Researchers in Australia are hard at work on technology that could finally integrate prosthetics with our nervous systems. This system reads signals from our brain using light and could allow for much more accurate recordings of neural activity.

Although the research is still in its early stages, the team has demonstrated that their approach is sound. With backgrounds in biomedical and electrical engineering, the researchers at the University of New South Wales (UNSW) Sydney have developed a new approach of measuring neural activity using light rather than electricity. This bypasses several of the issues plaguing other interpretation technologies that are currently available.

When matured, such an approach could allow for a fundamental shift in how medical technologies such as nerve-operated prosthetics or brain-machine interfaces function.

Reading minds

“[The new approach bypasses] thorny issues that competing technologies cannot address”, explains Prof. François Ladouceur from UNSW’s School of Electrical Engineering and Telecommunications, co-author of the paper. “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. As you shrink thousands of electrodes and put them ever closer together to connect to the biological tissues […] 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.”

In their new paper, the team shows that they can use optrodes — ‘light electrodes’ — to accurately measure neural impulses as they travel through the nerve bundles of living animals.

For this purpose, they connected an optrode to the sciatic nerve of an anesthetized animal. They then stimulated the nerve using a gentle electrical current and recorded the signal it carried using one of their optrodes. In order to have data against which to test the effectiveness of their approach, the team also performed this step using a conventional electrode and a bioamplifier.

Overall, the team explains that the nerve responses recorded by these different devices “were essentially the same”. The optical sensor recorded the impulse with stronger signal than the electrode, but that wasn’t surprising given that it is still in its earliest development phases. Crucially, however, the experiment did showcase that optrodes can be used to interpret neural activity reliably.

The nerve impulses measured here had magnitudes measured in microvolts, making them relatively weak. In the future, the team plans to test arrays of optrodes against more complex networks of nerves, and other types of excitable tissues. The end game is to produce interfaces that can link prosthetics to an active neural system.

The team estimates that a fully-functional hand — one that can move, manipulate objects, return full sensorial data, and perform all the myriad functions we take for granted such as changing speed or pressure — would require about 5,000 to 10,000 connections between it and a user’s neural system. Each one of us has a nerve bundle that travels down from the motor cortex into our arms and hands, and this eventually breaks apart into 5,000 to 10,000 individual fibers innervating each and every muscles.

In other words, if a chip with this number of optical connections can be implanted in the brain, or in some place in the arm before the nerve bundle separates into individual fibers, it could allow for the integration of a prosthetic hand with the same ability as a biological one — definitely an exciting thought!

With that in mind, it will likely be decades before such a device is even remotely close to being ready for use. For starters, we would need to refine the optrodes, and to make them bidirectional (i.e. allow them to send signals into nerves, not just receive them).

Beyond functional prosthetics, such research opens up new avenues through which technology can be integrated directly into our nervous systems. Devices such as brain-machine interfaces, which would allow us to control computers or receive sensory input from our hands, could be built on top of these optrode chips.

“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. Nigel Lovell, corresponding author of the paper.

The paper “Liquid crystal electro-optical transducers for electrophysiology sensing applications” has been published in the Journal of Neural Engineering.