If one Googles “self-healing coatings”, one will find an extraordinarily expansive listing of activity in terms of chemistry, proposed applications, products, and development status.  When you consider mankind’s history, self-healing, as a phenomenon, has certainly been appreciated in biological systems, but only in recent times has the concept been recognized as an advantageous property to have in manufactured materials.  

Self-healing as a concept is not foreign to us, as we well appreciate it as a wondrous thing, natural to ourselves as human beings.  We injure ourselves, scratching, ripping or piercing our skin, and miraculously within a short period of time the damage is healed.  Similarly broken bones heal themselves, albeit sometimes with the need and help of setting and immobilization.  

Simply, this healing occurs in three phases: inflammatory, proliferative, and maturation stages.  In the first occurring inflammatory stage,certain cells kill bacteria, debride tissue that is damaged, and release chemicals that cause new capillaries to move to the area and divide.  The proliferative phase proceeds with fibroblasts, producing a type of collagen that fills the void caused by the wound.  In the maturation stage, a stronger type of collagen is produced, cross-linked and aligned for strength.

Continuing, the concept of self-healing is now a popular subject in materials science, and is seen by many to be a desired function in regard to man-made manufactured materials.  Self-healing coatings by definition are representative of developing materials that possess the internal capability to repair sustained damage by themselves, or with some outside stimulation.  Some polymers, when damage is sensed, have the ability to repair themselves.  This is defined as “intrinsic” healing.  

Currently however, many self-healing materials are produced by the addition of substances to existing materials.  This is defined as “extrinsic” healing.  An example of the latteris a polymer coating, developed for the hulls of ships.  This coating has the ability to sense cracks and repair is initiated when microcapsules of resins contained in the coating rupture and react.  

Some of the self-healing products being discussed and being developed are truly bizarre and strain our imagination.  For example, what would be the impact of a self-healing concrete?   Work is progressing on so called  “bio-concrete”. This product concept mixes a certain bacteria into concrete. When cracks and holes appear the contained bacteria grow, providing a calcium carbonate filler material. This material (a waste product of the bacteria) is somewhat like limestone.

Further, can we imagine a self-healing material’s future where a scratched eye glass lens repairs itself, a scratch in vehicle paint disappears soon after the damage happens, painted buildings repaint themselves, flooring materials repair themselves, building panels don’t fail, space and aeronautical vehicle materials self-heal stress fractures?

Chemistry approaches  to self-healing are as numerous as proposed applications.