Tag Archives: metamaterials

From Cornell University, a liquid that remembers its shape

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Sometimes one experiences a frisson (shiver) when reading about a piece of research. Let’s see how you do with this Dec. 4, 2012 news item on Nanowerk,

A bit reminiscent of the Terminator T-1000, a new material created by Cornell researchers is so soft that it can flow like a liquid and then, strangely, return to its original shape.

Rather than liquid metal, it is a hydrogel, a mesh of organic molecules with many small empty spaces that can absorb water like a sponge. It qualifies as a “metamaterial” with properties not found in nature and may be the first organic metamaterial with mechanical meta-properties.

The Dec. 3, 2012 Cornell University news article by Bill Steele, which originated the news item,goes on to explain the interest in hydrogels and what makes this particular formulation so special,

Hydrogels have already been considered for use in drug delivery — the spaces can be filled with drugs that release slowly as the gel biodegrades — and as frameworks for tissue rebuilding. The ability to form a gel into a desired shape further expands the possibilities. For example, a drug-infused gel could be formed to exactly fit the space inside a wound.

…

The new hydrogel is made of synthetic DNA. In addition to being the stuff genes are made of, DNA can serve as a building block for self-assembling materials. Single strands of DNA will lock onto other single stands that have complementary coding, like tiny organic Legos. By synthesizing DNA with carefully arranged complementary sections Luo’s [Dan Luo, professor of biological and environmental engineering] research team previously created short stands that link into shapes such as crosses or Y’s, which in turn join at the ends to form meshlike structures to form the first successful all-DNA hydrogel. Trying a new approach, they mixed synthetic DNA with enzymes that cause DNA to self-replicate and to extend itself into long chains, to make a hydrogel without DNA linkages.

“During this process they entangle, and the entanglement produces a 3-D network,” Luo explained. But the result was not what they expected: The hydrogel they made flows like a liquid, but when placed in water returns to the shape of the container in which it was formed.

“This was not by design,” Luo said.

See the material for yourself,

Hydrogels made in the form of the letters D, N and A collapse into a liquid-like state on their own but return to the original shape when surrounded by water Provided/Luo Lab

Nature Nanotechnology published the team’s research online Dec. 2, 2012 and, unusually, the article is open access (at least for now),

A mechanical metamaterial made from a DNA hydrogel by Jong Bum Lee, Songming Peng, Dayong Yang,  Young Hoon Roh, Hisakage Funabashi, Nokyoung Park, Edward J. Rice, Liwei Chen, Rong Long, Mingming Wu & Dan Luo in Nature Nanotechnology  (2012) doi:10.1038/nnano.2012.211 published online Dec. 2, 2012

Depending on your reading interests and time available, Bill Steele’s Cornell University article has more detail than I’ve provided here or you can check out the well illustrated article in Nature Nanotechnology. As these things go, it’s quite readable as you can see with the abstract (Note: I have removed footnotes),

Metamaterials are artificial substances that are structurally engineered to have properties not typically found in nature. To date, almost all metamaterials have been made from inorganic materials such as silicon and copper, which have unusual electromagnetic or acoustic properties that allow them to be used, for example, as invisible cloaks superlenses or super absorbers for sound. Here, we show that metamaterials with unusual mechanical properties can be prepared using DNA as a building block. We used a polymerase enzyme to elongate DNA chains and weave them non-covalently into a hydrogel. The resulting material, which we term a meta-hydrogel, has liquid-like properties when taken out of water and solid-like properties when in water. Moreover, upon the addition of water, and after complete deformation, the hydrogel can be made to return to its original shape. The meta-hydrogel has a hierarchical internal structure and, as an example of its potential applications, we use it to create an electric circuit that uses water as a switch.

For anyone not familiar with the Terminator movies, here’s an essay in Wikipedia about the ‘franchise’. Pay special note to the second movie in the series, Terminator 2: Judgment Day which introduced a robot (played by Robert Patrick) that could morph from a liquidlike state into various lethal entities.

Flipping chirality at the Lawrence Berkeley National Laboratory

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First, it might be a good idea to define chirality. From the Lawrence Berkeley National Laboratory (Berkeley Lab) July 10, 2012 news release by LynnYarris,

Chirality is the distinct left/right orientation or “handedness” of some types of molecules, meaning the molecule can take one of two mirror image forms. The right-handed and left-handed forms of such molecules, called “enantiomers,” can exhibit strikingly different properties. For example, one enantiomer of the chiral molecule limonene smells of lemon, the other smells of orange. The ability to observe or even switch the chirality of molecules using terahertz (trillion-cycles-per-second) electromagnetic radiation is a much coveted asset in the world of high technology.

As for why anyone would want  to flip molecules back and forth between left- and right-handedness (from the news release),

A multi-institutional team of researchers that included scientists with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has created the first artificial molecules whose chirality can be rapidly switched from a right-handed to a left-handed orientation with a  beam of light. This holds potentially important possibilities for the application of terahertz technologies across a wide range of fields, including reduced energy use for data-processing, homeland security and ultrahigh-speed communications.

Here’s how the technique works, from the July 10, 2012 news item on physorg.com,

Working with terahertz (THz) metamaterials engineered from nanometer-sized gold strips with air as the dielectric – Zhang [Xiang Zhang, one of the leaders of this research and a principal investigator with Berkeley Lab’s Materials Sciences Division] and his colleagues fashioned a delicate artificial chiral molecule which they then incorporated with a photoactive silicon medium. Through photoexcitation of their metamolecules with an external beam of light, the researchers observed handedness flipping in the form of circularly polarized emitted THz light. Furthermore, the photoexcitation enabled this chirality flipping and the circular polarization of THz light to be dynamically controlled.

“In contrast to previous demonstrations where chirality was merely switched on or off in metamaterials using photoelectric stimulation, we used an optical switch to actually reverse the chirality of our THz metamolecules,” Zhang says.

The researchers describe in more detail the potential for this new technique,

“The observed giant switchable chirality we can engineer into our metamaterials provides a viable approach towards creating high performance polarimetric devices that are largely not available at terahertz frequencies,” says corresponding author Antoinette Taylor. “This frequency range is particularly interesting because it uniquely reveals information about physical phenomena such as the interactions between or within biologically relevant molecules, and may enable control of electronic states in novel material systems, such as cyclotron resonances in graphene and topological insulators.”

Taylor and her co-authors say that the general design principle of their optically switchable chiral THz metamolecules is not limited to handedness switching but could also be applied to the dynamic reversing of other electromagnetic properties.

From what I understand metamaterials are very expensive and difficult to produce which means this exciting advance is likely to remain in the laboratory of at least 10 years.