Transparency is the key to many technologies. Thin conductive films, like those made from ITO (indium tin oxide) for example, can carry currents or create electric fields critical for displays or solar panels without blocking all the light. The most powerful brain implants being built today have exactly this same requirement. Namely, they need to record fast electric signals with conductive arrays while permitting light to pass out through them for high-resolution imaging — and just to take it up a notch — let light pass in to permit optogenetic control directly under the implant for the icing on the cake.
Unfortunately, ITO is generally too stiff and too brittle for brain implants. Even if it could be made flexible, the high temperatures required to process it are incompatible with many of the materials (like parylene) that are used in the implants. Furthermore the transparency bandwidth of ITO is insufficient to fully exploit the wide spectrum of new UV and IR capable optogenetic proteins that have researchers fairly excited. The solution, now emerging from multiple labs throughout the universe is to build flexible, transparent electrode arrays from graphene. Two studies in the latest issue of Nature Communications, one from the University of Wisconsin-Madison and the other from Penn, describe how to build these devices.
The University of Wisconsin researchers are either a little bit smarter or just a little bit richer, because they published their work open access. It’s a no-brainer then that we will focus on their methods first, and also in more detail. To make the arrays, these guys first deposited the parylene (polymer) substrate on a silicon wafer, metalized it with gold, and then patterned it with an electron beam to create small contact pads. The magic was to then apply four stacked single-atom-thick graphene layers using a wet transfer technique. These layers were then protected with a silicon dioxide layer, another parylene layer, and finally molded into brain signal recording goodness with reactive ion etching.
The researchers went with four graphene layers because that provided optimal mechanical integrity and conductivity while maintaining sufficient transparency. They tested the device in opto-enhanced mice whose neurons expressed proteins that react to blue light. When they hit the neurons with a laser fired in through the implant, the protein channels opened and fired the cell beneath. The masterstroke that remained was then to successfully record the electrical signals from this firing, sit back, and wait for the Nobel prize office to call.
The Penn State group used a similar 16-spot electrode array (pictured above right), and proceeded — we presume — in much the same fashion. Their angle was to perform high-resolution optical imaging, in particular calcium imaging, right out through the transparent electrode arrays which simultaneously recorded in high-temporal-resolution signals. They did this in slices of the hippocampus where they could bring to bear the complex and multifarious hardware needed to perform confocal and two-photon microscopy. These latter techniques provide a boost in spatial resolution by zeroing in over narrow planes inside the specimen, and limiting the background by the requirement of two photons to generate an optical signal. We should mention that there are voltage sensitive dyes available, in addition to standard calcium dyes, which can almost record the fastest single spikes, but electrical recording still reigns supreme for speed.
One concern of both groups in making these kinds of simultaneous electro-optic measurements was the generation of light-induced artifacts in the electrical recordings. This potential complication, called the Becqueral photovoltaic effect, has been known to exist since it was first demonstrated back in 1839. When light hits a conventional metal electrode, a photoelectrochemical (or more simply, a photovoltaic) effect occurs. If present in these recordings, the different signals could be highly disambiguatable. The Penn researchers reported that they saw no significant artifact, while the Wisconsin researchers saw some small effects with their device. In particular, when compared with platinum electrodes put into the opposite side cortical hemisphere, the Wisconsin researchers found that the artifact from graphene was similar to that obtained from platinum electrodes.
At this point both groups are busy characterizing the performance of their new devices in exacting detail. If workable as more permanent brain implants they may offer a nice compliment to other new approaches we have recently seen — flexible materials like silk for example. Where silk may offer biodegradability and reversibility, graphene may offer biocompatible permanence and reliability. The significant hype regarding optogenetics, well-founded in our opinion, seems to have died down for the moment. New advances like those just described may help refocus general attention on the huge potential benefit optogenetics holds for humans.