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

Future of inks, paints and coatings takes shape: Researchers determine that particle shape affects the 'coffee ring effect'

 If you've ever spilled a drop of coffee on a surface, you might have noticed the curious way the color concentrates at the edges when the coffee dries. This is known as the "coffee ring effect," and recently, researchers have determined that the shape of the particles in the liquid is an important factor in creating this pattern. The research results could eventually translate into new techniques or formulations for product coatings, or better inks and paints.

This work, published in the August 18 issue of the journal Nature was performed by Arjun Yodh and colleagues at the University of Pennsylvania.

"We found that if you change the shape of the particles in the solution, the coffee ring effect goes away, and you end up with a uniform coating," said Peter Yunker, a graduate student in Yodh's lab.

First, a little fluid dynamics: As the liquid in a droplet evaporates the edges remain fixed, so as the volume decreases fluid flows outward from the middle of the droplet to its edges. This flow carries particles to the edges, and round particles at the edge will pack closely. By the time all of the liquid in the droplet evaporates, most of the particles will be at the edge, producing the coffee ring effect.

Both the shape that liquid droplets take, and the way the shape changes as the droplets evaporate, is greatly influenced by surface tension at the air-liquid interface. This tension is a property of the interface, based on how the molecules in the liquid interact with one another versus the air. For example, liquids with a high surface tension, like water, may form a raised droplet, because the molecules are very attracted to one another and not so attracted to the air. In contrast, liquids with lower surface tension, like alcohols, are more likely to form flat spots instead of curved droplets.

The Yodh group found that elongated particles in a liquid behave differently than round ones because of the way they are affected by the surface tension of the air-liquid interface. The forces at work are even observable in a common breakfast cereal.

"If you make the particles elongated or ellipsoidal, they deform the air-water interface, which causes the particles to strongly attract one another. You can observe this effect in a bowl of cheerios-if there are only a few left they clump together in the middle of the bowl, due to the surface tension of the milk," explained Yunker.

This clumping changes the way the particles distribute themselves within the droplet. Even if the clumped ellipsoidal particles reach the edge of the droplet, they do not pack as closely as round particles. The loosely packed clumps eventually spread to cover the entire surface, filling it so an even coating of particles is deposited when evaporation is complete.

"This work gives us a new idea about how to make a uniform coating, relatively simply. If you change the particle shape, you can change the way a particle is deposited. You can also make mixtures. In some cases, even just a small amount of ellipsoids can change the way the particles deposit when they dry," said Yodh.

In future studies, the research team will explore drying and deposition of different types of fluids. They will also investigate different particle sizes and shapes, and the interplay of particle mixtures.

"This is an exciting scientific result with potential commercial applications, which was in part enabled by support of the Materials Research Science and Engineering Center at the University of Pennsylvania," said Mary Galvin, program director for the division of materials research at the National Science Foundation, which partially funded the research. The centers program, recently renamed Materials Research Centers and Teams, provides support for interdisciplinary materials research and education while addressing fundamental problems in science and engineering.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by National Science Foundation.

Journal Reference:

Peter J. Yunker, Tim Still, Matthew A. Lohr, A. G. Yodh. Suppression of the coffee-ring effect by shape-dependent capillary interactions. Nature, 2011; 476 (7360): 308 DOI: 10.1038/nature10344

Nano fit-ness: Helping enzymes stay active and keep in shape

Proteins are critically important to life and the human body. They are also among the most complex molecules in nature, and there is much we still don't know or understand about them.

One key challenge is the stability of enzymes, a particular type of protein that speeds up, or catalyzes, chemical reactions. Taken out of their natural environment in the cell or body, enzymes can quickly lose their shape and denature. Everyday examples of enzymes denaturing include milk going sour, or eggs turning solid when boiled.

Rensselaer Polytechnic Institute Professor Marc-Olivier Coppens has developed a new technique for boosting the stability of enzymes, making them useful under a much broader range of conditions. Coppens confined lysozyme and other enzymes inside carefully engineered nanoscale holes, or nanopores. Instead of denaturing, these embedded enzymes mostly retained their 3-D structure and exhibited a significant increase in activity.

"Normally, when you put an enzyme on a surface, its activity goes down. But in this study, we discovered that when we put enzymes in nanopores – a highly controlled environment – the enzymatic activity goes up dramatically," said Coppens, a professor in the Department of Chemical and Biological Engineering at Rensselaer. "The enzymatic activity turns out to be very dependent on the local environment. This is very exciting."

Results of the study are detailed in the paper, "Effects of surface curvature and surface chemistry on the structure and activity of proteins adsorbed in nanopores," published last month by the journal Physical Chemistry Chemical Physics.

Researchers at Rensselaer and elsewhere have made important discoveries by wrapping enzymes and other proteins around nanomaterials. While this immobilizes the enzyme and often results in high stability and novel properties, the enzyme's activity decreases as it loses its natural 3-D structure.

Coppens took a different approach, and inserted enzymes inside nanopores. Measuring only 3-4 nanometers (nm) in size, the enzyme lysozyme fits snugly into a nanoporous material with well-controlled pore size between 5 nm and 12 nm. Confined to this compact space, the enzymes have a much harder time unfolding or wiggling around, Coppens said.

The discovery raises many questions and opens up entirely new possibilities related to biology, chemistry, medicine, and nanoengineering, Coppens said. He envisions this technology could be adapted to better control nanoscale environments, as well as increase the activity and selectivity of different enzymes. Looking forward, Coppens and colleagues will employ molecular simulations, multiscale modeling methods, and physical experiments to better understand the fundamental mechanics of confining enzymes inside nanopores.

More information: The paper may be viewed online at: http://dx.doi.org/10.1039/C0CP02273J

Provided by Rensselaer Polytechnic Institute (news : web)

Nano Fitness: Helping enzymes stay active and keep in shape

Proteins are critically important to life and the human body. They are also among the most complex molecules in nature, and there is much we still don't know or understand about them.

One key challenge is the stability of enzymes, a particular type of protein that speeds up, or catalyzes, chemical reactions. Taken out of their natural environment in the cell or body, enzymes can quickly lose their shape and denature. Everyday examples of enzymes denaturing include milk going sour, or eggs turning solid when boiled.

Rensselaer Polytechnic Institute Professor Marc-Olivier Coppens has developed a new technique for boosting the stability of enzymes, making them useful under a much broader range of conditions. Coppens confined lysozyme and other enzymes inside carefully engineered nanoscale holes, or nanopores. Instead of denaturing, these embedded enzymes mostly retained their 3-D structure and exhibited a significant increase in activity.

"Normally, when you put an enzyme on a surface, its activity goes down. But in this study, we discovered that when we put enzymes in nanopores -- a highly controlled environment -- the enzymatic activity goes up dramatically," said Coppens, a professor in the Department of Chemical and Biological Engineering at Rensselaer. "The enzymatic activity turns out to be very dependent on the local environment. This is very exciting."

Results of the study were published last month by the journal Physical Chemistry Chemical Physics.

Researchers at Rensselaer and elsewhere have made important discoveries by wrapping enzymes and other proteins around nanomaterials. While this immobilizes the enzyme and often results in high stability and novel properties, the enzyme's activity decreases as it loses its natural 3-D structure.

Coppens took a different approach, and inserted enzymes inside nanopores. Measuring only 3-4 nanometers (nm) in size, the enzyme lysozyme fits snugly into a nanoporous material with well-controlled pore size between 5 nm and 12 nm. Confined to this compact space, the enzymes have a much harder time unfolding or wiggling around, Coppens said.

The discovery raises many questions and opens up entirely new possibilities related to biology, chemistry, medicine, and nanoengineering, Coppens said. He envisions this technology could be adapted to better control nanoscale environments, as well as increase the activity and selectivity of different enzymes. Looking forward, Coppens and colleagues will employ molecular simulations, multiscale modeling methods, and physical experiments to better understand the fundamental mechanics of confining enzymes inside nanopores.

The study was co-authored by Lung-Ching Sang, a former Rensselaer graduate student in the Department of Chemical and Biological Engineering.

This research was supported by the National Science Foundation, via the Nanoscale Science and Engineering Center for Directed Assembly of Nanostructures at Rensselaer. The project was also supported by the International Center for Materials Nanoarchitectonics of the National Institute for Materials Science, Japan.

Story Source:

The above story is reprinted  from materials provided by Rensselaer Polytechnic Institute.

Journal Reference:

Lung-Ching Sang, Marc-Olivier Coppens. Effects of surface curvature and surface chemistry on the structure and activity of proteins adsorbed in nanopores. Physical Chemistry Chemical Physics, 2011; 13 (14): 6689 DOI: 10.1039/C0CP02273J