Biomimetics: smart geometry at work
by George Jeronimidis, 24 February 2012
Organisms have been fine-tuned under evolutionary pressures over millions of years, a small step at a time. These time-scales may be different from ours but design constraints and objectives are very similar. Functionality, optimisation and cost-effectiveness—or more appropriately—energy-efficiency are the primary drivers. In order to survive, plants and animals have been particularly smart in exploiting materials and geometry
Compared to many engineering materials, the substances/materials of biology do not have any especially outstanding characteristic. They are successful not so much because of what they are but because of the way in which they are put together. The bulk of mechanical loads in biology are carried by polymer fibres such as cellulose (plants), collagen (animals), chitin (insects, crustaceans) and silks (spiders's webs). The fibres are bonded together by various substances (polysaccharrides, polyphenols, etc.), sometimes in combination with minerals such as calcium carbonate (mollusk shells) and hydroxyapatite (bone). Their geometrical organization and the degree of interaction between them provide the means of tailoring properties for specific requirements, meeting the necessary functional performance.
Hierarchical structure of butterfly scale (magnification x 5000). http://en.wikipedia.org/wiki/Butterfly.
The use of fibres for making structural materials offers a great deal of scope and flexibility in design and we can learn from nature how best to exploit the intrinsic anisotropy and heterogeneity that fibrous composites possess. 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 and in the direction in which it is needed–this is a powerful 'design' principle where geometry and materials aspects are highly integrated. The best known examples of this are the 'adaptive' mechanical design of bone and trees.
Numerous geometrical patterns of load-bearing fibrous materials are found in nature, each one of them being a specific answer to a specific set of mechanical conditions and requirements. Every organism is "optimal" in space and time only in relation to the specific environment. The real important aspect of biological optimization is the possibility of adaptation in order to be able respond to changes.
To reinforce the message that often geometry is as important as the material system, if not more important, recent developments in "structural colours" inspired by the scales of butterfly wings have shown that the key is the architecture of the nano-, micro-structures that biology can produce. The pattern shown in the figure interacts with light in such a way that only specific colours are reflected, depending on the dimensions of the pattern. The structure is made of chitin (the fibrous polymer of insects) but it can be made of any substance; provided the dimensions are right, the physics does the rest.
Closeup of the scales of the Inachis io butterfly. http://en.wikipedia.org/wiki/Butterfly.
Many plants are capable of movement, sometimes slow (as in the petals of flowers which open and close, the tracking of the sun by sunflowers, the convolutions of bindweed's around supporting stems, snaking of roots around obstacles), sometimes visible to the eye (as in the drooping of leaves when mimosa pudica is touched), occasionally very rapid (as in the closing of the leaves of the venus flytrap). Movement and force are generated by a unique interaction of materials, structures, geometry and energy sources. The materials are the fibrous cell wall of perenchyma cells (non-lignified, flexible in bending but stiff in tension); the structures are the cells themselves (shape dimensions) which, with the biologically active membrane, can control the passage of fluid in and out of the cells; the energy source is the chemical potential difference between the inside and the outside of the cells.
These systems are essentially working as networks of interacting mini-hydraulic actuators, liquid filled bags which can become turgid or flaccid and which, owing to their shape, geometry and mutual interaction translate local deformations to global ones and are also capable of generating very high stresses. The pressures that can be generated can be as high as 20 bar. Several recent projects in materials, structures and architecture have taken inspiration from turgid plant systems.
There are many instances in engineering where variable stiffness materials and structures would be beneficial. This is particularly true in applications where one would like to alter the shape of a rigid structure, or element of structure, and then re-stiffen it (conformable wings, portable soft-rigid-soft structures). It is an ideal example for biomimetic approaches and, indeed, a great deal of progress has been made in developing "artificial muscle systems" based on active polymer gels, ionic and dielectric.
3-D arrangement of muscles and connective tissue in the octopus arm. From Kier and Stella, 2007.
More recently, work has been carried out on what are called "hydrostats", biological systems, such as the octopus arms, where by contracting various sets of muscles within an incompressible medium, can vary the stiffness of the structure. In the arms of the octopus there is a three-dimensional arrangement of muscle and collagen fibres, as shown in the figure above. The specific orientations of the muscle fibres are longitudinal, radial, circumferential and at ± 450 and they control the 3D shape and rigidity of the arm. By contracting the radial and circumferential muscles, for example, the arm gets longer because it has to work at constant volume. Contracting differentially the longitudinal muscles induces bending. If all the muscles contact simultaneously, because of the incompressibility condition the arm becomes a rigid structure.
Schematic of the bio-inspired octopus arm. From: octopusproject.eu
The biomimetic implementation of this system is a real challenge because the coordinated muscle action needs to be implemented via available types of actuators such as electroactive polymers or shape memory alloys. A schematic of how the artificial system may work is shown in the figure above; more information is available at www.octopus project.eu.
The examples presented above suggest that the design of biological systems relies heavily on geometrical attributes of the hierarchical structures, over several length scales, rather than on materials performance. One reason for this is that the production of fibrous materials is metabolically expensive and extracting maximum performance at minimum energy cost is vital for survival. This design constraint, coupled with the ability of organisms to adapt, has led to the extraordinary range of design solutions we observe in nature. Learning from those can provide significant benefits, especially in developing our own approaches and strategies for higher levels of integration between materials, structures and geometry for added functionality.