Whether it's to allow for the control of robotic limbs or just to watch neurology in action, to really get at the inner workings of the brain it's still necessary to access the gray squish of the brain itself. Unfortunately, accessing the brain up close means brain surgery, and most of us would rather avoid having our skulls opened up if at all possible. Alternatives, however, are limited.
This might not be the case for much longer. As described in a study published this week in Nature Biotechnology, researchers from the University of Melbourne have developed a matchstick-sized stent that can be inserted through a vein in a patient's neck and the delivered to a suitable eavesdropping location within their skull. From this outpost, the stent (a small mesh tube meant to be implanted in a blood vessel) sends signals through wires to a transmitter in the patient's chest.
The Melbourne group, led by neurologist Thomas Oxley and funded in part by DARPA, tested their technology in a sheep brain for a period of six months, comparing electrical signals received via the stent with those received via a more traditional brain implant. The signals matched.
Brain interfaces are often pitched as consumer-grade technology, with EEG headbands going for all of $50 on Amazon. That's cool and all, but the skull greatly attenuates the brain's electrical signature, allowing for the exterior detection of frequencies of only up to about 60 Hz. The most useful frequencies for brain-machine interfaces are between 70 Hz and 200 Hz, however, because this is the activity linked to the brain's motor cortex. Getting at these frequencies means penetrating the skull.
"Although penetrating cortical arrays have achieved the highest spatial resolution for neural recordings, they are invasive, may cause trauma to brain tissue during insertion, and may result in chronic inflammation, gliosis and disruption of the blood-brain barrier," Oxley and co. write. "Furthermore, existing cortical implant design and implantation procedures are associated with difficulty in accessing deep cortical regions, such as within sulcal folds, which may be information-rich regions. Therefore, there is a need for interfaces with improved biocompatibility, high-resolution signal transduction and long-term functionality that can be implanted with minimal trauma."
Image: Oxley et al
The brain requires a whole lot of blood to do its thing—accounting for some 15 percent of total cardiac output—which means some beefy blood vessels pass through even its darkest neuro-corners. The idea of inserting electrodes into the brain via these vessels is itself not brand new, but until now they've only been implanted for a few hours at a time. Meanwhile, stents have been implanted in the brain for other purposes, such as reinforcing aneurysm-damaged arteries, and pacemaker electrodes have been implanted in cardiac tissue, but putting the two technologies together is only now becoming a reality.
The stent itself consists of a 3 centimeter-long, 3 millimeter-wide mesh tube made from a nickel alloy called nitinol. Its net-like surface is then covered in an array of electrodes, with each electrode registering the activity of around 10,000 neurons. In the first six days of implantation, the signals sent back by the stentrode were fairly poor, but they then improved over the next few weeks. The group attributes this initial noisiness to the interfering effects of blood flow, which would have been mitigated as the mesh and the surrounding blood vessel began to grow together.
"We envisage that future applications of endovascular arrays may include motor cortex sensors in brain-machine interfaces and seizure prediction in epilepsy," Oxley and his group note. "Applications in neural stimulation open the possibility of achieving deep and superficial brain stimulation therapies without the requirement for craniotomy. Multiple deep brain stimulation targets have been identified as being accessible via arteries and veins, with targets for Parkinson's disease and obsessive-compulsive disorder being particularly suitable."
You probably won't be able to buy a stentrode on Amazon anytime soon, but some things, like invasive brain operations, are probably best left to the experts.