To see what you can do with these sensors, check out these videos.
Silicone embedding is the trickiest part of the process of building your own muscle sounds sensor. This is why I chose to make this a completely separate instructable. This instructable will definitely test your patience and manual ability, so be prepared to screw up a couple boards before you get the hang of it.
So without further delay... let's get started!
Step 1: Complete part one of the instructable
Step 2: Bill of materials
Part: Nylamid spacer (Diam: 13mm, Thick: 2mm)
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
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.
Step 3: Glue the spacer on the microphone
The air chamber is quite small (equivalent to a cylinder of 13mm in diameter and 2mm in height). This is where the nylamid spacer comes in. You need to glue the nylamid spacer directly to the microphone (figures 2 and 3). You also have to make sure the spacer is centered with the microphone. Note that the microphone is not centered on the board so the plastic spacer will be slightly offset and the board cannot be used as a reliable reference. Be careful not to let glue into the microphone because this will definitely make it useless and you will have to start all over!!. You can carefully glue a small piece of paper to the microphone first to protect it.
In terms of the glue, this is still an unresolved problem (suggestions welcomed). I have only used glue sticks because you have to make sure you will be able to remove the spacer afterwards (i.e. you need a weak glue), but if the glue is too weak, you won't be able to hold the spacer in place during embedding and the whole thing will become really difficult and frustrating. I encourage you to try a few different glues and post your results here, a glue stick is definitely NOT the best solution.
Step 4: Mix the 20A shore silicone
You will need about 2g total of the 20A shore silicone (i.e. 1g of part A + 1g of part B) for each sensor. If you are making more than one sensor at a time, it is ok to mix all the silicone required at once. However, once mixed, the silicone must be used ASAP. You can store the unused mixed silicone in a plastic bag in your freezer, but I don't recommend it (I would rather use the extra silicone to model something else, like a fake finger to mix with candies for halloween).
Spend a few minutes mixing the silicone with a clean metal roller in the same way you would mix pasta or cookie dough. In fact, a pasta roller may work pretty well to prepare the mix. Make sure there is no grease/oil on any of the surfaces the silicone touches. Grease will prevent the silicone from vulcanizing so wash your hands or wear surgery gloves. Also, make sure no air bubbles are left in the mix.
Step 5: Create the silicone case
You will get a "sandwiched" spacer where the "top slice" is the sensor and the "bottom slice" is the flat surface. At this point, you are ready to start placing some of the mixed silicone all around the sensor and spacer. You want to seal every single space and make sure there will be no air bubbles trapped. Don't worry about the electronic components, they work with very low currents so they won't overheat or short-circuit even when embedded.
Start by carefully shoving small quantities of silicone in tight spaces using the different modeling tools, then pack some more silicone around those spaces. Pack tightly, especially in the space between the spacer and the PCB around the microphone, and ensure the microphone is always centered on the spacer before continuing. Repeat until the silicone shows up around the board and use a flat spatula to pack the silicone some more from all 4 sides. My cases usually end up more square than the one shown (I didn't make this one).
Also make sure to cover the whole top of the board. It is not necessary to cover the accelerometer completely, so you can use it as a reference to even out the silicone at the top. Be prepared to spend up to 2 hours in this step, lousy embedding will make your sensor useless.
Step 6: Cure (vulcanize) the silicone case
After curing, carefully remove the sensor from the plastic board and then remove the plastic spacer. The case should look like the second figure below but with no bubbles!!!
Step 7: Mix the 65A shore silicone
Step 8: Roll membrane on indented board
The thickness of membrane should be approximately 0.5mm, although I have no idea how to ensure this to happen (more suggestions welcome).
Step 9: Apply solvent to case edges
Step 10: Apply membrane and cure
Allow the sensor to cure at 60 deg C for eight hours.
Step 11: You are done!
By embedding in silicone, the sensor sensitivity has been passively increased. You have also enabled the microphone to measure really low frequency vibrations such as those produced by contracting muscles. Try placing the sensor in your chest and watch/listen your heart beats!
Check out the videos:
And above all, have fun!
NOTE: The techniques presented in this instructable are not optimal and still pretty rudimentary. Some of the disadvantages include a lack of consistency in the performance and sensitivity of the sensors built (due to the lack of precision in manufacturing). You are encouraged to suggest different, easier and/or more appropriate ways to solve any of the steps of this instructable. Your input will be extremely valuable in making these sensors affordable and useful in prosthetics.