When DuPont released their first nylon stockings in 1939, the strength, elasticity and affordability of the world’s first synthetic fabric took the fashion industry by storm. Originally named “no-run”(before they realised that nylon did in fact ladder and hastily renamed it), the material has a plethora of uses in modern-day society, from screws to circuit hardware. Even today, the list of potential uses continues to expand as Cambridge scientists now believe nylon could also play a crucial role in “nanogenerators.” These are devices that convert ambient vibrations, like sound waves, to electricity, in order to power monitoring devices in homes, manufacturing industries, and even in our own bodies.

In August, a Cambridge material science research group, led by Sohini Kar-Narayan, developed a method of fabricating nylon so that it adopted a certain ordered arrangement of polymer chains (referred to in materials science as a phase). Described by the team as an “unprecedented discovery”, their nylon-based device resulted in a power output 34 times better than conventional aluminium-based nanogenerators. This article explores the science of nanogenerators and what this unusual phase of nylon can bring to the table.

“Kar-Narayan’s team found that the dipoles could be made to line up with each other without needing an electric field.”

There are several types of nanogenerator. Kar-Narayan and her team were working with triboelectric devices, which exploit the triboelectric effect to generate electricity. Many of us, in a moment of mistaken curiosity, have rubbed a balloon against our hair and rapidly regretted the static mess that results. This is an example of the triboelectric effect, a phenomenon that causes electron transfer between materials. In this case, it leaves your hair positively charged and repelling itself. All materials have different abilities to “hold onto” electrons, depending on how charged the nuclei are and the atmospheric conditions. When two materials are in contact, the difference between these abilities is the origin of the electron flow between them.

So how do we use this effect in a device? We shrink our sights to the very small. First, layers of two materials with very different abilities to “hold onto” electrons are placed close to each other. Surface roughness on the scale of a billionth of a metre creates lots of surface contact between the two. This means when a vibration hits the layers, bringing them into close contact, the triboelectric effect causes electrons to flow between them. Since one layer is an insulator, the electrons are exchanged via a circuit between the layers, giving a current and leaving one layer positively charged and the other negatively charged. This charged state is only stable when the oppositely charged layers are in extremely close proximity and attracting each other. This means when the vibration passes and the layers move apart, they return to their uncharged states as electrons flow around the circuit in the opposite direction. The periodic nature of the ambient vibrations means that this process repeats, resulting in an alternating current.

“The technology could be used instead of batteries to power devices in which batteries are difficult to replace or recharge.”

Nylon’s role in this device is as one of the layers, typically paired with Teflon. Nylon is excellent at releasing electrons, and Teflon at accepting them, so the pairing works well. Speaking to Varsity Science, Kar-Narayan explained: “One of the properties we are trying to exploit is [nylon’s] high surface charge density”. This means that the device can store a large quantity of charge on only a small surface area and that the resulting current has a large voltage. The way that nylon achieves this is by aligning its molecules so that their dipoles align in the same direction, giving opposite surfaces of the material strong opposite charges. We say that the material is now in a polar state.

Historically, scientists have used the γ’ phase of nylon in nanogenerators. All phases of nylon contain hydrogen bonds between the nitrogen-bonded hydrogens and carbonyl oxygens in neighbouring polymer chains. In the γ’ phase, the polymer chains and the hydrogen bonds have a random orientation, leading to a reasonably wide-spaced arrangement that allows chains a good degree of freedom to rotate. When placed in an electric field, the dipoles rotate to line up with each other, making the material polar, as described above.

The α phase of nylon is more ordered as regular hydrogen bonding arranges the chains into compact layers that are less free to rotate. An advantage of the compact arrangement is that when all the dipoles are aligned, the surface charge density that results is very high, and strong hydrogen bonding makes it resistant to temperature change. However, when α nylon crystallises naturally, there will be many individually oriented regions of dipole alignment, giving the material overall no polarity. However, Kar-Narayan’s team found that the dipoles could be made to line up with each other without needing an electric field using the “Template-Assisted Nanotemplate Infiltration” (TANI) method.

The process involves slowly growing nylon crystals inside cylindrical templates mere tens of nanometers in diameter. Kar-Narayan explained: “Using our template-assisted method, you can control crystalline properties, and get better surface charge density.” One way in which crystalline properties are controlled is through the tiny template diameter. This causes “nanoconfinement” effects to come into play, meaning the nanoscale crystals can be made to grow in certain directions. “There is also very little room for things to go wrong,” noted Kar-Narayan, since only very few crystal defects are likely to occur in a structure so small. The researchers found that they could make the nylon grow in uniform layers, with polymer chains following a snake-like path up the template to give an overall dipole alignment and polarity along the length of the nanowire.

The researchers are already looking at the commercial potential of these devices as Kar-Narayan and fellow researcher Yeon Sik-Choi patented some of the group’s ideas two years ago with Cambridge Display Technology. One possibility is that the technology could be used instead of batteries to power devices in which batteries are difficult to replace or recharge. The technology should effectively last indefinitely. Kar-Narayan explained further: “You can scale it up quite easily, just increase the number of your templates. The fact that you can grow it from solution is also great because typically solution-based methods are cheaper.”

Later this year, the team is also releasing a review of their work applying the TANI method to the negative Teflon layer of the nanogenerator. “It’s not quite as dramatic as what you get with nylon,” admitted Kar-Narayan, “but there is definitely an improvement.” With clear market potential, a scalable manufacturing process and ongoing research, this rapidly developing area of technology is definitely one to watch.