Helicoidal architectures

After a brief introduetion of helicoidal architecture and their optical properties, we will describe examples observed in nature. Then, we will discuss liquid crystals and biomimetic photonics. Finally, we will discuss the future of the field, its extension into templating, sensors, and the use of cellulose derivatives. 

Helicoidal architectures of polysaccharides, like chitin and cellulose are found both in the animal and plant kingdoms with a great variety range. Since the pioneering work on these structures, mainly preformed in the... 

Helicoidal architectures have also been found in the epicarp of different fruits (Figure 17.5). The first observation of this phenomenon was reported by D. Lee in 1991 in Elaeocarpus. Recently, similar structures have been observed in the fruit of Margaritaria nobilis and Pollia condensate. While all the other published examples of helicoidal architectures (both in cellulose and chitin) are left-handed, the fruit of Pollia condensata has also cell walls with right-handed orientation of the cellulose structures. 

The constituent parts that form liquid crystals can exist over a wide range of sizes, from molecules to colloids. At each size scale different forces control the interactions of the liquid crystal constituents, but the principles of liquid crystalline formation remain the same. In order to understand the use of liquid crystals in biomimetic structural colour for helicoidal architectures, a brief introduction into the huge and varied field of liquid crystals is given here. 

Using self-assembly of a chiral nematic phase in a biopolymer liquid crystal of cellulose nanociystals, a well-controlled technique has been developed to create solid helicoidal architectures for structural colour and for further functionalisation. This section describes the self-assembly process and the control parameters of tuneable helicoidal cellulose films, and the prospects for future development. 

So far, the creation of structural colour by helicoidal architecture has been described. This unusual architecture has however much potential in extended applications beyond its optical characteristics, and huge advantages remain to be exploited. Pathways to these functionalities lie in the inclusion of other materials in the helicoidal structure, and the transfer of the stmc-ture into entirely new materials through the creation of porosity, and hard and soft templating of the original structure. 

Organoclay materials with higher-order organization can also be prepared by template-directed methods involving self-assembled supramolecular structures. In this approach, preformed organic architectures in the form of tubes, fibers, hollow shells, gyroids, helicoids, and so on are transferred into hybrid materials exhibiting structural hierarchy, complex form and ordered mesoporosity [47-55]. For example,... 

While extremely widespread in beedes, other chitin architectures are less prominent in other species, and only a few examples are reported in the literature, probably because helicoidal components in some Crustacea (crayfish, crabs) are not individual chitin ciystallites, but bundles with a diameter of 25-50 nm. Such filaments form non-optically active helicoid with pitches on the order of microns. However, in the case of Panulirus argus, chitin calcified architectures produce green colouration. ... 

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