Biophysical and Biomechanical Adaptation and Bioinspired Engineering


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IUPS 2005

Caltech

March 28-30, 2005

Abstracts and Contributed Papers

Locomotion and Motility

Muscle

Internal Flows

Materials

Contributed Papers


Locomotion and Motility

Jellyfish Swimming and the Dynamics of Animal Vortex Wakes, Revisited
Dabiri, J.O.
California Institute of Technology, Pasadena
A primary challenge in the study of animal swimming and flying is to deduce instantaneous locomotor forces based on measurements of the animal wake. Recent studies of birds, fishes, and insects illustrate the inherent complexity of these flows, which currently limits progress toward this goal. This talk will describe studies of jellyfish swimming that suggest their morphology, propulsor kinematics and wake structure are simple enough to facilitate accurate measurements and quantitative models of their locomotion. Experiments conducted with free-swimming jellyfish and mechanical analogues illustrate unsteady fluid dynamic force contributions that have not been previously appreciated. The results suggest that these animals can serve as a useful model for testing new methods and theories in experimental and computational studies of animal locomotion. The fluid-structure interactions that characterize the motion of these animals are also common to animals with more complex mechanisms of locomotion. Therefore, a natural extension of these studies to animal swimming and flying in general can be anticipated.

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The Control of Aerodynamic Maneuvers in Fruit Flies
Dickinson, M.
California Institute of Technology, Pasadena
Like all forms of locomotion, flight behavior results from a complex set of interactions, not just within circuits in the brain, but among neurons, muscles, skeletal elements, and physical processes within the external world. To control flight, the fly's nervous system must generate a code of motor information that plays out through a small but complicated set of power and steering muscles. These muscles induce microscopic oscillations in an external skeleton that drive the wings back and forth 200 times each second producing a time-variant pattern of aerodynamic forces that the fly modulates to steer and maneuver through the air. The animal's motion through space alters the stream of information that runs through an array of visual, chemical, and mechanical sensors, which collectively provide feedback to stabilize flight and orient the animal towards specific targets. Drosophila, like many flies, search and explore their environment using a series of straight flight segments interspersed with stereotyped changes in heading termed saccades. Each saccades is a rapid maneuver in which the fly turns 90° in less than 50 ms. Using a combination of tethered and free flight methods, we have investigated both the sensory signals that trigger these rapid turns as well as the aerodynamic means by which the animals produce the required torque. The results suggest that the saccades represent a collision avoidance reflex initiated by the visual system. In tethered flight simulators, flies vigorously turn away from poles of visual expansion. The reflex is so strong that under closed-loop conditions flies actively orient toward poles of visual contraction – a result that is counterintuitive considering the visual flow flies would expect to encounter during forward flight. Once triggered, hard wired sensory-motor circuitry executes a rapid all-or-none program that directs a saccade either to the left or to the right. Although angular acceleration during the turns approach 20,000 degs s-1 , both the changes in motor output and the resultant alterations in wing motion required to produce saccades are quite subtle. Further, a high speed analysis of saccades indicates that flies must generate torque to start the turn, and counter-torque to stop. This result suggests that, despite their small size, fruit fly flight body dynamics are dominated by inertia and not friction during the brief saccades. In addition, free flight saccades are much shorter than fictive saccades in tethered flight, underscoring the importance of sensory feedback in regulating saccade duration. Evidence suggests that whereas the visual system triggers the saccade, the signal to initiate the counter-turn that terminates the maneuver arises from the mechanosensory halteres, which are more sensitive than the eyes to rapid rotation. This research illustrates how processes within the physical world function with neural and mechanical features of an organism's design function to generate a complex behavior.

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Ecological Consequences of Biomechanical Constraints on Swimming and Sensing in Protists
Grunbaum, D.
University of Washington, Seattle
Some of the most interesting applications of biomechanics follow from the importance of constraints on locomotion and sensation for ecological interactions between populations of consumers and resources. In the ocean, modern sampling techniques have shown that most resources are highly patchy. Consumers typically experience feast or famine: superabundant resources if inside a patch, and no resources otherwise. Ecological interactions are then strongly determined by locomotory and sensatory capabilities of specific consumers, and the behavioral strategies these consumers employ to rapidly locate and exploit resource patches. This talk will illustrate some of the connections between morphology, swimming and sensing performance, foraging behaviors and consumer-resource interactions in protists and other planktonic organisms. I will present population-level models derived from individual-level biomechanical and behavioral models, and present non-dimensional ecological indices that may concisely summarize complex dynamics in patchy resource landscapes.

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Median and paired fin controllers for biomimetic marine vehicles
Kato, N.
Tokai University, Shizuoka, Japan
This paper reviews the studies on the kinematics, hydrodynamics and performance of median and paired fin (MPF) in fish and biomimetic mechanical systems from the viewpoint of enhancing the propulsive and maneuvering performance of marine vehicles in at low speeds. Precise maneuverability and stability at low swimming speeds by use of MPF propulsion are seemed to be advantageous in complex habitats such as coral reefs. MPF propulsion in fish consists of undulatory fin motion and oscillatory fin motion. The kinematics of MPF in fish and mechanical system in both groups is discussed. Hydrodynamic models and experimental data of undulatory and oscillatory motions of MPF in fish and mechanical system are reviewed comparing with the experimental data. Pectoral fin propulsion has two categories which represent biomechanical extremes in the use of appendages for propulsion: drag-based and lift-based mechanisms of thrust production.?The hydrodynamic characteristics of two mechanisms are compared. The performance of fish and vehicles with MPF is reviewed from the viewpoint of maneuverability. Especially, performance of recently developed fish-like body with a pair of undulatory side fins, a model ship with a pair of ray-wing type propulsors and an underwater vehicle with two pairs of mechanical pectoral fins are discussed.

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The aerodynamics of small wings: performance measurements and analysis
Spedding, G.R.
University of Southern California, Los Angeles
Birds, bats and practical small­scale flying machines find themselves occupying an aeromechanical niche where small variations in the boundary­layer flow over a lifting surface can have significant consequences for the instantaneous and time­averaged lift and drag. The consequences may be good or bad, but in either case, the sensitivity of this system makes its study both challenging and interesting. A number of questions are being addressed in current research and will be summarised in this talk. Questions about what flying animals actually do are being investigated on live birds trained to fly in the Lund University wind tunnel. Questions about how the simplest possible lifting surface works are being answered on a study of flat and cambered plates in the USC Dryden wind tunnel. Both tunnels have extremely low, well­documented turbulence properties so there is some chance that agreement can be reached with companion state­of­the­art computations, with a Large Eddy Simulation model using immersed boundary methods (Univ. Maryland), and with vortex dynamics methods (Caltech) on appropriate 2D, and subsequently 3D problems.

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Fore-hind wing interactions in dragonfly flight
Wang, Z.J.
Cornell University, Ithaca, NY
Dragonflies are one of the most maneuverable insects. A distinctive feature of dragonflies, which reflects their ancient origin, is their use of two pairs of wings instead of one pair. As such, understanding the coupling between their fore and hind wings might shed light on the evolution of flight based on four wings to that based on two. In this talk I will report the effects of wing interactions on the forces and the efficiency seen in our computational and experimental study of dragonfly flight.

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Muscle

Power modulation during locomotion
Askew, G.N.
University of Leeds, Leeds, UK
Animals must be able to modulate the power output of their muscles to vary their locomotor performance. For many vertebrates, the muscles that power locomotion have different fibre types that vary in their physiological properties and oxidative capacity. These different fibre types can be differentially recruited for different types of activity, depending on the power required for the activity and the duration over which it must be sustained. However, the locomotor muscles in some vertebrates such as small birds have muscles with homogeneous fibre composition. To vary muscle power output, these species must adopt alternative power modulation strategies. There are a number of possibilities. The power source can simply be turned on and off to control the average power output. The amount of muscle work done per cycle can also be adjusted via changes in recruitment intensity, strain trajectory and operating frequency. This paper will review the power modulation strategies adopted by various animals.

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Muscle dynamics during locomotion: from active power modulation and force economy to passive dynamics
Biewener, A.
Harvard University, Cambridge, MA
The force-length behavior of animal muscles reflects the dynamics of oscillatory motion and propulsion of their appendages. In some cases, such as flight, muscles must generate considerable power, operating at high frequencies and contractile strains. In terrestrial locomotion, however, muscle work may be allowing muscles to develop force economically. In both cases, force enhancement via stretch activation or isometric force development increases power production and force economy (force/ATPconsumed). Muscles must also stabilize the motion of an animal’s limbs and body. Perturbation studies of running animals indicate that passive-dynamic mechanisms may help stabilize them in addition to neuromotor feedback control. Passive mechanisms offer immediate stabilization until longer latency active mechanisms can be implemented. Perturbation trials indicate that running guineafowl stabilize themselves from an unexpected sudden loss of PE via passive dynamics of the limb’s motion, in combination with short and longer latency neural feedback. The passive dynamics of limb extension minimizes PE loss, enabling the limb to function as a strut, with differences in limb contact angle influencing the stabilizing strategy. Differing strategies reflect the inherent variability of the motor task for stabilization.

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Elements of recovery of locomotion following spinal cord injury
Edgerton, V.R.
University of California, Los Angeles
Considerable insight can be gained in understanding the neural control of locomotion by examining the potential sources of modulation that are available and to examine these sources of control in a range of animal species and in experimental preparations differing in the degree to which they have been reduced surgically, pharmacologically or genetically. For example, full weight-bearing stepping can be generated by mice, rats and cats following midthoracic complete spinal transaction. Further, this stepping can readily adapt to changing environmental conditions such as loads and the speed of a treadmill belt. Several sources of stimulation of the neuromusculature of the hindlimbs are capable of evoking stepping. Stepping can be evoked following complete spinal transaction at the midthoracic region if the animal is trained to step properly, if the appropriate amount of neurotransmitter modulator is administered, e.g., strychnine which blocks glycinergic inhibition, or quipazine which activates serotonergic receptors, if the appropriate combination of sensory axons are activated and when the spinal cord is stimulated electrically by electrodes placed on the dura overlying the lumbosacral spinal cord. Insight is also gained when examining the interaction of these sources of neural control. For example, the specific electrode site for the most effective epidural stimulation of the spinal cord can vary depending on whether the stimulation is presented in the presence of quipazine and whether and how fast the treadmill belt is moving or whether there is any weight support of the hindlimbs taking place. Some of these same stimuli that induce stepping in rodents and in the cat can have a similar effect in humans with incomplete or complete spinal cord injury. Evidence will also be presented that the spinal cord is capable of performing and learning to execute standing and postural control movements.

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Extending the Preflex: Perturbation Rejection, Distributed Feet and Task Level Control.
Full, R.J.1
Cowan, N.2
Dudek, D.1
Goldman, D.1
Libby, T.1
Revzen, S.1
Sponberg, S.1
1University of California, Berkeley
2Johns Hopkins
Preflexes - the zero-delay, intrinsic responses to a perturbation - are often considered to be musculo-tendon reactions. We extend the concept of a preflex to skeletal structures, feet and legs to demonstrate how they simplify control in legged animals and robots. Spring-loaded, inverted pendulum and lateral leg spring models of terrestrial locomotion show passive, dynamic self-stabilization. Rapid recovery from brief, lateral perturbations during high-speed running support the hypothesis that insects use a feedforward control architecture. Tripping cockroaches running on a treadmill did not alter leg phase and amplitude. Animals running over a randomly rough terrain showed no significant difference in EMGs compared to flat terrain. Isolated insect legs rapidly recover passively from perturbations within and out of the plane of joint rotation. History-dependant viscoelasticity makes joints stiffer to rapid perturbations, but less stiff to control inputs over several cycles. Spiders and cockroaches cross simulated debris (mesh) rapidly even when 99% of the surface contact has been removed. Contact was not concentrated at the animals' terminal leg segments, but was distributed along the leg. Large cuticular leg spines showed asymmetrical flexibility - easily collapsing in one direction to pull legs out of debris, but stiffening in the other to provide a foothold during leg extension. Adding artificial spines to animals that lack them like crabs and to a legged robot significantly increased performance on rough terrain. Task level feedback for navigation can take advantage of these preflexive behaviors. To demonstrate the advantage, we tested a mechanosensory template for tactile feedback control of high-speed locomotion. Cockroaches using their antennae to follow walls fit a simple proportional derivative controller. Directly implementing this task level controller allowed a robot to wall follow effectively.

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Functional, structural and molecular consequences of eccentric muscle work
Hoppeler, H.
Steiner, R.
Klossner, S.
Daepp, C.
Flueck, M.
University of Bern, Switzerland
In most cases we think of muscle tissue as a machine designed to produce tension and doing work while shortening. This type of muscle action is known as shortening or concentric contraction. However, when the magnitude of the applied force exceeds that produced by the muscle, the muscle lengthens and thus undergoes what is defined as an eccentric muscle contraction. It is known that concentric and eccentric muscle contractions differ in important functional aspects. In eccentric contractions muscles can accommodate forces up to four-times higher than during concentric contractions. Moreover, the metabolic cost of developing tension in eccentric contractions (i.e. doing negative work) is only a fraction of that necessary to develop the same tension concentrically. Eccentric contractions show a different recruitment pattern, are more difficult to coordinate and can lead to muscle damage. Eccentric muscle damage is clinically known as delayed onset muscle soreness (DOMS). In the context of DOMS, a suit of structural modifications of the muscle cell including regenerative events such as satellite cell activation is known to occur. In locomotion eccentric muscle contractions are an integral part of each stride cycle, whereby eccentric contractions are used in braking and for storing elastic energy (Dickinson et al., Science 288:100-106, 2000). In general, eccentric contraction has received little academic attention. In view of the fact that eccentric muscle contractions allow for high mechanical loads on muscle tissue at a low metabolic cost we currently explore chronic eccentric exercise as a novel option to modify muscle phenotype. It is hypothesized that chronic eccentric exercise has specific functional, structural and molecular consequences.

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Sodium channel defects: Molecular mechanisms underlying myopathies
Ruben, P.
Utah State University, Logan
Non-dystrophic myotonias are a group of inheritable diseases that are caused by mutations in voltage-gated ion channels in skeletal muscle. One of these channels, the voltage-gated sodium channel, is responsible for the rising phase of action potentials in electrically excitable cells. Most studies on sodium channel mutations associated with paramyotonia congenita (PC) and potassium-aggravated myotonia (PAM) have tended to focus on defects in fast inactivation to explain episodes of cold-induced or exercise-induced muscle stiffness and weakness, although defects in inactivation were insufficient to explain the cold sensitivity of PC or the K+ sensitivity of PAM. In addition to impaired fast inactivation, we found that defects in sodium channel deactivation (closing) rates are also associated with PC and PAM mutations, and contributed to the difference in phenotype severity between two specific PC mutations involving the same residue, R1448C and R1448P. We also found that the temperature dependent changes in deactivation in R1448C and R1448H, mutations associated with cold-aggravated muscle stiffness and weakness, are consistent with the hypothesis that defects in deactivation are an important component of exacerbation by cold in PC. Deactivation in another PC mutation, I693T, also shows a significantly greater sensitivity to cold. When we tested PAM mutations for deactivation defects, we also observed that changes in the rate of this transition define the S804F, I1160V and G1306V mutations. Although increased extracellular K+ may indirectly exacerbate these defects by membrane depolarization, there were only minor effects on deactivation gating. Taken together, our results show a consistent link between mutations that cause myotonia and defects in sodium channel deactivation. Our results have also led us to formulate a new model of sodium channel gating that includes multiple open states.

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Designing Molecular Motors: Myosin Structural Regions that Determine Muscle Mechanical Properties
Swank, D.
Rensselaer Polytechnic Institute, Troy, NY,
We are investigating the relative importance and kinetic mechanisms by which the four variable structural regions of Drosophila myosin heavy chain (MHC) determine muscle fiber type mechanical properties. Previously we found that substitution of an embryonic MHC (EMB) for the native fast myosin (IFI) in Drosophila indirect flight muscle (IFM) transformed the IFM from a high power-generating muscle that performs optimally at high oscillation frequencies (fmax =150 Hz), to one that produces less power and functions best at low frequencies (fmax = 20 Hz). There are 4 variable domains that differ between the EMB and IFI MHC head regions due to alternative splicing from a single Mhc gene. We have systematically exchanged these regions and transgenically expressed the resulting chimeric myosins in the IFM. The converter domain had by far the largest influence on power, work, tension and muscle kinetics (fmax decreased 50%) of skinned IFM fibers. Correlations of muscle kinetics with properties of isolated myosin, i.e. actin motility and ATPase, revealed the relative influence of strong vs weakly bound cross-bridge states. Further details of IFI and mutant cross-bridge kinetics are being studied by varying ATP and Pi concentrations in fibers combined with sinusoidal analysis. For example, an unusually high ATP concentration, > 7.5 mM, is needed to saturate IFI fiber kinetics during work production, suggesting a very low affinity for ATP. Drosophila expressing IFI chimeric myosins that significantly alter muscle kinetics compensate by tuning wing beat frequency to be closer to fiber fmax. We conclude that the alternative MHC domains differentially influence muscle mechanical properties, but must cooperate to yield the specific EMB or IFI fiber types.

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Internal Flows

Masters of microfluidics: hydrodynamics of fluid transport in trees
Holbrook, N.M.
Harvard University, Cambridge, MA
Design a machine that can lift hundreds of liters of water per hour tens of meters in the air through tubes 100 μm in diameter -- without making a sound and without using any moving parts. Then add a second system, closely coupled to the first, in which even narrower tubes move concentrated sugar solutions. Make sure that your system is self-assembling and robust against environmental challenges such as drought, insect attack, mechanical damage, and freezing temperatures. Also include the capacity to adjust the transport capacity to the needs of your system as it changes in size from only a few cm in size to tens of meters in length. Answers will be judged by natural selection, meaning that there will be a high premium on the ability to perform these tasks in a cost-effective manner. Aspects of the fluid and solid mechanics of the winning entry will be discussed.

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Flow-Induced Morphogenesis
Hove, J.R.
University of Cincinnati, OH
There are few relationships as intimate as the one existing between living organisms and their fluid environments. Whether its a salmon swimming upstream, a sapling bending in a strong wind, or blood coursing through the heart of an Olympic sprinter, flow-structure interactions have an enormous impact on the form and function of all living things. This is particularly true during development as internal fluid flow serves to transport heat, gases and a myriad of biomolecules throughout the growing organism. Fluid flow also imposes mechanically transducible forces (e.g. shear, pressure, stretch) on adjacent and underlying cells, regulating their gene expression patterns. Research on biological flow/structure interactions has only recently begun to span the length scale from micro to macro. This is due, in large part, to the development of improved optical techniques and quantitative flow visualization technologies that allow unprecedented views into the world of biological flow. Quantifying the spatial and temporal characteristics of flow/structure interactions within the zebrafish model system has substantially advanced our understanding of both normal and pathophysiological vertebrate development. Progress in this area will be reviewed with an emphasis on cardiovascular and renal development.

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A bioengineering model of coronary circulation
Kassab, G.
University of California, Irvine
In a bioengineering approach to coronary blood flow analysis, one should use the branching pattern, vascular geometry and mechanical properties of the coronary vasculature, apply the basic laws of physics (conservation of mass and momentum) to write down the governing equations, specify the appropriate boundary conditions, and solve the appropriate boundary-value problems. A complete set of data on the branching pattern and vascular geometry of the pig’s coronary vasculature, from arteries to capillaries and capillaries to veins, have been obtained in our laboratory. We have also recently determined the distensibility of the coronary blood vessels. With the distensibillity of the blood vessels known, the mechanics of the blood vessel is coupled to the mechanics of blood flow to yield a pressure-flow relation for each vessel segment. Once the pressure-flow relation of all vessel segments is known, the anatomical topology of the vascular circuit is specified by the connectivity matrix. A definite connectivity of the vascular circuit is necessary so that a repeated application of the pressure-flow relation in different segments of the circuit can synthesize the longitudinal pressure and flow distributions.

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Information Processing by Stomatal Networks
Mott, K.A.
Utah State University, Logan, UT
Stomata (the tiny pores on the surfaces of leaves) must respond to environmental factors such that they open to admit enough CO2 for photosynthesis, yet close sufficiently to prevent excessive water loss. Much of this regulation occurs at the level of the individual pore, through signal transduction pathways in the two cells, termed guard cells, that form the pore. However there is increasing evidence that stomata interact with each other over short distances and can therefore be said to form a locally-connected network. We present evidence showing that stomatal networks may be processing information in a manner similar to artificial networks that perform distributed emergent computation. This information processing may allow stomata networks to optimize gas exchange for an entire leaf or plant despite the fact that each individual stoma has only local information.

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Biological vs. industrial crossflow filtration: ways to avoid a dead end
Sanderson, S.L.
College of William and Mary, Williamsburg, VA
Suspension-feeding fish, such as menhaden, anchovy, and carp, filter massive volumes of water to extract suspended food particles that are as small as 5 microns to 1 mm in diameter.  A miniature fiberoptic endoscope has been used recently to observe particle movement inside the oral cavities of live suspension-feeding fish.  These data, combined with computational fluid dynamics simulations in fish oral cavities, have led to the development of a new model involving crossflow filtration.  Although crossflow filtration is a multi-billion dollar industry for the manufacture of products that we use every day (e.g., dairy products, beer and wine, pharmaceuticals), this mechanism had not been recognized previously in any vertebrate.  Crossflow filtration in suspension-feeding fish differs from industrial crossflow filtration in several important respects.  The filter pores are larger in fish, the channel length is shorter, and particles do not accumulate on fish filtration surfaces.  Fish routinely retain particles that are small enough to be lost with the filtrate, and these particles rarely contact the filter surface as they are transported directly to the esophagus for swallowing.  The implications of such differences between biological and industrial crossflow filtration will be assessed, and future research directions for the analysis of biological crossflow filtration will be outlined.

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Materials

Fiat lux: The convergent 'design' of animal tissues that interact with light
McFall-Ngai, M.J.
Crookes-Goodson, W.J.
University of Wisconsin, Madison
Anatomists have long been aware that the eyes and photophores of animals show remarkable convergence in the arrangement of their tissues. Although eyes are sensory organs that receive light and photophores are effectors that emit light, at their most complex, they share lens, choroid, iris, and tapetum analogues. We have been exploring the underlying biochemical and molecular character of the eyes and photophores of the sepiolid squid Euprymna scolopes to determine whether the convergence in these organs extends beyond their anatomy. The results of these studies have shown striking similarities, including identical lens crystallins, high expression of the blue-light sensing cryptochromes, and a cephalopod-specific mechanism by which tissue reflectivity is achieved. Further, analysis of an EST-database of the juvenile E. scolopes light organ has revealed the expression of other genes that have previously been thought to be eye specific.

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Bioinspired attachment devices: what we can learn from evolution
Gorb, S.N.
Max Planck Institute for Metals Research, Stuttgart, Germany
Many animals bear leg attachment pads with an excellent ability to adhere to a smooth surface as well as to a variety of natural surfaces with rough profiles. There are two alternative designs of such systems: smooth and hairy. The smooth systems consist of soft deformable structures with a relatively smooth surface.Pads of geckos, flies, beetles, and spiders are covered by relatively long, deformable setae which, due to individual bending, increase the number of contacting points with the surface. Most recent data on hairy systems demonstrated their excellent adhesion and high defect tolerance of contact. The size of single points gets smaller and their density higher as the body mass increases. We provided the first experimental evidence of adhesion enhancement by division of contact area. A patterned surface, made out of polyvinylsiloxane (PVS), has significantly higher tenacity on a glass surface than a smooth sample made out of the same material. This effect is even more pronounced on curved substrata. An additional advantage of patterned surfaces is the reliability of contact on various surface profiles and the increased tolerance to defects of individual contacts.

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Spider silk or hagfish silk: Alternate routes for the production of high-performance protein fibers
Gosline, J.
Fudge, D.
Guerette, P.
Levy, N.
University of British Columbia, Vancouver, Canada
Spider dragline silks are renowned for their exceptional strength and toughness, and there is considerable interest in genetically engineered materials based on the protein sequence designs of these silks. The properties of dragline silks arise from the β-sheet crystal-reinforced protein network in these materials. Hagfish slime threads contain intermediate filaments (10 nm diameter filaments that self-assemble from α-helical, coiled-coil proteins) that bundle into parallel arrays to form macroscopic threads. Wet slime threads tested in tension show an initial, low-modulus elastic zone, but beyond 30% extension an irreversible, α-helix → β-sheet transformation causes deformation to become plastic. The threads fail at about 200% extension, but extension beyond 100% results in dramatic strain-hardening due to the formation of β-sheet crystals. This process suggests a novel strategy for the formation of β-sheet crystal-reinforced protein networks based on the self-assembly of α-helical proteins but is controlled by draw-processing. Indeed, when wet slime threads are stretched by 150% and dried, their properties become virtually indistinguishable from those of dragline silks. Thus, genetically engineered materials based on hagfish IF proteins offer an effective, alternate way to produce high-performance protein materials.

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Spider silk: design, performance, and evolution
Hayashi, C.Y.
University of California, Riverside
Spider silk is well known for having remarkable extensibility, tensile strength, and toughness. Much of this reputation is based on investigations of dragline silk spun by two species of orb-web weaving spiders. These works hint at how much can be learned about the molecular adaptation of biomaterials by studying spider silk. Spiders are extremely diverse (>37,000 species), inhabit nearly every terrestrial habitat, and unlike most other arthropods, spiders use silk for a variety of tasks throughout their lifetime. These tasks include safety draglines, prey capture nets, protective retreats, and coverings for eggs. Primitive lineages of spiders make only a few types of silk while more derived lineages, such as orb-web weaving spiders, synthesize seven types of specialized silk. Silk, being largely made up of protein, is thought to have superb mechanical properties because of natural selection for maximization of silk performance with minimal amounts of protein. Members of a gene family encode the silk proteins, and there can be substantial sequence variation among family members. Thus, studying this system will lead to a better understanding of how, over evolutionary time, structural motifs in silk proteins have been maintained, remodeled, or reinvented depending on the different functional demands of silks.

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Structural properties of calcification patterns in elasmobranch cartilage
Schaefer, J.T.
Summers, A.P.
University of California, Irvine
The cartilage of the skeleton of elasmobranchs is covered in a thin layer of calcified blocks, called tesserae. Though their function has not been definitively determined, we believe that tesserae serve to stiffen the cartilaginous skeleton. Batoid elasmobranchs (skates and rays) possess varying patterns of tesserae on the skeletal elements that form an internal and external support system for the elements of the wing. This variation is correlated phylogenetically and with swimming style. We used a materials testing system to measure structural variables in joints and individual skeletal elements. Here, we present data showing variation in stiffnesses associated with the change in calcification pattern. Based on the localization, both phylogenetic and morphological, of these patterns, we further hypothesize that this variation is an adaptation to different stress regimes imposed by different swimming styles.

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A Bioengineering Approach to Materials Synthesis and Design
Tirrell, D.A.
California Institute of Technology, Pasadena
We are exploring three approaches to the synthesis of natural and artificial proteins containing novel amino acids. In the first approach, we replace every copy of one of the natural amino acids by an analogue, in effect building proteins from an altered set of twenty starting materials. This approach is most useful when one is interested in changing the overall physical properties of the protein. A second approach uses mutant transfer RNAs to break the degeneracy of the genetic code, and offers the prospect of a protein chemistry based on a substantially expanded set of amino acid building blocks. A third method combines amino acid replacement with in vitro evolution, and explores the prospects for adapting proteins to novel amino acid constituents. This lecture will describe the most important elements of each of these strategies as well as some thoughts on the design of wholly artificial proteins with potential relevance to biotechnology and materials science.

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Contributed Papers

Alternative versions of the myosin S2 hinge affect the functional and structural properties of indirect flight muscle
Cammarato, A.
Suggs, J.A.
Dambacher, C.M.
Bernstein, S.I.
San Diego State University, CA
Myosin, consisting of two heavy chains (MHC) and four light chains, is the molecular motor that drives muscle contraction. Alternative splicing of transcripts from Drosophila melanogaster's single Mhc gene produces MHC isoforms in a stage- and tissue- specific manner. For example, use of alternative exons 15a or 15b, which encode part of the S2 hinge, correlates with contractile properties of the corresponding muscles. Expression of 15a in the fast adult oscillatory muscles (indirect flight muscles (IFM)) leads to an isoform containing hinge-A. Expression of 15b in slow embryonic muscles leads to isoforms containing hinge-B. To examine the functional and structural significance of the alternative hinges, fly lines were engineered that express MHC with embryonic hinge-B in adult IFM. The flight ability of hinge-switch adults was severely impaired. Ultrastructural analysis revealed IFM myofibrils with increased sarcomere lengths. Contour length measurements of rotary shadowed myosins showed that hinge-switch tails are greater than 4 nm longer than wild-type tails. SDS-PAGE confirmed identical relative mobilities for the two myosins. Thus, tail length differences result solely from different hinges and rod conformations. The inability of hinge-B to substitute for hinge-A may result from differing hinge structural properties that could influence sarcomere structure and muscle contraction.

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Reflection in the squid Euprymna scolopes is achieved by structural platelets composed of a highly unusual family of proteins called reflectins
Crookes-Goodson, W.J.1
Ding, L.2
Horwitz, J.2
McFall-Ngai, M.J.1
1University of Wisconsin, Madison
2University of California, Los Angeles
The biochemical basis of reflective animal tissues has interested researchers for decades. In general, animal reflectors are constructed of alternating layers high- and low- refractive index materials. In most animals, the layer of low refractive index is cytoplasm while the high refractive index layer is composed of guanine and/or hypoxanthine crystals. In cephalopods like the Hawaiian bobtail squid (Euprymna scolopes), reflective tissues are similarly constructed. However, we have found that in this animal, the layer of high refractive index is composed of a family of proteins that we have named reflectins. Reflectins are 36-37 kDa proteins that have a highly unusual amino acid composition. Of the total amino acid residues, 19% were tyrosine, 14% were methionine and 11% were arginine; six residues (Y, M, R, N, G and D) constituted 74% of the total amino acids. Reflectins were composed almost entirely of 5 repeating domains and were highly insoluble. Despite the absence of transmembrane domains, a signal peptide, or evidence of other membrane anchors, the proteins partitioned to the insoluble/membrane fraction and could only be solubilized by denaturants. Reflectins localized exclusively to reflective tissues in the squid, where they were packed into hundreds of structural platelets.

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Uniform strain in broad muscles: A new twist on tendons
Dean, M.N.
Summers, A.P.
University of California, Irvine
Myofilament overlap determines tension generation in all vertebrate skeletal muscle. The range of muscle fiber strains used to generate a given movement (i.e., sarcomere lengths) is therefore linked to force production. As a result, regions of a muscle experiencing different strains operate in different regions of the length­tension curve, likely decreasing whole­muscle force output. The anterior jaw adductor muscle of the cartilaginous fish, Hydrolagus colliei, exhibits a morphological solution to ensuring similar strains. The muscle's tendon flips 180 degrees on its longitudinal axis, such that anterior fibers insert more posteriorly and vice versa. Since insertions closer to the jaw joint experience smaller excursions during mouth opening, the anterior face of the muscle strains less than in an unflipped tendon system (the inverse is true for the posterior face). This results in nearly homogenous strain across the muscle with a flipped tendon, compared with a 10% inhomogeneity between anterior and posterior faces in the unflipped condition. We illustrate that Hydrolagus' morphology functions effectively in strain homogenization. The human latissimus dorsi muscle exhibits a similar morphology, indicating that this may be an ideal anatomical mechanism for strain homogenization in broad muscles attached to rotating structures and inserting relatively far from the joint.

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Spider Silk-Moisture Interactions
Garza-Gossett, A.
Li, C.Y.
Ravi, V.A.
Cal Poly Pomona, California
The mechanical properties of spider silk have been extensively studied and key properties such as strength, stiffness, extensibility and toughness have been well documented. However, studies of the interaction of spider silk with moisture appear to be sparse. In particular, an understanding of the physics of absorption and desorption of moisture into and from spider silk could have potential implications for smart materials and systems, e.g., in actuation mechanisms. In this study, still in its preliminary stages, we report on qualitative observations of contact angles of water on spider web fibers. These observations indicate that contact angles for viscid fibers-water combinations are less than 90°, implying that the viscid web fibers are hydrophilic. In addition to developing and refining methods for contact angle determinations, we also intend to investigate the effects of moisture interactions on mechanical properties of spider silk. While literature reports of supercontraction in silk fibers and the attendant mechanisms are available, the focus of this effort will be on the kinetics of absorption and desorption and the consequent effect on the mechanical behavior of spider silk.

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Arthropod Locomotion on a Challenging Substrate Using a Distributed Foot
Goldman, D.I.1
Spagna, J.C.1
Full, R.J.1
Lin, P.C.2
Koditschek, D.E.2
1University of California at Berkeley
2University of Pennsylvania
The natural environment is rough on a range of length-scales and reliable foot-holds can be sparsely distributed; arthropods including insects and spiders demonstrate remarkable stability and performance in such environments. Foot-substrate interactions must contribute to this ability, yet foot placement during dynamic movement remains largely unknown. We study locomotion on a surface with 1% of the area remaining, wire mesh. Remarkably, we find that spiders (Hololena adnexa), cockroaches (Periplaneta americana), and beetles (Cicandela repanda) can maintain speeds up to 40 body lengths/sec. Such performance is comparable to typical speed over solid ground. At these speeds, we have discovered that the animals use not only their traditional feet but distribute contact events along much of the leg, including large spines and hairs. The hairs/spines share an asymmetrical flexibility – they bend easily toward the leg axis, but resist movement in the opposite direction. Use of a “distributed foot” may be a useful general leg-design principle for fast-moving, stable animals that must traverse surfaces where careful foot placement is improbable–removal of the feet of cockroaches had little effect on their performance or stability during locomotion on the mesh. By adding spines to the leg of the hexapod robot RHex, we have demonstrated a distributed foot can be used to enhance robot negotiation of obstacles. We have also demonstrated that the locomotion of ghost crabs that otherwise perform poorly on the mesh can be dramatically improved by the addition of prosthetic spines with properties similar to those found in animals; the spines eliminate miss-steps and reduce the number of crashes to zero.

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Batoid Feeding Behavior: Prey preference, detection, and capture mechanics
Laura K. Jordan
University of California, Los Angeles
Studies of feeding behavior in elasmobranch fishes are frequently focused on sharks rather than their batoid relatives. Here, early findings on prey preference, anatomy of sensory systems, as well as prey capture and manipulation are presented for three batoid species. Dasyatis violacea, the pelagic stingray, inhabits the open ocean and is found offshore worldwide. It is reported to feed on pelagic fishes, squid, and some crustaceans. Urobatis halleri is a benthic ray which lives near shore on soft bottoms of the west coast of North and Central America and feeds on benthic invertebrates. Myliobatis californica is a benthopelagic ray also inhabiting the eastern Pacific Ocean off the coast of California and Oregon. M. californica is a powerful swimmer and feeds on invertebrates buried deeply in sand and mud sediments. Preliminary data shows that M. californica may preferentially consume certain invertebrate prey species over others. Electroreception and lateral line canal distributions and their potential effects on prey detection are explored. The use of fins and jaw movements in prey capture and manipulation are discussed in all three species.

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Bolus Rebound Kinematics During Engulfment Feeding In The Rorqual Whales
Kot, B.W.
University of California, Los Angeles
Rorqual whales (Family: Balaenopteridae) employ unique filter-feeding structures and techniques to controllably engulf massive amounts of prey-laden seawater while feeding. Engulfed water containing a concentration of prey items forms a bolus that enters the whales' distensible ventral pouch through the buccal cavity. The two-dimensional kinematics of this bolus was analyzed using digital video from three species of surface-feeding rorqual whales in the Gulf of St. Lawrence, Canada. Individual still frames were extracted from video sequences and landmarks were placed on consecutive frames to calculate movement, relative to the animal, of the bolus over 1/30 second frame intervals. The bolus initially translated posteriorly inside the ventral pouch then rebounded off the posterior end of the pouch before traveling back toward the buccal cavity. Velocities and directional changes were quantified, and the momentum of the bolus is thought to help initiate the ram filtration process through the baleen plates. Results showed that Balaenoptera physalus has the fastest bolus rebound velocity, followed by B. musculus and B. acutorostrata, respectively. Further work will be performed to help understand the implications of these findings, including how they contribute to scaling relationships between the sizes of the different species and the volumes of the boluses.

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Swimming biomechanics and kinematics in aracanin boxfishes
Lauritzen, D.V.
Gordon, M.S.
Wiktorowicz, A.M.
University of California, Los Angeles
The temperate water Australian boxfishes (family Ostraciidae, subfamily Aracaninae) differ morphologically from the widely distributed and more familiar tropical boxfishes (subfamily Ostraciinae) in two major respects: (i) The bony carapaces covering most of their bodies end posteriorly just anterior to the insertions of the dorsal and anal fins, rather than ending behind those insertions; (ii) The carpaces have only single mid-ventral keels rather than two or more variously positioned lateral keels. These differences correlate with, and probably permit, a major difference in swimming modes between the two groups. Aracanins swim using tail-wagging body and caudal fin (BCF) locomotor modes rather than the rigid-bodied, median and paired fin (MPF) modes characteristic of the ostraciins. Aracanins are also more strikingly sexually dimorphic than are ostraciins. We have studied swimming biomechanics and kinematics in both sexes of two aracanin species. As compared with four different ostraciins studied previously, aracanins have significantly lower upper critical swimming speeds. Rectilinear swimming movements are substantially more dynamically stable than is usually the case for fishes using BCF modes, however significant yawing recoil movements occur. Synchronization between pectoral fin and dorsal/anal fin movements appears significantly more variable than is usual for ostraciins swimming over similar ranges of speed. Gait changes occurring as fishes swim more rapidly appear less dramatic and clear-cut than is usual for ostraciins. Measures of maneuverability appear similar in the two groups. We will discuss possible ecological and evolutionary implications of these differences.

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Dynamic control of thrust in the paddling of cormorants during horizontal submerged swimming
Ribak, G.
Weihs, D.
Arad, Z.
Technion -- Israel Institute of Technology
Cormorants use their feet for propulsion during underwater swimming. The thrust supplied by the feet provides cormorants the forces needed to overcome both drag and buoyancy during horizontal swimming. The buoyancy of cormorants decreases with the increase in diving depth while drag of the birds depends on swimming speed. To control swimming over a wide range of swimming speeds and swimming depths the cormorants must control the partition of the thrust produced by the feet between forward motion and buoyancy offset. We modeled the hydrodynamic forces produced by the paddling motion of cormorants and used computer simulations to study the effect of swimming speed, swimming direction, tilt of the body and speed of the feet on thrust production. The model shows that at the average swimming speed observed in cormorants swimming voluntarily at shallow depth, vertical thrust magnitude is maximal. The model also predicts an upper limit of swimming speed of 2.5 ms ­1 . The tilt of the body during the stroke and the speed of the feet control both the magnitude and the direction of thrust for a given swimming speed. Experimental results supporting the predictions made by the model are also shown.

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Biomechanics and Kinematics of Swimming in Two Species of Pufferfish, Diodon holocanthus and Arothron hispidus
Wiktorowicz, A.M.
Gordon, M.S.
University of California, Los Angeles
The Porcupinefish (D. holocanthus) and Stars and Stripes Puffer (A. hispidus) are tropical marine fishes in the family Tetraodontidae. They both swim with rigid bodies and are multipropulsor median and paired fin swimmers (MPF). Each species uses different combinations of their fins during swimming, resulting in species-specific gaits. These differences may be to due to the following factors: i) body morphology; ii) fin area; and iii) habitat. D. holocanthus swims with undulations of its pectoral, dorsal, and anal fins at low swimming speeds, and switches to oscillation/undulation of its dorsal, anal, and caudal fins at medium and high speeds. It also has a box-like body shape, relatively large fins, and inhabits reefs, rocky areas and soft bottoms. A. hispidus swims at all speeds by oscillating/undulating its pectoral, dorsal, and anal fins. It reserves its caudal fin for steering and for burst and coast swimming at very high speeds. A. hispidus has an elliptical body and relatively small fins (except the caudal fin), and inhabits reef slopes and lagoons. In either case, both puffers appear to be stable swimmers that show little or no recoil movements. These fishes also have similarities and differences with A. meleagris and A. nigropunctatus used in Gordon et al (1996). The latter puffers developed significant anterior body deformations and swam with their medial and paired fins. D. holocanthus and A. hispidus did not develop anterior body deformations, and D. holocanthus used its dorsal, anal, and caudal fin most of the time. All puffers beat their pectoral fins 180° out of phase, and beat their dorsal and anal fins in phase.

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