The longevity of materials is a topical issue of the utmost importance. The durability of the materials we use in our daily lives, their performance, fatigue and service life, have emotional, economic, ecological and vital aspects, and researchers are striving to discover and develop materials that can be repaired, mended, healed and even self-repaired, just like those found in nature. On an industrial and technological level, the (long-term) durability of materials is a priority, and many fields of application can benefit from research into self-healing materials (construction, transport, furniture, medicine, sporting goods, etc.). This lesson summarizes the state of the art in the field referred to by our Anglo-Saxon colleagues as " self-healing materials ", and analyzes the various concepts involved in the tailor-made development of self-healing materials and materials with stimulated repair.
Damage to a material is a multi-scale process that begins at the molecular level with the breakdown of chemical bonds, followed by sliding and cracking phenomena at mesoscopic and microscopic scales, and ends with the outright failure of the material at the macroscopic level. The secret to self-healing materials is that their design must involve adaptive properties involving the use of reversible chemistry, dynamics, so as to program a response to damage, a response coming from the most fundamental level, the molecular level, the chemical bond. In this context, self-repairing materials are built by bringing into play reversible covalent or iono-covalent bonds, coordination bonds and all supramolecular interactions (hydrogen bonds, van der Waals π-π, electrostatic interactions). Knowing that self-healing can be passive, autonomous or ballistic, we described and discussed different examples of materials. The most widely used self-healing systems today are those containing microcapsules that fracture under mechanical, thermal, chemical or photochemical stress, releasing an active principle (catalyst, active molecule) that generates a chemical healing process. These strategies enable the development of self-sustaining polymer coatings on steel, epoxy resins that heal through the reversibility of Diels-Alder-type reactions, for example, hybrid sol-gel anti-corrosion coatings on metal, and ultra-high-performance concretes to which cement grains with very low hydration levels confer a self-healing character. The ballistic self-repair of materials is associated with the existence of ionic networks (ionomers, polyelectrolyte multi-layers) that exhibit strong bond dynamics ensuring material cohesion. These dynamics can be exalted by the impact of an object, generating friction and energy transfer, followed by heating. These mechano-thermochemical processes enable us to design materials that absorb the impact of a projectile in a very short time (from a few seconds to a few tens of minutes). We also presented self-adapting hybrid membranes made of gold nanoparticles and polyelectrolytes which, when subjected to the pressure of an object, fully recover their shape and mechanical properties in just a few hours. To conclude this lesson, we focused on bio-inspired self-repair, both by describing the repair mechanisms of certain materials from the living world (fibronectin, mussel byssus, abrasion-protective cuticle of flexible byssus filaments) and synthetic materials constructed using the general assembly principles of natural materials. Taking the example of connectin, a protein found in striated skeletal muscle, we can develop materials that combine mechanical properties such as elasticity, robustness and hardness, and that can recover their initial mechanical properties in just a few hours. It is the subtle coupling between the covalent bonds of the primary structure and the inter-module junctions (this part deforms, stretches but doesn't break) and the supramolecular bonds (this junction reversibly breaks and reforms) ensuring the formation of a secondary structure that makes it possible to generate a solid material that deforms plastically and absorbs energy. similarly, it is the combination of soft, deformable precollagen domains and hard lateral domains connected by sacrificial zones, in which the complexation of metal cations with histidine functions ensures filament coherence, that enables the material making up the byssus fibers to be both highly extensible, very hard and a good shock absorber. We should point out that the coordination-decoordination of metals by biological complexing agents is often at the heart of many biorepair phenomena. It is on the basis of these simple concepts that, very recently in the literature, hybrid polymers with modulable mechanical properties have appeared. Professor Leibler's seminar illustrates the latter theme by presenting new hybrid materials based on epoxy resins and zinc salts, which can be shaped at will, repaired and recycled under the action of heat.
