Jelly-based battery for brain implants
Researchers at the University of Cambridge have developed soft, stretchable batteries that are based on a jelly-like substance that could be used in wearable devices, soft robotics, or even implanted in the brain to power implants to treat conditions like epilepsy or depression. The bendy batteries work by using special polymers full of holes that can be filled up with charged particles, which generate a current when they flow from one polymer layer to the other. I went to meet Stephen O’Neill who is working on the project as part of his PhD at the Melville Laboratory for Polymer Synthesis at the University of Cambridge…Stephen - So what we're trying to do is make a soft and squishy power source or a battery, and this is to be able to interface with tissues in our brain or anywhere in our bodies where the materials aren't stiff and rigid, like your typical batteries in your phone or your laptop. And this will allow the body to really take it in and stop the body from rejecting these devices because they're soft and squidgy and have a lot of water just like our brains do.Chris - That's an issue though, isn't it? To a certain extent these things are stiff and rigid in order to give them the properties they've got and store lots of energy. So you're almost trying to reinvent the wheel here.Stephen - Yeah, exactly. That's true. So your typical things that conduct electricity or conduct electrons are metal or silicon. And these can conduct electrons because they're so stiff, electrons can flow across them. Whereas it's very difficult to make them soft and still conduct electrons. So what we've done here is we've made hydrogels, we've put polymers, dissolved them in water and tried to make these polymers charged to be able to conduct charge or conduct electricity.Chris - And is that what you brought along? Because you've got this enticing looking Petri dish here <laugh> Stephen, with this blue stuff in it. What's that?Stephen - Yeah, so I brought along one of the hydrogels and it's very soft and stretchable, almost like the jelly that people might know and actually eat. And we can stretch it to large amounts, actually over 10 times. Its initial strain, very flowable and movable.Chris - And that is a mixture of those polymers you're talking about that you've given a charge to. And are they stretched out in there almost like mini wires within the jelly then? Is that how that works?Stephen -Yeah, exactly. It's kind of like you can imagine a 3D network of wires in water. So about 60% of it is water and the other 40% is these wires that are stretched out and connected to each other in kind of a 3D network in that water. And these are charged.Chris - And how does that actually store charge then, if you've got these charged particles? Because I could say, well, salt has got sodium and chloride in it and they're charged particles. Doesn't store electricity though. So how does this actually store the charge and how does it then discharge?Stephen - What we have is we have salt within these gels and we have numerous compartments of the gels that we stick together. And some parts have lots of salts in them, and other parts have very low amounts of salts in them. And when we put two of these together, one hydrogel with high salts and one with low salts, the salts actually start flowing from the high to the low in a process that's called diffusion. So they kind of balance out the salts in the gels. Now if we, in between, we put something that can only let positive salts go through because salts usually are made up of positive and negative ions. For example, table salt is sodium chloride, the sodium is positive and the chloride is negative. So if we put something in between these two gels that will only let the sodium go through. Then on one we'll have a lot of sodium flowing from one side to the other and we'll get a buildup of these positive sodium salts just on one side and it will leave negative chloride salts on the other side. And this charge difference is really where we're getting this voltage.Chris - And how do you charge the battery? How do you force the ions to have that disequilibrium in the first place?Stephen - Yeah, so to be able to recharge the battery, we have to force them back over by applying a higher voltage. So that will be a recharging process and that's not that favorable, so we have to recharge it with another battery.