Engineering Inspired by Nature: Janus Particles and Self-Healing Circuitry

While not actually programmable robots in the traditional sense, these nanomotors enable a circuit to heal itself of tiny cracks. Here's how Janus Particles have enabled circuits to mend themselves similarly to human cells.

Try as we might, our electronic gadgets don't last forever. Electronics are subject to wear from normal use and damage from outside sources, whether physical or electronic. The lifetime of a device can be extended with care or by increasing the quality of the components.

Recently, though, a group of scientists have given us another option for increasing a device's longevity. They've created a special type of particle that can "heal" a circuit of specific types of damage, allowing it to last longer without the need for repairs or replacements. These are called Janus particles.

 

A bust of the god, Janus, displayed in the Vatican Museum. Photo courtesy of Fubar Obfusco.

Janus Particles

The science behind this technology is deceptively simple. Each tiny "robot" is a single special particle, known as a Janus particle. Named for the two-faced Roman god Janus, the particles are tiny spheres with a chemically different surface on each half.

 

Graphical representation of the fabrication of Janus particles. Image from Wikipedia

 

First created in the 1980s, Janus particles can be made in many different ways, depending on the materials of which they are composed.

One method works by trapping a tiny sphere of the first material, usually just a few hundred nanometers in diameter, in wax or oil so that one hemisphere is exposed, then coating that hemisphere in the second material, before dissolving the wax or removing the oil to free the particles. They can be made from many different materials depending on their intended functionality in a variety of industries. For example, Janus particles can be found in imaging cells and biosensing, theranostics, waterproofing textiles, dielectrophoresis, and more.

In the case of self-healing circuits, however, the particles are gold spheres with one half also coated in platinum. 

Bridging the Gap

When the particles are mixed into a solution of hydrogen peroxide, the platinum half reacts with the solution, releasing oxygen at a rapid enough pace to propel the particle forward, almost like a firework or bottle rocket. Thanks to this behavior, the particles are also referred to as nanomotors. 

The nanomotor solution can be applied to the the surface of a circuit and, as the particles shoot off in random directions, they will occasionally stick. The researchers tested the concept with a circuit which consisted of gold traces on a silicon wafer prepared with tiny gaps in a trace, which prevented the circuit from working properly. They found that, when the particles stick in a tiny crack in the circuit, it effectively bridges the crack. Since the particles themselves are conductive, this bridge allows the circuit to function as normal.

 

Microscopy (left) and simulated (right) images show particles gathering on a break in a circuit. Image from Nano Letters

 

Random distributions of the particles alone wouldn't be enough to bridge the gaps. However, the breaks in the circuit create differences in surface energy that to some extent attract the particles and hold them in position once they are in the right place.

Biological Parallels

One of the coolest parts of this self-healing system is how it mimics the way that our body heals injuries. The particles sticking to the crack work in much the same way that platelets adhere to the edges of a cut in a blood vessel, eventually sticking to each other and plugging up the hole. In fact, the researchers were inspired by this very process.

 

Platelets form a plug to stop bleeding. Image from Merck Manual

 

The simple principles that allow the system to work mean that no complex programming or complicated fabrication is required. It makes sense, really. Our bodies have been fixing tiny cuts like this for a long time. Why not mimic the process for damage to circuits? It certainly isn't the first time that we have found an elegant, effective solution in nature, and it definitely won't be the last.