Cellulose-based hierarchies are among the most abundant polymer fibre systems in biology and in the world. All plant life depends on these composites for a wide range of functions needed for survival: load bearing structural performance, shape adaptation, movement, transport of water and nutrients, etc. The intrinsic physical and mechanical properties of the fibres (nano-fibrils, microfibrils) are one the reasons for their success. With an elastic modulus and a tensile strength which compare favourably with those of high performance technical fibres such as carbon and aramid in absolute term - and often superior when the lower density of cellulose is considered - it is not surprising that biology has arrived at a rich, diverse and successful range of design solutions using these fibres [1, 2]. The other reason, more relevant in the context of this paper, is the organisation of the fibres in many hierarchical structures covering an extraordinary range of scales, from 10-8 to 102 metres.
(a) Cellulose nano-fibres (scale bar = 3μm; from www.cheme.cornell.edu);
(b) Tree in Stockholm park (15 metres high)
Plants represent a vast potential as renewable sources of high performance fibres, over and above
the traditional use of wood, bast and leaf fibres, etc. In the past ten years there has been a great deal of scientific and technological progress is extracting cellulose nano-fibres from a variety of sources and this has been done in order to “capture” the intrinsic mechanical properties of the fibres which are “diluted” as hierarchies are built up. Compare the Young’s modulus of a cellulose nanofibre, 134 GPa with that of wood along the grain, 10-15 GPa. However, there are many challenges in exploiting fully the properties of cellulose nanofibres. Being slender columns, they cannot carry loads efficiently in compression. As in the case of many polymer-fibre composites based cellulose, aramid and highly oriented polyethylene, micro-buckling results in very poor compressive strengths. In all situations which are not purely tensile, the properties of the matrix become critical, as does the bonding the fibres together. High modulus/high strength fibres require a reasonably stiff matrix in order to be able to “express” their high tensile properties. In other words the modulus ratio between fibre and matrix should not be too high otherwise reinforcement will require very high volume fractions of fibres which are difficult to achieve. A high performance fibre in a very low performance matrix is not a good composite system. The other aspects which present a significant challenge concern fibre orientation and fibre volume fraction. In plant cell walls the orientation and the volume fraction of reinforcement needed to achieve performance is the result of the bottom-up assembly of the cell walls. A composite with a two-dimensional planar distribution of fibres has a modulus of about one-third of the equivalent unidirectional system, dropping to about one-sixth if the fibres are statistically distributed in three dimensions . Technical solutions for aligning cellulose nanofibres will be needed in order to benefit from the properties that the fibres can deliver.
The use of fibres for making structural materials offers a great deal of scope and flexibility in design but it also presents a few problems. Anisotropy of physical and mechanical properties and heterogeneity must be accepted and but also properly exploited in the design of composite materials. In biology this is extremely common and happens as a result of “growing under stress” [4, 5]. The magnitude and direction of the loads that the organism experiences as it develops provide the blueprint for the selective deposition of new material, where it is needed, how much is needed and in the direction in which it is needed.
The assembly of the fibres into more “efficient” hierarchies, at higher levels of scale than the sub-micron, requires compromises between different performance requirements such as high stiffness, high strength or high toughness.
George Jeronimidis is Professor Emeritus in the School of Construction Management and Engineering at the University of Reading, England. He is presently Chair of the Academy Board and presented the Academy lecture at the 2011 annual meeting of IAWS in Stockholm.
His current research interests cover biomimetics ("The abstraction of good design from nature"), plant and animal biomechanics, smart materials and structures, mechanics of composite materials, design of composite structures and the application of biomimetic concepts to architecture. Biomimetic projects include developments of smart textiles, muscle-type actuators, integral strain sensors, impact-energy absorbing materials, natural fibre-based composites, development of cellular materials from renewable starch sources, bio-inspired air-flow sensors. Composite-related projects include the design of smart, self-regulating composite wind turbine blades, friendly suspension systems for road and rail transport which minimize damage to infrastructure, design and development of composite flywheels for "hybrid" urban mass transport systems.