Boosting brain-stimulating implants with nanowires

We’ve seen some pretty impressive results with brain-computer interfaces based on pincushion arrays of stiff electrodes. However, a major problem with these arrays is that they are temporary. Natural events in the brain inexorably degrade the connection between the living and the non-living until the device ceases to operate, and needs to be extracted. If we may draw a brief parallel here with the nuclear power industry, there is typically much more enthusiasm for installation of devices like these than there is for their removal.
There are two main processes responsible for the relatively short electrode lifetime. The first can be summed up as “reactive gliosis.” In a nutshell, the neurons you want to record from are eventually displaced from the electrode surfaces by more aggressive glial cells. The second major issue is that the natural movements of the brain further aggravate this displacement. The combined actions of heart, lungs, volume changes, and body movements wear on the tenous neuron connections until they are compromised.
Researchers at Lund University have been developing new electrode technologies to address both of these problems. To solve the glial cell issue, one team has built 3D nanostructured axon guides out of gallium phosphide (GaP). The key innovation here is to create a vertically-oriented bed of 80nm diameter nanowires that preferentially exclude the glia, and support the axons. As described in their recent publication in Applied Materials and Interfaces, the team built their devices using a combination of electron beam lithography and metal organic vapor phase epitaxy, a chemical process used to make thin films. GaP, as some may recognize, is the stuff of LEDs. Pure GaP LEDs emit green and doping them with nitrogen or zinc oxide generates a few additional colors.
The GaP is used here purely as a substrate. They is no fancy optogenetic applications, for example, yet incorporated. To convert the GaP base into an electrode, researchers would typically metalize the contact surfaces with with a thin film of something like gold. The researchers patterned the GaP into strips where the glial cells mostly stayed in the valleys in between. It is not yet fully understood why the neurites prefer the nanowire topology, or why the glial don’t, but it seems that the more motile and mitotically active glial do better on the flat areas.
In the meantime, another group at Lund University has been working on the motion artifact problem. They too have new work to report, this time in the Journal Frontiers in Neuroscience. Their approach is to use a soft paralene insulator material over a laser-milled gold electrode core. As the video below shows, their electrodes are encased in a rigid biodegradeable gelatin shell that provides stiffness when the electrode gets inserted. Afterwards the shell is dissolved to expose the flexible electrode.
These two technologies are fairly dissimilar and at least initially will likely have different applications. The GaP electrodes had first been shown to work in the peripheral nervous system. In the study mentioned here, the researchers used a culture of glia and retinal ganglion cells. These neurons generate the optic nerve, which technically speaking is the only ‘cranial nerve’ considered to be part of the central nervous system. This qualification comes along with all the protections and nourishments of the meninges that canvas the brain. It also includes the uniquely central myelinating system comprised of oligodendrocytes, as opposed to the peripheral Schwann cells. This is why, for example, we find central maladies like Optic Neuritis in Multiple Sclerosis, but not peripheral neuropathy like Guillain-Barré syndrome affecting the optic nerve.
It remains to be seen whether optic nerves growing on these different kinds of electrodes would be readily myelinated in the context of a GaP device. The researchers will no doubt be anxious to test their device as an implant rather than just in culture, and to integrate it with other technologies to extend its operation

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