Environment-sensing cell-sized robots go with the flow
Sometimes you've just got to kick back and collect the data. That seems to be the motto of what may be the world's smallest robots. Under development by MIT engineers, the 0.1 mm microrobots consist of miniature electronic circuits formed out of two dimensional materials fused with equally tiny colloid particles that float along as they gather information about their immediate environment.
The quest for ever smaller robots is from more than just the impulse to meet a technological challenge. Autonomous machines on the scale of a billionth to a millionth of a meter in diameter have a huge range of potential applications in science, medicine, and engineering because of their ability to monitor everything from the human body to oil refineries.
The tricky bit is to identify just what the strengths of such microbots are, and to find the best way to exploit them. One of the key is how to develop propulsion systems for microbots, and an array of approaches from building paddles to spinning tails have already been investigated.
The only problem is that at such a tiny scale these microbots are essentially colloids. That is, they are insoluble particles that remain suspended indefinitely in liquids or air. This is because in the world of the very small the random motion of the surrounding liquid or air molecules dominate gravity and the colloid particles can never settle.
The practical upshot of this is that these microbots become like zooplankton – the tiny larvae and other animals that form a major food group for sealife. These animals are so small that, though they may have fins, jets, or other forms of propulsion, can't really do much to go anywhere and have to drift where the currents take them.
Such is also the case with microbots. Though they can move a little, it's rather like trying to swim through tar at that scale. Rather than literally swimming against the current, the MIT team decided to focus on passive robots that pay more attention to gathering data rather than getting somewhere.
To do this, the team worked out a way to graft complete electronic circuits onto colloid particles. This would allow the microbots to be self-powered using tiny photodiodes to feed sips of electricity to the robot's computer, sensors, and memory circuits. The idea is that the microbots would gather data and then transmit it to base on command.
But why place the microcircuits on colloid particles? Why not just use the electronic packages themselves? According to Strano, such circuits without a substrate would be too fragile and would easily fracture under stress. In conventional microcircuits these substrates would be silicon, but the MIT team found this very energy thirsty and unsuitable for colloid particles. Instead they opted for new materials like graphene and transition-metal dichalcogenides that do better with colloid particles and use only nanowatts of electricity with subvolt voltages.
The MIT team sees many applications for the new technology in both the medical and engineering realms. These colloidal microbots could be, for example, be released into oil pipelines and record data as they drift along. When the reach the end of the line, they could be traced by their embedded micro-reflectors and commanded to transmit their findings, which could be used to detect leaks or contamination much faster, cheaper, and with greater accuracy than by human teams. Alternatively, they could be swallowed or injected as a diagnostic tool for tracking diseases.
MIT says that the technology is still in its infancy, but the hope is that one day it will be possible to create sufficiently small and powerful circuits that can be aerosolized or suspended in a colloidal liquid.
The research was published in Nature Nanotechnology.