Catch connective tissue (CCT) is the connective tissue that shows large stiffness changes in response to stimulation under nervous control. The dermis of sea cucumbers is a typical example of CCT. Mechanical properties of the dermis are determined by the extracellular materials that are made of collagen fibrils embedded in a hydrogel of proteoglycans. The dermis takes 3 mechanical states soft (S_a), standard (S_b) and stiff (S_c). Different molecular mechanisms of stiffening have been found in the transition S_a→S_b and in the transition S_b→S_c. In this article I will review my works on this intelligent material.

The aim of this study was to clarify postural control in the pitch direction using a combination of the flexion angles of the root and fin tip of the pectoral fin in Mobula japanica using Three-D-Computational fluid dynamics analysis. We made Mobula models that allow flexion of the tip of the fin and the root of the fin independently. It was revealed that independent pectoral fin flexion promotes a change in the velocity distribution around the body and, as a result, the pitch moment is generated.

To ensure separation control at a low Reynolds number, the wing with sinusoidal leading edge (SLE) has been used. However, it is not clear whether an SLE wing is effective for all wing cross sections. In this study, we focus on the effect of the maximum wing camber and the maximum wing camber position on the efficiency of the SLE wing. We found that if a flow separation occurs, the SLE wing enhances the flow reattachment of the separated flow and improves the C_L by generating longitudinal vortices from the leading edge independent of the maximum wing camber and its position.

Formation flight control is an effective method for small unmanned air vehicles (UAVs) to improve the limited performance of an individual UAV in a powerful aggregated system as a group. In this study, formation flight control with two fixed wing airplanes as examples of UAVs were investigated based on the collective motion of organisms where simple local interactions, such as, attraction, repulsion, and parallel orientation, form an orderly motion. A control system for the aforementioned interactions was developed using a microcomputer, motion sensor, direction sensor, GPS, and communication devices and was implemented on two fixed wing type airplanes. Flight tests were conducted for the attraction-repulsion and the parallel orientation controls, and the cooperated flight of two fixed wing airplanes was successfully accomplished, indicating the feasibility of the bio-inspired formation flight control.

In collective swimming, vorticity and pressure fields near a fish may be modified through hydrodynamic interactions between fish, and eventually influence swimming performance. We developed a three-dimensional (3D) computational approach and implemented a parametric study to: 1) make comparisons of the vorticity and pressure fields topology between a single fish and a pair of fish; 2) investigate the change of vorticity and pressure fields topology by varying the relative position and phase shift between the two fish in a pair; 3) investigate the perceivable pressure signals on the lateral line of fish due to the interference between the two fish.

The swimming motion of Tuna type fishes has excellent ability in its speed and efficiency. On the other hand, some studies on the most efficient swimming motion have been reported using numerical analysis of a two-point hinge mechanism model. However, since most conventional fish robots hold their caudal fin in a spring, the caudal fin works only passively and can not confirm theoretical results in an experimental way. Therefore, we developed a fish type robot that combines caudal fin angle actuating mechanism and tail oscillating mechanism. Using this robot, we experimentally investigated the change in swimming speed due to the difference in swimming motion.

It is shown that the wings of bumblebees during flapping undergo pitching (feathering angle) rotation that can be characterized as a fluid-structure interaction problem. Measurements of shape, size and inertial properties of the wings of bumblebees Bombus ignitus are described that provide the necessary input data for numerical modelling. A computational fluid dynamics (CFD) solver is combined with a dynamical model that describes the time evolution of the feathering angle. An example result of the numerical simulation is shown.

To address the deterioration of bridges and buildings, we have developed a robot, HORNET, to inspect building walls. This robot travels on the wall surface using propellers with wheels on the wall. As the running duration is short on smooth surfaces, the cross-sectional shape of the rotor was designed to reduce the power consumption using two-dimensional numerical analysis using a genetic algorithm. Furthermore, we manufactured several kinds of rotors imitating birds and performed experiments to determine the ideal shape to reduce electricity consumption. Experiments were also conducted to evaluate the performance when the rotor was attached to HORNET. First, we developed HORNET's dynamic model and evaluated the performance by running a simulation on a route imitating the actual inspection route. Additionally, by installing a new rotor in HORNET, we evaluated the running performance on the up and down paths.

There are few studies that compare thrust power P_T induced by tail beating with net metabolic power P_net obtained from oxygen consumption and mechanical efficiency η (= P_T P_net ^-1) of fish thrust. In this study, P_T and P_net were obtained using two species of chub mackerel and Japanese dace, and η was calculated by comparing kinetic energy and metabolic energy. P_T was calculated by multiplying swimming speed V_swim and thrust force T obtained using two calculation methods, the Milne-Thomson principle and Kutta-Joukowski theorem. Comparing each value, η showed a certain ratio irrespective of fish species and swimming speed. Linear approximation resulted in P_T = 0.21 P_net for Milne-Thomson and P_T = 0.44 P_net for Kutta-Joukowski.

Bacterial cells exhibit chemotaxis by repeating a straight swimming (run) and an abrupt change of the swimming direction (tumble). Cells detect the change in the concentration of a chemical attractant during the run and decrease the frequency of the tumbles if the cells have swum toward the favorable direction. As for the chemotaxis mechanism, a mathematical model has been proposed where the frequency of the tumble is correlated with the chemotaxis intensity. In this study, we observe the chemotactic behaviors of bacterial cells and compare the measurement with the mathematical model to quantify the chemotaxis intensity.

In this study, we describe a palm-sized robotic fish that can automatically track a goldfish. A robotic fish is suitable for ecological surveys because it is difficult to be noticed by aquatic animals. However, so far, there is no palm-sized robotic fish that can automatically track an aquatic animal. Automatic tracking by the robot is carried out by recognizing the goldfish with a camera and changing the swimming direction towards the goldfish. To follow the agile movement of the goldfish, the robot has a high turning ability with multiple joints. Finally, we have confirmed the robot can track a goldfish.

The objective of this study was to realize the backstroke with the swimming humanoid robot. By realizing the backstroke, the backstroke performance of the robot was assessed regarding the speed and the swimming behavior. The motion for the robot was generated based on the motion of an actual swimmer. However, since the robot did not have sufficient degrees-of-freedom to realize the motion, the actual motion was modified. The motion was investigated by simulation and later by the experiment. In the experiment, the robot successfully realized the backstroke at the stroke cycle 2.3 s with a swimming speed of 0.28 m/s.