New electrodes

Have been messing around a lot with EMG lately and one of the biggest problems I've faced is with the electrodes. Currently all the electrodes available on the market are disposable wet electrodes that generally only last for one use which is a problem for ongoing projects I'd like to be longer lasting and is also a hassle to have to try and find the exact same spot again every time so as to not skew the results.

The way I see it, there are two main options to get around this issue. Either to come up with cheap and easily manufactured reusable dry electrodes or to design small implantable ones. I think the former solution would be easier but I have no idea how to make that work. The implantable ones would be a lot easier said than done as you'd have to make them wirelessly transmit the data and because of the small size would likely have to have them wirelessly powered as well. And then you still have to ensure that they're both conductive and biocompatible.

Anyone got any ideas or suggestions?

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Comments

  • Implantable EMG sensors have already been developed.
    https://www.ncbi.nlm.nih.gov/m/pubmed/18002310/
    Unfortunately, it would be some serious work to make your own. Conversely, they also make reusable EMG sensors, but I don't think they meet the low cost requirement. While it's not a trivial task, it would be much more achievable in my opinion.

    That said, I'm all for making implantable EMG sensors, especially if you have the tools, skill set to pull it off, and the time to devote to the project.
  • I think an easier solution would be to figure out how to make your own conductive gel. Metal plates work fine unless you're trying for active electrodes. I'm sure you could mod the open eeg projects active electrodes to work this way.
  • I've tried out stainless implants to facilitate muscle contraction. Some of the signal would leak across skin and hurt like hell.
  • @Ironalex @Cassox I'm currently working on two versions to see if I can get anywhere with either. First is a reusable dry flexible electrode using conductive polymers and second is an implantable one.

    With the dry one, I'm just working on a better design and and a better composite material for the electrode itself as the polymer I'm using unfortunately is powdered and insoluble. Should have a prototype done within a month or two hopefully.

    The implantable one is still very much just a concept however as I still haven't figured out all the details or got the estimated size down small enough. @Cassox , you have quite a bit of experience with bio-compatible coatings, right? Do you have any ideas as to what might work for an implantable electrode? I'd imagine you'd either have to go with something conductive or find a way to integrate the electrode surface into the outer coating whilst having the whole thing still properly sealed.

  • I hope the new electrodes will not be sticky too much and will be painful once you remove it. :)

  • You could use something like this reference design for communication between the implant and a phone:

    http://www.ti.com/tool/TIDA-00217

    Put these two chips (NFC controller and low power MCU) on a 0.6mm thick PCB with some kind of convex electrode mounted. Read the analog signals off of the electrode with the microcontroller. If that's not precise enough, maybe use the ICs used on this sensor:

    https://www.adafruit.com/product/2699

    Once everything is working to your satisfaction, pot the whole thing in the biopolymer of your choice, leaving part of the electrode exposed. It might end up a little large in the X-Y axis, but short in the Z axis.
  • I finally have an update on the electrodes. Have settled on a design for wireless implantable ones that use ultrasound to communicate with and power them so will be putting those together and trying them out over the next couple of months. I may also try out using RF instead in the future. My only problem is that, limited by only using commercially available parts, the smallest I've been able to get them is 4mm in diameter (and about 9mm high). It would be great if they could be injectable however, but I unfortunately know nothing about that. Does anyway know what the size limit is when it comes to injectable implants?

    @cassox , if I remember correctly, you've used injectable magnets in the past, right? How did that work?

  • Following.

  • Hi. I really want to work on this project. I'm have time soon! Sorry!
  • Hey, no problem @cassox! There's no rush, just let me know when you do have the time :)

  • I've made some more progress on the electrodes. Have got the dry electrodes sorted which is great (am getting a batch made so should have some pictures and data for you all soon!) but am still stuck on the implantable ones. The main problem is, as per usual, powering the damn things.

    Having it purely battery powered was quickly ruled out as any battery reasonably small enough to implant wouldn't power it for more than a few days at most so I spent a lot of time looking into ways to either passively power it or recharge the battery. Having it powered thermoelectrically by body heat seemed initially promising, but alas, turns out it doesn't produce nearly enough power either. Biobatteries seem promising as well but there are no currently available commercial solutions so that was also ruled out. Inductive charging is unideal as it's not the safest and would be inconvenient for electrodes in many locations due to the very close proximity required for recharging. That left me with powering it kinetically which, although a working solution, is still far from ideal as there are few commercial solutions that aren't extremely expensive. The smallest I've been able to find is still far bigger than I'd like and leaves the implant's dimensions at around 10mmx20mmx5mm which, whilst tolerable, is still too big to be easily and comfortably implanted in all places and to not cause discomfort to the individual.

    I'd really like to get them small enough to be injectable as I feel that would make them accessible to far more people (realistically, I don't think nearly so many people will be willing to go the scalpel insertion route). Unless I can find a way to get the size down though (which I sadly doubt), I think I'm going to have to go back towards having them wirelessly powered which is somewhat annoying as I feel that having to use an external device in conjuction with them to some degree defeats the point of having a fully implantable device as the functionality is no longer something you intrinsically have.

    If anyone has any ideas or suggestions then that would be much appreciated!

  • Possibly some sort of RFID EMG chip?

  • @Drop That would require an external power source, which he's trying to avoid.

    Personally, I still like the wireless idea, the IMES sensors I linked to were a step up from regular surface emg. I know you want something self contained, but I don't see a better way until bio batteries become feasible. That's just my opinion though.

  • @Psyber how much power do you need for your device?

  • edited October 28

    @Drop Have already looked into that option but that unfortunately still isn't enough power unless you're using NFC but then you have the problem that the external transceiver has to be right above the skin on top of the electrode at which point you might as well just use a surface electrode anyway.

    @Ironalex Yes, I think you're right unfortunately. Looks like I'm going to have just stick with wireless ones for now which is still admittedly better than traditional gel surface electrodes even if they're not the completely self-contained solution I was hoping for. Still should be nice though as it removes the need to have to replace your electrodes every session and they can be connected to any device via bluetooth which allows you to easily use the data for whatever you want without having to deal with filtering and importing it via serial or anything. Also they should be small enough to be injectable which is a plus.

    @TheGreyKnight Each one uses about 6mA when actively transmitting data and about 2-5uA when in standby. Voltage supply can be between 1.8V-3.5V so it can use around 11-40mW when active depending on the supplied voltage. That's the lowest I've been able to get it to so far but I'm working on further reducing that.

  • I'm not sure how much help it would be, but using a resonant inductive system might afford you a longer range solution for external power supply than NFC. some of the designs I saw were good for like 100 mm, but that could be heavily dependent on coil orientation. I'm currently looking into making a centralized wireless power supply system for implants, but I have to do a lot of learning about how the systems work before I can actually start testing.

  • I've heard of that one, yeah. Admittedly, I didn't look into that all too much but it didn't seem an ideal solution either. Seems far too finicky and too dependent on the coils and their orientation to be practical as getting it precise enough to have a decent range would be quite hard I imagine. Ideally, you'd want something that has a greater range too so that you wouldn't be limited to having the transceiver right next to the electrode and could instead have it in your pocket or something. I think wireless implantable electrodes have such potential for so many different applications though so I'm really keen on finding a practical and functional solution.

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