*2.2. Prototype*

The base of the StressFoot prototype is a pair of sport shoes (Figure 1A). To sense the pressure distribution of the foot, we insert a pair of pressure sensitive insoles (UK size 9–10). These insoles are commercially available from sensing.tex [80] and offer 16 pressure sensors that rely on a force sensitive resistance technology (Figure 1B). In addition, the sensor placement is aligned with the critical pressure points discovered in previous work [81,82]. To drive the insoles, we developed a voltage divider circuit interfacing the insole with the Sparkfun Razor board that consists of an SAMD21 microprocessor manufactured by Atmel Corporation in San Jose, CA, USA. To greater understand the angle and motion of the leg and feet, we also queried the accelerometer of 9 DOF IMU (MPU-9250) manufactured by InvenSense in San Jose, CA, USA that comes with the microcontroller board, which was tied to the user's ankle. We decided to only utilise the Accelerometer (X,Y,Z) data, since it provides sufficient information. The prototype has a 3.7 V, 400 mAh LiPo battery and an SD card, to allow a fully wireless operation, without constraining the user's movements. The prototype can be used for approximately 16 h when it is fully charged.

**Figure 1.** Prototype: (**A**) shoe, (**B**) sensing.tex insole, (**C**) ankle strip, (**D**) SAMD21 microprocessor board with inertial measurement unit (IMU), secure digital (SD) card, voltage divider circuit and (**E**) 400 mAh Battery.
