Biomimetics of photonic nanostructures

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Biomimetics of photonic nanostructures
Biomimetics is the extraction of good design from nature. One approach to optical biomimetics focuses on the use of conventional engineering methods to make direct analogues of the reflectors and anti-reflectors found in nature. However, recent collaborations between biologists, physicists, engineers, chemists and materials scientists haveventured beyond experiments that merely mimic what happens in nature, leading to a thriving new area of research involving biomimetics through cell culture. In this new approach, the nanoengineering efficiency of living cells is harnessed and natural organisms such as diatoms and viruses are used to make nanostructures that could have commercial applications.

Department of Zoology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK; 2Department of Biological Sciences, University of Sydney, NSW 2006, Australia; 3Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK e-mail:;


Three centuries ofresearch, beginning with Hooke and Newton, have revealed a diversity of optical devices at the submicrometre scale in nature1. These include one-dimensional multilayer reflectors, two-dimensional diffraction gratings and threedimensional liquid crystals. In 2001 the first photonic crystal in an animal was identified2 and since then the scientific effort in this subject has accelerated. Now we knowof a variety of twodimensional and three-dimensional photonic crystals in nature, including some designs not encountered previously in physics. However, some of the optical nanostructures found in nature have such an elaborate architecture at the nanoscale that we simply cannot copy them using current engineering techniques or, if they can be copied, the effort involved is so great thatcommercial-scale manufacture would never be cost-effective. An alternative approach is to exploit the fact that plants and animals can make these designs very efficiently (for example, ref. 3). Therefore we can let nature manufacture the devices for us through cell-culture techniques. Animal cells are about 10 μm in size and plant cells can have sizes of up to about 100 μm, so they are both of a suitablescale for nanostructure production. Although the overall size of the structures produced by a single cell is about the size of the cell itself, these structures can involve sub-structures on a smaller scale (typically of the order of 100 nm). The success of cell culture depends on the species and on the type of cell. Insect cells, for instance, can be cultured at room temperature, whereas anincubator is required for mammalian cells. It is also necessary, where solid media are involved, to establish a culture medium that the cells can adhere to in order for them to develop to the stage where they can make photonic devices. This article will first describe progress in the conventional engineering approach to optical biomimetics, and then discuss the more recent cell-culture approach atgreater length.
nature nanotechnology | VOL 2 | JUNE 2007 |

Some insects benefit from anti-reflective surfaces, either on their eyes to see in low-light conditions, or on their wings to reduce surface reflections from transparent structures for the purpose of camouflage. Anti-reflective surfaces are found, for instance, on the corneas of moth and butterfly eyes4and on the transparent wings of hawkmoths5. These surfaces consist of cylindrical nodules with rounded tips arranged in a hexagonal array with a periodicity of around 240 nm (Fig. 1a). Effectively they introduce a gradual refractive index profile at an interface between chitin (a polysaccharide with a refractive index, n, of 1.54 that is often embedded in a proteinaceous matrix) and air (n =...
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