Towards the next generation of vision implants

Researchers at Chalmers University of Technology, the University of Freiburg and the Netherlands Institute for Neuroscience have collaborated on an ultrasmall implant with electrodes the size of a single neuron that can remain intact in the human body — a combination that holds promise for future vision implants for the blind. Their work has been published in the journal Advanced Healthcare Materials.

Often when a person is blind, some or part of the eye is damaged, but the visual cortex in the brain is still functioning and waiting for input. When considering brain stimulation for sight restoration, there needs to be thousands of electrodes going into an implant to build up enough information for an image. By sending electrical impulses via an implant to the visual cortex of the brain, an image can be created, and each electrode would represent one pixel.

“This image would not be the world as someone with full vision would be able to see it; the image created by electrical impulses would be like the matrix board on a highway, a dark space and some spots that would light up depending on the information you are given,” said Chalmers Professor Maria Asplund, who led the technology development part of the new project. “The more electrodes that ‘feed’ into it, the better the image would be.”

By creating a really small electrode the size of a single neuron, researchers have the potential to fit lots of electrodes onto a single implant and build up a more detailed image for the user. The vision implant created in the new study can be described as a ‘thread’ with many electrodes placed in a row, one after the other.

“Miniaturisation of vision implant components is essential — especially the electrodes, as they need to be small enough to be able to resolve stimulation to large numbers of spots in the ‘brain visual areas’,” Asplund said. “The main research question for the team was ‘can we fit that many electrodes on an implant with the materials we have and make it small enough and also effective?’ and the answer from this study was: yes.”

The major obstacle was not to make the electrodes small, but to make such small electrodes last a long time in a moist, humid environment such as the human body. Corrosion of metals in surgical implants is a huge problem, and because the metal is the functional part, as well as the corroding part, the amount of metal is key. The electrical implant that Asplund and her team have created measures only 40 µm wide and 10 µm thick, like a split hair, with the metal parts being only a few hundred nanometres in thickness — and since there is so little metal in the super tiny vision electrode, it cannot ‘afford’ to corrode at all, otherwise it would stop working.

The research team have solved this problem by creating a unique mix of materials layered up together that do not corrode. This includes a conducting polymer to transduce the electrical stimulation required for the implant to work, to electrical responses in the neurons. The polymer forms a protective layer on the metal and makes the electrode much more resilient to corrosion, essentially a protective layer of plastic covering the metal.

Image credit: Chalmers University of Technology/Maria Asplund.

“The conducting polymer metal combination we have implemented is revolutionary for vision implants as it would mean they hopefully could remain functional for the entire implant lifetime,” Asplund said.

The method was implemented at the Netherlands Institute for Neuroscience, where mice were trained to respond to an electrical impulse to the visual cortex of the brain. The study showed that not only could the mice learn to react to the stimulation applied via the electrodes in just a few sessions, but the minimal current threshold for which mice reported a perception was lower than standard metal-based implants. The research team further reported that the functionality of the implant stayed stable over time, for one mouse even until the end of its natural lifespan.

“We now know it is possible to make electrodes as small as a neuron and keep this electrode effectively working in the brain over very long timespans, which is promising since this has been missing until now,” Asplund said. “The next step will be to create an implant that can have connections for thousands of electrodes.”

Top image credit: iStock.com/bombuscreative