(Invited) Design of Hydrogel Material and 3D-Printed Molding for Imitating the Tactile Textured Properties of Moon Jellyfish
Authors: Ogawa Jun , Yamada Naoya , Watanabe Yosuke , Khosla Ajit , Kawakami Masaru , Furukawa Hidemitsu 
Published: December 2020

The current research on jellyfish robots designs a mechanical body that can assemble a waterproof and flexible drive system consisting of cables, motors and actuators to reproduce a swimming mechanism of jellyfish. On the other hand, research on jellyfish robots as visual art focuses on soft materials and demonstrates a technique to produce jellyfish swimming by the drag force received from the running water of the pump without using a special control mechanism. Most jellyfish robots that have been developed as toys imitate only their appearance, and they are being developed as commercial products with a low economic burden by using inexpensive materials such as vinyl and fishing line. The swimming of such jellyfish robots is far from the actual jellyfish. From this, it can be seen that the mechanism or material factor is largely due to the imitation of jellyfish. This study proposes a hydrogel jellyfish robot that approximates shape, water content, color and texture to a jellyfish. The body composition of moon jellyfish is gelatinous with a water content of 95% or more. The texture mimicry of jellyfish-inspired robots can be realized by using an equivalent hydrogel as a constituent material. Therefore, we develop a DMAAm gel that can artificially reproduce the tactile characteristics depending on the constituent materials. The chemical composition of the gel contains dimethylacrylamide (DMAAm) as the monomer, N, N'-Methylenebisacrylamide (MBAA) as the cross-linking agent, diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide (TPO) as the photopolymerization initiator, polyethylene glycol and water as the solvent. The gel of this formulation produces a white translucent color due to the solvent PEG and has a swelling ratio of about 2.5 times. Additionally, this study shows a three-dimensional modeling technology that molds this gel into the appearance of a jellyfish. Our hydrogel modeling adopts silicone resin using 3D printer. This mold consists of both an umbrella-shaped silicone and a silicone with the geometric pattern of a jellyfish stomach.The two molds are combined and the gel solution is poured from the solution inlet provided in the upper mold and cured by UV irradiation for molding. In addition, a gel film is created by sandwiching the pre-gel solution in a PET plate and UV curing. By irradiating the gel film and the gel jellyfish umbrella with UV again, we can develop a jellyfish robot with tentacles. Our jellyfish robot has a thickness accuracy on the order of a few millimeters and a hardness that can be crushed by its own weight outside water, and circulates in a liquid at a flow rate of about 40 L / min. Moreover, the tentacles have a thickness accuracy of the order of a few micrometers, and they are gel body structures with different thicknesses. We show that hydrogel jellyfish can imitate swimming by approximating the constituent material and shape to the actual moon jellyfish. Then, using our DMAAm gel and Wizard gel, we will compare and verify the visual effect caused by the difference in the texture of the blended gel from the time series change of the projected area by binary image processing. The results show that the wizard gel jellyfish does not deform under the conditions of circulating water flow in a water tank used for breeding commercial jellyfish, and it is difficult to exhibit jellyfish-like behavior. However, the millimeter-scale DMMAm gel reproduces the periodic contraction due to the elastic change, and the micrometer-order DMMAm gel has an aperiodic fluctuation behavior. Therefore, our gel jellyfish robot can simultaneously observe jellyfish contraction and tentacle fluctuation behavior. Finally, this study discusses a method for quantitatively evaluating from swimming by the gel jellyfish robot developed by soft body simulation based on Position Based Dynamics in virtual space.

Figure 1