Biomedical and electrical engineers at the University of New South Wales (UNSW), Australia, developed a new way to measure neural activity using light rather than electricity, which they say could lead to a complete reimagining of medical technologies like nerve-operated prosthetic devices and brain-machine interfaces.
François Ladouceur, PhD, with the university’s School of Electrical Engineering and Telecommunications, says the multidisciplinary team demonstrated that sensors built using liquid crystal and integrated optics technologies, called optrodes, can register nerve impulses.
Not only do the 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,” said 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.
“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,” Ladouceur said.
The researchers connected an optrode to the sciatic nerve of an anesthetized 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,” said Nigel Lovell, PhD. “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.”
The next step will be to scale up the number of optrodes to be able to handle complex networks of nervous and excitable tissue.
At the beginning of the project, the research team first considered how many neural connections are necessary to operate a hand, so “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 5,000 to 10,000 connections,” Ladouceur said.
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. The researchers say that 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 potentially be able to function with much the same ability as a biological one.
The study, “Liquid crystal electro-optical transducers for electrophysiology sensing applications,” was published in the Journal of Neural Engineering.
Editor’s note: This story was adapted from materials provided by UNSW.