Engineers at the University of New South Wales (UNSW Sydney) demonstrated an approach to measure neural activity using light. The team’s optical sensors, called optrodes, achieved accurate registers of the neural impulses traveling along a nerve fiber in a living animal.

According to the team, optrodes could provide a more comprehensive interface between the nerves and neural prosthetics than traditional electrodes.

“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,” said professor François Ladouceur.

According to Ladouceur, optrode technology resolves several issues that electrode technology does not address. “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,” he said.

The researchers developed passive, fluorophore-free optrodes based on the birefringence property of liquid crystals operating at the microscale. They showed that the optrodes had the appropriate linearity, relative responsivity, and bandwidth necessary for transducing electrophysiology signals into the optical domain.

To demonstrate the optrode’s conversion capabilities, the researchers connected an optrode to the sciatic nerve of an anesthetized rabbit. They stimulated the nerve with a small current and recorded the neural signals 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,” professor Nigel Lovell said. “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 researchers also demonstrated the miniaturization potential of optrode technology. The use of liquid crystals to convert bioelectric signals into the optical domain could enable high-bandwidth optical telecommunications techniques to be deployed in ultraminiature neural and cardiac interfaces.

Optrode technology also has the potential to enable brain-machine interfaces to help people with paralysis. On a broader scale, these interfaces aim to integrate artificial intelligence into human brain activities. Now that the team has demonstrated that optrodes can register nerve impulses, its next step will be to scale up the number of optrodes to handle complex networks of nervous and excitable tissue — for example, the 5000 to 10,000 neural connections required for hand movements.

The researchers believe that an optically driven prosthetic hand could potentially function in much the same way as a biological one.

To achieve this goal, the optrodes need to be bidirectional — that is, able to receive signals from the brain and send feedback back to the brain. The team plans to publish research shortly that will show that the optrode technology can both read and write neural signals.

In vivo devices that use conventional wire electrodes to capture neural activity are currently constrained to about 100 electrodes. Before electrode technology is able to support thousands, if not millions, of connections between the brain and a device, it has many challenges to overcome.

When thousands of electrons are shrunk and placed close together to connect to the biological tissues, the electrons’ individual resistance increases, degrading the signal-to-noise ratio and making the signal difficult to read.

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,” Ladouceur said.

“We don’t have these problems in the optical domain,” he said. “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.”

The research was published in the Journal of Neural Engineering (www.iopscience.iop.org/article/10.1088/1741-2552/ac8ed6).