Three years ago, scientists at the Swiss Federal Institute of Technology (EPFL) reported success in getting rats with severed spinal cords to walk again. They did so by suspending the animals in a harness, then using implants to electrically stimulate neurons in their lower spinal cord. Although this ultimately resulted in the rats being able to run on their previously-paralyzed hind legs, the technology still wasn't practical for long-term use in humans. Thanks to new research conducted at EPFL, however, that may no longer be the case.
In the original study, the rats were first injected with chemicals that replaced the neurotransmitters that could no longer reach their hind legs. Electrical stimulation was then delivered below the location at which the spinal cord had been cut, using electrodes that had been implanted onto the outermost layer of the spinal canal in that region.
This caused their hind legs to move, albeit involuntarily. After a period of training, however, the rats learned to activate the electrical impulses with their brains, allowing them to walk and run voluntarily – while still being supported by the harness, that is. Eventually, they even started forming new neuronal connections between the brain and the lower spine, circumventing the cut in the spinal cord.
Although the researchers hoped that the technology could eventually find use in a rehabilitative neuroprosthetic system for humans, there was at least one stumbling block – the implants, which weren't as soft and flexible as the biological tissue surrounding them. Over time this could cause irritation, which would in turn lead to inflammation, the build-up of scar tissue, and ultimately rejection.
Now, however, the scientists have created a new type of implant which addresses that issue. It's known as the e-Dura, as it's designed to be implanted on the spinal cord or cortex, beneath the dura mater – that's the protective envelope that surrounds the nervous system.
The implant consists of a stretchy silicone substrate, covered in cracked-gold conducting tracks leading to electrodes made from a silicon/platinum microbead composite. Those electrodes deliver a current, plus they can detect electrical impulses (such as those that would be used to move the legs) in the brain. Additionally, a microfluidic channel in the substrate is able to deliver the chemicals that were formerly injected by hand.
All of these components remain functional while also being highly flexible, allowing them to stretch and deform with the dura mater instead of rubbing or pressing against it. In lab tests, e-Duras implanted in rats caused no problems even after two months – according to EPFL, traditional implants "would have caused significant nerve tissue damage" within that same amount of time.
The scientists are now looking towards human trials, and are further developing the e-Dura for commercialization. A paper on their research was recently published in the journal Science.