Autonomous Robots Smaller Than a Grain of Salt

Robots are becoming more capable and more autonomous as time goes by. Today, robots can do everything from powering self-driving vehicles to navigating complex city streets and performing challenging surgical procedures in the operating room. However, as the scale of a robot decreases, so do its capabilities. The tech that powers cutting-edge robotic systems simply can’t fit into small packages. This means microscale robots, of the sort that could be deployed inside the human body to treat medical conditions, are still firmly in the realm of science fiction.

But a team led by researchers at Penn State University has taken us one step closer to that ultimate goal. They have developed microscopic autonomous swimming robots — smaller than a grain of salt — that can sense, think, act, and compute. They are barely even visible to the naked eye, yet they can do all of this without tethers, magnetic fields, or the human oversight that is normally hidden in the details of a report on this type of work.

Each of these microrobots measures roughly 200 by 300 by 50 micrometers, making them smaller than a grain of salt and comparable in size to single-celled organisms. Despite their tiny dimensions, the robots are fully self-contained systems. They carry onboard power generation, sensors, memory, computation, communication hardware, and propulsion, all integrated into a flat chip fabricated using standard semiconductor manufacturing techniques.

One of the key challenges at this scale is power. Traditional batteries are far too large, so the robots rely on microscopic photovoltaic cells to harvest energy from light. Operating on an extreme power budget of around 100 nanowatts, the robots are built using a 55-nanometer CMOS process and subthreshold digital logic to minimize energy consumption. Even with these constraints, each robot contains a tiny processor, instruction memory, data memory, temperature sensors, actuator control circuits, and an optical receiver for communication.

Programming the robots is done optically. A base station shines patterned light pulses onto the robots, which decode the flashes as instructions and store them in onboard memory. Special passcodes prevent accidental reprogramming and allow different robots to receive different instructions in the same environment. Once programmed, however, the robots operate completely autonomously, making decisions based on their stored code and sensor readings without any further external input.

Movement at the microscale presents another major obstacle. At these dimensions, water behaves less like a fluid and more like thick syrup, rendering wheels, legs, or propellers useless. To get around this, the researchers use electrokinetic propulsion. Each robot is equipped with simple platinum electrodes. When small voltages are applied, electric fields cause nearby ions in the surrounding fluid to move, dragging the fluid along and pushing the robot in a controlled direction. By selectively activating different electrodes, the robot can move forward, rotate, or trace curved paths, all while consuming only tens of nanowatts.

The onboard computer allows these basic motion primitives to be combined into intelligent behaviors. In demonstrations, the robots measured local temperature using integrated sensors and then altered their movement based on those readings. In one experiment, they encoded temperature data into subtle changes in motion that could be decoded by a camera, effectively transmitting sensor readings without radios or antennas. In another, the robots autonomously climbed temperature gradients, exploring their environment until they found warmer regions and then adjusting their behavior accordingly.

While these microrobots are still confined to laboratory environments, their capabilities hint at a future where swarms of programmable machines could monitor individual cells, explore microfluidic systems, or assist in building microscopic devices.