First, they needed to create adhesive foot pads that would keep the robot attached to the surface even when upside-down, but release to allow the robot to “walk” by lifting and placing its feet. The pads consist of a polyimide-insulated copper electrode, which enables the generation of electrostatic forces between the pads and the underlying conductive surface. The foot pads can be released and re-engaged easily by switching on and off the electric field, which operates at a voltage similar to that required to move the robot’s legs, thus requiring very little additional power. The electroadhesive foot pads can generate shear forces of 5.56 grams and normal forces of 6.20 grams — more than enough to keep the 1.48-gram robot from sliding down or falling off its climbing surface. In addition to providing high adhesive forces, the pads were designed to flex, thus allowing the robot to climb on curved or uneven surfaces.

The scientists also created new ankle joints for HAMR-E that can rotate in three dimensions to compensate for rotations of its legs as it walks, allowing it to maintain its orientation on its climbing surface. The joints were made of layered fiberglass and polyimide and folded into an origami-like structure that lets all the legs rotate freely, and passively align with the terrain as HAMR-E climbs.

Finally, the researchers created a special walking pattern for HAMR-E, as it must have three foot pads touching a vertical or inverted surface at all times to keep it from falling or sliding off. As one foot releases from the surface, swings forward, and reattaches, the remaining feet stay attached to the surface. At the same time, a small amount of torque is applied by the foot diagonally across from the lifted foot to keep the robot from moving away from the climbing surface during the leg-swinging phase. This process is repeated for the three other legs to create a full walking cycle, and is synchronized with the pattern of electric-field switching on each foot.

When HAMR-E was tested on vertical and inverted surfaces, it was able to take more than 100 steps in a row without detaching. It walked on inverted surfaces at speeds comparable to other small climbing robots and slightly slower than other climbing robots on vertical surfaces, but was significantly faster than other robots on horizontal surfaces, making it a good candidate for exploring environments that have a variety of surfaces in different arrangements in space. It is also able to perform 180-degree turns on horizontal surfaces.

HAMR-E also successfully maneuvered around a curved, inverted section of a jet engine while staying attached, and its passive ankle joints and adhesive foot pads were able to accommodate the rough and uneven features of the engine surface simply by increasing the electroadhesion voltage.

The team is continuing to refine HAMR-E, and plans to incorporate sensors into its legs that can detect and compensate for detached foot pads, which will help prevent it from falling off vertical or inverted surfaces. HAMR-E’s payload capacity is also greater than its own weight, creating the possibility of carrying a power supply and other electronics and sensors to inspect various environments. The team is also exploring options for using HAMR-E on nonconductive surfaces.

“This iteration of HAMR-E is the first and most convincing step toward showing that this approach to a centimeter-scale climbing robot is possible, and that such robots could in the future be used to explore any sort of infrastructure, including buildings, pipes, engines, generators, and more,” said corresponding author Robert Wood, a founding core faculty member of the Wyss Institute and the Charles River Professor of Engineering and Applied Sciences at SEAS.

“While academic scientists are very good at coming up with fundamental questions to explore in the lab, sometimes collaborations with industrial scientists who understand real-world problems are required to develop innovative technologies that can be translated into useful products. We are excited to help catalyze these collaborations here at the Wyss Institute, and to see the breakthrough advances that emerge,” said Wyss Founding Director Donald Ingber, the Judah Folkman Professor of Vascular Biology at Harvard Medical School and the Vascular Biology Program at Boston Children’s Hospital, and professor of bioengineering at SEAS.

This study was co-authored by Benjamin Goldberg, Neel Doshi, Kaushik Jayaram, and Jack Zhou from the Wyss Institute and SEAS.

Support for this research was contributed by the Wyss Institute for Biologically Inspired Engineering at Harvard University, Rolls-Royce, and the U.S. Army Research Office.