NEWS
Manufacturing the Invisible
A dopant addition technique delivers more consistent transparent conductive thin films—good news for future generations of smart devices.
Written by Poornima Apte
Graduate researcher Luiza Aguiar do Nascimento (left) and associate professor Wren Greene (right) show a newly developed electricity-conducting material that could revolutionize smartphones and wearable technologies. Photo: La Trobe University
A TOUCHSCREEN DEVICE like a smartphone needs to be made of a material that is conductive enough to register the electrical signals caused by finger movements. Similarly, biosensors must be sensitive to slight changes in concentrations of specific chemicals.
Conductive polymers meet these requirements well, but are challenging to manufacture with consistent properties. Researchers at La Trobe University in Melbourne, Australia, have developed a technique that enables large-scale manufacturing of transparent and thin conductive polymers.
Conducting Polymers
Conductors such as metals have long been reliable. However, they don’t always work for the next generations of advanced biosensors and electronics. It’s why conductive polymers, which are lightweight, flexible, and compatible with biological systems, are gaining traction.
Polymers are usually insulators, but conductive polymers are manufactured by intentionally adding a dopant—a molecule that increases the number of charge carriers—so that the final product facilitates the movement of electrons.
Conductive polymers are widely used in smart devices. Photo: La Trobe University
“Two different batches of the same polymers might have different properties and even different regions within a polymer film might have high and low conductivity.”
—Wren Greene, associate professor of chemistry and biochemistry at La Trobe University
While conductive polymers have many advantages, they also come with their own challenges. First, they lose conductivity as layers grow thinner. In addition, “conductive polymer electrodes are notoriously variable in their properties. Two different batches of the same polymers might have different properties and even different regions within a polymer film might have high and low conductivity,” said Wren Greene, associate professor of chemistry and biochemistry at La Trobe, who has supervised research into more efficient production of conductive polymers.
Dopant Addition Protocol
The traditional method of manufacturing conductive polymers is to mix the dopant in with the raw material during production. In such methods, dispersal of the dopant is difficult to control, as are the final properties of the conducting polymer.
To overcome the problem, Luiza Aguiar do Nascimento, a doctoral candidate in Greene’s lab, has developed an alternative production process: tether or attach the dopant directly onto a conductive surface like gold. While the researchers used a gold electrode, pretty much any conductive material can work instead.
The resulting polymer, which is transparent and invisible to the naked eye, is called 2D PEDOT, and grows directly on top of this tethered dopant. The researchers selected hyaluronic acid as the dopant because it has a high concentration of negative charges and is biocompatible, which means it can also work for biomedical devices meant for the human body.
Tethered Doping
In traditional manufacturing methods, two-dimensional conductive polymers grow in a confined space between two solid surfaces so their growth is geometrically restricted. There’s not much control over the lateral area in which the polymer is grown, and the process is very slow.
On the other hand, dopant tethering enables greater control of the resulting polymer because it restricts where and how much polymer gets deposited.
“Our polymer is grown without confinement so it’s unrestricted and we can grow the polymer within seconds,” Greene said. The polymer stops growing automatically once the dopant charges are used up. This allows for the possibility of extremely thin and uniform films, only a few molecules thick that cover exceptionally large areas.
Future Directions
This research offers a window into the development of nano films with programmable structures and properties.
“By engineering the dopant you can not only achieve very good conductivity and electrochemical properties, you can also manipulate morphology and mechanical stiffness,” Nascimento said. “If you need a wavy polymer you can engineer a wavy dopant layer and the polymer will follow that morphology. Likewise, if you need a stiff or soft polymer, you engineer the dopant to achieve that physical characteristic.”
Printing a nanoporous film would require printing the dopant with nanometer-sized holes.
“With tethered doping you get better performance and less batch-to-batch variability.”
As a result these polymers are more reliable and their production is scalable,” Greene said. “You get more uniform films with higher conductivity, which means you can sense currents at a much lower level and without as much noise in the measurement. You get more sensitive sensors that are easier to manufacture.”
Potential use cases for such conducting polymers, in addition to smartphones, are as electrochemical biosensors that detect specific chemicals in the body. The polymer can also be part of drug delivery systems with the drugs stored in the film and released on demand using an electrical signal.
Poornima Apte is a technology writer based in Walpole, Mass.
The material, which is called 2D PEDOT, is invisible to the naked eye. Photo: La Trobe University
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