Ultrasounds dance the 'moonwalk' in new metamaterial

Silicone beads embedded in a water-based gel (photograph is ~2 cm across).

Metamaterials have extraordinary properties when it comes to diverting and controlling waves, especially sound and light: for instance, they can make an object invisible, or increase the resolving power of a lens. Now, researchers at the Centre de Recherche Paul Pascal (CNRS) and the Institut de Mécanique et d'Ingénierie de Bordeaux (CNRS/Université de Bordeaux/Bordeaux INP/Arts et Métiers ParisTech) have developed the first three-dimensional metamaterials by combining physico-chemical formulation and microfluidics technology. This is a new generation of soft metamaterials that are easier to shape. In their experiment, the researchers got ultrasonic oscillations to move backwards while the energy carried by the wave moved forwards. Their work opens up new prospects, especially for high-resolution imaging (ultrasonography). It is published on 15 December 2014 in the journal Nature Materials.

Since the 2000s, the international scientific community has seen interest in metamaterials and their extraordinary properties grow exponentially. A metamaterial is a medium in which the phase velocity of light or sound waves can be negative (the material is said to have a negative refractive index).. In such a medium, the phase of the wave (the successive oscillations) and the energy carried by this same wave move in opposite directions. This property is not found in any natural homogeneous medium.

To obtain a metamaterial, it is necessary to make a heterogeneous medium that contains a large number of inclusions (known as microresonators). The usual way is to use micromechanical methods (etching, deposition, etc) to machine solid supports that will have the properties of metamaterials in one or two dimensions. However, this method cannot be used to work with soft matter at the micrometer scales required for ultrasounds, and the materials obtained remain limited to one or two dimensions.

In this study, the researchers developed a new type of metamaterial, in the fluid phase, formed of porous silicone microbeads embedded in a water-based gel. This metafluid is the first three-dimensional metamaterial to work at ultrasonic frequencies. In addition, due to its fluid nature, it can be made using physico-chemical processes and microfluidics technologies, which are much easier to implement than micromechanical methods.

One of the properties of porous media is that sound travels through them at very low speed (a few tens of meters per second) compared to water (1500 meters per second). Due to this sharp contrast, the whole suspension has the properties of a metamaterial provided the bead concentration is sufficient: when the researchers studied the propagation of ultrasonic waves through this medium, they directly measured a negative refractive index. Within such a metafluid, the energy carried by the wave travels from the emitter to the receiver, as expected, whereas the oscillations appear to move backwards in the opposite direction, rather like a dancer doing the 'moonwalk'.

These results open the way to numerous applications ranging from high-resolution ultrasound imaging to sound insulation and stealth in underwater acoustics. In addition, the soft-matter physico-chemical techniques used to make this metamaterial makes it possible to produce fluid or flexible materials with adaptable shapes, potentially at the industrial scale.

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Squid supplies blueprint for printable thermoplastics

Squid, what is it good for? You can eat it and you can make ink or dye from it, and now a Penn State team of researchers is using it to make a thermoplastic that can be used in 3-D printing.

"Most of the companies looking into this type of material have focused on synthetic plastics," said Melik C. Demirel, professor of engineering science and mechanics. "Synthetic plastics are not rapidly deployable for field applications, and more importantly, they are not eco-friendly."

Demirel and his team looked at the protein complex that exists in the squid ring teeth (SRT). The naturally made material is a thermoplastic, but obtaining it requires a large amount of effort and many squid.

"We have the genetic sequence for six squid collected around the world, but we started with the European common squid," said Demirel, who with his team collected the cephalopods.

The researchers looked at the genetic sequence for the protein complex molecule and tried synthesizing a variety of proteins from the complex. Some were not thermoplastics, but others show stable thermal response, for example, the smallest known molecular weight SRT protein was a thermoplastic. The results of their work were published in today's (Dec. 17) issue of Advanced Functional Materials and illustrates the cover.

Most plastics are currently manufactured from fossil fuel sources like crude oil. Some high-end plastics are made from synthetic oils. Thermoplastics are polymer materials that can melt, be formed and then solidify as the same material without degrading materials properties.

This particular thermoplastic can be fabricated either as a thermoplastic, heated and extruded or molded, or the plastic can be dissolved in a simple solvent like acetic acid and used in film casting. The material can also be used in 3D printing machines as the source material to create complicated geometric structures.

To manufacture this small, synthetic SRT molecule, the researchers used recombinant techniques. They inserted SRT protein genes into E. coli, so that this common, harmless bacteria could produce the plastic molecules as part of their normal activity and the thermoplastic was then removed from the media where the E. coli lived. Wayne Curtis, professor of chemical engineering and Demirel collaborating on this project together with their students worked on this aspect of the project.

"The next generation of materials will be governed by molecular composition -- sequence, structure and properties," said Demirel.

The thermoplastic the researchers created is semi-crystalline and can be rigid or soft. It has a very high tensile strength and is a wet adhesive; it will stick to things even if it is wet.

This thermoplastic protein has a variety of tunable properties, which can be adjusted to individual requirements of manufacturing. Because it is a protein, it can be used for medical or cosmetic applications.

"Direct extraction or recombinant expression of protein based thermoplastics opens up new avenues for materials fabrication and synthesis, which will eventually be competitive with the high-end synthetic oil based plastics," the researchers report.

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