New neural implant prevents scar tissue
A better way to couple up computers and prosthetic devices to the nervous system has been unveiled this week. Researchers have turned stem cells into muscle cells, and linked those muscle cells with tiny electrodes. When they’re implanted into a nerve, the nerves wire themselves up to the muscles and can activate them. The electrodes detect the muscle responses and send the signals to a computer or a prosthetic device such as an artificial arm. This technique has allowed the team to get around the existing problem that, normally, when you implant electrodes alongside nerves, a build-up of scar tissue subsequently introduces interference and blocks the detection of the signals. Damiano Barone is a neurosurgeon at the University of Cambridge and one of the brains behind the new approach…
Damiano - The nerve is not just one single structure, the nerve is a collection of wires - the axons. At the moment, most devices get what is called a compound signal. So they get all the information of all these axons together rather than the single axon that will go to a single muscle cell, for example. So what we want it to do is how do we get information rather than from a group of axons to a single axon.
Chris - So you're saying when you look at a nerve, there's a whole bunch of different wires in there, and present devices are listening to all the wires and mixing all the signals together and calling what they detect. You want to listen specifically to individual signals going down individual wires and do it in a way that won't scar up?
Damiano - Absolutely. And that's exactly what we've been trying to do. So what we did, we used stem cells that we pushed to become muscle cells, and what the nerve likes to do is to connect to muscle and muscle cells because that's what they're meant to do. They're meant to go to the muscle, make a junction, and then tell the muscle, please do contract. And the nerve was connected to our muscle cells, which were connected to our electrodes, and every time one of these wires was sending the signal, one of these muscle cells was contracting and it was telling one of the electrodes what was happening.
Chris - How do you get the muscle cells to talk to the electrodes without forming a scar? Because it seems like you are kicking the can down the road a bit where you put the electrodes into a nerve and it scars and that hampers performance. You've put muscle cells between the electrodes and the nerves, and you are saying that it doesn't scar. Well, why not?
Damiano - It's an interesting part because the muscle cells don't come from the body. So the muscle cell is a biological tissue but it's no biological tissue that was part of the body before. So all the, what we call the immunological reaction that caused that scarring, does not happen.
Chris - So how much better is this then, when you do these sorts of studies? And I presume that what you're doing with this is your model would be a person who, for instance, has lost an arm and you want to put a prosthesis on and control it with the same resolution as one could have moved their own fingers and you're putting these devices in into the severed nerve in order to pick up the signals. How much better is this than if you just took existing electrodes that you would couple up to that nerve stump?
Damiano - There is not really an electrode that is currently using clinical practice to power a neuro prosthetic arm. The best way in clinical practice is to take the nerves, implant it to the big muscle of the chest and the shoulder, and then they put cutaneous electrodes to pick up the signal. And that gives, to give an idea, six to eight signals to basically close and open the arm, move the wrist into a direction, same for the elbow. What we've done, we went up to 32 channels already. So we increase by at least one or two orders of magnitudes the number of signals we can get from a standard state of the art.
Chris - And how big is your device? Is it something that's relatively miniature and therefore could comfortably be implanted into a patient?
Damiano - Absolutely. I mean, at the moment the device we used was one to two millimeters squared.
Chris - And what about the duration of action? How long have you looked for to see how long this effect lasts for, of the superior performance and the failure of any scar tissue to form?
Damiano - In this project, we went up to four weeks. And the reason why we chose four weeks is because that is the time that the nerve takes to make those junctions muscles. And in four weeks we demonstrated that the cells do survive, but also that they kept functioning normally. What we need to do now is extend this time and connect the signals they were getting to an artificial limb.