engineering
without limit

Mechanical engineers can stake a claim for the nanoscale frontier. But the only way to truly own it is to dissolve the old boundaries among disciplines.

By Steven L. Girshick and Arun Majumdar

Over the past decade, there has been a remarkable increase in interest in nanotechnology among the science and engineering communities, the media, and private investors. The U.S. government now funds about $1 billion per year for the National Nanotechnology Initiative, and many states are making significant contributions as well. Japan, Taiwan, China, and countries in the European Union have begun funding at comparable levels. Clearly, the worldwide perception is that nanotechnology holds tremendous promise and is an important area for research investment.

Last year, the National Science Foundation held a two-day workshop in Arlington, Va., on mechanical engineering at nanometer scales. After a status report from Mihail C. Roco, chair of the National Science and Technology Council's Subcommittee on Nanoscale Science, Engineering, and Technology, 59 participants attended 29 talks and numerous poster sessions and panel discussions.

By the end, there was a growing consensus about the areas within nanotechnology where mechanical engineers were poised to make the greatest contributions and about how to change engineering education to address the new challenges that nanoscale technology represents. Indeed, nanotechnology may change the way we think about mechanical engineering altogether, making it a more multidisciplinary field, one as concerned with atomic-level effects as electrical engineering or chemistry is. It is crucial not to let the old barriers between disciplines discourage mechanical engineers from exploring this new frontier.

Many of the challenges of working at nanoscale stem from the way physical properties change at that level. When solids, liquids, and gases are confined to regions smaller than 100 nm, for instance, their behavior can be modified by the confinement. Properties such as thermal conductivity, electrical conductivity, optical absorption and emission spectra, mechanical strength, and viscosity are size dependent.

In the future, a nano-biological device may detect pathogens.

Many researchers envision that material organized at this scale could form the basis for structures, devices, and systems that could have tremendous impact on parts of the economy as diverse as information, energy, health, agriculture, security, and transportation. They see possibilities for data storage at densities greater than one terabit per square inch; for high-efficiency, solid-state engines; for analysis of single cells that could form the basis for diagnosis of complex diseases such as cancer; and for ultralight and ultrastrong materials for vehicles.

It is fair to ask what the role of mechanical engineering in nanotechnology will be. In fact, quite a bit of nanoscale science and engineering is already performed by mechanical engineers.

For example, mechanical engineers have been essential in developing instruments such as nanoindentors and atomic force microscopes, which are used for mechanical testing, nanoscale imaging, and metrology. Issues of feedback control of such systems are unique because of the nanoscale precision required in positioning and the ability to measure forces down to piconewton levels.

Mechanical engineering issues extend to instruments for nanoparticle and aerosol detection and characterization, as well as to various forms of nanoscale imaging. Magnetic data storage technology already has many features that fall well into the nanometer size range, and requires mechanical engineering knowledge and expertise to further its development.

It is important to recognize some unique features about nanotechnology. First, it is the amalgamation of knowledge from chemistry, physics, biology, materials science, and various engineering fields. It epitomizes the concept of the whole being greater than the sum of the parts.

Second, nanoscale science and engineering span different scales. Nanostructures and nanoscale phenomena are generally embedded in micro- and macrostructures, and their interactions are important. The connection between scales—nano to micro to macro—is also a critical aspect of integration.

In addition, it is often difficult to isolate nanoscale phenomena as we do at customary scales. That is, thermal, electronic, mechanical, and chemical effects are often related to each other. By changing one, it is possible to influence the others. This, of course, emphasizes the need for interdisciplinary knowledge.

There are many concepts in mechanical engineering that are critical in the development of nanotechnology. It is incumbent upon mechanical engineers to provide depth in these areas.

At present, nanotechnologists can create simple structures, like this silicon carbide tower.

One of the most important issues related to nanotechnology is systems integration and packaging. Researchers have been able to study individual nanostructures and have even synthesized building blocks such as nanoparticles and nanowires. But how do we integrate these building blocks in a rational manner to make a functional device or a system? This step requires design based on the understanding of nanoscale science, and on new manufacturing techniques.

One of the biggest challenges in nanotechnology is manufacturing. Assembling large quantities of nanostructures in a rational and rapid manner requires tooling, imaging systems, instrumentation, sensors, and control systems. After nanostructures are assembled into functional devices, they need to be packaged so that they can interact with their environment and yet retain the nanoness that provides the unique function and performance.

These concerns are similar to those found in conventional manufacturing, though there is a call for a level of precision that is not required by macroscale designers.

POWER PIPS

The ability to convert energy between different forms—and the capacity to use it—is the hallmark of modern civilization. Humanity will face a crisis in the coming decades due to the rate at which fossil fuels are being used and the impact this is having on the environment. Nanotechnology almost certainly has a role in resolving this crisis, and mechanical engineers are perfectly situated to capitalize on the opportunity.

It is widely recognized that renewable sources of energy such as solar electricity and biomass will gain importance in the future. However, cost is a hurdle to the effectiveness of such technologies. For example, the cost of photovoltaics must be an order of magnitude lower than its current value to make the technology competitive with fossil fuels. This could be possible if, for example, silicon-based devices, which are currently manufactured in high-temperature processes, were replaced by nanostructured plastic-based photovoltaics.

If thermoelectric devices made of materials such as silicon and germanium perform about 10 to 20 percent of the Carnot limit, they could be as competitive as cost-effective solid-state energy conversion devices. This can only occur if the semiconductors are nanostructured to control heat and charge transport. Mechanical engineers who understand these challenges better than other technologists, will almost certainly devise the solution.

The particles making up a silicon carbide film at top average only 20 nm across. A polymer material (second from top) is made of nanoscale layers. Each "carrot" made by Georgia Tech researchers (third from top) contains thousands of silica wires grown from a gallium droplet. Another group grows nanowires by depositing metals on a porous alumina membrane (bottom).

One of the biggest environmental challenges that humanity faces today is clean water. Nanostructured filters used for ion exchange hold promise for removing contaminants. Their manufacture however, must be inexpensive, and the science of nanofluidics must be understood to make these filters effective for cleaning water. Mechanical engineers can collaborate with biologists and public health researchers to resolve both these issues.

Another area of expertise for mechanical engineers, instrumentation, is also key to tapping the potential of nanotechnology. Instruments that can probe the environment with increased resolution and sensitivity lead to breakthroughs in science and engineering. The scanning probe microscopes invented in the 1980s are electromechanical devices, which require precision actuation with Angstrom resolution, microfabrication of cantilever probes, force sensing with resolution measured in piconewtons, and a fundamental understanding of dynamics and control to increase imaging speed and spatial resolution.

These stringent requirements are not limited to the microscopes, but apply to any nanoscale measurement. For example, there is a tremendous need for instrumentation in high-throughput imaging and measurement in nanomanu-facturing processes to enable automation and process control. These issues offer opportunities for mechanical engineers to provide a system-level understanding of such instruments.

Other potential applications require a mix of skills. Nanoparticles and nanowires exist on a scale similar to biomolecules such as DNA and proteins. This suggests that the biological sciences can provide crucial insights to the behavior of such material and that nanoscale devices may be used for medical applications. For instance, nanoparticles may be used as markers to study very small samples of DNA or proteins. Achieved in a high-throughput manner, this could form the basis for biomolecular analysis in extremely small volumes (on the order of single cells), which has major implications for diagnosis of disease.

In addition, the combination of nanostructures and mechanical sensors such as cantilever beams could be used in chemical or biological defense. Nanostructures such as particles and polymeric dendrimers could be designed as drug delivery systems.

Biomolecules could be used to perform non-biological tasks. Possibilities include manufacturing, energy conversion, signal amplification, and information processing. Many of these functions are already achieved in an extremely efficient manner within a cell, thanks to the genius of natural selection, but to exploit them in non-biological conditions is a nontrivial problem.

Nevertheless, applications such as manufacturing and energy conversion have always been strengths of mechanical engineers. Can we harness the power of the biomolecular machinery for mechanical applications? Research in this direction has already started.

Another field where nanotechnology may need mechanical engineers is information processing and storage. When transistors reach the scales of 20 to 30 nano-meters (a scale that will be necessary to keep up with Moore's law) quantum effects such as electron tunneling will lead to electron leakage, and this will cost power. Higher speeds will also require electromagnetic isolation, which will necessitate the use of materials that have extremely low thermal conductivities. In addition, novel cooling technologies that directly interface with electronic and optoelectronic chips must be developed.

To create chip designs that solve these thermal problems, technologists will need a basic understanding of how heat flows in nanostructures and across interfaces. Mechanical engineers have just this sort of expertise.

EXQUISITE DESIGN

Mechanical engineering concepts also come into play in designing magnetic data storage, which currently requires heads to fly over a disk with spacing of about 10 nm. Maintaining such flying heights without crashing calls for exquisite design and manufacturing of disks and heads, and fundamental understanding of dynamics, non-continuum fluid mechanics, and surface forces. This has always been part of mechanical engineering and is expected to remain so even as the scales involved shrink.

One of the biggest challenges in magnetic recording is the so-called superparamagnetic limit, which occurs when the volume of a magnetic domain is sufficiently small that thermal fluctuations randomize its polarization. This can be overcome by patterning the magnetic medium. How does one manufacture highly regular magnetic bits with sizes in the range of 20 to 100 nm over a disk surface with diameter of about 3 to 10 cm? The ultimate solution to this problem will be derived from mechanical engineering.

But with all the ways in which mechanical engineering will be crucial to unlocking the potential of nanotechnology, there are challenges as well. University engineering departments must change the way mechanical engineers are educated.

Although some universities claim to have modernized their curricula, a deeper look would suggest that in most cases courses of study reflect the technological needs of the Sputnik era or perhaps an earlier time. Mechanical engineering programs need to ensure that their students are given a solid grounding in the fundamentals of physics, chemistry, and biology.

Approaches must be developed that cultivate a different way of thinking, so that students can develop intuition for phenomena occurring at the nanoscale, as well as gain an understanding for connections that bridge the nanoscale, the mesoscale, and the macroscale.

A gene gun developed at the University of Minnesota sprays a mist of DNA-bearing particles into cells. Similar devices could be used one day to spread manufactured nanoscale objects across relatively broad surfaces.

For this to happen, nanoscience and engineering concepts will need to be integrated into existing undergraduate curricula. Topics such as solid state physics, chemical thermodynamics, surface forces at the atomic and molecular scale, nanofluidics, and motion and behavior of nanoscale structures—most of which receive little if any attention in the traditional undergraduate ME curriculum—will need to be integrated into core courses such as thermodynamics, heat transfer, fluids, statics and dynamics, and manufacturing.

Textbooks need to be written or revised to incorporate this type of material with the core mechanical engineering subjects. Requiring professors of mechanical engineering to take graduate-level refresher courses on these topics is not inconceivable.

Taken together, these changes will represent a new paradigm for the education of mechanical engineers, one that, if done right, will increase disciplinary depth. At the same time, at both the undergraduate and graduate levels, students should be exposed to courses that bring in concepts from multiple disciplines, and faculty and programs must find ways to reduce the barriers to interdisciplinary dialog.

What's more, there should be a strong ethical component to this new teaching paradigm. Like any other technology, nanotechnology can have many unintended consequences that are harmful to our society and to the environment. It can also be used in counterproductive ways that could pose risks to the society.

There are many questions that we engineers must openly discuss: How could nanostructures or manufacturing of nanostructures be harmful to human health? Are there any environmental effects? Could nanotechnology reveal information that infringes on privacy? If improved health diagnostics and therapeutics facilitated by nanotechnology increase lifespan, what effect would the result have on demographics and productivity? Would this technology be accessible to the whole population, or be available to only a certain segment of our society?

It is incumbent upon us engineers to pay close attention to these societal and ethical issues related to nanotechnology. We also need to educate ourselves, the public, and the media about what is realistic and what is not, and in what time frame we could expect nanotechnology to affect our lives. It is our responsibility to do so.

Last year's workshop confirmed the emerging consensus within the mechanical engineering community that nanotechnology will have a profound impact on society and on industry, and that MEs can play a crucial role. Some major recommendations include:

• Sustained support from the National Science Foundation and other funding agencies to maintain long-term, fundamental research in nanoscale science and engineering;

• A focus on research in nanoscale science and engineering that addresses the grand challenges that affect society and humanity;

• Development of education programs that incorporate the essence of nanoscale science and engineering into undergraduate and graduate mechanical engineering curricula;

• Collaboration across disciplines by both NSF and university departments to expose graduate and undergraduate students to interdisciplinary research; and

• Research that seeks integration across scales to exploit nanoscale effects at the micro- and macroscales.

We live at an exciting juncture in history, one where mechanical engineers can take the lead. What we need, though, is the foresight and initiative to embrace important aspects found in heretofore separate disciplines. If we can do that, the future of this technology—indeed, the future itself—will belong to us.

Steven L. Girshick is professor and director of graduate studies in mechanical engineering at the University of Minnesota in Minneapolis, and co-chair of the Nanomanufacturing Committee of the ASME Nanotechnology Institute. Arun Majumdar is the Almy and Agnes Maynard Professor of Mechanical Engineering at the University of California, Berkeley, and a member of the scientific staff of the Materials Science Division at Lawrence Berkeley National Laboratory. He chairs the Advisory Board of the ASME Nanotechnology Institute and is a member of the Nanotechnology Technical Advisory Group to the President's Council of Advisors on Science and Technology.



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