Self-Healing Materials: A Review of Intrinsic and Extrinsic Healing Mechanisms and Their Engineering Applications

Engineering materials accumulate small cracks during service, and these cracks grow until a component must be repaired or replaced. Self-healing materials are designed to repair such damage on their own, or with only a mild external stimulus, in a way inspired by the healing of wounds in living organisms. This review explains the principles behind self-healing and classifies the approaches into extrinsic systems, in which a healing agent is stored in microcapsules, hollow fibres or vascular channels, and intrinsic systems, in which the material itself contains reversible chemical or physical bonds that re-form after fracture. Representative studies on polymers, composites, cement-based materials, coatings and electronic materials are reviewed and compared. A general functional architecture is presented that describes a self-healing system as a sequence of damage, triggering, transport of healing material, repair reaction and property recovery, and the healing efficiency used to measure performance is defined. Applications in civil engineering, aerospace, automotive, coatings, electronics and biomedical devices are discussed. The review also examines current limitations, such as high cost, a limited number of healing cycles and difficulty in repairing large damage, and outlines directions for future research.