A new generation of composite materials that can be ‘healed on demand’ may be one step closer thanks to work from US-based researchers.

Even the toughest polymer composites are susceptible to damage, shortening their lifetime and limiting their use in a wide range of applications. And so, for decades, the search has been on to find low cost, durable materials that can heal themselves. But re-establishing chemical bonds across a crack efficiently and cheaply is not without its challenges. Self-healing materials have always attracted a lot of interest, but research has shown that liquid healing agents are only practical for micron or sub-micron-scale cracks. For larger cracks, another approach is needed, so called, healing-on-demand, where extrinsic stimuli, such as thermal treatment, are used to trigger the healing process. Most of these processes also need an external load, in order to first narrow the crack before healing can begin.

In a recent paper in Polymer [DOI: 10.1016/j.polymer.2015.03.022], researchers have looked towards biological systems for inspiration for their heal-on-demand composite. Muscles, and their contraction and relaxation, are what allow all animals to move. For composite researchers, they could also be the key to narrowing wide cracks without applying an external load. Fine wires of shape memory alloys have long been tested in polymers, but their high cost and poor interfacial bonding have limited them to niche applications.

For Zhang and Li, a widely-used commercial product held the key to developing more reliable polymer muscles - fishing line! The line was tightly-twisted and coiled in order to provide a well-defined expansion upon the application of heat. The spring-like fishing wire was embedded in a thermoset resin, and a fine-grained thermoplastic powder added. Mechanical testing of the sample showed that by applying heat (using a standard heat gun), the polymer muscle could actuate, relaxing and contracting in a repeatable manner. When a crack was induced in the material, this actuation narrowed it. The melted thermoplastic healing agents then filled in the narrowed crack, re-establishing the damaged molecular bonds. After the composite was cooled to room temperature, the crack (original diameter 0.24 mm) was found to be healed, with almost no detectable separation between the two edges.

It is hoped that the low cost, high healing efficiency, good compatibility, and excellent flexibility of this artificial muscle will enable a new route to designing novel, healing-on-demand polymer composites.

Polymer 64 (2015) 29-38,” Healing-on-demand composites based on polymer artificial muscle” DOI: 10.1016/j.polymer.2015.03.022