A (Jellyfish?) Robot Takeover: Hydrogel-based Soft Robotics Inspired by Jellyfish

When asked to think of the typical robot, you probably imagine a clunky BB-8-type of android made of metal. Or, if you are more familiar with electrical engineering or involved with your robotics team at school, you might be thinking of the claw device you assembled and coded. Either way, no matter what came to mind, I’ll take a wild guess and say the robot you envisioned was probably a conventional “hard” robot—one made of steel, aluminum, or a hard plastic that is strong and sturdy, but also rigid and inflexible. While important for a litany of industrial applications that require durability, in the realm of marine and ocean-exploring applications, the traditional “hard” robot simply doesn’t cut it. Scientists are turning to alternative materials to create robots that can adapt better to ocean environments, enabling them to crawl through reefs and crevices within deep ocean floors and trenches. This is especially important since, fun fact: only 5% of our oceans have been explored by humans! And of course, what better place to find that solution than in nature itself?

In this blog post, we’ll be discussing bioinspired hydrogel-based robots that mimic jellyfish for mechanical flexibility and motion in ocean environment applications.

What are hydrogels? 

A hydrogel is a biphasic material that mainly consists of hydrophilic (water-loving) cross-linked polymers and at least 10% water or another kind of liquid. This forms what you can visualize as a "squishy" polymer-based network surrounded by fluid. With the texture of a stress-ball toy or gel, hydrogels are extremely beneficial due to their toughness, flexibility, and extreme freedom level. Hydrogel polymers can swell and absorb a great deal of water without dissolving or breaking down. Hydrogels are already widely used in a huge range of applications, some of the most important being medical uses such as wound care and medical implants.

The Jellyfish: Understanding Its Anatomical Structure

Despite their rather misleading name, jellyfish are only marginally made up of "jelly" (or, more scientifically speaking, the gelatinous substance mesoglea) and contain about 95% water. You could consider jellyfish as almost like a natural kind of hydrogel! Their water-based composition, combined with being an invertebrate (having no backbone), makes jellyfish blob-shaped, floppy, and yielding to the touch.

In terms of motion, jellyfish use an internal jet-propulsion system that is founded on a delicate coordination of dynamic fluid interactions that create suction. They do not have a rigid structure to move or a centralized nervous system to coordinate motion. In order to swim via jet propulsion, ring muscle fibers surrounding the subumbrellar surface (subumbrellar muscles) are contracted. The subumbrella muscles tighten the bell and squeeze the subumbrellar volume. This movement ejects fluid from the subumbrellar region either in the form of a train of vortices (scyphomedusae) or in the form of a jet along the velar opening (hydromedusae). Essentially, these contractions result in the formation of low-pressure pockets under the jellyfish's "umbrella," where the in-flowing water sucks the jellyfish forward. Simultaneously, the discharging jet's energy produces thrust, propelling the creature through the water.

Two Different Bioinspired Approaches

Now that we have determined the jellyfish's structure and properties—specifically its fluid dynamic body and gelatinous motion system—it is clear why these animals are the first choice for bioinspired flexible-robot design. Below you'll find two exciting bioinspired designs for soft robots.

1] A team led by Jinhu Zhang, Tianye Zhang, et al. created a hydrogel-based artificial jellyfish model, dubbed the HABH for short. The HABH uses not muscles but rather six soft polyvinyl alcohol (PVA) hydrogel actuators, made in 3D-printed molds. As the actuators fill with water, the latter expand and buckle the body, finally ejecting a water jet out through a small hole at the end—like a natural jellyfish. Back-pumping causes the actuators to pull, completing the continuous hydraulic cycle to drive the robot forward.

To compare the HABH with real jellyfish, mechanical and acoustic experiments were conducted. First, the research demonstrates that the hydrogel jellyfish exhibits almost perfect acoustic impedance matching with water, which enables more than 99.5% transmission of sound across broadband frequencies. Acoustic impedance, or the quantity that is the product of the density of a material and the speed of sound in that material, is what determines the quality of how sound waves propagate from one medium to another. Having this almost ideal match is important for allowing the hydrogel jellyfish to be acoustically transparent in water and blend in with surroundings, a dramatic contrast to metals and silicone materials that exhibit major impedance mismatches and lower transmission rates.

Secondly, the scientists also wanted to measure the swimming ability of the HABH in a glass water tank, setting up an obstacle test with three rings (137 mm inner diameter, 100 mm apart) to mimic a natural ocean environment with narrow underwater channels. Despite occasional contact with the rings due to turbulence of water flow, the hydrogel jellyfish successfully propelled itself upwards with each contraction and generally glided smoothly through the openings even at various angles. Using a high-speed camera to track the device, they confirmed that the jellyfish's behavioral motion was in agreement with finite element simulations to guarantee the accuracy of its simulated behavior. Notably, the HABH jellyfish was indeed able to pass through orifices narrower than their original body length, demonstrating a capacity for greater mechanical flexibility under a system of hydraulic control—a significant advantage over conventional rigid robots that are unable to deform and move through constricted areas.  

Design and fabrication of HABH jellyfish (Zhang et al., 2022)

2] Another bioinspired approach, pioneered by Tianlu Wang and Keyjong-Joon Joo, et al., is the HASEL (hydraulically amplified self-healing electrostatic actuators) design, based on the moon jellyfish Aurelia aurita's shape and swimming manner. With diameters of 160 mm and six actuated lappets, the robot utilizes soft electrohydraulic actuators (HASELs), which are highly flexible, fluid-filled pouches that get deformed when applied with high voltage, acting like artificial muscles. Each actuator is a polymer shell filled with a liquid that contains carbon-lined electrodes. Under the application of high voltage, the corresponding stress causes a zipping motion, which then drives the dielectric fluid and rotates the lappet joints. In combination, they create the umbrella-like contraction-relaxation cycles that are responsible for jellyfish swimming. For the fabrication process of the robot device, 2D polymer films, electrodes, waterproofing, and stiffeners were folded into a 3D bell shape, and density was adjusted with care to 1.23 × 10³ kg/m³ for ideal stable buoyancy.

The HASEL robot was then paired with control electronics using the ROS (robot operating system) framework to provide high-voltage signals in order for users to be able to control lappet movement and generate natural paddling motions. When brought together, these traits created a successful bioinspired technology where the soft electrohydraulic actuation delivered strong, yet controlled jet propulsion while also maintaining the flexibility and adaptability that is inherent to natural jellyfish, addressing the concerns associated with hard robots.

Want to learn more? For further information, check out some of these papers and resources that I used to make this post:

1] The HBAH robot - Zhang, Jinhu, Tianye Zhang, Erqian Dong, Chuang Zhang, Zhonglu Lin, Zhongchang Song, Hongquan Li, Nicholas X. Fang, and Yu Zhang. 2022. “Bioinspired Hydrogel Jellyfish with Mechanical Flexibility and Acoustic Transparency.” Cell Reports Physical Science 3 (10): 101081. https://doi.org/10.1016/j.xcrp.2022.101081

2] The HASEL robot - Wang T, Joo HJ, Song S, Hu W, Keplinger C, Sitti M. A versatile jellyfish-like robotic platform for effective underwater propulsion and manipulation. Sci Adv. 2023 Apr 14;9(15):eadg0292. doi: 10.1126/sciadv.adg0292. Epub 2023 Apr 12. PMID: 37043565; PMCID: PMC10096580.

3] Bahram, Morteza, Naimeh Mohseni, and Mehdi Moghtader. 2016. ‘An Introduction to Hydrogels and Some Recent Applications’. Emerging Concepts in Analysis and Applications of Hydrogels. InTech. doi:10.5772/64301.

4] Chou, JS., Molla, A. Recent advances in use of bio-inspired jellyfish search algorithm for solving optimization problems. Sci Rep 12, 19157 (2022). https://doi.org/10.1038/s41598-022-23121-z.

5] World Economic Forum. 2022. “How Jellyfish Could Inspire the Next Generation of Robots.” World Economic Forum, October xx, 2022. https://www.weforum.org/stories/2022/10/bio-inspired-machine-intelligence-artificial-robotics/