The nature of progress in nanopatterning.

Developments in nanotechnology are making increasing demands on our ability to manipulate materials to designed patterns and structures. Patterning at increasingly diminished scales becomes very challenging, particularly with a view to manufacturing reproducible structures en masse. In fact Kelly recently argued that 'There are strict limits for which one-off fabrication is possible, but manufacture is not' [1]. Kelly was referring to top-down manufacturing processes and while activity in the field remains high and fruitful, there is at least as much research into bottom-up processes including self-assembly. In this issue, researchers at SASTRA University in India report programmable biomimetic self-assembly of carbon nanotubes (CNTs) using proteins [2]. The process uses changes in the surface charge and conformation of an unfolding protein, showing promise for biomedical applications and nanobiotechnology. Since the discovery of carbon nanotubes in 1990, their exceptional properties have inspired a cornucopia of applications, from the realms of science fiction with space elevators [3, 4], to more commercial merchandise such as tennis rackets [5]. A lot of applications based on CNTs use composites, and the properties of these composites can vary significantly depending on the manner in which the nanotubes are assembled. Researchers in the US recently reported experimental and simulation studies of thermal transport in carbon nanotubes [6]. Their work dealt with the discrepancy between the exceedingly high conductivity predicted and observed in single nanotubes and that reported for large assemblies. The work highlights some of the factors which diminish thermal conductivity in carbon nanotube assemblies, as well as identifying some optimal structures for maximizing thermal properties. The possible biomedical properties of carbon nanotubes have also attracted interest. However there has been debate over the potential for carbon nanotubes to support cell growth versus their potential toxicity. Researchers in New Jersey in the US have tackled these issues with a study of primary calvaria osteoblastic cell growth on single-walled carbon nanotube thin film substrates [7]. They describe a mechanism through which carbon nanotubes not only induce toxicity but also promote bone cell differentiation, leading to the formation of bone nodules. Bioapplications of nanomaterials often require bio-sympathetic methods for their production. The self-assembly achieved by viruses attracted the attention of Bancroft and his colleagues in the late 1960s [8, 9]. Since then a number of biomimetic approaches inspired by nature have been developed that allow fabrication of controlled complex structures with the potential for mass production [10, 11]. The tobacco mosaic virus has been demonstrated as one example of a useful biological template for simple and versatile nanopatterning [12]. Structures patterned using this biological toolbox may have applications in fields as broad as gas sensing and Li-battery electrodes. Researchers in Finland have also demonstrated the fabrication of complex protein structures using a DNA origami technique [13]. As the authors point out, controlled assembly of proteins has great potential for a range of biological applications and is also important for understanding fundamental biomolecular interactions. Many approaches to CNT assembly taken so far have required pre-patterning of a suitable substrate with a structure-directing agent to promote assembly of the nanotubes. However, as Nithiyasri et al in India point out in this issue, 'Assembly of CNTs in the dispersion medium would be more advantageous in terms of overcoming patterning hardships and cost as well as being suitable for a wide range of applications like biosensors, tissue engineering scaffolds and devices requiring high-density aligned domains of CNTs'. Their biomimetic approach uses thermally reversible denaturation of bovine serum albumin without the need for pre-patterning. In 1959 in his famous lecture 'There's plenty of room at the bottom', Feynman said 'What I want to talk about is the problem of manipulating and controlling things on a small scale' [14]. Fifty years later people are still talking about this, and as the work reported in this issue demonstrates so well, research in the field is still brimming with exciting developments. However, the paper by Nithiyasri et al does more than report an interesting phenomenon; it also proposes a mechanism for explaining how it happens. Feynman was also famous for his ability to explain things at an elementary level, and perhaps that was largely attributable to his genius for in-depth understanding of complex subjects. In fact he once remarked 'I couldn't do it', after an attempt to prepare a first-year student lecture on why spin one-half particles obey Fermi–Dirac statistics. He added, 'I couldn't reduce it to the freshman level. That means we don't really understand it.' What makes the developments in nanotechnology today so exciting is not just the advances in our ability to manipulate very small things, but our increasingly deepening understanding of how these processes operate.