Towards Sustainable Smart Orthotics: Acoustic Characterization of Bio-Filled, Additively Manufactured Perforated Conductive Polymer Composites^† <span style="font-size: 10.5pt; font-family: 'Times New Roman',serif; mso-fareast-font-family: 宋体; color: black; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA;">

This study presents a unified framework for tuning the acoustic absorption of 3D-printed conductive panels&mdash;made from polylactic acid (PLA) and thermoplastic polyurethane (TPU)&mdash;by systematically varying polymer type, infill ratio, cavity depth, panel thickness, and bio-filler. In unfilled configurations, TPU achieves peak mid-band absorption (&alpha; &asymp; 0.94 at 2.5 kHz; mean &alpha;̄ up to 0.55) only at 100% infill with cavities &ge;30 mm, while partial infill yields much lower performance (&alpha;̄ &asymp; 0.27&ndash;0.32 above 2 kHz). PLA exhibits micro-perforated panel behavior, with optimal broadband absorption (&alpha;̄ = 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 (&alpha; &gt; 0.60 at 800 Hz; peak &alpha; = 0.96 at 1.3 kHz; &alpha;̄ = 0.73 to 6.3 kHz), granular charcoal (GC) provides consistent broadband damping (&alpha; = 0.62&ndash;0.78 at 30 mm; &alpha;̄ = 0.71 at 50 mm), and wood sawdust (WS) yields narrow resonance peaks (e.g., &alpha; &asymp; 0.91 at 2.5 kHz for 15 mm gap), ideal for mid-band noise targeting. In TPU composites, AC maintains high broadband absorption (&alpha;̄ &asymp; 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 (&eta;&sup2; &asymp; 0.10&ndash;0.27), cavity depth (&eta;&sup2; &asymp; 0.10&ndash;0.20), and thickness (11&ndash;29% variance) as dominant factors, enabling targeted, EMI-shielded, sustainable orthotic designs.