[Excerpt:] In nature, shape is cheaper than material. This has been shown a number of times and is manifested in the remarkably high performance, both absolute and specific, of biological materials (wood is one of the most efficient of materials; antler bone is tougher than any man-made ceramic composite) which is achieved not by the use of high performance components but by the degree of detail and competence in their design and construction.
Smart by Nature
An essay from: Lightness — The Inevitable Renaissance of Minimum Energy Structures
There is a duality between engineering and nature which is based on minimum use of energy. This is because animals and plants, in order to survive in competition with each other, have evolved ways of living and reproducing using the least amount of resource. This involves efficiency both in metabolism and optimal apportionment of energy between the various functions of life. A similar situation obtains with engineering, where cost is usually the most significant parameter. It seems likely, then, that ideas from nature, suitably interpreted and implemented, could improve the energy efficiency of our engineering at many levels. This transfer of technology, variously called bionics, biomimetics or biognosis, should not be seen so much as a panacea for engineering problems as a portfolio of paradigms.
In nature, shape is cheaper than material. This has been shown a number of times and is manifested in the remarkably high performance, both absolute and specific, of biological materials (wood is one of the most efficient of materials; antler bone is tougher than any man-made ceramic composite) which is achieved not by the use of high performance components but by the degree of detail and competence in their design and construction. The implication is not only that animals and plants have to work hard to win the raw materials – sugars, amino acids, salts – from their environment, but that their control over the assembly and shaping of these materials is much more complete than ours. An essential part of this control is the cellular feed-back mechanisms which direct the accretion of material to places where it is most needed, resulting in adaptive structures. The shape of a tree is the history of the forces which were acting on it while it grew. These same sensory mechanisms, allied to a more mobile effector system as found in animals, lead on to structures whose lightness and apparent fragility are made robust by the ability to adapt shape and structure quickly to changing loads. This adaptiveness not only reduces the energy input into the production of the structure, but also allows it to adapt to changing forces and circumstances during its lifetime, many of which may be unpredictable. Such adaptiveness has also been called smart or intelligent behaviour.
The concept of Smart or Intelligent materials (and systems and structures) has been around for a number of years. A “smart” material (or system or structure – the one word takes all) interacts with its environment, responding to changes in various ways. A simple example is photochromic glass, darkening on exposure to light. In order to be responsive to its environment a material must have structure (for example, the molecular mechanism underlying photochromic glass) and in most instances is a system since it needs a receptor or range of receptors, a central processor which can differentiate between the inputs and integrate them into a single output, and an effector. This system could be considered as a material if it were integrated within a single lump of stuff (rather than having wires going from and to the central processor) and were being used or observed in a size range at least (somewhat arbitrarily) ten times larger than the size of the individual components. Smartness can be a simple response which follows on directly and inevitably from the stimulus; or the outcome of an if-then construct in which a decision is made based on balancing the information from two or more inputs; or the ability to learn, which is probably the smartest thing of all, since learning can lead to a patterned model of the world (the brain is “stored environment”) allowing informed prediction. It can be argued that the successful organism is the one which knows what is going to happen next and that prescience is more important than smartness, or at least subsumes it. How can smartness be implemented, and what might it do for technology? I think it’s worth first comparing the design philosophy of nature with that of an engineer. Consider a robot such as might be found in a factory. It is carefully designed so that the arms are of the correct length and stiffness for their purpose. The joints are carefully made and give the arm(s) well-defined arcs and planes of movement. Compare the animal equivalent, which has arms of undefined length and varying stiffness, joints with very well designed bearing surfaces, and arcs and planes of movement which are relatively vaguely defined. The skeleton is defined purely functionally and can have a relatively wide variety of shapes and still work properly. For instance, in learning to walk you have a general aim and make adjustments until you manage it. But everyone walks differently due to their individual technique and adaptations to their own particular design of skeleton. So the structure of a robot need be defined only in terms of its load bearing ability and the positions and places in which it needs to hold or place things. It is necessary to have very good bearings at the joints but the material and structure of the arms and other parts are far less critical. But there is no way the robot can tell where the end of its arm might be. It has to be taught by example. Move the bits of the robot to where you want them to be and let the robot remember how it did it. Such a concept would be far cheaper to produce. For “robot”, read “any sort of machine or structure”. The important point is that you don’t have to engineer every part of the structure to very high tolerance if the structure is smart and can learn how to cope functionally with what it is.
The concepts of robotics can be applied to buildings, making them mechanically adaptive. This is already done in a relatively primitive way to give some protection against earth movements. But a truly responsive building would be prestressed, converting compressive loads into tensile ones, gathering all the residual compressive loads into a single mast. It would then respond to changes in internal and external forces by adaptively changing its state of prestress, giving lightweight stability. The technology for implementing such designs is with us, at least in “bolt-on” mode. Piezoelectric elements can provide strain sensing and small-scale actuation, as can strain gauges and linear motors. Integration of input and output is a trivial problem with modern technology, although precisely what action might be taken in response to a given input is not always obvious.
Nature’s technology involves miniaturisation and integration. Sensing necessarily occurs at the molecular level. Sensitivity is much higher – in the campaniform sensillum of insects, for instance, nanometer displacements can be registered. The sensillum is integrated into the fibrous composite material which makes the exoskeleton of the insect in such a way that it can transmit displacements to the sensor cell, without compromising the mechanical continuity of the exoskeleton. This gives a model for strain sensors which could be built into a composite skin such as is used in fighter aircraft to form the basis of a health monitoring system or form part of a smart control and feedback system. The UK Defence community is also working on a reconfigurable aerofoil whose material is based on the design of the skin of the sea cucumber. This skin is a fibrous composite material (collagen in a mucopolysaccharide matrix) which can change its stiffness. Thus it can soften, change its shape, and stiffen again. The actuation system for the aerofoil could be based on another current project – worms. A gel inside a suitably engineered compliant container with properly designed fibre orientations in the wall. The gel can be stimulated chemically (or electrically or thermally), change its volume by absorbing a solvent, and change its shape as a function of the geometry of its enclosure. Quite large forces can be generated in this way (plants lift concrete slabs using the same mechanism), yet the specific gravity of the system is only 1.
The ultimate smart structure would design itself. Imagine a bridge which accretes material as vehicles move over it and it is blown by the wind. It detects the areas where it is overstretched (taking into account a suitable safety factor) and adds material until the deformation falls back within a prescribed limit. We have the technology to detect the overload, but lack the means to add material automatically. We are part-way there with Carolyn Dry’s self-repairing concrete structures, in which fractures cause reinforcing material to be released from embedded brittle containers and added to the structure. The ideal would be for the material to be added from an external source so that the structure was not compromised by having to contain its own salvation, necessarily reducing its load-bearing ability. Combine this with adaptive prestressing and the ability to remove material from areas which are underloaded, and we have a truly adaptive architecture. This approach would result in lighter and safer structures, since stress concentrations would never occur, and the safety factor could be reduced as the structure reached its design optimum – registered as a reduction in the rate of internal reorganisation. The paradigm is our own skeleton.
Nature is smart – are we smart enough to learn its lessons?