Engineers light the way to nerve-operated prosthetics of the future

François Ladouceur, Associate Professor at UNSW Sydney

Biomedical and electrical engineers at UNSW Sydney have developed a new way to measure neural activity using light – rather than electricity – which could lead to a complete reimagining of medical tech like nerve-operated prosthetics and brain-machine interfaces.

Prof François Ladouceur, with the School of Electrical Engineering and Telecommunications at UNSW, says the multi-disciplinary team has just demonstrated in the lab what it proved theoretically shortly before the pandemic: that sensors built using liquid crystal and integrated optics technologies – dubbed ‘optrodes’ – can register nerve impulses in a living animal body.

How critical is optrodes technology in neuroscience?

“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,” commented Prof. François 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 thus a problem reading the signal. We call this ‘impedance mismatch’. Another problem is ‘crosstalk’ – when you shrink these electrodes and bring them closer together, they start to talk to, or affect each other because of their proximity.”

But because optrodes use light and not electricity to detect neural signals, the problems of impedance mismatch is redundant and crosstalk minimised. “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 measure the neural impulses as they travel along a nerve fibre in a living animal. Scientia Prof Nigel Lovell, who heads the Graduate School of Biomedical Engineering and Director of the Tyree Foundation Institute of Health Engineering, was part of the team that sought to show this in the lab.

He says the team connected an optrode to the sciatic nerve of an anaesthetised animal. The nerve was then stimulated with a small current and the neural signals were recorded with the optrode. Then they did the same using a conventional electrode and a bioamplifier.

“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 tech. Ultimately, we could identify the same characteristics by measuring electrically or optically,” says Prof. Lovell.

What does the discovery mean for prosthetics?

The team has been able to show that nerve impulses, which are relatively weak and measured in microvolts, can be registered by optrode. The next step will be to scale up the number of optrodes to be able to handle complex networks of nervous and excitable tissue.

Prof. Ladouceur says at the beginning of the project, his colleagues asked themselves, how many neural connections does a human 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 – 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, between your brain and your hand there is a bundle of nerves that travels down from your cortex and eventually divides into those 5000 to 10,000 nerves that control the delicate operations of your hand. If a chip with thousands of optical connections could connect to your brain, or some place in the arm before the nerve bundle separates, a prosthetic hand could be able to function with much the same ability as a biological one.

That’s the dream, anyway, and Prof. Ladouceur says there are likely decades of further research before it’s a reality. This would include developing the ability for optrodes to be bidirectional. Not only would they receive and interpret signals from the brain on the way to the body, they could receive feedback in the form of neural impulses going back to the brain.

The long game: brain-machine interface

Neural prosthetics isn’t the only space that optrode tech has the potential to redefine. Man has long fantasised about integrating tech and machinery into the human body. Some of this is now a reality, such as Cochlear implants, pacemakers and cardiac defibrillators, not to mention smart watches and other tracking devices giving continual biofeedback.

But one of the more ambitious goals in biomedical engineering and neuroscience is the brain-machine interface that aims to connect the brain to not only the rest of the body, but potentially the world. “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.

There are many biotech firms taking this very seriously. Elon Musk was one of the co-founders of Neuralink that aims to create brain-computer interfaces with the potential to help people with paralysis as well as incorporating artificial intelligence into our brain activities.

The Neuralink approach uses conventional wire electrodes in its devices so it must overcome impedance mismatch and crosstalk – among many other challenges – if they are to develop devices that host thousands, if not millions, of connections between the brain and the device. Recently Mr Musk was reported as being frustrated at the slow pace in developing the tech.

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. We don’t have these problems in the optical domain. If there is neural activity, its presence influences the orientation of the liquid crystal which we can quantify by shining light on it. It means we don’t extract current from the tissues as the wire electrodes do. And so the biosensing can be done much more efficiently,” Prof. Ladouceur says.

Since the researchers have shown that the optrode method works, they will publish research that shows the tech is bidirectional – it can’t only read neural signals, but can write them too.