Variable stiffness biological and bio-inspired materials

This article reviews the main mechanisms of stiffness variation typically found in nature. The temporal changes in stiffness may be fully or partially reversible, or completely irreversible, and can be very slow or fast in time depending on the strategy adopted to alter the mechanical properties. It is also possible to observe changes in the stiffness in order to recover the original mechanical properties in damaged natural materials by means of self-healing mechanisms. In addition to stiffness variations in time, natural materials can also exhibit stiffness changes in space. These variations can be represented by alterations in the spatial distribution of the microscopic constituents across multiple hierarchical scales, from very small physical scales to large macroscopic dimensions. In order to optimise the strength and multifunctionality of these materials, spatial changes can also occur over larger areas at one single scale. In addition, several examples are provided to illustrate how natural materials have been exploited further in order to develop new bio-inspired materials. © 2012 The Author(s)

Controlled accessibility Lewis acid catalysed thermal reactions of regenerated cellulosic fibres

A combination of techniques have been used to characterise lyocell regenerated cellulose fibre subjected to low-moisture thermal-catalytic reactions with zinc chloride Lewis acid. Application from non-swelling ethanol reduces catalyst accessibility, but at high temperatures migration takes place through the internal fibre morphology. The extent of chain scission is reduced at lower temperatures, leading to a higher leveling-off degree of polymerisation (LODP). In contrast, application of zinc chloride from water results in a lower LODP, due to the more even distribution of catalyst. The weights of extractable polymer material increase according to two separate rate constants, following established semicrystalline models. A higher Arrhenius activation energy for chain scission is seen for zinc chloride application from ethanol, which may be due to the physical mobilisation of the cellulose polymer at high temperature, associated with a cellulose Tg. This may also aid recrystallisation. Cellulose dehydration endotherms and pyrolysis exotherms are shifted to lower temperature for application of zinc chloride from ethanol compared to water, which may be the result of a higher local concentration of catalyst and a faster reaction onset