New depths in neuroscience: advances may help meld man with machine
Neural probes have long been a staple of neuroscience research, but in 1978 Dr. William Dobelle demonstrated they could do much more. Inserting 68 platinum electrodes into the visual cortex of a blind patient, Dobelle developed a medical device that was able to convey visual stimuli by transmitting information from a computer directly into the brain. While neural probes continue to be an indispensable tool in today's research, recent advances in microfabrication techniques have resulted in the production of ultra-thin, flexible probes that could supercharge the field of neuroprosthetics.
A handful of hurdles regarding traditional probes' biocompatibility have limited its widespread adoption for therapeutic purposes. Foremost, typically rigid probes have had a large footprint, a two-pronged problem. These probes normally slice and kill neurons upon insertion, a process that stimulates an acute response resulting in tissue swelling. Chronically, specialized immune cells in the brain called microglia will migrate to the source of insertion where they will release a number of factors in an attempt to encapsulate the probe, further isolating it from the surrounding neurons. The later process, termed glial scarring, interferes with a probes' ability to effectively transmit or receive electrical impulses with the surrounding neurons.
Recently, researchers at the University of Texas, Austin, have used specialized photolithography techniques to make ultraflexible nanoelectric thread (NET) probes that successfully integrate with neuronal tissue without resulting in glial scarring. In their 2017 paper published in Science Advances, Luan et al. fabricated two NET probes, the NET-10 and the NET-50. The NET-50, sitting at 1µm thick and 50µm wide, housed a linear array of eight electrodes. The NET-10, however, stands to be one of the smallest reported probes to-date, sizing up at 10µm x 1.5µm. These probes had nanonewton scale probe-tissue interfacial forces, and upon insertion into a mouse brain caused only minor blood brain barrier leakage that healed by one month post-surgery. These probes were able to take consistent recordings of the mouse brain for four months. After this period, histological analysis of the brain revealed that the microvasculature surrounding the probe had recovered. Such findings are paramount to the development of brain-machine interfaces for neuroprosthetics, as the researchers pointed out, in which case neural probes may be expected to require these features of ultraflexibility and reduced footprint that enhance biocompatibility.
Those with paralysis may stand to greatly benefit from advances in neural probe technology. The Christopher & Dana Reeve Foundation revealed statistics in 2013 showing roughly 1 in 50 people, or 5.4 million US citizens, are currently living with some form of paralysis. For those with lost limb function, restoring mobility has the potential to improve their overall quality of life. For example, in 2012 researchers at the University of Pittsburgh School of Medicine demonstrated using two implanted electrodes, one quadriplegic woman was able to feed herself by using her thoughts to control a robotic arm. While devices for this purpose such as that developed by BrainGate (a neuroprosthetic company formed during the 2000's) are currently in the investigational stage, advances in probe biocompatibility may help aid both overall effectiveness and approval of these devices in the future.
Top image: BrainGate Technology