Self-healing material

A self healing material is a material which has the built-in ability to partially repair damage occurring during its service life time. Usually materials' properties degrade over time due to the initiation of damage (like micro cracks) on a microscopic scale that tends to grow and will ultimately lead to failure of the material. From the biological world we know about mechanisms that continuously sense damage and repair it. In the field of materials science researchers are now trying to engineer this type of behaviour into man-made materials, so called self healing materials.

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The first report of a man-made self healing material was by the group of prof Scott White of the University of Illinois at Urbana-Champaign ^[1]. They reported an epoxy system containing microcapsules. These microcapsules were filled with a (liquid) monomer. If a microcrack occurs in this system, the microcapsule will rupture and the monomer will fill the crack. Subsequently it will polymerise, initiated by catalyst particles (Grubbs catalyst)that are also dispersed through the system. This model system of a self healing particle proved to work very well: the service life time of a structure made of such material will be significantly higher.

Currently a number of research groups world wide is developing self healing mechanisms for essentially all materials classes (metals, polymers, ceramics, cemetitious and fibre-reinforced composite^[2] materials). Besides the mechanism of micro encapsulation from White and co-workers, many other 'healing strategies' are being explored (e.g. reversible chemistries, dynamic precipitation etc).

It was only recently, that the possibility of using supramolecular polymers, a typical example of reversible chemistries, was explored for self-healing applications. In supramolecular polymers relatively short prepolymers are reversibly connected by for example quadruple hydrogen bonding units. This type of bond is sensitive for heat. By elevation of the temperature, self-healing can be induced because the reversible bonds will disconnect, and when lowering the temperature new bonds will be formed. An advantage of this method is that no reactive chemicals or (toxic) catalysts are needed.

References

1. ^ S.R. White, N.R. Sottos, P.H. Geubelle, J.S. Moore, M.R. Kessler, S.R. Sriram, E.N. Brown, S. Viswanathan: "Autonomic healing of polymer composites", Nature. 2001 409, 794-797.
2. ^ R.S. Trask, H.R. Williams, I.P. Bond: "Self-healing polymer composites: mimicking nature to enhance performance", Bioinspiration and Biomimetics. 2007. 2. 1-9.