Reliability Strategies in Living Organisms by Stuart Burgess, B.Sc., Ph.D., CEng., FIMechE Bristol, U.K.   Page:  1 | 2 | 3 Living organisms represent an important source of reliability strategies for engineers and scientists. The natural world contains millions of species of living organisms, and these contain a great range of components and processes that have been proven to function very reliably in harsh environments. For example, a human heart can function as a self-maintaining subsystem for 75 years or more, during which time it beats on the order of 2.5 billion times and pumps some 300 million liters of blood [Ref. 1]. This performance is superior to any man-made pump working in similar conditions, and indeed it is very difficult to design a man-made replacement heart with anywhere near the same capability as a living heart. An important advantage of exploring biological systems is that many engineering systems are taking on more of their characteristics, such as a large number of parts, high integration, large hierarchy (nano-scale to macro-scale) and extreme complexity. Many modern engineering systems, such as aircraft, process plants and motorcars, are also taking on these kinds of characteristics. As engineering systems become more biological in concept, there is an obvious potential benefit in copying biological reliability strategies. Optimum Design One advantage of optimum design is that internal forces can be minimized for a given loading condition, and this can minimize the demands put on material properties. For example, some structural concepts generate inherently lower stresses than others when subjected to a given loading condition. A key advantage of optimum design is that it can minimize the quantity of material required to meet a given design goal. In nature, this means that components can be constructed quickly and with minimal energy. The camel is a good example of optimal heat exchanger design for hot climates. The creature has a tall, thin body that gives minimal projected area against the sun. However, it also has a large surface area for heat loss perpendicular to the sun. The storage of fat in the hump means that the body skin of the camel is able to have very small amounts of fat. The optimum design of heat exchange in the camel minimizes the demands put on the camel’s internal cooling system. The principle of optimum heat exchanger design is also seen in many engineering systems. For example, the use of large cooling fins on the casings of electric motors sometimes alleviates the need for complicated internal cooling systems. The prevalence of optimum design in nature provides strong evidence that optimum design can play an important role in attaining high reliability in engineering systems.   “As engineering systems become more biological in concept, there is an obvious potential benefit in copying biological reliability strategies.” Redundancy Examples of redundancy in essential organs include the kidneys and lungs in the human body. The human body has two kidneys and two lungs but is quite capable of surviving (albeit with reduced capacity) with only one of each. Examples of redundancy in non-essential organs include the eyes and ears. Living organisms generally have live redundancy – i.e., the redundant elements are all fully functioning in parallel, and the load is shared between them while they are all working. The fact that redundancy is seen widely in nature gives support to the principle that this is an important reliability strategy in engineering systems. Automatic Control Living creatures generally have an autonomic nervous system that controls involuntary actions in the body, including the beating of the heart, movements in the gut and the secretion of sweat. A great advantage of having an autonomic nervous system is that the voluntary part of the brain can be free to think about high-level actions such as movement and planning. Also, automatic control allows a speed and consistency that are difficult to match with voluntary control. The speed and consistency of the mammalian autonomic nervous system are undoubtedly a key reason why mammals function very reliably. Automatic control has had a dramatic influence on the efficiency of engineering systems over the last few decades, enabling large increases in efficiency, productivity and reliability. Planned Maintenance An example of planned maintenance in nature is found in the flight feathers of birds. The activities of birds in flight and nesting mean that feathers are prone to damage, so the replacement of feathers is vital to the birds’ survival. It is remarkable that birds replace their feathers while maintaining normal flying activity. Planned replacement is common in engineering systems. Planned Inspection Many creatures carry out self-inspection to maintain a high standard of fitness. Birds pass their beaks through their feathers to repair unzipped barbs and to lubricate the feathers. Humans inspect for all kinds of problems from teeth ailments to cancerous growths. The great advantage of inspection is that dormant problems can be identified and dealt with before they cause significant problems. Planned inspection is common in engineering systems. Storage Most living organisms store fat in order to store energy for lean times. The polar bear provides a particularly extreme example of such storage. After feeding during the spring, a pregnant female’s body contains up to 50% pure fat [Ref. 2]. This results in a 15-cm thick layer of fat around the thighs and body, enabling the polar bear to survive without food for many months. An example of the storage of fuel in an engineering system is seen in the storage of large amounts of coal at power stations. Such storage means that the power station is not vulnerable to breaks in coal supply. Minimal Number of Macro Parts In most cases, biological components are made up of a minimal number of macro parts. For example, a man-made positive displacement pump generally contains several different elements that are assembled together with various fasteners. However, the mammalian heart is largely a one-piece structure where the muscles and nerves are fully integrated. The use of a minimal number of macro parts is an important aid to reliability because there are a low number of interfaces where wear, misalignment and separation can take place [Ref. 3]. Considering the functions that a mammalian body can perform, the number of macro parts and interfaces is remarkably minimal. Biological systems show that there is the potential for reducing the number of macro parts in current engineering systems. One-piece structures are now more feasible with the availability of high-strength and formable materials like carbon fiber reinforced plastic (CFRP). Minimal Number of Sliding Parts As well as having a minimal number of macro parts, biological systems also have a minimal number of sliding parts. The advantage of minimizing the number of sliding parts is that these are vulnerable to excessive wear and movement, and this can lead to poor reliability. The human heart valve is an example of a component with a design that avoids sliding parts. Its flaps act as a non-return valve, and they hinge through flexing without any sliding taking place. In contrast, engineering non-return valves usually contain a bearing device to allow relative movement at a hinge. Relative sliding does occur in the joints of mammals. However, considering the functions that a mammalian body can perform, the number of sliding joints is minimal.   NEXT PAGE

Editor's Note: The human body is amazing. We recognize the process of self-healing when we cut ourselves and watch the body fight off infection by attacking the invading cells and overwhelming them, healing the cut. Self-healing takes place because there is an ample supply, or redundancy, of like cells to be copied, which initiates the healing process. But what happens in a case when there’s no redundancy? How does the body self-heal if there are no model cells to replicate? A study published in the June 19, 2003, issue of Nature describes how the self-healing process works when cells don’t have a defect-free sample from which to make a copy. In the body, chromosomes are inherited in pairs, with one half of every pair coming from the mother and the other from the father. It is this pairing that lets them swap corresponding pieces of themselves, a mechanism that allows the body to rid itself of damaged genes. (Ref. CNN.com/Health, June 19, 2003) The Y chromosome, however, uses a trick to self-heal. Since the singular Y chromosome doesn’t have a match to swap with, it ingeniously uses one of its back-up copies to replace the flawed gene. In engineering, a parallel to the self-healing process occurs when we enlist parts from similar equipment to make a repair, if there are no redundant back-up units. The other alternative is to bypass the "broken" circuits and switch in another segment that will keep the system functioning. When we design automatic alternate routing as part of a system, this is "equipment self-healing" at its best.