Soft Gold Electrode Helps Nerves To Connect With Electronics

If you believe the old German fairytales, somewhere out there is a devilish imp named Rumpelstiltskin who can spin straw into great threads of gold. But making long, thin threads of gold is a surprisingly difficult feat – as modern science has discovered, metal nanowires are usually hard and rigid.

Now, researchers at Linköping University have succeeded in creating soft, stretchable electrodes using silicon and a new process for manufacturing gold nanowires. The team believes that this “soft gold” could be used to bridge the connection between the body’s nervous system and electronics, opening up new avenues for treating medical conditions such as epilepsy, Parkinson’s disease or paralysis. The research is published in Small.

A new approach to making gold nanowire

To create the ideal meld between electrical systems and the human body, researchers want to have electrodes with a high enough conductivity to be useful for electronics, while still having mechanical properties that are similar to the natural softness of the body.

“The classical conductors used in electronics are metals, which are very hard and rigid. The mechanical properties of the nervous system are more reminiscent of soft jelly,” said senior study author Klas Tybrandt, professor of materials science at the Laboratory of Organic Electronics at Linköping University.

“In order to get an accurate signal transmission, we need to get very close to the nerve fibres in question, but as the body is constantly in motion, achieving close contact between something that is hard and something that is soft and fragile becomes a problem,” he explained.

Ideally, researchers would be able to take long threads of gold nanowire, embed them in some kind of silicone rubber jelly and that would solve the problem, creating a soft and squishy version of the traditional gold wires on a silicon computer chip. But in reality, there are a lot of obstacles that must be overcome to make this possible. Namely, the inherent properties of gold make it extremely difficult to work into long and narrow nanostructures.

Laura Seufert, a doctoral student in Tybrandt’s research group, is credited with coming up with their clever workaround.

Unlike gold, silver is a very good metal for making nanowires; it is already used in some other types of stretchable nanomaterial. But silver is chemically reactive, and in the same way that silver jewelry might tarnish over time, the silver in nanowires will start to break down so that silver ions leak out. In high enough amounts, this silver could present a health risk.

Seufert’s solution was to start with a thin nanowire made of pure silver, which could then be used as a scaffold for making long gold nanostructures, effectively sidestepping the struggles involved in growing gold nanowires from scratch.

“As it’s possible to make silver nanowires, we take advantage of this and use the silver nanowire as a kind of template on which we grow gold. The next step in the process is to remove the silver. Once that’s done, we have a material that has over 99% gold in it. So it’s a bit of a trick to get around the problem of making long narrow gold nanostructures,” Tybrandt explained.

Soft electronics are kinder on the body

Using this technique, Tybrandt’s team has successfully synthesized gold nanowires, which they sandwiched between two sheets of soft silicone rubber to form a soft, stretchable microelectrode.

Mechanical tests demonstrated that the gold nanowire–silicone composite has a high conductivity, reversible stretchability and a nerve-like softness. Based on the results of these tests, the team estimates that soft multielectrode nerve cuffs based on this composite could remain stable in the body for at least three years.

The team also evaluated peripheral nerve stimulation using a rat sciatic nerve model, which showed that the new soft electrode can stimulate a rat nerve as well as capture signals sent from the nerve.

“We’ve succeeded in making a new, better nanomaterial from gold nanowires in combination with a very soft silicone rubber. Getting these to work together has resulted in a conductor that has high electrical conductivity, is very soft and made of biocompatible materials that function with the body,” said Tybrandt.

The Linköping University researchers are now focused on refining this material, with the goal of creating different types of electrodes that are even smaller and can come into closer contact with nerve cells.

Reference: Seufert L, Elmahmoudy M, Theunis C, et al. Stretchable tissue‐like gold nanowire composites with long‐term stability for neural interfaces. Small. 2024:2402214. doi: 10.1002/smll.202402214

This article is a rework of a press release issued by Linköping University. Material has been edited for length and content.