  The backswimmer fly is an aquatic insect, capable of regulating its buoyancy
underwater. Its abdomen is covered with hemoglobin cells, used to bind and
release oxygen, reversibly. Upon entering water, the fly entraps an air bubble
in a superhydrophobic hairy structure on its abdomen for respiration. This
bubble, however, can change its volume through regulated oxygen flow from the
abdominal hemoglobin cells. In this way, it can reach neutral buoyancy without
further energy consumption. In this study, we develop a small, centimeter
scale, backswimmer inspired robot (BackBot) with auto-buoyancy regulation
through controlled nucleation and release of microbubbles. The bubbles nucleate
and grow directly on onboard electrodes through electrolysis, regulated by low
voltage. We use 3D printing to introduce a three-dimensional bubble-entrapping
cellular structure, in order to create a stable external gas reservoir. To
reduce buoyancy forces, the bubbles are released through linear mechanical
vibrations, decoupled from the robot's body. Through pressure sensing and a
Proportional Integral Derivative control loop mechanism, the robot
auto-regulates its buoyancy to reach neutral floatation underwater within
seconds. This mechanism can promote the replacement of traditional and
physically larger buoyancy regulation systems, such as pistons and pressurized
tanks, and to enable the miniaturization of Autonomous Underwater Vehicles.
