KEY CONCEPTS
• A lionfish-inspired robot was prepared with soft materials that used power stored in an aqueous-based redox flow battery to enable movement in a manner similar to that of animals. 
• The redox flow battery also acted as an electrolyte hydraulic system to power pumps that moved the pectoral fin and tail of the robot. 
• The robot consisted of almost entirely fluid (approximately 90% by volume) and used a zinc iodide flow cell. 

The rapid development of artificial intelligence is leading to interest in determining how to develop robots that can act, work and function in a similar fashion to animals. But there are challenges faced in designing a robot that can exhibit the multifaceted way that animals act and react to their environment. 

Researchers in a previous TLT article (1) overcame the complex three-dimensional motions of bat wings using a technique known as principal component analysis to develop a robotic bat that can fly in a similar manner. In this process, the more than 40 active and passive joints in bat wings were reduced to nine joints in the robotic bat. Self-lubricating bearings were used to enable the robotic bat to fly.

Robert Shepherd, assistant professor in the Sibley School of Mechanical and Aerospace Engineering at Cornell University in Ithaca, N.Y., says, “The biggest problem in emulating animals is the need to optimize the endurance and adaptability of the robot.” 

Shepherd believes there are three issues that need to be overcome in the development of better robots. “Robots do not have the energy density of animals who use energetic fuels such as fats and sugars. They lack the distribution of a large number of actuators and sensors as well as the brainpower and controllers to coordinate motion in response to their environment. A third factor, and one that we address in our recent work, is that robots store their energy centrally rather than distributed throughout the body like fat in animals.”

To have access to power, robots need to have some type of energy-storage device that can be utilized when needed. Batteries are a viable option, but traditional types such as lead-acid and structural contribute too much weight for robots to move effectively. An attractive storage option for Shepherd is redox flow batteries (RFBs). 

In a previous TLT article (2), a RFB containing a non-aqueous solvent was described. The RFB is divided into two compartments that contain liquid electrolytes known as the catholyte and the anolyte, which surround their respective electrodes (cathode and anode, respectively). An anion-exchange membrane separates the two compartments. During charging and discharging periods, half of the cell is oxidized while the other half is reduced.

Shepherd felt that using a RFB may not only give a robot a more flexible energy storage system but also enable the power to be utilized by pumps turning the electrolyte into a hydraulic system. He says, “We envision developing a system that is similar to the heart’s function in an animal’s cardiovascular system. The heart acts as a pump to move blood throughout the body, which provide the power needed for the body to function.”

The researcher’s approach was to convert electrical instead of chemical energy into mechanical energy. 

Lionfish-inspired robot
Shepherd and his colleagues developed a robot modeled after a lionfish that employs a RFB as both battery and hydraulic system. He says, “Our intention was to develop a robot from soft materials that contained an aqueous system to emulate how living organism function.”

An image of a lionfish-inspired robot is shown in Figure 1. 


Figure 1. A lionfish-inspired robot was developed that uses a redox flow battery to both store energy and act as a hydraulic system. (Figure courtesy of Cornell University.)

The RFB used by the researchers was a hybrid that contained a liquid catholyte and a solid anode. Shepherd says, “We were able to maximize the energy density of the battery by using this approach. The liquid catholyte contained triiodide anions and the solid anode was composed of zinc. We used a zinc iodide flow cell because of its simplicity, neutral pH, low viscosity and high-energy density. For reasons of safety, the ionic concentration was kept low.”

Two pumps were used to enable the electrolyte hydraulic system to move the pectoral fin and tail of the lionfish-inspired robot. The researchers used soft materials including flexible electrodes and a cation-exchange membrane encased in a silicone skin to allow for flexible movements such as bending. The lionfish-inspired robot consisted of almost entirely fluid (approximately 90% by volume) that mimics the composition of animals. 

Energy is discharged in the RFB when zinc is oxidized generating electrons and zinc ions are produced. The electrons move to the catholyte enabling the RFB to power the pumps. At the same time the zinc ions flow to the anolyte leading to the reduction of triiodide to iodide anions. 

The researchers achieved an energy density of 124 Wh per liter with an average discharge voltage of 1.00 V. Testing of the lionfish-inspired robot was accomplished through the use of cyclic voltammetry and by placing the robot in a fish tank. The lionfish inspired robot had sufficient power to swim for more than 36 hours untethered at a speed of 1.56 body lengths per minute in a tank of water. 

Shepherd says, “Our initial design was inefficient because we needed to move the ions and electrolytes one way through the RFB and then reverse the process. By improving the power density, we believe the lionfish-inspired robot will be able to move more quickly over a longer period of time.”

Ultimately, the researchers are looking to use this hybrid RFB soft design to create other robots. Shepherd says, “Our objective is to develop a robot that can walk on land.”

Additional information on this work can be found in a recent article (3) or by contacting Shepherd at rfs247@cornell.edu. 

REFERENCES
1. Canter, N. (2017), “Robotic bats: Learning how bats fly,” TLT, 73 (5), pp. 14-15.
2. Canter, N. (2018), “Non-aqueous redox flow batteries,” TLT, 74 (5), pp. 20-21.
3. Aubin, C., Choudhury, S., Jerch, R., Archer, L., Pikul, J. and Shepherd, R. (2019), “Electrolytic vascular systems for energy-dense robots,” Nature, 571 (7763), pp. 51-57.