Towards Sustainable Smart Orthotics: Acoustic Characterization of Bio-Filled, Additively Manufactured Perforated Conductive Polymer Composites†

This study presents a unified framework for tuning the acoustic absorption of 3D-printed conductive panels—made from polylactic acid (PLA) and thermoplastic polyurethane (TPU)—by systematically varying polymer type, infill ratio, cavity depth, panel thickness, and bio-filler. In unfilled configurations, TPU achieves peak mid-band absorption (α ≈ 0.94 at 2.5 kHz; mean ᾱ up to 0.55) only at 100% infill with cavities ≥30 mm, while partial infill yields much lower performance (ᾱ ≈ 0.27–0.32 above 2 kHz). PLA exhibits micro-perforated panel behavior, with optimal broadband absorption (ᾱ = 0.40) at 50% infill and a 30 mm cavity, while full infill reduces effectiveness. Introducing bio-fillers into conductive PLA significantly alters performance: activated carbon (AC) boosts low-frequency absorption (α > 0.60 at 800 Hz; peak α = 0.96 at 1.3 kHz; ᾱ = 0.73 to 6.3 kHz), granular charcoal (GC) provides consistent broadband damping (α = 0.62–0.78 at 30 mm; ᾱ = 0.71 at 50 mm), and wood sawdust (WS) yields narrow resonance peaks (e.g., α ≈ 0.91 at 2.5 kHz for 15 mm gap), ideal for mid-band noise targeting. In TPU composites, AC maintains high broadband absorption (ᾱ ≈ 0.73), GC performs well even at shallow cavities, and WS produces dual-band peaks (~1 kHz and ~4 kHz at 50 mm). Thinner panels favor high-frequency absorption; thicker ones enhance low-frequency response. Statistical analysis identifies filler type (η² ≈ 0.10–0.27), cavity depth (η² ≈ 0.10–0.20), and thickness (11–29% variance) as dominant factors, enabling targeted, EMI-shielded, sustainable orthotic designs.