The lifetime of man-made materials is controlled largely by the wear and tear of everyday use, environmental stress and unexpected damage, which ultimately lead to failure and disposal. Smart materials that mimic the ability of living systems to autonomously protect, report, heal and even regenerate in response to damage could increase the lifetime, safety and sustainability of many manufactured items. There are several approaches to achieving these functions using polymer-based materials, but making them work in highly variable, real-world situations is proving challenging.
The life cycle of plastics and other materials used for engineering begins with the extraction of raw materials, followed by the synthesis and processing of the polymer building blocks, which are manufactured into a product that has a particular use or function. The product is eventually degraded or damaged during use, and is ultimately disposed of or recycled. Polymers and polymer-based composites are designed and manufactured to be as robust as possible for a given application, but failure is eventually inevitable. In the case of high-volume and simple, low-cost products, such as the ubiquitous plastic bag, the materials will ideally be recycled after use. But in many instances, the life cycle of polymeric materials could be expanded by programming them with biologically inspired, autonomous functions to protect them from, and to limit, damage, or even to reverse damage and regenerate in response to environmental stress. The attraction of this approach is not only waste reduction, but also the ability to create products with increased safety and superior reliability — a particularly appealing feature for applications such as medical implants, undersea pipelines or structures in space, where damage is difficult to detect, and repair is costly or even impossible.
Research that encompasses chemistry, polymer science, processing and engineering has delivered polymeric materials that have remarkable self-healing, sensing and reporting properties. In this Review we sketch our vision of how such functions could extend the life cycle of functional polymeric materials, and outline the basic performance criteria and material-design principles that should guide the development of practically useful systems. We then examine in more detail the different biologically inspired functions that have been realized, and explore how they can endow materials and devices with improved performance. Finally, we discuss the problems that need to be overcome for this class of polymeric materials to fulfil its promise and find commercial uses.