Are nanotechnology machines a match for nature's biomotors?
(Nanowerk Spotlight) While the concept of a 'machine' – a mechanical or electrical device that transmits or modifies energy to perform a certain task – can be extended to the nano-world, these nanomachines can not be built by just further miniaturizing machine blueprints from the macro-world. On the nanoscale, the nanomachine components would be atomic or molecular structures each designed to perform a specific task which, all taken together, would result in a complex function. The problem is that functional nanomachinery will need to take into account the quantum effects that dominate the behavior of matter at the nanoscale, affecting the optical, electrical and magnetic behavior of materials.
An alternative approach to miniaturizing machines down to the nanoscale is to borrow from the highly successful design shop of Mother Nature. Until a few years ago, catalytic micro- and nanomotors have been more or less unknown outside biology. With the explosive growth in nanotechnology, however, catalyzed nanoscale motion has become a heavily researched phenomenon. Scientists find that there is much to be learned from nature's motor systems for the development of artificial nanoscale machinery (see: Catalytic nanotransporters for nanotechnology applications outside biological systems).
"The use of nanomotors to power nanomachines and nanofactories is one of the most exciting challenges facing nanotechnology" Wang tells Nanowerk. "Nature has created efficient biomotors through millions of years of evolution and uses them in numerous biological processes and cellular activities. Nanoscale biomotors rely on spontaneous reactions of energy-rich biomolecules, such as hydrolysis of the biological fuel adenosine triphosphate (ATP). The energy released from the ATP hydrolysis results in linear or rotational movement induced by small conformational changes."
Nanoscale transport highway based on directed motion of artificial catalytic nanomotors and cargo manipulation (loading, transport, and delivery) along predetermined microfabricated tracks. (Reprinted with permission from American Chemical Society)
This efficient conversion of chemical energy into mechanical work makes biological nanomachines the active workhorses in cells and enables numerous
functions, ranging from intracellular transport of organelles and vesicles to large-scale muscle contractions. The efficiency, size, and complexity of these biomotors are currently beyond the capabilities of nanotechnology researchers.
Nevertheless, scientists have made great progress over the past few years in developing artificial nanomotors and Wang's review specifically summarizes the advances made with chemically powered catalytic nanowires. He describes the vision of applying them for demanding activities, and discusses the challenges facing the realization of such operations. In particular, he addresses several key questions related to the development
of synthetic nanomotors:
Can we transform the basic principles of biomotors for designing powerful man-made nanomachines?
Can artificial nanomotors compete with biological motors?
In what environments can they function? What fuels can they use?
Can synthetic nanomotors be powerful, versatile, and 'smart' enough to perform demanding tasks and complex self-regulating operations?
Can we integrate our nanoengines with more complex architectures, performing multiple functions?
In his review, Wang focuses on the self-propulsion of fuel-driven bimetallic catalytic nanowire motors and then goes on to discuss a path towards autonomous microsystems.
"The greatly improved velocity, motion control, cargo-towing force, and lifetime of modern catalytic nanomotors offer great promise for creating powerful onchip microsystems powered by autonomous transport" he says. "By transporting analytes or cargo without bulk fluid flow, such nanomotors may eliminate the need for external pump or power common to pressure-driven or electrokinetic flow-based microfluidic devices and may address the challenge of fluid transport in nanofluidic systems."
One of the major current limitations of chemically powered catalytic nanomotors is that scientists have managed to operate them only within a very narrow range of environments (low ionic strength aqueous solutions) and limited fuels (hydrogen peroxide, hydrazine). Wang says that this limitation currently precludes many potential applications of artificial nanomotors, particularly biomedical ones. He explains that extending the scope of synthetic nanomotors to diverse operations and wide range of environments would require the identification of new fuel sources and further improvement in the power and efficiency.
Notwithstanding all the impressive progress that has been made in this field, scientists have their work cut out for them – current man-made nanomachines are still primitive compared to their biological counterparts, leaving much room for improvement.
In particular, says Wang, artificial nanomotors lack the sophisticated functionality of biomotors and are limited to a very narrow range of environments and fuels. "Extending the scope of such motors to high ionic strength media is particularly crucial for realizing exciting biomedical opportunities. The energy-conversion efficiency of artificial nanomotors is significantly smaller (by orders of magnitude) than the energy transduction of biomotors. In addition, synthetic nanomotors require further increase in force and versatility, along with substantial size reduction."
Wang and his fellow nanomotorists are optimistic about the great promise offered by catalytic nanomotors: "In the not-so-distant future, we expect to see self-regulated nanomachines delivering drugs or destroying toxic pollutants, motion-based ultrasensitive biosensing of disease markers or chemical agents, or nanorobots cleaning out clogged arteries," he says. "These and other exciting future applications of man-made nanomachines will be limited only by our imagination."