Tensile test. Specimens were fabricated and the tensile test was performed to investigate the effect of cartilage inclusion on anisotropic stiffness. Specimens of two types of silicone elastomers were tested: Ecoflex 00–20 (Smooth-On) and Sylgard 184 (Dow Corning). The former was a compliant elastomer (tensile strength ~ 1.1 MPa) used as the soft tissue. The latter was a rigid elastomer (tensile strength ~ 6.7 MPa) and used as the cartilage. Based on these materials, four types of specimens were prepared as shown in Fig. 3(a): A) specimen with soft tissue and vertically (i.e., tensile direction) arranged cartilages, B) specimen with soft tissue and horizontally arranged cartilages, C) specimen consisting of soft tissue only, and D) specimen consisting of a randomly arranged cartilage materials in the soft tissue. In the specimen Type-D, the amount and mass ratio of the materials are the same as those in the types A and B. Every specimen had the dimensions of 70 mm (width) × 70 mm (length) × 3 mm (thickness). The results of the tensile test are shown in Fig. 3(b). The details of this experiment are explained in the Method section. The Young’s modulus values of each specimen obtained from the experimental data are summarized in Fig. 3(c). The data showed that type-A specimen has the largest modulus among the samples (0.22 MPa). Type-B and Type-C exhibited similar modulus: 0.05 MPa and 0.03 MPa, respectively, whereas Type-D showed the modest rigidity with the Young’s modulus of 0.13 MPa. The difference in Young's modulus in the specimens was clearly correlated with the presence of the cartilages. This result also suggests that the anisotropy of stiffness is called by the alignment of cartilages.
Fabrication of the robots. We developed three types of stingray robots based on the same set of materials used in the previous section. The first was with cartilages (Type-A indicates the stiffness that transmits the power. Type-B indicates the softness that generates the traveling wave.), the second was without cartilages (type-C), and the third comprised a mixture of soft tissue and cartilage materials (type-D). In the last one, the amount and ratio of the materials were the same as in the first one. The cartilages were designed by the analogy with the actual skeleton of a stingray that has radially distributed cartilages. Figure 4(a), (b) illustrates the structure of the robot having a circular shape with a diameter of 160 mm and a fin thickness of 3 mm. The cartilages were 1 mm thick and 6 mm width. The robots were made of the three main parts: the cartilages, the body (soft tissue), and the servomotors (FS 0403, FEETECH). The servomotors were powered externally through electrical wires. The servomotors were coated with a silicone bond (BathbondQ, KONISHI) to ensure water resistance. The use of waterproof servomotors can also be considered. However, they were not used in this study because of the robot size limitation. A pressure-tube consist of rubber and very light and bond coat plug joint were used to prevent short circuits. For the fabrication of robot body and cartilages, the molds were used to solidify the liquid materials. The molds were made of an acrylic resin, and consisted of multiple parts. A CNC router (MDX-540S-AP, Roland) and a laser cutting machine (Speedy 360, trotec) were used to produce the molds. Figure 5 summarizes the fabrication process of the robot based on the molding. The cartilage parts were fabricated by injecting Sylgard 184 into the mold, and then solidified in an oven at 80°C for 1 h. Bubbles inside the silicone were removed using a vacuum chamber. For fabrication of the robot body, Ecoflex 00–20 was injected into the mold to fix the servomotors, cartilages, and other parts. After the silicone was completely solidified, the robot was removed from the mold. The fabricated robots are demonstrated in Fig. 4(c)-(e).
Compression test. We performed compression test of the robots to examine their overall stiffness (see the Method section for the details of the experiment). Figure 6 shows that the stiffness of the robot with cartilages (type-A) is similar to that in the robot made of the mixture of soft tissue and cartilage materials (type-D). Therefore, by comparing the performance of these two robots it is possible to distinguish the effect of cartilage inclusion on the swimming efficiency disregarding the overall stiffness. In addition, the type-C robot with the much lower stiffness made only of soft tissue material (type-C) illustrates the effect of overall stiffness on the swimming efficiency.
Swimming test. We conducted swimming test of the developed robots in the experimental environment shown in Fig. 7(a) where the robot is immersed in a water tank filled with tap water. Figure 7(b) depicts a sequence of swimming movements of the robot with cartilages. Following the expectations, traveling waves were generated along the fins that push the robot forward. We measured the swimming speed and electric power consumption for each type of the robots while varying the frequency of the driving voltage from 0 to 9 Hz (7.5 V). At every tested frequency, the measurements were performed 10 times and the average value was reported. Figure 8(a) plots the measured swimming speed of the robot with cartilages. A peak speed of 13.1 mm/s appeared at a frequency of 4 Hz at a. The peak electric power consumption was 3.48 W. The presence of peak in the swimming speed suggests that there is a resonance vibration of the structure which enhances the amplitude of traveling waves. Regarding the robot without cartilage (i.e., made of the soft tissue only), the swimming speed was lower compared with the other robots as can be seen in Fig. 8(b). This may be due to the high compliance of the fin structure where the water pressure reduces the transmission efficiency of the traveling waves. As for the robot made of the mixture (Fig. 8(c), it exhibited a trend similar to that of the robot with cartilage. A peak speed of 11.2 mm/s was observed at a frequency of 4 Hz. The peak electric power consumption was 3.73 W.