Synthetic blood to power vascularized robots

New soft robots can flex their muscles with synthetic “robot blood” and multipurpose circulatory organs.

Original paper: Aubin, C. A. et al. Electrolytic vascular systems for energy-dense robots. Nature 571, 51–57 (2019)

The circulatory or vascular system of animals is a complex organ that performs multiple functions simultaneously. Take the human circulatory system for example: it transports oxygen and nutrients throughout the body, regulates temperature, helps in fighting diseases, etc. However, robots don’t usually function the same way. Despite the vast improvements in robot design, they still cannot do much multi-tasking. Part of the problem is that robots are typically built from rigid parts that only do one thing. A battery usually does not play any other role aside from providing energy, and moving parts are typically controlled by independent actuators.  These single-purpose components (such as batteries) are typically less efficient than their biologic counterparts, and add to the weight of the robots (limiting their performance, maneuverability, speed, and autonomy).

In order to create multi-functional robots, we can take inspiration from biology and build them out of multi-purpose components. A research team led by Robert F. Shepherd from Cornell University (NY, USA) has found a way around this problem by integrating a multifunctional circulatory system into untethered, autonomous soft robots. In a recent issue of Nature, they present an aquatic soft robot with a synthetic circulatory system which provides chemical energy. The synthetic blood has zinc and iodide ions that make it electrically conducting (like an electrolytic blood), and powers an artificial heart (a pump that circulates “blood” throughout the robot body). Rather than using traditionally rigid materials to build the robot, the circulatory system is encased in a soft, flexible body made out of stretchable silicone. The circulating “blood” can deform the flexible body of the robot when pumped through the vascularized fins, like electrolytic “robot blood” flexing a muscle, moving the fins of the fish and propelling the robot forward  (Figure 1, Video 1). By bringing materials and robot design closer to complex biological systems, their multifunctional synthetic vascular system can combine hydraulic force transmission, mechanical actuation, and energy storage (killing not two but three birds with one stone).

Figure 1. Lionfish-inspired robot powered by an electrolytic vascular system. a) Schematic shows synthetic blood in the tail fin (red) and in the dorsal and pectoral fins (yellow). b) The synthetic blood circulates through the robot body and actuates the tail fin by expanding and contracting the silicone parts at each side (adapted from Aubin et al.)

Although flow batteries are less energy dense than their lithium ion equivalents, their integration in soft robotic platforms has significant advantages: the electrolyte can fill up most of the volume of the robot and act as the hydraulic system (without the need of additional space), and the surface area of the flow batteries inside the soft robot can be maximized for an increased energy capacity (hence the large area of dorsal fins). With this approach, the autonomous robot can swim upstream for long operation times (up to 36 hours). 

Video 1. Autonomous lionfish-inspired robot swimming.

The proposed bio-inspired approach reduces the gap between biological complex systems and synthetic, traditionally bulky robotic platforms. According to the authors, “this concept can be generalized to other machines and robots”, which opens a wide design space for multifunctional soft machines, from materials design to actuation and control. Such improvements in energy storage and performance could advance the development of soft robots for search-and-rescue operations, marine exploration, inspection of underwater pipelines, and coral reef health monitoring (all applications where autonomy and safety are critical). Although still in its infancy, “robot blood” could one day power, actuate, and control untethered soft robots that can safely and autonomously interact with humans in delicate environments. And just like in science fiction, robots with synthetic organs and circulatory systems could one day live among us.

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