Medicine these days entertains all kinds of ambitious plans for reading off brain signals to control wheelchairs, or using electronics to bypass spinal injuries. But most of these ideas for implants that can interface with the nervous system run up against a basic materials problem: wires are stiff and bodies are soft.
That motivated some researchers at the École Polytechnique Fédérale, in Lausanne, Switzerland, to design a soft, flexible electronic implant, which they say has the same ability to bend and stretch as dura mater, the membrane that surrounds the brain and spinal cord.
The scientists, including Gregoire Courtine, have previously showed that implants can allow mice with spinal injuries to walk again. They did this by sending patterns of electrical shocks to the spinal cord via electrodes placed inside the spine (see “Paralyzed Rats Take 1,000 Steps, Orchestrated by Computer”). But the rigid wires ended up damaging the mice’s nervous systems.
So Courtine joined electrical engineer Stéphanie Lacour (see “Innovators Under 35, 2006: Stéphanie Lacour”) to come up with a new implant they call “e-dura.” It’s made from soft silicone, stretchy gold wires, and rubbery electrodes flecked with platinum, as well as a microchannel through which the researchers were able to pump drugs.
The work builds on ongoing advances in flexible electronics. Other scientists have built patches that match the properties of the skin and include circuits, sensors, or even radios (see “Stick-On Electronic Tattoos”).
What’s new is how stretchable electronics are merging with a widening effort to invent new ways to send and receive signals from nerves (see “Neuroscience’s New Toolbox”). “People are pushing the limits because everyone wants to precisely interact with the brain and nervous system,” says Polina Anikeeva, a materials scientist at MIT who develops ultrathin fiber-optic threads as a different way of interfacing with neural tissue.
The reason metal or plastic electrodes eventually cause damage, or stop working, is that they cause compression and tissue damage. A stiff implant, even if it’s very thin, will still not stretch as the spinal cord does. “It slides against the tissue and causes a lot of inflammation,” says Lacour. “When you bend over to tie your shoelaces, the spinal cord stretches by several percent.”
The implant mimics a property of human tissue called viscoelasticity—somewhere between rubber and a very thick fluid. Pinch the skin on your hand with force and it will deform, but then flow back into place.
Using the flexible implant, the Swiss scientists reported today in the journal Science that they could overcome spinal injury in rats by wrapping it around the spinal cord and sending electrical signals to make the rodent’s hind legs move. They also pumped in chemicals to enhance the process. After two months, they saw few signs of tissue damage compared to conventional electrodes, which ended up causing an immune reaction and impairing the animal’s ability to move.
The ultimate aim of this kind of research is an implant that could restore a paralyzed person’s ability to walk. Lacour says that is still far off, but believes it will probably involve soft electronics. “If you want a therapy for patients, you want to ensure it can last in the body,” she says. “If we can match the properties of the neural tissue we should have a better interface.”