Colloids — insoluble particles or molecules anywhere from a billionth to a millionth of a meter across — are so small they can stay suspended indefinitely in a liquid or even in air. Robots about the size of a human egg cell were created that can sense their environment, store data, and carry out computational tasks. They consist of tiny electronic circuits made of two-dimensional materials, piggybacking on colloids. By coupling these tiny objects to complex circuitry, the researchers hope to lay the groundwork for devices that could be dispersed to carry out diagnostic journeys through anything from the human digestive system to oil and gas pipelines, or perhaps to waft through air to measure compounds inside a chemical processor or refinery.
Colloids can access environments and travel in ways that other materials cannot; for example, they are small enough that the random motions imparted by colliding air molecules are stronger than the pull of gravity. Similarly, colloids suspended in liquid will never settle out.
The tiny robots are self-powered, requiring no external power source or even internal batteries. A simple photodiode provides the trickle of electricity that the robots’ circuits require to power their computation and memory circuits. That's enough to let them sense information about their environment, store those data in their memory, and then later have the data read out after accomplishing their mission.
Such devices could ultimately be a boon for the oil and gas industry. Currently, the main way of checking for leaks or other issues in pipelines is to have a crew physically drive along the pipe and inspect it with expensive instruments. In principle, the new devices could be inserted into one end of the pipeline, carried along with the flow, and then removed at the other end, providing a record of the conditions they encountered along the way, including the presence of contaminants that could indicate the location of problem areas.
The initial proof-of-concept devices did not have a timing circuit that would indicate the location of particular data readings, but adding that is part of ongoing work. Similarly, such particles could potentially be used for diagnostic purposes in the body; for example, to pass through the digestive tract searching for signs of inflammation or other disease indicators.
Most conventional microchips, such as silicon-based or CMOS, have a flat, rigid substrate and would not perform properly when attached to colloids that can experience complex mechanical stresses while traveling through the environment. In addition, all such chips are very energy-thirsty. Two-dimensional electronic materials, including graphene and transition-metal dichalcogenides, could be attached to colloid surfaces, remaining operational even after being launched into air or water. And such thin-film electronics require only tiny amounts of energy — they can be powered by nanowatts with sub-volt voltages.
The nanodevices produced with this method are autonomous particles that contain electronics for power generation, computation, logic, and memory storage. They are powered by light and contain tiny retroreflectors that allow them to be easily located after their travels. They can then be interrogated through probes to deliver their data. In ongoing work, the team hopes to add communications capabilities to allow the particles to deliver their data without the need for physical contact.