Structural polymers are susceptible to damage in the form of cracks, which form deep within the structure where detection is difficult and repair is almost impossible. Cracking leads to mechanical degradation of fibre-reinforced polymer composites; in microelectronic polymeric components it can also lead to electrical failure. Microcracking induced by thermal and mechanical fatigue is also a long-standing problem in polymer adhesives. Regardless of the application, once cracks have formed within polymeric materials, the integrity of the structure is significantly compromised. Experiments exploring the concept of self-repair have been previously reported, but the only successful crack-healing methods that have been reported so far require some form of manual intervention. Here we report a structural polymeric material with the ability to autonomically heal cracks. The material incorporates a microencapsulated healing 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).

Mechanochemical transduction enables an extraordinary range of physiological processes such as the sense of touch, hearing, balance, muscle contraction, and the growth and remodelling of tissue and bone. Although biology is replete with materials systems that actively and functionally respond to mechanical stimuli, the default mechanochemical reaction of bulk polymers to large external stress is the unselective scission of covalent bonds, resulting in damage or failure. An alternative to this degradation process is the rational molecular design of synthetic materials such that mechanical stress favourably alters material properties. A few mechanosensitive polymers with this property have been developed; but their active response is mediated through non-covalent processes, which may limit the extent to which properties can be modified and the long-term stability in structural materials. Previously, we have shown with dissolved polymer strands incorporating mechanically sensitive chemical groups-so-called mechanophores-that the directional nature of mechanical forces can selectively break and re-form covalent bonds. We now demonstrate that such force-induced covalent-bond activation can also be realized with mechanophore-linked elastomeric and glassy polymers, by using a mechanophore that changes colour as it undergoes a reversible electrocyclic ring-opening reaction under tensile stress and thus allows us to directly and locally visualize the mechanochemical reaction. We find that pronounced changes in colour and fluorescence emerge with the accumulation of plastic deformation, indicating that in these polymeric materials the transduction of mechanical force into the ring-opening reaction is an activated process. We anticipate that force activation of covalent bonds can serve as a general strategy for the development of new mechanophore building blocks that impart polymeric materials with desirable functionalities ranging from damage sensing to fully regenerative self-healing.

Polymeric materials exhibit an extraordinary range of mechanical responses (Figure 1a), which depend on the chemical and physical structure of the polymer chains. 3 Polymers are broadly categorized as thermoplastic, thermoset, or elastomer. Thermoplastic polymers consist of linear or branched chains and can be amorphous or semicrystalline. The mechanical response of thermoplastic polymers is highly influenced by the molecular mass, chain entanglements, chain alignment, and degree of crystallinity. Thermosetting polymers consist of highly cross-linked three-dimensional networks. The mechanical properties of these amorphous polymers depend on the molecular mass and the cross-link density. Elastomers (e.g., rubber) are highly deformable elastic networks that are lightly cross-linked by chemical or physical junctions. Mary M. Caruso (center) was born and raised in Tampa, FL. She received a B.S. degree in Chemistry from Elon University (Elon, NC) in 2006, where she worked under the direction of Prof. Karl D. Sienerth. Her research included the synthesis and electrochemical characterization of a novel palladium complex. She is currently pursuing her Ph.D. in Organic Chemistry at the University of Illinois at Urbana-Champaign under the guidance of Prof. Jeffrey S. Moore and Prof. Scott R. White. Her research interests include the development of new catalyst-free self-healing polymers, microencapsulation, and mechanical testing of bulk polymers. Douglas A. Davis (second from left) was born in Martin, TN. In 2004, he received his B.S. degree in Chemistry from the University of Tennessee, Knoxville, where he worked on surface enhanced Raman spectroscopy (SERS) in an electrospray plume with Professors Charles Feigerle and Kelsey Cook. He joined Prof. Jeffrey Moore's group at the University of Illinois, Urbana-Champaign in 2005 to pursue his Ph.D. in Organic Chemistry. His current research interests include designing and synthesizing mechanophores, which can be induced to undergo chemical reactions with mechanical force. Qilong Shen (third from left) was born in 1974 in Zhejiang Province, China. He received his B.S. degree in Chemistry (1996) from Nanjing University, China, and his M.S. in Chemistry (1999) from Shanghai Institute of Organic Chemistry, the Chinese Science Academy, China, under the supervision of Prof. Long Lu. After a two-year stay at the University of Massachusetts at Dartmouth with Prof. Gerald B. Hammond, he moved to Yale University, where he received his Ph.D. under the guidance of Prof. John F. Hartwig in 2007. He is currently a postdoctoral researcher with Prof. Jeffrey S. Moore at University of Illinois at Urbana-Champaign. His research interests include the discovery, development, and understanding of new transition metal-catalyzed reactions, and the mechanochemistry of polymers.

Self-healing polymers and fiber-reinforced polymer composites possess the ability to heal in response to damage wherever and whenever it occurs in the material. This phenomenal material behavior is inspired by biological systems in which self-healing is commonplace. To date, self-healing has been demonstrated by three conceptual approaches: capsule-based healing systems, vascular healing systems, and intrinsic healing polymers. Self-healing can be autonomic—automatic without human intervention—or may require some external energy or pressure. All classes of polymers, from thermosets to thermoplastics to elastomers, have potential for self-healing. The majority of research to date has focused on the recovery of mechanical integrity following quasi-static fracture. This article also reviews self-healing during fatigue and in response to impact damage, puncture, and corrosion. The concepts embodied by current self-healing polymers offer a new route toward safer, longer-lasting, fault-tolerant products and components across a broad cross section of industries including coatings, electronics, transportation, and energy.

Self-healing polymers composed of microencapsulated healing agents exhibit remarkable mechanical performance and regenerative ability, but are limited to autonomic repair of a single damage event in a given location. Self-healing is triggered by crack-induced rupture of the embedded capsules; thus, once a localized region is depleted of healing agent, further repair is precluded. Re-mendable polymers can achieve multiple healing cycles, but require external intervention in the form of heat treatment and applied pressure. Here, we report a self-healing system capable of autonomously repairing repeated damage events. Our bio-inspired coating-substrate design delivers healing agent to cracks in a polymer coating via a three-dimensional microvascular network embedded in the substrate. Crack damage in the epoxy coating is healed repeatedly. This approach opens new avenues for continuous delivery of healing agents for self-repair as well as other active species for additional functionality.