Introduction: self-healing polymers and composites.

Self-healing materials are inspired by biological systems in which damage triggers an autonomic healing response. This new paradigm in materials design has led several exciting interdisciplinary approaches involving chemistry, mechanics and materials processing to successfully incorporate self-healing functionality in polymers and composites. The first clear demonstration of self-healing in an engineered materials system occurred in early 2001 with the introduction of a microencapsulated healing agent and suspended catalyst phase in a polymer matrix. Since that time, rapid advancement has been made in the field following this conceptual approach, while alternative concepts have emerged in the scientific literature. From molecular-based reversible bonding schemes to macroscale structural approaches using hollow glass fibres, there currently exists a number of concepts at various stages of development for self-healing in polymers and composites. Owing to the increasing interest and activity in the field of self-healing, we believe that it is an opportune time for a special issue of the Journal of the Royal Society Interface. This themed supplement draws together current research from eight leading research groups. The papers encompass a range of topics with the common aim of developing self-healing functionality. Verberg et al. (2007) are seeking the implementation of ‘synthetic leucocytes’ via microchannel delivery networks and microcapsules capable of releasing nanomaterial repair agents. These active agents will not only sense the state of the walls of the vasculature, but also direct the vital components to the damaged sites. Once there, the diffusion of the nanoparticles out of the microcapsules and deposition at the damage site due to the enthalpic environment leads to an autonomic ‘repair and go’ function. A means to ascertain instantaneous knowledge of a material’s structural integrity and, more importantly, when it has been compromised, remains a considerable challenge in current systems and materials. In order for the full potential of self-healing materials to be realized,