One area of technology taken very seriously here in the ExtremeTech bunker is optoelectronics. While tethers for electricity and electronic circuitry aren’t going away any time soon, all-optical communications and computing increasingly loom on the horizon. Parallel developments have taken hold in the field of neurotechnology, another area we take seriously at ET. Interfacing neurons to electronics has already given us tremendous technologies including brain-computer interfaces (BCI) and deep brain stimulation (DBS). The bridges to optical interfaces for the brain have already begun to be built — first with optogenetics, and more recently, directly through optically active materials.

A new paper published in Nature Photonics has demonstrated reliable photoactivation of neurons grown on glass slides coated with poly(3-hexylthiophene) polmer (P3HT). Furthermore the authors show that the bio-organic interface restores light sensitivity in explants (transplants to culture) of rat retinas with light-induced photoreceptor degeneration. In other words, the material can be used to make an artificial retina. We recently reported on the Alpha IMS bionic eye implant which is implanted in the eye and therefore moves and focuses with the eye itself. While this is a tremendous advance over external camera based systems, the Alph IMS still needs to have a bulky stimulator box implanted nearby for power and control. The beauty of the P3HT polymer is that it needs no external power, other than the incident light itself.

In a paper from two years ago the team first demonstrated that the polymer retains its optical properties over time in the sometimes harsh conditions of a tissue environment. They have now also demonstrated that the polymer is not toxic to the neurons. More importantly they show that the neurons are photoactivated in a way that does not cause them undue stress. Other research has shown that infrared light can activate neurons by itself, however it made for an indiscriminate stimulator whose primary mode of activation was due to heat — not an ideal way to befriend neurons. When grown on patterned P3HT polymer, neurons can be reliably activated at power levels of just 4mW per mm2, as long as the pulse lasted for 10 ms. The neurons were able to sustain repetitive firing up to 20Hz using this protocol. Beyond 20Hz and the cells started to drop spikes, or produce spikes of significantly reduced amplitude.

It could be argued that a 10-millisecond pulse width is too long, after all the neurons that control eye movements can fire signals at several 100 hertz, indicating that neurons have much more bandwidth available to them than could be accessed with P3HT photoactivation. Well, then don’t use this technique for those neurons. However the vast majority of the main output neurons of the cortex, the pyramidal cells — which by the way make ideal targets for BCIs — seem to be content to fire at just a few hertz or at most, tens of hertz.

The researchers here have done a tremendous job at trying to get to the bottom of how the polymer activates the neurons. This is important to help establish the longer term viability of the technology, particularly in light of the potential for the polymer to oxidize and change behavior. The ion displacement that leads to neuron activation is thought to be due both to capacitive coupling and to faradic currents at the interface. However despite decades of research, researchers still do not even fully understand how the action potentials of neurons themselves are generated and propagated. Better understanding of the fundamental behavior of neurons will help establish new technologies, like direct photoactivation, and complement the many developments in its sister technology, optogenetics.

Now read: The past, present, and future of bionic eyes

Research paper: doi:10.1038/nphoton.2013.34 – “A polymer optoelectronic interface restores light sensitivity in blind rat retinas”