Using a phenomenon known as micro-emergent behavior, MIT engineers have created elementary microparticles that can collectively produce sophisticated activities, such as an ant colony building tunnels or foraging for food. When microparticles cooperate, they can create a clock that oscillates at a very low frequency. Researchers have demonstrated that it is possible to use these oscillations to power small robotic devices.
“This behavior can be translated into a built-in oscillating electrical signal, which, besides being of interest to physics, could be highly effective in microrobotic autonomy. Many electrical parts need this kind of oscillatory input, including Jingfan Yang, a recent MIT graduate and one of the main authors of the study," adds.
The component particles of the new oscillator engage in a simple chemical mechanism that allows them to communicate with each other by forming and bursting tiny gas bubbles. These interactions, under the right conditions, result in an oscillator that beats at intervals of a few seconds, just like a clock.
According to Michael Strano, Professor of Chemical Engineering at MIT, "We're trying to look for very simple rules or properties that you can code into relatively simple microrobotic machines so we can have them perform highly sophisticated tasks en masse."
Thomas Berrueta, a graduate student at Northwestern University under the guidance of Professor Todd Murphey, is a co-author of the study with Yang.
Insect colonies such as ants and bees can perform tasks that a single member of the group could never complete, which is an example of emergent behavior.
“Ants have small brains and perform extremely basic cognitive functions, but when they work together they can do amazing things. They can gather food and create these complex tunnel systems,” Strano says. “Physicists and engineers like me want to understand these rules because it means we can create tiny beings that work together to accomplish complex tasks.”
In this project, the goal was to create particles that could produce oscillations or rhythmic movements at very low frequencies. Until recently, creating low-frequency micro-oscillators required expensive, complex electronics or special materials with complex chemistry.
For this study, the researchers created disks of 100 microns in diameter as elementary particles. The platinum patch on SU-8 polymer-based discs can accelerate the conversion of hydrogen peroxide to water and oxygen.
Particles tend to move towards the top of a hydrogen peroxide droplet when placed on the droplet surface on a flat surface. They interact with other particles in liquid-air contact. Each particle creates a small bubble of oxygen, and when the two particles get close enough to interact, the bubbles burst and the particles separate. The process then restarts with the formation of new bubbles.
When particles work together, Yang says, "they can do something quite fantastic and useful, which is actually difficult to achieve at the microscale. A particle on its own remains motionless and does nothing fascinating.
Scientists have discovered that two particles can make a fairly reliable oscillator, but the rhythm becomes erratic as more particles are added. However, the addition of one particle that is slightly different from the others can serve as a "leader" that rearranges other particles in a rhythmic oscillator.
This leader particle is the same size as the other particles, but because it contains a slightly larger patch of platinum, it can produce a larger bubble of oxygen. This allows this particle to migrate to the center of the cluster, where it controls the oscillations of all other particles. The researchers discovered that they could create oscillators with at least 11 particles using this method.
This oscillator has a frequency ranging from 0,1 to 0,3 hertz, depending on the amount of particles; this is on par with low-frequency oscillators that control biological processes such as walking and heartbeat.
The researchers also demonstrated how they could use the rhythmic beats of these particles to create an oscillating electric current. To achieve this, they used a platinum and ruthenium or gold fuel cell instead of a platinum catalyst. The voltage of the fuel cell is converted into an oscillating current by mechanical oscillation of particles that rhythmically change the resistance from one end of the fuel cell to the other.
In some cases, such as when powering miniature walking robots, it can be advantageous to generate an oscillating current rather than a constant current. This method was used by MIT researchers to demonstrate that they could power a micro-actuator that served as the legs of a small walking robot previously created by Cornell University researchers. The laser source of the first model required the current to be oscillated by the human, alternately aimed at each set of legs. By using a wire to transmit the current from the particles to the actuator, MIT researchers demonstrated that the built-in oscillating current created by its particles could power the cyclic motion of the microrobotic leg.
According to Strano, he demonstrates how a mechanical oscillation can be transformed into an electrical oscillation, which can then be used to power robotic tasks.
Controlling swarms of small autonomous robots that could be used as sensors to monitor water pollution is one of the potential uses for this type of technology.