Step 2: Bill of Materials

This instructable requires a few custom materials and tools. The lab where I made the sensors already had most of the materials/tools available or they were custom-made there, so I apologize for not being able to direct you to specific suppliers/part #. If you try the instructable, please post where you got your materials from!

Part: Nylamid spacer (Diam: 13mm, Thick: 2mm)
Supplier: Custom-machined
Qty: 1 per sensor (reusable)

Part: 20A Shore RTV Silicone
Qty: ~2g per sensor

Part: 65A Shore RTV Silicone
Qty: <1g per sensor

Part: Acrylic (transparent plastic) board with circular indentations
Specs: 25mm minimum width and length
Centered indentation depth: 0.4 - 0.6mm
Centered indentation diameter: 4 - 6mm
Additional indentations at a minimum 30mm distance in any direction
Supplier: Custom-machined
Qty: 1

Other: Assorted modeling hand tools, precision scale, metal roller, silicone solvent.

NOTE: You can get the silicone and solvent from Nusil Technology. The modeling tools, you can probably find in a good art supplies store.

<p>It may sound silly and weird, but have you tested the latex of condoms to be used as the membrane instead a self-made-one? The latex is very thin yet durable and for sure biocompatible (in THIS applicational field).</p>
to get the right thickness, put a spacer the same thickness that you want the membrane to be under each end of the roller and roll across.
That is a very good idea, thanks!
hi, is there a reason you used silicon to seal the chamber as opposed using a tape or film to seal it? this seems a bit more complicated than it needs to be, to me at least.
No particular reason. We thought we would eventually manufacture the whole thing automatically, so it wouldn't really matter if it was a bit more complicated doing it manually, but that hasn't happened so I think it is worth to try simpler methods. I think tape or film would work ok but I would still suggest to ensure there is some kind of bump so the skin is in contact with the membrane at all times. If you make it with tape and get decent results, it might be worth adding you as a collaborator so you can add your alternative to the instructable.
hi i resolved the problem of the membrane. use two pieces of plexiglass separated by something (i used a 0.5mm pcb) if you give the pcb the shape you want you will get perfect membranes. i also did the same for the case of the microfone . if somebody is interested i can send some pictures. i have some question: 1) why do we use on oven considering that rtv silicone solidificates at room temperature? 2) why is the membrane made of sil. shore 60 ?'can we use the shore 20? 3) how are the dimension of the air chamber calculated? Thank's and ciao
That's great marco... I have added you as an author so please do add your info, steps, edits and/or pictures. My responses: 1) You just have to wait more... the oven is to speed up the curing process 2) I made some tests where shore 60 came out working better than 20, but I don't think the difference is too significant. I think I attached the paper with the data to this or the first instructable. 3) mmm.. I am not sure what you mean... again, this was done empirically, I just tested a whole bunch of sizes and the one reported here worked best.
Hmmm...to deal with the consistency in the performance and sensitivity, it might be possible to compensate using a neural network. Maybe, just maybe, some of my friends can help deal with this type of problem. I can provide more information in the next year or so, no promises, this is not an easy problem to solve or maybe you solved it already since post. :)
Hello Operon, I read about the selection of silicone hardness from your thesis. Is there any alternative if I can't get the silicone with hardness of 20A and 65A? As in one of your experiments, you concluded that "The softest or the hardest silicone types (shores 20A and 65A) are recommended for MMG measurement, as they exhibited the least variability during the tests." I wonder if a difference of ±5A would affect the measurement much. What do you think? Did you use any mold while embedding the components? Packing the silicone to the sensors like you described gives me an idea that the viscosity of the silicone is very high like modeling putty. I assume the silicone you used is for potting and encapsulation purpose, which generally has low viscosity. Is it not? As I know most of the silicone encapsulation use pouring method which requires mold. By the way, thank you so much for sharing this instructable. That's really a cool stuff. I wish I'll be able to build one.
I build something similar to your microphone enclosure and amplified the signal. I noticed something puzzling. When then the enclosure is air tight and the membrane is pressed there is low frequency high voltage rumble generated by the microphone. When amplified it sounds like rain on a large drum. If i make a small vent that rumble disappears. Is there something about electret microphone which make them act like that at high pressure? Are you getting these results.
That is a bit weird... I remember getting high frequency noise when touching the microphone's case but it disappeared after embedding. I don't remember any low frequency rumble. Just out of curiosity: how did you enclose your microphone? Mine was not enclosed at high pressure (I imagine that could damage the mike)
I used a bottle cap to make the air chamber and a stiff plastic membrane to close the cap. I noticed from your pictures that air chamber is actually just a bubble in front of the mic.
The <a rel="nofollow" href="https://web1.instructables.com/ID/FLA/2AHF/0BDEXCFNTVT/FLA2AHF0BDEXCFNTVT.pdf">paper linked</a> from the <a rel="nofollow" href="https://www.instructables.com/id/E6UNFPT2VXEXCFH2JY/">other instructable</a> doesn't really explain the advantages of this type of system. In which situations is it better/cheaper than surface EMG?<br/><br/>I am specifically interested in detecting independent movement (preferably just the beginnings or intent to move) of the muscles in the forearm that control fingers. Is this realistic?<br/>
<strong>Is this better than EMG?</strong> This is still an open question. Some say it is (in theory), but it hasn't really been sufficiently proven. The main assumption is that muscle sound sensors are less sensitive to placement (because sound travels farther than electricity through tissue). They are also supposed to be cheaper, but these are the only two reasonable advantages I have heard about. They have disadvantages as well, like being slower than EMG and VERY sensitive to movement. The idea was abandoned by NASA early after they proposed (and patented) it, but they may still have some good uses, and if the limitations are resolved, they could definitely become a great alternative to EMG. For now, we just need to perfect their construction.<br/><br/><strong>Is this cheaper than EMG?</strong> Depends, EMG sensors for prosthetics run at about US$700. So, if you make your own muscle sounds sensor it will definitely be cheaper, although I bet that is also true if you make your own EMG sensor.<br/><br/><strong>Can it detect finger movement?</strong> This is actually something for which muscle sound sensors could be more useful than EMG sensors because the EMG sensors can't really detect deep muscle activity, but the tendon movement of finger flexors and extensors can be easily detected by the muscle sounds sensors. The only big problem is always limb movement (or any movement other than what you want to measure). You would have to figure out how to filter it out, perhaps by using the embedded accelerometer. You can check <a rel="nofollow" href="http://komodoopenlab.com/jsilva/pdfs/silva_mmg_thesis.pdf">my thesis</a> to get some ideas. Also, <a rel="nofollow" href="http://www.prismlab.org">the PRISM</a> lab has recently done some research into this.<br/>
Electrode maintenance in EMG is a pain. Electrodes are good for a short while, but for long duration you start looking at implantable electrodes. You get a lot of motion artifact in emg which in your case you deal with by using an acc. But in emg the microphonics (change in impedance of the electrodes due to motion) are random and hard to filter. You get a lot of line noise with EMG which in your system is definitely not a problem. check out my dual power supply to power your system.
I added you as a collaborator... maybe you want to add a link to your instructable and some instructions on how your power supply may be used.
Thanks, I amt thinking of build your device... but a bit simpler.
<strong>&quot;like being slower than EMG&quot;</strong><br/><br/>What does &quot;slower&quot; mean in this context?<br/><br/><strong>&quot;although I bet that is also true if you make your own EMG sensor.&quot;</strong><br/><br/>Yeah. I was thinking of building something based on the <a rel="nofollow" href="http://openeeg.sourceforge.net/">OpenEEG</a> design.<br/><br/><strong>&quot;but the tendon movement&quot;</strong><br/><br/>Oh, excellent. I hadn't thought of that. EMG only measures muscles, while sound is created by the tendons moving around, too?<br/><br/>I'm not really sure how movement and muscle sound would be measured differently by the accelerometer and the microphone. They'd both show up as the same kind of vibration in my mind. I'll read through your thesis and see if I can get a better grasp of this.<br/>
Can you use hot glue in place of silicon? Silicon is nasty to work with.
You sure can try... the problem is always the precision though (getting good enough measurements in a consistent manner)... I agree, silicone is horrible to work with, but it is the best I have been able to do, although I am sure there must be a whole bunch of materials worth to try.
You need to use new amputees to model your system. New amputees muscles are not atrophied and changed and they still controlled in a natural fashion. New amputees can learn intrinsically to use a prosthesis if it is fitted IMMEDIATELY after surgery. This is because the subtle myofeedback loops that go to the spine are still alive. You take these for granted but they can make a difference of 6 month in phys-hab. This was observed by a polish doctor in the Napoleonic war. The retreating French army could not stop and so new amputee had to be immediately fitted with a stump and made to walk. They walked quite well.
Thank you so much Jorge!!! I can't wait for this instructable and, I used a similar procedure and I have ready some specimes for doing characterization. In my sensor, the 13x2mm air chamber only reports a max SNR of 19.0 dB. But, as you write, this procedure with the silicon is not precise. I have had thinking about another geometry in the air chamber, maybe as a cone, but now I don't have the time for trying. But in a couple of months I will try to test with conic or another geometries. Thanks again!!
No problem!! I have added you as a collaborator in the instructable. It would be great if you could add steps on how to measure the SNR. You can post some pictures of your sensors as well. Don't hesitate to make any recommendations you consider useful!

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