Acoustic Cloaking
Design for meta­materials that deflect sound waves

Source: “Acoustic cloaking in two dimensions: a feasible approach”
Daniel Torrent and José Sánchez-Dehesa
New Journal of Physics 10: 63015-63025

Results: Designs have been drawn up for a material that could lead to the first acoustic cloaking device. Computer models suggest that alternating layers of two types of patterned, elastic rods, called sonic crystals, would direct sound waves around an object so that they re-formed on the other side with no distortion, as if the sound waves had never encountered the object.

Why it matters: The cloak could make ships invisible to sonar and improve the acoustics of concert halls by allowing sound to pass around load-bearing columns. Buildings covered in the material would be shielded from street noise. Other researchers have designed and built materials that can cloak objects from microwaves, but they divert only particular wavelengths. The new research predicts that an acoustic cloak would shield objects from a broad spectrum of sounds, from high pitches to low.

Methods: The researchers developed computer models based on previous theoretical work and used them to simulate the movement of sound waves around acoustic cloaks with varying numbers of layers. The models showed that sound waves flow best around materials made of 200 layers of composite sonic crystals.

Next steps: The designed material would work only in two dimensions–with sound waves traveling in a plane. The researchers will extend their theoretical work, developing new designs for materials that work in three dimensions, and then build and test them.

Testing Nanotoxicity
A rapid assay offers a much-needed way to evaluate nano­materials’ safety

Source: “Perturbational profiling of nanomaterial biologic activity”
Stanley Y. Shaw et al.
Proceedings of the National Academy of Sciences 105: 7387-7392

Results: Researchers have developed a way to evaluate the safety of nanoparticles by quickly comparing them to nanoparticles already tested for toxicity. They determined the effects of different doses of nanoparticles on a ­variety of cell types in culture. Then they performed tests in mice, showing that their tests on cells could predict which nanoparticles would have effects in animals similar to those of previously screened nanoparticles.

Why it matters: Hundreds of products containing nanomaterials are already on the market, and more are under development. Few if any of the materials have been thoroughly tested. The new assay is faster and cheaper than testing in animals but appears to give a good approximation of the results; it represents an important step toward speeding up the process of evaluating new nanomaterials. The approach could help researchers choose between similar nanoparticles on the basis of potential safety risks.

Methods: The researchers tested 50 nanoparticles, most of which are being developed for medical imaging, in the four cell types that they are most likely to encounter in the body. Each nanoparticle was tested at four different concentrations in mouse immune cells, human liver cells, and two types of human blood-­vessel cells. Automated systems collected data on cell death, metabolic changes, and other signs of toxicity.

Next steps: The experiment, which focused mostly on iron-containing nanoparticles and tiny semiconductor particles called quantum dots, now needs to be extended to other nanomaterials. The assay works well for nanoparticles entering the body intravenously, but to test the properties of nanomaterials that might enter in other ways, including inhalation, future assays will need to use different cell types, such as lung cells.