To protect vital components from intense pressure, such as a robot’s computer brain, engineers often enclose them in spherical or cylindrical pressure housings. This is because circular shapes evenly distribute the water’s pressure over their entire area, whereas shapes with sharp edges (e.g. cubes or pyramids) concentrate stress at those points, increasing the risk of implosion. To ensure the pressure-resistance of these housings, engineers must also use strong materials. But there’s usually a tradeoff: the stronger the housing, the heavier the robot. And heavier robots require greater buoyancy, which can make it awkward and more cumbersome for them move at depth, and cause battery life to drain more quickly.
Traditionally, engineers have solved this problem by relying on flotation devices that compensate for increased weight, allowing heavy robots to still float. The famed submarine Trieste, which was the first to dive the deepest point in the ocean, Challenger Deep in the Mariana Trench, got its buoyancy from a huge tank of gasoline sitting on top of a steel sphere that housed the robot’s pilots—similar in design to a blimp. The gasoline created an internal atmosphere that kept the submarine from collapsing under pressure. And since gasoline is less dense than water, it ensured the vehicle would be buoyant enough to move freely. Big tanks, however, are not ideal for maneuvering along the dark, unknown topography of the seafloor, which ocean robots often need to do.
To avoid such bulk, many engineers today opt for a material called syntactic foam. It features a mixture of glue-like epoxy and hollow microscopic glass spheres. The hollow spheres lower the density of the foam, making it lighter than water and creating buoyancy, while the glue provides strength and durability to hold its shape. Syntactic foam can be shaped into any form, allowing ocean robots to move in a streamlined manner.