Solar-powered robots can walk autonomously

Researchers at Cornell University have integrated electronic brains into solar-powered robots that range in size from 100 micrometers to 250 micrometers. Because they are smaller than an ant's head, they can walk autonomously without being controlled externally.

Although Cornell researchers have previously developed microscopic machines capable of crawling, swimming, walking, and folding themselves up, they were permanently attached to strings; to generate motion, wires were used to provide electrical current or laser beams had to be focused on specific areas of the robots.

According to Professor Itai Cohen of the College of Arts and Sciences, "Before, we had to manage these strings to elicit any sort of response from the robot."

"However, now that we have these intelligent individuals on board, it's like removing the ropes from a puppet. Similar to when Pinocchio becomes awake."

This development opens the door to a new generation of tiny gadgets capable of tracking germs, detecting chemicals, eliminating pollutants, performing microsurgery, and removing arterial plaque.

Collaborating on the study were Cohen's lab, Alyosha Molnar's lab in Cornell Engineering, and Paul McEuen's lab in Physical Science (A&S), who are all co-senior authors.

CMOS clock circuits with a thousand transistors and diodes, resistors, and capacitors are the brains of the new robots. The integrated CMOS circuit produces a series of phase-shifted square wave frequencies that determine the robot's stride. The legs of the robot are equipped with platinum actuators. In addition to the circuit, the legs are also powered by photovoltaics.

Itai Cohen, a professor of physics, compares the breakthrough of placing CMOS circuitry on microrobots to "Pinocchio's awakening."

"In a way, the electronics are rather primitive. "This clock circuit is not an advancement in the capability of circuits," remarked Cohen. "However, all of the electronics must be constructed with minimal power consumption in mind, so that we didn't have to use enormous photovoltaics to power the circuit."

Molnar Group research made low-power electronics feasible. Alejandro Cortese, Ph.D. '19, collaborated with Reynolds to develop the CMOS brain that was subsequently manufactured by XFAB.

Silicon-on-insulator 8-inch wafers contained the completed circuits. According to Reynolds, each robot's brain - which is effectively its body - measured 15 microns in height, compared to the flat wafer-sized circuitry. In collaboration with Cornell NanoScale Science and Technology Facility (CNF), he designed a complex photolithography procedure to etch the brains into an aqueous solution and pattern the actuators for the legs.

According to Cortese, CEO of OWiC Technologies, which he co-founded with McEuen and Molnar to commercialize optical wireless integrated circuits for microsensors, one of the main factors that facilitate this is the use of microscale actuators that can be controlled by low voltages and currents. This is the first time we have demonstrated that all of those legs can be controlled by a single circuit in a CMOS process.

This robot is named after Edward Purcell, a physicist who proposed a similar model to explain how microorganisms swim; an antbot with six legs that walks in a tripod gait similar to that of an insect; and a dogbot with four legs that can be controlled by a modified circuit that receives laser commands to change its walking speed.

"Eventually, the capacity to convey a command will allow humans to give the robot instructions, and the internal brain will choose how to execute them," Cohen added. "Therefore, we are conversing with the robot. The robot may provide us with information about its surroundings, and we might respond by instructing it to investigate the situation.

Compared to macroscale robots with inbuilt CMOS circuitry, the new robots move faster than 10 micrometers per second.

Using Reynolds' fabrication process, which involves the customization of foundry-built electronics, other researchers can now equip microscopic robots with their own applications, including chemical detectors and photovoltaic "eyes" that enable robots to navigate by sensing changes in light intensity.

Reynolds stated, "This allows us to conceive extremely complicated, highly functioning, programmable microrobots that are integrated with not only actuators but also sensors." "We're intrigued about the potential in medical – something that could move about in tissue, recognize healthy cells, and destroy bad cells – and in environmental remediation, such as if you had a robot that could break down pollutants or detect a hazardous chemical and eliminate it."

To manage the flow of fluids, the scientists attached their CMOS clock circuits to platinum-based, electrically driven actuators in May.

Cohen stated, "The exciting part is, just as we didn't know what the iPhone would be about until it was released to the public, we're hoping that now that we've shown the recipe for linking CMOS electronics to robotic actuating limbs, people can design low-power microchips that can do all sorts of things." This is the concept behind releasing it into the ether and allowing people's imaginations to run wild.

MRSEC, Cornell Center for Materials Research, Air Force Office of Scientific Research, Army Research Office, and Kavli Institute at Cornell for Nanoscale Science funded the study.