Silk substrate hugs brain's curves

Tagged: Brain Science, Technology
Source: Ars Technica - Read the full article
Posted: 4 years 14 weeks ago

We've found lots of technically challenging ways to monitor brain activity, but scientists may have come up with an easier one. A paper published in Nature last week describes a new method for placing electrodes onto soft, curvilinear biological surfaces: embed them them on a flexible, silk-based substrate that can be resorbed into tissue. Because of the flexibility, there's increased contact between the electrodes and tissue, enabling scientists to get a highly detailed study of the signals in the underlying cells. However, the longevity of the devices hasn't been studied much, and its ability to hang on through physical changes bears further scrutiny.

Researchers can record the activity of individual neurons in the brain, but it involves the use of rigid electronics that clash with our squishier insides, and embedding monitoring devices can sometimes be difficult and involve complicated surgery. Neuroscientists would have a much easier time if it were possible to just lay electrodes directly over the surface of the brain and watch from there.

The problem is that rigid electrodes aren't especially useful for brain-surface monitoring, as they don't conform to the the brain's peaks and valleys, so the contact points produce an incomplete signal. To fix this, scientists needed to come up with an electrode substrate that was flexible yet sturdy, and wouldn't interfere with the electrodes' readouts.

They found an ideal candidate in a special silk derived from silkworm cocoons. The cocoon silk was prepared in a sodium carbonate solution, which removes a glycoprotein that can interfere with immunological responses. It was then shifted to a lithium bromide solution that made the fibers soluble. This resulted in a material they called silk fibroin. Once processed, it was firm enough for electrode arrays to sit on, but very pliable.

The authors placed a small sheet of 30 contact points encased in silk fibroin on the brains of some test animals, and then flushed the area with a saline solution to flatten the silk onto the brain's surface. The electrodes adhered to the brain, filling in the crevices much better than previous substrates have allowed.

As a result, the silk substrate arrays performed much better than their more rigid counterparts. When tested with anesthetized cats, a 2.5-micrometer thick non-silk electrode array made bad contact at half of its contact points, whereas an equally thin silk substrate electrode had contact at almost all of its points, and produced much stronger signals.

The implications for a highly detailed monitoring device like the one described here are quite big—scientists could monitor brain surfaces over larger areas in higher detail with a device that is far less likely to shift around. They could also apply the electrodes to other organs, such as the heart, to look for any irregularities in electrical activity. However, once the array has conformed to a surface, it stiffens a bit, so it wouldn't be great for use on surfaces that move around too much, like the stomach.

There are additional possibilities for active electrodes in the brain, which can not only monitor, but can also induce impulses in the neurons they contact. For example, an array could detect a characteristic pattern of brain impulses that precede a seizure, then send out countering impulses that cancel the seizure before it happens. (It's highly likely that this is the kind of active brain hardware you would need to be able to connect to the Matrix.)

One possible limitation not discussed in detail was the longevity of the arrays. The authors mention briefly in the paper that electrodes of a similar composition implanted in tissue didn't cause any inflammation after four weeks. Still, if they intend to use the monitoring arrays for the long term, they may need to take into account the growth and shrinkage that human organs can sometimes experience over long periods of time.