10.1098/rsif.2006.0169)

Kersey, F. R., Loveless, D. M. & Craig, S. L. 2007 A hybridpolymer gel with controlled rates of cross-link rupture andself-repair. J. R. Soc. Interface 4, 373380. (doi:10.1098/rsif.2006.0187)

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Trask, R. S., Williams, G. J. & Bond, I. P. 2007 Bioinspiredself-healing of advanced composite structures using hollowglass fibres. J. R. Soc. Interface 4, 363371. (doi.10.1098/rsif.2006.0194)

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Williams, A., Boydston, J. & Bielawski, W. 2007 Towardselectrically-conductive, self-healing materials. J. R. Soc.Interface4, 359362. (doi:10.1098/rsif.2006.0202)

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