Outlook

A wealth of stable nanotubes form from simple peptide building blocks, ranging from 0.7 nm to 1 000 nm in diameter and of varying rigidities. The shortest peptides (dipeptides) form the widest NTs, while the narrowest NTs are prepared by cyclic peptides. Control of surface properties has been demonstrated, however, in almost all the cases the inner and outer surfaces of NTs are the same and it does not seem possible to modify them independently of each other. Cyclic peptide NTs are a notable exception. Here the inner surface is always the same since it is defined by the peptide backbone, while the outer surface is distinctly different, lined by the side-chains and tunable by peptide design.

Another crucial issue in device fabrication is the ability to gain precise control over the positioning of the nanostructures relative to a substrate, to each other, and to other functional components. Significant progress has been made in this domain, but further developments are needed. However, it is obvious that the current state-of-the-art in peptide NTs already offers a wide range of morphologies, chemistries, and macroscopic arrangements to hopefully match the requirements of specific applications. This is possibly the reason why there seems to be an extensive and impressive array of applications that are currently explored using peptide NTs, such as drug delivery, biosensors, biomolecular filters, scaffolds, and microelectronics.

Despite the numerous investigations on peptide NTs, there is a need to expand studies to other factors that can determine potential commercial usefulness, such as compatibility with the final product formulations, processability, and robustness during processing. Another area that seems less advanced is the assessment of biocompatibility of peptide NTs; ideally such studies, together with rigorous quality control and peptide purification, should go hand-in-hand with the parallel escalation of their applications. Another remarkable observation is that most reported peptide NTs (apart from the cyclic peptide ones) seem to have started out as a serendipitous observation, i.e. the researchers stumbled on them during the course of other studies. This demonstrates that we understand very little about peptide NTs, so are often not in a position to design the final product of an experiment. There also seems to be a notable lack of theoretical insight into peptide NT formation. This can be a very fruitful approach in that it captures the knowledge generated by experiments with the aim of efficient future quantitative predictions, rational design, and precise control of the properties of the self-assembling structures.