Wednesday, March 28, 2012

New gecko insights inspire even stronger adhesives

But researchers at the University of Massachusetts Amherst have made the connection.

First they showed  the previously unappreciated role of geckos' tendons and bones in the little lizards' ability to climb up walls without slipping. Then they used that knowledge -- plus a large helping of human ingenuity -- to create an adhesive device that can hold the television securely on a wall.

"Our 'Geckskin' device is about the size of an index card and can hold a maximum force of about 700 pounds while adhering to a smooth surface such as glass," said Alfred Crosby, associate professor of polymer science and engineering at the University of Massachusetts Amherst.

To produce it, Crosby added, "We focused on the properties and attributes of the gecko: high capacity, easy release, reliability, and the ability to stick to a variety of surfaces."

"This is definitely an important contribution," said Metin Sitti, professor of mechanical engineering at Carnegie Mellon University and an expert on small-scale locomotion and manipulation, who did not participate in the project.

Crosby carried out the research with his doctoral candidate Michael Bartlett and biology professor Duncan Irschick, with support from the Pentagon's Defense Advanced Projects Research Agency.

Scientists have long recognized that so-called van der Waals forces, which produce weak electrical attraction among molecules, cause adhesion between tiny hairs in geckos' toes, known as setae, and vertical surfaces on which the climb.

However, efforts to apply that process on a large scale have had limited success. Scotch tape gains its stickiness through the van der Waals forces.

"But you can't make the forces stronger," Crosby said. "People have tried to produce artificial setae," Irschick added. "But they don't scale up effectively."

To develop a different approach, the UMass team studied the large-scale structure of geckos' feet.

Expanding on research by University of Calgary biologist Anthony Russell, the team discovered how tendons, bones, and skin work together to produce the easily reversible adhesion that causes a gecko's feet to stick to a wall briefly and then release from it as the tiny lizard moves up, down, or sideways on the wall. The process works in large part because of the role of the tendons. In most creatures, tendons connect bones to muscles.

"But in geckos' feet, uniquely, the tendons stretch from bone into skin," Irschick explained.

The group used that knowledge as the basis of an adhesive system stronger than any relying on van der Waals forces.

"We wanted something that would cover a large area and would become increasingly stiff," Crosby recalled. "But those demands are contradictory."

Scotch tape, for example, covers a large area, but is soft and thus unable to hold significant weight. The geckos' anatomy suggested that the team could overcome the contradiction by using a specially treated fabric. A fabric can be both soft and stiff. A tablecloth, for instance, can drape over a table and conform to the shape of anything underneath it while remaining stiff if you try to pull it. For their Geckskin, the researchers mimicked the anatomy of geckos' feet.

"We took a fabric, put a bit of rubber around it, and sewed another piece of fabric -- the 'tendon' -- into that 'skin'," Crosby explained.

Since the fabric is stiff and the rubber soft, the combination yields a stiff but flexible system that drapes over a large surface area, permitting maximum contact and adhesion.

Geckskin's strength does not apply in all directions. While it is almost impossible to move it along any surface on which it is mounted, Crosby said, "a gentle peel from one edge allows it to be effortlessly removed from the surface on command."

It can be removed and stuck onto another surface as often as needed without leaving any residue or losing adhesive strength.

The team has used a variety of ingredients for the rubber component. In particular polydimethylsiloxane, a component of silly putty, holds the promise, in combination with fabric, of developing an inexpensive, strong, and durable dry adhesive.

The researchers also tried a variety of fabrics.

"Those with the greatest load capacity use the fibers such as Kevlar and fiber-based fabrics that are most stiff," Crosby said.

According to Crosby, Geckskin stacks up well against current commercial adhesives.

"The force per area is definitely higher than all the pressure-sensitive ," Crosby said. "The combination of high force and user release is not there in available adhesive systems. And unlike Velcro, Geckskin doesn't need a matching surface."

The team, which reported its advance in the journal Advanced Materials, is now discussing possible commercialization of the technology. 

Source: Inside Science News Service (news : web)

Study finds how bacteria resist a 'Trojan horse' antibiotic

The study appears in the Proceedings of the National Academy of Sciences.

Bacteria often engage in with one another, and many antibiotics used in medicine are modeled on the they produce. But also must protect themselves from their own toxins. The defenses they employ for protection can be acquired by other species, leading to antibiotic resistance.

The researchers focused on an enzyme, known as MccF, that they knew could disable a potent "Trojan horse" antibiotic that sneaks into disguised as a tasty protein meal. The bacterial antibiotic, called microcin C7 (McC7) is similar to a class of drugs used to treat bacterial infections of the skin.

"How antibiotics work is that the antibiotic portion is coupled to something that's fairly innocuous – in this case it's a peptide," said University of Illinois biochemistry professor Satish Nair, who led the study. "So susceptible bacteria see this peptide, think of it as food and internalize it."

The meal comes at a price, however: Once the bacterial enzymes chew up the amino acid disguise, the liberated antibiotic is free to attack a key component of protein synthesis in the bacterium, Nair said.

"That is why the organisms that make this thing have to protect themselves," he said.

In previous studies, researchers had found the genes that protect some bacteria from this class of antibiotic toxins, but they didn't know how they worked. These genes code for peptidases, which normally chew up proteins (polypeptides) and lack the ability to recognize anything else.

Before the new study, "it wasn't clear how a peptidase could destroy an antibiotic," Nair said.

To get a fuller picture of the structure of the peptidase, Illinois graduate student Vinayak Agarwal crystallized MccF while it was bound to other molecules, including the antibiotic. An analysis of the structure and its interaction with the antibiotic revealed that MccF looked a lot like other enzymes in its family, but with a twist – or, rather, a loop. Somehow MccF has picked up an additional loop of amino acids that it uses to recognize the antibiotic, rendering it ineffective.

"Now we know that specific amino acid residues in this loop are responsible for making this from a normal housekeeping gene into something that's capable of degrading this class of antibiotics," Nair said.

With this information, researchers – and eventually, doctors and other clinicians – will be able to scan the genomes of disease-causing bacteria to find out which ones have genes with the loop in them, Nair said. "If we know what type of are causing an infection we know what kind of antibiotic to give and what kind not to give," he said.

Nair is also an affiliate of the Center for Biophysics and Computational Biology, the department of chemistry and of the Institute for Genomic Biology at Illinois. The research team included scientists from the Russian Academy of Sciences and Rutgers University.

Provided by University of Illinois at Urbana-Champaign (news : web)

Nano rescues skin: Shrimp shell nanotech for wound healing and anti-aging face cream

 Nanoparticles containing chitosan have been shown to have effective antimicrobial activity against Staphylococcus saprophyticus and Escherichia coli. The materials could be used as a protective wound-healing material to avoid opportunistic infection as well as working to facilitate wound healing.

Chitosan is a natural, non-toxic and biodegradable, polysaccharide readily obtained from chitin, the main component of the shells of shrimp, lobster and the beak of the octopus and squid. Its antimicrobial activity is well known and has been exploited in dentistry to prevent caries and as preservative applications in food packaging. It has even been tested as an additive for antimicrobial textiles used in clothing for healthcare and other workers.

Now, Mihaela Leonida of Fairleigh Dickinson University, in Teaneck, New Jersey and colleagues writing in the International Journal of Nano and Biomaterials describe how they have prepared nanoparticles of chitosan that could have potential in preventing infection in wounds as well as enhancing the wound-healing process itself by stimulating skin cell growth.

The team made their chitosan nanoparticles (CNP) using an ionic gelation process with sodium tripolyphosphate. This process involves the formation of bonds between polymers strands, a so-called cross-linking process. Conducted in these conditions it precludes the need for complex preparative chemistry or toxic solvents. CNP can also be made in the presence of copper and silver ions, known antimicrobial agents. The researchers' preliminary tests show the composite materials to have enhanced activity against two representative types of bacteria.

Understanding the mechanism of inhibition of bacteria by these particles may lead to the preparation of more effective antibacterial agents. The team has also demonstrated that the CNP have skin regenerative properties in tests on skin cell fibroblasts and keratinocytes, in the laboratory, which might even have implications for anti-aging skin care products.

Story Source:

The above story is reprinted from materials provided by Inderscience Publishers, via EurekAlert!, a service of AAAS.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Mihaela D. Leonida; Sudeep Banjade; Thong Vo; Gloria Anderle; Gerhard J. Haas; Neena Philips. Nanocomposite materials with antimicrobial activity based on chitosan. International Journal of Nano and Biomaterials, 2012 DOI: 10.1504/IJNBM.2011.045885

Molecular graphene heralds new era of 'designer electrons'

Researchers from Stanford University and the U.S. Department of Energy's SLAC National Accelerator Laboratory have created the first-ever system of "designer electrons" -- exotic variants of ordinary electrons with tunable properties that may ultimately lead to new types of materials and devices.

"The behavior of electrons in materials is at the heart of essentially all of today's technologies," said Hari Manoharan, associate professor of physics at Stanford and a member of SLAC's Stanford Institute for Materials and Energy Sciences, who led the research. "We're now able to tune the fundamental properties of electrons so they behave in ways rarely seen in ordinary materials."

Their first examples, recently reported in Nature, were hand-crafted, honeycomb-shaped structures inspired by graphene, a pure form of carbon that has been widely heralded for its potential in future electronics. Initially, the electrons in this structure had graphene-like properties; for example, unlike ordinary electrons, they had no mass and traveled as if they were moving at the speed of light in a vacuum. But researchers were then able to tune these electrons in ways that are difficult to do in real graphene.

To make the structure, which Manoharan calls molecular graphene, the scientists use a scanning tunneling microscope to place individual carbon monoxide molecules on a perfectly smooth copper surface. The carbon monoxide repels the free-flowing electrons on the copper surface and forces them into a honeycomb pattern, where they behave like graphene electrons.

To tune the electrons' properties, the researchers repositioned the carbon monoxide molecules on the surface; this changed the symmetry of the electron flow. In some configurations, electrons acted as if they had been exposed to a magnetic or electric field. In others, researchers were able to finely tune the density of electrons on the surface by introducing defects or impurities. By writing complex patterns that mimicked changes in carbon-carbon bond lengths and strengths in graphene, the researchers were able to restore the electrons' mass in small, selected areas.

"One of the wildest things we did was to make the electrons think they are in a huge magnetic field when, in fact, no real field had been applied,"Manoharan said. Guided by the theory developed by co-author Francisco Guinea of Spain, the Stanford team calculated the positions where carbon atoms in graphene should be to make its electrons believe they were being exposed to magnetic fields ranging from zero to 60 Tesla, more than 30 percent higher than the strongest continuous magnetic field ever achieved on Earth. The researchers then moved carbon monoxide molecules to steer the electrons into precisely those positions, and the electrons responded by behaving exactly as predicted -- as if they had been exposed to a real field.

"Our new approach is a powerful new test bed for physics," Manoharan said. "Molecular graphene is just the first in a series of possible designer structures. We expect that our research will ultimately identify new nanoscale materials with useful electronic properties."

Additional authors included Kenjiro K. Gomes, Warren Mar and Wonhee Ko of the Stanford Institute for Material and Energy Sciences. Francisco Guinea is a researcher at the Madrid Materials Science Institute. The research was supported by the U.S. Department of Energy's Office of Basic Energy Sciences, the National Science Foundation and the Spanish Ministry of Science & Innovation.

Story Source:

The above story is reprinted from materials provided by DOE/SLAC National Accelerator Laboratory.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Kenjiro K. Gomes, Warren Mar, Wonhee Ko, Francisco Guinea, Hari C. Manoharan. Designer Dirac fermions and topological phases in molecular graphene. Nature, 2012; 483 (7389): 306 DOI: 10.1038/nature10941

Killer silk: Making silk fibers that kill anthrax and other microbes in minutes

Rajesh R. Naik and colleagues explain that in adverse conditions, bacteria of the Bacillus species, which includes anthrax, become dormant , enclosing themselves in a tough coating. These spores can survive heat, radiation, antibiotics and harsh environmental conditions, and some have sprung back to life after 250 million years. Certain chemicals — most popular among which are oxidizing agents, including some chlorine compounds — can destroy bacterial spores, and they have been applied to fabrics like cotton, polyester, nylon and Kevlar. These treated fabrics are effective against many bacteria, but less so against spores. The researchers tried a similar coating on to see if it could perform better against these hardy microbes.

They developed a chlorinated form of silk, which involves soaking silk in a solution that includes a substance similar to household bleach and letting it dry. Silk treated for just an hour killed essentially all of the E. coli bacteria tested on it within 10 minutes and did similarly well against spores of a close relative used as a stand-in. "Given the potent bactericidal and sporicidal activity of the chlorinated silk fabrics prepared in this study, silk-Cl materials may find use in a variety of applications," the authors say. Other applications, they add, include purifying water in humanitarian relief efforts and in filters or to mitigate the effects of toxic substances.

More information: Sporicidal/Bactericidal Textiles via the Chlorination of Silk, ACS Appl. Mater. Interfaces, Article ASAP, DOI: 10.1021/am2018496

Bacterial spores, such as those of the Bacillus genus, are extremely resilient, being able to germinate into metabolically active cells after withstanding harsh environmental conditions or aggressive chemical treatments. The toughness of the bacterial spore in combination with the use of spores, such as those of Bacillus anthracis, as a biological warfare agent necessitates the development of new antimicrobial textiles. In this work, a route to the production of fabrics that kill bacterial spores and cells within minutes of exposure is described. Utilizing this facile process, unmodified silk cloth is reacted with a diluted bleach solution, rinsed with water, and dried. The chlorination of silk was explored under basic (pH 11) and slightly acidic (pH 5) conditions. Chloramine-silk textiles prepared in acidified bleach solutions were found to have superior breaking strength and higher oxidative Cl contents than those prepared under caustic conditions. Silk cloth chlorinated for ?1 h at pH 5 was determined to induce >99.99996% reduction in the colony forming units of Escherichia coli, as well as Bacillus thuringiensis Al Hakam (B. anthracis simulant) spores and cells within 10 min of contact. The processing conditions presented for silk fabric in this study are highly expeditionary, allowing for the on-site production of protein-based antimicrobial materials from a variety of agriculturally produced feed-stocks.

Provided by American Chemical Society (news : web)