Vincenzo Amendola and Moreno Meneghetti, at the University of Padova, Italy, take inspiration from nature to design materials that can repair themselves.

Nature uses self-healing in all living systems to repair damage caused by environmental interactions. A simple case is repairing a skin wound - without this mechanism, we could not live. DNA repair, which must occur routinely in every living organism, is another example. But at what level do repairing mechanisms occur? Looking at the components of a living system, we find cells, which typically have micrometre dimensions. But we have to zoom in further, namely to the nanoscale, to see the sub-cellular structures on which nature's self-healing mechanisms work. There we can see why natural systems are inspirations for the world of nanostructures.

Self-healing of a polyelectrolyte-gold nanoparticle composite after damaged caused by external pressure (ref. 1)

Nanoscience and nanotechnology are advancing rapidly. We can now see nanoscale matter using microscopies based on accelerated electrons (transmission and scanning electron microscopies). These provide much greater resolution than photons, which form the basis of optical microscopy. Probe microscopies, such as scanning tunneling microscopy and atomic force microscopy, also help to complete the picture. Armed with these techniques, scientists are developing new strategies to control the synthesis of structures at the nanoscale.

But the same problem faced by living systems is now also the problem of synthetic nanoscale structures - they must self-heal when they interact with their environment. Nanosystems have a large surface with respect to their volume and their many surface atoms are prone to defects. So it is very important to develop and understand strategies for self-healing new nanostructured materials. Three general approaches have emerged - auto-assembling materials, shape-memory materials and materials capable of responsive chemical reactions - although they are still in their infancy.

Auto-assembling materials spontaneously organise themselves. Following an external perturbation, their structure reassembles itself automatically. Shape memory materials recover their original structure by exploiting their characteristic phase transitions. The material's form is usually programmed at high temperature. Damages occurring at room temperature are repaired by heating the material above its phase transition temperature. There are more examples in the third type of self-healing nanostructures, which exploit a chemical reaction, inspired in some cases, although with much less complexity, by natural processes. For example, polymers with multiple fractures can be repaired using an inbuilt 3D microchannel network which transports two reactive components. The two components come into contact only when a fracture breaks the microchannels. They then react to heal the break. The idea is similar to what happens with our vessels when a wound occurs although, clearly, the complexity of the natural process is far from being realised.

In nature, many healing processes occur because a functional property of the system must be recovered - DNA healing is a good example. Scientists recently found that a similar mechanism occurs for gold nanoparticles. The particles possess a property known as multiphoton absorption, which means that they can absorb more than one photon from high intensity laser pulses at once. Researchers believe this property could be exploited, for example to protect eyes from intense laser pulses. But the nanoparticles quickly lose the property because the laser pulses fragment them. Now scientists have found that phthalocyanine mixed in with the nanoparticles encourages fragmented particles to aggregate. Laser pulses then fuse the aggregated structures into larger nanoparticles. This self-healing mechanism, promoted by the same laser that inflicts the damage, preserves the multiphoton property of the nanoparticles.

While self-healing of functional properties is rare at present, researchers will be forced to look in this direction in the future to obtain nanomaterials with improved properties. Natural processes could provide the inspiration required to meet these nanotechnological challenges.

 

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