Research

I started my research by checking the sensor section of Kobakant's "How to Get What You Want". There were a few pressure matrix designs, but the machine sewn version caught my eye because it is machine sewn and thus faster to fabricate. The pressure matrix uses a piezo resistive conductive fabric with conductive fabric sewn horizontally on one side and vertically on the other. The resistance between any horizontal/vertical pair of conductive threads can be read to determine how much pressure is being applied at the crossing point of the two threads (because the conductivity of the fabric decreases when under pressure). From the pictures provided by Kobakant, I noticed a few things that I will need to take into account when designing the pressure matrix. First of all, there is a picture of fingertips with a black residue from the conductive fabric. This means I will likely need to layer regular fabric on either side of the pressure mat to avoid this residue getting onto the brace or the patient (this will also help to insulate the threads from moisture that might build up under the brace). The second thing I noticed was that when pressure is applied to one thread crossing, pressure is sensed at several points (but localized at the point where the pressure is actually applied). This means there may need to be some data processing to determine where the pressure is actually applied (although I don't know what this processing will actually be). 

I thought that the Kobakant sample would be a good jumping off point, so I  looked into the details of the implementation. I was able to find machine sewable conductive thread on Adafruit quite easily. Finding piezo-resistive conductive fabric was a bit difficult as all the suppliers seemed to be out of stock. Luckily, I found some on Mouser. The next issue was how to read all of the conductive threads. This depended on how big the pressure matrix was going to be. If the pressure map didn't have many points, I could read it using analog inputs on a microcontroller. If there were many, many points, I would have to use a multiplexer to allow me to read many of the threads with few analog pins. I will assume that we will need a multiplexer, because even a 10x10 pressure matrix requires 10 analog inputs (For a nxm pressure matrix, (n) analog inputs need to be read for a total of (n*m) points).