Using light instead of electricity, biomedical and electrical engineers from UNSW Sydney have created a novel approach to quantify cerebral activity, which might completely rethink medical products like brain-machine interfaces and nerve-operated prosthetics.
The multidisciplinary team has just proven in the lab what it theoretically proved just before the pandemic: sensors made using liquid crystal and integrated optics technologies called “optrodes” can register nerve impulses in a living animal body, according to Professor François Ladouceur of UNSW’s School of Electrical Engineering and Telecommunications.
Not only do these optrodes perform just as well as conventional electrodes that use electricity to detect a nerve impulse but they also address “very thorny issues that competing technologies cannot address,” says Prof. Ladouceur.
“Firstly, it’s very difficult to shrink the size of the interface using conventional electrodes so that thousands of them can connect to thousands of nerves within a very small area.”
“One of the problems as you shrink thousands of electrodes and put them ever closer together to connect to the biological tissues is that their individual resistance increases, which degrades the signal-to-noise ratio so we have a problem reading the signal. We call this ‘impedance mismatch’.”
“Another problem is what we call ‘crosstalk’ when you shrink these electrodes and bring them closer together, they start to talk to, or affect each other because of their proximity.”
However, since optrodes detect neural impulses using light rather than electricity, impedance mismatch issues are unnecessary, and crosstalk is reduced.
“The real advantage of our approach is that we can make this connection very dense in the optical domain and we don’t pay the price that you have to pay in the electrical domain,” Prof. Ladouceur says.
In vivo demonstration
In research published recently in the Journal of Neural Engineering, Prof. Ladouceur and fellow researchers at UNSW wanted to show that they could use optrodes to accurately measure the neural impulses as they travel along a nerve fibre in a living animal.
We demonstrated that the nerve responses were essentially the same. There’s still more noise in the optical one, but that’s not surprising given this is brand new technology, and we can work on that. But ultimately, we could identify the same characteristics by measuring electrically or optically.
Professor Nigel Lovell
Scientia Professor Nigel Lovell, who heads the Graduate School of Biomedical Engineering and is Director of the Tyree Foundation Institute of Health Engineering, was part of the research team that sought to demonstrate this in the lab.
He says the team connected an optrode to the sciatic nerve of an anaesthetised animal. Next, a little current was used to activate the nerve, and the optrode was used to record the neural signals. After that, they repeated the procedure using a regular electrode and a bioamplifier.
“We demonstrated that the nerve responses were essentially the same,” says Prof. Lovell. “There’s still more noise in the optical one, but that’s not surprising given this is brand new technology, and we can work on that. But ultimately, we could identify the same characteristics by measuring electrically or optically.”
New dawn for prosthetics
The team has so far been able to demonstrate that optrode technology can record nerve impulses that are measured in microvolts and are relatively faint. The next stage will be to scale up the quantity of optrodes to support intricate networks of excitable and nervous tissue.
Prof. Ladouceur says at the beginning of the project, his colleagues asked themselves, how many neural connections does a man or woman need to operate a hand with a degree of finesse?
“That you can pick up an object, that you can judge the friction, you can apply just the right pressure to hold it, you can move from A to B with precision, you can go fast and slow all these things that we don’t even think about when we perform these actions. The answer is not so obvious, we had to search quite a bit in the literature, but we believe it’s about 5000 to 10,000 connections.”
In other words, there is a bundle of nerves between your brain and your hand that travels from your cortex and eventually separates into those 5000 to 10,000 nerves that regulate the subtle movements of your hand.
A prosthetic hand would be able to perform nearly identically to a real one if a chip with hundreds of optical connections could be connected to your brain or a location in the arm before the nerve bundle divides.
That’s the dream, anyway, and Prof. Ladouceur says there are likely decades of further research before it’s a reality. This would include making optrodes capable of bidirectional operation. They could receive feedback in the form of neuro impulses that travel back to the brain in addition to receiving and interpreting information the brain sends to the body.
The long game: brain-machine interface
Optode technology has the ability to alter many areas, not just neural prosthetics. For a very long time, people have fantasized about repairing or enhancing the human body by merging technology and engineering into it.
Cochlear implants, pacemakers, cardiac defibrillators, as well as smart watches and other tracking devices that provide continuous biofeedback, are examples of some of this technology that is now available.
The brain-machine interface, on the other hand, aspires to connect the brain to not just the rest of the body but also, theoretically, to the rest of the world. This is one of the more ambitious objectives in biomedical engineering and neuroscience.
“The area of neural interfacing is an incredibly exciting field and will be the subject of intense research and development over the next decade,” says Prof. Lovell.
While many biotech companies are treating this very seriously, it is currently more fantasy than actuality. One of the co-founders of Neuralink, a company that seeks to develop brain-computer interfaces that may aid patients with paralysis and incorporate artificial intelligence into human mental processes, is entrepreneur Elon Musk.
If the Neuralink technique is to create devices that support thousands, if not millions, of connections between the brain and the implanted device, it must overcome a number of obstacles, including impedance mismatch and crosstalk. Recently Mr. Musk was reported as being frustrated at the slow pace in developing the technology.
Prof. Ladouceur says time will tell whether Neuralink and its competitors succeed in removing these obstacles. However, given that implantable, in vivo devices that capture neural activity are currently constrained to about 100 or so electrodes, there is still a long way to go.
“I’m not saying that it’s impossible, but it becomes really problematic if you were to stick to standard electrodes,” Prof. Ladouceur says.
“We don’t have these problems in the optical domain. In our devices, if there is neural activity, its presence influences the orientation of the liquid crystal which we can detect and quantify by shining light on it. It means we don’t extract current from the biological tissues as the wire electrodes do. And so the biosensing can be done much more efficiently.”
Since the optrode technique has now been demonstrated to be effective in vivo, researchers will soon publish findings demonstrating the technology’s bidirectionality that is, its ability to not only read but also write brain impulses.
