Creating Nanotubes as Strong as DiamondsCreating Nanotubes as Strong as Diamonds

Originally Published MDDI November 2001R&D DIGESTA monthly review of new technologies and medical device innovations

November 1, 2001

12 Min Read
MDDI logo in a gray background | MDDI

Originally Published MDDI November 2001

R&D DIGEST

A monthly review of new technologies and medical device innovations

THIS MONTH: Creating Nanotubes as Strong as Diamonds |Devices Synthesized from Artificial Molecules | New Technologies Aid Nanostructure Development | Unconventional Gluing Method Seals Microdevices

Making Stronger Carbon

A semiconducting/metal junction formed from two carbon nanotubes.

The creation of nanotubes in 1991 enabled researchers to explore a number of new possibilities in materials science—from development of new structures with exceptional strength to synthetic muscles for use as device actuators (see August 1999 MD&DI, p. 46). Now, a team of researchers led by Vincent Crespi, Downsborough associate professor of physics at Pennsylvania State University (University Park, PA), has generated computer simulations of carbon nanotube fibers that would have mechanical strength comparable with that of diamond. At the time, the researchers were using supercomputers at the San Diego Supercomputer Center, the University of Michigan, and the University of Texas to simulate the electronic states and total energies of various carbon molecules. Although the new fibers have yet to be synthesized, the group theorizes that nanotubes about 0.4 nm in diameter could be made from simple starting materials.

The researchers' discovery was made during studies of unrelated features of carbon compounds. Crespi comments, "This is one of those sideways inspirations that comes when you're looking at one thing and you suddenly realize it has a different application." The discovery prompted a shift in the research focus to concentrate on simulations of the strong nanotubes.

The key discovery, according to Crespi, was that a particular type of tetrahedral carbon atom had special properties. He explains, "Structurally, the carbon atoms are bonded to four neighbors instead of three, as in previous tubes. Performance-wise, they are electrically insulating instead of semiconducting or metallic—as previous tubes are. Also, they are stiffer."

Says Crespi, "Based on our calculations, these new nanotubes are about 40% stronger than others formed using the same number of atoms. The nanotubes we simulated may well be the stiffest one-dimensional systems possible."

The researcher suggests that the main challenge now is synthesis. "We've designed the material to be synthesizable from certain precursors, but our work itself is a calculation of properties based on first-principles theory."

Forming Devices from Artificial Molecules

Research has often focused on creating synthetic materials that are not only capable of mimicking the function of natural ones, but also of extending the material's capabilities, creating new potential applications. For example, an artificial protein-like molecule created at Ohio State University (Columbus, OH) could be the foundation for developing new drugs and medical treatments.

For some time, scientists have tried to synthesize the shape of proteins using thin plastic filaments called dendrimers. Jonathan Parquette, assistant professor of chemistry at the university, and a group of his students are considered to be the first to coax the thin filaments to maintain a shape that suits needed applications.

Depiction of a protein-like molecule made of plastic filaments, or dendrimers.

The molecule is shaped like a sphere, supported by branching beams of polymer material inside. Hollow portions of the structure could theoretically hold drugs or other chemicals. According to the researchers, the synthetic proteins eventually could be made to function as devices to deliver medicine to specific disease sites in the body. They could also act as catalysts for chemical reactions that produce drugs, or form computer chips for light-responsive molecular electronics.

Says Parquette, "The work is primarily directed at developing a fundamental understanding of how we can design and synthesize molecules that fold or 'zip up' into predictable shapes. We have made considerable progress in that regard, so our efforts are now beginning to bifurcate into both developing function from the well-defined structure of these molecules and also continuing to control the shape and folding of molecules on increasingly greater nanometer-length scales." Nature controls biological function by modulating protein shapes, he explains. "One can imagine developing nonnatural molecules with functions that expand beyond that of the biological realm, which is limited of course by evolution."

Parquette speculates that the method could aid in developing treatments for diseases such as diabetes. "One could imagine developing sensors for biologically relevant chemicals in the body, such as glucose in diabetics, among many others," he says. "If the synthetic systems could be developed to function like a protein in that the 'sensing' of one molecule increased the sensitivity of the sensor to another molecule, a phenomenon in proteins called cooperativity, then these sensors could be used to detect very small amounts of a chemical of interest." He believes that it may also be possible "to design these structures to recognize cell surfaces; if we could achieve that goal, then it may be possible to deliver drugs encapsulated in the dendrimers to a particular cell type such as a tumor cell."

Parquette notes that another important function of proteins is catalyzing various chemical reactions in biological systems. "We are currently exploiting the shape of our dendrimeric molecules to develop catalysts of nonbiological reactions that may be of use in pharmaceutical development," he says, adding that "there are many other potential applications that I am excited about. But only more research will determine which of these will ultimately materialize into something useful."

Parquette explains that antibodies appear to be effective in delivering drugs or radiation to specific cell types, adding that such methods have great potential. "Developing synthetic analogs of antibodies would permit greater flexibility in the design of drug-delivery agents," he says, "but it will be quite a while before any synthetic molecule can recognize a cell type with the selectivity of an antibody. Nevertheless, there is great potential in learning how to achieve this selectivity."

New Tools for Nanotechnology

Discussion of the latest advances in medical technology often entail nanotechnology. Observers speculate that the future is in minute devices, from lab-on-a-chip analyzers to implantable diagnostic and therapeutic systems. Some researchers, however, suggest that a gap of sorts still exists between devices that can be designed and those that can actually be manufactured using current technology.

Now, a team of researchers at the University of Michigan (Ann Arbor, MI) hope to develop new tools for closing this gap. The group, led by Bradford Orr, PhD, and Duncan Steel, PhD, has demonstrated a technique they hope will greatly improve the study of nanostructures and shorten the development time for quantum computers and similar devices.

The method being explored uses elements of both coherent nonlinear optical spectroscopy and low-temperature near-field microscopy. Specifically, the technique combines the direct optical probe and spectral selectivity of coherent nonlinear optical spectroscopy with the spatial selectivity of near-field microscopy. According to the researchers, the technique is capable of both optically inducing and detecting quantum coherence in an extended structure with subwavelength resolution. Says Steel, "This puts us another step closer to closing the gap between our present-day capabilities and the sophisticated nanodevices and quantum computers of the future."

The new technique is a significant departure from conventional imaging methods. As Steel explains, it uses the same imaging technology, "but combines it with the power of coherent laser spectroscopy. Prior to this demonstration, the signals that were used to build up the image were incoherent. In this case, the signal is coherent." He adds that there are two implications to this. "First, we build up a spatial map that is directly related to what we are exciting. The earlier maps were indirectly related to what was excited. Hence, we are now able to map out the center of mass motion of a quantum wave function in a semiconductor heterostructure. The second implication is that by exploiting the features inherent in coherent spectroscopy, we can follow the quantum dynamics that occur in these structures."

He explains that the technique is applicable to any optically active system, enabling it to be adapted easily to different applications. Steel suggests that such applications could encompass medical use. "This methodology is useful in general in the development of any nanotechnology. This particular advance could certainly see applications in the medical setting for designs that exploit nanooptical features and possibly some of the new ideas that exploit the quantum features that can occur on the nanoscale," says Steel. "This kind of spectroscopy may play a role in some aspects of biomolecular spectroscopy. This is preliminary, but I am aware of some groups that are working to try to use the additional information at the quantum level as an analytical tool to identify biomolecules. This has a long way to go and is highly speculative."

Gluing Microsize Medical Device Components

Micrograph of an outlet in a tiny plastic medical device. The two halves of the structure were bonded together with adhesive, filling the sharp corners of the opening.

Biomedical electromechanical devices currently under development could one day be used to treat tumors and other conditions by delivering therapeutic drugs directly to the disease site. The actual fabrication of such devices, which can be smaller than a human hair, poses a considerable challenge. Engineers at Ohio State University (Columbus, OH), however, believe they have overcome a critical hurdle by developing a new technique for sealing plastic casings of medical nanodevices.

In addition to aiding device construction, the technique promotes the flow of medicine and other fluids through nanochannels. The group suggests that the method, resin-gas injection-assisted bonding, can also be used to alter the material on the surface of a device to suit different medical applications.

According to L. James Lee, professor of chemical engineering at the university, "Plastics have great potential for use in these devices, because they are inexpensive and easy to shape into individual parts. But sealing a tiny casing poses a special challenge. So does altering the characteristics of the plastic to suit different medical tasks. Our method allows someone to do both in one shot." He explains that conventional methods for plastic bonding rely on such welding methods as ultrasonic, laser, infrared, or thermal, or on adhesive techniques, including glue or tape.

Lee further suggests that "these are good for feature sizes in the order of a hundred microns." But, the researcher adds, "for smaller microfluidic channels, these conventional methods may not be applicable because they tend to block the channels or make it difficult to control channel dimensions."

Lee says the research was initiated to address such limitations. "Since packaging of polymer-based biochips is a challenging task and there didn't seem to be any good available method, my group initiated this effort about a year ago," he explains.

The group tried several different techniques for sealing such devices, including welding, gluing, and double-sided tape. Although gluing seemed the most promising, traditional adhesives only gummed up the tiny channels found in microdevices. The researchers then tried using traditional liquid adhesive in a nontraditional way.

Top view of two fluid reservoirs connected by a narrow channel. The structure could be used in implantable devices to dispense medications.

For their initial work, the group molded a plastic device about the size of a small matchstick. The device consisted of a 100-mm-wide channel with a fluid reservoir at each end. The device was molded in two pieces, including a bottom platform containing the channel and reservoirs and a lid. After both parts were coated with a few drops of hydroxyethyl methacrylate (HEMA), they were fitted together. A short burst of nitrogen gas was then blown in one end of the device and out the other. This forced the adhesive to coat the inner surfaces on its way out. Finally, the entire device was cured using UV light.

According to the researchers, tests revealed that liquid traveled through the tiny channel between the two reservoirs with no leaks. The device appeared to be sealed successfully inside and outside. Examination using electron microscopy showed that while most of the HEMA had flowed cleanly throughout the device, some of it had filled in the corners of the reservoirs. As a result, all the sharp corners were smoothed out, which promotes good fluid flow, says Lee.

The researcher adds, "This method allows for simultaneous device bonding and surface modification. Using a mask, it can also achieve local surface modification (e.g., some locations of the microfluidic channels can become hydrophilic while other locations can become hydrophobic)." Likewise, Lee explains, the surface can be made to bind with certain proteins in the body, or to reject proteins.

Lee suggests that the same basic technique could be applied with other adhesives. "We have used HEMA a lot in our lab for other research projects, so it was convenient for us to use it to test the idea. We have also tried other photocurable resins such as SU-8 (an epoxy-based photoresist). They all worked well. As long as an adhesive won't dissolve the plastic substrate, it can be used. We prefer photocurable adhesives because photocure is fast and can achieve local surface modification by using the masking technique. However, the bonding can also be done by thermal cure."

The researchers are continuing to study the new method. Says Lee, "We have successfully bonded channels with widths of 10 mm or larger. One of our current efforts is to extend this technique to bond microfluidic platforms with smaller channel sizes and complicated patterns. We will also try to add more functions to surface modification, such as making the channel surface electric or magnetic conductive, and adjusting the surface static charge to facilitate electro-osmotic-induced flow through the channel." In the future, the group plans to investigate how to make the coating conduct light or electricity, which they believe could prove useful in devices intended to perform a chemical reaction, Lee explains.

Copyright ©2001 Medical Device & Diagnostic Industry

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