The MPR121 is a really nice IC, which is unfortunately now a legacy chip that you need to order in large quantities if you want to mount it on your own board. If we get to any kind of production status we'll need a full review of this component in the Bill of Materials but for now, it's a wonderful device to use on the various break-out modules available for a few £ each.

One of the characteristics of capacitive sensing is that pretty much any conductor will act as part of the capacitive plate if electrically connected. In the case of #SoleSense, there were clear differences in the readings that were obtained by each sensor when no-one was near the copper tape. The jumper wires bundled together were definitely contributing to the varying readings. They also affected readings when they rubbed past each other. I knew that I needed to make a pair of insoles that would allow comparison of the patient's weight being transmitted through each foot, so I needed to make the readings more repeatable.

The first thing I decided was to use a uniform pad layout between each foot. To ensure repeatability, I took a photo of my foot out line with some notional pad areas over key sections of the foot.

I imported this photo into Inkscape and drew round it. Then I exported to dxf into Onshape and made a thin insole with raised pad sections.

You can see that I have left minimal material under the arch of the foot to allow flex, and there are recessed cable runs to try and get the same length and route of wire between each pad and the MPR121 board. Of course, once this is in CAD, it is easy to mirror to make the other foot. The only change is that the other end's terminals are used (11,10,9,8,7 as opposed to 0,1,2,3,4) on the MPR121. This needs adjusting in code, which is trivial.

I printed this out (in hot pink PLA) and ended up with suitable physical versions.

I also printed out a cutting template for each pad so that I could make uniformly sized copper tape pads. 

I tested soldering the wire (printer cable style ribbon wire this time) to the copper tape once it was stuck to the PLA and to my pleasant surprise it was possible to make a good solder connection without damaging the PLA (noticeably).

The MPR121 module is held in place with a piece of double sided foam tape. I soldered a female header to the other side of the module but for now, I'm happy that I have got a physical design with a much better chance of producing repeatable and comparable readings between left and right food, regardless of the cable routing of the power and I2C bus to the Arduino.