Soft gold creates a connection between nerves and electronics.

The soft electrodes developed by Claes Tibrant's research group at Linköping University are stretchable so they can follow body movements without damaging tissue. They consist of extremely thin threads of gold and soft silicone rubber.

In the future, it may be possible to use gold in soft interfaces to connect electronics to the nervous system for medical purposes. This technology could be used, for example, to alleviate conditions such as epilepsy, Parkinson’s disease, paralysis, or chronic pain. But creating an interface where electronics can meet the brain or other parts of the nervous system presents its own challenges.

– Classic conductors used in electronics are metals, which are very hard and rigid. The mechanical properties of the nervous system are like soft jelly. In order to get accurate signal transmission, we have to get very close to the nerve fibers involved, but the body is constantly moving and so trying to get close contact between something hard and something soft and delicate becomes difficult, says Klas Thiebrandt, professor of materials science at the Laboratory for Organic Electronics at Linköping University (LiU), who led the research, in a press release.

Therefore, researchers want to create electrodes that have good conductivity and at the same time have mechanical properties that mimic the softness of the body. In recent years, several studies have shown that soft electrodes do not damage tissue as much as hard electrodes do. In the current study, published in the journal Small, a group of researchers at Linköping University developed gold nanowires – a thousand times thinner than a hair – and embedded them in a flexible material to create soft microelectrodes.

“We have succeeded in creating a new and better nanomaterial of gold nanowires mixed with a very soft silicone rubber. By making them work together, we have created a conductor that has high electrical conductivity, is very soft and is made of biocompatible materials that work with the body,” explains Klaas Tibrant.

Silicone rubber is used in medical implants, such as breast implants. Soft electrodes also contain gold and platinum. Both metals are common in medical devices for clinical use.

However, making long, narrow gold nanostructures is extremely difficult. This has been a major hurdle until now, but now researchers have come up with a new way to make gold nanowires. And they do it using silver wires.

Silver has unique properties that make it a very good material for creating the kind of nanowires that researchers want. That’s why silver is used in some stretchable nanomaterials. The problem with silver is that it’s chemically reactive. In the same way that silver cutlery changes color over time when chemical reactions occur on the surface, the silver in nanowires degrades so that silver ions leak out. In high enough concentrations, silver ions can be toxic to us.

When Laura Sievert, a PhD student in Claes Thiebrandt’s research group, tried to make, or “grow,” gold nanowires, she came up with a new approach that opened up new possibilities. At first, it was difficult to control the shape the nanowires took. But then she discovered a method that led to very even strands. Instead of trying to grow gold nanowires from scratch, she started with thin nanowires of pure silver.

“We take advantage of the fact that you can make silver nanowires and use the silver nanowires as a kind of template on which we grow the gold. The next step in the process is to remove the silver. When you’re done, you have a material that’s more than 99 percent gold. So, getting around the problem of making long, narrow gold nanostructures is a bit of a challenge,” says Klaas Tibrant.

In the study, the researchers, in collaboration with Professor Simon Farnebo in the Department of Biomedical and Clinical Sciences at LEU, showed that soft, flexible microelectrodes could stimulate a nerve in mice and pick up signals from the nerve.

In applications where soft electronics are integrated into the body, the material needs to last a long time, preferably a lifetime. The researchers tested the stability of the new material and concluded that it has a lifespan of at least three years, which is better than many nanomaterials developed so far.

The research team is now working further to improve the material and create different types of smaller electrodes that can come into closer contact with neurons.