Flexible Smart Insole and Plantar Pressure Monitoring Using Screen-Printed Nanomaterials and Piezoresistive Sensors

Abstract

Individuals experiencing gait dysfunction─such as the elderly, those with peripheral nervous system damage, or individuals with Parkinson's disease─face a heightened risk of physical injury due to imbalanced weight distribution. Despite recent advancements in wearable movement trackers, there remains a significant need for a reliable long-term plantar pressure monitoring system. While some existing devices measure pressure characteristics, many are hindered by limitations in spatial resolution, sensitivity, and the presence of bulky peripherals. Here, we introduce a flexible smart insole system that integrates screen-printed nanomaterials to create a high-density piezoresistive sensor array, enabling accurate plantar pressure measurement during daily activities. To ensure scalable and cost-effective manufacturing, we utilize a screen-printing method to fabricate 173 carbon-based sensors directly onto a flexible insole circuit. The printed sensors demonstrate a remarkable sensitivity of -0.322 kPa^-1, surpassing previous benchmarks. When combined with a wearable mobile communication circuit, this system offers a comprehensive analysis of the user's plantar pressure distribution. Experimental studies conducted with human subjects showcase the smart insole's real-time monitoring capabilities in common daily ambulation scenarios. The integration of high spatial resolution, exceptional sensitivity, and a fully mobile wearable system holds significant promise for enhancing outcomes across various applications, from healthcare to athletics.

Keywords: flexible smart insole; gait analysis; piezoresistive sensors; plantar pressure monitoring; screen-printed nanomaterials; wearable electronics.

1

Overview of a flexible smart insole and plantar pressure monitoring system using screen-printed nanomaterials and piezoresistive sensors. (A) Illustration of the smart insole system in use. (B) Example of the user interface when connected to the insole system, showing pressure heatmap, gait characteristics, and CoP trajectory tracking. (C) Exploded view highlighting the screen-printed multilayered functional materials, integrated wireless electronics, and its low-profile form factor for user comfort.

2

Optimization of CE² ink mixture and fabrication parameters. (A) Illustration outlining primary fabrication steps of the piezoresistive sensor array. (B) Optical microscope images and surface characterization of the bare and printed IDEs. (C) Resistivity of printed CE² when mixed at different ratios of carbon-epoxy ink and PDMS (n = 10). (D) Overall height of the sensor when printing 1–5 layers (n = 10). (E) Resistivity of printed CE² when printing 1–5 layers (n = 10).

3

Mechanical and electrical characterization of pressure sensors. (A) Schematic illustration showing the piezoresistive working principle of the sensor. (B) Equivalent circuit for a single sensor array of the insole. (C) Theoretical response curve of a sensor during loading and unloading of applied pressure. (D) Single sensor (carbon-epoxy:PDMS = 70:30 (wt %)) response upon applied pressure (n = 1). (E) Durability test over 400 bending cycles at 90° (n = 1). (F) Calibration test results of a single sensor (carbon-epoxy:PDMS = 70:30 (wt %)) using four different calibration weights (n = 1). (G) Sensor sensitivity as a function of printed layer number (1–5 printed layers; n = 1).

4

Real-time wireless pressure monitoring system. (A) Overview of the test setup for continuous pressure monitoring with the fabricated smart insole and integrated electronics. (B) Schematic of data acquisition and processing flow throughout the smart insole system. (C) Example of the heatmap as viewed in the user interface. (D) Validation of high spatial resolution with a human subject. Heatmaps show regional plantar responses during different phases of the gait cycle.