We know of no competing system of synthesis that has the potential to achieve such a broad range of predictable structures for bottom-up assembly of nanostructured materials for science and industry.

The closest competition appears to be a group of researchers at Temple University (Christian Schafmeister, et al.) [see external links to Schafmeister and Temple] who have succeeded in altering one of the 22 natural amino acids so that it can form an extra bonding feature in protein-like polymers. This extra, monomer-to-adjacent-monomer bond prevents free rotation of the amide linkages to create small, semi-rigid, spiro-carbon-based “ladder” polymers (see external link to image).

Although innovative and imaginative, this approach has several critical limitations:  1) it does not provide much variation in structural features (the reported monomers produce only gentle curvatures) [see Note #1 below], 2) the structures are only softly rigid [see Note #2 below], 3) they require an aqueous polymerization environment (like proteins do) [see Note #3], and 4) they require a two-step process to form the nanostructure, the second of which cannot be begun until the first is fully completed.  However, their approach has one clear advantage in being potentially “programmable” from DNA and RNA templates just like natural protein polymers. If the resulting polymers are biologically non-immunogenic, that will be an additional strong feature to promote.

Two other systems of nanostructural assembly known to us are even less commercially suited.

Dr. Nadrian Seeman and colleagues at New York University [external link to Seeman at NYU] have been building geometrical nanoconstructs such as cubes and octahedra out of DNA and RNA. Although these efforts are academically elegant, we do not consider this “close” competition because DNA/RNA is quite bulky, very soft and “squishy,” and largely unsuitable for the kind of robust structural applications described herein. However, like Schafmeister’s approach, the DNA-RNA templates are “transcribable” through DNA-RNA replication techniques and can be mass produced fairly cheaply.

Another nanostructural assembly system worthy of mention was developed by Professor Bing Gong and colleagues at the the New York State University (at Buffalo) using a hydrogen-bond donor but not a corresponding hydrogen-bond acceptor [see external link to Gong at UB].  This approach does constrain the linkages in a nanostructurally favorable way, sufficient to form pi-stacked helical coils [see illustrations in PNAS article], but there is substantial electronic repulsion between the remaining adjacent neutral ring-hydrogen atom and the amide linkage groups, thus creating two thermodynamically favored conformations rather than one. This repulsion causes “canting” of all nearest-neighbor aromatic systems. While this canting is not sufficient to prevent pi-stacking and the formation of nanocoils, it does produce structural indeterminacy, which is a critical weakness for a de novo nanostructural design system.

Unlike the natural and unnatural proteins mentioned above, NSI’s nanopolymers are synthesized using organic solvents, precluding any requirement for water as solvent. Synthesizing with organic solvents allows us the potential to incorporate nanostructural elements that are incompatible with water, neutral pH or biological colloids. As a result, our polymers are better adapted to addressing the variety of markets requiring nanostructured materials—for biological or non-biological applications.

Perhaps our stiffest competition is from living systems.  The biological self-assembly platform is programmable, exceedingly inexpensive, and readily scaleable.  If a prospective application does not require robust materials, does not need high or low tenperatures, can operate well in an aqueous solution at neutral pH with electrolytes, a biological nanomaterials solution may well be the best choice.  But it is our opinion that such narrow constraints are for too restrictive for the current sophistication demanded by nanomaterial scientists and engineers. 

Note #1:  This lack of variation can be overcome by synthesizing various analogs of the core amino-acid monomers. [Return to main text.]

Note #2:  This flexing is caused by partly aliphatic (non-aromatic) 6-membered rings, which connect the rigid spiro-carbon centers into a polymer backbone.  Aromatic 6-membered rings, used by NSI's monomers, are rigid.  But with two aliphatic carbon atoms in a 6-membered ring, there are actually two thermodynamic minima, one on either side of true planarity.  So the resulting structures have a fundamental indeterminacy (a metastable “twisting”) that accumulates over the length of the polymer strand.  This might be a serious limitation for a nanostructural self-assembly system. [Return to main text.]

Note #3:  It remains untested whether or not materials made in this way may also be susceptible to enzymatic digestion by microbes. [Return to main text.]