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[PCB Desktop Test with an Arduino as temporary Power Source]

Motivation

Diabetic Foot Ulcers are a severe and common sympton of diabetes. With the reduced peripheral sensory capabilities many diabetic individuals struggle with, small injuries or blood flow reducing positions can lead to tissue death and infection on the surface of the foot. This device is a multimodal sensing insole designed to create a uniquely comprehensive view of the plantar surface, through pressure, shear force, temperature, humidity, and acceleration. This data is collected with the intention of detecting the precursor conditions to DFUs, in an attempt to warn the wearer and prevent formation.

Approach

I independently designed and built the multimodal rigid/flex PCB in Altium Designer, integrating:

• Temperature and humidity sensing
• Accelerometer for motion/orientation detection
• Vertical force measurement
• A novel shear force detection method using force-sensitive resistors (FSRs)

The board is driven by a Nordic nRF5340 microcontroller module with Bluetooth Low Energy (BLE) connectivity for wireless data transmission.

For validation, I designed custom experiments to generate Bland-Altman plots for the temperature and humidity sensors and created models for the shear sensor response. Iterated through shear sensor designs to optimize accuracy and repeatability.

[Insole functional block diagram]

Results

The full PCB was verified to be functional, with sensor readings near or within manufacturer guidelines, and Bluetooth functioning as expected. While iterations continue to improve the shear sensor capabilities and grow the dataset to prevent overfitting, an initial model produced good results:

• Detects on-axis shear forces of approximately 5 N
• Bill of materials as low as $20
• Measures vertical force to within 1–2 N accuracy

This work is being prepared for submission to an IEEE journal.

[Manufactured PCB (left) and Altium PCB (right)]