Bio Focus: Neural implant mimics mechanical properties of neural tissue
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eural implants are important and useful for studying the nervous system and even help treat certain neurological issues. However, while compliant, today’s implants are still stiff relative to neural tissue, making them unsuitable for long-term use. Researchers at the École Polytechnique Fédérale de Lausanne (EPFL, or Swiss Federal Institute of Technology in Lausanne) have now designed a soft neural implant that mimics the shape and elasticity of dura mater, the protective membrane of the brain and spinal cord. Described recently in the journal Science, the new implant, called e-dura, doesn’t induce inflammation or spinal cord damage after nearly two months of continuous use in rats, and has been shown to restore locomotion, using electrochemical stimulation, to rats suffering from paralyzing spinal cord injury. “We’ve developed a technology where the neural implant has the mechanical signature of the natural dura mater, and this allows the implant to be almost transparent to the neural tissue,” says Stéphanie Lacour, Bertarelli Foundation chairwoman of neuroprosthetic technology at EPFL and co-corresponding author of the new study published in the January 9 issue of
Science (DOI: 10.1126/science.1260318; p. 159). “This is an exciting new technology, which we hope will have use as a long-term neural prosthetic solution.” Neural implants serve a wide range of functions in medicine. For example, cochlear implants can help restore hearing to some people who are hearing impaired, and other types of implants can help alleviate Parkinson’s disease symptoms, epileptic seizures, and neuropathic pain. These neuroprosthetic devices are often made of soft silicon, but they contain metallic foil electrodes, which make them rigid compared to biological tissue. This biomechanical mismatch between implants and neural tissues can cause sheering and rubbing to occur, potentially resulting in inflammation and neural tissue damage over time, and cause the device’s electrodes to eventually fail. To address this issue, Lacour and her colleagues developed e-dura, which contains interconnects, gold electrodes, and “chemotrodes” (microfluidic channels that can deliver drugs) that can all bend, stretch, and deform with the movement of the neural tissue. To make their soft implant, the team began by fabricating a transparent substrate by spin-coating polydimethylsiloxane (PDMS) onto a silicon carrier wafer, which they cured overnight. They evaporated tracks of a 35-nm-gold film
full of microcracks—which provide a meshlike structure improving flexibility and stretchability—onto the substrate to serve as the electrode interconnects. They also created a secondary PDMS substrate consisting of a thick PDMS slab and two thin (20 μm) PDMS layers, which were full of puncture holes corresponding to sites of the electrodes. The researchers encapsulated the electrode wiring by bonding the two substrates (making sure to align the puncture holes with the underlying electrodes) and then peeling off the thick PDMS slab, leaving behind two 20-μm-thick
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