Smart materials on the tip of a fishing hook

When Johan Foster, a group leader from the Adolphe Merkle Institute (AMI) puts an artificial worm at the end of a fishing hook, the bait is perfectly inert. But once it is in the water it starts to wiggle, thus wondrously imitating its natural counterpart. The explanation: when it comes into contact with water, this piece of shape memory polymer regains its initial geometry.

An intelligent invertebrate

This innovation from the AMI researchers led by Professor Christoph Weder and Johan Foster was inspired by sea cucumbers, a soft marine organism whose skin contains multiple collagen fibres and becomes stiff upon contact. When the animal is calm, these fibres are independent of each other. But as soon as it is touched, it secretes peptides that enable the fibres to bond together and form a sort of scaffolding which makes its skin rigid. This mechanism is reversible at will and makes the skin of the sea cucumber a natural smart material.

In the case of the artificial worm, the AMI researchers inserted crystalline cellulose nanofibres in a polymer. These nanofibres are of natural origin and can be gained by dissolving cotton or paper. Despite being structurally simple, their mechanical properties resemble those of carbon nanotubes. When integrated in a polymer, they join together by means of what chemists call hydrogen bonds. Depending on the size and the concentration of fibres, the composite can be as hard as a CD jewel case. By adding water, however, you can weaken the hydrogen bonds and make the polymer as soft as rubber. Here again, the mechanism is reversible at will, making this composite a smart material.

Shape memory

Hence, in order to create an artificial worm, all you have to do is moisten a piece of this material, then stretch and bend it before letting it dry. When it dries, the hydrogen bonds in-between the fibres regain the upper hand and solidify the polymer in its deformed state. If we again dip it in water, this loosens the bonds between the fibres, causing them to revert to their initial form.

Although fishing lures might indeed be an easy-to-realize first product, the artificial worms were primarily made to demonstrate the properties and potential of the new water-activated shape-memory materials. Christoph Weder and Johan Foster are considering more sophisticated applications of their new materials in the biomedical area. They could, for example, serve as a substrate for microelectrodes implanted in the brain, which can only be positioned precisely if extremely rigid. However, it is this very rigidity that accelerates their rejection by the organism. As the intracranial fluids are essentially composed of water, the materials devised by AMI can play a dual role: rigid for implantation, and later soft in order to slow down rejection.

National Research Programme "Smart Materials" (NRP 62)

As a cooperation programme between the Swiss National Science Foundation (SNSF) and the Commission for Technology and Innovation (CTI), the National Research Programme "Smart Materials" (NRP 62) is committed to the development of new intelligent materials and the advancement of promising projects to the R&D stage. It strives to promote scientific excellence and contribute to the successful industrial exploitation of smart materials and their applications. NRP 62 intends to combine the expertise and resources of various research institutions across Switzerland. The researchers will devise the technologies needed for the development of smart materials and for their application in intelligent systems and structures within strategically important sectors of Swiss industry. NRP 62 will operate with 11 million Swiss francs and run until 2015