Table of contents

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.

Nacre is the iridescent layer found inside a large number of mollusk shells. This natural composite has a very high mineral content, which makes it hard and stiff. However it is the toughness of nacre which is the most impressive: it is three orders of magnitude tougher than the mineral it is made of. No manmade composite material can boast such amplification in toughness, and for this reason nacre has become a biomimetic model material. The mineral in nacre comes in the form of microscopic polygonal tablets, which have the ability to 'slide' on one another in large numbers when the material is loaded in tension. This key mechanism makes nacre a quasi-ductile material, which in turn greatly increases its toughness and makes it damage tolerant. Numerous 'artificial nacres' were developed in the past but none of them can truly duplicate the remarkable mechanism of tablet sliding. In this work selected structural features of nacre were implemented in a PMMA-based composite, which for the first time could replicate the collective tablet sliding mechanism. This material demonstrates that the powerful toughening mechanism operating in natural nacre can be duplicated and harnessed in engineering materials.

Textile manufacturing is one of the largest industries in the world, and synthetic fibres represent two-thirds of the global textile market. Synthetic fibres are manufactured from petroleum-based feedstocks, which are becoming increasingly expensive as demand for finite petroleum reserves continues to rise. For the last three decades, spider silks have been held up as a model that could inspire the production of protein fibres exhibiting high performance and ecological sustainability, but unfortunately, artificial spider silks have yet to fulfil this promise. Previous work on the biomechanics of protein fibres from the slime of hagfishes suggests that these fibres might be a superior biomimetic model to spider silks. Based on the fact that the proteins within these 'slime threads' adopt conformations that are similar to those in spider silks when they are stretched, we hypothesized that draw processing of slime threads should yield fibres that are comparable to spider dragline silk in their mechanical performance. Here we show that draw-processed slime threads are indeed exceptionally strong and tough. We also show that post-drawing steps such as annealing, dehydration and covalent cross-linking can dramatically improve the long-term dimensional stability of the threads. The data presented here suggest that hagfish slime threads are a model that should be pursued in the quest to produce fibres that are ecologically sustainable and economically viable.

A comprehensive understanding of the control of flexible fins is fundamental to engineering underwater vehicles that perform like fish, since it is the fins that produce forces which control the fish's motion. However, little is known about the fin's sensory system or about how fish use sensory information to modulate the fin and to control propulsive forces. As part of a research program that involves neuromechanical and behavioral studies of the sunfish pectoral fin, a biorobotic model of the pectoral fin and of the fin's sensorimotor system was developed and used to investigate relationships between sensory information, fin ray motions and propulsive forces. This robotic fin is able to generate the motions and forces of the biological fin during steady swimming and turn maneuvers, and is instrumented with a relatively small set of sensors that represent the biological lateral line and receptors hypothesized to exist intrinsic to the pectoral fin. Results support the idea that fin ray curvature, and the pressure in the flow along the wall that represents the fish body, capture time-varying characteristics of the magnitude and direction of the force created throughout a fin beat. However, none of the sensor modalities alone are sufficient to predict the propulsive force. Knowledge of the time-varying force vector with sufficient detail for the closed-loop control of fin ray motion will result from the integration of characteristics of many sensor modalities.

We have built a simple mechanical system to emulate the fast-start performance of fish. The system consists of a thin metal beam covered by a urethane rubber, the fish body and an appropriately shaped tail. The body form of the mechanical fish was modeled after a pike species and selected because it is a widely-studied fast-start specialist. The mechanical fish was held in curvature and hung in water by two restraining lines, which were simultaneously released by a pneumatic cutting mechanism. The potential energy in the beam was transferred into the fluid, thereby accelerating the fish. We measured the resulting acceleration, and calculated the efficiency of propulsion for the mechanical fish model, defined as the ratio of the final kinetic energy of the fish and the initially stored potential energy in the body beam. We also ran a series of flow visualization tests to observe the resulting flow patterns. The maximum start-up acceleration was measured to be around 40 m s⁻², with the maximum final velocity around 1.2 m s⁻¹. The form of the measured acceleration signal as function of time is quite similar to that of type I fast-start motions studied by Harper and Blake (1991 J. Exp. Biol. 155 175–92). The hydrodynamic efficiency of the fish was found to be around 10%. Flow visualization of the mechanical fast-start wake was also analyzed, showing that the acceleration peaks are associated with the shedding of two vortex rings in near-lateral directions.

Most wind farms consist of horizontal axis wind turbines (HAWTs) due to the high power coefficient (mechanical power output divided by the power of the free-stream air through the turbine cross-sectional area) of an isolated turbine. However when in close proximity to neighboring turbines, HAWTs suffer from a reduced power coefficient. In contrast, previous research on vertical axis wind turbines (VAWTs) suggests that closely spaced VAWTs may experience only small decreases (or even increases) in an individual turbine's power coefficient when placed in close proximity to neighbors, thus yielding much higher power outputs for a given area of land. A potential flow model of inter-VAWT interactions is developed to investigate the effect of changes in VAWT spatial arrangement on the array performance coefficient, which compares the expected average power coefficient of turbines in an array to a spatially isolated turbine. A geometric arrangement based on the configuration of shed vortices in the wake of schooling fish is shown to significantly increase the array performance coefficient based upon an array of 16 × 16 wind turbines. The results suggest increases in power output of over one order of magnitude for a given area of land as compared to HAWTs.

Behavioral and kinematic properties and capacities of wild migratory salmonid fishes swimming upstream and jumping up waterfalls generally have played only minor roles in the design and construction of passageways intended to help these fishes get past dams and other human-made obstacles blocking their movements. This paper reports the results of an experimental study of relevant behavioral and kinematic properties of adult kokanee salmon (Oncorhynchus nerka) jumping up waterfalls as they migrate upstream. We used a portable, adjustable apparatus to study in the field fish responding to artificial waterfalls under a range of flow conditions. We observed fish under conditions of varying water flow rates, pool depths, fall heights and fall angles. We analyzed digital video recordings of their behaviors. Kokanee salmon spontaneously jump up waterfalls within a relatively narrow range of conditions, including low flow speeds, near vertical angles and pool depth to fall height ratios near 1.0. Preferred values for each parameter are, to some extent, dependent on other parameters. In contrast to previous misconceptions, jumping behavior is initiated by running S-start accelerations from beneath the boils formed in the plunge pools below waterfalls, as opposed to C-start standing jumps from the surface. S-starts are immediately followed by burst swimming to the point of takeoff at the surface. These results can contribute to an improved basis for developing designs of fish passageways that may ultimately make them more effective and efficient.

Motivated to develop a technique for producing many high-fidelity replicas for the sacrifice of a single biotemplate, we combined a modified version of the conformal-evaporated-film-by-rotation technique and electroforming to produce a master negative made of nickel from a composite biotemplate comprising several corneas of common blowflies. This master negative can function as either a mold for casting multiple replicas or a die for stamping multiple replicas. An approximately 250 nm thick nickel film was thermally deposited on an array of blowfly corneas to capture the surface features with high fidelity and then a roughly 60 µm thick structural layer of nickel was electroformed onto the thin layer to give it the structural integrity needed for casting or stamping. The master negative concurrently captured the spatial features of the biotemplate at length scales ranging from 200 nm to a few millimeters. Polymer replicas produced thereafter by casting did faithfully reproduce features of a few micrometers and larger in dimension.

We present a small single camera imaging system that provides a continuous 280° field of view (FOV) inspired by the large FOV of insect eyes. This is achieved by combining a curved reflective surface that is machined into acrylic glass with lenses covering the frontal field that otherwise would have been obstructed by the mirror. Based on the work of Seidl (1982 PhD Thesis Technische Hochschule Darmstadt), we describe an extension of the 'bee eye optics simulation' (BEOS) model by Giger (1996 PhD Thesis Australian National University) to the full FOV which enables us to remap camera images according to the spatial resolution of honeybee eyes. This model is also useful for simulating the visual input of a bee-like agent in a virtual environment. The imaging system in combination with our bee eye model can serve as a tool for assessing the visual world from a bee's perspective which is particularly helpful for experimental setups. It is also well suited for mobile robots, in particular on flying vehicles that need light-weight sensors.

The effect of the velocity program and duty cycle (St_L) on the propulsive efficiency of pulsed-jet propulsion was studied experimentally on a self-propelled, pulsed-jet underwater vehicle, dubbed Robosquid due to the similarity of essential elements of its propulsion system with squid jet propulsion. Robosquid was tested for jet slug length-to-diameter ratios (L/D) in the range 2–6 and St_L in the range 0.2–0.6 with jet velocity programs commanded to be triangular or trapezoidal. Digital particle image velocimetry was used for measuring the impulse and energy of jet pulses to calculate the pulsed-jet propulsive efficiency and compare it with an equivalent steady jet system. Robosquid's Reynolds number (Re) based on average vehicle velocity and vehicle diameter ranged between 1300 and 2700 for the conditions tested. The results indicated better propulsive efficiency of the trapezoidal velocity program (up to 20% higher) compared to the triangular velocity program. Also, an increase in the ratio of the pulsed-jet propulsive efficiency to the equivalent steady jet propulsive efficiency (η_P/η_{P, ss}) was observed as St_L increased and L/D decreased. For cases of short L/D and high St_L, η_P/η_{P, ss} was found to be as high as 1.2, indicating better performance of pulsed jets. This result demonstrates a case where propulsion using essential elements of a biological locomotion system can outperform the traditional mechanical system equivalent in terms of efficiency. It was also found that changes in St_L had a proportionately larger effect on propulsive efficiency compared to changes in L/D. A simple model is presented to explain the results in terms of the contribution of over-pressure at the nozzle exit plane associated with the formation of vortex rings with each jet pulse.

Much spider silk research to date has focused on its mechanical properties. However, the webs of many orb-web spiders have evolved for over 136 million years to evade visual detection by insect prey. It is therefore a photonic device in addition to being a mechanical device. Herein we use optical surface profiling of capture silks from the webs of adult female St Andrews cross spiders (Argiope keyserlingi) to successfully measure the geometry of adhesive silk droplets and to show a bowing in the aqueous layer on the spider capture silk between adhesive droplets. Optical surface profiling shows geometric features of the capture silk that have not been previously measured and contributes to understanding the links between the physical form and biological function. The research also demonstrates non-standard use of an optical surface profiler to measure the maximum width of a transparent micro-sized droplet (microlens).

This paper presents direct measurements of the aerodynamic forces on the wing of a free-flying, insect-like ornithopter that was modeled on a hawk moth (Manduca sexta). A micro differential pressure sensor was fabricated with micro electro mechanical systems (MEMS) technology and attached to the wing of the ornithopter. The sensor chip was less than 0.1% of the wing area. The mass of the sensor chip was 2.0 mg, which was less than 1% of the wing mass. Thus, the sensor was both small and light in comparison with the wing, resulting in a measurement system that had a minimal impact on the aerodynamics of the wing. With this sensor, the 'pressure coefficient' of the ornithopter wing was measured during both steady airflow and actual free flight. The maximum pressure coefficient observed for steady airflow conditions was 1.4 at an angle of attack of 30°. In flapping flight, the coefficient was around 2.0 for angles of attack that ranged from 25° to 40°. Therefore, a larger aerodynamic force was generated during the downstroke in free flight compared to steady airflow conditions.

It has come to the attention of the authors that, due to a typographical error, the values for mass in table 1 were ten times larger than the correct values. The correct version of table 1 is reproduced in the PDF with the corrected values shown in a bold. The calculations in the rest of the paper were based on the correct mass values, so they are unaffected.