Special issue on the biomimetics of aquatic life: applications for engineering

This special section gained its impetus from the International Conference on 'Biological Applications for Engineering' chaired by Professor Robert Allen (Southampton, UK, 17–19 March 2008) and the resulting publications that followed, reflecting its major topic areas (Robert Allen 2009 Bioinspiration, Biomimetics 4 010201 ). Continuing interest in bioinspired engineering and biomimetics is addressed in the context of models for engineering applications inspired by aquatic life. A collection of six papers bear upon four subject areas: biomaterials (both mostly inorganic hard structural materials and mostly organic fibres), propulsion (biorobotic fins relevant to understanding the performance and sensory control of manoeuvre and steady swimming of fish, and mechanical models that mimic rapid accelerations), group behaviour (employing fish schooling hydrodynamics to model wind turbine farm performance) and ecologically important engineering structures in river systems (improving fish passageway design). A brief synopsis of the content and significance of the papers follows. Barthelat explores the basis and mechanisms of toughness (ability to resist crack propagation) of natural mollusc shell nacre. Nacre is about three orders of magnitude tougher than the mineral (calcium carbonate) of which it is made. This toughness amplification far exceeds that of man-made composites, making nacre an excellent biomimetic model for a new generation of composite ceramics. Nacre's toughness resides in the form and dynamic behaviour of its principal components. Polygonal microscopic tablets slide collectively when loaded in tension, making the material 'quasi-ductile', increasing toughness. Barthelat experimentally mimics the key structures and mechanisms of the sliding process for the first time, allowing for the prospect of the mechanism in natural nacre to be utilized in engineering materials. In a broader sense, he also demonstrates that a biomimetic approach need not completely replicate a biological model to achieve practical engineering ends. Fudge, Hillis, Levy and Gosline show that draw-processed hagfish slime threads yield fibres comparable in mechanical performance (e.g. strength, toughness, extensibility) to spider dragline silk. This might seem to suggest that hagfish slime protein fibres may simply constitute an alternative biomimetic model for engineered high performance protein fibres suitable for comparable applications (e.g. bullet proof fabrics, suspension cables, artificial ligaments). Both offer alternatives to conventional petroleum based synthetic fibre production. However, Fudge et al explain current difficulties and disadvantages regarding the practical utility of the long touted spider silk model. Among them, the expression of spider silk proteins (spidroins), or parts thereof, is difficult because spider silk genes are large and repetitive and dragline silk is subject to marked shrinking when wet. Also, spider silk protein fragments cannot be spun into fibres using conventional technologies. The authors emphasize potential advantages of the hagfish model. In particular, slime thread genes are much smaller and less repetitive than spidroin genes making them more suitable for bacterial expression. In addition, post-drawing steps (e.g. annealing, dehydration, cross-linking) facilitate thread long-term stability. Fudge et al suggest that hagfish slime may be a superior biomimetic model to spider silk. Phelan, Tangorra, Lauder and Hale develop a sunfish based biorobotic model platform of pectoral fin propulsion that generates the principal forces associated with steady forward swimming and manoeuvre. It allows for the assessment of the relationships between sensory information, fin ray motions and propulsive forces. Hitherto, little consideration has been given to the sensory basis of this common form of fish locomotion; nor to its implications for the design and operation of biomimetic autonomous underwater vehicles (AUVs). A small set of sensors represent the fish's sensorimotor system (lateral line and other receptors). Based on experiments, Phelan et al imply a role for receptors intrinsic to the pectoral fins. They find that no single sensory modality is sufficient to predict propulsive force, implying an integration of many sensor modalities. The findings of this elegant preliminary study have important biological implications for understanding the relationships and integrations of fish swimming behaviour, sensory systems and performance. It provides a somewhat unique example of how an engineered robotic system can bear upon the function of the biological model on which it was based. In addition, pectoral fin propulsion of a rigid body is currently a preferred model for smaller AUVs designed for missions requiring low speed and a high degree of manoeuverability. The results of this study bode well for the incorporation of a similar sensory system in AUVs. Conte, Modarres-Sadeghi, Watts, Hover and Triantafyllou construct a simple biomimetic fish, designed to emulate the rapid accelerations from rest (fast- start) motions of fish. The authors point out that the accelerations of specialist 'fast-starters' can far exceed those of man-made vehicles. Potentially biomimetic automated underwater vehicles incorporating the capacity for rapid acceleration could greatly improve start-up, braking and manoeuverability in turbulent aquatic environments. Remarkably, the simple mechanical pike model (a thin metal beam covered by a urethane rubber body with a low aspect ratio tail fin) is sufficient to mimic the basic form of the time-versus-displacement, velocity and acceleration patterns measured for actual pike. Conte et al show that efficiency values (ratio of the final kinetic energy to initial stored potential energy of the body) are also similar to those experimentally determined for fast-starting pike. The broad correspondence in performance pattern of the mechanical model and pike is likely real (as opposed to spurious or fortuitous) and all the more remarkable, given that the model system lacks many of the attributes of a real pike (e.g. posteriorly placed median fins, a rearward travelling body wave). Doubtless, further refinements of the experimental system that increase its fidelity relative to the real fish, will be associated with commensurate performance increases. Whittlesey, Liska and Dabiri employ a bioinspired model of the possible energetic advantages of fish schooling hydrodynamics to bear upon an assessment of the relative performance of arrays of horizontal versus vertical wind turbines (HAWTs and VAWTs, respectively). Whittlesey et al point out that HAWTs in close proximity suffer from a reduced power coefficient relative to an isolated turbine and that VAWTs may experience small decreases or even increases in power coefficient circumstances giving high power output per unit area of land. A potential flow model, based on the configuration of the shed vortices in the wake of schooling fish, suggests power output increases of an order of magnitude for a given land area for VAWTs relative to HAWTs. Given the socio-economic importance of wind turbine farms as power sources, the need to maximize their efficiency and effectiveness is obvious. However, approaching such objectives from the standpoint of a model of the hydrodynamics of fish schooling is far from an obvious point of departure. In addition to thoroughly addressing its purpose, this innovative study illustrates a general point: namely, that biological systems can provide good models for engineering applications, even when there is no obvious correspondence between the purpose of the structures and functions of the biological model and engineering application. Lauritzen, Hertel, Jordan and Gordon point out that, generally, the behavioural or kinematic capabilities of migratory salmonids have not been taken into account in the design and construction of the engineering structures (i.e. passageways) that function to facilitate their upstream movements by negotiating dams and other man-made obstructions. Lauritzen et al employ an ingenious portable adjustable waterfall generator to determine the responses of adult kokanee salmon to flow rate, pool depths, fall heights and angles. They show that kokanee initiate fast-start accelerations from below waterfall plunge pool boils (as opposed to surface C-starts) and burst swim to surface take-off. Clearly, understanding the behaviour and performance of fish is key to the effective function of passageways and other structures intended to facilitate their movement. Put differently, engineering structures in this context should be bioinspired by the capacities and capabilities of the organisms that they are designed to accommodate. In closing, I would like to thank Professor Robert Allen for inviting me to be guest editor for this special edition, Ms Maggie Howls, Mr Richard Kelsall, Dr Andrew Malloy and the publishing team for their support.

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