Partial Weight Bearing Insole

BMED 2310 design project, Georgia Tech, team of 5, Jan to Jul 2025

What

A wearable that measures how much weight a patient puts on an injured leg and shows it live on their phone, so they stay inside the partial weight bearing limit their physical therapist sets.

Why

After a lower-limb fracture, patients are told to bear only a percentage of their body weight, but even trained physical therapists misjudge it by up to 30%, and training on a bathroom scale does not carry over to walking. The few biofeedback devices cost about $700 to $7,000 or are not sold in the US.

How

Placed three 50 kg force-sensitive resistors at the foot's three critical contact points, sewn into a sock liner worn in the patient's own shoe
Built the processing unit around an ESP32, with the sensor circuit and a rechargeable 3.7V LiPo with a charging booster, in a 3D-printed PLA housing
Wrote the Arduino C++ firmware and calibrated the three sensors with a linear regression against a bathroom scale, across users of different weights
Streamed calibrated load over WiFi to Arduino Cloud for a mobile app with a live gauge and progress over time
Helped model the looks-like CAD for the next version, a double-layer sock with a miniaturized processing unit

ESP32
Arduino C++
Force-sensitive resistors
Calibration
Linear regression
Arduino Cloud
LiPo charging
3D printing
CAD

Red 3D-printed PLA housing open, showing the ESP32 processing unit, wiring and battery — Processing unit: ESP32, LiPo and charging in a PLA housing

Foot on a bathroom scale with the phone app reading load during calibration — Calibrating against a bathroom scale

Phone app showing a live load gauge and a load-over-time chart — Live gauge in the Arduino Cloud app

Scatter plot of sensor reading against scale reading at 50% partial weight bearing, with a linear trend line — Sensor vs scale at 50% body weight

Results

Within 3% of a reference scale at 25, 50 and 75% body weight (n = 20, paired t-tests)
Working prototype from foot sensors to a live phone gauge

Modeling

The CAD for the next version: wiring sealed inside a double-layer sock and a processing unit small enough to sit at the ankle.

CAD render of the double-layer sock with the processing unit at the ankle — Double-layer sock

CAD render of the sock with internal wiring exposed — Internal wiring

CAD render of the sock sole showing sensor placement — Sensor placement

CAD renders of the miniaturized processing unit from several angles — Miniaturized processing unit