Until now, so-called "surface implants" have reached a roadblock: they cannot be applied long term to the spinal cord or brain, beneath the nervous system's protective envelope also known as "dura mater": When nerve tissues move or stretch, they invariably rub against rigid implants. After a while, this repeated friction causes inflammation, scar tissue buildup, and ultimately rejection.
Flexible and stretchy, the new implant developed at EPFL is placed beneath the dura mater directly onto the spinal cord. Its elasticity and flexibility are almost identical to the living tissue surrounding it. This reduces friction and inflammation to a minimum. When implanted into rats, the e-Dura prototype caused neither damage nor rejection, even after two months. More rigid traditional implants would have caused significant nerve tissue damage during this time period.
The researchers tested the device prototype by applying their rehabilitation protocol - which combines electrical and chemical stimulation – to paralysed rats. Not only did the implant prove its biocompatibility, but it also did its job perfectly, allowing the rats to regain the ability to walk on their own again after a few weeks of training.
"Our e-Dura implant can remain for a long period of time on the spinal cord or the cortex, precisely because it has the same mechanical properties as the dura mater itself. This opens up new therapeutic possibilities for patients suffering from neurological trauma or disorders, particularly individuals who have become paralyzed following spinal cord injury," explains Prof. Lacour, team leader and holder of EPFL's Bertarelli Chair in Neuroprosthetic Technology.
Developing the e-Dura implant was quite a feat of engineering. As flexible and stretchable as living tissue, it nonetheless includes electronic elements that stimulate the spinal cord at the point of injury. The silicon substrate is covered with cracked gold electric conducting tracks that can be pulled and stretched. The electrodes are made of an innovative composite of silicon and platinum microbeads. They can be deformed in any direction, while still ensuring optimal electrical conductivity. Finally, a fluidic microchannel enables the delivery of pharmacological substances – neurotransmitters in this case – that will reanimate the nerve cells beneath the injured tissue.
The implant can also be used to monitor electrical impulses from the brain in real time. This way, the scientists were able to learn about the animal's motor intention before it moved at all. "It's the first neuronal surface implant designed from the start for long-term application. In order to build it, we had to combine expertise from a considerable number of areas," explains Prof. Courtine, team leader and holder of EPFL's IRP Chair in Spinal Cord Repair. "These include materials science, electronics, neuroscience, medicine, and algorithm programming."
For the time being, the e-Dura implant has been primarily tested on paralysed rats. But the potential for applying these surface implants is huge – for example in epilepsy, Parkinson's disease and pain management. The scientists are planning to move towards clinical trials in humans, and to develop their prototype in preparation for commercialization. (© École Polytechnique Féderale de Lausanne EPFL, AcademiaNet)