Characterization and Design of 3D Printed Composite Insole Fabricated by Dual-Nozzle 3D Printer Using CB-PLA and Fe-PLA Filaments

Technical Summary: Characterization and Design of 3D Printed Composite Insole Fabricated by Dual-Nozzle 3D Printer Using CB-PLA and Fe-PLA Filaments

Problem Statement
Existing smart insole technologies for monitoring plantar pressure and gait parameters often rely on externally mounted sensors, wired electronics, or multi-layered assemblies. These conventional designs frequently suffer from reduced comfort, increased structural complexity, and susceptibility to mechanical failure under repeated loading. While Fused Deposition Modeling (FDM) offers a pathway for complex geometries, traditional single-material printing limits functional integration. Furthermore, assembling separate components via adhesives or mechanical fasteners creates weak interfacial bonds that compromise durability and performance. There is a need for integrated insole systems where sensing functionalities (electrical and magnetic) are directly embedded into the structural body to enhance robustness and user experience.

Methodology
This study utilized a dual-nozzle FDM 3D printer (Ultimaker S3) to fabricate composite structures and smart insoles using three types of polylactic acid (PLA) filaments: standard PLA (sheath), carbon black-filled PLA (CB-PLA, conductive core), and iron-filled PLA (Fe-PLA, magnetic core).

The research proceeded in two phases:

1. Composite Structure Fabrication: The authors compared two fabrication strategies:
   • Assembled Structures: Sheath and core components were printed separately and physically joined post-printing.
   • Integrated Structures: Sheath and core materials were co-printed simultaneously in a single continuous process.
     Samples were created with core diameters of 1.25, 2.50, 5.00, 7.50, and 10.00 mm, and a fixed thickness of 5.00 mm.
2. Smart Insole Fabrication: Based on the performance of the composite structures, the integrated strategy was selected for insole production. Insoles were designed with six sensing regions corresponding to primary plantar pressure zones (metatarsal, midfoot, heel). These were fabricated with core diameters of 2.50, 5.00, 7.50, and 10.00 mm, and varying thicknesses of 1.00, 2.50, and 5.00 mm.

Characterization and Testing
The fabricated samples were evaluated based on:

• Printing Efficiency: Actual printing time and total weight.
• Electrical Properties: Current-voltage (I-V) characteristics measured using a source meter (Keithley 2450) to assess conductivity in CB-PLA structures.
• Magnetic Properties: Magnetic field strength measured using a Gauss meter (LANDTEK GS-100D2) for Fe-PLA structures.
• Mechanical Properties: Compressive strength and deformation behavior tested via a universal testing machine (Instron 5567), including analysis of stress-strain curves and post-compression dimensional changes.

Key Results

• Fabrication Efficiency: Assembled structures required approximately 1 minute less printing time than integrated structures due to the separate fabrication of components. However, integrated structures were consistently lighter than their assembled counterparts.
• Electrical Conductivity: Only PLA/CB-PLA structures exhibited electrical conductivity; PLA/Fe-PLA structures were electrically insulating. In integrated PLA/CB-PLA structures, electrical performance improved with larger core diameters. The integrated PLA/CB-PLA-10.00 structure (1.0 mm thickness) achieved the highest current of 0.10 A at 24 V. Thinner insoles generally demonstrated higher conductivity.
• Magnetic Field Strength: PLA/Fe-PLA structures generated magnetic fields, while PLA/CB-PLA structures showed negligible magnetic response. Magnetic strength increased with core diameter and decreased with insole thickness due to magnetic flux attenuation. The integrated PLA/Fe-PLA-10.00 insole (1.0 mm thickness) achieved a maximum magnetic field strength of 351 ± 8.84 Gs.
• Mechanical Performance: Integrated structures demonstrated superior mechanical integrity compared to assembled structures. Assembled samples exhibited weak interfacial bonding, leading to sudden stress drops and irregular stress-strain curves during compression. In contrast, integrated structures showed stable S-S curves, higher load-carrying capacity (up to 10.22 kN for integrated PLA/Fe-PLA-10.00), and uniform deformation. The continuous bonding in the dual-nozzle process facilitated efficient stress transfer and reduced internal voids.

Significance and Claims
The study concludes that the dual-nozzle FDM printing of PLA-based composite filaments offers a practical and scalable method for producing multifunctional insoles. The primary significance lies in the integrated sheath-core design, which eliminates the need for post-printing assembly and adhesive bonding, thereby reducing structural complexity and improving mechanical durability.

The authors claim that by controlling core size and thickness, it is possible to tailor the electrical and magnetic properties of the insoles for specific sensing applications. The integrated approach successfully combines structural integrity with embedded sensing capabilities (electrical for CB-PLA, magnetic for Fe-PLA), establishing a foundation for future research into fully integrated, wearable footwear for gait monitoring and rehabilitation without the limitations of external sensor mounting.