As every mechanical engineer knows, the stiffness of a material — its elastic modulus — is often a critical property; likewise in nanomechanical engineering, though in part for a different reason. I’d like to say a few words about this, then discuss some materials of interest in implementing nanosystems. And there is something I must say about beef, too.
Stiffness determines how much a component will deform in response to a force, or equivalently, how much elastic energy will be stored in the process of distorting a component into a given shape. In nanomechanical engineering, the energy is of special importance, because the probability of a distortion declines exponentially with increasing elastic energy — in other words, stiffness limits the amplitude of thermal fluctuations. (This post was getting too long, so I moved further discussion of this topic to another page: Elastic Modulii, Stiffness, and Thermal Fluctuations.
The chart below compares the (Young’s) modulus of some very different materials.
Toward advanced nanosystems
These materials differ in stiffness by orders of magnitude, and this, together with related facts, will be important in charting paths toward advanced nanosystems. A few observations:
- The range of modulus values for structural proteins is like that of engineering polymers: Some are like rubber; many are like epoxy, polystyrene, or polycarbonate; and silks can rival Kevlar.
- DNA, although valuable as a medium for design and fabrication of large, atomically precise structures, is much softer than structural proteins.
- In comparison to a typical structural protein, the stiffness of pyrite (FeS2, also known as fool’s gold), rivals that of diamond. A number of metal oxides have elastic moduli that are somewhat greater than pyrite.
- Finally, except for steel and diamond, all these materials are readily synthesized at low temperatures from substances dissolved in water. The same is true of several metals and semiconductors, and of many silicates and metal oxides.
To me, these facts suggest several conclusions:
- DNA will serve better as a means for organizing components than as a means for establishing tight control on geometrical relationships.
- The greater stiffness of structural proteins is yet another property that complements the properties of DNA, suggesting that the two materials can play quite different mechanical roles in biomolecular composite nanosystems.
- The ability to form complex, atomically precise structures of pyrite or of various metal oxides would be an enormous step forward in engineering stiff nanomechanical structures and machines.
- The feasibility of water-based synthesis suggests that advances in the design of biomolecular composite nanosystems can open relatively direct paths toward this ability.
A multitude of organisms use crystal-binding proteins to direct the growth of stiff, inorganic, crystalline materials, and the principles of this process are being adapted for applications in nanotechnology. To quote from the abstract of a 2007 paper:
Nature has long used peptide- and protein-based manufacturing to create structures whose remarkable mechanical, transport, optical, and even magnetic properties are determined by a fine control of composition and architecture extending from the nanoscale to the macroscale…. In recent years, portable amino acid sequences selected from combinatorial libraries and supporting the assembly, nucleation, and geometrical organization of solid phases have emerged as attractive tools for bionanofabrication.
Protein is not like meat
When people think of protein, there’s a temptation to think of beef — to imagine a soft, tender material, and perhaps to salivate. This impression gives structural protein engineering a faintly ridiculous air.
When you imagine a protein structure, think not of meat, but of horn, a weapons-grade biomaterial made of keratin.
As the expanded graph below shows, confusing structural proteins with meat is is not just wrong, but enormously wrong — wrong not by a factor of ten, or a thousand, but by a factor of more than a million.
It’s time to free the idea of protein engineering from this conceptual contamination.
Title updated 10 Feb 09
- Modular Molecular Composite Nanosystems
- Polyoxometalate Nanostructures
- Making vs. Modeling: A paradox of progress in nanotechnology