Powering microrobots has long been a chemical engineering problem. A 1.7 g microrobot can crawl for only about two minutes on commercial batteries. To run for two hours, it would need a battery weighing 94% of its total mass, with an energy density of 5382 Wh kg −1 , far beyond today’s lithium-ion technology. Below 1 cm 3 or 1 g, microbatteries lose energy density even faster because packaging and conductive materials take up a growing share of their mass.
From carrying batteries to walking on energy
A team led by Yue Gao at Fudan University’s Department of Macromolecular Science tried a different route. Instead of making batteries smaller and denser, the researchers asked whether the robot could harvest energy from the surface it walks on. Active materials such as aluminum, zinc, silicon, tin, and lead are common in pipes, buildings, data centers, and equipment. If a robot’s foot could trigger a chemical reaction on contact, its runtime would no longer be limited by onboard storage.
The team’s answer is an open electrochemical power system: a crosslinked potassium polyacrylate membrane attached to the microrobot’s foot. It converts chemical energy from the substrate and from atmospheric moisture and oxygen into electricity.
A skin that powers, grips, and senses
The membrane is made in one step by polymerizing acrylic acid monomer, crosslinker, initiator, and potassium hydroxide directly on the robot’s foot. A flexible platinum/carbon film acts as the reaction site for oxygen and water. The skin can be as thin as 135 micrometers and as small as 1 square millimeter. On zinc, it delivers a power density of 133 mW cm −2 ; on aluminum, 103 mW cm −2 . These values are about an order of magnitude higher than those of typical microbatteries. The system also retains its current capability when scaled down to 0.0004 cm 3 . The design must satisfy several conflicting needs at once. It has to hold water, stay flexible, stick to surfaces, release easily, and sense the ground.
Holding water and surviving extreme temperatures
The membrane’s carboxylate groups, potassium ions, and hydroxide ions interact strongly with water molecules. Even at 20% relative humidity, similar to the Sahara Desert, it retains most of its water and continues to discharge after 48 hours. At −20°C, potassium ions disrupt the ordered hydrogen-bond network of water, keeping the membrane ionically conductive. The voltage drops by only about 0.2 V compared with room temperature. The crosslinked structure also allows operation at 80°C. After water evaporates, adding water restores performance.
Power on contact, off on lift-off
The membrane generates electricity when it touches an active substrate and stops when the foot lifts. The substrate loses electrons. On the platinum/carbon side, oxygen and water accept electrons to produce hydroxide ions. The reaction starts in about 0.1 milliseconds and stops when contact ends. Each walking step refreshes the reaction interface. Discharge products remain on the substrate rather than accumulating on the membrane, avoiding the performance decay seen in static batteries.
Sticky in the right direction
The membrane combines strong shear adhesion with easy normal detachment. Carboxylate groups create electrostatic adhesion to metal substrates. In the walking direction, adhesion is strong enough to prevent slipping. In the vertical direction, detachment resistance is low, so the robot can lift its foot efficiently. This allows the microrobot to crawl on inclined aluminum surfaces.
Sensing the ground through ions
The membrane can also sense the surface beneath it. Different materials cause different electrochemical reaction rates and hydroxide consumption, leading to directed migration of potassium ions and a characteristic potential signal. Rough surfaces deform the membrane and change ion distribution, producing another signal. Material changes generate stable square-wave signals, while roughness changes produce millivolt-level spikes. The two signals can be naturally decoupled. With an X-Y electrode array, the membrane can also locate where contact occurs. This helps the robot recognize its environment and actively seek energy-rich paths.
National Science Review
Experimental study