Experimental Hydrodynamics and Evolution: Function of Median Fins in Ray-finned Fishes

The median fins of fishes consist of the dorsal, anal, and caudal fins and have long been thought to play an important role in generating locomotor force during both steady swimming and maneuvering. But the orientations and magnitudes of these forces, the mechanisms by which they are generated, and how fish modulate median fin forces have remained largely unknown until the recent advent of Digital Particle Image Velocimetry (DPIV) which allows empirical analysis of force magnitude and direction. Experimental hydrodynamic studies of median fin function in fishes are of special utility when conducted in a comparative phylogenetic context, and we have examined fin function in four ray-finned fish clades (sturgeon, trout, sunfish, and mackerel) with the goal of testing classical hypotheses of fin function and evolution. In this paper we summarize two recent technical developments in DPIV methodology, and discuss key recent findings relevant to median fin function. High-resolution DPIV using a recursive local-correlation algorithm allows quantification of small vortices, while stereo-DPIV permits simultaneous measurement of x, y, and z flow velocity components within a single planar light sheet. Analyses of median fin wakes reveal that lateral forces are high relative to thrust force, and that mechanical performance of median fins (i.e., thrust as a proportion of total force) averages 0.35, a surprisingly low value. Large lateral forces which could arise as an unavoidable consequence of thrust generation using an undulatory propulsor may also enhance stability and maneuverability. Analysis of hydrodynamic function of the soft dorsal fin in bluegill sunfish shows that a thrust wake is generated that accounts for 12% of total thrust and that the thrust generation by the caudal fin may be enhanced by interception of the dorsal fin wake. Integration of experimental studies of fin wakes, computational approaches, and mechanical models of fin function promise understanding of instantaneous forces on fish fins during the propulsive cycle as well as exploration of a broader locomotor design space and its hydrodynamic consequences.     

Experimental hydrodynamics of fish locomotion: functional insights from wake visualization

Despite enormous progress during the last twenty years in understandingthe mechanistic basis of aquatic animal propulsion—a taskinvolving the construction of a substantial data base on patternsof fin and body kinematics and locomotor muscle function—thereremains a key area in which biologists have little information:the relationship between propulsor activity and water movementin the wake. How is internal muscular force translated intoexternal force exerted on the water? What is the pattern offluid force production by different fish fins (e.g., pectoral,caudal, dorsal) and how does swimming force vary with speedand among species? These types of questions have received considerableattention in analyses of terrestrial locomotion where forceoutput by limbs can be measured directly with force plates.But how can forces exerted by animals moving through fluid bemeasured? The advent of digital particle image velocimetry (DPIV)has provided an experimental hydrodynamic approach for quantifyingthe locomotor forces of freely moving animals in fluids, andhas resulted in significant new insights into the mechanismsof fish propulsion. In this paper we present ten "lessons learned"from the application of DPIV to problems of fish locomotionover the last five years. (1) Three-dimensional DPIV analysisis critical for reconstructing wake geometry. (2) DPIV analysisreveals the orientation of locomotor reaction forces. (3) DPIVanalysis allows calculation of the magnitude of locomotor forces.(4) Swimming speed can have a major impact on wake structure.(5) DPIV can reveal interspecific differences in vortex wakemorphology. (6) DPIV analysis can provide new insights intothe limits to locomotor performance. (7) DPIV demonstrates thefunctional versatility of fish fins. (8) DPIV reveals hydrodynamicforce partitioning among fins. (9) DPIV shows that wake interactionamong fins may enhance thrust production. (10) Experimentalhydrodynamic analysis can provide insight into the functionalsignificance of evolutionary variation in fin design.     

Functional Morphology of Aquatic Flight in Fishes: Kinematics, Electromyography, and Mechanical Modeling of Labriform Locomotion

Labriform locomotion is the primary swimming mode for many fishesthat use the pectoral fins to generate thrust across a broadrange of speeds. A review of the literature on hydrodynamics,kinematics, and morphology of pectoral fin mechanisms in fishesreveals that we lack several kinds of morphological and kinematicdata that are critical for understanding thrust generation inthis mode, particularly at higher velocities. Several needsinclude detailed three-dimensional kinematic data on speciesthat are pectoral fin swimmers across a broad range of speeds,data on the motor patterns of pectoral fin muscles, and thedevelopment of a mechanical model of pectoral fin functionalmorphology. New data are presented here on pectoral fin locomotionin Gomphosus varius, a labrid fish that uses the pectoral finsat speeds of 1 –6 total body lengths per second. Three-dimensionalkinematic data for the pectoral fins of G. varius show thata typical "drag-based" mechanism is not used in this species.Instead, the thrust mechanics of this fish are dominated bylift forces and acceleration reaction forces. The fin is twistedlike a propeller during the fin stroke, so that angles of attackare variable along the fin length. Electromyographic data onsix fin muscles indicate the sequence of muscle activity thatproduces antagonistic fin abduction and adduction and controlsthe leading edge of the fin. EMG activity in abductors and adductorsis synchronous with the start of abduction and adduction, respectively,so that muscle mechanics actuate the fin with positive work.A mechanical model of the pectoral fin is proposed in whichfin morphometrics and computer simulations allow predictionsof fin kinematics in three dimensions. The transmission of forceand motion to the leading edge of the fin depends on the mechanicaladvantage of fin ray levers. An integrative program of researchis suggested that will synthesize data on morphology, physiology,kinematics, and hydrodynamics to understand the mechanics ofpectoral fin swimming.     

Functional morphology of the fin rays of teleost fishes

Ray‐finned fishes are notable for having flexible fins that allow for the control of fluid forces. A number of studies have addressed the muscular control, kinematics, and hydrodynamics of flexible fins, but little work has investigated just how flexible ray‐finned fish fin rays are, and how flexibility affects their response to environmental perturbations. Analysis of pectoral fin rays of bluegill sunfish showed that the more proximal portion of the fin ray is unsegmented while the distal 60% of the fin ray is segmented. We examined the range of motion and curvatures of the pectoral fin rays of bluegill sunfish during steady swimming, turning maneuvers, and hovering behaviors and during a vortex perturbation impacting the fin during the fin beat. Under normal swimming conditions, curvatures did not exceed 0.029 mm^?1 in the proximal, unsegmented portion of the fin ray and 0.065 mm^?1 in the distal, segmented portion of the fin ray. When perturbed by a vortex jet traveling at approximately 1 ms^?1 (67 ± 2.3 mN s.e. of force at impact), the fin ray underwent a maximum curvature of 9.38 mm^?1. Buckling of the fin ray was constrained to the area of impact and did not disrupt the motion of the pectoral fin during swimming. Flexural stiffness of the fin ray was calculated to be 565 × 10^?6 Nm². In computational fluid dynamic simulations of the fin‐vortex interaction, very flexible fin rays showed a combination of attraction and repulsion to impacting vortex dipoles. Due to their small bending rigidity (or flexural stiffness), impacting vortices transferred little force to the fin ray. Conversely, stiffer fin rays experienced rapid small‐amplitude oscillations from vortex impacts, with large impact forces all along the length of the fin ray. Segmentation is a key design feature of ray‐finned fish fin rays, and may serve as a means of making a flexible fin ray out of a rigid material (bone). This flexibility may offer intrinsic damping of environmental fluid perturbations encountered by swimming fish. J. Morphol. 274:1044–1059, 2013. © 2013 Wiley Periodicals, Inc.     

A new system for analyzing swim fin propulsion based on human kinematic data

The use of swim fins has become popular in various water sport activities. While numerous models of swim fin with various innovative shapes have been subjectively designed, the exact influence of the fin characteristics on swimming performance is still much debated, and remains difficult to quantify. To date, the most common approach for evaluating swim fin propulsion is based on the study of “swimmer-fins” as a global system, where physiological and/or biomechanical responses are considered. However, reproducible swimming technique is difficult (or even impossible) to obtain on human body and may lead to discrepancies in data acquired between trials. In this study, we present and validate a new automat called HERMES which enables an evaluation of various swim fins during an adjustable, standardized and reproducible motion. This test bench reliably and accurately reproduces human fin-swimming motions, and gives resulting dynamic measurements at the ankle joint. Seven fins with various geometrical and mechanical characteristics were tested. For each swim fin, ankle force and hydromechanical efficiency (useful mechanical power output divided by mechanical power input delivered by the motors) were calculated. Efficiencies reported in our study were high (close to 70% for some swim fins) over a narrow range of Strouhal number (St) and peaks within the interval 0.2<St<0.4, as shown in previous studies on flying or swimming animals. Therefore, an interesting prospect in this work would be to accurately study the impact of adjustable fin kinematics and material (design and mechanical properties) on the wake structure and on efficiency.     

Computation of Unsteady Flow Past a Biomimetic Fin

The unsteady hydrodynamics of a biomimetic fin attached to a cylindrical body has been studied numerically using a computational fluid dynamic (CFD) simulator based on an in-house solver of the Navier-Stokes equations, combined with a recently developed multi-block, overset grid method. The fin-body CFD model is based on a mechanical pectoral fin device, which consists of a cylindrical body and an asymmetric fin and can mimic flapping, rowing and feathering motions of the pectoral fins in fishes. First the multi-block, overset grid method incorporated into the NS solver was verified through an extensive study of unsteady flows past a single fin undergoing rowing and feathering motion. Then unsteady flows past the biomimetic fin-body model undergoing the same motions were computed and compared with the measurements of forces of the mechanical pectoral fin, which shows good agreement in both time-varying and time-averaged hydrodynamic forces. The relationship between force generation and vortex dynamics points to the importance of the match in fin kinematics between power and recovery strokes and implies that an optimal selection of parameters of phase lags between and amplitudes of rowing and feathering motions can improve the performance of labriform propulsion in terms of either maximum force generation or minimum mechanical power.     

Mechanosensation in an adipose fin

Adipose fins are found on approximately 20% of ray-finned fish species. The apparently rudimentary anatomy of adipose fins inspired a longstanding hypothesis that these fins are vestigial and lack function. However, adipose fins have evolved repeatedly within Teleostei, suggesting adaptive function. Recently, adipose fins were proposed to function as mechanosensors, detecting fluid flow anterior to the caudal fin. Here we test the hypothesis that adipose fins are mechanosensitive in the catfish Corydoras aeneus. Neural activity, recorded from nerves that innervate the fin, was shown to encode information on both movement and position of the fin membrane, including the magnitude of fin membrane displacement. Thus, the adipose fin of C. aeneus is mechanosensitive and has the capacity to function as a ‘precaudal flow sensor’. These data force re-evaluation of adipose fin clipping, a common strategy for tagging fishes, and inform hypotheses of how function evolves in novel vertebrate appendages.     

Musculoskeletal morphology and regionalization within the dorsal and anal fins of bluegill sunfish (Lepomis macrochirus)

Ray‐finned fishes actively control the shape and orientation of their fins to either generate or resist hydrodynamic forces. Because of the emergent mechanical properties of their segmented, bilaminar fin rays (lepidotrichia), and actuation by multiple muscles, fish can control the rigidity and curvature of individual rays independently, thereby varying the resultant forces across the fin surfaces. Expecting that differences in fin‐ray morphology should reflect variation in their mechanical properties, we measured several musculoskeletal features of individual spines and rays of the dorsal and anal fins of bluegill sunfish, Lepomis macrochirus, and assessed their mobility and flexibility. We separated the fin‐rays into four groups based on the fin (dorsal or anal) or fin‐ray type (spine or ray) and measured the length of the spines/rays and the mass of the three median fin‐ray muscles: the inclinators, erectors and depressors. Within the two ray groups, we measured the portion of the rays that were segmented vs. unsegmented and branched vs. unbranched. For the majority of variables tested, we found that variations between fin‐rays within each group were significantly related to position within the fin and these patterns were conserved between the dorsal and anal rays. Based on positional variations in fin‐ray and muscle parameters, we suggest that anterior and posterior regions of each fin perform different functions when interacting with the surrounding fluid. Specifically, we suggest that the stiffer anterior rays of the soft dorsal and anal fins maintain stability and keep the flow across the fins steady. The posterior rays, which are more flexible with a greater range of motion, fine‐tune their stiffness and orientation, directing the resultant flow to generate lateral and some thrust forces, thus acting as an accessory caudal fin. J. Morphol., 2012. © 2011 Wiley Periodicals, Inc.     

A predictive model of cell traction forces based on cell geometry

Recent work has indicated that the shape and size of a cell can influence how a cell spreads, develops focal adhesions, and exerts forces on the substrate. However, it is unclear how cell shape regulates these events. Here we present a computational model that uses cell shape to predict the magnitude and direction of forces generated by cells. The predicted results are compared to experimentally measured traction forces, and show that the model can predict traction force direction, relative magnitude, and force distribution within the cell using only cell shape as an input. Analysis of the model shows that the magnitude and direction of the traction force at a given point is proportional to the first moment of area about that point in the cell, suggesting that contractile forces within the cell act on the entire cytoskeletal network as a single cohesive unit. Through this model, we demonstrate that intrinsic properties of cell shape can facilitate changes in traction force patterns, independently of heterogeneous mechanical properties or signaling events within the cell.     

A comparison of pectoral fin ray morphology and its impact on fin ray flexural stiffness in labriform swimmers

The organization of tissues in appendages often affects their mechanical properties and function. In the fish family Labridae, swimming behavior is associated with pectoral fin flexural stiffness and morphology, where fins range on a continuum from stiff to relatively flexible fins. Across this diversity, pectoral fin flexural stiffness decreases exponentially along the length of any given fin ray, and ray stiffness decreases along the chord of the fin from the leading to trailing edge. In this study, we examine the morphological properties of fin rays, including the effective modulus in bending (E), second moment of area (I), segmentation, and branching patterns, and their impact on fin ray stiffness. We quantify intrinsic pectoral fin ray stiffness in similarly sized fins of two closely related species that employ fins of divergent mechanics, the flapping Gomphosus varius and the rowing Halichoeres bivittatus. While segmentation patterns and E were similar between species, measurements of I and the number of fin ray branch nodes were greater in G. varius than in H. bivittatus. A multiple regression model found that of these variables, I was always significantly correlated with fin ray flexural stiffness and that variation in I always explained the majority of the variation in flexural stiffness. Thus, while most of the morphological variables quantified in this study correlate with fin ray flexural stiffness, second moment of area is the greatest factor contributing to variation in flexural stiffness. Further, interspecific variation in fin ray branching pattern could be used as a means of tuning the effective stiffness of the fin webbing to differences in swimming behavior and hydrodynamics. The comparison of these results to other systems begins to unveil fundamental morphological features of biological beams and yields insight into the role of mechanical properties in fin deformation for aquatic locomotion.     

Work that body: fin and body movements determine herbivore feeding performance within the natural reef environment

Herbivorous fishes form a keystone component of reef ecosystems, yet the functional mechanisms underlying their feeding performance are poorly understood. In water, gravity is counter-balanced by buoyancy, hence fish are recoiled backwards after every bite they take from the substrate. To overcome this recoil and maintain contact with the algae covered substrate, fish need to generate thrust while feeding. However, the locomotory performance of reef herbivores in the context of feeding has hitherto been ignored. We used a three-dimensional high-speed video system to track mouth and body kinematics during in situ feeding strikes of fishes in the genus Zebrasoma, while synchronously recording the forces exerted on the substrate. These herbivores committed stereotypic and coordinated body and fin movements when feeding off the substrate and these movements determined algal biomass removed. Specifically, the speed of rapidly backing away from the substrate was associated with the magnitude of the pull force and the biomass of algae removed from the substrate per feeding bout. Our new framework for measuring biting performance in situ demonstrates that coordinated movements of the body and fins play a crucial role in herbivore foraging performance and may explain major axes of body and fin shape diversification across reef herbivore guilds.     

Evidence of force exchanges during the six-legged walking of the bottom-dwelling fish, Chelidonichthys lucerna

Locomotion in terrestrial vertebrates is supposed to be derived from preadaptation in bottom-dwelling fish. A few fish species have been assumed to walk on the substratum, on the basis of coordinated movements of their paired fins. However, the validity of this assumption has remained uncertain, because of a lack of evidence that their fin rays actually exert a force on the substratum. Here, we provide the first conclusive evidence that a benthic teleost fish, the gurnard, Chelidonichthys lucerna (Triglidae), exerts forces on the substratum during its temporary bottom-dwelling hexapod locomotion. This demonstration was achieved by the use of a photoelastic gel technique combined with a force calibration device. The movement patterns of the three first pairs of rays of the pectoral fins were analysed in relation to the forces exerted on the substratum, by measuring deformations of the photoelastic gel substratum produced by the rays. The rays were shown to produce a force pattern that confirmed the existence of a hexapod locomotion in a vertebrate that was consistent with body propulsion and voluntary substratum walking.     

Computational Research on Modular Undulating Fin for Biorobotic Underwater Propulsor

Biomimetic design employs the principles of nature to solve engineering problems. Such designs which are hoped to be quick, efficient, robust, and versatile, have taken advantage of optimization via natural selection. In the present research, an environment-friendly propulsion system mimicking undulating fins of stingray was built. A non-conventional method was considered to model the flexibility of the fins of stingray. A two-degree-of-freedom mechanism comprised of several linkages was designed and constructed to mimic the actual flexible fin, The driving linkages were used to form a mechanical fin consisting of several fin segments, which are able tO produce undulations, similar to those produced by the actual fins. Owing to the modularity of the design of the mechanical fin, various undulating patterns can be realized. Some qualitative observations, obtained by experiments, predicted that the thrusts produced by the mechanical fin are different among various undulating patterns. To fully understand this experimental phenomenon is very important for better performance and energy saving for our biorobotic underwater propulsion system. Here, four basic undulating patterns of the mechanical fin were performed using two-dimensional unsteady computational fluid dynamics （CFD） method. An unstructured, grid-based, unsteady Navier-Stokes solver with automatic adaptive re-meshing was used to compute the unsteady flow around the fin through twenty complete cycles. The pressure distribution on fin surface was computed and integrated to provide fin forces which were decomposed into rift and thrust. The pressure force and friction force were also computed throughout the swimming cycle. Finally, vortex contour maps of these four basic fin undulating patterns were displayed and compared.     

Integrated diversification of locomotion and feeding in labrid fishes

An organism''s performance of any ecological task involves coordination of multiple functional systems. Feeding performance is influenced by locomotor abilities which are used during search and capture of prey, as well as cranial mechanics, which affect prey capture and processing. But, does this integration of functional systems manifest itself during evolution? We asked whether the locomotor and feeding systems evolved in association in one of the most prominent and diverse reef fish radiations, the Labridae. We examined features of the pectoral fins that affect swimming performance and aspects of the skull that describe force and motion of the jaws. We applied a recent phylogeny, calculated independent contrasts for 60 nodes and performed principal components analyses separately on contrasts for fin and skull traits. The major axes of fin and skull diversification are highly correlated; modifications of the skull to amplify the speed of jaw movements are correlated with changes in the pectoral fins that increase swimming speed, and increases in force capacity of the skull are associated with changes towards fins that produce high thrust at slow speeds. These results indicate that the labrid radiation involved a strong connection between locomotion and feeding abilities.     

A Three-Dimensional Kinematics Analysis of a Koi Carp Pectoral Fin by Digital Image Processing

Pectoral fins fascinate researchers for their important role in fish maneuvers. By possessing a complicated flexible structure with several fin rays made by a thin film, the fin exhibits a three-dimensional (3D) motion. The complex 3D fin kinematics makes it challenging to study the performance of pectoral fin. Nevertheless, a detailed study on the 3D motion pattern of pectoral fins is necessary to the design and control of a bio-inspired fin rays. Therefore, a highspeed photography system is introduced in this paper to study the 3D motion of a Koi Carp by analyzing the two views of its pectoral fin simultaneously. The key motions of the pectoral fins are first captured in both hovering and retreating. Next, the 3D configuration of the pectoral fins is reconstructed by digital image processing, in which the movement of fin rays during fish retreating and hovering is obtained. Furthermore, the method of Singular Value Decomposition (SVD) is adopted to extract the basic motion patterns of pectoral fins from extensive image sequences, i.e. expansion, bending, cupping, and undulation. It is believed that the movement of the fin rays and the basic patterns of the pectoral fins obtained in the present work can provide a good foundation for the development and control of bionic flexible pectoral fins for underwater propeller.     

Functional morphology of the pectoral fins in bamboo sharks, Chiloscyllium plagiosum: benthic vs. pelagic station-holding

Bamboo sharks (Chiloscyllium plagiosum) are primarily benthic and use their relatively flexible pectoral and pelvic fins to rest on and move about the substrate. We examined the morphology of the pectoral fins and investigated their locomotory function to determine if pectoral fin function during both benthic station-holding and pelagic swimming differs from fin function described previously in leopard sharks, Triakis semifasciata. We used three-dimensional kinematics and digital particle image velocimetry (DPIV) to quantify pectoral fin function in five white-spotted bamboo sharks, C. plagiosum, during four behaviors: holding station on the substrate, steady horizontal swimming, and rising and sinking during swimming. During benthic station-holding in current flow, bamboo sharks decrease body angle and adjust pectoral fin angle to shed a clockwise fluid vortex. This vortex generates negative lift more than eight times that produced during open water vertical maneuvering and also results in an upstream flow that pushes against the posterior surface of the pectoral fin to oppose drag. In contrast, there is no evidence of significant lift force in the wake of the pectoral fin during steady horizontal swimming. The pectoral fin is held concave downward and at a negative dihedral angle during steady horizontal swimming, promoting maneuverability rather than stability, although this negative dihedral angle is much less than that observed previously in sturgeon and leopard sharks. During sinking, the pectoral fins are held concave upward and shed a clockwise vortex with a negative lift force, while in rising the pectoral fin is held concave downward and sheds a counterclockwise vortex with a positive lift force. Bamboo sharks appear to sacrifice maneuverability for stability when locomoting in the water column and use their relatively flexible fins to generate strong negative lift forces when holding position on the substrate and to enhance stability when swimming in the water column.     

Biomechanics of swimming in the pufferfish Diodon holocanthus: propulsive momentum enhancement is an adaptation for thrust production in an undulatory median and paired-fin swimmer

A form of large-amplitude elongated-body theory appropriate for the analysis of undulatory fins attached to a rigid body of elliptical section suggests a benefit due to momentum enhancement relative to the fins on their own. This theoretical prediction is experimentally confirmed for the first time. Theoretical momentum enhancement factors for Diodon holocanthus (2.2 and 2.7 for the median and pectoral fins, respectively) compared well to inferred thrust values determined from particle-image velocimetry (PIV) wake measurements (2.2-2.4 and 2.7-2.9). Caudal fin mean theoretical thrust was not significantly different from measured (PIV) values (n = 24, P > 0.05), implying no momentum enhancement. Pectoral-fin thrust was half that of the median and caudal fins due to high fin-jet angles, low circulation and momentum. Average total fin thrust and fish drag were not significantly different (n = 24, P > 0.05). Vortex rings generated by the fins were elliptical, with size dependent on fin chord and stroke amplitude. Hydrodynamic advantages (thrust enhancement at no cost to hydrodynamic efficiency, reduction of side forces minimizing energy wasting yawing motions and body drag) are probably common among rigid-bodied organisms propelled by undulatory fins. A trade-off between momentum enhancement and the rate of momentum generation (thrust force) sets a practical limit to the former. For small fins whilst momentum enhancement is high, absolute thrust is low. In addition, previously suggested limitations on thrust enhancement set by reductions in propulsive force associated with progressive reductions in fin wavelength are found to be biologically unrealistic.     

Resolving Shifting Patterns of Muscle Energy Use in Swimming Fish

Muscle metabolism dominates the energy costs of locomotion. Although in vivo measures of muscle strain, activity and force can indicate mechanical function, similar muscle-level measures of energy use are challenging to obtain. Without this information locomotor systems are essentially a black box in terms of the distribution of metabolic energy. Although in situ measurements of muscle metabolism are not practical in multiple muscles, the rate of blood flow to skeletal muscle tissue can be used as a proxy for aerobic metabolism, allowing the cost of particular muscle functions to be estimated. Axial, undulatory swimming is one of the most common modes of vertebrate locomotion. In fish, segmented myotomal muscles are the primary power source, driving undulations of the body axis that transfer momentum to the water. Multiple fins and the associated fin muscles also contribute to thrust production, and stabilization and control of the swimming trajectory. We have used blood flow tracers in swimming rainbow trout (Oncorhynchus mykiss) to estimate the regional distribution of energy use across the myotomal and fin muscle groups to reveal the functional distribution of metabolic energy use within a swimming animal for the first time. Energy use by the myotomal muscle increased with speed to meet thrust requirements, particularly in posterior myotomes where muscle power outputs are greatest. At low speeds, there was high fin muscle energy use, consistent with active stability control. As speed increased, and fins were adducted, overall fin muscle energy use declined, except in the caudal fin muscles where active fin stiffening is required to maintain power transfer to the wake. The present data were obtained under steady-state conditions which rarely apply in natural, physical environments. This approach also has potential to reveal the mechanical factors that underlie changes in locomotor cost associated with movement through unsteady flow regimes.     

Establishment of a biomimetic device based on tri-layer polymer actuators--propulsion fins

We propose to use bending type tri-layer polymer actuators as propulsion fins for a biomimetic device consisting of a rigid body, like a box fish having a carapace, and paired fins running through the rigid body, like a fish having pectoral fins. The fins or polymer bending actuators can be considered as individually controlled flexible membranes. Each fin is activated with sinusoidal inputs such that there is a phase lag between the movements of successive fins to create enough thrust force for propulsion. Eight fins with 0.125 aspect ratio have been used along both sides of the rigid body to move the device in the direction perpendicular to the longitudinal axis of the body. The designed device with the paired fins was successfully tested, moving in an organic solution consisting of solvent, propylene carbonate (PC), and electrolyte. The design procedure outlined in this study is offered as a guide to making functional devices based on polymer actuators and sensors.