Building biological systems from scratch traditionally involves copying the original, but simply mimicking an animal’s shape and muscle alignment isn't enough. In a new study published today (July 22) in Nature Biotechnology, a team from California Institute of Technology and Harvard University worked to understand how a jellyfish’s motions and interactions with the surrounding water lead to swimming, then used available bioengineered tissues to construct a jellyfish mimic that could move through the water just like the real thing.
"I was amazed at how effectively they allowed the vehicle to emulate swimming by natural organisms," said Jack Costello, a jellyfish expert from Providence College, Rhode Island, who was not involved in the study. "I was so impressed with the attention to getting all the important variables lined up so that they emulated the animals." Costello added that the technique could aid vehicle design, as well as help researchers make more realistic replacement tissues, among other biomedical applications.
John Dabiri from Caltech and colleagues used the juvenile of the jellyfish Aurelia aurita as a model for jellyfish propulsion. Juvenile Aurelia have a simple body plan consisting of a central disc with eight radiating thick, flat tentacles called lobes. Observing their movement, the team learned that the jellyfish propel themselves through the water by simultaneously and completely contracting the bell that forms the central bulk of their body. The contraction is achieved by activating lines of muscles through their lobes synchronized by a system of pacemakers. The team mimicked this action by building their model, named a medusoid, out of a sheet of cultured rat heart muscle tissue, applying an electrical field to the water to serve as the pacemaker.
After fast muscle contraction, jellyfish slowly recoil back to their original flat shape. To mimic this action, the team molded the rat muscle cells on a synthetic elastic silicone sheet that naturally recoiled. Lastly, the researchers identified the shape of the jellyfish as an important factor for pushing the water with maximum efficiency by creating patterns of fluid movement between the lobes of the animal. By mapping the swimming muscles of the Aurelia with chemical stains, the team determined the specialized alignment of cells, which they reproduced with the rat heart muscle cells that conformed to a designed pattern on the silicone sheet. In the end, the researchers created a medusoid jellyfish mimic that could swim by pumping fluid away from its body center, much like the real animal.
"This paper clearly shows that if you don't copy the way animals have evolved to do things, you don't get the results that they do either," said Costello. "To faithfully copy what animals do is not always something we could try to do before. It's a very clear representation that you need to get the details right."
Dabiri said they used a jellyfish as a concept model because its pumping action is similar to that of a beating heart. The interaction of the jellyfish pump with water is also analogous to the blood flow in the heart, and Dabiri and his colleagues hope tissue engineering could soon be used to replace components like heart valves that could gain an advantage by being active, rather than passive, components of the circulatory system.
"That movement is different to many current tissue replacements where a passive system is replaced by another passive system," said Dabiri. "There could be some advantage to allowing that component to move, to change the blood flow or to respond to some other signal."
The medusoid can only swim in one direction for now, because all the muscle cells are identical, but Dabiri hopes future models can incorporate multiple cells types that will allow the model to turn and maneuver. The team also wants to reconstruct the jellyfish's feeding habits. At the moment, the current created by swimming does attract some water towards the medusoid, as in the real animal, but the medusoid lacks the ability to capture any prey and consume it. Dabiri said this could be important for extending the lifetime of a system. By adding this capability to bioengineered systems, an active heart component could gain energy from sugar in the blood flow, for example, such that no outside energy source would be needed.
"Now that we've established a design process that begins by looking for certain features within the real biological system and extracting those design principles, the next organism that we go on to can be more complex than this one, and I think the mimics can be achieved in much faster time," said Dabiri.
Watch a video of the team explaining their research and see the medusoid swim next to a real jellyfish:
J. Nawroth et al., "A tissue-engineered jellyfish with biomimetic propulsion," Nature Biotechnology, doi:10.1038/nbt.2269, 2012.