Biological archetypes for self-healing materials

Abstract

Damage and fatigue are ever-present facts of life. Given enough time, even the most robust material, whether man-made or natural, succumbs to the deleterious effects of cracks, fissures, and defects during normal use. Traditionally, materials engineers have approached this problem by creating damage-tolerant structures, intensive quality control before use, vigilant inspection during use, and designing materials to function well below their theoretical limit. Living organisms, on the other hand, routinely produce materials that function close to their theoretical limit as a result of their remarkable ability to self-heal a range of non-catastrophic damage events. For this reason, many researchers in the last 15 years have turned to nature for inspiration for the design and development of self-healing composites and polymeric materials. However, these efforts have so far only scratched the surface of the richness of natural self-repair processes. In the present review, we provide an overview of some paradigmatic and well-studied examples of self-repair in living systems. The core of this overview takes the form of a number of case studies that provide a detailed description of the structure–function relationships defining the healing mechanism. Case studies include a number of examples dependent on cellular action in both animals (e.g., limb regeneration, antler growth, bone healing, and wound healing) and plants (e.g., latex-based healing, plant grafting, and wound closure in woody vines and succulent plants). Additionally, we examine several examples of acellular self-repair in biopolymeric materials (e.g., mussel byssus, caddisfly silks, and whelk egg capsules) that are already inspiring the development of a number of self-healing polymers.