Materials Science Blossoms in Boston

MRS Meeting

Winter arrived fashionably late to this year's Materials Research Society (MRS) annual meeting, giving conferees three balmy-for-Boston days before settling into the chilly drizzle they've come to expect from the event, held for the past 28 years in Beantown.

The season's tardiness was appropriate, though, as fashion took center stage at this year's meeting. During the afternoon breaks on the second and third days of the event, held from Nov. 28 to Dec. 2, attendees were treated to a geek-chic fashion show that featured "technologically advanced clothing made possible by materials research."

Professional models and MRS student members strutted side by side down the catwalk sporting silver-coated antibacterial textiles, denim jackets equipped with solar cells, and tank tops illuminated by nanostructured optical fibers attached to light-emitting diodes. In the words of Boston Globe reporter Hiawatha Bray, "All the clothes showed off recent developments in materials research, an unsexy but vital field in which scientists strive to invent materials with extraordinary physical properties."

Although hardly slaves to fashion, the meeting's 5,100 conferees would probably disagree with Bray's characterization of materials research. As the nearly 2,600 talks and more than 2,000 poster presentations demonstrated, there's plenty of alluring science going on in the materials world. As the following highlights suggest, chemists could find a bounty of interesting research in the meeting's 42 technical symposia, with talks on nanoparticles, electrochromics, and sensor-filled fabrics made by ink-jet printing.

• Tooth Whiteners Weaken Enamel
• Electrochromic Plastics For Cyclists And Skiers
• Sensor-Studded Textiles, Courtesy Of Ink-jet Printing
• Proofreading Nanoparticle Assemblies
• For Sale: 3BR/2BA With Power Windows
• Questioning Common Perceptions About Nanoparticle Toxicity

Tooth Whiteners Weaken Enamel

Brightening up your pearly whites may give you a healthier looking smile, but according to a new study, the bleaching process may actually be weakening your teeth. Michelle E. Dickinson, a staff scientist with Minneapolis-based instrument maker Hysitron, found that over-the-counter and professional dental bleaching solutions reduce the hardness of tooth enamel and weaken teeth's nanomechanical properties.

Before going to market, commercial tooth whiteners undergo safety testing as part of the approval process, and there have been several studies suggesting that whiteners may weaken teeth. Dickinson wondered if the testing methods these studies used were really addressing how bleaching agents may be affecting the tooth's surface as well as how consumers actually use tooth whiteners.

"My opinion was that maybe they haven't done the right tests," she explained. Conventional testing of tooth strength and hardness gives an average value taken over the entire tooth. Whitening products, however, only act upon a tooth's surface, where they oxidize discolored enamel.

Dickinson used nanoindentation-a way of testing mechanical properties over a very small area of material-to study how bleaching agents alter the top 100 nm of a tooth's surface.

Dickinson took extracted human teeth, coated them in clear nail polish, and then "opened little windows" in the varnish in different regions of the tooth's surface. "I decided to study two parts of the tooth," she said. "You only want one side of the tooth whitened-the part that everyone sees-but the functional chewing surface could be affected by the bleach as well."

She then treated the teeth with a 10% carbamide peroxide [CO(NH₂)₂•H₂O₂] solution she purchased from a local drugstore. That concentration is standard for over-the-counter whitening products, although they can go as high as 15%, Dickinson noted. The bleach's instructions called for twice-daily, hour-long treatments for two weeks, but she limited exposure to one hour a day for seven days.

To mimic the prescription whiteners that dentists use, Dickinson prepared a 35% carbamide peroxide treatment. "I made my own solution because no one would give me any," she said, adding that she consulted with dentists to get an accurate formulation. She also used this treatment one hour a day for a week, although she pointed out that dentists typically use longer bleaching times or accelerate the process with ultraviolet light.

Dickinson's tests showed a dramatic decrease in tooth hardness. The over-the-counter whitener decreased tooth hardness by 22% on both regions of the tooth. "I thought that was pretty significant," Dickinson told C&EN. Even more striking, the 35% carbamide peroxide solution modeled on the prescription treatment decreased the hardness of the tooth's chewing surface by a whopping 82%, from 5.09 gigapascals to 0.94 GPa, which, Dickinson said, is close to the hardness of dentin, the soft material beneath a tooth's enamel.

Dickinson also observed a change in the tooth's microstructure with increasing treatments. Originally, the tooth was polished and appeared to have a flat surface under a scanning probe microscope. After bleaching, the tooth's microstructure could be seen-evidence that some of the tooth material had eroded. Furthermore, she noted an extreme difference in nanowear properties. It took far less force to scrape portions of the whitened teeth versus unwhitened teeth.

Despite the remarkable changes in nanomechanical properties, Dickinson was quick to point out that damage from the over-the-counter whiteners was not all that different from what would be seen if a person "sipped on orange juice or cola all day." Teeth can recover from whitening treatments by remineralizing, she said. In the future, Dickinson hopes to redo the studies with fluoride-containing artificial saliva to get results that more closely mimic teeth's natural environment.

N. Dorin Ruse, a professor of dentistry at the University of British Columbia, Vancouver, told C&EN he found Dickinson's talk very interesting, although he pointed out that others have used different tools to show that whitening agents can weaken teeth. "Nevertheless, any article that raises awareness regarding possible detrimental effects of bleaching is a welcome paper," he said. "Public awareness, and dentists' as well, should be raised."

Dickinson acknowledged that her results are preliminary. Nevertheless, she thinks they indicate that nanomechanical testing should be de rigueur for approval of tooth-bleaching products.

This story appeared in Latest News, November 29, 2005

Electrochromic Plastics For Cyclists And Skiers

Motorcyclists and skiers may no longer have to squint at blinding sunlight as they speed around treacherous turns, thanks to an electrochromic "smart foil" that darkens in seconds and lightens just as quickly. Motorcycle helmet visors and ski goggles made with the material could be appearing on the market as early as next year, according to Claes-Göran Granqvist, one of the foil's inventors.

For decades, scientists have been trying to capitalize on electrochromic materials, which can reversibly change their optical properties with the application of an external voltage. Their ability to lighten and darken on command seems ideally suited for energy-efficient windows and certain eyeware applications. But electrochromic products have been slow to make it to market, primarily because of high manufacturing costs and low durability.

Now Granqvist, an engineering sciences professor at Sweden's Uppsala University, and coworkers have developed the first electrochromic products based on a flexible plastic substrate. Because these foils could be manufactured by inexpensive roll-to-roll technology, Granqvist thinks they may overcome the obstacles that have hamstrung other electrochromic products (Mater. Sci. Eng. B 2005, 119, 214).

Granqvist's group builds the electrochromic foils by taking two transparent polyester foils and coating them with an electrically conductive coating of indium tin oxide. Then they coat one of the foils with tungsten oxide and the other foil with nickel oxide. Finally, they sandwich an ion-conducting polymer between the two foil components.

"It's a little bit like a battery," Granqvist explained. The oxide coatings are akin to a battery's anode and cathode, and the polymer between them works like an electrolyte, shuttling ions from one to the other. The tungsten oxide coating is dark in its charged state and bleaches as it discharges. The nickel oxide coating, on the other hand, is transparent in its charged state and darkens as it discharges.

Applying just 1.4 V to Granqvist's device, via a switch, photodetector, or voice control, charges the tungsten oxide, discharges the nickel oxide, and darkens the foil. Reversing the voltage turns the material transparent. The material has a "memory," Granqvist said, so that only a voltage needs to be applied to change the material's properties. He also pointed out that the foils don't just go from dark to light but can take on intermediate shades.

ChromoGenics, a company that grew out of Granqvist's research, has already made prototype motorcycle helmet visors and ski goggles using the smart foils. "I have tried the ski goggles in a snowfield, and it's really amazing," Granqvist said. Without any eye protection, you have to squint hard in the bright glare of the snow, but, he added, "if you put on the ski goggles, your whole face relaxes. It's very comfortable."

Granqvist thinks the material's most promising applications will be in smart windows that moderate light to keep buildings cool in the summer and warm in the winter. "Even in Sweden, most commercial buildings are cooled most of the time," he joked. Granqvist envisions a day when we'll turn off our windows as easily as we turn off our lights.

Earlier this month, ChromoGenics secured $2.2 million in funding from Volvo and "a very large U.S. company" that Granqvist declined to name. The money will be used to set up a pilot plant for making the foils.

This story appeared in Latest News, November 30, 2005

Sensor-Studded Textiles, Courtesy Of Ink-jet Printing

From an engineering perspective, a body's exterior-whether human, animal, or insect-is basically a collection of millions of stress and strain sensors that follow movement and give the organism information on its surroundings. Even the humble spider monitors its environment by using thousands of tiny sensing hairs, which detect vibrations in its skeleton and in the air.

Paul Calvert, a professor in the materials and textiles department at the University of Massachusetts, Dartmouth, thought that it should be possible to make textiles that likewise have many thousands of sensors. This kind of smart clothing could monitor patients' vital signs as they stroll around the hospital. Physical therapists could use information from smart textiles around joints to teach athletes and rehabilitation patients to move without straining delicate knees, elbows, and ankles.

To create sensor-filled textiles, Calvert employed his expertise in using ink-jet printers to fabricate electronic devices. "Think of ink-jet printing as a tool that lets you do chemistry on surfaces," Calvert said. "What we wanted to do was to print highly conducting lines onto a textile."

Calvert's group formed conducting "wires" throughout the fabric by printing lines of an aqueous solution of silver nitrate onto a woven fabric. Each line was printed over about 500 times to accumulate an ample amount of silver nitrate into the textile. They then dipped the fabric in a reducing electroless plating bath of silver, ammonia, and glucose.

"The glucose reduces the silver ions in solution on the existing seed layer by an autocatalytic process to produce a metal line," Calvert explained.

PEDOT-PSS [poly(3,4-ethylenedioxythiophene)-poly(4-styrene sulfonate)] conducting polymer is then printed between the silver lines on the fabric. The chemicals don't just sit atop the fabric, Calvert noted. Rather, they penetrate into the textile's fibers. This way, the conductive system is protected against weather, dirt, and washings. The PEDOT-PSS polymer is piezoresistive-its resistance changes with strain. The silver carries the signal to an external detector that records the data.

Calvert has already used the fabric to sense motions like the bending of a knee and the twisting of a wrist. The next step, he said, is to develop programming that can deal with all the data streams the textile provides. "If something as dumb as a spider can deal with these data streams, certainly we can figure out a way to," he joked.

In the future, Calvert envisions incorporating a wireless antenna and a power supply within the textiles. "Ultimately, we would like to be able to ink-jet print all of the components into the fabric." He also imagines that biomedical implants equipped with a sensing system could be made in a similar manner. "I think to have a medical implant that's actually feeding data back to the outside world could be very useful," Calvert said.

Proofreading Nanoparticle Assemblies

Mistakes are inevitable. but it only takes a tiny mistake in a nanoparticle assembly to render molecular electronics, photonics, or computational devices built with the technology useless. Most nanoscientists have tried to tackle this problem by optimizing assembly processes. University of Illinois, Urbana-Champaign, chemistry professor Yi Lu had an entirely different idea.

Instead of trying to avoid defects, Lu thought it made more sense to accept defects in the assembly process and then use some type of editor that finds and removes these defects. He took his inspiration from biological processes, such as messenger RNA-templated protein synthesis, where a transfer RNA synthetase proofreads and hydrolyzes incorrectly incorporated amino acids.

"Nature still makes the best materials," Lu said, "so let's learn from nature." For several years, scientists have been using DNA to string nanoparticles together. In this DNA-templated nanoparticle assembly process, single strands of DNA are covalently attached to nanoparticles. A complementary strand of DNA acts as a template, holding the particles together by forming the classical DNA double helix.

As much as DNA-templated assembly has advanced, the technique still has drawbacks, Lu noted. Sometimes, the synthesized DNA strand is too short or incorrect nucleotides are accidentally incorporated into the molecule. Mistakes like these lead to defects in the nanoparticle assembly.

Lu reasoned that by using DNAzymes-DNA molecules that catalyze reactions as enzymes do-his group would be able to detect mistakes in the assembly and remove them. The DNAzyme works like a proofreader. Part of the molecule coordinates with a portion of the template DNA. When nanoparticles with shortened or mismatched DNA hook up with the template, the catalytic region of the DNAzyme loops around, and upon addition of lead ions, catalyzes removal of the incorrect nanoparticle via cleavage of the phosphodiester.

Alternatively, if a nanoparticle with DNA of the appropriate length and sequence joins onto the template, there's no space for the DNAzyme to loop around and the active structure of the catalytic DNA can't form. "As a result, the template is not cleaved and the particle is incorporated into the assembly," Lu explained.

Lu and his team, postdoc Juewen Liu and graduate student Daryl P. Wernette, found they could use the DNAzyme system to proofread nanoparticle assemblies both during and after the assembly process. "This was a small, but definite, step in the right direction," Lu said, adding that the work was intended to be a proof-of-concept demonstration (Angew. Chem. Int. Ed. 2005, 44, 7290).

In the future, Lu hopes to use the proofreading technique to assemble other types of nanomaterials, such as quantum dots and nanotubes. He also hopes to expand the error-correcting technique so that other, less expensive biomolecules and biomimetic systems can be used to control nanomaterial assemblies. "Introducing concepts from biology can really make a difference in nanotechnology," he said.

For Sale: 3BR/2BA With Power Windows

The days of the zippy roadster are gone. Americans, it seems, don't want to drive cars anymore. They want to drive houses-big sport-utility vehicles, equipped with plush seats, TVs, and DVD players. Architect Craig Konyk took note of the 21st-century automobile's trend toward domesticity and decided to turn it on its head.

Konyk, head of Konyk Architecture and an architecture professor at Columbia University and the Parsons School of Design, New York City, is the visionary behind the UP!house, a house that's built like a car. "With options ranging from power windows to home theater entertainment packages," Konyk explained, "the UP!house is an attempt to make the purchase of the single-family house more akin to that of the latest model Volkswagen."

The UP!house originally grew out of a project to create affordable, design-savvy prefab housing. Given the automotive industry's ability to produce millions of automobiles that are roughly the size of a Manhattan studio apartment, Konyk decided to adopt similar materials and manufacturing processes.

Like a car, the UP!house is built upon a chassis made of a factory-welded, lightweight unibody steel. The house's exterior-of tinted glass, Kalwall panels, and a Formawall "skin"-sits on the chassis. "The chassis is always the same, but the body might evolve," Konyk noted. Homeowners could freshen up the look of their house by simply replacing this exterior shell. The design allows the homeowner to add or remove sections of the house with relative ease. And the entire structure can be disassembled and reassembled in a new location.

Konyk told C&EN that commercial availability of several materials was integral to the UP!house's design. "All architects owe a debt to the chemists who improve the specifications of the products we use," he said.

The exterior end panels of Kalwall-a lightweight, translucent fiberglass composite bonded to an aluminum frame-insulate the house but let natural light filter in. Konyk also pointed out the Formawall skin. This highly insulating material is composed of a 3-inch, high-density-polyurethane-bonded sandwich panel. Konyk said it's the same stuff used to make walk-in refrigerators in restaurants.

UP!house owners can choose from one of 24 fashionable colors, such as moss or Ferrari red, for the Formawall. The colors won't fade for at least 20 years, Konyk added, thanks to a polyvinylidene fluoride coating and an ultraviolet-resistant finish. While these coatings are applied in the factory, Konyk noted that PPG has recently introduced a similar coating that can be applied in the field and could be added on as home maintenance to renew the factory coating.

The house's interior also makes good use of chemistry. Polycarbonate panel walls and ceilings hide the home's plumbing and wiring. Floor and cabinets coated with high-gloss white epoxy coating give the sensation of living in an iPod.

Once ready to roll off the assembly line, an UP!house would cost about $350,000, appliances included, according to Konyk's estimate. It should go from factory to backyard in about 10 weeks, with another two weeks for on-site assembly. While there's been plenty of interest in the structure, at the moment the UP!house exists only in cyberspace. Konyk hopes to raise about $750,000 to build a prototype.

Questioning Common Perceptions About Nanoparticle Toxicity

The conventional wisdom about the toxicity of small particles is that the smaller the particle, the more toxic it is. Surface area increases as particle size decreases, and the greater the surface area, the greater the number of exposed reactive groups. Or so the logic goes.

The notion is practically dogma among those concerned with toxicity of nanoparticles, but David B. Warheit, a toxicologist at DuPont in Newark, Del., argues that that conclusion is based on just a handful of studies. "We can't just assume because it's nano it's going to be more toxic than a larger particle," he said.

While some studies have shown that nanoparticles are more toxic than larger fine particulate matter of the same composition, Warheit noted that these reports only examine a handful of materials. At the moment, he said, there simply isn't enough data to conclude that decreased particle size leads to increased toxicity for all materials.

"Particle size is not the only variable that affects toxicity," Warheit added. He listed nine other factors-shape, charge, chemical composition, surface area, crystal structure, presence of surface coatings, aggregation properties, particle number, and method of synthesis-that can also influence how a material behaves in the body.

Warheit presented some preliminary pulmonary toxicology studies in which he compared the effects of fine-sized particles to nanoparticles of the same composition. In one experiment, he instilled different types of TiO₂ particles into the lungs of rats-an experiment that approximates what happens when particles are inhaled. He found that nanoscale particles of TiO₂ were no more toxic than larger fine-sized particles.

Warheit also examined the effect of particle shape on pulmonary toxicity. TiO₂ nanorods and TiO₂ nanodots exhibited no significant difference in toxicity when compared with the 300-nm fine-sized particles.

The pigment-grade titania that Warheit used is a low-toxicity material. He wondered if the effects would be any different if he did similar experiments using quartz-crystalline silica-a material that's known to be toxic. As expected, the 1.6-µm fine-sized quartz particles were toxic, but the nanoscale quartz particles gave some peculiar results. Particles that were 50 or 130 nm in size were less toxic to the lungs than the fine-sized particles, but 10-nm particles were more toxic.

"Maybe this experiment raises more questions than it answers," Warheit said. "It just goes to show that you can't generalize from just a couple of studies."

Warheit also emphasized that his studies are hazard studies, as opposed to risk studies, a distinction that's often misunderstood. A material can be hazardous but still be classified as low risk if there's little chance of exposure. To call for a moratorium on nanoparticle research, as some activist groups have done, fails to acknowledge that level of exposure defines how potentially dangerous a material can be, Warheit added. "That's the reason it's important to have good product stewardship."