Biological systems have the ability to sense, react, regulate, grow, regenerate, and heal. Recent advances in materials chemistry and micro- and nanoscale fabrication techniques have enabled biologically inspired materials systems that mimic many of these remarkable functions. This issue of MRS Bulletin highlights two promising classes of bioinspired materials systems: surfaces that can self-clean and polymers that can self-heal. Self-cleaning surfaces are based on the superhydrophobic effect, which causes water droplets to roll off with ease, carrying away dirt and debris. Design of these surfaces is inspired by the hydrophobic micro- and nanostructures of a lotus leaf. Self-healing materials are motivated by biological systems in which damage triggers a site-specific, autonomic healing response. Self-healing has been achieved using several different approaches for storing and triggering healing functionality in the polymer. In this issue, we examine the most successful strategies for self-cleaning and self-healing materials and discuss future research directions and opportunities for commercial applications.

In this contribution, we briefly discuss how various physicochemical properties of surfaces affect the wettability and self-cleaning character of both naturally occurring and synthetic surfaces. Using a few selected examples from nature, we discuss the superhydrophobic effect and antifouling character of such surfaces and how these properties are associated with variations in surface chemical composition and topography. We also review a few special case studies aimed at adopting these biomimetic schemes to design and fabricate functional self-cleaning surfaces.

The wettability of several superhydrophobic surfaces that were prepared recently by simple, mostly single-step methods is described and compared with the wettability of surfaces that are less hydrophobic. We explain why two length scales of topography can be important for controlling the hydrophobicity of some surfaces (the lotus effect). Contact-angle hysteresis (difference between the advancing, θA, and receding, θR, contact angles) is discussed and explained, particularly with regard to its contribution to water repellency. Perfect hydrophobicity (θA/θR = 180°/180°) and a method for distinguishing perfectly hydrophobic surfaces from those that are almost perfectly hydrophobic are described and discussed. The Wenzel and Cassie theories, both of which involve analysis of interfacial (solid/liquid) areas and not contact lines, are criticized. Each of these related topics is addressed from the perspective of the three-phase (solid/liquid/vapor) contact line and its dynamics. The energy barriers for movement of the three-phase contact line from one metastable state to another control contact-angle hysteresis and, thus, water repellency.

Recent experiments have revealed that the wax on the lotus leaf surface, by itself, is weakly hydrophilic, even though the lotus leaf is known to be superhydrophobic. Conventional understanding suggests that a surface of such waxy composition should not be able to support superhydrophobicity and high contact angles between a liquid and the surface. Here, we show that the unexpected superhydrophobicity is related to the presence of “reentrant texture” (that is, a multivalued surface topography) on the surface of the lotus leaf. We exploit this understanding to enable the development of superoleophobic surfaces (i.e., surfaces that repel extremely low-surface-tension liquids, such as various alkanes), where essentially no naturally oleophobic materials exist. We also develop general design parameters that enable the evaluation of the robustness of the composite interface on a particular surface. Based on these design parameters, we also rank various superhydrophobic and superoleophobic substrates discussed in the literature, with particular emphasis on surfaces developed from inherently hydrophilic or oleophilic materials.

Over the past ten years, a broad range of self-healing materials, systems that can detect when they have been damaged and heal themselves either spontaneously or with the aid of a stimulus, has emerged. Although many unique compositions and components are used to create these materials, they all employ basic chemical reactions to facilitate repair processes. Kinetically controlled ring-opening reactions and reversible metal–ligand interactions have proven useful in autonomic self-healing materials, which require no stimulus (other than the formation of damage) for operation. In contrast, nonautonomic self-healing materials, which require some type of externally applied stimulus (such as heat or light) to enable healing functions, have capitalized on chemistries that utilize either reversible covalent bonds or various types of noncovalent interactions. This review describes the underlying chemistries used in state-of-the-art self-healing materials, as well as those currently in development.

Self-healing polymers have experienced rapid technological advancement over the past seven years. They have moved from a conceptual demonstration to practical application in this time frame and have grown from a single design to a generic paradigm for modern materials development. Potential applications of self-healing polymers are quite broad, including microelectronic substrates and encapsulants, polymeric paints and coatings, structural composites, and biomedical devices. In this article, we focus on polymeric systems that heal in an autonomic fashion, that is, automatically and without human intervention. The types of systems under development and the future of this paradigm in advanced materials are discussed.

Self-healing is receiving an increasing amount of interest worldwide as a method to address damage in materials. In particular, for advanced high-performance fiber-reinforced polymer (FRP) composite materials, self-healing offers an alternative to employing conservative damage-tolerant designs and a mechanism for ameliorating inaccessible and invidious internal damage within a structure. This article considers in some detail the various self-healing technologies currently being developed for FRP composite materials. Key constraints for incorporating such a function in FRPs are that it not be detrimental to inherent mechanical properties and that it not impose a severe weight penalty.