New electrodes could make brain surgery less invasive

Swiss researchers have developed flexible brain electrodes inspired by soft robotics that can spread out after being inserted through a small hole in the skull.

Scientist Stephanie Lacour came up with the device in response to a request from a neurosurgeon who gave her team the challenge of inserting a large cortical electrode array through a small hole in the skull, deploying the device in the 1mm gap between the skull and the surface of the brain. 

“Minimally invasive neurotechnologies are essential approaches to offer efficient, patient-tailored therapies,” says Lacour, professor at EPFL Neuro X Institute. “We needed to design a miniaturized electrode array capable of folding, passing through a small hole in the skull and then deploying in a flat surface resting over the cortex.  We then combined concepts from soft bioelectronics and soft robotics.”

There were many forms of novel engineering required, from the shape of the spiralled arms to the deployment of each arm on top of highly sensitive brain tissue. The first prototype consists of an electrode array that fits through a hole 2 cm in diameter, but when deployed, extends across a surface that’s 4 cm in diameter. It has 6 spiral-shaped arms, to maximize the surface area of the electrode array, and thus the number of electrodes in contact with the cortex. Straight arms result in uneven electrode distribution and less surface area in contact with the brain.

This electrode array is neatly folded into a cylindrical tube, in the way a butterfly is squeezed inside its cocoon. After deployment through a small hole in the skull, each spiralled arm is gently deployed using an everting actuation mechanism inspired by soft robotics. 

The electrode array actually looks like a kind of rubber glove, with flexible electrodes patterned on one side of each spiral-shaped finger. The glove is inverted, or turned inside-out, and folded inside of the cylindrical loader. For deployment, liquid is inserted into each inverted finger, one at a time, turning the inverted finger right side out as it unfolds over the brain.

“The beauty of the eversion mechanism is that we can deploy an arbitrary size of electrode with a constant and minimal compression on the brain,” says Sukho Song, lead author of the study.  “The soft robotics community has been very much interested in this eversion mechanism because it has been bio-inspired. This eversion mechanism can emulate the growth of tree roots, and there are no limitations in terms of how much tree roots can grow.”

Song also explored the idea of rolling up the arm of the electrode as a strategy for deployment. But the longer the arm, the thicker it becomes when rolled up. If the rolled-up electrode becomes too thick, then it would inevitably take up too much room between the skull and the brain, placing dangerous amounts of pressure on the brain tissue.

The electrode pattern is created by evaporating flexible gold onto elastomer materials. So far the technology has been tested in a mini-pig, and will now be taken to market by spin-out company Neurosoft Bioelectronics.