One-Upping Nature in a Quest for New Materials

By Monte Basgall, Office of News and Communications

Taking their inspiration from the “soft and wet” natural world, engineers and scientists are designing new tools and devices that aim at practical applications. The goal is to “reverse engineer” scores of millions of years of natural evolution.

Over this span, molecules have assembled themselves into cells and cells have organized into plants, animals and the complex biomechanisms necessary to support life. Now, in a promising new initiative, interdisciplinary teams of engineers, chemists, biologists and physicists are asking whether humans can mechanistically dissect some of nature’s ideas in order to borrow from them in fashioning future products.

Humans have followed some of biology’s blueprints in the past, perhaps most notably in emulating the shapes of bird wings to learn the secrets of flight. But the ambitions of Duke’s new Center for Biologically Inspired Materials and Material Systems (CBIMMS) are much broader and in some ways even more bold. This initiative, based in the Pratt School of Engineering, seeks to use some of biology’s materials and lilliputian self-assembly methods to design and build some strikingly different kinds of devices.

These devices might include molecule-sized “nanotechnology” machines and electrical components, and heat-triggered drug delivery systems far smaller than blood cells. Tools developed during this basic research effort could help biological and medical investigators learn more about how nature did it first, and how to “engineer around” bioprocesses that can cause disease.

Learning How Nature Works

About 30 investigators at Duke, plus nearly 20 at other institutions or companies in the United States and Europe, are involved in research at CBIMMS. All of them are taking an engineer’s look at nature in order to learn how to improve on its designs for human uses, says Robert L. Clark, the center’s director, who also is the Jeffrey N. Vinik Professor of Mechanical Engineering and senior associate dean for research at the Pratt school.

With the sophisticated tools already being developed by the researchers, “we can pull apart a biological system and see how it works,” Clark says. “We may get some ideas in that process, but we’re not going to try to do it in the same way. We don’t have millions of years over which to evolve our designs. What we have learned from biologists is that biology operates on the ‘just good enough’ principle. The general perspective is that things aren’t optimized. In biology, things work just well enough to survive. As engineers, we can use our optimization techniques to create better structures for specific purposes.”
David Needham, center co-director and a professor of mechanical engineering and materials science, adds: “Nature created what it has with a limited set of molecules that were available at the time of the evolutionary process. Now, it is a matter of working with synthetic chemistries and biobased synthesis to create new molecules that have these principles but reproduce their functions in novel, improved materials.”
At this early stage in the center’s life, Needham probably has led his research group farthest along in creating biologically inspired innovations. This despite the fact that his background is more in chemistry than engineering; he has learned about materials science by teaching it. In 2001, Duke received a patent on a new type of capsule developed in his laboratory. Called liposomes, these capsules are only two molecules thick and about 100 billionths of a meter in diameter, or one-hundredth the size of a red blood cell. In one medical application, these tiny capsules carry payloads of a chemotherapy drug to a tumor and then dump their cargo when triggered by the kind of focused heating used in other kinds of cancer therapy.

The liposomes, which are about to go to clinical trials, are inspired by the two-layer fatty membranes that enclose every cell. But in the liposomes, the fatty material has been chemically modified to make it more waxy. When exposed to heat, this material undergoes a temporary “phase change,” much like melting candle wax does, rendering the sphere porous until the heat is removed. “Nature doesn’t have wax membranes,” Needham says. “This is a good example of a biologically inspired structure that was modified to suit certain needs of a specific application--in this case, releasing a drug on cue in a tumor.”

Needham’s liposomes are examples of the kind of “soft and wet” materials that abound in nature. These contrast with the “hard and dry” metals, ceramics and plastics that usually make up human-engineered products. Nature also is the domain where molecules such as proteins and DNA assemble themselves, using their own forces, into the structures of life. This contrasts with the way manufactured products are made by workers on assembly lines.

Taking a pencil, Needham plots the scales of traditional engineering as a line moving downward from the hundreds-of-feet dimensions of the largest manufactured products. He then plots the scales of biological structures as a line moving upward from the most submicroscopic. The two lines “meet at the nanometer [billionths of a meter] level,” he says. “This is the scale where manufacturing and self-assembly are going to touch, and where soft and wet are going to touch hard and dry.”

Hot Research Field

The nanometer scale is also the realm of nanotechnology, a hot research field that is inspiring investigators to delve into atomic dimensions in their quests toward ever greater miniaturization. Technologists already have crafted structures measuring in microns (millionths of a meter) out of such hard-and-dry materials as silicon, producing computer microcircuits and various other devices.

But CBIMMS director Clark contends that biology has been the most successful at nanoscience, which involves scales 1,000 times smaller. “I’m sure there will be successes with hard, dry materials,” he says. “But the self-assembly that takes place in the aqueous medium of life has already set a precedent for how to engineer at that level.”

Authorized in January 2001, the center is in keeping with Duke’s strategic plans “to try to focus on developing the things that we do best,” says Vice Provost for Academic Affairs John Harer, who also is a professor of mathematics. “It’s something that everybody recognizes as an exciting new frontier. And it fits what we are so good at, recognizing our own strength in the biological sciences, including our medical school.” Information about the center is available on its website at mems.egr.duke.edu/CBIMMS.

The center already has applied for numerous funding grants from federal agencies. In May 2002, the National Science Foundation (NSF) announced that CBIMMS will receive $1.2 million from its Nanoscale Interdisciplinary Research Teams program.

An earlier $170,000 NSF Materials Research Grant is underwriting the construction of four newly designed atomic force microscopes (AFM’s), instruments used to measure the minuscule strengths of bonds that link atoms together in molecules. These new microscopes represent a significant improvement on previous AFM technology, Clark says: “We designed the system from the ground up.” Stefan Zauscher, an assistant professor of mechanical engineering and materials science, led the team that designed the new instruments.

Tiny Beams and Laser Tweezers

AFM’s use a tiny cantilevered beam--akin to a fishing rod--to pull on molecules, breaking their bonds one by one to measure their strength. Innovations incorporated in the center’s new devices include digital rather than analog controls, meaning that handlers can test different molecular samples simply by modifying computer software rather than by having to modify the instruments. There also are special feedback controls to minimize vibrations, sometimes a problem in earlier instruments. The AFM’s will be sensitive enough to accurately pick up and drop individual molecules in patterns “like an ink jet printer,” Clark says--steady enough “to draw in a straight line.”

Center researcher and assistant chemistry professor Jie Liu earlier used a commercial AFM to form a nanoscale “DU” pattern, thereby demonstrating the abilities of the microscope, while also becoming Duke’s first “nanoartist.” (The pattern can be seen on Liu’s research page at www.chem.duke.edu/~jliu/labgroup/research.html.) He created this design by using a technique called “dip-pin nanolithography” to lay down clusters of gold atoms on a silicon surface. This was no easy task, Liu says, because the tiny tips of commercial AFM’s are not very stable. “They move,” he says. “When we try to draw in a straight line, it forms a curved line. This means you cannot probe very small features.”

Now, with more practical goals in mind, Liu is waiting for the center’s engineers to finish building a more stable AFM to help him modify the carbon “nanotubes” that his research group earlier learned to produce in large quantities. For this and other basic research goals, a Duke-made microscope will “give me exactly the positioning capabilities that current devices do not have.” The nanotubes are nanometer-width cylinders of carbon atoms that possess both exceptional strength and unusual electronic properties, he says. With the advanced capabilities of the new AFM’s, he hopes to learn, for instance, how to deposit tiny amounts of metal salts or polymers directly onto the nanotubes. Liu envisions crafting such deposits into “nanowires,” which then could be used to connect the nanotubes to electrodes in order to measure changing electronic properties in chemical or biological environments. These tiny circuits would thus serve as “nanosensors,” says Liu, whose group is now working on one such sensor for sugar.

Another goal might be to fabricate structures of polymers with desired biological functions. “One idea,” Liu says, “would be to see how polymers might change their structures under tiny currents, much like muscles are controlled by signals though our nervous systems.”

In an effort to learn some of nature's basic secrets, Harold Erickson, a professor of cell biology at the Medical Center, is collaborating with other center investigators to study a set of events that occurs in bacteria. In particular, the group wants to understand how certain protein molecular subunits assemble themselves into natural polymers in a central ring within bacteria, and how the ring then constricts when the bacteria divide.

In other work, Erickson is exploring the use of another new twist in technology--a “laser tweezer” system being developed by researchers in Pratt’s department of mechanical engineering and materials science. Previous laser tweezers have used intensified light beams to apply a force to an object--say, a molecule--but they can apply their force only in a linear direction. The new tweezers have the ability to “put a twist on molecules,” Erickson says. . For example, using the laser tweezers to twist proteins in the molecular meshwork that binds cells together might yield an understanding of how those critical molecules work.

Stephen Craig also welcomes the research capabilities offered by the new center. An assistant professor of chemistry, his work focuses on creating novel kinds of self-assembling molecules with new and interesting properties. “I think the concept behind the center is brilliant,” he says. “And the timing is right. There is exactly the correct critical mass of people.”

The CBIMMS, he adds, “puts me in touch with other people on campus who have expertise and equipment that will help me greatly. But probably even more important is the intellectual firepower ready at hand. The center provides a very nice forum and rallying point for meeting with people in other departments who I wouldn’t talk to otherwise.”

“Duke can be a major player in this emerging area,” says Ashutosh Chilkoti, an assistant biomedical engineering professor. His research interest is in using amino acids, the building blocks of proteins, to construct other “bioinspired” materials with interesting properties.

“There have not been that many universities that have focused on this area,” Chilkoti says. “Duke’s traditional strengths in biology and the life sciences would leverage that nicely.”

(From Duke Research 2001-2002.)