New way to shape thin gel sheets proposed

Ryan Hayward, Christian Santangelo and colleagues describe their new method of halftone gel lithography for photo-patterning polymer gel sheets in the current issue of Science. They say the technique, among other applications, may someday help biomedical researchers to direct cells cultured in a laboratory to grow into the correct shape to form a blood vessel or a particular organ.

"We wanted to develop a strategy that would allow us to pattern growth with some of the same flexibility that nature does," Hayward explains. Many plants create curves, tubes and other shapes by varying growth in adjacent areas. While some leaf or petal cells expand, other nearby cells do not, and this contrast causes buckling into a variety of shapes, including cones or curly edges. A lily petal's curve, for example, arises from patterned areas of elongation that define a specific three-dimensional shape.

Building on this concept, Hayward and colleagues developed a method for exposing ultraviolet-sensitive thin polymer sheets to patterns of light. The amount of light absorbed at each position on the sheet programs the amount that this region will expand when placed in contact with water, thus mimicking nature's ability to direct certain cells to grow while suppressing the growth of others. The technique involves spreading a 10-micrometer-thick layer (about 5 times thinner than a human hair) of polymer onto a substrate before exposure.

Areas of the gel exposed to light become crosslinked, restricting their ability to expand, while nearby unexposed areas will swell like a sponge as they absorb water. As in nature, this patterned growth causes the gel to buckle into the desired shape. Unlike in nature, however, these materials can be repeatedly flattened and re-shaped by drying out and rehydrating the sheet.

To date, the UMass Amherst researchers have made a variety of simple shapes including spheres, saddles and cones, as well as more complex shapes such as minimal surfaces. Creating the latter represents a fundamental challenge that demonstrates basic principles of the method, Hayward says.

He adds, "Analogies to photography and printing are helpful here." When photographic film is exposed to patterns of light, a chemical pattern is encoded within the film. Later, the film is developed using several solvents that etch the exposed and unexposed regions differently to provide the image we see on the photographic negative. A very similar process is used by UMass Amherst researchers to pattern growth in gel sheets.

Santangelo and Hayward also borrowed an idea from the printing industry that allows them to make complicated patterns in a very simple way. In photolithography, just as in printing, it is expensive to print a picture using different color shades because each shade requires a different ink. Thus, most high-volume printing relies on "halftoning," in which only a few ink colors are used to print varied-sized dots. Smaller dots take up less space and allow more white light to reflect from the paper, so they appear as a lighter color shade than larger dots.

An important discovery by the UMass Amherst team is that this concept applies equally well to patterning the growth of their gel sheets. Rather than trying to make smooth patterns with many different levels of growth, they were able to simply print dots of highly restricted growth and vary the dot size to program a patterned shape.

"We're discovering new ways to plan or pattern growth in a soft polymer gel that's spread on a substrate to get any shape you want," Santangelo says. "By directly transferring the image onto the soft gel with half-tones of light, we direct its growth."

He adds, "We aren't sure yet how many shapes we can make this way, but for now it's exciting to explore and we're focused on understanding the process better. A model system like this helps us to watch how it unfolds. For biomedicine or bioengineering, one of the questions has been how to create tissues that could help to grow you a new blood vessel or a new organ. We now know a little more about how to go from a flat sheet of cells to a complex organism."

Provided by University of Massachusetts at Amherst

Smart, self-healing hydrogels open new possibilities in medicine, engineering

Hydrogels are made of linked chains of polymer molecules that form a flexible, jello-like material similar to soft-tissues. Until now, researchers have been unable to develop hydrogels that can rapidly repair themselves when a cut was introduced, limiting their potential applications. The team, led by Shyni Varghese, overcame this challenge with the use of "dangling side chain" molecules that extend like fingers on a hand from the primary structure of the hydrogel network and enable them to grasp one another.

"Self-healing is one of the most fundamental properties of living tissues that allows them to sustain repeated damage," says Varghese. "Being bioengineers, one question that repeatedly appeared before us was if one could mimic self-healing in synthetic, tissue-like materials such as hydrogels. The benefits of creating such an aqueous self-healing material would be far-reaching in medicine and engineering."

To design the side chain molecules of the hydrogel that would enable rapid self-healing, Varghese and her collaborators performed computer simulations of the hydrogel network. The simulations revealed that the ability of the hydrogel to self-heal depended critically on the length of the side chain molecules, or fingers, and that hydrogels having an optimal length of side chain molecules exhibited the strongest self-healing. When two cylindrical pieces of gels featuring these optimized fingers were placed together in an acidic solution, they stuck together instantly. Varghese's lab further found that by simply adjusting the solution's pH levels up or down, the pieces weld (low pH) and separate (high pH) very easily. The process was successfully repeated numerous times without any reduction in the weld strength.

Ameya Phadke, a fourth year PhD student in Varghese's lab said the hydrogel's strength and flexibility in an acidic environment – similar to that of the stomach – makes it ideal as an adhesive to heal stomach perforations or for controlled drug delivery to ulcers.

Such healing material could also be useful in the field of energy conservation and recycling where self-healing materials could help reduce industrial and consumer waste, according to Varghese. Additionally, the rapidity of self-healing in response to acids makes the material a promising candidate to seal leakages from containers containing corrosive acids. To test this theory, her lab cut a hole in the bottom of a plastic container, "healed" it by sealing the hole with the hydrogel and demonstrated that it prevented any leakage of acid through the hole.

Moving forward, Varghese and her lab hope to test the material in its envisioned applications on a larger scale. The team also hopes to engineer other varieties of hydrogels that self-heal at different pH values, thereby extending the applications of such hydrogels beyond acidic conditions.

Provided by University of California - San Diego (news : web)

Butterfly molecule may aid quest for nuclear clean-up technology

The distinctive butterfly-shaped compound is similar to radioactive molecules that scientists had proposed to be key components of nuclear waste, but were thought too unstable to exist for long.

Researchers have shown the compound to be robust, which implies that molecules with a similar structure may be present in radioactive waste.

Scientists at the University of Edinburgh, who carried out the study, say this suggests the molecule may play a role in forming clusters of radioactive material in waste that are difficult to separate during clean-up.

Improving treatment processes for nuclear waste, including targeting this type of molecule, could help the nuclear industry move towards cleaner power generation, in which all the radioactive materials from spent fuel can be recovered and made safe or used again. This would reduce the amount of waste and curb risks to the environment.

The Edinburgh team worked in collaboration with scientists in the US and Canada to verify the structure of the uranium compound. They made the molecule by reacting a common uranium compound with a nitrogen and carbon-based material. Scientists used chemical and mathematical analyses to confirm the structure of the molecule's distinctive butterfly shape.

The study, funded by the Engineering and Physical Sciences Research Council, the EaStCHEM partnership and the University of Edinburgh, was published in Nature Chemistry.

Professor Polly Arnold of the University of Edinburgh's School of Chemistry, who took part in the research, said: "We have made a molecule that, in theory, should not exist, because its bridge-shaped structure suggests it would quickly react with other chemicals. This discovery that this particular form of uranium is so stable could help optimise processes to recycle valuable radioactive materials and so help manage the UK's nuclear legacy."

Provided by University of Edinburgh

Exotic material shows promise as flexible, transparent electrode

The result could open this class of unusual materials, called topological insulators, to its first practical applications: flexible, transparent electrodes for solar cells, sensors and optical communications devices.

“It’s rare for a good conductor to be both transparent and durable as well,” said Zhi-Xun Shen of SLAC and Stanford’s Institute for Materials and Energy Sciences (SIMES).

Researchers led by Shen, Zhongfan Liu and Hailin Peng of Peking University in China, and Yulin Chen of Oxford University in England published their results last week in Nature Chemistry. Until recently, Peng and Chen were graduate students and postdoctoral researchers at Stanford and SIMES. They have continued to collaborate with Shen’s research team after being named professors at their current universities.

The basic structural unit for bismuth selenide is a five-layer sandwich made up of alternating single-atom sheets of selenium (orange) and bismuth (purple). Units are stacked on top of each other as thicker samples are made. The selenium-selenium bonds between the units are weak, allowing the overall material to flex durably without being damaged, unlike conventional electronic circuits. Credit: Hailin Peng, Peking University

The researchers made and tested samples of a compound in which sheets of bismuth and selenium, each just one atom thick, alternate to form five-layer units. The bonds between the units are weak, allowing the overall material to flex while retaining its durability. And as a topological insulator – a new state of quantum matter – the material conducts electricity only on its surface while its interior remains insulating, an unexpected property with unknown potential for fundamental research and practical applications.

Since surface atoms dominate the structure of bismuth selenide, it is an exceptionally good electrical conductor – as good as gold. Unlike gold, however, bismuth selenide is transparent to infrared light, which we know as heat. While about half the solar energy that hits the Earth comes in the form of  infrared light, few of today’s solar cells are able to collect it. The transparent electrodes on the surfaces of most cells are either too fragile or not transparent or conducting enough.  The new material could get around that problem and allow cells to harvest more of the sun’s spectrum of wavelengths.

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This informal in-lab video shows several different samples of bismuth selenide formed atop thin mica insulator substrates as they are tested for conductivity before and after flexing. In the first segment, the sample is placed on a piece of paper showing the Chinese characters for ?topological insulator.? In the second segment, the material?s conductivity is essentially unchanged when flexed. In the third segment, some 60 cycles of deep flexing causes only a very small change in the material?s conductivity. The researchers also used a mechanical rig (not shown) to bend the material 1,000 times reproducibly, after which the material still showed minimal deterioration in conductivity. Credit: Hailin Peng, Peking University.

The researchers’ experiments also showed that bismuth selenide does not degrade significantly in humid environments or when exposed to oxygen treatments that are common in manufacturing.

“In addition to being a scientific success,” Chen said, “this demonstration should alert engineers and companies that topological insulators can also be important commercially.”

Peng added, “Infrared light pulses carry phone calls and data through optical fiber networks, so bismuth selenide may be useful in communications devices. This material could also improve infrared sensors common in scientific equipment and aerospace systems.”

Peng and colleagues made the bismuth selenide samples and conducted the flexing, conductivity and transparency tests in China. The researchers confirmed that the samples were topological insulators at the Stanford Synchrotron Radiation Lightsource’s Beam Line 5-4 at SLAC.

Theorists first proposed topological insulators in 2004, and experimentalists made the first examples, using mercury telluride at very low temperatures, two years later. Guided by theory, Chen, Shen and colleagues proved in 2009 that cheaper, more abundant and easier-to-handle bismuth telluride and similar compounds containing antimony and selenium are topological insulators at room temperature. Also in 2009, Peng, Shen and colleagues discovered important electrical conduction behavior in bismuth selenide nanoribbons.

Provided by SLAC National Accelerator Laboratory (news : web)